Displacement: the 2.5 to 5 tons per person rule - SailNet Community
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Displacement: the 2.5 to 5 tons per person rule

I'm a little confused about the 2.5 to 5 tons displacement per person for a cruising boat logic. As I was reading I saw that this was expected to be considered independent of the LWL or LOA of the craft. My problem is that if you select a boat purely by its displacement with no regards to its length you might wind up with a boat that is 10000# disp. and 45 ft long! Thats not seaworthy! So my question is this: is there a displacement to LOA to Beam ratio? I could come up with one I guess but why reinvent the wheel when I have such a darn knowledgable resource at my finger tips!

The reason why I'm so interested is that I'm currently trying to calculate the seaworthyness of a particular fin keeled 8750# disp 31 ft LOA, 8'9" beam hull. Acording to this she fits the bill fine, but part of me thinks that this is too light. Ah! Someone set me straight!

Last edited by deckhanddave; 11-10-2006 at 03:52 AM.
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Old 11-10-2006
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Whether the craft's displacement is "light" or not depends upon what she was intended for, as well as what you intend for her to do.

I'll be happy to look at this a bit later for you (I'll reference Dave Gerr's Nature of Boats), but I'd need to know what you are intending to do with any sailboat. 'Course, someone else here may beat me to it...

Sincerely,

/s/ Jon C. Munson II
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Old 11-10-2006
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This is just a "rule of thumb" (q.v.) meant to serve as a general guideline. It has to be applied along with a lot of common sense.
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I'd want to use a rule of thumb like the 2.5 -5 ton rule to help me choose between boat options for some extended cruising. The cruising planned breaks down to about 70% being relitively costal work which has made me take a closer look at medium displacement boats.

So far I'm looking at the
seafarer 31 (31' LOA, 23 LWL, beam 8'9,", disp 8800#, 3500#keel)
Tartan 30 (29'9" LOA, 24 LWL, beam 10", disp 8750#, 3700# keel)
Alberg 30 (30 LOA, 22 LWL, 8'9" beam, disp 10000#, 3450# kell) [slow?]
deckhanddave is offline
Old 11-10-2006
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is there a displacement to LOA to Beam ratio? I could come up with one I guess but why reinvent the wheel when I have such a darn knowledgable resource at my finger tips!

Sorta, ITs called the displacement to length ratio Im sure JeffH can give a better explanation than I can.
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Ok, from Gerr's Nature of Boats, page 100:

Quote:
Calculating a Cruiser
Let's figure the displacement required for a crew of four on an ordinary 10-day vacation cruise:

4 x 160 lb. = 640 lb. crew

4 crew x 10 days x 14.5 lb./day x 1.5 reserve = 870 lb. food and water

4 crew x 10 days x 5 lb./day = 200 lb. personal gear

TOTAL: crew, food and water, personal gear = 1,710 lb (3/4 ton !)

1,710 lb. x 7.5 = 12,825 lb. displacement

Plus or minus 10% = 11,540 lb. to 14,100 lb.
required displacement
You should be able to deduce whatever you need from here. You'd really need to look at what you mean by "extended" etc., defining those things will significantly help you. Planning the boat you need around what you wish to do will determine what craft is suitable. Alternatively, you could do the opposite, but the boat will then dictate what you can do - naturally.

Sincerely,

/s/ Jon C. Munson II

Last edited by jmunson2; 11-10-2006 at 02:43 PM.
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