Hydrodynamics of a wooden ponton boat design

Dear boat friends,

Some friends and I are building a Pontoon Boat, but we're concerned about some hydrodynamics of our boat.

We have the following situation: We're using 10 floaters of 90cm*60cm of about 200 liters, which generates, according to Archimedes' law, about 2000KG of floating power.

Our construction will weigh around 450 Kilogram and will (probably) be thrusted with a 9,9HP Yamaha 2-stroke engine.

That's for the background information. Now to the core of the question.

We have an initial design, which looks like this (render):

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And an alternative design looks like this (render):

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Since we're more basic construction type of guys and not expert woodworkers, I'd like to know how much you guys think the front construction will affect the speed. And will it be safe to skip the wooden skewed front boxes. Or maybe stated another way. Will the skewed front boxes increase the hydrodynamics a lot?

Hope you guys can help! Would be much appriciated

 

The "blunt" ends to the pontoons will slow you down, but the rear end of of it, moreso, and especially if your passengers are all seated back there, depressing the stern further. You need a streamlined fairing, 4 of them.

 

*****

 

Agreed on the streamlining, and the position of the weight.

There was a recent post (somewhere) about a guy who used barrels but cut them and welded them into a streamlined shape.

Also if those are wooden shapes it would be easier to make them streamlined out of plywood in a "square" shape.

The skewed front boxes will have very little effect.

 

I fully agree.

 

The bow (front) skewed boxes will probably have little effect on speed, and may slow the boat because they are larger than the cylinders behind them.

Streamline fairings may have a small effect on the top speed at the expense of considerably more complicated construction.

 

For the front, all you need is a bullet-shaped rounding. Say a 4" radius on the edge of a 4" foam disk stuck to the front. That will gain most of what is to be had, and it is easy. The aft should be tapered. About 7 degrees of taper would be good. Leave the transom sharp edged, but smaller than the midsection.

It would be better if the deck did not extend past the centerline of the pontoons. That makes it too easy to capsize. I like the idea of (light) furniture around the sides to prevent people from crowding to one side.

You need to calculate the weight of everything and find the center of mass of the craft in three dimensions before you begin building. This is an afternoon's work, but it will have a larger impact on performance than anything else you could do because getting the boat to trim correctly is very important.

 

Needed because ????? The boat won't work with square ended pontoons? To make it look like a "real" boat? To maximize speed?

Why would it "be better if the deck did not extend past the centerline of the pontoons"? What is special about the centerline of the pontoons?

Where the passengers sit or stand will have a major effect on trim.

 

One of the drawbacks of using cylindrical pontoons, is that the flotation decreases rapidly once they are immersed past the half mark. Usually half of the volume of the pontoon is used on the calculation for maximum displacement and the rest is used as reserve flotation. The bows on pontoon usually fall into two types of designs. One looks like a spoon and is almost a section of a cone. The other has a sharp bow that tapers into the hull. Google images of pontoons and it will give you a good idea of how successful designs have solved the bow problem. The stern is usually left square.

 

Woodehh, do you have access to a wood lathe, especially anything like a Shopsmith that will let you work from either side?

I ask because a way to achieve your needed streamlining, forming the outer shell, might be achieved by metal spinning. If you are not familiar with metal spinning it is a way of making bowls, cones or domes from sheet metal over a wood form.

Here's a video of a set up cobbled together to help you with the idea:



I believe 60cm round may be too large to anneal after working in an ordinary range oven, but if you know someone with a big enough range that may be a possibility worth looking into. Doing so would help resolve stress build up issues with the cold worked metal.

If you can spin the end caps you need they can more closely match the basic barrels. You could reinforce them with an internal framework. If you wanted to get fancy you could fill them with a good floatation foam and seal them up.

A cone shape on the trailing end would probably work best.

 

I wonder how many designers of 200 litre drums over the years have factored into their thinking, "I must make these things suitable to make pontoons, in case someone gets a 'bright' idea to do it ! ". The answer has to be, none. It is really a case of making extra work for yourself, rather than saving anything by adapting these things for use as part of a boat. The whole thing is basically a dud idea, imo. If you think you are getting a cheap boat out of it, you are right, it will look cheap, and when you go to sell it, the price will need to be cheap to attract a buyer. And the safety of it is suspect at best, even if you manage to keep it all together structurally, which would not be easy, but puncture or split one or two of those old drums, and it turns into a menace. Make a proper pontoon boat, not one you made only because you couldn't resist some cheap or free materials, of dubious utility.

 

To be frank, you don't want to know the hydrodynamics of your design, from an efficiency stand point. Starting with using cylinders and ending with quite blunt bow, well, it's going to plow pretty good, to stay the least. The only advantage of cylinders or drums for a set of hulls is their cost. If you have to make these hulls, you're much better off shaping them more appropriately, with the goals of the boat, rather than what's available.