March 22, 2012

Calculating boat size

IN THE VERY BEGINNING there was the dream.  The dream was of white puffballs of cloud in an azure blue sky, and soft warm seas curling up beneath the transom as the friendly trade winds urged your little boat across the wide ocean.  And as the dream became stronger, you wondered, "How big a boat do I need?" And it boiled down to two things: the number of crew and the weight of stores. The old rule said that to find the minimum required displacement for long-distance cruising with reasonably comfortable living spaces and amenities, all you had to do was multiply the combined weight of crew and stores by 7.

You already knew the number of crew. Just the two of you. But the weight of stores? How on earth do you estimate that? Well, you come to  me, of course. I know these things. Here are the rules of thumb:

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. (That's a little more than 1 gallon U.S.)

Safety reserve:  Add it all up, then add 50 percent.

Personal gear: Allow 5 pounds per day, or a maximum of 120 pounds per person. For permanent liveaboards, make that a maximum of between 500 and 1,000 pounds.

So here's an example. Find the smallest boat needed for two people with water and provisions for six weeks.

—Displacement (within 10 percent) = (weight of crew and stores) x 7.

—Longest time between provisionings = 42 days.

—Number of crew = 2. Weight = 2 x 160 = 320 pounds.

—Daily stores = 6 pounds x 2 crew x 42 days = 504 pounds.

—Water = 8.5 pounds x 2 crew x 42 days = 714 pounds.

—Safety reserve = 504 (stores) + 714 (water) = 1,218 x 1.5 = 1,827.

Personal gear = 120 pounds x 2 = 240 pounds.

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

—Displacement required = 2,067 x 7 = 14,469 pounds, or 6.5 tons.

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

Now you know how big a boat to look for. So let the dream proceed.

(Or else, if you're like most of us, you can just wing it, and go in the boat you've already got.)  

Today's Thought
I believe it to be true that dreams are the true interpreters of our inclinations, but there is art required to sort and understand them.
— Montaigne, Essays III

If an S and an I and an O and a U
With an X at the end spell Su,
And an E and a Y and an E spell I,
Pray what is a speller to do?

Then if, also, an S and an I and G
And H E D spell side,
There's nothing much else for one to do
But go and commit sioux-eyesighed.

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


Greg Ross said...

Mr. Vigor,
Shared these astute comment of yours with the Ericson Owners' community.
Should generate a few hits for you!

Deb said...

Totally unrelated subject but I couldn't find a contact you link - we had our boat renaming today and used your ceremony. It was a great time and I just wanted to say thanks! Our blog post on it is here if you're interested.


Jordan, Contessa 26 #312 said...

Hi John,
I'm a big fan of your book, "Twenty Small Boats to Take you anywhere." It kept my dream alive while waiting to find my Contessa 26. We finally bought our little boat last year and sailed it for 6 weeks around much of the Great Lakes.

If I have understood your post above then you may have made a slight omission in the math of your example. The 2x160lbs=320lbs of crew weight is excluded from the crew & stores sum. Once included, it raises the required displacement to 17,829 pounds.
16,000 to 20,000lbs using +/- 10%.

I hope to have the chance to put this theory to the test one day when I have the time and funds to do some long-distance cruising.

Thanks for posting.