Propeller design

[skipper_rj]

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.

[Guest625101138]

Use this site to determine the hull drag:
http://illustrations.marin.ntnu.no/h...ing/index.html

Use this site to determine the best prop:
http://colaco.freeshell.org/mhepperl.../jp_applet.htm
You have to set Options pagto suit water.

If you can use these two sites you will be close to the required design. The "optimum" will depend on the constraints and practical realities about strength of materials, draft etc.

Rick W.

[skipper_rj]

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.

[Jimboat]

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.

[Guest625101138]

Originally Posted by skipper_rj

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.

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

Rick W.

[skipper_rj]

Thank you again Mr Rick.

[skipper_rj]

Originally Posted by Jimboat

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.

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

[Jimboat]

Originally Posted by skipper_rj

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

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.

[afrhydro]

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 ???

[afrhydro]

Originally Posted by Rick Willoughby

Use this site to determine the hull drag:
http://illustrations.marin.ntnu.no/h...ing/index.html

Use this site to determine the best prop:
http://colaco.freeshell.org/mhepperl.../jp_applet.htm
You have to set Options pagto suit water.

If you can use these two sites you will be close to the required design. The "optimum" will depend on the constraints and practical realities about strength of materials, draft etc.

Rick W.

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

[Guest625101138]

Originally Posted by afrhydro

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 ???

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.

[Guest625101138]

Originally Posted by afrhydro

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

There is a reasonable amount of explanation on using JavaProp on this thread:
Torque and fuel effiency

Rick W.

[Jimboat]

Originally Posted by afrhydro

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 ???

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.

[Guest625101138]

Originally Posted by Jimboat

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.

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.

[afrhydro]

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