A little help please for an old first-time builder

Hi guys,

Novice here, enthusiastic though limited woodworker, looking to make a pontoon boat for carrying up to 800lb of people and gear very slowly around a local lake. I hope to be able to move the boat with my 55lb Minn Kota trolling motor, but if that won’t work I’ll have to find a small outboard.


I am thinking perhaps 12ft x 6ft, ½ in plywood floats and deck, floats 2 or 3 ft apart. I have estimated materials at 400lb tops. My floatation calculation suggests each float would need to have a cross section approaching 240 sq in.

Given my limited woodworking skills I am looking at a basic box shape, perhaps with tapered ends. To make the most of the very limited power, would an approximately square section be best, or should I go for rectangular, and if so, would narrow and deep (say 12 in wide by 20in) be preferable to broad and shallow?

What would the preferable treatment be for bow and stern? Would there be a noticeable advantage in having curved sides or bottom?

I have read around the forum a bit and seen similar questions, but for slightly different configurations. Any and all tips will be much appreciated.

 

Hi Stranded.
Good to see a beginner with reasonable and practical questions.

If you are "going slow", then maybe you don't have to worry about an efficient shape. All of those points are important.

But, if you are using batteries, then the power drain of an inefficient hull could be important.

I think you could find some free plans for you project that would optimise all of those points.
By extending your skills a bit, you would end up with a much more satisfying results.

Free pontoon boat plans http://www.jemwatercraft.com/pontoon.php

 

The 55# kota would work for lakes under 200 acres or so with an extra battery. Lakes larger would get to be tough due to windage.

I have a plan in mind that would do well with a small gas motor.

Study plans are free. Full plans are 105$.

The plan seller also has a forum with lots of helpful boat builders.

Pontoon Cat 20 (PC20) - Study Plans https://bateau.com/studyplans/PC20_study.php?prod=PC20

 

Stranded.... a twelve foot catamaran, ne pontoon boat, is on the verge of being impractical. A twelve footer could be built with some confidence from quarter inch ply. A 16 footer would be far more useful, safer, more comfortable and just as easily propelled. A sixteen footer could be built inside of 250 pounds. add 350 pounds for two passengers, two batteries and a Min kota 55 adds about 125 pounds for a wild guess of a total of 725 pounds.

Each pontoon needs to have the capacity to support that kind of weight while still having some degree of safety. Lets say that you wish to have the maximum immersion of either of the pontoons at 12 inches. Very well, lets tinker with the math. If the actual length of the immersed part of the pontoon is ....say 15 feet then 180 inches. A conventional shape of a catamaran hull will have a prismatic coefficient in the region of 0.52 more or less. Prismatic coefficient can be thought of as the average area of any or all of the sections of the hull. Here we go. We need 725 pounds of displacement in one hull. if we calculate the number of cubic inches of displacement for that displacement we get 725 divided by the constant 0.03611....or 20,077 cubic inches. The length of the waterline is perhaps 180 inches. Divide 20, 077 by 180 to get 111.5 cubic inches. But that is the average section area that we need. So divide the average by the prismatic coefficient to determine what the main or mid section of the pontoon needs to be. 111.5/0.52 to get 214.5 square inches. That is to be for a maximum single hull draft of 12 inches.... therefor 214.5 divided by 12 = 17.875 inches width. That is conveniently 17 and seven eights inches. What the hell make it 18 inches for convenience. Are we having fun yet?

It would be far more practical to make your boat a simple skiff of about 16 feet by perhaps 48 inches wide at the chines... maybe 56 inches wide at the sheer. That boat well be cheaper to build, easier to build, and probably more efficient at low power than the catamaran. Such a boat will accommodate the two passengers inside the boat, not on top of the boat, and it will be easier to transport and launch than the cat. Take up less storage space too. For low power propulsion the bottom needs a suitable amount of rocker. That is to say that the ends of the bottom will curve up slightly at both ends. That boat is going to be lighter more maneuverable and better suited to human convenience. You could row he boat if you have a desire to get some fun exercise. Not as practical an exercise with a cat. The 55 Minkota will move it at a modest pace that is probably as fast or faster than the cat. It will also float in far less water than the cat and thus be easier to load or unload on your trailer. Something like 5 inches or less draft.

I hope that this is helpful and not overly critical of the pontoon boat option. Please continue with your questions or comments.

 

The drawing I mentioned can be reduced by 10% to 18' iirc.

 

Thanks so much to all for your contributions.

rwatson: I hear what you're saying but I'm not sure this old dog is up to learning stitch and glue tricks. It would need a huge leap of faith for me to go out on the water in something I'd cobbled together without knowing what I was doing. (Though this applies a bit to any option I choose ). I do like the idea of some sort of professionally designed hull though. I suppose I couldn't use something like 2 pdracer hulls (Free Plans for the Puddle Duck Racer Sailboat http://www.pdracer.com/free-plans) suitably lengthened and narrowed? That would be a lot easier for me to build.

fallguy: Thanks for the tip on the motor capacity. I am now more confident that it may do the job. Wind is almost never a factor on the lake I'm thinking of - nearly always dead calm. I will study the plan you recommend.

messabout: Funnily enough, I was already thinking that 12ft was probably too small after looking at the pics of the model rwatson proposed! It's good to know I could go to 16 ft using the same power input. I appreciate your math lesson - I somehow suspected there was more to this than the simple calculation I used. As for the skiff, I see the advantages you're pointing to, but I still feel a pontoon is more stable and comfortable and better suited to the activities I would be using it for: fishing and swimming.

There is another factor I haven't mentioned, but just so you know you are dealing with a complete nutcase: I have the idea that I can build something that can be knocked down and transported without a trailer. So, hulls in two parts bolted together, etc. I don't have the money, space or driving skills for a trailer.

Thanks again for all your ideas: this forum is awesome.

 

1/4" ply for a 16 footer ?

Cripes - that WOULD be heavy.

Ocean going Kayaks are only between 4 -5 mm (1/8 -3/16") assuming you epoxy/glass the outside which small boats like this would do.

"STANDEDINMIX It would need a huge leap of faith for me to go out on the water in something I'd cobbled together without knowing what I was doing. "
Yes, well I did suggest you "stretch yourself". That's the beauty of following a design - your mental picture of the boat you think you need is erased for the boat you really DO need.

 

I was hoping that someone would post a reply such as yours. You have brought some math, experience, and (un)common sense to this proposal.

 

While I much admire the science and the sheer poetry of streamlined boat designs, it would take me a long time to build (and time is not on my side), and I am far from sure I could achieve an acceptable result.

As I have said, I am taking a modular approach, and am now considering an 8ft x (approx) 12in wide x 24in deep, vertical-wall box mid section, with two 4ft end sections.

I’d appreciate opinions on which of the illustrated options would offer least drag, or would the differences be negligible?

 

A simple skiff will take less time and material than any of the sketches you have drawn. It is easier to bend a piece of ply in a gentle curve than it is to make hard joints like the straight slope design shows in two places per hull.

If you make the boxes only 12 inches wide the boat will have less initial stability than a typical flat bottomed skiff. If you make the boxes wider the side to side stability will improve. That is because the narrow but taller box will have a smaller displacement per inch of immersion than a wider box would. The box shape pair of pontoons are easy enough to assemble but a downside is that there will be excess wetted surface for a given displacement.

The electric troller will have to work harder to push a larger wetted area. That is because of skin friction which is a major factor when propelling a floating object at slow speed.
Any of the shapes that you have drawn will suffice. None of them are ideal. The bullet shape is perhaps the better of the three but the front end needs to also have some"rocker". The stem does not need to be slanted as in the second drawing.

 

Messabout, I really appreciate your patience and detailed information and advice. For sure you have saved me from rushing into something that would have had poor results. I have to give the whole project a lot more thought.

On the building question, it's not bending ply that bothers me particularly, it's more the stitch and glue method, which is totally foreign to me. Perhaps after all I will have to do a few trial projects to see if I can get to grips with it, and then maybe I can build proper hulls after all. I was really hoping to avoid a lengthy learning process and achieve a quick build of a very basic design, but heigh ho, back to the drawing board.

Once again, thanks for your invaluable contribution.

 

Stich-n-glue is probably the easiest building method to learn. It is also easy to practice by making miniature models of your project. Simply cut scaled versions out of stiff paper or cardstock, then tape together. Wire twist ties hold the plywood together until the resinated cloth "tapes" cure.

 

The drawings are not great.

There are a ton of reasons.

Just get a proven plan from a designer. There is a very helpful builder's forum on bateau.com and this forum is helpful where bateau builders fall down on more technical stuff.

The most common failure I have seen is guys underbuilding the toons. Build it not for minimum, but for maximum loads.

I have a two man canoe and if we want a third guy, we need a rental canoe.

 

Strandedinmx: if you can make a peanut butter and jelly sandwich you can make a stitch and glue boat. The sandwich involves the ability to spread peanut butter evenly over a slice of bread. Stitch and glue joining method works pretty much like that. You spread some epoxy, that is about the consistency of peanut butter, against a piece of plywood. That spreading is usually made easier by using a tool of rigid plastic, sort of like a credit card. Pretty easy to do. The stitching part is only a method for bringing two pieces of ply together before you apply the peanut butter/epoxy. Stitching is often done with short lengths of wire or plastic wire ties. It can actually be done with string like a needle and thread method. Many designs will let you do the joining part with ordinary packaging tape. All you need do is bring the matching parts together and then glue them in place. There are many instructive videos on You Tube.

 

This is actually good to take many instructive videos on you tube or other sources of instructions. but i think i would be better to take advice from a experienced. So i would love to suggest you for further considerations to Astra management group.