Pedal Powered Boats

Actually there was a NASA test plane that rotated the the wing to allow the same capability as the F-111 swing wings, just at a lot lower cost. It was built by Burt Rutan. The wing was straight, which is terrible at near supersonic speeds, the right tip swept forward, the left aft. This allowed enough lift at take off and landing (slow speed) but reduced the lift at high speed (and decreased shock wave effects - not applicable to water, although it might have benefits if you could go fast enough to cavitate the foil).

http://www.fiddlersgreen.net/models/Aircraft/Ames-Dryden-AD1.html

 

that is some crazy stuff looking at back in the day, I can only wonder what they're up to now! If that kind of research and design, and testing of course, was put to the water we might have some real cool stuff out there now!

 

In some ways it is more simple than that.

If you assume the boat weighs say 20kg, additional appendages etc say 5kgs, and person peddling say 75kg...so an all up weight of 100kg.

Taking an optimistic lift coeff of say 0.8, the required area to lift 100kgs at say 4 knots is 0.60m^2. Doesn't sound much, but, if the chord is just 250mm, for a high aspect ratio, the span is 2.40m. Hardly small.

So, not a small foil and this is not even calculating the drag and not to mention the stabilisation of the whole dynamic system which requires some thought once foil borne too.

Achieving a higher speed, obviously helps reduce the size, such at 6knots which is not slow, it still requires a span of 1.17m....not exactly small. However, if one was going for a record, and could produce a burst to overcome the drag, then it could be possible, but I think one would have to train hard to maintain such a uniform acceleration/speed to ensure the lift to become foil borne...and investigate the dynamics once foil borne too. Solving the dynamics shall end up causing more drag.

 

Likewise on your opinion Tom, you have made some very good calls. One of those situations where I would love to be proven wrong! My basis is on a net 75 watts to the water, under optimum conditions....

Porta

 

Cg

Glen
Your last message, quote...
"have a couple questions about positioning of the center of mass vs center of bouyancy (fore and aft). Would you expect the relative position to be the same for cat and a mono? Is there an advantage to having the bow a little lighter in rougher conditions if the hulls are the same on both ends? And I imagine the outriggers need to be included in this calculation for the mono....."
A good rule of thumb is to have the base of the back of the seat about 150mm aft of midships. This is assuming it is a canoe shaped hull. You want the boat trimming evenly at rest as it will trim by the stern when you start moving.

Ian

 

Glen,

I'm not sure I would be as "precise" as I57. Since it really depends upon your hull shape (profile shape mainly) and the speed at which you wish to go. Coupled into that is the L/D ratio and L/B ratio....so its horses for course. Unless you provide more data of your hull form.

 

But 4 kts is very slow for a pedal powered boat. Even fairly mundane pedal boats are capable of speeds of around double this, which reduces the required foil area to around 1/4, which is within the bounds of feasibility.

The issue then is one of trading foil area, AoA and induced drag to find the best compromise between being able to get foil borne at a speed just below hull speed, yet not have too much drag at maximum cruise speed. This would seem to suggest designing the foils to lift at the left-most edge of the drag bucket, with cruise speed being around the centre of it.

It was this I wanted to spend some time looking at, because it's a complex trade off between foil L/D (largely determined by the section chosen, and we're working at very low Re here, where laminar flow dominates and the pressure distribution across the foil will have a significant impact on performance), wetted aspect ratio and foil loading. The latter is also critical, and probably dominates performance once the section is optimised, I believe.

Unfortunately, a simplistic analysis doesn't really get close to what might just work to increase the cruise speed of a pedal boat without needing more "engine power".

For example, if we take Rick's V15 design as being a reasonable starting point, then we know that a reasonably fit pedaller can drive this hull at around 3.5 m/S or more for prolonged periods. If the foils were designed to provide enough lift at this speed, with their highest allowable AoA (and hence highest induced drag) and then the foil AoA was reduced as the velocity increased as the hull lifted clear of the water, it would seem probable that a significantly greater velocity could be sustained on the same power. It should be possible to use a high lift, laminar flow section, like the 63-209, at this relatively low loading, as cavitation shouldn't be an issue. This might allow a Cl of better than 0.9 at maximum AoA. The total foil area would need to be around 0.18 m², split evenly between two foils of about 0.09 m². This seems to be practical, to me, as a 0.09 m² foil with an aspect ratio of, say, 9 (which is on the low side) would have a span of 0.9 m and a chord of 0.1 m (assuming weight could be held down to 100 kg).

 

This is one reason for the "simplistic" analysis.

If looking at the 63-209, to get a Cl of 0.9, that is at an AoA of almost 8 degrees, not a small value. To control this up to the AoA and then to lower the AoA once foil borne to reduce drag is not an easy task for someone who is more focused upon peddling to maintain speed. Not saying it can't be done though. But this also then adds more modes of failure into the system.

The other issue is that of the foil area is assumed to be submerged. This being the case, there is no resorting force following a disturbance, it is all vertical lift. So a slight heel is not favourable, unless the foils have some dihedral of sorts, but more drag again. Thus a disturbance could become potentially unstable and in which case she'll come off her foils. So this requires at least 3 foils to provide stability, more drag. And if getting the Cl of 0.9 is required to keep the area low, then controlling 3 foils becomes even more complex.

So you could opt to make the foils surface piercing and make them "W" shaped to be naturally restoring, but this adds more drag than fully submerged.

And of course the old chestnut, can the foil, once made, take the loads. We are taking about very thin foil sections when looking at the 63-209. On the chord of just 100mm, the thickness is just round 3mm or so, very prone to damage of the leading edges. And so on..

Hence the rather simplistic review...interesting thought though. Not given it a lot of attention...may be a nice conundrum for Eater Monday

 

It's certainly challenging, but the foil section on that rowing shell looked to be pretty thin, with a high aspect ratio, so I'm guessing that is can be done. NACA 63-209 is a 9% section, BTW, so the foil thickness would be 9 mm for a 100 mm chord, which seems a bit better, structurally.

Depth/AoA control would be essential, so an automatic system would be needed. The surface feeler systems always look to me as if they run with an underdamped control response, as they seem unable (based on the few I've seen videos of) to differentiate between waves and the mean surface level. A system that measured the mean hull flying height and adjusted the foil AoA to maintain it at a near-constant level would seem to be needed, perhaps using ultrasonic distance sensing and a fairly simple servo loop controlling foil pitch.

Certainly the system would have limited application, and be subject to damage from floating or submerged objects, but for competition purposes on sheltered water it might well be OK.

I had a quick look at V foils, because of their natural ability to adjust wetted area with velocity. The downside seems to be the doubling of drag from the surface piercing struts (when compared to an inverted T foil). The natural stability of a V foil, perhaps with a horizontal lower section to give a slightly better L/D, might win out though, as at these speeds the drag is probably dominated by induced and form drag, with drag from the surface piercing elements probably quite modest.

 

It seems that what you're talking about is spamming the forum with your first post...........

(I deleted your spam link, BTW)

 

Well spotted, helps if i bother to check in my book first!
But looking at the co-ords, it is only 9mm for about 35-40% at best of the section...so the tips/edges have little supporting them in reality. So 9mm seems ok, but looking at the section properly now..hmm...that trailing edge is awfully thin for a long part of the chord.

Ive designed/built such things on larger vessels, albeit faster too. The structural aspects usually dominate. Disappointing too really, didn't do what it said on the tin!

I think that is a bridge to far, for such a boat. It could be a simple push pull type cable or hydraulic line to a predefine AoA, rather like cycling a bike with a small/finger hand size lever when changing gears.

But what would concern me the most would be the overall effect on the GM when foil born. That would require a bit more study, and hence it effects. You would really need a stiff restoring system when foil born, so the foils really need to be far apart and either 2 up fwd one aft or vice verse. Too much roll would not be good, nor would be too much pitching.

Thus the hull selection would play a big part. A high L/D ratio would be needed, which may render the static condition rather difficult even before getting to foil borne.

 

It may well be that V foils would be a reasonable compromise for this boat. The AoA could be fixed at the cruise speed angle, and the varying immersed depth of the foils used to control flying height for any given speed (or power input). Tom Speer has written a useful paper here: http://www.tspeer.com/Hydrofoils/generic.pdf that works through some of the trade-offs, although I've not had time yet to go through all of them for the speed and loading range that a pedal boat would be working at. What is clear is that the foil would be working at values of Re that are down around 300 to 800 at most, well below the sort of operating area that Tom Speer's looking at in that paper.

The near-constant drag characteristic of a tapered chord V foil looks to be an interesting option to explore further for this power-limited application, particularly as the drag associated with the surface piercing elements seems quite a small proportion of the total.

I believe that the V foil configuration should provide a fairly strong righting moment, as well as automatically controlling height in relation to power input. What I'm not sure about is the effect of turning, as I can see the possibility of the out-of-turn force exceeding the restoring moment of the greater immersion depth of the outboard foils. It may well be preferable to use trim tabs on the foils for steering rather than rely on a conventional rudder.

Looks like a need to spend a bit of time looking at this in more detail!

 

You sure you have your maths correct?

Re, for the 100mm chord, at 3.5m/s in 15c SW = 295x10^3.

300 is a bit on the low side!

 

Whoops! my turn for a mistake, I was a factor of 1000 out on the dynamic viscosity of water! (comes from normally working with air......)

 

Laddering is effective that way as is "V"-ing or dihedral.