« Reply #1 on: Jul 04, 2010, 6:07 pm »
Building a Sevtec Scout

I chose to build a Sevtec Scout (http://sevteckits.com) mainly because, as far as I know, 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 main reason was that I couldn't believe a hovercraft could work with only a 10hp engine!

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

The plans consist of eight sheets of A3 size with set of general instructions.  They are very clear but do not include every detail (like most plans) and some of the pages are a bit cluttered.  They look CAD generated but are actually produced using a drawing program and are not accurately scaled.  There were only a couple of errors that I came across while using them.

I chose to built the 11-foot version of the Scout (the plans allow 8 to 12 foot) and fit it with a Tecumseh 13.5hp lawn mower engine (electric start, fuel and oil pump single cylinder OHV).  The other major change was I decided to use a 1170mm diameter fan (Hasconwing 5Z type, 3 blade in a twelve blade hub rotating at 1358rpm) in place of the Ultraprop propeller specified - the fan was available in the UK, was cheaper and, theoretically, produces more thrust (it didn't quite work out that way - see below).

All of the Sevtec craft hulls are built using thin (12mm or 25mm) rigid polyurethane structural foam (80Kg/m3).  The foam is brittle and powdery but makes amazingly stiff and lightweight panels when bonded to fibreglass skins.

The hull is an open bathtub shape made from a set of flat panels.  There are no internal hull bulkhead panels or braces used.  Wood is used in the construction along the outside top edges as an outer skirt attachment point and is also used to form the top deck frame.  No wood used below the outer hull edges - instead, 'hardpoints' are created in the foam core by boring out foam plugs and filling with microballons.  The wood that is used is included during the composite panel construction and is covered by glass/resin and is therefore fully protected.  Some builders have used high-density foam (320Kg/m3) to replace the wood entirely.

The bag skirt system looks very ugly, as it seems to be made from a vast amount of loose floppy material!  It has a unique partitioned cushion with a main cushion and a bow cushion.  The bow cushion is formed using two simple skirt curtains, one at the bow and the other a couple of feet inboard.  Opening a flap valve in the front curtain can deliberately deflate the bow cushion.  This causes the nose of the craft to drop and results in a controlled braking action (it can also be used, in conjunction with the rudders, to make very quick turns).

The actual hull construction was very quick.  Laminating the panels is very easy as they can be worked on at tabletop height.  The panels are then stuck together to make the hull using hot melt glue before the joints are glassed.  The hull construction took about two weeks

The superstructure is formed using timber stringers.  The plans specify that the deck should be covered with heat shrink aircraft fabric.  I wasn't that happy with the durability of this type of material so I decided to skin the deck using thin (9mm) foam panels lightly glassed on the inside.  I covered the entire outer deck with glass and coloured resin to save having to paint.

The complete thrust/lift drive system and engine is made as a single assembly that can be easily lifted from the craft by undoing four bolts.  The engine used is a vertical shaft type that drives the lift fan directly and the thrust via a simple V belt passed around a pair of idlers to change the belt direction to vertical.  The engine frame and fan supports are simply 4 lengths of 25mm square tubing bolted together with a bit of angle as a brace.

The lift duct is made from the same foam as the hull.  The duct is 75mm deep for the 500mm lift fan.  It only took a couple of hours to make a complete duct.
The rudders are made from foam panels that are formed into aerofoil shapes and glassed with a single layer of 6oz woven both sides.  The rudder pivots are stainless bolts embedded in filler included in the laminated panel.  Like the duct, this is a very quick and easy way to make a lightweight part for a hovercraft.

The fan guard is made from 16mm aluminium tube formed as two loops with heavy mesh around the periphery.  A lighter mesh is used to enclose the entire engine, lift fan and the front of the fan guard in a cone shape.  The rudder supports are attached to the rear duct ring.  The entire guard, including rudders only weighs 6.5Kg.
It took around six weeks to complete the craft before testing started.  A lot of construction time is taken up by the usual small fiddly stuff like control systems, wiring and guards - the big bits like the hull seem to take very little time!  The front and partition skirts need to be trimmed (literally by chopping bits off the bottom edge!) to get the craft to ride correctly - no other trim adjustments are needed.  The way the front cushion works makes the craft self-level so you don't need to shift the load around.

You may be thinking that the garden chair looks a bit naff!  I spent some time trying to find something better and couldn't - it's comfortable, weatherproof, lightweight and easily available (just pop round to the back garden and pick one up!).

The initial static thrust tests were very disappointing.  The fan only produced about 38lbs of thrust (the design spec for the craft using a prop was 44lbs and the theoretical fan thrust should have been 70lbs).  I tested the static thrust with a variety of pulley ratios and pitch settings to establish that the theoretical calculations were accurate (they were 2.63:1 drive ratio and 37.5 degrees blade pitch).  I then decided that the poor thrust must be caused by the lack of a duct.  I made a temporary straight-sided short duct (no inlet radius) using 12mm foam sheet, glue, tape, etc inside the guard frame.  It had 6-12mm tip clearance, which I thought was pretty good considering what it was made from!  I managed to get a couple of thrust tests done before it self-destructed! There was no measurable improvement in thrust (the actual test technique had about a +/-7% accuracy due the measuring device and the instability of the craft under full thrust so there may have been a small improvement).

During this time the Scout actually performed pretty well with only 38lbs of thrust - the plane out time was about 8 seconds on smooth deep water with 110Kg payload. The lack of thrust showed if you attempted to plane out against a wind of much more than 10mph or against a tide or river flow - not surprising really! If you ran onto the water from land above hump speed (about 6mph) it stayed on plane fine - I cruised along inshore for about ten miles or so with two adults on board staying above hump speed.

I bought a 48-inch Ultraprop as specified in the plans.  After changing pulleys again I tested the static thrust.  It now measured 58.5lbs.  This was far more than I expected - the main reason may be that the engine I used was slightly higher output than the plans specify (6.3hp available for thrust instead of 5.5).  The craft now planed out very easily in a few seconds with a payload of 200Kg and would reach a speed of around 33mph on smooth water.

Another major advantage of using the prop was a reduction in noise levels.  The fan measured 79dBa at 25 metres and the Prop 74dBa in spite of the fact that the prop was rotating faster than the fan!

I, and several others, am now totally convinced that 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 enough reason to use one on a cruiser!

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 (you can fully open the front vent at maximum speed with no danger of plough in).  In fact, the upturned bow design and double front plough planes make plough in virtually impossible (at least I've never got it to do it!).  The only real downside of a craft with this small an engine is that the performance on anything other than smooth 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!  It performance on water is very impressive - I've used it on some pretty rough chop (2-3foot) with no feeling of imminent disaster!
The composite construction techniques used on this craft are ideally suited to hovercraft (light weight, strong and cheap!).  The hull weight came in at just 47Kg and the complete craft loaded with everything except driver and passenger at 117Kg.
For someone looking for a safe, economical (less than 0.5 gallons an hour!) and reliable craft for water or mudflat/beach cruising then the Scout may be the answer.
John Robertson
« Last Edit: Jul 09, 2010, 7:59 pm by John Robertson »