Hydrofoil Paddle Downwind Board

[FoilAddict]

Hi! long time lurker, first time poster. I’m interested in your guy’s thoughts on board design for a standup paddle or prone paddle downwind foil board. Most current designs are far from good and hard to get on foil in the best of conditions plus I think it’s an interesting conversation and possibly applicable/has parallels to foiling boat design. In my limited experience designing and riding foils and foil boards, downwind is by far the hardest discipline to design for.

My design goal is a board made for the most advanced riders riding the fastest wings possible. I have wing designs that easily will average over 17kn but are just out of range of my gear and ability to get on foil. The board doesn’t need low drag on touchdown as I’m not touching on my current gear. Speaking of, my current board is 6’ long and 24” wide with a rounded nose and fairly pulled in tail. My thoughts are 6’2” x 22” wide will be the first change. Another option is prone paddling, which offers less power but boards down to maybe 17” wide.

foil assistance starts around 5kn and takeoff is around 8kn. Takeoff angle is between +12 and +18 degrees. Because of the limited power of a paddler, large peaks in resistance should be avoided (like a full planing hull). Acceleration is super important, waves move around 6-7kn and usually last 3-7 seconds.

mid lobe to hear your thoughts and any other info that could help. As these boards become more hull-like your knowledge becomes more relevant and valuable.

Mahalo

 

[BlueBell]

Welcome Hawaii Foiler!

You will be much closer to those answers than any of us.
And, you have the luxury of a test sight right there in the middle of the Pacific.
Longer and narrower is generally faster so you are moving in the right direction.
Weight of course is a speed eater.

What is it exactly you are trying to achieve,
a faster board for higher take off speeds
using smaller foils resulting in higher top speeds?

 

[FoilAddict]

Yes a faster board is key to using more efficient foils that can use the energy in groundswell bumps. Most foils currently need to be at the very top of their range in these conditions, and are limited by the thickness and chord needed to get up. My goal is to push for higher aspect ratio, thinner foils that make better use of the available energy. I assume a semi planing high L/B ratio hull will pass hull speed with the least spike in resistance? Problem is as soon as we pass hull speed, wetted area is the biggest factor until takeoff and that drag is surprisingly high. Many boards use some sort of step to combat this but compromise with increased drag in the displacement to planing transition.

 

[BlueBell]

How about a little electric drive with a collapsing prop like on R/C gliders.
It would only be used to get you on foil.
So, not much needed in the way of power, battery or controller.
On/off should do it.
This would allow for smaller, faster foils, with a higher lift off speed.
-OR-
An inflatable bladder that surrounds your hub where the strut meets the foils.
A small CO2 canister inflates it at speed providing momentary buoyancy assisting lift-off.
Then deflate once on foil.

 

[FoilAddict]

Those are interesting, a co2 cannister wouldn’t fit in the wings or fuselage of the foil unless the diameter was increased, and lots of volume would be added to get that kind of buoyancy. Electric 100% works and I’ve tested efoils with that configuration, tons of fun especially because you can ride all the way upwind so no need for a drive up the coast!

I’ve been sitting in 2 ideas for a while-
Flap adjustment could reduce the stall speed and flatten, or even reflex at high speed to reduce drag. This is probably the best way but hard to do in wings under 15mm thick, especially since the area servos or wires would run are the most structurally stressed parts.

Water ballast is another one. It’s been well proven that on any given wingset, a heavier rider is faster. If there was good a way to ride a bigger wing to get up, then increase your weight say 5kg+ that would be awesome, especially since most races end in a pumping section where a bigger wing and lighter weight has a huge advantage!

 

[BlueBell]

You don't already have flap adjustment?
That would be a good place to start.

 

[FoilAddict]

No we don’t, here is an example of a fast downwind foil, since the vertical strut is so far back it’s hard to run an active control system without significantly increasing the fuselage diameter

 

[tspeer]

...Flap adjustment could reduce the stall speed and flatten, or even reflex at high speed to reduce drag. This is probably the best way but hard to do in wings under 15mm thick, especially since the area servos or wires would run are the most structurally stressed parts....

A flexible trailing edge, or flap mounted with a flexure, can act passively to transition smoothly and automatically between high and low speed positions without any control system. There are basically two design points to consider. The first is the high speed shape you want to have. The second is the flap deflection at takeoff.

Start with the high speed shape and calculate the hinge moments. The moments will be mainly a function of speed and only secondarily a function of angle of attack. From the high speed hinge moments, calculate the deflection that resulted to get you to the high speed position. Subtract the deflection from the high speed shape to get the zero speed jig shape. Adjust the stiffness of the flexure to get the desired deflection at the takeoff speed.

I like to additionally calculate the point load on the trailing edge that will result in the same deflection as the high-speed distributed load. This load can applied to the foil in a jig to tune the stiffness of the as-built flexure to get the right deflection.

 

[Kayakmarathon]

Olympic paddlers have experimented with hydrofoils for fun. One prototype was called the "flyak"

Hydrofoil Kayak - Flyak - Human Powered Hydrofoil http://human-powered-hydrofoils.com/hydrofoils/hydrofoil-kayak-flyak/