most efficient propulsion?

OK this has probably been asked different ways 100 times but all other factors aside which propultion system has the least power loss. The outboards have stright propultion but makes the torque turn 90 degrees. Inboards do not turn the torque but they by thier nature do not apply the thrust streight since they are angles down, and they have a seperate rudder. I don't know the inner workings of a sterndrive but I would imagine they make the torque turn twice aprox 90 degrees each. Surface drives apply power in a streight line (some do anyway) but they have to break the surface tension of the water and they turn a very large heavy prop. Jet drives have very little surface area in the water, but they work on a small, high rpm impailer which probably has a 88% or less thermal efficency. I woud guess that the surface drive might have the edge, but the fastest and most efficient boats I have ever seen are both powerd by sterndrives which seem to be the worst for efficiency. So I am totaly confused any feedback would help.

 

Depends on the loading conditions and design speed for water propellers. Often draft constrains efficiency in lower speed applications. At high speed appendage drag means the lower efficiency of the surface prop wins out.

If you are only interested in efficiency at speed then air props provide the best solution providing you can tolerate the large sweeping blades.

It is horses for courses really.

Rick W.

 

Which would be the most efficient at a 35-40 mph crusie speed?

 

Hard to beat the simplicity of an outboard drive. Possibly depends on the size of boat but you can get some large outboards and engine efficiency is not too bad either.

To get relative merit of things. A simple gearbox will have efficiency up around 98%. An outboard propeller might be 75%. So getting the prop right has bigger potential for gain in efficiency.

The fastest boats usually have surface drives but this is well above your speed of interest.

Rick W

 

Look at the Seabright skiffs of William Atkin and you might come away with a different opinion about this 'fact' ...

Only at high planing speeds. For low planing speeds I think tunnel-stern Seabright skiffs have the edge.

 

Suppose we have a two-bladed propellor with a radius of 12 inches turning at (say) 600 RPM. This gives it a swept area of 36 x PI square inches and produces a certain amount of thrust.

If we increase the diameter of the propellor (same efficiency) to 18 inches we get 81 x PI square inches of swept area. To produce the same thrust can we reduce the RPM to 36/81 x 600 (about 266 RPM)?

 

The simple answer is yes but the question does not have a great deal of meaning unless you nominate a boat speed and motor power or hull drag.

The reason you would go to a larger diameter prop, spinning slower would be to improve efficiency so the question needs context. You are battling basic physics to get the larger diameter prop to operate at the reduced efficiency of the smaller diameter prop.

For example, if you do the comparison for a dinghy able to do 6kts with 1kW the 12" prop will have an efficiency of around 65%. The prop would be 12 X 20, which is a large pitch for the diameter. For the same conditions of drag an 18" prop at 266rpm would only require only 900W. The prop would be 18 X 40. Again a very unusual pitch to diameter ratio.

As you can see the question needs to be directed at some specific operating condition to make sense. In practical terms you will not be able to get a small prop to achieve the same efficiency as a larger diameter prop unless you are looking at higher rpm than nominated. Of course you could butcher the blades on the bigger diameter prop to reduce its efficiency but that would make even less sense.

Rick W

 

Thank you Rick. I am trying to minimise the propellor RPM of a pedal drive system for a cruising boat. The target is about 3 knots at about 100 watts. I don't know if that is possible but I am only in early days. Once I get the thrust/speed of the propellor sorted, I can gear it to suit my preferred cadence. What you say about the better efficiency of the larger prop heartens me.

Some aircraft use "fences" to stop airflow from sliding sideways over the wing surface. These are vertical aluminium strips about 150mm high running fore and aft over the wing. Others use "winglets" on the tip to improve lift. Would using "fences" or tip turbulence tamers similar to winglets be useful in improving prop performance or would the extra rotational drag soak up any benefits?

 

You should easily get 3kts with 100W if you have a hull more than about 3.5m long.

As a starting point you could use 250rpm and a 400mm prop.

Bolly make a 15X25 prop for human powered boats that is quite good and reasonable price.

If you have welding and grinding shop facilities then you can make the ideal prop in a few hours but it needs to be designed to suit thee application.

Rick W

 

JaredT

"...which propultion system has the least power loss.."

Are you referring to the least amount of losses from engine all the way to jet/prop etc, ie mechanical losses, or are you referring to the efficiency of the conversation of the power being delivered at the jet/prop into forward motion ...?

Also the answer also depends upon many other factors such as the type of vessel, ie length displacement ratios and the froude number you're running at to name a few. Since a vessel of 5m length boat doing 30knots is somewhat different to a 100m vessel doing 30knots. The solution for one may not be applicable for another.

Just not 100% clear in what you're asking.

 

Thank you gentlemen,
I am planning a long journey by pedal powered catamaran. The hulls will be 4.8 metres (16 feet) long with a length to beam ratio just over 15:1. The all-up weight of craft, rider and supplies will be about 200 KG (440 pounds), the rider being the largest lump. To make the journey feasible I need a minimum cruising speed of 6KPH. As all the propulsive energy will come from my battered 62 Y.O. body, I need to make it as efficient as possible. I think I can sustain 100 Watts for useful periods of time without killing myself and so that is my target power.

I saw some effective drive systems using 12" props and thought that I'd see what I could do to improve the theoretical efficiency in propellor design and drive train losses before finally putting pencil to drawing paper. Propellor diameter is not a problem up to half a metre. I want to use the largest diameter propellor as possible to maximise efficiency and to reduce drive train losses by keeping the RPM down.

I have a fairly well equipped workshop and can make props from aluminium or iron.

 

uggliozzi

In that case, the main driver, excuse the pun, is the boat and hence the lines. You're using a 15:1 L/B which is ideal, if you can afford to squeeze a bit more (maybe up to 20:1) that'll help. Also, make your hull shape semi-circular, for minimum wetted surface area, ie to reduce your skin friction resistance component.

Will you attempt to make a model of the hull to establish rough total resistance and hence the power you need? This shall tell you whether it is worth all the effort, ie what overall "savings" can be made and their effects.

Designing props to be 'more' efficient is not that easy, many have been trying it for years; many factors affects prop effeciency. Even more so for your application, since you will not able to maintain a constant speed, you'll have slight variations with each cycle, so trying for an optimum maybe just a nice theoretical idea, but in practice you wont notice any difference owing to minor changes in delivered power which which is ostensibly cyclic rather than steady state, as in theory.

 

resistance

uggliozzi

Attached is some research my colleague and I did 10 years ago, when investigating different hull forms for lower resistance. (All published at RINA conference).

Once you get in to high L/B's with low length displacement ratios, the difference is very very small. Shapes has pretty much zero effect. You can see that semi-circular and classic 'canoe' hulls were marginally better, but only at certain speed range.

If you can do a few model tests, it may greatly assist in your quest. Then focus on mechanical losses.

 

Ad Hoc,

I'm afraid my hull cross-section shape will be pretty crude due to my poor skills in this area, but I am heartened by your PDF and that I will be going quite slowly.

It sounds like I need to build at least one hull and do a few tow tests to see just what its drag will be before making any further plans.

This is the reverse of what I had planned. Just goes to show how much listening to experts can help we beginners.

 

uggliozzi

If the shape is pretty crude, as noted above, if the L/B is high and your length disparagement ratio is low, the actual shape wont matter too much. So, don't worry about that. But, if you can, semi-circular will help a bit.

It is far better to build a few models to test your ideas and parametric studies, than build the full scale thing once and and test it. Since you may not have the resource/time to build another full scale should you encounter problems, nor will you know if other "more simple" changes will effect the overall design.

Focus on what you can change easily that does affect the resistance and the delivered power. Hull length/displacement and the method of delivering the power. Prop design, just select one that is suitable for your average speed and that can absorb the power your delivering.

Finally, obvious, just make it all as light as possible!