# High Speed Boats. Center of Gravity

Discussion in 'Boat Design' started by mvd535, Jul 22, 2006.

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### mvd535Junior Member

Dear all,
I am Chilean Naval Architect and I spent most of my 27 years working on Ferries and Fishing Vessels projects. Lately I am involving in high speed boat design and I need help to find the right book, or papers or direct help from somebody who could know the recommended longitudinal center of gravity location in a high speed boats to get a good dynamic trim. Boats starts from 10 m. to 20 m. length. All are in aluminum hull and mostly waterjets propulsion system.
I will appreciate very much any help on this.
Mario Villa

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### GilbertSenior Member

Lord's book on planing monohulls gives a simple rule of thumb of about 1 and one quarter times the planing waterline beam ahead of the aft end of the planing surface.

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### im412Junior Member

give tad a pm..he sounds like he knows and may suggest some books
go to this post, click on his name, drop down list, click send personal message

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### mvd535Junior Member

HSB Center of Gravity.

Many thanks to Gilbert and IM412 for the quick replays. I will try all your suggestions. Many thanks.

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### mvd535Junior Member

I have some more questions base on your important notes.

OK, I can minimize the deadrise as much as my desition, but I assuming that I must let a minimum angle. ¿ What in your opinion this angle must be ?, Ideally no zero, right ?.
2.- Reduce chine beam as much as possible.
Could you clarify were this reduction must be applied relative to the boat length ?. I mean in what length percentage ?.
3.- Could you explain a little bit what "hump drag" means ?.
4.- Do you use any software to run series of drag calculations or a simple formulas ?. Any suggestion on this ?.
5.- What "isolate hump speed" means ?.
6.- Surface area vs weight: OK, Do you know the recommended (surface are)/(weight) recommended ratio I must ideally get ?.
7.- I can map thrust vs drag at hump speed, but please clarify "hump speed" meaning. Is this the speed when planing just start ?.
8.- Yes. no question to keep the bottom clear forward of the intake and no lifting strakes in line. Acctually this is a waterjets manufacturers recommendation too. Totally agreed ¡. Thanks.
9.- Bottom hull shape will not offer any important transition from vee to flat since theses are twin type of propulsion system, so shell continues to the bottom waterjet base at the same deadrise angle at both side of the boat.
10.- Absolutely ¡¡¡, weight is the most sensitive factor that I must watch it. I am totally agreed.

Mario

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Mario,

The quick smart alec reply to the LCG question is, "as far aft as possible". IE put the engines on the transom and the fuel just forward of that, shape the hull to suit. But of course it depends on the definition of "high-speed", how fast is that really? Running waterjets pushes the engines forward, then you need to crowd the fuel aft if you can find a spot.

As a rough guide on LCG, using the at rest waterline and speed in knots.

Speed/sqrt L = LCG as % at rest LWL from bow
0.8 = 47%
1.1 = 51%
1.4 = 54%
2.2 = 58%
3.0 = 60%
6.8 = 62%
12.5 = 66%
Which is about as far as it will go!

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### RANCHI OTTONaval Architect

According to Hatch:

Optimum L.c.g. static position expressed as % L.w.l. measured from aft end of planing surface :

V/(LWL)^0.5 where V in knots and L.w.l. in feet vs (Lcg/Lwl)*100
---------------------------------------------------------------

1 - 44
2 - 43.8
3 - 43.1
4 - 42.1
5 - 40
6 - 37
7 - 33

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### mvd535Junior Member

To Ranchi Otto,
Many thanks for your cooperation by giving me this information.

LGC expressed as % of the LWL of the planing surface.
But ¿ How I can determinate de planing surface if I do not know the final dynamic trim angle ?. Or maybe I am wrongly understanding, and I must use the static LWL to get that % ?.

Thanks
mvd535

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### mvd535Junior Member

HSB Center of Gravity.

It seems to me that both answer are the same, but there is a difference that I must clarify:

2.- RO answer: LCG as % of the planing LWL

I would say that the answer number 1.- could be the valid one, since I can not get the length of the planing surface unknowing the final trim angle at the design stage. To me "at rest LWL" is an easy data to know at the designing stage ??.

many thanks,
mvd535

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mario,

I would guess that both LCG figures are indeed the same, one measured from aft and the other from forward. You really will not easily come up with planing surface unless you run the hull in a tank.

Thanks for the drawing, in that configuration you will have a hard time moving LCG aft of 62% DWL. The only comment I would make is you probably want to move the fuel aft, in tanks right across the boat just forward of the engine room.

Here is a planing performance spreadsheet that you can play with.

View attachment Planing new.xls

Try different deadrise and LCG positions and note the results.

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Minimum Deadrise will depend on speed and sea-state, you state 40 knots in the drawing, and patrol/law enforcement vessels need to carry on whatever the weather. Thus one expects high deadrise, but government vessels are often heavy (equipment, structure, and endurance) so lower deadrise (better lift, lower drag) would be better.

In this type of vessel I would be tempted to go with a low deadrise (5-10 degrees) narrow pad running along the centerline, between the jets. Then a high deadrise section (18-22 degrees), then a reverse deadrise lifting strake outboard. All angles at the transom, amidships should be higher angles. Minimum deadrise for 40 knots would be about 16-17 degrees at the transom, 20+ would be better.

Mount the jets as close together as possible without their buckets hitting.

In working with the Savitsky spreadsheet you'll see that drag goes down as you reduce beam. The narrower form (higher aspect) for a given weight reduces the drag hump and trim changes.

Again go into the sheet and look at the graph of drag vs speed, it starts out steep, then flattens off and drops, then goes up steeply again. The "Hump" is obvious, this is where the boat is gaining momentum and coming up out of the hole onto a plane.

Every hull form has different drag character, thus you need to be sure your jets have the thrust at that slow speed to overcome the drag over the hump.

Best current book on practical application, Principles of Yacht Design 2nded, Larsson and Eliasson. Best current book on Hydrodynamics, Hydrodynamics of High-Speed Marine Vehicles, Odd M. Faltinsen.

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### RANCHI OTTONaval Architect

The length L.w.l is static not dynamic.
The relationship is (l.c.g./l.w.l.)*100.

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### mvd535Junior Member

HSB C. of G.

To Ranchi Otto,
mvd535

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### mvd535Junior Member

HSB C. of G.

Many thanks for your complete information and suggestions.
Many thanks for the Excel file
I will see and try all.
mvd535

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I see I neglected to answer the surface area vs weight question.

I don't usually worry about it much, just keep the weight as light as possible and length/beam ratio within averages. Keep the displacement length ratio as low as possible.

I do worry about planing surface area vs weight in planing catamarans, in small boats (say <20') and in boats that have big holes in their planing surface, outboard wells, prop pockets, notches, transom bevels, or jet intakes.

I have an old chart from George Crouch published in the 1930's, which graphs bottom loading in pounds of displacement/sq. ft. bottom area at rest vs LWL at rest. It ranges from a low of 25-37 lbs/sqft for a 15' LWL to 49-70 lbs/sqft for a 50' LWL. Lindsay Lord covers this subject in depth in his book, Naval Architecture of Planing Hulls. But the forms he references are so far from typical modern planing hulls as to make his detail useless except in a very general way.

A reasonable rule of thumb is that 40 pounds/sqft is fairly low bottom loading and 50+ will be fine.

Obviously every factor in a particular hull form will affect the efficiency of its planing and how much weight it can support. Wetted length, beam, deadrise, trim, etc. will all afect how well the boat planes.

Again the Savitsky sheet is a good guide, try different numbers and watch the affects.