Displacement/Semi/Planing

[Hotel Lima]

Hello again,

Does anyone have an idea to what would be the best method for getting a 160ft~ vessel in the neighborhood of 22+~knots cruise and 28-30+knots top speed? By best method I mean efficiant and stable. A 28-30ft beam is planned to keep resistance to a minumum. I'm not looking for anything stupidly fast but getting out of the 12knt neighborhood would certainly be nice.

Let me know your thoughts...

thanks, Jake

[Brands01]

A displacement hull of this length will travel at a top speed of nearly 17 knots. If you want to go any faster, your hull needs a prismatic coefficient that reflects your ideal speed length ratio, and plenty more horsepower.

[eponodyne]

I'm thinking the best way to achieve this would be a sizeable inheritance, and unlimited funds. Both for the construction, and for dealing with the damage caused by the wake this Behemoth will throw.

[Pericles]

KCook pointed this out to me very recently. It should give you some guidance.

http://www.yachtforums.com/forums/sp...et-yachts.html

Pericles

[water addict]

Originally Posted by eponodyne

I'm thinking the best way to achieve this would be a sizeable inheritance, and unlimited funds. Both for the construction, and for dealing with the damage caused by the wake this Behemoth will throw.

First sentence is entirely correct. You forgot fuel costs though.

In the semi-planing realm, carefully designed catamaran will give less resistance than a monohull for a given displacement, provided the displacement target is not too high. A good treatise of this is given in Principles of Naval Architecture.
Not sure about the restricted beam though. If you can relax that, a cat may be worth consideration for this speed target.

[Hotel Lima]

I think you've got the wrong idea. I'm looking at the 20's and possibly very low 30's, not freeway speeds.

[PAR]

Your cruise and target top speed aren't realistic, unless you have an unlimited purse to draw your fuel bills from. Not knowing what vessel you're referring, it's displacement, hull configuration, etc. makes all this pure speculation, but craft of that size which achieve those speeds, require many thousands of HP and drink gallons per minute. The designer of the vessel would be best to consult, if interested in reasonable estimations for a performance envelope of that range. Low 30's is "freeway speeds" on the water.

[EStaggs]

Low 30s puts the fuel costs in the "EXPONENTIAL" range....

E

[Hotel Lima]

I was thinking 80-100gph, twin 5000hp diesels. Could it be more?

[Landlubber]

Hotel Lima,
Have you ever looked at fuel consumption figures, you are sooooo way off course i do not believe it.
A marine diesel uses about 1 gallon for every 20 horsepower, so
500 hp would be using 25gph at 20 knots and at this sort of speed, a 5000hp engine would use about 250gph. BUT fuel consumption is to the power of 3 from 20 to 30 knots, so you would have to expect something in the order of 750gph to do 30 knots in the boat you describe. All rough figures, but at least it is something to go by, you can definately count on 80-100 as being impossible.

[Landlubber]

To give you an idea of figures, this is quoted from a site that has a Ferretti:

Length: 30.60 Mtr (100 Feet)
Beam: 7 Mtr (23 Feet)
Draft: 2.26 Mtr (7 Feet)
Year Built: 2004
Main Engines: MAN 2 x 1300 kw
Cruising speed: 16 Knots
Max Speed: 19 Knots
Cruising Range: 620 NM
Fuel Consumption: 450 Ltr/Hr (119 Gall/Hr)
Built by: Ferretti