Semi hydrofoiling

Why do you think designers are stupid?

What would your "surface skimmer" amas look like?

 

not only that. planing amas are not _that_ rare. Definitely not some kind of secret solution.

 

No I don't think designers are stupid. I meet and talk to many naval architects in my day job. They are simply, in the main, not going after the same problems that I am.
But that is changing, fast. With decarbonisation and electrification now at the top of the agenda, the luxury of throwing more power at the problems (of foiling and planing), is less and less possible.

Very few naval architects I meet have the remotest interest in human powered boats. In my view though, they should be absolutely obsessed with them.
The problems of low power, having to minimise weight etc. loom very much larger in human powered boats than in any other craft.

Of course, human powered boats don't yet pay the bills for people who make their living designing boats.

If 'no rules' ( apart from human power only) human powered boat competitions over a variety of distances were very much more of 'a thing' than the impossibly niche activity they are, we, as a species would have much, much more efficient boats of all sizes.

Nothing focusses the mind quite like only having 0.3 hp to play with!

 

Yes, I did state that in my post.
However, you do also see a lot of displacement shaped amas that are planing right across the whole speed range, which begs the question of why they didn't just use a flat planing ama and optimise for that condition.

 

That looks like a nice design. I'm getting off topic, but I also have experimented with a small lift foil on a SUP fin on a 14 ft race SUP. I was using it on flat water only and could find nothing measurable.

Reading between the lines, it sounds like you're going for downwind racing. And as you know, putting any kind of foil on it will put you in Unlimited class.

We don't have any Unlimited class downwind racing in the UK, so I soon decided to work on other areas- little point in developing an illegal board!

But I do think that
A) race SUPs all suffer terribly from squat.
B) Due to history, evolution of the sport they are still 'boards' when they need to be 'boats'. It looks like your design is one that has got over this though. (We've only just started getting the first production hollow hulls for goodness sake)
C) The rooster tail of a fast moving SUP has to be energy that can be redirected into lift or even forward propulsion on flat water.

I raced some flat water SUP races pre Covid, and when drafting people, you'll often see a leaf circulating about for some time a foot behind their board. This is of course the effect that enables drafting to take place, but really, with better hull design, drafting ought to be consigned to the history books.

For making an aft SUP foil work for increased flat water efficiency, I just don't think placing it on the fin is the way. I tried it for the same reasons you did, I guess. No extra mast drag ( you need the fin anyway), nice strong fin box already present to take loading.

But for optimising for flat water, the fin is too far forward to interact with that water that's leaving the stern. It's in the free stream, which is part of the reason they put the fin there.

I'm convinced that the solution for flat water is somehow related to Hull Vane's technology, where the foil is slightly below and aft of the transom.

I actually have engaged with Hull Vane on this, and they were initially interested in what I was doing, but they they ultimately thought there was no carry across of their tech into race SUPs.

I wouldn't rule it out though, they have patented the technology and while they were very engaged and helpful with what I was thinking about, If I had a patented technology that was selling well into the commercial marine and super yacht sectors, I probably wouldn't want some guy in a shed messing around with it on a SUP.

So I wonder if it might still be worth a play with for someone with the time and inclination.

In terms of just making a SUP fully foil. Possible for very short duration, but for UT2 type intensity, a very, very hard problem.

The pump foil boards are up to about 6 minutes of foiling now, but these are low aspect foils and not the most efficient.

And the hope of having something that will foil at lower intensity, and be able to climb back onto its foils from a water start half an hour later at the hands of a half spent athlete is still some way off.

I know of several people, and indeed teams, that are working on this problem though.

SUP is such a big pastime now and sustainable foiling on flat water such an obvious future path to a lot of people. But sheesh, it's a heck of a problem, when you consider how hard it is with a (more efficient) kayak and kayak paddle.

 

Doesn't that depend upon how much weight you're asking your hull to support?

A kitesurfer on a wakeboard can plane off the beach from standstill on a board with a displacement of what? 5 kgs max?

He does it because he's getting hauled upwards by a kite.

Isn't this what I'm talking about, only doing that upwards hauling with a foil?

 

Right but what would be the calculated power or force of the kite?

Human powered boats are interesting - but a tiny niche outside of the well served kayak and to lesser extent canoe market. Most of my play designs are about solar or human powered boats with "lightship" or around 140kg. However, those are of no interets to vast majority of people. Most people who own boats want to a) get somewhere at reasonable speed (definition of reasonable varies) b) spend time with family or friends and chill out.

Cranking out 250 watts for 20 minutes is not the leisure most seek when getting a boat. Now is that 250W really enough to fly foils or a wig? I seriously doubt it can be done in anything except maybe a record attempt style exotic and fragile vessel + super fit operator.

Nothing wrong in trying to find ways to get faster with the same limited power but I think you will find that most possible things have been done already.

With solar you can get faster than by human power and I think light skinny tri could get to 7 knots or similar quite reliably with decent range. Assuming LFP batteries and stored energy to provide a bulk of the power. when we start talking about 15 knots it is a bit different thing.

 

Maybe I missed it in the discussion, but every vessel we've been involved with that used foil lift - any lift at all - had a cross-over point speed where the added drag of the foil appendage(s) was finally less than the reduced drag from reducing the hull resistance. Many foil-assisted concept boats failed to achieve that critical cross-over speed point with the power installed. Thus, those hulls were actually slower with the foil(s) attached than without. I can recall several fast catamaran projects that ended up in that fail category. Even the successful foil-assisted super-slend monohull that we built as a demonstrator struggled to get over the "hump" before finally "taking off" with actual benefit of foil lift.

 

Thanks for the confirmation. That is a beautiful design and build.

It seems to me canoe hull style has not been taken as far up the speed/length ratio as it should. There is value in the larger range of displacement and greater efficiency at lower speeds. Maybe the higher cost of electric power will help people reconsider it.

 

BMcF Long time no see. While you are around, I had a couple questions about that cross over and how it scales with displacement, sea state, and most importantly metacenter. For small craft like HPBs, an acceptably stable (monohull) boat will be quite beamy and have both larger wave resistance and larger wetted surface than a boat with a displacement an order of magnitude larger and scaled in the normal way.

If a boat of a couple tons and very slender and low D/L has a wave drag of about 1/6 the skin friction drag at the design point, a boat of a couple hundred pounds would have wave drag more like 1/4 skin friction in spite of the scaling laws working in the other direction because of the stability constraint.

This stability constraint seems to be totally ignored in every calculation and scaling exercise I have seen, and it has a big effect on the hull's buoyancy to drag ratio when comparing it to a foil set. For a light recreational boat that needs a metacentric height somewhat above belly button height, how do you parameterize the scaling laws to produce one foil drag curve and one hull drag curve on a graph with nondimensional axes? what are the best nondimensional parameterizations for the two axes? I've got a couple of kludges for specific boats, but I figure someone has done some more fundamental work on scaling laws for foilers.

 

One of the first things I learned about designing and testing foil-borne craft were the pitfalls involved in trying to employ any of the more traditional scaling methods to the total craft. We always split the problem in to two parts..treating the foils completely separate from the hull, especially when dealing with foil-assisted craft that were not ever fully foil-borne. Then we'd fudge in something for foil-hull mounting junctions. That's underlying why never tested foil-supported craft in the model basin unless all we wanted to demonstrate was seakeeping performance and nothing to do with powering.

Even with a fair understanding and good tool set for designing a foil system, we've still been "bitten" by unexpected results on more than one occasion. Our 43-foot foil-assisted super-slender-monohull, for example, had a take-off drag hump that was far larger than anything we'd predicted from the initial design and analyses. Once past that hump, the actual and predicted total drag values came back in to good agreement.

I found an old plot of the predicted drag performance....the hump was much more pronounced in actual fact. We did a fair amount of investigation to try and better understand why the difference (it was a huge factor in the sizing and selection of waterjets for the proposed full scale vessel, obviously) but never got our analytical approach to completely match the measured results.

 

Indeed.
Ours had the zero sum gain ... but it was a fun project to try though.. back in those days at Wavy!