Load Limits

Inquiry; how does one determine the load limit of any given boat?* Is there a mathematical formula or maybe just a simple ratio?* Does the builder or designer provide that info? If you were provisioning for an extended cruise, would load limit even be a factor? How much do allow for weight of fuel?**Do you give much concern to*balancing the weight when loading non fuel items?********** KJ




-- Edited by KJ on Tuesday 18th of January 2011 12:53:41 AM

-- Edited by KJ on Tuesday 18th of January 2011 12:55:36 AM

In displacement boats, like trawlers, volume (where the heck*can I store this stuff)*has always been*the limiting factor, not weight.* Best to keep heavy items low and in the center of the boat as far as practical.

The only limit is how far you want to stick down in the water.

Many trawlers will sink about 1 inch for every thousand pounds of goodies.

Bigger boats will do about half that.

It is time for a couple of you guys to get a book on stability.

Every boat has different characteristics. Here's the calculated data for a 42' Krogen at a half load which is a displacement of 39,500 lbs with 2,500 lbs of ballast.

Hydrostatic data:

Displ/length ratio (D/L)........294
A/B ratio.......................2.2
Prismatic coef. (PC)............0.60
Lbs/inch immersion..............2,000 lbs/in
Moment to trim one inch.........4,250 ft-lbs
L.C.B...........................51% aft
L.C.F...........................56% aft

Stability:

GM (approx).....................3.9 ft
Range of positive stability.....85 degrees

If a NA was involved in the design, the designed waterline is a pretty important reference point.* There have been capsizes of fishing vessels when the waterline was painted someplace other than where it was designed to be, but if accurately located, fore and aft and beam trim is easy to determine.* The bonehead method of determining load capacity (my favored method, always) is to ask at haulout what the weight of the vessel is and subtract that from the design displacement and you have a pretty close estimate how much stuff you can load on before the boat sits below the waterline.* When gutted and with partially filled tanks, Delfin weighed 40 tons.* Her design displacement was allegedly 60 tons.* I added about 18 tons with full tanks, cabinetry and equipment and put her in the water. We then had to locate another ton of ballast to get her on her waterline fore and aft and port to starboard, so that method seemed to work ok for me. We left a ton of capacity in reserve for the contents of the liquor cabinet.

If you look at cruising sailboats, they are routinely stuffed with so much extra weight that the waterline gets re-painted higher.* Not such a great idea, but perhaps less of a problem with a sailboat up to a point.

Locating weight lower is better than higher for obvious reasons. If the weight is put where it was designed to be put, stability is what it was intended to be, whether great or marginal.


-- Edited by Delfin on Tuesday 18th of January 2011 10:29:08 AM

Larry,

Wow, great data.* How would you use that to determine your load?* What does the GM in the stability data represent (sorry I'm not a nautical engineer or a naval architect), not too sure about the prismatic coefficient either.* Stability is exactly what I am concerned with. I was hoping for more than just checking how low your stripe is in the water, although that does seem pretty straight forward.*

-- Edited by KJ on Tuesday 18th of January 2011 10:32:37 AM

-- Edited by KJ on Tuesday 18th of January 2011 10:33:28 AM

Carl (Delfin),** great info, thanks.*** KJ

-- Edited by KJ on Tuesday 18th of January 2011 10:53:30 AM

Here's a pretty good description of metacentric height (GM) and how irt affects stability.

http://en.wikipedia.org/wiki/Metacentric_height

And a blurb on understanding the Prismatic Coefficient

http://www.navweaps.com/index_tech/tech-004.htm

KJ, Buehler goes in to great detail of all of these specs in his book "Voyaging Under Power". It is a great book and a great read and somewhat of a "bible" for motorboat enthusiasts.

Thanks for the info.*** KJ

Baker wrote:

KJ, Buehler goes in to great detail of all of these specs in his book "Voyaging Under Power". It is a great book and a great read and somewhat of a "bible" for motorboat enthusiasts.

Click to expand...

I believe you are confusing Ferris Buehler with Robert Beebe.* Buehler's book on cutting highschool is a classic, but only tangentially related to ocean passages.

*