March 24, 2011

The right size boat for you

HOW BIG A BOAT do you need for long-distance cruising in reasonable comfort? I say reasonable comfort because there are those among us, usually the young and adventurous, who will go to sea in almost anything that floats, no matter how small and rudimentary. And good luck to them. I salute them. But them are not me. And I are not they. Not any longer.

To answer the question, though: it depends on three factors: the number of crew, the weight of the stores they need, and the number of days between provisioning stops.

So, to find the minimum required displacement of a cruising sailboat (within 10 percent) start by multiplying the combined weight of crew and stores (including water) by 7.

For planning purposes, use these guidelines:

Crew: Multiply number of crew by 160 pounds.

Stores: Allow 6 pounds per person per day.

Water: Allow 8.5 pounds per person per day.

Safety reserve: Multiply the total of stores and water by 1.5.

Personal gear: Allow 5 pounds per days or a maximum of 120 pounds per person. (For permanent liveaboards, a maximum of 500 to 1,000 pounds is more appropriate.)

Here’s an example:

1. Find the minimum boat displacement for two people with water and provisions for 42 days.

2. Displacement (within 10 percent) = (weight of crew and stores) times 7.

3. Longest time between provisionings = 42 days.

4. Number of crew = 2. Crew weight = 2 times 160 = 320 pounds.

5. Daily stores = 6 pounds times 2 crew times 42 days = 504 pounds.

6. Water = 8.5 pounds times 2 crew times 42 days = 714 pounds.

7. Safety reserve = 504 (stores) + 714 (water) = 1,218; times 1.5 = 1,827.

8. Personal gear = 120 pounds times 2 = 240 pounds.

9. Total weight of stores, water, safety reserve and personal gear = 1,827 + 240 = 2,067 pounds.

10. Displacement = 2,067 times 7 = 14,469 pounds or 6.5 tons.

11. Displacement within 10 percent = 13,000 pounds to 16,000 pounds (5.8 tons to 7 tons.)

A heavy displacement 30-footer, such as circumnavigators Eric and Susan Hiscock’s Wanderer III weighs in at 16,000 pounds but more modern designs could get that displacement down to 12,500 to 13,000 pounds and produce a faster hull with more room down below. And in each case, the boat could easily be handled by a crew of two.

Today’s Thought
It is a pleasure for to sit at ease
Upon the land, and safely for to see
How other folks are tossèd on the seas
That with the blustering winds turmoilèd be.
— Lucretius.

Boaters’ Rules of Thumb, #176
To estimate the sail area a boat needs, take three quarters of the square of the waterline length in feet — that is, multiply the waterline length by itself and take 75 percent of the result. The answer is in square feet. Some boats will vary considerably, of course, but this will give you a general idea.

“Sorry to hear about your husband. What happened?”
“Well, I asked him to pick a cabbage for supper and he went into the garden and keeled over suddenly and that was that.”
“Wow. What did you do then?”
“Oh, I opened a can of peas instead.”

(Drop by every Monday, Wednesday, Friday, for a new Mainly about Boats column.)


Anonymous said...

I was just running through the formula that you presented and noticed that in the example you neglected the weight of the crew. If we include it the estimated displacement would be 16709 lb which would give a range of 15038 to 18380 lb.

John Vigor said...

Anon: Item number 4 gives the weight of the two crew.

John V.

John Vigor said...

Anon: I see what you mean. Yes, indeed you are correct. With the crew weight (320 pounds) added and multiplied by 7, the estimated displacement would be 16,709 lb. as you point out.

Thanks for a good catch.

John V.