Head check on Max Capacity and Max Power Calc.

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Rooster368
Posts: 89
Joined: Mon Jul 26, 2010 9:23 am

Head check on Max Capacity and Max Power Calc.

#1 Post by Rooster368 » Sun Jul 24, 2016 11:34 pm

I apologize in advance for the tedious question on calculations, but I wanted a double check on my calcs.

I am adding a 3rd toon (with build int transom) to a 28ft pontoon as part of a restoration. All three tubes are 21 inch "U-style" with Styrofoam inserts. I assessed using the Canadian construction standard, but it wasn't clear if a tritoon is calculated the same way as a traditional pontoon. Here is what I came up with (keep in mind I had to covert everything to metric, then back to standard).

Max Capacity

GW = ((72m^3*1000kg)-1000kg)/2 = 3100kg = 6,834 lbs (~ 41 people)

Max Power

MP = 3(8.5m X8.5m)*0.5m = 108kW (~145 HP)

I ball parked the the total weight of the vessel at 1,000k (2,200 lbs), but honestly I am not sure what the final weight would be. I guess Iam concerned that 41 people seems like too much and 145 hp seems like too little.

Any help would be appreciated.....even if it is to go back and take high school algebra.

Thanks,
Rooster

NCMike
Posts: 3
Joined: Sun Mar 06, 2016 12:42 pm

Re: Head check on Max Capacity and Max Power Calc.

#2 Post by NCMike » Tue Jul 26, 2016 5:38 am

I wouldn't think its as easy as just doing doing a little math and coming up with new numbers. I know I am new around here but I am a Mechanical Engineer by trade and the final weight capacity would factor in grades of materials used at a minimum. And Max power would definitely take that into consideration as well. Grade and number of fasteners used to secure your new log with transom and also the grade of material it is fastened to. Maybe I am over thinking it but I wouldn't venture to far over the original specs of the pontoon. Use common sense and think about where your failure points will be. Again, I don't think your going to find just 1 equation to give you a definitive answer.
2015 Crest II 230 Tritoon w/ Suzuki DF150
2015 F150 King Ranch (tow vehicle)

Joseph Elliott
Posts: 56
Joined: Fri Aug 29, 2014 5:47 pm
Location: Alberta, Canada

Re: Head check on Max Capacity and Max Power Calc.

#3 Post by Joseph Elliott » Wed Jul 27, 2016 12:06 pm

Hi Rooster, those numbers look correct, with pontoons the calculations always overestimate the number of people (I'll qualify that by stating that according to the calculations I can put over 40 people on my little 16' toon even though i could never physically fit that many on there) and the max horsepower always looks low. The calculations are based entirely on buoyancy and volume of the pontoons. I don't know the laws in the states (although they are extremely similar) but up here in Canada it is indeed all based on just a couple equations as to the max passengers and max horse power.
Alberta, Canada
Home made 16' pontoon, 1980 60HP Johnson- work in progress

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rancherlee
Posts: 580
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Location: Eveleth MN

Re: Head check on Max Capacity and Max Power Calc.

#4 Post by rancherlee » Thu Jul 28, 2016 4:48 pm

Do a search for the ABYC H-35 worksheet, that covers calculations for flotation and person capacity. Generally a 3rd toon only adds 2-3 persons of capacity, and 50% more HP rating. Basically is comes down to the flotation of a single tube since someone could scream "Hump back whales" and everyone moves to one side putting all the passenger weight on one side submerging the one tube. A center tube adds a fulcrum that will only allows half the weight of the pontoon to help keep the outer tube from submerging in that situation. A center tube may add 3000+ lbs of flotation but only actually provide 1000# of useable capacity if the pontoon weighs 2000#. My 20' tritoon has just under 12,000# of floatation but only has 15 persons capacity and 160hp max according to the worksheet. I got a Capacity plate made for 14 people and 150hp to play it safe.
1988' Kennedy 20' "Haley's Comet"
Rebuilt 2016 with 25" single strake outer tubes and a 25x23" straked U-tube
2003 Suzuki DF140 - Yamaha 9.9HT kicker - 39.1@6300rpm

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rancherlee
Posts: 580
Joined: Tue Jul 13, 2010 5:59 am
Location: Eveleth MN

Re: Head check on Max Capacity and Max Power Calc.

#5 Post by rancherlee » Thu Jul 28, 2016 5:36 pm

Ok, I got curious and did the worksheet for a 2500# 28' tritoon with 21x23 U shaped logs and came up with a 19 person Capacity and 260hp max rating. Use the engine weight of 803# for all ratings over 220hp on that worksheet.
1988' Kennedy 20' "Haley's Comet"
Rebuilt 2016 with 25" single strake outer tubes and a 25x23" straked U-tube
2003 Suzuki DF140 - Yamaha 9.9HT kicker - 39.1@6300rpm

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