Fenced surface piercing V foil sail inflatable.

Doug did you mean wetted-surface are of boat or foils ? I thought you meant boat but could see that the v foils compared to a laser radial pylon set up which is most similar dinghy I can see would almost certainly have a higher wetted surface area. Radial 4.7m2 sail 60kg reasonably similar I think
A logical method would be to measure speed with outboard centre board down and pull behind my boat measureing bollard pull at various speeds and see what speed can get with latest sail rig. But it would be several months I think with covid and floods before could do that but would show if feasible or not I think.
Being locked down here for another two months or so I think I might just make something up with best stab.
I suspect a pylon wand controlled submerged foil would be more likely to suceed but a lot more hassle with maybe no outcome.
Roger

 

When I wrote that, I meant the boat's wetted-surface area, but I should have referred to its overall drag instead.

I hate being too negative, but I honestly don't think this is a suitable platform for a hydrofoil sailboat. You can probably get it foiling by towing it, or powering it with an outboard motor, but as a sailboat, I think you will be very disappointed.

 

OK thanks for the honest advice.
I might still have a go if only the new sail set up works OK will give me some satisfaction not to many inflatable conversions around I think.

 

I've been thinking about this some more and believe from the dinghy aspect the hull may actually be a low drag efficient planing hull. A lot of years ago I built a twin skin GRP sea sled dinghy bit like a Boston Whaler without the V the idea being to trap air under the hull at speed. It worked very well it was heavy but was still very fast and with a small 5hp engine there was nothing in the water when planing probably 15 knots. Biggest problem was manoeuvrability bit like a hovercraft went straight on in turns needed weight shift and took a while to stop. The reason for the story is the hull of this inflatable is similar the floor is attached to the tubes above the bottom forming two parallel spray rails and stern is low dead-rise 3deg or so and will trap air I believe. See pic attached. There is a YouTube video showing this hull doing 22kph (GPS) with a 6 hp (Prop Cp 50% 3hp)which I believe equates to 70-90 lbs drag (would include skeg drag). It will plane readily one up with 4hp at around 8kn equates believe 60lbs drag .I have found resistance curves on the forum for a similar size dinghy with 150kg all up showing a planing drag of 56lbs at 10 knots 68lbs at 12 knots. The laser bare hull is 15kg heavier or more than the inflatable and have seen quoted all up weight 80kg. A 4.7m2 laser will foil at around 10-12 knots I believe in a 12knot or so wind with an inverted T form foil. I have 5.5m2 or so a little more sail area and is perhaps 15kg lighter than the Laser.

I have found two small sailing dinghy's perhaps comparable size/weight that will do 10-12 knots with less sail area than I have the Manly Junior and Fire Bug. The Optimist has foiled with 3.3m2 but is lighter.

A crude estimate of wind force available 15knots wind on a reach with apparent wind perhaps 20knots N = 0.5 x 5.5 x 106 x 1.3 = 86lbsf which is similar to the outboard powered drag above.

That would seem to suggest approaching 10 knots or perhaps more could be possible with this sail set up. It does therefore seem to me to perhaps be feasible for a foil set up to work aiming at 6knots or so lift off. Rule of thumb hull drag = foil drag so IF lifts off then 12knots maybe possible ?

Looking at a V foil vs inverted T set up drag.

The projected area of V and T would be the same given same section same lift and presumably drag the same for just the foils ? Or is the drag related to the flow stream of the actual length of the foil which would be longer in the V form for a given projected length. The drag at the T connection or V connection would be similar I assume perhaps improved with a fairing.

The inverted T would need a centre support with additional drag so the V could perhaps be less drag than the inverted T set up for main foil ?