Custom Tritoon Tiki Boot --- will it roll over?

[Markmysite]

Hi all. Just joined the forum and have been reading up on center of gravity and buoyancy. I purchased a custom tiki tritoon and wanted to have some opinions from those that understand engineering better than I do.

Here is the background... The boat was originally a 1989 24' JC Pontoon. The previous owner stripped it down and added hardwood floors over the decking, rails, a bar, and a center pontoon that goes back 18'. The toons are U shaped, non foam, and they are all 23" tall X 21" wide. He also added a top deck made out of 2x4's and metal roofing panels. The top deck is 8' wide X 9'6" long. There is also a front window that is held up that is 8' wide X 3' long The previous owner built it 3 years ago and used it as a charter boat part of the time. He would take 10-12 people out and it never tipped over for him so at first I wasn't worried about it. I never let people go up top while underway of course.

The top deck is my concern obviously. I calculated the weight (using 16lbs per 8' 2x4). The wood and metal roof weighs about 500lbs. I'm thinking the CoG changed a lot with that addition. The hardwood floors also weigh 500lbs so maybe it counter balances the roof structure. I always try to keep the people load distributed evenly and it's been fine. The logs submerge to a little over the middle with full gear and 6-8 people.

I put a lightweight motor on it recently (2003 Yamaha 2 stroke... Only 262lbs). It has a 50 gallon fresh water tank that weighs about 400 lbs when full. I use it to counter balance the batteries and anchors on the SB side only filling it part way usually. Otherwise the boat does go down about 2 " more on the SB side when at stable float. I had some ventilation issues in turns and waves so I lowered the motor all the way down (20" shaft) and now it's much better.

My concern is will this thing tip over? I had it out for Memorial Day weekend with 6 people on it and the channels got pretty choppy as the drunk people on the bigger boats zipped by me. The 3rd toon helps keep it from submarining but water does come over the front sometimes even with everyone closer to the stern. When we're stuck in a side roll because of the width of the channel and boats passing us we can't cut into the wake at the right angle. She got rocking pretty good a couple times and it freaked me out and lifted my captains chair up on one side.

How can I calculate the CoG and max capacity on this boat? Is there anything I can do to keep it more stable (other than removing the roof top)?

 

[Markmysite]

Weight loads

Here are the weight loads... Including the boat hull, engine, gear... Everything

3,857lbs... With fresh water tank full

3,425lbs... Without water tank filled

Other
Beer, ice, cooler... 60
Food, towels, etc... 80

3,997... Water full and gear
3,565.. Without water tank

With 6 people... 960lbs (some kids)

Final Max Loads
About 5,000 lbs with 6 people and full water tank
About 4,500 lbs with 6 people and no water tank

 

[messabout]

Boat is overloaded. Each outer pontoon, if draft is 10.5 inches, would support roughly 1500 pounds. The middle one less than that. Actually the middle float tends to de stabilize the boat but with that excess weight you can not do without it.

The canopy at an estimated 500 pounds is absurdly heavy. The thick, heavy, added floor?? Why is that necessary unless the occupants are bent on doing frantic indian war dances. What is all that stuff under the roof? Whatever it is find a better, lower, location for it., leave it ashore if possible. Bottom line: put the boat on a strict and certain weight reduction program. That will take some time and money but it is better than drowning a half dozen nice people.

 

[Markmysite]

Boat is overloaded. Each outer pontoon, if draft is 10.5 inches, would support roughly 1500 pounds. The middle one less than that. Actually the middle float tends to de stabilize the boat but with that excess weight you can not do without it.

The canopy at an estimated 500 pounds is absurdly heavy. The thick, heavy, added floor?? Why is that necessary unless the occupants are bent on doing frantic indian war dances. What is all that stuff under the roof? Whatever it is find a better, lower, location for it., leave it ashore if possible. Bottom line: put the boat on a strict and certain weight reduction program. That will take some time and money but it is better than drowning a half dozen nice people.

Wow, if that's right then it would only float about 4,000 lbs. I looked at a chart and it shows a that two 22" diameter toons at 24' would support about 3,614 lbs. Using the same chart at 20' and dividing in 2 the center one would support 1,355lbs... So that would be 4,969lbs. Another chart had way higher numbers showing 6,232 for 21" diameter ones... Plus 3,219 for the 20 footer. That's 9,451 for round pontoon which seems crazy high.

Boat Buoyancy By Pontoon Diameter Versus Length, Number of Pontoons
Diameter 18′ 20′ 20′ Tri 22′ 22′ Tri 24′ 24′ Tri 26′ 26′ Tri
19″ 3,766 4,185 6,277 4,603 6,905 5,022 7,532 5,440 8,160
21″ 4,674 5,193 7,790 5,713 8,569 6,232 9,348 6,751 10,127
23″ 5,679 6,310 9,465 6,941 10,412 7,572 11,358 8,203 12,305
25″ 6,782 7,535 11,303 8,289 12,433 9,042 13,563 9,796 14,693


I just wasn't sure it was an accurate way to do it because all of the charts I found are for round pontoons and the JC pontoons are U shaped so they don't measure the same way do they? Wouldn't the U shaped ones be better capacity wise or no? JC claims their U shape of 21" x 23" would be equivalent to a 25" round tube. If so, then the chart above goes to 12,433 at 24' for 2 and with the center it would be a total of 16,200. Doesn't seem right.

I went to the sandbar today and had 4 people and lots of gear. It measured 8" toon draft. It wasn't as busy so I could cut the wakes and it rode great. It was a 60 footer last weekend that hit me with a side roller that worried me.

The top is way heavy yes. He had metal roof panels underneath and put hinges on them to store life vests is all. I moved them all under seats for easier and more safe access. The rack above the bar holds the stereo up high and is fairly light but that's something to look at.

The hardwood floors were done for aesthetic and theme reasons I guess... But it's a ton of weight yeah. It was stripped and then everything was bolted through that floor though so it'd need to be stripped to come out. I saved 100lbs going with the Yammie 90 2 stroke so that helped some. I also changed to lightweight resin wicker furniture to save weight.

Anyone know the formula for calculating capacity on these U shaped toons?

 

[Markmysite]

A site said this....

As seen from the stern (rear) end of a pontoon, you’ll often find two shapes that are popular: circular and u-shaped. The next difference would be the measurement in diameter across (or across plus height for u-shaped pontoons).

Another talked about U shaped pontoons not producing inverse buoyancy. Is that the failure risk when they fail and it just goes all the way over? Another talked about how a U shaped toons bouyancy doesn't start rapidly reducing after it's halfway submerged as a round tube does.


Can that be right? Add width plus height?
Mine are 21" wide and 23" high = 44

 

[messabout]

Yes that is correct. All things considered the U shaped pontoons are superior to the round ones.

I wrote that a single pontoon will support 1500 pounds at ten and a half inches draft. That was just a quicky number but it was in the ball park. I calculated that the pontoon U section was 20 feet long. For simplicity I did not even count the pointy part at the front end.

Ten and half inches is just half the circle that would be made with a 21 inch diameter section. That's what you have at the bottom of the U. Things get a lot better when your draft begins to exceed that ten and a half inches because above the rounded part we have the equivalent of a box shape. For every inch of additional sinkage past ten and a half your bouyancy will be 182 pounds. You have the potential of 12,5 times 182 pounds of buoyancy if the pontoon was sunk all the way to the top. 182 x 12.5 = 2275 pounds plus the 1500 pounds that the rounded section provides. A total of 3775 pounds for each pontoon.

Now in the case of a 23 inch diameter round pontoon, things are not so favorable. If we sink the 23 inch round one to half its diameter (11.5 inches) that 20 foot pontoon will be supporting 1800 pounds. Note that he is an inch deeper in the water. Now what happens is that if the round one is sunk beyond the half diameter mark, each additional inch of immersion will give less and less flotation per inch because the section is getting smaller and smaller as it goes deeper and deeper. The U shaped one does not do that,
it adds support steadily.

Note that I have used the flotation figures for fresh water. Since you are in Stuart you will be in salt water and the flotation is just slightly higher in salt water than in fresh water. About two and a half percent higher. Not enough to care about on a small boat like yours..

Get some weight off the boat if you can. It will work better and be a bit more economical to operate if you can lighten up a little bit. Most of all lighten anything that is high up.....like that canopy. One of the cleverest ideas that others have used is to call the guys that build screen enclosures for your house. They put up aluminum structures with insulated roofs that do not weigh much. The insulated roofing option is capable of supporting an ordinary sized man. The non insulated roofing will not support much weight. Best not go up there anyway.

 

[Jamie Kennedy]

Just as a triple check I will add my own calculations here.
I will use 19 feet to account for the pointy bow.
I will assume a 21" diameter half cylinder 10.5" high,
with a 21" wide x 12.5" high box on top of the half cylinder.
I will use fresh water for density on case you go up some rivers.

Displacement at 10.5" draft
= 19' x 21"/12 x 10.5"/12 x 3.14/4 x 62.4 x 3 = 4275 pounds

Increased displacement per 1" added draft
= 19' x 21"/12 x 1"/12 x 62.4 x 3 = 518 pounds per inch

The critical thing here is as you add weight you lose stability, because it will take less and less moment to heel the boat over to the point the leeward rail goes under and you tip over. Given 8" draft with 4 people and gear, lets estimate the displacement that day
= 4275 pounds - 518 pounds/inch x 2.5" = 3000 pounds with 4 people and gear.

On the day with 6 people, you might have drawn an extra inch, but you are lowering the metacentric height as you add more and more people, and depending on your centre of gravity it might become unstable even before you lose much of your freeboard. You also have the effect of the waves. As you add people the boats rolling period will increasing, meaning it takes more side to roll from one side to the other. For small motion this feels more comfortable to passengers, but it is actually a bad sign. As you add passengers it will roll slower from side to side and at some point it might roll slow enough that it will match with the timing of the waves and roll further and further with each wave, at some point, with that outer rail goes under, it could keep going over, especially of people fall over and add more weight to that side.

Estimate of Metacentric height, using just two outer pontoons at 37" from centerline.
Area Moment of Inertia = 19 feet x 21"/12 x 37"/12 x 37"/12 x 2 = 632 feet^4
Displaced Volume at 8" draft = 3000/62.4 = 48.1 feet^3
Displaced Volume at 10.5" draft = 4275/62.4 = 68.5 feet^3
Displaced Volume at 13" draft = 5570/62.4 = 89.3 feet^3
Metacentric Height at 8" draft = 632 feet^4 / 48.1 feet^3 = 13 feet
Metacentric Height at 10.5" draft = 632 feet^4 / 68.5 feet^3 = 9.2 feet
Metacentric Height at 13" draft = 632 feet^4 / 89.3 feet^3 = 7.1 feet

So you can see as you add passengers, you aren't just losing freeboard, and raising your centre of gravity, you are also lowering the metacentric height, which is that virtual point from which your total mass is suspended, making it easier for waves to tip you over past the point of return. The other thing that the weight aloft has done besides adding weight, and raising the centre of mass, and raising your metacentric height, is that it has also increases the mass moment of inertia, which also increasing your rolling period making it easier for waves to roll you over.

I would suggest you measure your rolling period with 4, 6, and 8 volunteers on board. You can leave them seated, but you yourself should stand and shift your weight from side to side to get it rolling. Do this away from the dock with no lines attached. Once it is rolling and you are shifting your weight just enough and in time with the rolling to keep it rolling at steady state, count how many rolls there are per minute. Do this first with 4 people plus yourself, and then with 6, and then with 8. You should get a sense of increasing instability as it rolls slower and slower but farther and farther with less and less effort as you add more people. You will also develop a good feel for whether or not the boat 'feels' as it should whenever you leave the dock, and how it might react to a series of large waves hitting you beam to.

caveat. I am not a naval architect, but these are the sort of calculations and sea trials we did for a 40 foot fishing boat when I worked for a naval architect after high school, 37 years ago. It was a great experience, and lots of fun. I also stayed at a Holiday Inn last night.

 

[Markmysite]

Thanks guys. That was the type of data I was looking for!

So it looks like the overall capacity per toon is better than I was thinking but the rolling waves are the issue (like we experienced on Memorial Day weekend). The most I had on the boat was 10 people and tons of gear and I checked the pontoon depth on each side before taking off and we were still pretty far above the water.

The time a rolling wave hit us on the side it definitely started to roll side to side quite a bit and that's when I decided it was time for some math. I took a 3 footer head on at a 30deg angle this past weekend and it took it no problem and cut through it with ease and didn't rock at all. I've learned to move over when a large boat passes by and then cut it back at the right angle.

I'll try to get rid of as much weight as I can... And it's more motivation for me to lose a few pounds as well.

 

[Jamie Kennedy]

lol. Yes a beer belly isn't always good for stability, but half a belly full of beer can be worse due to the free surface effect. ;-)

 

[Markmysite]

lol. Yes a beer belly isn't always good for stability, but half a belly full of beer can be worse due to the free surface effect. ;-)

Hahaha, I think some of my passengers certainly contributed to the free surface effect on Memorial Day weekend. Too much alcohol sloshing lol

It's hard being the captain and telling drunk people where they have to sit and why (as they give you the evil eye). This is my first pontoon and I learned really quick that they are very sensitive in regards to weight distribution. I can move 2 kids that weigh 70 lbs each and it can lift that side visibly.

 

[Jamie Kennedy]

Yes I think if my calculations are right then moving 1/3 of the 518 pounds it takes to increase displace one inch, or 173 pounds, from the top of one outer pontoon to the top of the opposite pontoon will cause one to rise 1" and the other to fall 1". That is over only 6 feet. So if you take 130 pounds and move it the full 8 feet you will get the same effect, which is actually 1.3" on the sides not 1" in the centers. So 400 pounds from one side to the other would cause a 4" change on each size, and on the high side you then start loosing width on that pontoon. And this is all supposing they walk softly, and there is no wake.

Always fun racing with ladies and asking them to sit to leeward or sit to windward. It's never a problem with women like my sister that grew up racing dinghies and keelboats all their lives, but I would hardly call them ladies. ;-)

 

[Heimfried]

Jamie, I used your numbers as an example for web calculation.

The pontoon is shown grafically and it is possible to play with weight, CG etc. and let it draw an heel arm curve.

http://www.bootsphysik.de/rechner/boot_engl_pont.php

(Link temporary)

Click i-Buttons for information.

 

[Heimfried]

Yes I think if my calculations are right then moving 1/3 of the 518 pounds it takes to increase displace one inch, or 173 pounds, from the top of one outer pontoon to the top of the opposite pontoon will cause one to rise 1" and the other to fall 1".

The heel angle caused by moving weight depends on the hight of the center of gravity.

Only 1" rise and 1" fall requires a very low CG height (about deck level), if I'm right. The pontoon with "roof", people, gear will have its CG at a greater hight.