I chose a Sevtec Scout (http://members.aol.com/sevtec/sev/SCOUT.html) mainly because no-one else in the UK had built a Sevtec craft and I was curious as to how the ugly looking skirt worked! The other reason was that I couldn't believe a working hovercraft could operate with only a 12hp engine (most "small" UK craft have 40hp+ engines fitted)!

Here is a partial copy of the first plane page just to give you an idea what the plans look like:

I chose to built an 11 foot version of the Scout (the plans allow 8 to 12 foot versions to be made) and fit it with a Tecumseh 13.5hp lawn mower engine (the lowest cost new engine I could find in the UK). The other main change was I decided to use a 1200mm diameter fan in place of the propeller specified - the fan was much cheaper and, theoretically, produced more than enough thrust (except it didn't quite work out that way - see below)

Scout

Hull size: 11ft x 5.5ft (3.35m x 1.68m)

Empty weight: 220lbs (100Kg)

Max payload: 250lb (114Kg)

Cruise speed: 24mph

Top speed: 31mph

Fuel use: 0.5GPH

Engine: 10-25HP lawn tractor engine, @ 2900rpm

Propeller: 48inch 3 blade UltraProp consuming 5.7HP @ 44lb static thrust, 1500rpm max

Fan: 20in diameter 6 blade 4Z consuming 5.3HP @ 104CFS and 8PSF, max 2900rpm

Hard hull clearance: 10in (254mm)

Cabin flat area: 3ft x 6.5ft (0.915 x 2m)

 

The conclusion I reached after building this craft is that it does do exactly what the advert says! The skirt/cushion system is excellent - the front curtain brake works very effectively for stopping and turning quickly and the craft planes out and reaches 33mph on smooth water. The only real downside of a craft with this small an engine is that the performance on anything other than flat ground is not that great (hill starts are definitely out!). Although, with a bit of a run, it can get up surprising steep slopes! Overall, I was so impressed with the design package that I decided to built a Prospector - a larger car-engine Sevtec design (http://members.aol.com/sevtec/sev/PROS.html)

The actual construction process was very straightforward and only took about six weeks to first hover. I was particularly impressed with the composite foam panels - they a very quick to make and result in a very strong and lightweight craft. A lot of construction time is taken up on small fiddly stuff like control systems and guards - the big bits like the hull seem to take very little time!

The fan versus prop debate is now very active in the UK - triggered partly as a result of my "experiment" using a fan on the Scout. I, and several others, are now convinced that a cruising craft should use a prop rather than a fan for thrust. The jury is still out on whether a fan or prop is more efficient (although there doesn't seem to be any significant difference) but the fact that a prop is quieter is plenty good enough reason to use one on in a cruiser!

 

Build Log

20/5/05

Bought foam panels and resin/glass from local plastic suppliers - much cheaper than online retailers!

After much research and talking to professional GRP laminators I've come to the conclusion that the foam density isn't that critical - what's much more important is the gap between the glass layers. I've decided to use 40Kg/m3 (2.5lbs/ft3) instead of the 4.5lbs/ft3 called for in the plans. None of the laminators I've spoken to use higher density foam and it is only available to special order at around £60 per sheet!

Started build

10/6/05

Cut out all panels and bonded wood strips to them - took about 2 hours.

15/6/05

glassed one side of all panels - 4 hours total

16/6/05

cut hardpoints and attached final wood edging sections

glassed second side of all panels - 5 hours total

used heavier glass (12oz?) on underside of floor and H8A outside face

 

17/6/05

beveled edges of panels ready for final assembly

trial assembly of all panels

Assembled all panels - 8 hours

Much hassle assembling front bow panels - they are very tricky to align properly!

 

18/6/05

fitted main stringers and hull supports

20/6/05

fitted dash strips and rear engine panel

Note - dimension 26" from rear to engine panel isn't correct - the filler panels are only 25" that fit inside this dimension!

fitted bow cross sections and rear foam insert

fitted rear quarter wood strips

21/6/05

fitted side torsion strips

dashboard angle sections

dash to bow supports

 

22/6/05

not much done today - fitted rear engine bay panels

Worked on getting a free 12.5hp Briggs twin working - duff coil I think!

23/6/05

spent 1/2 day trying to get a freebie b&s 12.5 twin started! Duff ignition unit!

sourced aluminium skids. Stripped lawn tractor the engine was fitted to (got a notched control lever I can use my UH18 as the lift control lever).

24/6/05

Cut and shaped rear deck and corner side panels - glassed one side of all of them

25/6/05

Fitted flotation compartment at one side and glassed inside - took quite a while!

26/6/05

fitted second flotation compartment and sealed it

27/6/05

sanded/planed wooden members to shape. Glassed, in one piece, engine bay and flotation compartment top using white pigment (to avoid having to paint!). Finish is dull and the glass shows but it should be more durable than paint.

28/6/05

fitted extra braces from floor to main stringers half way along cockpit - the stringers can flex a bit too much in the middle when pushed down - much stiffer now. Decided to foam skin the upper deck using 1oz woven on the underside and 1oz on the front top deck with 6oz on the top of the sides using thin <10mm foam panels rather than use the aircraft fabric specified in the plans. Started to cut and shape the foam panels (all 13 of them!)

29/6/05

Glassed the underside of all of the deck panels. Hot melt glued about half of them into place

30/6/05

Fitted remainder of deck panels. Shaped them to rough fit deck. Glassed top side of deck using white pigmented resin. Modified steering wheel mechanism to lighten it and make up pivot for steering cable.

 

1/7/05

Formed three rudders and glassed one side right over leading edge and onto second side by about 3". Sanded hull edges. Cut out sections of rudders ready to encapsulate rudder pivot bolts. Made jig to keep bolts aligned.

2/7/05

Decided to make lift duct by wrapping a section of foam round a former. Made former (and deck top sections) by cutting a 10" radius circle out of a foam sheet (actually three foam sheets altogether) - the inner circle is the former and the outer is the deck section of the duct! Hot melt glued together and used cheap (£1 per pair) nylon straps to wrap a 3" foam strip around the circular former. I just bonded the strap into place permanently (apart from the metal hooks and ratchet obviously!).

3/7/05

Formed inlet lip radius and glassed remaining inside and outside surfaces (using pigment in the resin).

4/7/05

Cleaned up workshop (again!). Machined two aluminium pulleys to take 26x8 bearings.

5/7/05

Setup lift duct in rear hull - filled in around duct to make a better fit in rear hull top. Glassed rear fill panel and underside of lift duct support.

6/7/05

Mounted engine to cross members and positioned it in duct to give minimum fan clearance all around.

7/7/05

Made up fan support 1" sections. Mounted thrust fan and bearing assembly to check fit.

8/7/05

Made idler supports. Cut grooves in duct to drop engine frame to correct height. Glued engine frame into duct and put in hard points in duct and engine bay panel to take air drive mount bolts. Made up rear air drive mount brackets (fitted to back of hull). Trial assembly of air drive - two problems - I can't get the idlers close enough to the engine cylinder head (it's an OHV engine with a cover over the valves which makes the engine longer) and belt is far too short (I need at least 9" longer belt to fit properly).

 

9/7/05

Painted inside of hull using blue boat deck paint - not very pretty but it should be durable. made up rudder pivot sections from T and elbow conduit joints.

10/7/05

Made up spacers for thrust fan mount to prevent engine mount tube compressing. Laid out screws on a board in a half circle to form plastic thrust frame tubing around.

11/7/05

Made up guard from PVC pipe. Glued joints and screwed together side struts. I needed to add extra cross braces to make the rudders supports rigid enough. Overall it bends too much to be a safe guard system so I've decided to re-make it in aluminium tubing

12/7/05

Got air drive running - the engine is very rough and it has vibrated the belt so much it dropped off. The belt was then chewed up by the lift fan! Tried again with a higher belt tension - this time it would run with the thrust fan fitted but when revved up the belt came off again.

13/7/05

Decided to get a larger belt (SPA instead of SPZ) so ordered new belt and pulleys.

Meanwhile I re-made the idlers as the old ones had loose bearings in them and wobbled a bit too much.

14/7/05

machined new pulleys and fitted larger belt. Ran much better but eventually it threw the belt again - this time it took a chunk out of one of the idler pulleys. Contacted Barry Palmer and asked for advice. He suggested fitting touch bars to stop the belt resonating

15/7/05

Made up touch bars and fitted them in three places - top and bottom of prop stand and on engine frame. Runs much better - I've left the partially damaged belt on for now - no point in destroying a new one until it works OK!

17/7/05

Started marking out the skirt - there are a lot of sections! The skirt shapes are quite complex.

18/7/05

Started gluing skirl together - pretty tricky stuff!

19/7/05

Finished skirt gluing and started attaching it to underside of hull. The fit around the back end isn't very obvious from the plans. Also the way the partition skirt outboard ends are attached to the side skirts isn't clear either - posted message asking for advice

20/7/05

Got info on partition skirt attachment - it's a straight line on the side skirt.

21/7/05

Shaped and fitted skids to underside - fitted remaining aluminium strip skirt attachment. - ready to go!

22/7/05

First hover! No thrust fan fitted but it hovers just fine at very low revs - just above tick over. It will hover with a bit more revs with three of us onboard. The skirt brake system works just as advertised.

23/7/05

I am going to change pulley sizes once more to get the optimum ratio for the fan - 35 degree with a ratio of 2.6:1 which should give a theoretical 75lbf thrust! The optimum ratio is actually 2.9:1 (78lbf) but the prop pulley would be huge - and very heavy!

24/7/05

Started re-making guard in aluminium. I have added extra front to back ties in PVC pipe to stiffen the structure. I also used PVC for the outside guard to engine frame braces.

25/7/05

Finished guard and assembled rudders. Connected controls and tested craft in long grass field - only just hovered on tarmac and wouldn't hover on grass at all! Problem is that engine was only revving to about 2200rpm because of the load of the thrust fan. Need to change the pulley ratio to get better fan loading and more engine power

 

26/7/05

removed guard and disassembled air drive belt system to change pulleys.

27/7/05

Changed to larger idler pulleys and correct drive/prop pulleys for a 2.63:1 ratio. I had to use a slightly longer belt (2682 instead of 2650) and fit the idler right angles on the back side of the fan frame instead of the front - also had to move the prop pulley a little further back to get belt alignment again.

9/7/05

Tried out in field again even though it was raining and the long (6") grass was very wet. Hovered OK this time and could just move sideways across the slope but I couldn't get back uphill again (it hovered but seemed 'stuck' in position). I could pull it uphill very easily so it must be very marginal. I re-measured the thrust and it was only 18Kg (38lbs)! The belt has gotten looser (it was new) so I think it may have been slipping (especially as it was raining!). I also have the lift fan set to 30degrees instead of the recommended 25 - this will also be losing thrust power.

30/7/05

re-built fan frame using 25mm tube instead of 20mm. Re-made idler brackets using 5mm thick angle to stop them flexing. Changed position of left hand side brace to higher up on the fan frame. The frame is now very rigid - it doesn't flex at all. Re-tensioned belt.

31/7/05

First real test at Netherdale sports pitches (about 5 rugby pitches together in one flat area). Very slow to pickup speed but OK once going - noticeable drag on longer grass (4" or so). Spent about four hours - had to re-tension belt after an hour or so.

1/8/05

Went to Paxton house on River Tweed. Launched from sloping beach - craft went OK upriver downwind but wouldn't get back on plane again upwind (or downwind if stopped on water). Nose also bobbed up and down indicating a skirt trim problem? Skirt looked inflated OK front and back but back end wouldn't get out of the hole.

 

2/8/05

Changed pitch of lift fan from 30 to 25 degrees - seemed slightly slower to get on hover but not much difference. Built and tested tacho for engine. Now revs to 3550rpm with 25 deg lift fan pitch. Changed pitch of thrust fan from 35 to 40degrees now revs to 3250 and thrust is 18Kg max (39.6lbs).

3/8/05

Changed pitch on thrust fan to 37.5degrees. Re-attached skirt where it joins flotation compartments (needed fill panels added to make it conform correctly at the rear).

6/8/05

More testing at Netherdale. Measured thrust using spring balance instead of bathroom scales (I suspect the scales were measuring lift as thrust as the craft wanted to climb up the wall) tied to fence post - measure 15-16Kg static (33-35lbs).

12/8/05

Tested again on river at Paxton. Much better - it will now plane out me on my own or two kids together. Takes about 15 seconds or so to plane but it does work. Top speed now about 28mph. Cruised up and downriver for about four hours. Skirt flotation foam detached and came out of drain hole - also lost glass strip on rear panel bottom edge (an exposed foam edge). Noise measurement at 25m on river full throttle around 76dB.

14/8/05

Went to Longniddry and launched onto beach. Worked great, Planes out in deep water OK with me and bow bounce has now gone. Cruised about ten miles with me & son no problem at all (see video on Photos & Videos page)!

27/8/05

Measured thrust again in open space at 16-18Kg (35-39lbs) using new spring balance scales. Ordered a Prop direct from Ultra-Prop.

2/9/05

Fitted an Ultra Prop set to 14 degree pitch. Thrust now measures 24.3Kg (53.5lbs) at 2900rpm. Tested at Paxton in evening - now planes out upstream into 10mph wind OK.

Measured noise level has now dropped to 74dB. The prop rotates faster than the fan did yet, produces less noise and significantly more thrust!

4/9/05

Changed prop pitch to 13degrees - engine now revs to 3100rpm static with 26.4Kg (58lbs) thrust. Cruised from Berwick up to near Coldstream with both craft - Scout excellent even over 2ft waves and up 30degree beach exit ramp. Top speed about 33mph

That's it - all done!

To see how the craft performed look into the Hover Diary! For info on any modifications and maintenance see Scout Maintenance

 

Additional information