Electric Hydrofoil Powerboat

Discussion in 'Boat Design' started by intrepid71, Dec 28, 2015.

  1. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Yes, the fuel-consumption is higher and the top-speed is lower than I expected when designing the boat. There are also uncertainties in the ways we try to analyze here -propeller efficiency that we guess and fuel-consumption that is not linear with torque.
    A more reliable way to investigate this would be a direct drag-measurement. We tried with our previous prototype, but the measurements were disturbed by the towing-boat's wake, and the force was also very unsteady, so we did not get any reliable results. Next time we may put the load-cell in the towed boat, so we can use a very long towing rope. If we can use a hydrofoil as towing boat (that produces very small waves) will also help. One may also consider to have load-cells in e.g. the drive-unit support, so we can get components of the drag.
     
  2. Joakim
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    Joakim Senior Member

    As I said earlier one way to establish the specific fuel consumption is to compare to smaller engines. E.g. this shows the performance of BF50, BF30 and BF20 in the same boat http://www.sandstrombatar.se/wp/wp-content/uploads/K%C3%B6rfakta-Sandstr%C3%B6m-C-495-och-Honda-BF20-BF30-BF50.pdf

    Unfortunately the consumption is only given for BF50, but BF20/30 have been tested elsewhere and at full power BF20 consumes about 7 l/h and BF30 about 11 l/h. Also BF30 has been measured to deliver exactly 30 hp (at propeller shaft).

    So at 25.1 knots that boat requires 30 HP and BF50 consumes about 10 l/h at that point and is thus more efficient than BF30 at full power. About 0.33 l/HP.

    I didn't find any power measurement for BF20, but Honda seems to have quite accurate power ratings, thus it will likely be close to 20 HP. At 19.8 knots the boat requires 20 HP and BF50 consumes about 6 l/h and is thus clearly more efficient than BF20 at full power. About 0.30 l/HP.

    The latter is quite close to your operating point at 23 knots, thus 0.3 l/HP should be a very good estimate.

    Is it necessary to have such a low pitch propeller to get off the water? You would get better propeller and engine efficiencies with a higher pitch. E.g. with 16" pitch the predicted efficiency is 75% and lower rpm with higher torque will give lower speficic fuel consumption. Actually even above 77% would be possible by reducing the blade area to ~40%, but that may cause troubles at other speeds.

    Here are several specific fuel consumption maps to get an idea how they typically look like http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_%28BSFC%29_Maps

    As you can see from the maps, the specific fuel consumption gets quite bad at very low loadings. That will limit the possibilities of getting to extremely low l/nm values with an 50 HP engine at 23 knots.
     
  3. Joakim
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    Joakim Senior Member

    I also checked the predicted propeller efficiencies for that test with BF20-50. At 20 knots BF20 with 9.25*10 could have about 68% and BF50 with 11*15 about 73%. Thus BF20 outputting 20 HP equals in thrust only 18.6 HP from BF50. But that is more than compensated by 50 kg more weight of the engine and higher drag of the bigger drive in water.
     
  4. BertKu
    Joined: May 2009
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    Location: South Africa Little Brak River

    BertKu Senior Member

    Hi Alexander, I am impressed with your project and wonder whether you and your team ever considered to go for 2 or 3 or 4 separate battery/propeller systems to reduce the losses with high currents and switching of those currents. I personally would not buy a boat with voltage of greater than 48 Volt should there be a breakdown after a couple of years usage, with the risk factor to the crew. A car is a different issue, there one can walk home in a breakdown situation. One can't swim for hours in the sea.
    I truly hope that your team will get into large production volumes, looking at the future whereby also England is now mumbling that the law on diesel fumes for pleasure boats in their harbours has to be reduced. Bert
     
  5. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Yes, it is difficult to get off the water with higher pitch than 15". But the motor-support can be lifted up to pass the drag-maximum at 12 knots. At some 15 knots the motor has to be lowered to the regular flying position before take-off. We have not obtained any measurable improvement over the 15" prop with the 16" we have tried so far. This can be because our 16" is not as good individual as our modified 15", or it indicates that the strut down to drive-unit has a lot of drag.
    To summarize this discussion: We have a hydrofoil that can reduce the fuel-consumption near 50% compared to a planing boat of similar size at 23 to 25 knots. But a better result should be possible. To find out what we can improve, a direct drag-measurement in a towing test would be very helpful.
     
  6. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Hello Bert, so far we have only used 4-stroke outboards for our full-scale prototypes. But we have considered electric drives as well. Yes we have considered having a separate electric drive for redundancy, to go in shallow harbors and as a power-boost during take-off.
     
  7. Joakim
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    Joakim Senior Member

    I think the total drag can be quite acurately calculated from the data you have given. I would be very surprised if the total drag at 23 knots would be outside 800-1100 N range, most likely within 900-1000 N.

    I would think it is more beneficial to know the individual drag components than the total drag. If the drag of the foils really is only ~200 N, there are some other components that are much higher than they should be.

    How much does the consumption vary with load? You got 0.29 l/nm with minumum load. How much is it with maximum load you can "take off"? The difference should give the actual L/D of the foils, since nothing else changes, if the ride height is the same.

    Then you told that you can adjust the motor vertically. Measuring the consumption at different heights gives a good idea of the drive drag.

    Is it a must to get the boat planing first and then flying? You can't jump over the hump with foils? Too small area for that speed?

    Your project is very interesting and I had a long discussion (with you?) at the Helsinki boat show a few years ago. I hope you are able to succeed!
     
  8. AlexanderSahlin
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    AlexanderSahlin Junior Member

    Thank you Joakim, You probably met my partner Björn Ljungdahl at Vene-Helsinki. Yes, I think we have had a good discussion here, that can contribute to even more efficient hydrofoils than we have built so far.
    About air-drag: The result 0.5 l/m in ≈20 knot headwind is on protected water with quite small waves. The problem downwind is that you usually get a side-component, which increases the drag. I have not been able to drive downwind on protected water under so steady conditions that I have been able to measure how much the fuel-consumption is reduced, but I don't think it has been as low as 0.25 l/m, as it should be with constant fuel/power and constant prop-efficiency. Waves can increase the drag a little, which makes it hard to obtain so much drag-reduction downwind.
    The drag-coefficient may well be higher than 0.5 with a big open cockpit. Hoerner reports CD=0.95 for an open car in FluidDynamicDrag, with a cockpit-size/frontal area quite similar to ours. Our frontal area is ≈2.5 sqm. So this may agree quite well with my estimates.

    The fuel-consumption increases with flying-weight. This is what should be expected at the speed for minimum drag. But the measurements we have so far are not so accurate that we can use them to determine the wing-drag that way. My estimates for the wing-drag indicate that the minimum is near 25 knots rather than 22 knots, where we have the minimum fuel-consumption. Hence, the relative increase in wing-drag will be somewhat higher than the relative increase in weight.
    So, when trying to separate the drag-components this way we have to rely on: variation in propeller-efficiency, fuel-consumption/power and my numerical model for hydrofoil-drag. For me, there are too many uncertainties to draw any conclusion on that.
    I also checked my estimate for the lifting wing only: 800 kg flying-weight, span 2.1 m, wing 0.64 m below the surface, Profile-drag from X-foil with Ncrit=3, Induced drag multiplied by 1.3 for surface-effects and non-elliptic load, and 2-D wave-drag for the wing according to Hoerner. The results were: L/D=42.8 at CL=0.33 or 24.7 knots. At CL=0.4 or 22.5 knots, where fuel-consumtion is lowest, L/D=41.4. Setting Ncrit in X-foil to 1 decreases the max. L/D for the wing only to 35, which still corresponds to a very low drag from the wing.
     

  9. Joakim
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    Joakim Senior Member

    Assuming 20% of aerodynamic drag, constant engine efficiency and constant propeller efficiency, you should get 0.43 l/nm in 20 knots head wind and 0.23 l/nm in 29 knots downwind, if aerodynamic drag was the only thing that changes from 0.29 l/nm without wind.

    Driving +-20 degrees downwind would keep the apparent wind below 8 knots and should keep the consumption below 0.24 l/nm.

    Since you are getting about 0.5 l/nm and not below 0.25 l/nm, I think there is something else going on.

    According to Savitsky http://www.boatdesign.net/forums/attachments/hydrodynamics-aerodynamics/72559d1342801013-inverted-chines-strakes-question-oh-no-inclusion_of_whisker_spray_in_performance_prediction.pdf their (military style) planing boat models had measured Cd of 0.7. This shows Cd 0.6-0.7 for convertible cars and 0.7-0.9 for old cars like T Ford http://www.engineeringtoolbox.com/drag-coefficient-d_627.html

    In your case you have plenty of room below the boat for air flow and the underbody is very aerodynamic. It makes most of the frontal area. I don't think that the upper body with rather unaerodynamic windshield and open area can spoil the total Cd to much worse than the 0.6-0.7 given for the convertible cars.

    In your pictures and videos there is quite a lot of sprays to the hull. Could this be a major drag component like whisker sprays for a planing boat (in the link above) and explain the worse than expected fuel consumption? Maybe wind and waves increase the sprays considerably? Or the sprays increase the aerodynamic drag by blocking the air flow below the hull. Waves certanly increase the drag of the drive and the vertical parts of the main foil.

    How much would the drag and consumption of a normal planing boat increase in 20 knots head wind? From what I remember testing the top speed at head and down winds the effect was quite small.
     
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