Opinions on this plan......

Hi,

I saw a plan from a 1978 issue of Boat Builder, for a pontoon type boat called "Parti-O". You can see it at: http://www.svensons.com/boat/?p=HouseBoats/Partio

It would be pretty much everything I need. But my questions are, or advice I am looking for, has to do with how much weight it can carry, and how "tippy" the thing would be.

For displacement I get, (6" x 1' x 2' x 17') x 62.5 = 1062.5 ..... giving a displacement of 2,125 pounds considering both pontoons. Is this even close?

For "Tippy-ness", how do I even figure this? I would assume at some point, with some loading, this thing would just tip over... i.e. if everyone stood on the top, at the edge....how do I figure out what that point is?

Thanks,
Jim

 

Dear JF

In order to capsize any boat by improper loading, you have to move the center of gravity(CG) latteraly past the latteral center of bouyancey(LCB). With a top heavy boat, such as a canoe, this is easy because as the canoe leans over the latteral CG moves toward the low side. The LCB also moves to the low side too, but not as much. So over it goes.

With the boat you have in mind, you have TWO LCBs, one for each pontoon. As the CG shifts to one side, The pontoon on the low side sinks deeper and deeper, taking on more of the wieght as the high side pontoon rises higher and higher. Eventually, the low side pontoon is carrying all the weight. And the boat does't have to lean too far for this to happen.

What this means is that to capsize this vessel by improper loading, you have to put enough weight on the low side to completely submerge the pontoon on that side. So all you have to do is calculate the total volume of one of the pontoons, subtract half the weight of the empty boat as well as the engine and full fuel tank divided by 62.4 and multiply the remaining volume of the pontoon by 62.4 to get the weight in lbs needed to submerge the low side pontoon. I think you would need a lot. My guess is that you would need at least ten Texas sized revelers (averaging 200lbs each) all standing on the very edge of the boat to do the job. So it can happen. A sufficiant amount of foolhardieness can doom any vessel. The question is: would you ever let that many people aboard (Coast Guard would probably rate the boat for only six) and let them all behave that badly.

Of course, all this assumes there is no water in the low side pontoon. If the low side pontoon started flooding, the boat could capsize much sooner.

Hope to have been some help.

Bob