Large MY Prop Sizing

[tallen]

I am designing a 180' planing luxury yacht. This vessel is much larger then typical planing vessels and I am trying to reach speeds of about 40 knots. According to my calculations I need a combined horsepower of about 35,000 HP to get about 264,000 lbs of thrust. I would like to get this from two large high pitch propellers but I don't know where to go to get specs for propellers to meet these requirements. The propellers will be driven from two large electric motors which themselves will be powered by diesel turbines.

Any help would be much appreciated

[Jango]

Need to know the Total Displacement.

[tallen]

its comes in about 850 tons

[Jango]

I have software for determining Prop sizing "Propcalc", but unfortunately it will not accept your Huge numbers. If you could scale down,(ALL numbers) perhaps it might work.

[tallen]

I can scale down the ships dimension's and displacement, however, I forget exactly how to scale the velocity

[Jango]

Velocity should be directly proportional to size. I/4 length = 1/4 V or 10K.

[tallen]

Ah, ok i thought it was more complicated then that (using volumetric fruede number and whatnot).. anyway if i stick with 1/4 size the new dimensions come to:
LWL: 39.975'
BWL: 8.362'
Draft: 2.125'
Displacement: 14.177 tons

[gwboats]

Sounds like a great project!

My numbers give me props about 70" x 85" (2 props) or 60" x 80" (4 props).
I think you will definately need supercavitating types.

Don't forget tight weight control and LCG postion are imperative and keep waterline length maximum

My data comes from experience with:
www.bravechallenger.com
60knots - 95 tons - 103' - 12,750HP

Best of luck
GW

[RANCHI OTTO]

Why propeller? At such speed a lot of technical problems...

It would be better 3 waterjets Kamewa:

2 standard
1 booster in the middle

At low/medium speed you operate with the 2 lateral jets and for the full speed with all 3 together..

With submerged propeller you have at 40 knots a lot of appendages resistance...

I don't see advantages for such kind of design

[tallen]

the only reason I haven't really gone with jets was that I am looking for long range higher efficiency at low speeds and I heard that jets were only efficient at high speed. In addition I personally don't know much about designing with jets, especially of the size required.

[RANCHI OTTO]

Tallen,

for reach 40 knots you will need cavitating blade profile...at low speed this kind of propeller have a very low efficiency.

Waterjets aren't complicate ....you send to Kamewa a speed vs resistance curve and they will provide you all necessary informations concerning weight, intallation, thrust limits, thrust with 2/3 jets, etc.

Try is a good experience..

If you have propellers you will need a central skeg for protect them and at 40 knots...resistance...resistance...

[tallen]

Well I didn't think I would need cavitating blades. I did, however, believe that I would be looking for a much higher pitch blade then usual. That way its rotational speed would not be any different then other props, however, each full rotation would push more water.

[RANCHI OTTO]

The pitch is the last caracteristics of the screw...

Find at first your max. propeller diameter and corresponding gearbox ratio.
Than the blade area ratio and the corresponding pitch.

[charmc]

Tallen,

Otto has a point. There are good reasons why most high speed (40 knots +)vessels in the 50 - 80 meter hull length range use jets. As I understand the issue, even the first generation jets offered several benefits that offset any loss of efficiency, including reduced drag due to a cleaner profile below the waterline and the related reduced chance of damage from striking floating debris. The waterjet mfrs all have design calculation software to aid in selection of a package. It would be worth your time to research both alternatives and compare the benefits and drawbacks; regardless of your final decision you'll know you have evaluated all viable technologies.

[FAST FRED]

Another jet advantage is reduced draft props 5 ft in diameter may be trash magnets, or increase the vessels depth required.

Smooth water for anchoring is always closer to shore in the thin water.

Thrust is thrust , so perhaps a centerline jet could be selected/optimized for the displacement mode , when only a small fraction of the 40K power is needed .
Or a CL conventional prop and suitable diesel, , a ZF 2 speed tranny would make the cruise prop at least usefull at FLANK, and not a drag source.

FF