aircraft propeller on hydrofoil boat

[haybayian]

I am trying to design a 13 foot hydrofoil and because I want the thing to be beachable I plan to make the foils retractable and to use an airboat propeller.
I understand that HD 4 managed to reach 61.5 kts with air propellers and I have not read yet that these were a problem. Another reason to use an air propeller as opposed to a water one is the relative simplicity of the transmission from engine to prop in the case of an aircraft propeller.

What do you old timers think?

Haybayian

[messabout]

It is going to be noisy and will make you the object of scorn with the general population.

What kind of power do you anticipate using? Weight is a factor to be dealt with. You are proposing a pretty small boat at 13 feet. However the airboat maniacs here in Florida have 12 footers with 350 chevy power. Of course the airboats are not foilers and they are rather wide as boats go. Consider that you will have a substantial torque reaction from the prop. That can be accounted for in the design stage. Maybe a different or larger foil on one side, offset prop, or some other counter torque method.

Air prop as opposed to water prop, is the more expensive option. Seriously expensive actually. This sounds like a fun project. Lets see what the other forum people will chip in here.

[haybayian]

Thanks for your thoughts.
My hydrofoil features a main hull approximately 4 m x 1 m and two small outriggers designed to provide stalibilty. It is a 2 seats in tandem machine. The engine is a Mazda 1500 cc DOHC 92 bhp (potentially 128 bhp with new camshatfs). The weight of the engine is 225 lbs and that of the craft altogether 1000 lbs. The engine would run the prop through a belt and 2 to 1 reduction. The prop would be running subsonic and of a design that minimizes decibels. A four blade prop would cost about $ 600 US. To achieve my goal (to have a fully retractable undercarriage) if I wanted a water prop I would have to make a rather complicated and vulnerable gear or hydraulic system.
This is where I am coming from. But I am listening to what others more experienced have to say.

Haybayian.

[rasorinc]

Put a prop on something like this.

http://www.boatdesign.net/forums/att...n-wyjazd01.jpg

[Mild Bill]

What is the proposed diameter of the propeller, and what are the expected engine rpm's at cruise power and max power?

[haybayian]

I could keep the prop rpm to 3200 (half the max design speed of my engine). The prop dimension? I hope to keep it reasonably small (i.e. 45 to 60 " or smaller if I can find one that will deliver at least 300 lbs of thrust). Of course noise is a major consideration.

Haybayian

[haybayian]

Originally Posted by rasorinc

Put a prop on something like this.

http://www.boatdesign.net/forums/att...n-wyjazd01.jpg

Nice machine. Yes I could bolt a prop to it. Should it be above or below the waterline.
Haybayian

[FAST FRED]

The reason there are not many air props on boats is propulsive efficency.

My rule of thumb is you get 5lbs of thrust from one HP in a std air prop, with about 7 as theoretical Max.

20 Lbs of thrust , 25lbs with a good (usually 3rd try!) prop, with about 30 as max, pushing on water.

So if you need 300lbs of push a 15 hp engine and a wet prop vs a 60+ hp engine and air prop is the trade off.400% more fuel, same speed.

Yes, those 12 ft boats in Fl with a 350ci Chevy do go great , but at 10-15 gph , its not a cheap hobby to flatten grass.

FF

[kach22i]

Originally Posted by haybayian

I want the thing to be beachable

You live in Canada, build a hovercraft and use it year round.

Sevtec's are pretty quiet with a four stroke in them.

[messabout]

Sounds like you have glommed on to a Miata engine. A truly tough and sophisticated little power plant. I claim that a new set of cams will not only cost about as much the 600$ prop but you will be shooting yourself in the foot with that engine mod. What the cams will do is move the torque curve upward in the rev range. Not what you want to do. I also doubt that cams alone will boost power as much as your figures suggest. The Crower catalogue, or some other, will quote very optimistic figures. Much educated modification will be needed for the whole induction system not just the cams. Why am I being such a smart mouth about the engine output? I build and work on race engines every day, mostly SCCA type stuff. Leave the engine as it is, make useful power at a lower RPM, save money too.

If you plan to turn a 60" prop at 3200 the tip speed will be way up in the stratosphere. Like 25,000 ft/min. Subsonic yes, but way noisy and probably shaky. Also take Fast Freds arithmetic seriously.

Hey Fast Fred: Living around Ortona you have probably been shaken out of bed more than one time by the grass flatteners. Why do those dudes like to run at night?

[haybayian]

Thanks Messabout for you input.

I agree with some of what you said but not all of it. The Mazda engine is a Z5 delivering 92 hp @ 6000 rpm . A more agressive cam profile and change in valve overlap does move the output to 128 HP @ 7000 rpm. This is what Mazda Corp did on their sporty models, granted they also upgraded the injectors and the ECU. High rpms is not indeed what I need. However the engine to prop reduction is my choice so it would not be such a problem.

You are right about the $$$, wrong about the HP/rpm but chances are that I will leave the engine alone anyway.

Now the prop. I had an e-mail yesterday from a prop manufacturer. They can sell me the components to make a 6 blade composite prop for 450$. I would do the assembling. The prop would run up to 3100 rpm subsonic and take up to 95 hp. The key is " 6 short blades" which will make the prop (48" to 50" in diameter, 16 " pitch) as quiet as humanly feasible.

I enjoyed reading your arguments.

Haybayian

[haybayian]

FF Thanks for your points. Could you elaborate on your "rule of thumb"? I would like to understand the huge discrepancy between air props and water props efficiency. Yes I know that water is not compressible, etc But what else should I know. I am not sold to one system or another but I would like to get the whole picture or most of it. For instance in the water props cavitate, struts or shafts and bearings drag. Drag for an aircraft prop moving at less than 40 kts, even with a shroud around it is not significant. Is it possible that the airboat type fix pitch propeller would be less efficient than the water prop in the take off phase and then once cruising the water prop would become less efficient. Anyways, my point is not to challenge your views but to understand these things and make the right decision.

Haybayian

[messabout]

One source of information about water props is the propellor book by Dave Gerr. An interesting and informative read well worth its price.

[markdrela]

An air prop can be every bit as efficient as a water prop, provided the disk area is increased by 800x to account for the 800x smaller air density. Or equivalently, the air prop wants to have a diameter about 28x larger than the water prop. You can make it smaller of course, but the efficiency will decrease.

[Guest625101138]

There is a simple little prop design program that runs in explorer using Java applets. It is called JavaProp. You can run it from this link:
http://colaco.freeshell.org/mhepperl.../jp_applet.htm
I have attached a couple of the screen dumps of results using the parameters you have already listed.

The applet can also be used to look at a water prop by changing the parameters in the options page.

If you are playing around with props this is about the easiest you will find that gives valid results.

To get the results shown I selected the MH foils that have Re# of 500,000 using the thickest on the inside and the thinnest of 9.8% at the tip with the 13% in the middle. I set the AoA at 0 degrees so the blades are not too narrow. You can get better efficiency with higher AoA but the blades look to be too narrow to take the load. You can adjust the AoA to get the blade width that matches what you are looking to buy.

The six bladed prop with profile shown below will get efficiency around 57%. The thrust shown should be enough to get at least a 1000kg boat to 50kts. So if you can get the weight around 1500lb including load then it should be easy with the small diameter prop.

You can play around with JavaProp to see how things change. I tried a 3-bladed, 2m in diameter running at 1500rpm. It gets 66% efficiency.

Rick W