Electric Hydrofoil Powerboat

The voltage will drop below 3.7 as it discharges. The amperage will also drop in direct proportion. You need to look at the discharge curves to be able to calculate reserve power.

 

Yes and it will also start above 3.7 V. 3.7 V is close to the average voltage.

 

Dear Joakim, you are correct that the present Foiltwister prototype reduces the fuel consumption at best near 50 % compared with a boat of similar size. My best result in calm weather so far is 0.29 l/nm at 22 to 23 knots speed. The figure 75 % comes from the drag-reduction we expected, if we could have designed the drive-unit for hydrofoil use. On the present prototype we have used a standard lower unit, designed for use on planing boats. It creates a major part of the drag when submerged so deep that the stabilizer can be installed on the fin.
Why cannot our "simple foil design" be efficient? Our main foil has the same span as the boat's beam, tapered wingtips that will give quite close to optimum span-wise lift-distribution and I have optimized the hydrofoil section for cruise around 25 knots with cavitation-free operation up to 40 knots. Thanks to the Foiltwister lateral stabilization no ailerons are required. Hence, it will be as hydrodynamically clean as possible. The lifting hydrofoil alone has a L/D=40 at cruising speed. With the Foiltwister system the side-force on the struts is always very small, which minimizes also the drag of the struts.

 

I didn't say it can't be efficient and I didn't know the details of it. One could say that the current stabilazer foil installation makes it inefficient.

So you have the stabilazer foil installed to the "anti cavitation plate", thus above the propeller? And you need to keep the foil well below the water. That's a very bad thing, especially for higher speeds. Your video shows the foil on on tip of the fin (below the propeller) https://youtu.be/8bweBQHF3UY

The standard outboard fin is not strong enough? How stable is the height of the stabilizer fin? Is it even possible to get the propeller at optimum depth (=mimimum drag + still good grip) with this system or would that require an active control system?

What is the total weight of your prototype?

 

On our boat with the extended Honda outboard the stabilizer is at the fin, as you saw in the video. The fin is strong enough.
When you put the motor on a planing boat you can keep the water-surface very near the anti ventilation-plate, so the water-pump is always above the water e.g.
But on the hydrofoil application the water-surface is sometimes a half meter or so above the anti-ventilation plate. We will try to solve these problems on our next boat, so we can keep the drag low also when the propeller is some half meter or so below the water surface.
We have a mechanical altitude-control, like they use on the Moth dinghies, but we rotate the entire front foil unit, since we want to avoid flaps. But it is hard to make something follow the water-surface at 25 knots speed or more. that will require a very high vertical acceleration, 10 g or so. I know from developing paravanes for high-speed sailing.
http://www.trampofoil.com/speedsailing/index.html
We have also considered to move the drive-unit vertically during flight, but found it too complicated.
The all-up weight is 700 kg.

 

That's quite a big number. Is it a theoretical value, or it has been measured through tests? If theoretical, does it take into account the wave drag and the decrease of lift due to free-surface proximity?

 

It is quite hard to measure components of drag in our case. So I have only theoretical estimates for that. For the induced drag I have assumed a high Froude number, so the pressure-field is similar to a corresponding case with the free surface replaced by a plane of anti-symmetry. Hence, biplane-theory can be applied for the efficiency-factor. I have assumed that displacement-wavedrag is small, because deep submergence(h/c=4) and a thin foil-section(11%).

 

Why would water surface be half meter above the anti-ventilation plate? Or do you mean in over half meter waves which must be close to maximum you can foil in with such a low clearance and blunt hull.

In such waves added drag is maybe not such a big deal. Can you get the anti-ventilation plate slightly out of the water in flat water.

0.29 l/nm and 700 kg works out to something like 7:1 over all lift to drag ratio for the vessel and ~1000 N total drag using 0.3 l/hp and 70% propeller efficiency. If you have 40:1 for the main foil, it only creates ~200 N of drag. Where does the remaining 800 N come from? At 23 knots the drive should not have more than 100-200 N, if it is at optimum height.

 

Interesting Joakim, you seem to have some info that we don't have, e.g, the ratio between fuel-consumption and torque at 4500 rpm for the Honda BF 50. Have you measured torque and fuel-consumption at different rpm in a test-rig for 4-stroke outboards? Have you measured the drag-force on outboard drives as well? Also very interesting for us. I have only been able to do some rough theoretical estimates.
You are right that it should be possible to get better results for the fuel-consumption if you can fly with the anti-ventilation plate near the surface on flat water. Maybe we shall try to adjust the motor for such conditions some calm day when we also can avoid wakes from other boats.
You asked for other components of the drag: there are struts, attachments between wing and struts, stabilizer, and air-drag.

 

There are quite a few measurements in boating magazines. E.g. in Finnish Vene 3/2001 the specific fuel consumption of BF50AX is measured to be 215-245 g/hph (0.29-0.33 l/hph) at 4000-6000 rpm full throttle (lowest at 4500 rpm, highest at 6000 rpm). You probably have a newer model, which might consume slightly less.

At 4500 rpm the full power is about 41 HP thus you are operating at about 50% load. Usually a smaller outboard at full power consumes a bit more than a larger one with partial load in the same boat and at the same speed, thus specific fuel consumption is still good at 50% load.

So 0.30 l/hph should be the correct order of magnitude.

I don't have the dimensions of BF50 drive, so my estimates are based on guessed dimensions. I have a rather simple model, which gives ~100 N for 23 knots. Then you can estimate using the ORC sailboat rating rule, which has a measurement based model for a saildrive. From that I got ~200 N.

Then you can also estimate based on known performance of BF50. You can quite easily reach ~45 knots with BF50 in a small racing boat still having the drive at normal depth. You know that the drive drag is proportional to v^2, thus at ~45 knots drive drag will be 4x compared to 23 knots. At 45 knots BF50 can produce ~1100 N thrust. If all that thrust would go to drive drag, the drive drag at 23 knots would be less than 300 N. So that is the absolute maximum for the drag.

For a similar size and weight V hull boat my Savitsky model predicts 0.40 l/nm at 23 knots. The measurement for BF50 shows 0.45 l/nm. In the prediction the total drag is 1440 N, with the following components:
Hull friction drag 670N
Whisker spray friction drag 54 N
Drag due to lift 530 N
Aerodynamic drag 96 N
Drive drag 85 N

Probably drive drag is a bit underestimated in this model. Also the whisker spray drag in the prediction assumes optimal spray rail design for this operation point. The latter is the most likely the main cause for the 10% error found.

 

Thank you very much for the info Joakim. The optimum efficiency at 4500 rpm agrees well with our results, where we have got the lowest fuel-consumption with a 15" prop, that cruises at 23 knots at that rpm. But does the fuel-consumption vary linearly with power at constant rpm?
Your estimates for the drag of the drive are also reasonable. In our case we still have the water-pump at it's ordinary place, so it has to be enclosed into the part of the strut that usually goes in the water.
You also assumed a propeller efficiency at 70 %. This is probably valid at the propeller's design-point. But we operate at some half of that load. I reduced the fuel-consumption some 10% by removing the cupping from the 11-1/4x15" Solas-Honda aluminium prop. This indicates that the propeller is somewhat over-size for hydrofoil-flight.
You compare our measured fuel-consumption with a numerical model based on Savitsky's method.
But the fuel-consumption of our hydrofoil has been compared with that on a planing boat with the same motor in a test on the water by Staffan Westerling, published in the Swedish magazine Båtnytt #2,2013. The planing boat was a Drive Open 50, slightly smaller than the Foiltwister and 100 kg lighter. At 23 knots the fuel-consumption for the Drive-50 was 0.52 compared with 0.31 for the Foiltwister.
My conclusion after this is that it is hard to reduce the drag of the main hydrofoil and it's struts very much from our present, but the drag of the strut down to the drive-unit and the air-drag can be reduced. Probably, also the propeller-efficiency can be increased from our present.
On our next boat we will try to do these improvements. I can not tell how it will look on this forum now, but it will not be a bow-rider!

 

Yes the specific fuel consumption depends on power and rpm. The values I gave were at full power. Usually the specific fuel consumption is a bit better with slightly lower load and then starts to get worse and worse while lowering the load.

It is hard to know the propeller efficiency. It can be around 70% both at full power and at your operating point.

As far as I know Drive 50 Open has no spray rails. According to my Savitsky predictor this increases drag and thus consumption by about 20% at 23 knots compared to optimal spray rails.

You said "the all-up weight is 700 kg". Drive 50 Open weighs 500 kg + 100 kg for the engine + 100 kg for the driver and some fuel, thus about 700 kg in total.

 

E.g. a B-series based propeller calculator gives 2175 rpm (propeller shaft) and 73.4% efficiency for 1000 N thrust at 23 knots for the pitch and diameter you gave. For 1700 N thrust (Drive Open 50) 2380 rpm and 71.9%.

 

The 700 kg for the Foiltwister was for fully equipped boat with motor and fuel but without crew.
In Staffan Westerling's test we were 2 onboard, =180 kg-> 880 kg total with crew onboard. When testing Drive 50 they were also 2 onboard. When I got 0.29 l/nm I was alone (700+100=800 kg total flying weight). Sorry for any confusion.
So, if the propeller is that efficient we have to focus on the strut down to the drive-unit and air-drag for further improvements. I estimate the air-drag in calm weather to about 20 % of the total. (the fuel-consumption increases to some 0.5 l/nm in a 20 knot headwind)

 

For 800 kg weight my Savitsky predictor gives 0.48 l/nm for a V hull boat at that size. But I guess your boat has some extra weight due to foils and structures needed by them, thus a fairer comparison is to a bit lighter "normal" boat.

It's hard to say what is the efficiency of the propeller you have. It can be a bit over 70%, but it could be clearly worse as well (not below 60% I hope).

Are you sure all the extra drag in 20 knots head wind is aerodynamic? No waves? Did you get below 0.25 l/nm downwind as well? How much is the frontal area? ~200 N sounds a lot. If Cd is say 0.5, you would need to have 4.4 m2 of frontal area. The average beam is probably around 2 m, thus the average height would need to be about 2 m as well. I'm guessing you only have about half of that and I don't think Cd can be anywhere near 1.0 for that boat flying with very aerodynamic lower side.

What is your top speed? Was it only around 30 knots?