The Cordless Canoe Challenge

[liveforphysics]

So, if I'm understanding things correctly Jeremy, your boat almost matched the fastest with much less power/energy than the other entries?

[Jeremy Harris]

So, if I'm understanding things correctly Jeremy, your boat almost matched the fastest with much less power/energy than the other entries?

I believe so, yes. I was only running a single Makita drill, the winner was running four Makitas coupled to a common shaft. I don't think the winner was using all that power effectively though, both because the prop seemed a bit small for the power, plus the boat was hard up on the wave making drag hump, as was I.

The problem with making boats go fast is that initially they are like bikes, with drag being roughly proportional to the square of speed and power being roughly proportional to the cube of speed. Once you hit the hull speed limit (in kts it's typically about 1.3 x sq. rt. waterline length in feet) then drag increases massively; it's like hitting a brick wall as the boat tries to climb over its own bow wave. Hull speed for my boat (and the winning boat) would be about 4.8 kts, so we were both pushing hard up against the limit by running at 6 to 7 kts. It needs a heck of a lot of extra thrust to get over this hump and on to the plane, plus it also needs a different hull shape, one with a fairly flat area aft to give some lift. The problem with having a big flat area aft is that at low speeds it increases drag a lot, meaning you may not be able to then get fast enough to plane on the available thrust. It's a bunch of difficult compromises, but I have a cunning plan for next year...............

[Lebowski]

lol I saw that graphic and immediately liked the styling. Rocket-ship themed.

and there I go thinking it was a pen!s drawn on the side of the shark

[flathill]

Awesome! To keep ahead of the pack next year you might try one of hitachi's new brushless drills

Quote: Only in England would you see this level of eccentricity, I suspect

We have our own wacky Amphibious races here in norcal

[lpbman]

I could be way off here and misunderstanding the drag issue you're having, but could one of these help you out?

[Jeremy Harris]

I could be way off here and misunderstanding the drag issue you're having, but could one of these help you out?

Unfortunately not, for a couple of reasons. The first is that the rules set a limit on maximum overall hull length, including the propulsion unit, of 5 metres (around 16ft). If I were to shorten the hull and add a bulb bow then I'd just run into the Froude limit at the same speed. The second reason has to do with viscous drag and displacement. The added volume of the bulb bow would need a lot of ballast to keep it submerged, and weight = displacement which is proportional to wetted area. Resistance at low speeds is proportional to the square of wetted area, and power required is proportional to the cube of wetted area, so even a small increase in it adds to the power required by a lot. This doesn't matter for a tanker or bulk carrier, as they are designed to carry heavy loads, but one key requirement for a boat running from just a few hundred watts is light weight.

[lpbman]

That makes perfect sense... thanks for the explanation.

I guess there is always this:)

http://www.youtube.com/watch?v=Ksiq9d0M ... =endscreen

Almost winning with 1/2 the power of the winner bodes well for your plucky quacker

[Jeremy Harris]

That makes perfect sense... thanks for the explanation.

I guess there is always this:)

http://www.youtube.com/watch?v=Ksiq9d0M ... =endscreen

Almost winning with 1/2 the power of the winner bodes well for your plucky quacker

That's my cunning plan, mentioned before! I've made one T foil for the stern and just need to get out on the water and test it.

[richmpdx]

Extremely interesting. I just did a little browsing on the web about the human powered decavitator so my knowledge of hydrofoils is extremely limited. Is a T foil fully submerged? Will that require control surfaces for stabilization.
Rich

[Jeremy Harris]

Extremely interesting. I just did a little browsing on the web about the human powered decavitator so my knowledge of hydrofoils is extremely limited. Is a T foil fully submerged? Will that require control surfaces for stabilization.
Rich

The plan at the moment is to use the submerged T foil right at the stern, to lift it up. The hull is flat bottomed, but canoe-shaped, so has little planing area aft, but enough area forward to support the total displacement at a speed of about 7 1/2 kts. The canoe shape is good for acceleration, as it has a pretty low hull resistance at low speeds, it's just not the right shape for low resistance at higher speeds. If I can get the stern to lift on the foil and take around 30% of the total displacement, then there's a reasonable chance that I can get the rest of the hull to plane on the wider centre section, so reducing the resistance and allowing the boat to go faster.

At these low speeds, planing needs slightly less power than running with the boat completely supported on foils, plus doing it this way makes it self-stabilise. When the stern squats under acceleration, the aft T foil will run at a high angle of attack and give enough lift to pull the stern up. As the boats starts to pitch bow down and get on the plane, the foil angle of attack reduces, reducing drag and allowing the boat to accelerate.

That's the theory, anyway, I still have to see if it will work...............................

[Hillhater]

electric hydofoil canoe..
http://www.youtube.com/watch?v=jh_Rhkej ... re=related

[richmpdx]

Jeremy,
Sounds like a very clever plan to use a single foil and take advantage of the canoe shape. To adjust lift, you will just change the mounting angle of the foil or possibly have a mount that permits adjusting fore-aft position?
Rich

[Jeremy Harris]

Jeremy,
Sounds like a very clever plan to use a single foil and take advantage of the canoe shape. To adjust lift, you will just change the mounting angle of the foil or possibly have a mount that permits adjusting fore-aft position?
Rich

Thanks, yes that's pretty much the plan. The inverted T foil mounts on an aerofoil section strut that's attached to the stern post. The plan is to have this vertical strut adjustable, a bit like an outboard mount, so I can trim the angle of the foil.