Calculating loads?

[Bubba1235]

Anyone have a link that will help me calculate how much weight a tube will safely float? Say length x diameter, etc.

 

[5150abf]

Re: Calculating loads?

What do you have and I will see if I can match that up to a boat at work, should give you a rough idea.

 

[Rancherlee]

Re: Calculating loads?

Round tubes are Easy.
Say you have 21" Tubes. 3.14(PI) x 10.5 ^2 (radius Squared) = 346 Cubic inches per inch of tube length.

346x12 = 4152 cubic inches per foot of length
There are 231 cubic inches in a gallon of water.
4152 / 231 = 17.98 gallons of displacement per foot of pontoon
Water = 8.3#
17.98 x 8.3 = ~149 # of load capacity per foot of length

Say you have 20' logs , knock off a foot or two for the pointed fronts for saftey since its hard to calculate there capacity and different toons have different fronts.
20-2 = 18
18x149 = 2682# of flotation per pontoon, 2 toons give you ~5300# THEN subtract the pontoon, furniture, deck, outboard,fuel, ect........ and come up with a number. Lets say you come up with 3800# of capacity left after you subtract all the weight, NOW you divide by 2 to give you a TRUE capacity of 1900#. This is encase someone on the pontoon sees something and EVERYONE runs to one side and looks over the railing to get a look as your whole load is now putting all there weight over one pontoon. The 1900# capacity in this case is MAX. I would put in a 10% safety factor also incase there is some water in the tubes ext. so in this case, 1900 x .90 = 1710# capacity.


I'll run some numbers quick here for various common diameter pontoons.

19" = 114.5# per foot
21" = 149# per foot
23" = 179# per foot
25" = 211.5# per foot

 

[smclear]

Re: Calculating loads?

Round tubes are Easy.
Say you have 21" Tubes. 3.14(PI) x 10.5 ^2 (radius Squared) = 346 Cubic inches per inch of tube length.

346x12 = 4152 cubic inches per foot of length
There are 231 cubic inches in a gallon of water.
4152 / 231 = 17.98 gallons of displacement per foot of pontoon
Water = 8.3#
17.98 x 8.3 = ~149 # of load capacity per foot of length

Say you have 20' logs , knock off a foot or two for the pointed fronts for saftey since its hard to calculate there capacity and different toons have different fronts.
20-2 = 18
18x149 = 2682# of flotation per pontoon, 2 toons give you ~5300# THEN subtract the pontoon, furniture, deck, outboard,fuel, ect........ and come up with a number. Lets say you come up with 3800# of capacity left after you subtract all the weight, NOW you divide by 2 to give you a TRUE capacity of 1900#. This is encase someone on the pontoon sees something and EVERYONE runs to one side and looks over the railing to get a look as your whole load is now putting all there weight over one pontoon. The 1900# capacity in this case is SAFE.


I'll run some numbers quick here for various common diameter pontoons.

19" = 114.5# per foot
21" = 149# per foot
23" = 179# per foot
25" = 211.5# per foot

Correct me if I'm wrong, but your calcs are assuming the entire tube is under water (full displacement). This would not be a safe assumption IMO. You seem to add a "safety factor" when you divide by 2 to get a "true" capacity but that's flawed as well. (where did you come up with the "2") Most tubes only submerge 4-5 +/- inches. You would need to calculate the water displaced at the normal waterline on each tube to get a true weight carrying capacity.

 

[OldePharte]

Re: Calculating loads?

Off hand I would tend to agree with rancherlee.

Cut the submerged volume in half to find the maximum capacity for a tube half in the water. As long as the maximum is not exceeded, then the tube shouldn't go any deeper.

I did not check the actual calculations, but they looked correct.

 

[smclear]

Re: Calculating loads?

Off hand I would tend to agree with rancherlee.

Cut the submerged volume in half to find the maximum capacity for a tube half in the water. As long as the maximum is not exceeded, then the tube shouldn't go any deeper.

I did not check the actual calculations, but they looked correct.

I agree that rancherlee's process is correct AND his calcs appear correct. I was questioning 1. His first step was for a fully displaced tube - which I believe is an unsafe condition. and 2. His random use of "2" as a divider for achieving a maximum weight. I know on every pontoon I have seen, the waterline does not come up to half the tube.

 

[bowman316]

Re: Calculating loads?

but you could submerge the whole tube, and then you would reach you absolute max load. in his calcualtion, at 5300 lbs, you would be submerging the whole tube. It is not safe, but that is the max weight it will float.

you can calculate your weight if you take the waterline, and calculate the volume of air inside. The deeper the waterline, the more weight.

 

[cornercanyon]

Re: Calculating loads?

I read Rancherlee to be dividing by two in case everyone moved to one side of the craft with two pontoons. I don't think Rancherlee took into account the true displacement of a pontoon as noted by others: you do not want the entire toon to be underwater do you?

To correctly figure a safe load the amount of water actually displaced is necessary, not the total volume of the pontoon and if the weight isn't evenly distributed an additional factor of safety should be added.

 

[pontoonokie]

Re: Calculating loads?

i have no clue on the numbers.. but i think what he is trying to convey is if you add the amount of weight he said .. it would put the waterline over the top of the toon.. thus sinking your ship... please correct me if i have step over my simple knowledge...
curtis

 

[Rancherlee]

Re: Calculating loads?

Yes, my calculations would put one tube to the point of submerging IF ALL the weight in the calculation was up against the rail on one side of the pontoon. chances of this happening are pretty slim but possible. I person could always put in a 10% safety factor also, this would knock my 1600# rating to 1440# in my above calculations. I checked out a few manufactures websites that list hull weight, load capacity, total capacity, pontoon diameter, ect. and one came pretty close to my calculation for load capacity, between my "10% safety and max" numbers and the rest played it pretty conservative with there numbers being at least 15% lower than my calculated numbers. My personal 20' pontoon boat is from the mid 80's and has the octagon equivalent of ~19" pontoons. and has a load capacity of 8 people or 1200# and my equation shows a 1300# capacity so its pretty close. I'll go back and edit my first post to include a 10% "safety" also. I know I looked at several older pontoon with missing capacity plates, most people don reinstall them when they redo the furniture since they are not required on 20'+ boats in my state (and I also think thats a coast guard regulation also)

 

[cornercanyon]

Re: Calculating loads?

Thanks for the doing the homework Rancherlee. This is a great site.