Technical Data

Now, if one wants more reading on Sevtec craft all one has to do is "Google" up and click on the "groups" button and ask for Sevtec, or alt.rec.hovercraft.Sevtec or other things and just about anything will come up, including some technology discussions, as well as catfights and whatever usually goes on in newsgroups.

This paper, published in the Canadian 1984 International Conference on Air Cushion Technology, Sept 25, 26, 27, at Vancouver, BC may be old, but the numbers are just as valid today. A systems approach was taken to compare the Sevtec design philosophy directly to the design of large surface skimmers (hovercraft) with some surprising results. (A wait may be required for download for dialup.)

Here is another paper, on the radiated noise of an 8hp craft of the author's, showing the overall far field noise, and a breakdown of the constituent contributors, engine, fan, propeller, to the far field noise through narrow band noise analysis.

And here is still another paper showing the most fundamental analysis of a small sev.

It's in the Numbers

Below is a numerical spread sheet model of the straight line performance of a surface skimmer.

The spreadsheet was generated from the equations shown in the first paper. The upper left section is the input file, (for the Sevtec Scout) where the surface skimmer is defined in the simplest of terms. The items backgrounded in yellow are absolutes, while the blue areas are input generated through test experience with the craft being studied or from experience from past craft. (To the right are "buttons" for navigating through the spreadsheet, which extends well beyond what is shown here, and have nothing to do with the analysis.) The blue block just below the input file is the results of calculating various overall surface skimmer parameters as generated by the input file data. The next block down shows the cushion performance. The red horizontal line is positioned at the fan maximum expected efficiency to pick up a daylight height under the skirt running surface. ( A more realistic place to put the line is just below 70% giving a lesser height than shown.) This number is a relative number, or it is used to compare craft, as there are other functions involved, (such as under skirt discharge coefficient) and is not an absolute measurement of the actual daylight under the skirt. The last block is thrust and drag breakdown, and a plot of propeller thrust and surface skimmer drag is shown. The top smooth overwater speed is where thrust crosses over the drag curve. This is only one of the spreadsheets used by the author to design craft. More of the spreadsheet (for Sevtec Vanguard prototype) follows. Decisions involving propeller and fan sizing, cushion size and craft powering can now be juggled with weight and trim analysis and lots of experience in designing, building and operating some two dozen different craft.