I installed the hood and hoop supports. The hood is made from PVC coated nylon (same type as the skirt) in 260gm/m2 (about 9oz/yd2) which is sold as groundsheet material. It doesn't have the wear resistance that the skirt has but that shouldn't be needed as I don't intend to hover on the roof ;-).
The hoops are made from 20mm PVC pipe with 15mm plastic water pipe internal reinforcing in the "soft" areas. It's not as stiff as steel or aluminium pipe but it weighs a lot less than either and won't rust or need painting.
Fitted a 25W marine radio and aerial (I thought I'd better have one as I seem to be spending quite a lot of time on the sea!). It's a digital type (DSC) with a GPS connection that automatically transmits your position and emergency call on channel 70 - all by pressing one button (carefully protected under a flap of course!).
I fitted the aerial to the top rear of the prop guard so that it folds down horizontal when the guard is folded forward. The aerial is supposed to be a minimum of 3metres away from anyone when transmitting - it's pretty difficult to do that on a small craft!
I finally discovered the source of the annoying squeak I've been hearing for some time. I removed the prop so I could get my head right into the engine bay while it was running (and to remove a source of noise). The noise was coming from the prop pulley area. I removed the prop bearings and shaft, etc to discover that the rear bearing was loose on the shaft! The bearing has worn a depression in the shaft surface of around 0.004" deep probably due to vibration or fretting. It wasn't obvious at all until I removed the belts as the belt tension holds the bearing against the lower part of the case. Further investigation showed that the lower half of the bearing retainer (a two part pressed steel shell) was bent down slightly in the centre - It wasn't gripping the bearing properly. I had supported the shell using 12mm nut "spacers" under the two bolts. As there was no support in the centre of the bearing shell, the belt tension and engine power had obviously pulled the shell downwards and deformed it.
I managed to clean up the shaft and moved the bearing forwards onto a "clean" section of shaft - I then bent the shell back into shape.
To prevent this happening again I've fitted a length of angle under the bearing shell with the nut spacer below it - that should provide enough support for the shell. With hindsight, I now remember having to tighten the belt after a couple of hours running time. I suspect that the shell was bent at this time and I was taking up the slack caused by the bearing moving downwards - I had thought it was just the belts slackening after first run! The moral to this is that if you are using a pressed steel bearing housing make sure it is well supported!
After having problems with the exhaust leaking (mainly due to me using a dodgy rusty second hand exhaust!) and having problems with the engine (due to muck in the carb) I decided it would be easier to swap out the engine for another one I have in a pickup (50K miles, runs sweet).
I stripped out the old engine and discovered that the rear main bearing on the extension shaft had slight wear in it. I removed the crank/flywheel extension and pulley assembly and put it in the lathe - I found that the bearing had a distinct wobble. I decided to replace the bearing and get a new extension shaft made up (this time the shaft started at 35mm diameter and was turned down to 30mm to make absolutely sure it was true.
I got the new engine installed complete with replacement bearing and shaft. I measured the vertical run-out from the rear cross member to the top of the bell housing at 0.005" (0.15mm). It's surprising how flexible the engine assembly is - you can deflect the bell housing by about 0.004" just by pulling on the prop during a compression stroke on the engine. After discussing a problem Mike (warx) had with belt slap I fitted touch bars to the frame near the top. The four separate belts all had slightly different tension and would resonate at different frequencies - the bars have stopped this almost completely.
I also decided to clean up the rudder and throttle controls. The rudder has always been a bit floppy and sometimes stiff so I removed the throttle cable and used it as one cable in a new push pull arrangement for the rudder. It's now easier to turn and more positive.
Double steering cable mechanism - the steering wheel end.
The throttle has always been a bit sticky due to the cable run - it had a 180 degree turn just before the control lever and was a 19 foot long cable. After re-using the long cable for the rudder I replaced the throttle cable by using the heater valve control cable (not really needed as you can leave the water circulating through the heater matrix and just use the air flap valve in the heater unit). I also changed the routing as this cable is a couple of feet shorter - it now has a single 90 degree bend at the lever end and a larger sweep at the engine end.
New engine installed and ready to go
It has a very smooth idle - it idles at 500-600rpm and is smooth as silk.
The Scout air drive sits in the Prospector deck for transit - the Scout hull rides on the roof rack of the car.
After the beach recovery on Sunday past (see Diary 2006), I discovered that the Powerfin prop was showing serious signs of erosion about 26" down from the tip on both blades. There were a line of pits along the leading edge with glass visible in them under a magnifier.
Above pic as an extreme close up - it isn't as bad as it looks! the pits are around 0.3mm (0.011) maximum. The factory fitted tip tape was in bad shape also - although it had protected the area underneath. The tape had what looked like air bubbles in a few places - when I removed the tape the 'bubbles' were filled with grit/sand that had eroded the prop under the tape
This pic should the erosion 'edge' between the area that was protected buy the tape and the unprotected area. You can also see the joint line between the upper and lower halves of the prop shell on the tape protected area (this looks like a weak point in the design as this rough joint is right on the leading edge where it will get the worst exposure to debris). This particular prop design is made in a two part mould with gel coat, a layer of glass then carbon fibre. The two halves are joined with epoxy soaked soft PU foam compressed in between. Erosion on the leading edge is potentially serious as it could compromise the shell joint and result in de-lamination of the blade.
After the first couple of hours from new, the prop rear surface (the pressure surface) started to lose it's gloss - it turned a light gray colour. The 'graying' has now spread around the leading edge onto the front face of the prop for about 1/2". The exposure to sand on Sunday must have accelerated the erosion - as you can see from the videos, I usually launch down a sand ramp at my 'normal' cruise site. I would estimate around 1.5-2 hours total exposure to dry sand in the 200 hours this prop has been used. The sand at this site is very fine - almost dusty - the type of sand may be contributing to the erosion.
After discussions with Barry Palmer, Stuart Gort of Powerfin and Jeremy Harris (microlight user), I decided to try to repair the prop. After sanding and cleaning the surface, I have overlaid the leading edge with a strip of biaxial glass tape and epoxy. I then coated both upper and lower surfaces with epoxy resin as a sacrificial layer - it should protect the underlying gel coat. I have also fitted 4" wide tape from the tip to almost the root (3" on pressure face, 1" on the front face. Hopefully, this will extend the life of the prop a bit - I can't afford to buy a new prop twice a year!!
Above is my curing poly-tunnel (with prop inside) is shown above. I use a fan heater and some really lightweight plastic sheet - it self-inflates around whatever you are trying to cure. You can get a good temperature rise inside - leave the end open to prevent it from overheating! I can cure polyester panels in ten minutes and epoxy in an hour or so regardless of the outside air temperature, The lightweight sheet means you an easily inflate it around any size or shape of object - I've even put the entire Prospector hull under the sheet during winter to cure panel joints quickly.
Powerfin say that this prop isn't really suitable for "harsh" environments - the gel coat is too hard and abrades easily. Stuart Gort suggested using a Warpdrive prop fitted with nickel leading edges - seriously expensive and heavy! I don't consider my use to be a harsh environment - a couple of minutes of sand exposure in four hours isn't what I would call harsh - in the UK plenty of hovercraft only every cruise on sandy beaches!
I have to say that, overall, I wouldn't recommend using a Powerfin prop on a hovercraft if you are ever anywhere near dry sand. With hindsight, fitting wider/longer/better quality protection tape would have prevented the serious damage, However, short of covering the entire pressure surface, the graying erosion would still occur and would eventually lead to failure.
I also have to say that I've had fewer problems with the UH wood prop and the Ultraprop (Scout). The softwood UH prop is epoxy glass covered, painted and has tape on the leading edge. I have replaced the tape a couple of times over the last two years and also had to make a minor repair to 1/4" of one prop tip but the remainder of the prop is in excellent condition. The Ultraprop has no tape and the leading edge is still as sharp as a knife! Multiwing type fan blades also seem to be almost immune to edge erosion. The hardness of the surface of the blade seems to be the critical factor. This is probably why soft prop tape works so well. If you use very hard metal tape it would probably also be OK - the problem is the in-between stuff which is too hard to absorb particle impact and too soft to prevent chipping.
Had engine problems for last few days - the engine would hesitate and run rough when opening the throttle from idle. It was OK when the second choke opened. Eventually discovered a bad plug (even though they had only done about 20 hours running. It was getting dark last night when I was running the engine and I could see an orange glow inside the plug ceramic - the resistor must have broken down and allowed the spark to arc across the gap internally.
I also stripped down the carb and cleaned it during this fault find. I decided to swap the carb and inlet manifold for an earlier version that didn't have all the emission control stuff on it (Exhaust gas re-circulation, etc, etc) - it's just more stuff to go wrong and isn't needed. The only emission stuff left is the PCV (positive crankcase ventilation) and the one way valve to collect vapour from the float bowl. I also adjusted the second choke opening gap - it was way out and is adjusted just by bending a tab on a small lever - very easy to 'accidentally' bend it is you dump the carb down a bit hard!
I tested the craft in the field - the engine ran great - throttle response was excellent. The new type prop tape started to come unstuck at the tip after about ten minutes - not a success!
I replaced the prop tape with a different brand which has a much better adhesive - after a trip on the river it showed no signs of damage or peeling at all. I also replaced the main prop shaft with a stainless steel version - It's impossible to remove the bearings using a plain steel shaft after a couple of sea water trips! While doing this, I noticed one of the fan shaft bearings was sounding noisy . I removed all four - three of them were OK but I replaced them all with new double sealed versions (the bad bearing had gritty grease although the seal seemed undamaged). As a test, I've 'sealed' the end of the bearing to the shaft using silicon sealant - I cleaned the bearing shell and greased the shaft so the sealant only sticks to the bearing case.
I've had a persistent oil leak (only when engine running) at the front of the new engine I installed. It's got to the point that it's losing about 1 litre of oil every 4-5 hours running time. There is no leak when the engine isn't running so it has to be on the pressure side of the oil system. I replaced the sump gasket with the engine in place (pretty tricky as there isn't much room!) but that didn't fix it. It looked like it was coming from someplace near the front centre as there was oil at the sump to block joint. I then removed the engine as it's impossible to see what's going on at the front (even with mirrors/lights/ etc). I replaced the O rings in the oil pump housing - new gaskets and oil filter and re-installed the engine.
It looked great for about 20 minutes - then the oil started coming out again!!!! The only thing left was the crankshaft oil seal so I removed the engine again (only an hour to do this time - practice makes perfect!) and got the front pulley off with a bit of a struggle. There was a small weep from one corner of the seal so I removed it only to discover that the seal I'd bought about 3 months ago was the wrong size! I ordered another one which arrived in a day but it was also the wrong size - it just dropped right through the crankcase hole. The crankcase measured 59mm and the seal was 58.9mm - not much in it. There is no inner lip on the crankcase, it's just the edge of the aluminium casting - I could have just glued/sealed the seal into place it there had been a lip. I re-ordered the seal from an aftermarket supplier AND from a Subaru dealer (delivery THREE WEEKS!).
Still waiting for the oil seal! I bought a couple of driving lights to fit to the craft (the under bumper 55W style. It would be very difficult to install them into the front of the nose as the surface slopes backwards and outwards - I'd need to make a deep cowling to fit them. I've installed then on the top deck just in front of the windscreen - that way I should be able to adjust them while driving. Hopefully I won't need them but, after my last nighttime experience, I'd rather have them ready!
Started to repair skirt from yesterdays little adventure (see 2007 diary for the full story).
First, I lifted the craft off the trailer using a tractor back hoe and a bit of pushing and shoving. I built an 18" high frame to support the back end and held the front end up using a pair of axle stands to give enough room to work underneath. The rear skirt was completely split top to bottom at the back and had been ripped from the rear and right side inner attach strips. I also had to repair two big slits I had cut myself to try to stabilize the craft off hover.
A lot of the skirt damage was probably caused by using the craft in "boat" mode for 30 miles!
The split in the rear skirt panel
There is also a damaged hull area half way along one side where I collided with a bridge pier.
The damage had crumpled the foam and de-laminated the foam/fibreglass (top and bottom skins over about a 6" area. I will cut out the damaged foam and epoxy a new section in and also epoxy the wood together again before re-laying the top skin.
I spent a couple of hours gluing the skirt back together - there were a few tricky bits on the curved outer back sections that you can't lay flat - I had to glue strips on in sections. I had to put patches over the areas where the screws had ripped through the skirt - I still have to do the last joint to connect the ends of the skirt together again.
Pic showing side deck foam that I cut out - it had failed and was covered in cracks and loose areas. I left the inner skin intact and will bond a new foam section to it then glass over the top.
I clamped the timber edging together (after gluing with thickened epoxy) using two lengths of wood screwed through the outer attach strip on top of some polythene sheet - the top and bottom decks are at an acute angle and can't be easily clamped. I'll just fill the screw holes before painting this section.
Finished hull repairs and re-attached the skirt. Craft hovers OK. I still have to paint the area where I repaired the foam but I'll have to wait until the weather improves a bit. I also fitted an all round white light to the top of the duct to comply with navigation regulations.
Fitted larger rectangular spotlights with Xenon bulbs. I was tempted to fit 100W or 130W bulbs but I would have to add a power relay and upgrade the power feed cables from the battery.
I've replaced the hood stays (PVC pipe) with aluminium tube (an old tent frame). the PVC worked OK but was a bit too flexible. The tent poles slot together - I can dismantle the entire hood and store it under one of the side decks. I've also started to make side screens to fit the hood. I bought some clear PVC - as used in tents, etc) and have glued it into cutouts in the side panels, I have ordered poppers to attach the top edge to the main cover and will hold the bottom edge to the hull using elastic cord around cleats. It's really designed to let me camp inside the craft rather than just providing weather protection when using it. The Prospector floor area is more than big enough for two people to lie down on!
Completed side screens for hood. I've also made small windows in the sides of the main hood at the front to improve driving visibility.
The cabin now gets very warm with the heater running! You also just see one of the repaired skirt tears halfway along the side in the pic above.
Finally decided it was time to swap engines again! The oil consumption was noticeably worse on the last outing - about 1/2 litre in under two hours cruising. I fully serviced the original engine (and remembered that I swapped it because the exhaust mount holes were stripped in the head!). I rep threaded the holes and fitted a new crankshaft seal.
When I removed the old engine I discovered that the crankshaft extension shaft wasn't properly locked to the rear pillow block bearing and had been chattering - the shaft had wear marks on it. I got a new shaft made up. The run out at the end of the shaft when everything is mounted in the craft is 0.05mm (0.002"). I reckon that's probably as good as you can get. It is pretty difficult to measure accurately. I tried using a dial gauge mounted on the rear bearing cross member with the pointer on vertical face of the aluminium flywheel adapter. This method includes both crankshaft journal bearing movement and any surface irregularities on the plate. One other problem you get is that the whole frame/engine bends slightly depending on which part of the engine stroke - compression strokes seem to be worse - not surprisingly! The most consistent method I found was to mount the dial gauge on the cross member with the pointer on the vertical face of the engine bell housing (right at the top). The bell housing is the same distance (vertically) from the flywheel centre as the rear bearing is from the flywheel centre horizontally. Therefore any movement in the vertical plane is the same amplitude as in the horizontal. The other things I had to do were to remove the spark plugs to make the engine turn easy and to polish the area where the dial gauge pointer touches the bell housing. I turn the engine slowly and steadily using the prop to avoid applying pressure to the engine or frame.
Tested craft in the field only to discover that the front engine pulley (water pump/alternator) had a bad wobble. I replaced it with another one and it's OK now. The engine ran very well - starts easy although it's a bit rougher than the old engine at idle.
I had a loan of a big MIG welder for a day (to make a trailer for Peter) so I decided to modify the prop shaft mount frame. I've chosen to put the prop pulley between two bearings rather than overhanging the end. In addition to providing better support it reduces the length of the prop shaft quite a bit.
For some reason the above pic shows the prop shaft as if it's out of line horizontally - it isn't!
It was actually pretty easy to do. I just chopped the bearing support section off just forward of the rearmost bearing, turned it around 180 degrees and welded it onto the rear support - it was side by side welded so that it was offset 1" or so.
I had to extend the supports that go forward and down to the lift duct and also add new angled braces to the rear. I used a cast pillow block as the rear bearing to take the thrust load - better than the pressed steel bearing housings originally used.
The main disadvantage of having the pulley between two bearings is that it's harder to remove the belts. In reality, it's a big job removing the prop belts anyway as you need to remove the lower engine bearing block and both fan belts - the prop belt is against the flywheel with the fan belts behind it!
Carried out noise tests in field - it was a nice still evening. The Prospector reads 85dBa at 25 metres. The highest reading is from directly behind the craft - not surprising as the exhaust and lift fans point backwards. The lift fans are the very obvious loudest noise source - they have the typical fan "howl" at high speed.
Replaced rear prop shaft bearing - it has some play in it. I also made up a belt tensioning frame (as per plans). I replaced both bearings with new ones and used plastic pillow blocks with sealing end caps. They are designed to hold stainless bearings in food processing machines - rust and leaking grease aren't a good idea in these places!
I filled the plastic covers with grease and also sealed them to the housing with silicon to prevent grit or water getting into the bearing.
Installed Powertwist belts for testing. I've replaced one of each of the fan drive belts and one prop belt with PowerTwist A section belts. These belts are made up using separate links that are locked together using tabs on each link.
They can be installed onto the drive as an open belt and re-connected - without removing fans/bearings/etc. I suspect that they will have better water slip resistance due the their non-continuous sidewall.
They supposedly have better vibration absorption characteristics. There is a short video clip (click here) showing the belts running at just above a cold engine idle. You can see the standard V belts flapping but the orange Powertwist belt hardly moves.
After around 20 hours of use I re-tensioned the Powertwist prop drive belt (by removing a link) - the two fan belts didn't need re-tensioning. The belts edges are a little black but otherwise there is no sign of wear.
I tested a four blade 59" UltraProp in place of the PowerFin prop. I carried out noise and thrust tests in a local field. the PowerFin was around 86dBa and the UltraProp 89dBa. The Ultraprop was actually noticeably quieter at under half throttle but noisier at full power. The Ultraprop seemed to produce slightly less thrust (I don't have a balance capable of measuring the actual thrust but the craft was slower accelerating uphill in the field). Overall, I would say that the Powerfin is a better prop in this setup (maybe because it is larger diameter and has a wider chord?). The only downside of the Powerfin is it's erosion resistance (see above!).
After discovering that I had to re-tighten the Tablink prop belt yet again I decided to remove it (it was slipping). I've left the two fan drive Tablinks belts fitted for the moment as they seem to be fine.
Before the last trip (to the Tay) I swapped the carb for another one I had in stock(!) - the one that was fitted had a very rough idle. The replacement carb idled fine but, during the trip, I discovered it had a nasty flat spot. It was smooth up to about 3100rpm then further throttle lever movement would do nothing until, all of a sudden, the engine would rev to 4100rpm. You couldn't get it to run anywhere between 3100 and 4100RPM. I stripped the original carb and got it cleaned at a local lawn mower repair shop that had an ultrasonic cleaning bath. Once re-fitted, the engine runs great - smooth idle and good smooth throttle response. Probably a good idea to get the other carb cleaned the same way!
Decided to replace the skirt. During the Severn trip when operating on smooth water, I was having problems steering at times - the back end would "stick" at one side then the other making it difficult to maintain a straight course. The skirt has several large repairs on it and is showing bad wear at the front corners. The new skirt has rivets fitted to the wear point between the front quarter, front curtain and side skirt joint.
I gave the under hull a good check over while the craft was skirtless - the only damage was a couple of small scrapes on the front lower plough plane (see above).
The craft looks really skinny without a skirt! You can also see why they have such good sea capability - the hull is very boat like
Making the skirt seemed to take a long time - this time I fitted two flotation foam strips instead of one (I've used thin camping mat foam instead of the thicker pool noodle type). I also extended the strips around the back and up into the forward quarter cones. Hopefully this should make water starts a bit quicker. It had better - it took me a whole day to install the flotation foam (126 skirt material loops to make and glue)!
Replaced a small section of one of the side decks where the foam had delaminated. There was a small crack extending under two of the anti slip strips. The crack had started at a point where I had obviously sanded through the glass fibre - this was clear once the tape had been removed as the surface filler just crumbled away! I used a new type of foam on this repair - Airex C52.60 (60 Kg/m3 about 3.7lbs/ft3). This foam is a modified polyurethane with fibre reinforced skins bonded to the foam. It is cold and hot formable and is far more elastic than the standard PU foam - you can bend it to more than 90 degrees with no sign of cracking. the skins also make it very easy to cut and bond as you don't need to carry out any surface preparation or cleanup. Even better - it's cheaper than the standard PU foam! Time will tell how good it is (I've also replaced the Scout lower plough plane with this foam)
Above pic shows the top skin removed from the affected area. You can clearly see where the top skin was cracked as dirt has gotten into the foam. I usually work out what area has delaminated by pressing on the top skin - you can hear a slight rustling sound caused by the foam rubbing on the skin if it has delaminated
Finished repair. It's important to sand off any paint for an inch or so around the repair to make sure the resin bonds to the surrounding skin rather than just to the paint!
Time for the yearly bearing check! I jack up the engine and remove the belts for the prop and fans so that I can rotate and feel all of the bearings. The prop bearings were fine (replaced last year with plastic housings - see above). The lift fan bearings had no obvious play on them but were a bit 'rumbly' so I replaced them. The rear engine bearing had some wear on it but was smooth so I left it alone (after re-greasing it pushed out a small amount of clear water).
It's almost impossible to remove these bearing from steel shafts (using a stainless shaft helps but the bearing still corrodes a bit) - the bearings are a pretty snug fit onto the shaft and it doesn't take much corrosion/etc to seize them solid. I usually cut through the beatring using a thin cutting disc almost right through to the shaft. (the cut is made along the shaft). If you then hammer a chisel into the cut (and while supporting the shaft on the other side), the bearing shell will usually split open and fall off the shaft (the bearing tracks are high carbon steel and are pretty brittle).
Replaced the rear inner skirt attach strip I used temporarily at the Loch Fyne Hoverin back in May. I had used a length of1mm steel plate and it was starting to seriously rust away! I got a length of 5mm aluminium plate bent to fit up inot the lift duct by about 50mm. I filled and glassed over the old screw holes and damage and alos fitted a section of polyurethane skirt material as a "damage reducing" flap to protect the inner rear skirt from abrasion.
The prop tape was showing signs of damage on one blade - there was obviously a pin hole near the hub and debris forced in there was being pushed along the blade under the tape to the tip by centrifugal force peeling the tap off as it goes! Removing the tape damaged the paint so I had to re-finish a blade and then re-balance before fitting new tape.
While putting the craft back onto the trailer without the prop fitted I noticed that it seemed very noisy. One of the fan bearings was VERY noisy (it had slight "cogging" when rotated by hand - a BAD sign! I removed both fan shafts and fitted all new bearings (one other of the four was a little bit noisy but the other two seemed fine). The bearings were replaced in July so they haven't lasted long at all!
The prop tape was showing some signs of wear so I removed the prop and the old tape. In doing so, the tape tool some of the epoxy coating and paint off with it. I re-finished the prop with epoxy and paint and applied the new tape.
I removed the front mounted spotlights (the were reflecting off of the front deck making it hard to see forward at night). I've fitted them to the top of the duct on a plastic plate - along with the nav light, strobe and aerial (it's getting a bit crowded up there ). I fitted the white nav light onto the top of the amber beacon to comply with the visibility zone in the Colliion Regulations. Wiring the lights in took nearly a day as i had to unwrap (and re-wrap) the rear wiring loom to get access to the connections.
After having problem at the Loch Fyne event with the engine, I stripped the carb and had it cleaned - it seemed to work much better. However, it played up again during the next trip so I stripped and cleaned again. Once again it seemed OK during a short river cruise but started misbehaving at the Hovershow (this time it wouldn't idle without stalling but was OK at mid & high range).
Stripped carb yet again and this time I found small piece of debris stuck in the end of the idle emulsion tube. It appears that this bit of muck has been moving around inside a transfer chamber and alternately blocking the main jet. idle or transition jets causing the problems I've been seeing (obviously trailering the craft has moved the debris around). Hopefully it' now fixed - the engine now idles nicely and picks up OK.
Decided to fit proper spray protections for the belt drive so I made a moulded prop pulley code (the plug was two kitchen bowls with some plastic bucket to make it deep enough). I also fitted belt covers by using rectangular section kitchen air duct and removing one side to make a deep U shape - perfect - light and cheap!)
After I found a couple of holes in the exhaust system,.I decide to replace it with a stainless system (salt water plus heat isn't a good combination for mild steel!!). I bought stainless pipe bends and a length of pie and welded it up using stainless wire in a MIG welder. Very strange stuff to weld - it produces good penetration and the weld pool forms OK but, during cooling it congeals into lumps. Doesn't look very pretty but it is actually welded properly! A TIG welder is apparently the best way to weld stainless.
I bought a Honda fireblade 900 silencer (second hand at £25 from ebay). It is a aluminium body silencers and is supposed ot be very quiet (bikers replace them because they are TOO quiet!!). I made stainless flanges for the engine and silencer (stainless is VERY difficult to machine). Installed on the craft it is quite a bit quieter than the old dual box system and hopefully will last a lot longer!
Noticed one of the lift fan bearings was sounding a bit noisy so I replaced it . These bearings have lasted a couple of years and it's all down to fitting deflectors or spinners to keep water and much away from them.
Over a three day period of pretty intensive hovercraft use (266 miles and 16 hours flying time) a couple of things failed.
Firstly, a rudder had an argument with a bush whilst I was backing the trailer around a corner - and the rudder lost!! It was an original rudder made from 3lb cheapo foam and has had a crack in the skin on one side for some time - it folded at the crack oddly enough! I repaired it by bonding a PVC tube up the outside using acrylic adhesive. Time for new rudders I think!
One of the steering cable plastic eyes failed at the rudder end. I use boat control cables (33c) with the plastic sleeve end fittings (Mercury type from memory). I have never had trouble with this before but I am a bit suspicious that the plastic may not be UV stabilised (it's meant to be inside an engine cowling or control box in a boat - not exposed at the back end as in a hovercraft).
Had a couple of small 4" skirt rips (only one needed fixing during the weekend).
The mechanicals all worked fine all weekend - the new belt and pulley covers did their job properly.
I calculated fuel mileage at 12.5mpg over the weekend.
I had the usual skirt-sticking issue on the very smooth water and we also had a very rough return up the east side of lower Loch Fyne one afternoon (Ian B and Trev will verify what the conditions can be like around the Portavadie area ).
Due to the damage caused to one rudder a couple of weeks ago, I decided to re-make all of the rudders using a new method I've been developing for some time (it's a quick and cheap way to make and fit rudders - more to follow )
I decided to make 5 smaller width rudders to replace the barn-door sized ones I was using (three at 42" x 20" each). They ended up 12"x36" and I fitted them staggered in height (the lower one was at the centre to avoid the "dead" area at the prop hub - with the higher ones at the outside.). Building the rudders was very easy and quick and it didn't take long to fit them. However, making the cranked rudder bar was a major job (it has to be formed in three dimensions rather than the usual two dimension which added a whole new level of complexity ).
Anyhow, they got fitted to the craft along with new steering cables and a new (bicycle based) steering column. Nice and smooth!
I tested the setup in a field and it worked really well - much more rudder authority made the craft very easy to manoeuvre. On stationary hover, you could tip the top of the prop guard side to side 5 inches at full rudder deflection.
The rudders have around 10% LESS area in total than the larger plans rudders but provide better control and are less vulnerable to the wind - overall a success. I just need to check the durability of the new construction technique (although, at less than £5 and 10 minutes per rudder maybe it isn't that important ). One other advantage is that I've just shortened the craft by 8 inches!
Ever since fitting the new exhaust system (see above) I've had problems with either soot on the rear deck or burn marks/holes in the skirt near to the silencer outlet. The skirt holes happen when stopped on water at idle - the craft is in "boat" mode but the skirt is trying to inflate so it pushes right up out of the water and touches or gets too close to, the hot exhaust.
I've tried various devices on the outlet pipe - extensions, heat shield, etc) but none of them were very successful so I cut my welded s/steel pipe and re-fitted the silencer pointing slightly upward (similar to the way it's fitted on a motor bike). It is now well clear of the skirt and the outlet points into free space above the lower rudder mount bar. With no extension or shields, it's also actually quieter than it was before!
I've had problems with the exhaust silencer-to-pipe joint loosening after a few hours running. i tried bigger clam bolts, better mount straps. etc but nothing worked. Eventually I realised that you can't bolt ANYTHING that is connected to the engine to anything other than the engine.
The silencer supports were bolted to the cross bar which wasn't connected to the engine block. The frame design in the Prospector "floats" the engine block and supports the engine using the small crankshaft extension (drive pulley) bearing. Effectively the engine block is free to move around the crankshaft (it's constrained by the rubber mounts on the front crossbar). The silencer, being rigidly mounted, was taking the rotaionl load of the engine body.
The solution was simple - I fitted a short flexi exhaust section 9stainless steel as used on most cars nowadays!