rebuilding a 30ft ply pontoon

its a plan and hopefully will follow my/a/anyones design...

got a pontoon houseboat, 2 stories at front, 1 at back I have at a place up north, its got badly rotted ply under glass pontoons- 4ft wide, 4 ft high on each side and 2ft across the bottom

I let her sink on a sand flat as the pontoons are so badly rotted they require 24/7 pumping to stay afloat

In oz, so american suppliers/prices are useless in this case apart from ones I know are sourced locally

I am looking at wilsons plastic pontoons as a maybe ($$$ but very quick less than 8 weeks delivered and 2-3 weeks fitted))

or replacement hulls (new ply built offsite and hauled into place..12-18 mth delivery onsite)

what I am trying to figure out is if the wilson will work?

I got a 4ft wide ply pontoon - each side is 4ft high, and its 2 ft across the bottom that sat 14" above the keel for the waterline with the houseboat

looking at a wilson (if the $ doesnt cost too much) but is their 36" in the same length (30 ft approx) the same as my ply hull???
where would the waterline be on the wilson hull?


my maths skills couldnt figure this one out- 2 and 2 is 4 (maybee????) but this is crazy-(my first attempt had 1/8" in the water.. I gotta plane not a houseboat!!!)

 

Boppa, the information that you gave us not quite enough to provide any certainty. If your ply pontoons are rectangular and they have no rocker (curvature of the bottom when viewed from the side) and perhaps a little bit of a point on the bow, then we have a basic box with a pointy end. WIth the numbers that you have given, a single pontoon would have been supporting roughly 3600 pounds.

Here is an easy way to figure the approximate displacement of a box like float. Multiply the width in inches times the immersion, in this case 14 inches, times the length of the box. Thirty feet would be 360 inches but the pointed part might be 60 inches long I used 300 inches and ignored the pointed end. so multiply 24 x 14 x 300 to get 100,800 cubic inches of displacement. Multiply the number of cubic inches displaced by the following constant. It is 0.03611 OK 100,800 x .03611 = 3639 pounds. The constant is merely the weight of one cubic inch of water. If the Wilson pontoon are 36 inches wide and they are rectangular then they will support the 3600 pounds easily and they will have less immersion...actually just a little over 9 inches of draft. Give us more detail so that we can do this more accurately.

And I am surprised that you are dealing in inches and pounds. Aussies came to their senses quite a while ago and they are more likely to use the metric system which is actually easier to deal with.

 

they are basically a trapaziod as viewed from the rear (I suspect they are entirely built out of 4x8 ft ply sheets uncut except for the bottom which is a single sheet cut lengthwise (2 x 8 ft)
Totally flat bottom from front to back all the way to the bows which have a point on them in both directions
tries to show
side view is
|--------------/
|-------------/

top view is
|--------------\
|--------------/

rear view is
\------/
..\---/

so yeah - its basically got no curves in it at all- everything is flat panels at angles

It's funny- I and many oher people use metric all the time-except when talking about boats- I use Kg and metric ton for weights, but on the boat it all goes back to feet (but still with metric weights, just to confuse the issue further)
altho I did grow up using both so I tend to swap back and forth a lot more than the younger people do
;-)

 

Many of us are more than willing to help you. Your project is sufficiently involved to take up a lot of time and education in order to resolve the issue advantageously. Matter of fact there are many very knowlegeable forum people who live at the bottom of the world (OZ). I don't know if any of them are in Queensland but if so you should contact them. They could guide you on an informed path and probably save a lot of time and more importantly a lot of money. Let us hope that one of the good Aussie mates will jump in here with some one on one advice.

If your pontoons are basicly flat on the bottom, that is to say that the rear end of the pontoons are at the same level as the front end, then you are wasting a lot of power, fuel, and money to move the boat. That is the stuff of interest to you.

Also if the pontoons are rotted, then they were not built by knowledgeable or caring boat people. The fact that the pointed ends are not curved is more evidence of that reality. I urge you to get some hands on help and advice from a professional boat designer or at least an acknowledged amateur. (That means that you need not take council from your second cousin in law or other pretender). You will surely save money, time and disappointment by doing so.

We will help you, within reason, even to instruction on flotation computations if it will help.

 

they are the old ones and yup- they were slow as even with 150hp merc on the back it struggled
I was looking at these Wilsons plastic pontoons
http://www.plasticpontoon.com/web/36.html
as a replacement
biggest advantage for me is the modular design, I can take them to the boat in the ute and assemble them onsite. Where the boat is, is flat sand and these arent as high as my existing ones- so my plan was to take them there in bits, chock the hull on stacked pallets and cut the old ones off the beams, assemble and put them into place with new beams alongside the existing ones.

The alternative is get new ones made offsite, but then that will involve large (semi) trucks and cranes etc which could be problematic where it is, or pump out the hulls and get her to a slipway (more $$ again)

Beause these ones arent as high as my existing ones it means no jacking or anything, just stack the pallets up until they are supporting the superstructure and attack the old rotten ply hulls with a chainsaw!!!

(Where it is atm the sand flats are fully exposed at low tide for quite a few hours and end up with nearly a couple of feet of water over them at high tide) so the idea is that when the new pontoons are under it, just smash up/ pull out the pallets and idle out on high tide


I just havent been able to figure out where the new waterline on the wilson ones would be compared to the old hulls (I dont want to use them and end up finding them almost totally submerged!!)

Thats kinda the plan of attack at the moment and was hoping it was a feasible thing to do

 

Bit more info on the wilsons- they are modular plastics in 6ft lengths that plug into each other like lego at the ends and structural rails bolt them together. Have a circular outline but with a flared bowpiece. I would be using the bow piece and 4 center sections to make each hull (so 2 bows and 6 centers and 2 sterns) to make up a hull 31 ft in total- so a foot longer than my existing ply hulls. Just not sure how to figure what the final waterline is likely to be using these- the old ply hull drew 14" of water, but how to figure it out on these wilson hlls I dont know

 

Applying the SWAG system (Scientific Wild Assed Guess) to the numbers for the old pontoons gave us a flotation of about 3600 pounds per pontoon. A total of 7200 pounds. For the sake of prudence let's add a small fudge factor and say that the displacement total was 8000 pounds.

The Wilsons are round and 36 inches in diameter by 72 inches long. At full immersion they will each have a displacement of 2646 pounds. If only half immersed they will displace 1323 pounds. Four of them would give us 5292 pounds at half immersion or 18 inches of draft.. Of course the pointy ends will contribute a little bit of bouyancy too. We can ignore them for now. More bouyancy than you require so they would draw a bit less than 18" of water. SO far so good.

Wilson will surely have a chart that describes the bouyancy for every inch of immersion. Why not contact them for the chart. They'd ought to be glad to provide that for a prospective customer. We can calculate such a chart for ourselves but it involves some trigonometry. I will spare you the pain.

In any case the Wilsons will probably do the job very nicely. The boat will go with somewhat less power and ride more easily too.

Caveats: You will be more affected by cross winds with the round pontoons. Maybe not seriously but a reality worth knowing about. Also since the modules are foam filled, you can expect the foam to absorb some water, thus weight, over the years. Do not expect to make a lot of speed with the 150 HP
8000 pound boat. Maybe 6 to 8 MPH. T

 

OK I got some more info from the Oz dealer- they didnt have a displacement chart available, they are contacting the manufacturer to see if they can get one for me

to make up the lengths of my new hulls I need 2 nose pieces, 6 center pieces and 2 stern pieces to make the 2 new hulls.
Acccording to the dealer- each of these pieces has a capacity of 2600 lbs fully submerged- there are 10 in total (5 in each hull) giving a total lift of 26000lbs with both hulls fully submerged
I only have 8000lbs on the old boat by your estimates( I tried it out myself with a online calculator and got the same near enough, a little less)

so less than a third of the pontoons would be in the water ie less than 12", is this right?

hardly anything in the water at all for a 4 ton (metric) boat- getting stuck on sandbars wont be an issue thats for sure

 

No not quite. If the numbers we are using are about right, then the round floats will need more than 12 inches of immersion but not 18 inches. The geometry of a cylindrical object is not the same as a rectangular one. That is to say that the flotation potential versus immersion dimension of a cylinder is not a linear function as it is for a rectangular object.

To understand this concept a little bettr get out a pencil and paper. Draw a square box and a circle inside the box that just touches the sides of the box. The bottom of the box will touch the bottom of the circle. Now draw a line line parallel to the bottom line of the box but about one third of the way up the box/circle. Observe that the area of the box, bounded by the lines, is larger than the area encompassed by the circle. Draw some more parallel lines (these are imaginary water lines) and you can see that the area of the circle increases in a manner that is different from that of the box.

As mentioned before, the circle area can be calculated with precision but the calculations involve taking angles, finding areas of a triangle and some other tedious stuff. The pointed sections will also contribute to the bouyancy of the assembled lot. The volume per inch of immersion of those parts is also calculable but even more of a complexity. I do not wish to put too fine a point on this so get the chart from the manufacturer. The chart is a fundamental necessity for assessing the usefulness of the product. The dealer is slightly remiss for not having such a chart on hand.

 

AHHA!!

thank you for that, it slowly sinks in....

Now I get where I was going wrong (I deal a lot better with hands on things) and your explaination plus some scribbling on a notepad made sense finally
finally get how it works

(It's been a long time since school and drawing shapes,figuring areas and stuff, I was making some mistakes on figuring out things)

many thanks