Low power semi-hydrofoil?

Thanks very much for those links, I'll check that book out. I know absolutely nothing about planing boats, so it looks like that might be a good start.

 

Thanks again. I checked out the Bartender designs and see better what you mean; presumably they added those wings aft for the same reason.

The length of all the boats in this race is about the same, close to the 16ft maximum allowed under the rules. My boat has a waterline length of about 14ft 9ins, the one in the photo might be maybe 6 ins longer. I was running at an S/L of between 1.56 and 1.69 (I was doing between 6 and 6.5 kts). My best guess is that the boat pictured was running at an S/L of around 2, I don't think that, based on her speed, she got over an S/L of 2, as I'm reasonably sure she wasn't doing more than 8kts, as she was racing a boat with a known top speed of 7kts at one point, and was only marginally faster.

I wonder how the low aspect ratio rails would compare with a high aspect ratio foil fitted under the stern? In terms of practical fixing, a T foil would be easier than adding rails, as the hull is a very light foam sandwich on the sides. The transom is beefed up with a timber insert, because I was originally thinking of mounting the motor back there.

I did a few rough calculations last night, and it seems I can get around 70 to 80lbsf of lift from a 1m long, 100mm chord foil with an Eppler 817 section at a speed of about 6kts. That's around 1/3rd of the total displacement, and would undoubtedly lift the aft end of the boat up a lot, especially if the speed increases as I hope it will. The drag penalty from this foil seems reasonable, around 100 W of added power would be needed at 6kts, but I'm guessing that this might well be offset by the drag reduction from reducing the level of squatting at the stern..

 

If the maneuverability requirements effectively limits your length to, say, 20 feet or so, you need a stable low speed maneuverable hull and a sprint boat between turns. A foil could certainly do that if it picked a broad stable stern section up out of the water as speed picked up. You could have a 20:1 L/B at 10 knots and a 7:1 L/B at 5 knots in the turn. You could even design it to pitch by 10 degrees and get a few feet of the bow out of the water in the turn. Bimodal boat.

But if you up the power to 2kw, you should be looking at 13-14 knots, and a nine footer would doubtless be an easier thing to live with. If you go faster, the run time drops, so power goes up faster than energy. If you clear the hump, energy might not increase at all from 7 to 10 knots. I'd seriously look for data on the old minihydros.

Somewhere there must be a chart of best hull loading and angle of attack and aspect ratio for a given speed and minimum drag for a flat plate.

Hey, what happened to the Flettner idea.

 

As in Flenttner Rotor? I'm not sure what you are suggesting?

 

Thanks, Phil. The hull overall length, including drive system, rudder etc, is limited under the competition rules to 5m (16ft 5ins). My hull is shorter, at 4.7m, as I was originally going to put the drive on the small transom.

Those figures are encouraging, as the power is probably achievable with a couple of drills, and what you've said about the total energy to get around the course probably not going up by much, if at all, when over the hump makes sense.

A chart of hull loading, A/R and AoA for a flat surface would be pretty useful, I'll do some digging around.

The Flettner rotor is still on for some experiments this year, I'm just enthused by the boats performance in the Cordless Canoe Challenge over the weekend and thinking ahead to making it faster for next years event!

 

Over in the design and build thread for this boat I mentioned that I want to experiment (not for the race) with a Flettner rotor on this hull. Just for fun really, and only for use on sheltered waters, as the thing intrigues me.

 

Are you thinking of the Flettner rotor as an air drive propellor or in place of a prop in the water? I have also been interested since I saw a Popular Mechanics article in the 60's (I think). What is the name of the thread?

Did you ever see the article on the Coanda effect in Scientific American around 1965. One of the applications was a powered hydrofoil (T-foil) which demonstrated fwd thrust and lift at the same time.

 

I am planning on having it as a sail, as Flettner did on his experimental boats. The design and build thread where I mentioned this is here: http://www.boatdesign.net/forums/boat-design/duck-punt-twist-42466.html#post542515 and there's more in a long-running thread here: http://www.boatdesign.net/forums/bo...ner-rotor-ship-launched-24081.html#post225166 and particularly these posts: http://www.boatdesign.net/forums/bo...-rotor-ship-launched-24081-10.html#post396581 and http://www.boatdesign.net/forums/bo...-rotor-ship-launched-24081-12.html#post446611 I managed to glean some useful information from Stephen Thorpe, a chap who has built a rotor-powered sailing dinghy and want to try something similar but with some better rotor speed and direction controls.

 

Jeremy,

Good to see you moving ahead with this evolution.

Remember my hydrofoil-kayak design?

Going full hydrofoil may be a few steps further down the line but if you're interested in applying it to your boat I have made good progress with my "pedal powered hydro for the layman" design.

It would stay on foil with as little as 100 watts which is easily sustainable for your average Joe.

In your application the flooding/purging bulb and the foils could be made much smaller.

It does get a little complicated with microprocessor controls which I dearly wanted to avoid but they are the ticket for staying upright on a single strut and maximizing efficiency.

The components are readily available through the R/C world and total about £150.

My concept utilizes a mechanical surface sensor forward hydrofoil and a main, centre foil with a small canard off the bottom of the pod. The leveler controlled servo activates both the main foils (port and strd) independantly and the canard to keep the vessel upright. The main foils are also controlled from a surface sensor to maintain ride height (lift and drag) as velocity increases by flattening the AOA.

I've calculated anywhere from 8 - 14 knots with only 100 watts input!

It seems like you've got 3 or 4 times that power.

 

Sorry for the delay in getting back to this thread, I've been making up some moulds for foils. As luck would have it, I was sent this link: http://www.youtube.com/watch?v=yLboyOqi6R8&feature=player_embedded Seems pretty impressive to me, as the bike stays up on the foils easily without much more than 100 W or so I'd guess.

I'm going to start off with a small foil, around 1.2 m x 0.12 m, with tapered ends, using the Eppler 817 section, stick it on the aft end of the boat on an adjustable angle bracket and see what happens. My guess is that it'll lift the stern up and hopefully allow the wider flat bottomed section of the hull further forward to plane. Testing will tell!

Tom, those speed and power figures seem encouraging! I've potentially got maybe 5 to 6 times that much power, as I know I pulled between 800 and 1000 W from the drill during the race, as I can completely discharge a 54 Ah lithium battery pack in between 3 and 4 minutes!

 

The guy in the video looked quite exhausted at the end.

 

Another way to alter trim is to use an interceptor, ie a vertical, shallow "fence" projecting below the bottom of the boat. My guess is you'd want to try depths from 3 mm to maybe 10 mm along both sides for various distances from the stern forward.

 

Consider a simple "thought" experiment with the foil.

Add the foil but set the angle of attack so it provides zero lift. Total drag will increase due to the wetted area of the foil.

Set the angle of attack so the foil provides lift. The drag of the foil system will increase due to the "induced drag" of the finite span lifting surface and the increase in sectional drag with lift.

What happens with the hull when the foil is providing lift is not as clear and depends in part on the location of the foil. If the foil provides lift but is positioned so that trim does not change then the wetted surface of the hull will decrease which reduces drag from that source. There should also be a reduction in drag due to wave making but it may be relatively small.

If the foil is positioned so that trim is changed due to the lift of the foil then hull wetted surface area may increase or decrease with a corresponding increase or decrease in skin friction drag. Likewise wave making resistance may increase or decrease due to the combined change in trim and reduced weight supported by the hull.

The comments above ignore any direcdt interaction of the foil system with the free surface. Depending on its proximity to the sides of the hull and the surface it may generate its own significant wave system. The interaction of the foil's wave system with the hull's wave system could result in an increase or decrease in net drag.

The effectiveness of the foil will also be very speed dependent. At sufficently slow speeds there would be a net drag increase due to the increase in wetted area while the foil provides little lift. As speed increases and the lift generated by the foil increases there may be a speed reached where the net drag decreases compared to the hull without the foil. Or the net drag may never decrease with the foil system.

Even with a system which provides a net decrease in drag above a certain speed the net benefit of the system depends on the boat's "mission". With the foil system in place the boat will accelerate at a slower rate at slow speeds even though it may be faster at high speeds. This means for a leg which begins with the boat stopped in the water there will be a minimum length of the leg for the boat with the foil to be faster over that leg than for the boat without the foil.

 

Thanks for that input, I realise that, at the moment, I'm trying to optimise a hull that is less than ideal, simply because I underestimated the speed potential that can be obtained from a single cordless drill. The Duck Punt hull was chosen because it had an acceptably low resistance at, or below, the onset of significant wavemaking resistance (being pretty much canoe-shaped), it had good turning ability (being flat bottomed), and it was easy and quick to build.

Had I realised that it could get close to planing speed on such little power, then I think I'd have gone for something shorter and with more beam. However, I am where I am, so am now faced with seeing if I can stop the narrow aft section squatting down and burying itself as the bow tries to climb over the bow wave and get on to the plane.

I have a feeling that interceptors will be of little benefit, because of the limited planing area aft of amidships (although I fully accept I may be wrong!). I may try them, later, but for now I'd like to experiment with a foil mounted right at the small transom, to act as a substitute for the lack of hull area aft.

The lift/drag vs speed curves seem to show that I have enough "spare" power to work with it OK, what I don't know (but will have to find out from an experiment) is whether the total hull resistance with the foil will be less than without it. It's a complex multi variant problem, as, at the moment, the propeller is optimised for a higher boat speed than I'm actually getting, which means it's absorbing too much power at the maximum speed at the moment. I'm hoping that, by reducing resistance at, or around, the onset of planing, I can get the propeller up into its best operating region. It's complicated by the cordless drill being most efficient when run at close to its maximum rpm.

If this experiment with a simple inverted T foil looks promising, then I may re-think the approach to next year's competition. I'm certain that we'll see planing hulls in the race next year, so the question is, "would a full hydrofoil be both practical and competitive?". One potential problem is that the rules for next year's competition won't be published until three or four months before the race, so any design can't be finalised until then - I may find that next years rules will preclude using foils!

 

Jeremy, thats really unfortunate about the rules. All your work may go down the drain, at least for this particular competition. Can't see why they wouldn't finalize rules a year ahead? Good Luck-I think you're on the right track......