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lasiemartins · 02-06-2013, 16:34

How to calculate the load capacity supported a catamaran and a sailboat,
* * the maximum weight you can carry these boats??

Andrew Troup · 02-06-2013, 17:10

Work out how deep in the water you can live with, work out how much underwater volume that represents, work out how much that volume of water would weigh. Subtract the weight of the empty hull(s)

What's left over is how much extra weight you can carry to bring the boat as deep in the water as you chose in step 1

If you break the boat, or make it un-manageable, you picked a figure in step 1 which was too deep

To work out reliably in advance whether the boat will be unmanageable, you need to train yourself in naval architecture and design a few boats, and to work out in advance whether the structure will break, you need a lot of detailed understanding of engineering principles and practice.

Working out the loads on a boat, especially a multihull. is a lot more difficult than the loads on, say, a bridge or a crane.

**David M** · 02-06-2013, 17:16

In naval architecture there is what is called the MT1 number. That is the amount of weight to lower the waterline by one inch. The naval architect or the manufacturer would have this curve. It's a curve and not a linear function because it varies slightly with how far the vessel is submerged.

There are a whole bunch of other factors that determine what is safe. Sometimes safe is a gray area though. Generally speaking, the more weight you load on a vessel, the less safe it becomes because of less reserve buoyancy and quite often less overall stability.

On vessels that are classified by a society such as Lloyd's Register or the American Bureau of Shipping, there are load marks (Plimsoll marks) that you do not exceed for the seas that you plan on traveling. All larger commercial vessels have them, some larger yachts have them but then most smaller yachts do not.

What do you plan on doing? Don't think you can safely heavily load any boat without doing your stability and trim calculations or consulting with a naval architect who has the vessels stability curves.

On a cat there is going to be a torsion number relating to angular momentum for the hulls that you do not want to exceed. The more mass you load in each hull especially outwards from the hulls center of buoyancy, the worse the situation. You do not want to over stress the structural members that attach the two hulls

Teknav · 02-06-2013, 17:28

Fred Flintstones' scientific observation and calculation...if the ocean's water level is at the same level as the boat's deck, your boat is over-loaded; get rid of one brontosaurus steak!

Mauritz

svtrio · 02-06-2013, 17:49

lasiemartins,

The link below will provide a broad range of information that relates to your query. Such data was assembled by the builder (Prout) when my cat was built (under Llyod's supervision) in the 1970s and that original data correlates very well with data in this Web site. The site is a worthwhile visit from anyone who has a cat. Navigation requires a bit of effort...

Multihull Dynamics, Inc. - Home

Roger

barnakiel · 02-06-2013, 17:51

Some manufacturers publish 'light' and 'load' figures. These may be a guide.

I think we carried as much as 25% of our dry displacement on early stages our extended adventures (mono-hull boat, classic hull shape, light/medium SA/displ). But not all boats can carry this much and I think cats fall into this category.

It is important to notice performance drop, loads rise (more failure risk for your rigging, etc.) and the fact that the extra weight must be distributed so that it does not detract (or not too much) from your stability.

As others noticed - ultimately you are limited by your freeboard ;-)

b.

bobconnie · 02-06-2013, 18:41

I always figured if I had to raise the water line a second time I was over loaded!! Works for me LOL

muskoka · 02-06-2013, 19:24

There is a good thread on here somewhere started by a Lagoon 380 owner which addressed this in detail. It also listed the typical weights you might expect from various bits of kit.

In brief, all the Euro boats have CE plaques which list the loaded weight inclusive of crew, fuel & water. My boat can theoretically take 2250kg. Beware, if you load anything near that limit you'll be extremely slow. Cats are terribly weight sensitive and not much fun to sail when loaded up.

Cheers

daddle · 02-06-2013, 21:44

The rough calculation is based on the surface area at the waterline. Multiply the waterline length by its breadth and then by the prismatic coefficient of 0,6. That gives a good estimate of the area at the waterline. Catamarans multiply by two because of the redundant hull. That gives an area in square meters (or feet for the ancient mariners here) Multiply by .01 to give cubic meters displacement per cm of draft change. Or 1/12 to give cubic feet of displacement per inch of draft change. A cubic meter of displacement is a metric ton. A cubic foot of water is 64 pounds.

I know that simple arithmetic may be too tough for many ... Ask a high school kid ...

Simple!

The prismatic coefficient varies, but not by much.

Bash · 02-06-2013, 21:52

Quote:

Originally Posted by daddle

The prismatic coefficient varies, but not by much.

Correct. It takes longer to tape the waterline prior to applying bottom paint than to calculate its position.

chala · 03-06-2013, 07:58

Look at the LWL if that line is the true design LWL and has not been changed by some previous owners then if the water line of the water you are floating in is below the LWL you are under loaded. If that line is above the LWL you are overloaded. It is possible to calculate by how much you are under or over loaded by finding the available internal volume between the LWL and the under or over loaded line.
LWL = Load Water Line is the waterline at which a boat is calculated to float when fitted out and ready to sail, i.e. When carrying the load for which the boat was designed.

Cheechako · 03-06-2013, 08:20

It is probably too complex to actually calculate. It has a lot to do with the strength of the bridgedeck for a catamaran.... not just calculating displacement . I can tell you this, when I sold my Passport 47 mono and bought the lagoon 42 cat, we had both boats next to each other at the dock. We simply moved everything from the mono to the cat accross the finger pier. The Passport was down 5" on it's water line (we had previously moved the bottom paint up to above the boot stripe!) When loaded the Cat was only down about 1.5-2" max from the unloaded level. Not sure we should have put that much in the cat, but it sailed fine. We subsequently got rid of the gen set and 1.5 hp AC water maker on the cat which tok a bunch of weight out.