- Last Updated on Thursday, 31 January 2013 11:27
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A suggested method for trimming a Sevtec skirt
Sevtec hovercraft are very sensitive to the bow and partition skirt trim - they will not perform well if the trim isn't correct. The Sevtec videos and build instructions detail the trim procedure but it isn't that easy to get a good result IMO. The following procedures are based on information from various sources plus some stuff I've found out myself - it seems to give the most predictable results with the least effort. The only major difference between the Sevtec instructions and what I've done is that I trim the partition skirt to be higher than the bow skirt. This suggestion was made by Bryan Phillips of Amphibious Marine and, after testing it on both my Scout and Prospector I am happy to recommend it.
What you need:
Get a "wet" felt tip pen (a contrasting colour to the skirt!) and tape it to the end of a 2ft long thin stick or cane (so you can reach to mark the partition skirt on a larger craft. Cut a length of 2x4 (or 2x2 or some other reasonably chunky bit of timber) to fit under or onto the bow point of the craft (if you have a large craft then cut two lengths). This timber should be the height of the bow when the craft is on hover (specified on the plans). If your craft hull isn't built exactly to plan and the bow is slightly higher or lower than make the timber length equal to the bow height plus the hover height less the skid height!). You will also need some eye protection and a set of ear defenders are also a good thing to have.
Remove the prop from the craft (obviously you don't need to do this if you have separate thrust engine!). Find the most level and smooth piece of ground you can (tarmac or concrete are good) and move the hover onto it. Tie the craft to some fixed object to stop it drifting around when on hover (make sure the ties don't pull downwards on the craft hull when on hover).
Lift the craft nose up and tape or attach the timber length to it at the correct point (on a large craft use a length of timber at each bow corner). Make sure the timber doesn't interfere with the bow skirt. At this stage lift the bow skirt and fold/crumple/tape it up so it can't reach the ground when the craft is on hover (make sure you don't pull it hard enough to distort the side/forward quarter skirts).
Start the engine and increase throttle while watching the back end of the craft. At this point, the nose should be well up off the ground. Keep increasing the throttle until the back end stops rising. The craft is now on full hover. Put on you eye/ear protection, crawl under the bow skirt and place a couple of axle stands or blocks (set to slightly less than the hover height) under the front of the skids for safety - the last thing you want is an engine failure while you are crawling under the hull Make sure the stands don't interfere with the skirts. Jam open the bow brake vents and, if needed, add weight to the craft nose until the bottom of the timber stick just touches the ground (add buckets of water into the cockpit or a toolbox, rocks, whatever you have!). Reduce throttle until the rear end starts to fall again (the front is held up by the stick) then increase it until the rear has just reached it's full shape. The craft should be sitting level to the surface at this point - check it by measuring the height of the outer hull edge front and rear and side to side. If it isn't level then shift the weights around inside to get it as near level as possible. If the surface is smooth you will probably get skirt bounce (a fluttering or bouncing of the skirt against the surface) - to reduce this, slide a bit of timber under the skirt edge about half way along one side.
Crawl under the bow and mark a line along the partition skirt by sliding the stick with the pen along the ground while lightly touching the partition skirt. If this is the first trim you will probably find the lower edge of the partition skirt is crumpled and folded and it's difficult to mark a decent line - I found it useful to pull out the bottom of the skirt edge so it's to the front rather than folded backwards.
Stop the engine - the front of the craft should settle onto your axle stands. Crawl under the bow skirt so you can reach the partition skirt. Remembering to allow for the pen thickness (the ground contact line will be 1/2 the pen thickness BELOW the marked line) trim the skirt to about 1/4" (5mm) HIGHER than the ground contact line. It can be difficult to reach under and make this cut especially where the partition skirt joins the side skirts - in this area it's OK to taper the cut so that it ends right on the ground contact line rather than higher. To cut this area you may find it easier to do it from the sides of the craft rather than underneath (just pull the side skirt up and out to expose the end of the partition skirt). Once you have made the first cut, repeat the entire process again to check it. You will probably find that cutting the skirt changes the rear trim and the craft settles or lifts a bit more - you will probably have to repeat the mark-trim process two or three times until it trims out properly.
Once the partition skirt has been trimmed properly you can work on the bow skirt. This is much easier to mark and cut. It should be cut to be level with the ground (cut below your marked line by 1/2 the thickness of your pen). You will probably need to weigh down the bow to get your timber support to touch the ground during this process - you can jam open the brake vents to help lower the bow. Again, if was long to start with you will probably have to make several cuts to get it right.
Get the craft onto water for testing - land operation will tell you nothing. If it planes out OK then you need to look for the following symptoms:
1. Porpoising - the nose bobs up and down on smooth water - usually when going downwind.
This is usually caused by the partition skirt being too long (or the bow skirt too short). The partition skirt is 'sealing' the front compartment which then pressurizes and lifts the nose until the bow skirt comes up off the surface and releases the excess pressure causing the nose to drop again. This process repeats and produces the bouncing motion - it can be quite violent. If the bow skirt trim is correct the fix for this suggested by Barry Palmer is to trim the partition skirt about 1/4" higher in the centre and taper the cut towards the outside edged (like an inverted V shape). This will slow the rate at which the partition skirt seals to the surface and should damp the porpoising motion.
2. Plough in downwind or into rough water - it feels like the brake vent has been pulled - the nose drops and the craft slows.
I think that this is caused by insufficient pressure in the front cushion compartment. Again, caused by the partition skirt being too long. There needs to be air passing under the partition skirt to pressurize the bow compartment (there are small feed holes at the top corner of the forward quarter skirts but they don't appear to be able to provide enough air fast enough to keep the front compartment inflated if the water is rough).
3. Back end seems to stick or drag reducing speed and making turning difficult - usually shifting weight fixes it for a short time.
This is caused by too long bow and/or partition skirt. The bow is being held too high and pushing the rear skirt into the water causing the drag.
4. Front vent brake doesn't work properly or is very slow to start to drop the nose
Probably the partition skirt is trimmed too high (there is too large a gap between the bottom edge and the surface). The front partition vents cannot dump enough air to lower the bow enough to get the partition skirt to seal on the surface because too much air is being fed under the partition skirt from the main cushion.
5. Craft rides nose high with spray from the front side corners
The bow skirt is too trimmed too high. The front compartment is pushing the nose high. Double check the correct bow height (the timber support length) and re-trim both partition and bow skirts.
If you need to re-trim either the partition or bow skirt then check both of them again - every time you trim you are changing the craft attitude slightly and it will affect the other skirt to some degree. If you find you have cut too much off (too high a trim) then you can easily just glue a strip of material back on. Glue new material onto the rear surface of the skirt to prevent water un-peeling it. When adding new material it's best to cut another two or three inches (or more) away from the existing skirt and use a narrow overlap (1/2-1" is plenty) - if the glue joint is too near the surface it can make the lower edge of the skirt too stiff.