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Re: Achilles Bending

Proper inflation is key - that's all I could find on that site - but not necessarily the whole problem. Also, getting the keel tube inflated definitely helps with the ventilation issue - perhaps it helps keep the transom down by moving the bow and center up a bit, and moves aerated water off to either side of the center - but again does not solve the problem entirely with my boat. I think the basic issue is that the standard transom height for the SU-16 (at least my older one) is a little tall for (at least some) longshaft motors. It puts some of them at the limit of proper prop bite, so that small things like the added height from using a shield plate over the transom top, or weight distribution, make a noticeable difference in ventilation. I even notice a big difference between using the boat in salt vs fresh water - there's less than 3% difference in bouyancy and that is making a difference! I've experimented with a Stingray Jr foil and while it helps the boat plane at slightly lower speed - and since the ventilation only occurs at higher speeds and rpms that means more useful speed range on plane - I can't tell yet that it reduces ventilation at higher speed. It's possible that some foils will help more with this - they mostly seem designed for improved planing and I wonder if a different design would be needed to reduce ventilation. It looks to me like the ventilating air begins to be entrained at the forward end of the ventilation plate and most foils don't add much surface in that region, it's all behind and to the side of the ventilation plate to provide more lift. I'm not an engineer but it seems to me like you'd want the opposite effect - suck the motor down a bit to preserve the bite. Or maybe it's possible to extend the ventilation plate out in the right direction to increase the suction needed to entrain air into the prop. At least on my setup, with the Stingray, once you're on plane the foil is pretty well clear of the water. From what little I've seen of other foils, they're designed similarly.

For the OP's issues, I'd say there are two things going on - the periodic transom squat is probably made worse by some underinflation, though I've had this issue with my three 14-16' sectional floor inflatables, it's probably just a fact of life with a non-rigid hull form. My observation is that it's also made worse by having the motor trimmed down for getting on plane - once up, that pushes the nose and foward hull bottom down which then 'catches' the water, at which point the transom flexes under the motor thrust, which exacerbates the porpoising. If you trim up a bit this problem is reduced, but the ventilation problem increases as you trim up. The second aspect of the ventilation problem is probably just that the motor is not mounted low enough, due to the transom design.

Re: Achilles Bending

I had this same problem with a 14ft Saturn. I was operating the boat at about 85% inflation. I also had the motor completely tilted down. I found that by adding more pressure to the tubes and tilting the motor up slightly, the boat no longer felt like it was buckling.