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/hydrodynamics/resistance/planing/index.html

Use this site to determine the best prop:
http://colaco.freeshell.org/mhepperle/javaprop/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 (http://www.aeromarineresearch.com/stbd2.html)and the "Tunnel Boat Design Program" software (http://www.aeromarineresearch.com/tbdp6.html)- both specifically dedicated to detailed analysis of lift/drag/power characteristics of powered catamarans.

[Guest625101138]

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]

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 (http://www.aeromarineresearch.com/stbd2.html)and the "Tunnel Boat Design Program" software (http://www.aeromarineresearch.com/tbdp6.html)- 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]

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]

Use this site to determine the hull drag:
http://illustrations.marin.ntnu.no/hydrodynamics/resistance/planing/index.html

Use this site to determine the best prop:
http://colaco.freeshell.org/mhepperle/javaprop/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]

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]

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:
http://www.boatdesign.net/forums/showthread.php?t=22289&page=2

Rick W.

[Jimboat]

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 (http://www.aeromarineresearch.com/adverts/extreme3_june03.html)that I wrote that touches on methods for considering dynamic center of gravity (center of dynamic balance) for typical power catamarans.

[Guest625101138]

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 (http://www.aeromarineresearch.com/adverts/extreme3_june03.html)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

[afrhydro]

rick
if i could figure out how to use the info you posted i would have a lot more info to go on

[Jimboat]

afrhydro - Scott, i have emailed to you a copy of all 3 articles (http://www.aeromarineresearch.com/adverts/extreme_apr03.html)relating to the lift/drag/dynamic balance of hulls. Lift/Drags based on the principle of the Savitsky planing model. Dynamic balance outlined in the 3rd article. Call if you want to discuss more. This analysis is part of what the TBDP software (http://www.aeromarineresearch.com/tbdp6.html)does.

[afrhydro]

cool
going by the seat of paints so to speak for changes now
seems very tricky
i like the idea of making the after plane a small shallow tunnel hull slots
it would seem to be the simplest solution to more speed and fuel consumption
got to decrease hydro drag by half or close

[afrhydro]

jim or rick
look at this for me
you will see the two little wedges in the photo
those are gone
now my question is to get more lift
should i recess a 6 " x 1 1/2 x 8' tunnel slot up to the back of the sponsons or
go the other way and make a shoe

[afrhydro]

jim as soon as i get my printer working ill fill out that form and fax it to you

[oceanmaster66]

Jimboat,

Do you know of any publication or research paper that describes an analytical method for finding the Dynamic C of G of a monohull?

Thanks

[Jimboat]

oceanmaster66 - the paper that i wrote on dynamic CofG was aimed at power catamarans & tunnel hull designs, but the methods are the same for monohulls. The key is to do the analysis throughout the velocity range, so that any dramatic changes in the DCofG are identified (such as the typical "Hump Zone (http://www.aeromarineresearch.com/adverts/HotBoat_April2007.html)" that higher speed planing hulls experience).

Here's an article (http://www.aeromarineresearch.com/adverts/extreme3_june03.html)that I wrote that touches on methods for considering dynamic center of gravity (center of dynamic balance) for typical power catamarans.

Drop me a line if you have any questions.