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Old 25-03-2010, 21:03   #1
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Formula for Sailboat Displacement

I was looking thru John Vigor’s book “Things I wish I’d known before I started sailing”. One of the subjects is how big a cruising boat do you need? He says to take the total weight of the crew, food and water in pounds and multiply it by 7.5 to determine boat displacement.

As an example he uses a 30 day cruise with 495lbs food and water, 200lbs clothing and personal gear and crew weight of 320lbs = 1015lbs x 7.5 = 7612lbs displacement +/- 10%.

Has anyone actually used this formula to determine the minimum displacement for a cruising boat? I ask this question because there have been hundreds of newbie questions regarding “what boat size should I get” and I have never seen this formula mentioned in a response.

I am also a newbie who has spent the last couple of years sailing the Great Lakes and prior to this a couple years in SoCal.

Also spring is here. They just dropped the first boat in the marina last weekend here in Kenosha, WI. Air temps are a balmy 45 – 50 deg F. I think I'll wait a few more weeks.

Ray
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Old 26-03-2010, 01:42   #2
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Has anyone actually used this formula to determine the minimum displacement for a cruising boat? I ask this question because there have been hundreds of newbie questions regarding “what boat size should I get” and I have never seen this formula mentioned in a response.



Ray
I would put MINIMUM in capital letters regarding that formula...and I think that was his point...The boat has to be able to carry the load you intend safely .

But he is talking traditional cruisers in his book..some modern racer cruisers will not fit into or be restrained by his formula.
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Old 28-03-2010, 16:41   #3
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Has anyone actually used this formula to determine the minimum displacement for a cruising boat?
Interesting calculation: I think Donald Street used to advocate something like 5000# per crew for a minimum displacement, so Vigor is probably on the lower end of that – nonetheless, I’ve always thought Street was being overly generous, but, to my Admiral’s chagrin I’m more enamored with the austere end of things, so once beyond the minimums for life-support like food & water I think a lot depends on your personal luxury viewpoint or comfort philosophy…
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Old 28-03-2010, 16:55   #4
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The bigger the boat, the less the load matters. My boat, dry, displaces 65,000 pounds. A few pounds of clothing and food doesn't matter. On the other hand, a light displacement cat would choke on what we carry. Water and fuel alone weigh a ton.
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Old 28-03-2010, 17:14   #5
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a more useful formula

Ray, rather than worrying about minimum displacement, worry about ideal displacement.

ID = CD + 30%. In other words, ideal displacement is whatever your current boat displaces plus 30%. Your Cal 34, for example, displaces 9500 lbs. What this means is that for your current cruising plans you really need a boat displacing 12,350 lbs.

The futility of it all is that the moment you buy a boat displacing 12,350 lbs, your new ideal displacement will be 16,000 lbs.
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Old 28-03-2010, 17:19   #6
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* LOL * That's a wise observation, Bash.

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Old 28-03-2010, 18:38   #7
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Bash, I suppose thats what is meant by going down the proverbial rat hole. Maybe I should buy Speciald's 65,00lb Taswell and play it safe.
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Old 28-03-2010, 19:01   #8
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Those Oyster 655 are mighty nice. Now, where did I put that lottery ticket?
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Old 30-03-2010, 14:40   #9
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As a point of reference, our Aphrodite 101 is 6200#. Two people for two weeks of coastal cruising is about her maximum range.

We roll up the Avon and stow the 2 hp Merc safely below in suitable lockers. There are no jugs on deck as you see on so many yachts. There is enough space for food, the way we eat... which is very lightly by choice... for two weeks and I can imagine four weeks without trouble.

The problem is water. A water maker would be hard to fit and require added batteries, diesel fuel, etc.

A 10,000# 28 footer I sailed for many years had all the space you need to sail a couple across an ocean. Don Street has it right.
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Old 30-03-2010, 16:16   #10
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we used a different method to determine the size of boat needed.. My figuring was measured in feet of how far would I have to go to get out of shouting range of the wife if she was really pissed at me.. 42 feet just about does it...
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