After the experience of the first phase of the new bag skirt for Peter Venn’s TP6 craft, we had some new design criteria:

  1. reduce the spray coming over the bow
  2. contain the skirt on the sides to allow loading on to the trailer .

The first design showed that it is possible to make a bag skirt to replace extended segments. While Peter’s preferred outcome was a bag skirt, we learned that there were features of the pure bag that were undesirable and a hybrid skirt would be the outcome. He decided that the skirt should have extended segments around the bow and and that the ground contact for

So we went back to the drawing board.

I started by looking at the various forms of skirts that we could put on. I used Solidworks sketches as the design tool because I could create a graphical representation of the forces in the skirt profile. That allowed me to change key dimensions of the profile and ensure that the skirt balance was maintained.

Bag skirt profile

First I reviewed the idea of just a pure bag. As expected, it could be fitted but it’s obviously was never going to meet the requirement of fitting on the trailer. It just extends out too much.

To achieve the desired hard structure clearance the inside profile is very optimistic. In reality the skirt would crush down on the surface, lowering the hull clearance

Loop finger skirt profile

The second type considered was a loop finger skirt. It seemed a logical next step. The finger is contained under the loop and the loop is a smaller radius than a bag skirt.

This is a balanced profile. Peter had tried a form of loop segment trying to sort his original skirt and wasn’t particularly keen to try it. The profile at the bow would have to be raked back to minimse plough-in which would alter the pitch trim the wrong way

Loop bag profile

The loop bag profile was the selected solution. The loop and bag extend the same distance outside the outer gunwale and the inside profile of the bag is not too extreme. This profile requires a third attachment between the inner and outer.

The dashed lines in the drawing are a representation of the tension forces in the skirt.

Loop bag – front profile

This is the front profile that was considered, trying to meet the objective of the same approximate ground contact point a but providing some protect from the generated spray.

Like the original skirt, we thought that at first the skirt would be just sewn together so that sections could be replaced if they were unsatisfactory. We didn’t proceed with this design. The transition sections around the corner would have been difficult to design.

Loop segment profile

I also investigated fitting a segment to the same loop as a profile for the bow, in order to minimise spray. It looked to be a feasible solution but we decided that simply using segments at the front and rear was the simplest known approach.

The ground contact point of the segment needed to be further out than usual to match the ground contact point of the bag.

Segment profile

This is the profile of the extended segment designed to match the loop bag skirt. Again the ground contact is further outboard that usual. We have a loot of experience with this form and although it would probably be a little more bouncy than normal PMX profiles, it should be stable.

The only issue remaining was to provide a pressure feed for the loop bag when lifting off from water. With the bag only fitted along the side of the craft, cushion pressure would still be restricted when floating. We had originally intended to have sort of fabric pipe running through corner segments near the bag but Peter came up whit the vastly more pragmatic solution. He attached some PVC drain pipe to the hull to hold the segments off the hull surfaces. At liftoff on water, sufficient air at pressure is pumped to the bag to inflate it and lift the hull. Once enough of the hull is clear of the water, the cushion is fed to the rear and craft fully lifts off.

The following two videos show the performance of the skirt. It meets all the design intent.

The craft is much stiffer in roll and Peter doesn’t use use body movement as he previously had with extended segments. He had to re-learn his flying technique to push the craft through corners more. He also found that the bags tend to resist rudder control at speed. When the thrust pitches the nose sown slightly the front of the bag tends to plough the water a little and create extra drag.

Hover history - a look at a Turbo 225 Wedge
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