Flat Pontoons

hi, I'm working on a pontoon boat design, but for reasons i can't explain too much here, I need to go with a square shape profile for the pontoons instead of the typical round profile. The front end of each will slope upward, but will be flat like the rest of the pontoon. So no vee's anywhere. If I've figured things right, the tunnel height will be about 12" at best. I'm thinking dimensions might be about 30" sq for the pontoons and about 26' for overall length, set apart to create about 8'-6" overall width. Anyone have any feel or experience with what kind of issues am I going to face with this type of design? Thanks!

 

Like any flat bottom hull, it will pound more than a V. Except for that there is nothing wrong. The draft will be less than a round hull. Pedal boats are usually designed like that.

 

Assuming the deck is on top of the pontoons and you have 1 1/2 feet of draft, you will be displacing about 13,000 lbs. That's a lot of beer.

The 12" tunnel height can be an issue with waves slapping the underside of the deck, especially with exposed deck supports running crosswise.

 

Thanks for replying. That's encouraging - I had thought the draft would be a better too. And as far as the pounding, yeah, probably right there too although i'm thinking that being spread out between two 30" wide flat surfaces 6' apart it might not feel as bad as a full flat bottom boat maybe?

So here's the things that are jumping out to me as possible issues - but I could be (hopefully) wrong - and i'd love to get the opinions of some others on this:

1) MANEUVERABILITY? Like less maneuverable than round pontoons, but how much. Is it going to feel like a car on ice when turning? And if comparing a 8 x 26 flat bottom boat and a 8 x 26 flat pontoon boat, are they going to turn about the same? Seems like the pontoon would turn tighter but I don't know.

2) TUNNEL HEIGHT? For lakes and rivers, is 12" of height for the tunnel going to be enough? Could I get by with 8"? (if so i could get by with pontoons 24" wide pontoons). For instance, with 12" tunnel ht, driving perpendicular into another boats wake - are those waves going to be slamming the underside of the deck?

3) SPEED? This thing is not going to be pulling around tubers or anything, but I would love it if it could go as fast as a regular round pontoon boat with the same weight. I'll get more floatation out of a flat bottom pontoon, but what about all that surface in the water compared to round pontoons? I guess in theory, if a round pontoon has more draft, there may be almost as much surface against the water as a flat pontoon with less draft and therefore speed about the same in the end? Is that how it works?

Thanks!

 

If they weight the same and have the same lenght and beam, the flat bottom pontoon should plane easier and faster. I don't think turning is going to be much different

 

jdworld -
It's good to see that you're considering all these factors. I wonder, though, how much time you've spent on pontoon boats. Because, you see, many of the big manufacturers of these things don't seem to bother with the sort of stuff you're talking about.

The standard pontoon boat has a prismatic coefficient of 0.9 to 1. It doesn't really plane, it just bulldozes its way along using as much power as you put on it (although, being a skinny cat of sorts, this is still often fairly efficient compared to an overloaded planing hull). The bridgedeck height is determined, to a large degree, by how much room there has to be for the guy with the welding torch to get in between the crossbeams and the connections to the tubes. And so forth.

Recently there have been some more sophisticated V-hull pontoons on the market, but by and large, most of the fleet are basically party barges on floats, the design starting with the deck layout and the pontoons being selected from a stock pile to handle the expected weight of crew and beer.

So the fact that you're thinking through how your boat will work already puts you ahead of a fair chunk of the mass-produced fleet.

Now, as to your questions:

1) Handling on a pontoon boat, in anything more than a dead calm, is essentially dictated by windage. Round vs. square bilges is not going to make much of a difference.

2) A 12" bridgedeck will slap on the wakes of most planing powerboats 18' or larger. An 8" bridgedeck will slap on the wakes of smaller boats as well, and will give you a bit of a jolt if you hit a big wake when lightly loaded. This is just part of being a pontoon boat. You simply beef up the bridgedeck and stay out of anything stronger than a force 3 sea.

3) If you're fitting enough power to plane, flat bottoms will plane much better and give a higher cruise speed than round bottoms. If you're going to putter around at five knots all the time, round hulls will likely be more fuel efficient.

 

Haha - yeah, after scouring around here for any info on pontoons, it's pretty clear that pontoons are the red-headed step child of the boating community. But i'm going to change that and get the pontoon the respect it deserves! Actually, I dont really have any big love affair with pontoons, But you gotta admit it's pretty fascinating (and weird looking) when you see one pulling a skier.

That's REALLY good news to hear that a flat might plane better than a round! Thanks - That's exactly what I needed to know. I don't care as much about slow speed efficiency as I do getting from point A to B as fast as possible. And I'm not locked into this pontoon concept, it's just that for what I'm working on, it's either two flat pontoons, or a full flat bottom barge like hull. Which brings me to my next questions - two flat pontoons vs a flat bottom barge like "john boat" type hull. Between those two:

1) Which will plane better (ie get from point a to b faster)

2) With the tunnel ht at 12", both going the same speed, which is going to have a more "slamming" ride? It seems the barge might be smoother, due the entire hull being in the water and not able to be slapped by an airborne wave? right?

 

Expect more efficient planing, but a harsher ride, from a pure flat-bottom "giant jon boat" type compared to what you'd get from a square-hulled pontoon boat. When you're in waves, the hull isn't always in the water. Try to visualize how the water will flow when it meets the hull- a V hull cuts through the water, pushing it to the sides, while a flat bottom hits the whole wave at once in a single, jarring whallop.

 

Ok that visualization makes a lot of sense actually. A big wave getting spread between two narrow surfaces (the flat pontoons) to slam the water vs one big flat surface to hit the water. So then, more good news - two flat pontoons will ride smoother than a plain flat hull, and plane better than round pontoons. I like it! So now to solve the next prob - the tunnel height, and slamming effect.

 

I think the toons will ride softer, but a plain flat hull will get on plane faster, easier and will go faster using less gas. I think it will also turn easier. I can't really see where you will have 13,000 lbs of weight unless you're making a houseboat, and in that case a flat hull will have less draft for the same weight and the whole cabin structure will be close to two foot lower as your deck will be inside the hull, probably below the waterline instead of 1 1/2' above. A solid hull would use less plywood, but might have more framing.

 

Having just completed a wooden pontoon boat, I welcome and encourage your interest in giving the pontoon its deserved justice!

My hulls are round, so I can not compare them to the flat ones that you are planning. I am in the design stages of adding flat lifting strakes to my hulls to increase planing.

Have you considered adding a center log to your design (increased freeboard)? What about the addition of "belly skins" to cover the underside of your deck. These would reduced the slamming that would be encountered from waves.

 

I always thought that if I were going to build a pontoon boat, I'd scale a set of semi-dory plans to give me a max beam somewhere between 24-30", then just stretch the stations out to 20'. Build a pair of them with sealed decks, put a platform on top, and hang an outboard off the stern. Seems to me that couldn't be any less seaworthy, or harder to push across the water, than the round pontoons I see plowing along....

 

Thanks for the interest in my flat pontoon project. Still working away on the design. I'm sort of going on a wing and a prayer here. I see there's two conflicting posts about 2 flat pontoons vs a flat bottom hull. Would welcome any other thoughts on that. Although I somehow missed the 2nd post about 2 flats not being as good as a one big flat, and In the meantime I've practically got the whole thing designed now based on two flat pontoons - oops!

My overall goal is to get this built, but I am in no financial position to experiment with hulls. I got one chance to get it right. But there's a HUGE side of me that's just dying to see how two flats would do. I read a bunch of stuff on line about the efficiency of catamaran type hulls. But I guess when you're talking pontoons it's a different world. I guess with cat hulls, the idea is the air trapped in the tunnel helps elevate the whole thing when going fast enough, but a pontoon can never hope to go that fast for that effect to make a difference, and therefore they aren't any more efficient. My main thing is speed and planing. I've kind of solved the height problem. I just want it to be able to get up on top of the water. I absolutely don't want it to be too heavy to plane and sit there and drag. That's kind of the big question mark I guess. I have calc'd the displacement stuff out and think i've got that right. Which takes care of it floating at a standstill. But I wish there was some way to "model" the planing capabilities. For some reason, visualizing it, it just seems like with two big wide flat pontoons as I have now, it will lift up nicely and sort of slide across the water on the back halves of those two flat surfaces. As long as it's not too heavy. Lots of physics involved in this boat design stuff though!

 

How much do you figure it will weigh? Are you making a houseboat or just a deckboat?

 

Here's one sort of like what you're trying...
http://www.svensons.com/boat/?p=HouseBoats/PlatformBoat

From here...
http://www.svensons.com/boat/