Hydrodynamics of small pontoon boat

Hi all

This is my first time posting on the forum, although I have read it many times over the years. Thanks for a great forum!

I want to build a small pontoon boat for fishing and I have a question about the hydrodynamics of the sterns of the pontoons. Everybody agrees that you need pointed (e.g. cone-shaped or spoon-shaped) bows on the pontoons. But I see some differing opinions about the sterns. Some people say a blunt (square) stern is okay, while others say you also need a pointed stern to reduce the turbulence.

If I look at commercial designs, I see a mix of both. I also see some designs that look like a compromise between the two with a somewhat pointed stern, but not as pointed or sharp as the bow.

I guess the bow needs to be able to cut through waves, so that may be a factor.

The two PVC pontoons will be about 12 feet long and 14 inches in diameter. I will use an electric trolling motor for propulsion so the speeds will be low. I have attached a picture of 3 options, just to guide the discussion. Option 1 shows a symmetrical design with pointed bows and sterns. Option 2 shows pointed bows with less pointed sterns. Option 3 shows pointed bows with blunt sterns.

My question boils down to this. Obviously I have a trade-off between the "sharpness" of the pontoon ends and the weight-to-length ratio. The more sharp I make the pontoon ends (bow and / or stern), the smaller the weight-to-length ratio, which means I will need longer pontoons to support my weight. Now let's say I worked out that I only have X feet available for the combined lengths of the pointed portions of the bow and / or stern. How do I split up those X feet between the bow and stern to minimize my drag going forward in the water? A 50% / 50% split as in option 1? A 100% bow and 0% stern as in option 3? Or some split in between, like 75% bow and 25% stern as in option 2? Thanks in advance for any advice!

 

If you have the power to plane you are better off chopping off the stern for a clean exit. With a trolling motor I think you want a cleaner exit. You don't necessarily need to go right to a sharp point though. I think you want something option 4, where it starts out towards the same pointiness as the bow, and then it is abruptly truncated. If it is 14" in diameter I think you can get a pointy enough bow over a distance of 2 feet. For the stern you would head off to a virtual point say 1 foot past the transom, so it's taper might start 1 foot from the transom and then abruptly truncate at the transom. So you would have 2 feet of bow, 9 feet of straight run, and 1 foot of taper at the stern before truncating. But I think I would do two feet at both ends, and taper the stern off a little smoother than the bow, but still truncate it at the transom. So taper it as though you were going for a point 1 or 2 feet past the transom, starting from a point 2 feet ahead of the transom. That will still give you 8 feet of straight run for as good a volume as you might need. I wouldn't want to sink it half way tough, because it will loose stability rapidly once the lower side is past half way. Also, I would place bulkheads at 2 feet, 6 feet, and 10 feet, because the ends are more likely to take a hit. So for a rough idea of your carrying capacity say 1/4 of the volume of just the straight run, or in other words just 1/2 the volume of the straight run of one of the pontoons. That works out to 533 pounds in fresh water. I you included the bow and stern bits maybe could them as half as much volume per foot, so they would add another 20% for 640 pounds displacement. You can sink a little deeper than that but you will be losing stability and reserve buoyancy.

The interesting part is how you want to taper your bow and stern sections. Do you make a vee notch down the centerline and then bend it in from the sides? That is the really fun part I think.

 

welcome to the forum Nexus77

IN a nut shell, it comes down to 2 things

1) Your length to displacement ratio
and
2) Your target speed.

Your target speed is the most crucial. Since the shape of the hull differs when a hull is below Fn 0.5 to one that is over Fn 0.5. In simple languages below planing speed and above planing speed.

You can read more about the Fn here.

and you can read a bit about length-displacement ratio (LD) here and here.

 

Thank you very much for the replies!

 

Just a thought Nexus. I know you didnt ask, (and I bet I get a few busybodies complaining about that) but is there any reason why you are restricting the materials to PVC pipe ?

Its true you get an 'instant' length of hull, but the engineering of the ends, the joins on the platform and a whole lot of other gotcha's make using PVC a lot more problematic than more standard plywood and epoxy, both in weight and strength. I speak from experience.

For example - if you bend the PVC to make tube endings, you have to make plugs of foam, and/or play with fibreglass. If you have compression rings or other fastenings to hold the central platform, you need to think about extra strengthening.

I would encourage you to think about a more reliable and better planned solution from plywood.

You might get some inspiration from


or

http://shantyboatliving.com/2011/videos-homebuilt-pontoon-raft-boats/

 

Rwatson, thanks for that. Do you guys think that Wooden Micro Catamaran in the clip should have pointed sterns for that trolling speed?

I nearly went with plywood and epoxy. But I did a project with PVC previously and that was quite successful by my modest standards. I made two outrigger pontoons for my Canoe so that I could sit on a chair or even stand and fish. Attached are some pictures. I want to scale that up to make a pontoon boat.

I hope you can make out from the pictures how I did it. I cut sections of the pipe at the exact angles so that I could fit the pieces together in a way that they go "uphill" at the ends. The pieces were then glued and stitched together. The stitch work is only visible on the inside. I drilled holes on either side of the seam and cut grooves on the outside to connect the holes on either side. I then stitched with cord so that the cord on the outside lies embedded in the grooves connecting the holes. The cord is therefore below the surface of the pipe to protect it from abrasion. The holes and grooves were then sealed. They are also properly sealed on the inside. Finally, a small elliptical section of "deck" was fitted to the top of each end. The deck is simply sealed with silicone and fastened with small screws. The decks can be fairly easily removed if a leak needs to be fixed and sealed on the inside, not that this was ever needed. The deck also has a drain hole that can be plugged with a wine cork.

If I follow this method, I think it will be less work than plywood and epoxy. And the pipe is fairly abrasion resistant. I'm a bit worried about the plywood and epoxy getting damaged if I have to drag the boat over rocks and branches. Although granted that it won't look nearly as nice as wood.

 

Thanks for the pictures - interesting work.

It seems that stitch and glue arn't total mysteries to you, so that's a good thing if you ever get to the plywood stage.

Yes, the plywood cat should have had better sterns, but maybe he is planning to mount larger outboards in the future.

The thing is - making nice shapes out of pipes is one thing - having them work as boats though can create problems.

Outriggers work fairly simply, and there are few pressures put on them. Making a whole boat to float you is another story. For example, on your outriggers, you have two bolts to attach the arms to. On a boat, these have to be fairly substantial to handle one or maybe two bodies, across waves that put pressure on the joints. Handling this in PVC is tricky, and will result in a lot of messy reinforcing.

Then there is buoyancy. When I was younger, I was happy just to float around. But I soon wanted to go further, carry more stuff, move faster with sail or motor. You soon find PVC sections limits you a lot.

Then cost. If you can get PVC for free, then great. However, generally you get a lot more value with ply and epoxy.

As for wear - PVC and epoxy are about the same softness. Epoxy is easy to recoat and paint, PVC as well. Where PVC is problematic is that its a more brittle. Dropping one end off the boat on a hard surface can crack it.

I guess in summary, you end up with a lot more useful boat, cheaper and longer lasting with marginal extra effort in ply and epoxy than PVC.

 

Thanks for the input. I'm now thoroughly torn between the two approaches, PVC vs wood. I'll give it some more thought. By the way, what I also meant with the PVC being more resistant to abrasion was that there are several millimetres of PVC that could be scraped off or damaged over time without causing a problem.

So it's safe to say that for low trolling or rowing speeds (below planing speed) I should make the bows and sterns equally pointed?

 

Yes, I see what you mean. For a small plywood craft that spends most of its time out of the water, even leaving deep scratches unattended for a while isn't the end of the world. But, taking five minutes to mix up some goo, and later putting a dab of paint over it, isn't a huge drama either.

Yes, thats preferable. The shape of the ends should be as gradual as possible.

But heres another thing if you are interested in efficient shapes. The whole shape of the hull is important too.

Compare the profiles of a rowing shell to a PVC pipe. Plywood will get you a better performing craft.