Propeller design

Discussion in 'Props' started by skipper_rj, Apr 14, 2008.

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skipper_rjJunior Member

I am not used to design of propeller for planning boats, and I would like to learn more about it. Does anybody have a hint of a book or an article that I could read, in order to know more about it? I would specially like to know more about propeller/hull and motor optimization. Thank you.

2. Guest625101138Previous Member

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skipper_rjJunior Member

Thank you Mr Rick,
I had some difficulties to use the page because of the java app required, but now it is ok.
I am a bit affraid of using ready to use algoritms, specially because I haven´t yet found any that fit to calculating resistance of powered catamarans. Do you think this should work well? Thank you again.

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JimboatSenior Member

skipper_rj - you're right about the uniqueness of powered catamarans. There is a need for analysis of the balance of planing loads of different surfaces combined with the issues of dynamic stability changes through the velocity range. It's certainly all possible, however, event though mathematically complex. Have a look at the "Secrets of Tunnel Boat Design" book and the "Tunnel Boat Design Program" software - both specifically dedicated to detailed analysis of lift/drag/power characteristics of powered catamarans.

5. Guest625101138Previous Member

Post the results you have and they can be checked to see if they are sensible.

Rick W.

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skipper_rjJunior Member

Thank you again Mr Rick.

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skipper_rjJunior Member

catamaran design

Do you believe it is a good book for beginers in the design of a powered catamaran?

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JimboatSenior Member

Well, I am a bit biased, since I wrote it....but, Yes I do think it's good for beginners...it starts at the beginning and progressively provides more detailed information.

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afrhydroSenior Member

how do you figure the lcg
i know it was 8 ft from the transom when i buit it
and then the vcg at my thickest point or side view its only 26 inches ???

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afrhydroSenior Member

now im really lost

wow
thats going to take some time for me to get the hang of that

11. Guest625101138Previous Member

If the boat is trailerable and well ballanced on the trailer then the lcg will be above the middle axle/s. You can also work it out by where it sits in the water.

Rick W.

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JimboatSenior Member

afrhydro - your LCG question is a good one. The first idea of "balancing" that many ownersmost is to consider the static measure of the CG. This is really only a small part of the story of balancing the design of your hull. Actually, with a hull design such as yours, this value and location is of much less importance than the Dynamic C of G (or Dynamic Center of Balance). The deadweight of the hull is counteracted by all of the lifting forces such as hydrodynamic lift and hydrodynamic drag (and at appropriate velocities, aerodynamic lift/drag too). These forces come from lifting surfaces, all drag components and the power application forces. The values and location of these forces all contribute significantly to the location of the Dynamic CofG.

The important issue to recognize is that the Dynamic CofG changes at each velocity. So it's important to check the Dynamic CofG throughout your operating velocity range. Balancing the hull on the trailer is really only of value if you intend to run your boat while it's on the trailer.

There are methods of establishing the dynamic CofG. Here's an article that I wrote that touches on methods for considering dynamic center of gravity (center of dynamic balance) for typical power catamarans.

14. Guest625101138Previous Member

I believe the question was in relation to using the Savitsky model presented on the previously posted link. This relies on the static lcg. Most of the other factors you state are taken into account by the input parameters.

No need to make it more complex than needed to get a good answer.

Note: The linked Savitsky model scale only goes to 45kts. Even at this speed other factors come into play so it is of limited value at the high end.

Rick W.

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afrhydroSenior Member

jim i coundnt read it the letters were fuzzy

ill try to blow it up

now as i sit in the water how do i find my lcg
i know i need more lift about a ft from the transom
i was thinking of a 1 1/2 inch step about a foot from the end of my transome step

or making a small shallow tunnel just in behind the sponsons but under and on the outside of the after plane
i could catch some air and drive it under my transome and out the back of the hull
but im not sure i would get the lift i need

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