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  #1  
Old 01-27-2008, 03:55 AM
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Displacement hull replace planing hull

What are the drawbacks if one uses a displacement hull instead of a planing hull for a power boat ?

So, instead of using a single planing hull you use two displacements hulls.
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Old 01-27-2008, 01:54 PM
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A displacement hull uses it's surface area for buoyancy. A planing hull tries to rise out of the water and skim across the surface....with the necessary HP.
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Old 01-27-2008, 02:04 PM
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A planing hull also uses it's surface area for buoyancy, same as the displacement hull.

A displacement hull cuts through the water (displacing water) while a planing hull skim across the water surface.

OK, now if I use 2 displacement hulls instead of the single planing hull what is the difference going to be ?
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Old 01-27-2008, 05:10 PM
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Sorry, but your question is too vague. Don't get the connection between two displacement hulls and one planing hull.
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Old 01-27-2008, 05:31 PM
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You walk into a shop to buy a boat, on the one side is a planing hull and outboards on it. On the other side is a displacement hull cat with an outboard on each hull. Both are classified as the 'same size' vessel, the displacement hull jobby would be wider than the planing hull, so less roll could be expected. Would the planing hull boat be faster ? I seem to have read that displacement hulls have a speed limit... does this apply to motor driven as well or only for sailing catamarans. My thinking tells me an outboard could put a displament hull on the plane as well, or am I misjudging this ?

Any other differences / similarities ?
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Old 01-27-2008, 07:45 PM
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Ah, so that is what you are talking about. You should have mentioned catamarans when you meant "two" displacement hulls. Catamarans have very high length to beam ratio which means that the "hull speed" definitions for normally proportioned monohulls do not apply to them. Thus a catamaran can run at speeds that could only be made in a monohull if it were planing.

I suggest some basic reading of boat design and performance characteristics. Dave Gerr's "Nature of Boats" and Ted Brewer's "Understanding Boat Design" would be a good place to start.
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Old 01-27-2008, 11:25 PM
masalai masalai is offline
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Get rick Willoughby to run some quick analysis on your preferred length W/L & tonnage via Michlet/godzilla....

Mine is 12m & 7.5 tonnes & is where I am working & 2 x 75 kw or LESS is more than enough - some old figures on an old (see my model) design may have been capable of 20Knots???? with 60kw/side or 30KW to give 15 knots (flat seas no wind), but I would not trust those figures as I did them & may -will- have plenty of errors and wrong assumptions - but it will give you a goal....)

I am sold on displacement in an efficient hullform for cruising cats. Most cruising will probably be at, or less than, 10knots in coastal trips for comfort and good seamanship reasons.

I have only just started to scratch the surface in learning anything on this hullform & understanding constraints etc...

I have found I can export some design parts to "Blender 3D" a linux package and will also try one of the other linux 3D cad options later. Ubuntu is becoming more friendly every day & is more stable & less worried by viruses etc....

But I can't run Michlet/godzilla - so sad, too bad - .... From evidence of similar displacement powercat designs, 15knots at around 1.5 and LESS litres per mile - long range capacity of 1500 PLUS miles - should be do-able
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Old 01-28-2008, 02:47 AM
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Quote:
Originally Posted by Fanie View Post
.... Would the planing hull boat be faster ?
At 20kts a 12m planing hull displacing 6 tonne would require twice the power of the best 12m cat of the same displacement.


Rick W.
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Old 01-30-2008, 02:27 PM
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Fanie,

There are planing catamarans as well as displacement catamarans. A planing mono hull and a planing catamaran of identical weights and horsepower will usually see the catamaran take line honours due mainly to the reduced wetted surfaces. http://en.wikipedia.org/wiki/F1_Powerboat_Racing

Displacement catamarans with slim hulls in ratios around 12 to 1 (hull length to width) do not plane, but as Tom mentioned, they will not be constrained by "hull speed" considerations and will travel surprisingly fast, either under power or sail.

Taking the Gunboat 62 as an example, the two hulls are less than 7 feet wide and 60 feet LWL, so she glides through the water under sail and has been clocked at 35 knots. I guess she will have lifted one hull at that speed.

If a power cruising catamaran has short stubby hulls it will be slow, unless it is able to plane, then its length does not matter. Otherwise it's the old, old story, the inches really do matter. http://www.boboramdesign.com.au/cata...e-30-38-range/

Best wishes,

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Old 01-30-2008, 03:08 PM
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Thanks guys, I've heard what I wanted to hear. Have an idea, it may work out
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Old 01-30-2008, 03:25 PM
jehardiman jehardiman is offline
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Just remember that there are other issues besides just speed/horsepower comparisons. There is seakeep, speed in a seaway, motion issues (I won't really open that barrel of tar), etc.

As I have said before.... There is no one best hull form, there is only the hull form that best fits your needs.
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Old 01-30-2008, 07:51 PM
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Thanks Jehardiman !
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