Sizing the long distance cruising boat for the "cruising" half of the audience

[Greg Ross]

I believe it's legally Ok to plagerize, er copy and share someone elses' composition if proper reference and linguistic credit is given.
I am a regular reader of Johns' Blog http://johnvigor.blogspot.com and thouroughly enjoy his range of topics and stellar sense of humour.
For those of you who aren't familiar with his work John is the author of a dozen boating books, authored many many articles, contributing editor for "Good Old Boat", author of the "Official Boat Renaming Ceremony" and sailed his own good old boat from South Africa across the Atlantic to North America. John lives and cruises in the Pacific Northwest.
And to balance my enthusium here's the link to the Sailnet discussion on Master Vigor;
http://www.sailnet.com/forums/general-discussion-sailing-related/74569-blog-month-john-vigor.html
What I wanted to share with you was Johns' view on sizing a long distance cruising boat and I, as carefully qualified above share with you, in his own words;

“How big a boat do I need?" by John Vigor<O></O>
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.<O></O>
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:<O></O>
Crew: Multiply number of crew by 160 pounds.<O></O>
Stores: Allow 6 pounds per person per day.<O></O>
Water: Allow 8.5 pounds per person per day. (That's a little more than 1 gallon U.S.)<O></O>
Safety reserve: Add it all up, then add 50 percent.<O></O>
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.<O></O>
So here's an example. Find the smallest boat needed for two people with water and provisions for six weeks. <O></O>
—Displacement (within 10 percent) = (weight of crew and stores) x 7.<O></O>
—Longest time between provisionings = 42 days.<O></O>
—Number of crew = 2. Weight = 2 x 160 = 320 pounds. <O></O>
—Daily stores = 6 pounds x 2 crew x 42 days = 504 pounds.<O></O>
—Water = 8.5 pounds x 2 crew x 42 days = 714 pounds.<O></O>
—Safety reserve = 504 (stores) + 714 (water) = 1,218 x 1.5 = 1,827.<O></O>
—Personal gear = 120 pounds x 2 = 240 pounds.<O></O>
—Total weight of stores, safety reserve, and personal gear = 1,827 + 240 = 2,067 pounds. <O></O>
—Displacement required = 2,067 x 7 = 14,469 pounds, or 6.5 tons.<O></O>
—Displacement within 10 percent = 13,000 to 16,000 pounds (5.8 to 7 tons)<O></O>
<O></O>
Now you know how big a boat to look for. So let the dream proceed.<O></O>
(Or else, if you're like most of us, you can just wing it, and go in the boat you've already got.)<O></O>

 

[toddster]

Conversely, someone slightly more bored than I am could go through the Ericson catalog and use the formula to produce a chart of hypothetical cruising ranges by model...

Since the largest item in the formula is water, it seems that a water maker might significantly increase the range of a given boat. Whether it would be smart to trust a device for a significant portion of requirements is another matter.

 

[Rocinante33]

Greg,

That is an interesting example in the calculation. 42 days between provisioning is quite a stretch, though, as in his example. I guess I would plan for that much if transiting from the west coast (whether from the U.S., or Central America) to the Marquesas. I can't think of many other passages on this earth that would be much longer without stops for reprovisioning. Anyway, if you use a more reasonable 3 - 4 weeks, the boat size you come up with is more reasonable and more achievable for many of our boats.

With respect to a watermaker in place of water in the tanks, I think you need quite a bit in the tanks for back up in case the watermaker fails. Of course they do have hand pumped watermakers for survival usage. You could pack one of those for a back up. You also need big battery banks to run a watermaker.

 

[Greg Ross]

Mr. Vigors' equation

Keith,
I read his book, "Small Boat to Freedom" some years ago and wouldn't but be surprised if that 42 days wasn't his transit time from South Africa to his landfall in the Caribbean/ South America. And that was non-stop. That trip to freedom of his was precipitated by "apartheid" that was rampant at the time in SA. When they did depart as I recall he indicated whilst on a boat his wife and son had never been out of sight of land, perhaps they'd never overnighted. Regardless, quite a story.
Of course if you go solo you'll only need 1/2 a short ton and that's something less than 1" on my draft!

 

[toddster]

There are watermakers listed at 1 gph @ 4 amps. You could run that on a couple of small solar panels. Still, I think I'd carry the full water ration in tanks.

A year or two ago, I chanced upon a website (can't find it now) showing a number of long-distance cruisers current position reports. If you clicked on any marker, it showed you the track history and short log entries for that boat. I recall there was one boat stuck between the galapagos and mexico, that had been at sea for over 30 days and just wasn't making much progress. I was beginning to wonder about those guys.