pontoon boat design

I plan to weld up an aluminum pontoon boat. My question is what shape and what size of pontoons should I make. The deck is only going to be 5'x10', and I would like it to have the ability to carry about 700-800 lbs. I have seen similar boats with two sets of small diameter round pontoons (two each of 12" diameter and 8" diameter), but this seems to just add addition weight to me. I was thinking of creating two V-bottomed pontoons (just a slight angle), each with a footprint of about 20". Any comments from anyone in the know about even basic principles of boat design would be appreciated---Thank you

 

I wonder if the double small pontoons are actually better for planing than the single pontoons and by how much? Seems like they would help create a lot more lift.

In my opinion, most pontoon boats are not really created for performance (either speed or efficiency) but are simply made the easiest way to manufacture and the least expensive they can be.

What speeds are you going for, and how much power are you going to supply?

At planing speeds I would think a generally flat pontoon which would then transition to a pointed section at the bow would work well in terms of speed. Also reducing draft. On the flip side, the potential problem with broad flat pontoons will be maneuverability... it might handle like a barge, both at slow speeds and the way it turns at faster speeds.

 

Thank you for your reply. I really am at a loss when it comes to any thing boating. I want to build this pontoon to enable me to putt around average sized lakes and fish. As far as power goes, I was hoping to use a 5 up to maybe 15 horsepower outboard--anything that will be easily taken on and off of the transom, but also provides enough power to not leave me struggling out on the lake. I would like to keep the amount of motor I need to a minimum and that is why I am so concerned with pontoon design. Most of the manufacturers use round ones, but one used a hexagon--claiming that it maneuvered better and rode higher, therefore getting maximum efficiency from its power source. I tend to agree with you that maneuverability would go down with such a design. I read something dealing with the design of canoes stating the fact that flat bottomed canoes were more stable in still water, but that the V-bottom canoes were easier to motivate and could cut through waves more easily. From this I came up with the idea off the very slight V-bottomed pontoons. I want the thing to be stable (that is why I am building a pontoon to begin with), but I also want it to be relatively efficient and not to spooky if the wind starts chopping up the lake. If anyone knows a formula that I can use to determine the size of the pontoons that I need (volume) in order to carry 700 to 900 pounds, it would be greatly appreciated. Sorry to bother you guys with such a trivial little boat, but I have had a hard time figuring this stuff out on my own! Thanks again

 

If you haven't already seen this thread: http://boatdesign.net/forums/showthread.php?threadid=243 I think you will find it interesting.

I think a flatter broader pontoon would be better for a faster planing hull with plenty of power (I see lots of pontoon boats with 30-50 hp outboards zipping around here). But for your use with a 5-15 hp engine and a load of 800 lbs I would suggest a better more selender displacement shape which would be more efficient at slow speeds (and cutting through the waves).

As far as displacment, you would simply "guestimate" the weight of the pontoons, structure, deck, people, and gear. Water weights 8.35 lbs per gallon and 1 cubic foot of space holds ~7.5 gallons of water (so one cubic foot of space would displace 62.5 pounds of water). You can use these basic numbers to size your pontoons (keeping in mind a liberal margin of safety and using only the volume that you want to be below the water line) and then as the design progresses you can refine your weights/displacement values.

 

Good link. Will the draft (amount of pontoon under water--I think that is what draft means?) be affected by the shape of the bottom of the pontoon or does that only have to do with how much water it displaces? So if I figure that the boat and everything that it will carry will way 1000 lbs. and divide that by 62.5 I will need pontoons that have a volume of sixteen cubic feet in order to be sitting at about water line, correct? Then I simply add volume to raise the deck up? I appreciate your help, one because I am a novice, two because aluminum is expensive and I don't want a "could of worked" laying around, and three this stuff is pretty damn cool!

 

Yes the shape will determine the draft.

At displacement speeds which is probably all we're concerned with here, it will be quite simple as the same amount of water will be displaced to support the given weight, so a slender/sharp v pontoon shape will sit deeper while a broad flat pontoon will be shallower to displace the same volume of water.

P.S. Here's a good summary of terms:
http://www.tedbrewer.com/yachtdesign.html

 

Thanks for your help. At present I think that I am going to go with the idea of V-bottomed pontoon (very slight though). My question now is regarding plywood. Most boats use Marine Grade plywood, but I have heard that pressure treated is better because Marine Grade is only treated on the outside layers--is there any truth to that (it think pressure treated is actually cheaper, as far as cost is concerned)? Another question is regarding safety. What can I do to ensure that the boat I build will hold up on a rough lake, how strong are the damaging forces I might encounter? Like I said this will be a welded aluminum design, and I want to keep it as light weight as possible--I do not want to overbuild it (like I am known to do), but I don't want it to be questionable as far as safety is concerned. I suppose that this is more of a question aimed at people with some experience with aluminum. One more question--will a trolling motor with say 50 to 55 lbs of thrust motivate this pontoon boat as a primary power source? Even if the wind comes up? Thanks again!

 

Check out www.rettey.com. They sell pontoon boat kits and parts thereof. They use plastic pontoons that have the flotation/buoyancy already figured.

 

Do yourself a favor, go down to the local lake where there are a few boats in the size range your looking to build and take some measurements. The local boat sales outlet will have a dozen or so laying about to look at, bring a tape measure. Stay pretty close to what you see and maybe the thing will float with the correct side facing the sunshine.

I say this because you're getting some poor advise from all above but Jeff. Guessing isn't how to do it with any level of safety or economy in material use.

5 - 15 HP isn't a lot, so as efficient a shape as can be designed will reap large rewards. Though this will require much more expertise then you currently have, it can be had. I have a 45 HP outboard that weighs in at about 150 pounds, that I regularly remove (by hand) and I'm no monster at 150 pounds and 50 years old, so maybe you could use some more HP if your up to it.

Sure a small trolling motor (50 pounds) can be your main engine, but you will not punch through waves over a foot nor go much faster then a mile an hour or two, assuming no wind is holding you in place like a 15 knot header or 3 knot current.

As for the people who are telling you about marine grade plywood, STOP TALKING TO THEM. They not only don't know what they are talking about they probably don't even know how the stuff is graded or what the preservative used is. Pressure treated ply is far inferior to marine grades for several reasons. It has less layers and those are of lesser quality, it has large voids and defects, and frankly except for building a tool shed next to the house, isn't good for much. I know all the George Buehler fans out there are going to lynch me for these comments. Most of the lumber (99%) and plywood (99%) found in the local Lowes Depot doesn't belong on a boat, unless you don't care how long it will last. I know lots of folks use PT as a decking between the hulls. This works for about 5 years if it sees any real time on the water. It'll last 10 years if it's kept in the yard and has some pitch to it for rain run off, but this also means you've built a boat you're not using.

For the record, I like George's work and his concepts. I've been on a few of his designs and they're nice and salty, even if they do need more sail area then designed or more refinement in the appendages.

Find a set of plans, stick to them (don't over build, a lot can go wrong if you do) and you'll have a fine little boat. Guess at it and you should stay as far away from the shore line as you care to swim back to . . .

 

The Coast Guard likes boats under 20' to meet several safety criteria. I suggest you at a copy from their web site.