How do I calculate the weight limit of a pontoon boat?

Can anyone help me find out the weight limit of a pontoon boat I recently purchased? It is a 1990 model, 24ft Riveria Cruiser and the tag that show the weight limit and capacity in number of passengers is old and unreadable.

I think it has 18" pontoons.

I know there must be an engineering formula for calculating the weight support capacities of pontoons but I cannot find one on the internet by using search engines.

Thanks,

 

Firstly, welcome aboard boatdesign.net

A pontoon boat is, for the purposes of load rating, considered a multihull vessel. In Canada, the relevant standard is TP1332 (1999) section 4, paragraphs 61-72 at http://www.tc.gc.ca/marinesafety/TP/TP1332/section4.htm#multi-hull American standards are closely harmonized with Canadian ones. There should be an equivalent American standard but it will probably be harder to read and will produce virtually identical results.
You will find the Canadian standard to be quite readable and the calculations easy to do by hand.
Some will undoubtedly say in response here "just take the volume of the pontoons, multiply by the density of water, there you go". That's essentially how the standard works, with some added criteria. It is better to follow the accepted government standard, because (a) it is what the Coast Guard will hold you to if you're stopped by them, (b) the authors of the standard have already computed appropriate safety factors and outlined how to perform and evaluate the stability tests, which makes your job a lot easier and safer, and (c) they give you the formulae and definitions, and outline exactly how to do it so that it's nearly impossible to screw up if you passed grade 8 math.

 

Also, any pontoon boat with the same length and pipe diameter should be the same capacity. Visiting a showroom you could either find one the same length, or calculate based on what lengths/diameters you do see.

Alan

 

If it also has the same dry weight and the same size engine, that is. Doing the actual calculations takes less than ten minutes if you know how much the boat weighs when it's out of the water.

 

True---- though I think a ballpark is what's wanted, for practical reasons. Recycling yards and dumps have scales--- takes two trips, but very accurate. I plan to weigh my boat that way.
You are correct, that the dry weight will vary somewhat.

Alan

 

surface area of a (virtual) slice of the pontoon (Pi x 9 (sqd) = 1.76625 sq ft x the length of the pontoon... 24 ft = 42.39 cu ft x 2 = 84.78 x 62.4 lbs/ cu.ft (water weight displaced) = 5290 lbs /2 (half the pontoons submerged) = 2645 lbs total all up weight. You will have to subtract the dry weight from that and I would also subtract another 10 % or 265 lbs just to be on the safe side. According to the CG, people weight 145 lbs so divide the remaining by that for # of people...then use common sense... 4 200+ lb "Bubbas" equals 5 to 6 CG people. And don't forget the Beer wt...gotta account for that...(just don't over do and drive).

Steve

Edited to add: Measure your pontoons...I checked on Riviera pontoon boats and all of them had 23-25" pontoons...which would significantly add to the capacity. Measure the circumference and divide by 3.14...should be close enough. Then plug half the remaining # into the spot where the 9 is in the above calculations and continue from there.

 

Thanks!

Thanks very much everyone who replied to my question. Amazing how helpful this site is!