Flat Pontoons

[jdworld]

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!

[gonzo]

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.

[SamSam]

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.

[jdworld]

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!

[gonzo]

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

[marshmat]

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.

[jdworld]

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?

[marshmat]

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.

[jdworld]

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.

[SamSam]

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.

[WoodenPontoon]

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.

[troy2000]

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....

[jdworld]

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!

[SamSam]

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

[SamSam]

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/

[jdworld]

It's gonna be a houseboat. Sort of a small houseboat, but still will all the stuff - kitchen, bath, etc. im actually trying to calc the weight right now. Item by item, material by material. Close to a grand total though. I calc'd the pontoons based on no more than about 6k of weight. That worked out to about 8-10" of draft, if I did things right.

[jdworld]

Those other plans look similar - but I notice they have a vee shape on the bottom. Not an option for mine - gotta be flat. And even with the vee, I think they said something about it only going 8 mph. Of course that's relative to the motor size. But then again, another thing I've heard and been keeping in mind is that I guess there is a point with a boat planing that it doesn't matter what size engine you have on there - if it doesn't plane it doesn't plane, right? But theirs looks like it would go faster with a bigger engine than the photos show. Esp with vee bottom pontoons. But then maybe weight is an issue with the size of their pontoons and all it can handle is a little 15hp or something. So many variables......

[PAR]

You do realize that a heavy, shoebox shaped boat will need lots of horse power to get up and plane. Lets say you hit your target at 3 tons. Add another ton for fuel, people, Fidel the wonder dog and a couple of coolers full of beer, so 4 tons. How may hundreds of HP do you plan on tossing at this girl? Making 4 ton houseboats get up and scoot takes lots of power. I'm just wondering what your HP ideas are.

The reason these types of boats don't have big engines is they can't get up and plane without hanging the engine from an F-16 on it butt. Rather then focus on flat bottom pontoons, focus on how light you can make it. Forget about 2x4 studs, 2x8 joists and 3/4" plywood siding, it'll never be light enough to plane (okay maybe with 500 HP you can get it over 10 MPH).

I'm not trying to dig on you, maybe just redirect your energy toward building as light as you can, so you have a chance at planning without needing four 250 HP Black Max's hanging of it's stern.

[jdworld]

No, this is great info - no digging taken - bring it all on because you guys are the experts and all i have is my intuition. So yeah, weight has been my primary focus throughout so far - although i had been designing under the assumption of getting up on plane. That will really suck if it can't plane, and I'd prefer to not build the whole thing and then find that out.

So the tech stuff... just figured it up to be about 5800 w/o peeps. With peeps and beer maybe 6800. If I calc'd things right, 6k gave me 8" draft with (2) pontoons 30" x 22' pontoons. So I might be at maybe 10" or so now? I have 24" to the underside of the deck, so I really don't wanna get to the 12" draft mark if I can help it. There's stuff I can cut if I have to but it won't make too much difference - maybe 500lbs. And yes, it's all using the lightest thinnest construction and material's I could think of.

So nevermind the draft, at that weight what size outboard might I need? I had hoped to be able to use a 90hp. Not gonna work, right?

[PAR]

Nope not even close. To get to 25 MPH you're going to want to be around a ton, total weight including fuel, supplies and crew (with 90 HP). What speed would be acceptable for you as the minimum? How are you building the deck, walls, interior furniture, etc?

[jdworld]

oh great. well, getting from 6k down to 2k is going to be pretty much impossible - unless i ditch the houseboat portion of it! I might be able to shave off 1k at best. I don't really have a min speed i guess. I'd like to get from point a to b without feeling like i'm pushing water foremost, and i'd like to get there faster than a kayak. I don't know, 10-15mph would be nice I think. So i guess efficiency and feeling like i'm up and gliding not plowing is bigger to me. I'm not too concerned with how fast I glide.

How are you calcing that 25mph at 2k weight? Is there an easy formula (ie rule of thumb) I could use so I can play around with the numbers? If I have to step up the engine I will, but then the weight goes up too. Looks like a Honda 90 is 365lbs. If I go up to the 115 - 150hp Hondas their all around 500lbs.....so I would add another 135lbs to upgrade the engine. Not sure how that plays in. I'm also wondering if 500lbs is too heavy for the back end and will sink it down? But I guess shifting more weight to the front probably evens that out.

This thing is sort of a comparison, although much longer at 32'. But they are quoting around 20mph with a 115hp. Not sure how many people they had onboard for that test. Longer pontoons, round, about 1200 lbs lighter unloaded - guess that all factors in:

http://www.suntrackerboats.com/boat/motors.cfm?boat=3033

[jdworld]

btw you asked about how i'm building stuff.......as much aluminum as possible. Interior furniture marine ply unless i can find something as strong that doesn't rot or warp and is lighter. Same for deck. Walls 2", probably luan interior and fg siding.

[SamSam]

This one apparently goes 15mph with 4 people and a 30hp motor.
http://www.svensons.com/boat/?p=MechanixIllustrated/BudgetHouseBoat

[PAR]

The 33' 6" Sun Tracker you mention weighs in at 2,25 tons (400 lbs. load), which gives you an idea how over weight your estimates are (2010 figures, 2009 are different). Their "shipping" weight figures, printed in the brochure, are a bit light. Just a full fuel tank adds 250 lbs.

I preformed no calculations for the speed estimate, with available power, but after several decades at this sort of thing, you get a feel for stuff. I intentionally estimated low, so if you did develop and run with similar spec's you'd be faster, pleased you'd "beaten the curve"

It takes a good bit of engineering to develop structures that are both light and strong. A taped seam 1/4" plywood box will hold up seated guests. How are you making the seats? Are they heavier then a 1/4" plywood box with filleted and taped corners? Would this be the trend around the rest of the boat?

[jdworld]

i'm not understanding...are you saying you think the 6000 weight estimate for mine is over, based on theirs being 4500? If so, I hope you are right - that would be great. But I kind of feel i might be under. I meticulously went material by material, item by item, researching weights online and two days came up with 6k. Plus I have some heavy stuff going on - big refridge, genset, 2 batteries, etc. But either way, even if I'm correct with my weight at 6k, based on that suntracker and your experience, do you think with a 150 on the back I would plane and maybe get 20mph?

As far as the cabs, yeah I based everything on 3/4". Pretty beefy I know, and yes 1/4" if put together right would probably work fine for the vertical planes. But with all the moisture around a boat, I would worry about warpage in a couple years, even with coat of paint or whatever. Doesn't happen?

[PAR]

A 33' boat should be easily twice as heavy as your boat, meaning your are several times heavier then necessary. The 1 by stock for cabinetry suggest just this. No, I don't think you'll plane. I think you'll have hulls very deeply immersed, because you've under estimated full up weight and a big block Chevy with big HP and heavy reduction, might get you up on plane, spinning a big wheel, but an outboard will likely just turn water into foam, attempting to push you fast enough.

[WoodenPontoon]

There are many things that would reduce weight in your design. Consider building trusses for your carrying beams that tie your pontoons together. These could be made out of lighter wood such as cedar. If your then put a light weight skin on the truss faces (1/8" plywood), your trusses would be super strong and have very little weight.

Have you considered commerically available furniture? Much of this furniture is roto-molded plastic and tends to be much lighter than a plywood equivalant.

You should be looking to save every pound that you can. 5/8" plywood decking in stead of 3/4" will save 1/6th the weight of your deck and will be strong enough to serve its purpose.

As you start oufitting your design, you will gain more weight than your initial design accounts for.

[jdworld]

OK OK i got it down to 4700lbs w/o people and gear! Now don't ask me to go any lower. I am not giving up my dishwasher, 1000w generator, and 7.3 cf refridge!

Actually that 4700 is with all that kind of crap on board, as well as 35 gals of water, and 15 gals of gas, full propane, full toilet tank, etc. Worst case scenario excluding people and beer.

Still probably not enough though, right?

[PAR]

The load should be full up, crew, guests, Fidel the wonder dog, cooler full of beer and the dancers on the fore deck. A boat that size can easily handle a ton of well fed friends, putting you in the 6,700 pound range. Don't lose the equipment, pare down the decorative surfaces, consider sandwich construction walls and roof, etc.

[jdworld]

Yeah i wanna make the walls as thin as I can get away with. However, consider there's a big sundeck roof where the babes are gonna hang out - hopefully more than one. So what ever I rest the roof on can't buckle or flex due to thin ness. What exactly is sandwich construction for walls?

[jdworld]

Also - I am just having nightmares about spending thousands to build this thing, only to find it's a slug. I want to model it as much as I can upfront. ANd maybe literally. Here's where it gets into some physics that I clueless on. If I built little scale versions of my flat pontoons and deck (maybe 1/2" = 1'), and then place weight around on it to simulate the motor, cabin, refer, tanks , and other heavy stuff - will it behave and float in water the same way the full size version would. Will the buoyancy and displacement behave proportionately the same when things are that small?

And then what kind of wood would I use to best simulate a hollow aluminum flat pontoon?

[jdworld]

.....and if the floatation test goes well i might even take it a step further and hang one of these babies on the back and see if she planes! :)


http://www.hobby-lobby.com/images_templ/swap-images/gr2370_xlg.jpg

[PAR]

No offense, but houseboats are slugs. It would be a very rare exception for one to plane, unless is was quite small. You're limiting the design further with pontoon shapes, which requires you to use even lighter weight structures, if you want them to plane.

I guess my point is there is a lot more then meets the eye to this yacht design thing. Many compromises go into each decision and you're just beginning to see some of these difficulties. To have a houseboat, as you've described it and get up an plane, you'll need considerably more "clever engineering" and then it's just an outside shot, unless given enough power. Theoretically, you could hang several large HP outboards on it's butt and away you go, but these ain't cheap to buy, install or operate. Have you priced out 250 HP outboards lately? How about two of them?

A better approach to your problem, would be a skiff or Jon boat hull, instead of the pontoons. You'd be able to carry much more weight and you'd have more area to help generate lift, so you could plane.

What you need is the help of a professional or very experienced amateur designer. Designing these types of structures (strong, light and able to safely have a dynamic relationship with the water) isn't for novices, at least not at this level. A 10' fishing skiff, sure, but not boats this size. There's just way too much to think about, engineer, develop, understand, the safety of loved ones, etc.

[jdworld]

yeah i know i'm pushing the limits of what a houseboat can typically do. But I 'm also part pontoon boat - so i'm comparing to those also. And in researching pontoons out there, there's some Benningtons for example that are about 5000 lbs and they put 150 hp engines on them (and ski behind them). Yes, round pontoons, not flat. But weight wise, not too far off from me. I'll have a big cabin on top, but weight wise still just as light (or heavy depending on your perspective). So I have that to go on. Also came across a company called Holiday Mansion that used to build them. Their's planed and did 30-40 mph. Videos on you tube. Flat wide hull, not pontoons, which I'm sure helped. And yes, they have twin v-8's. Another smaller HB called the Nomad which supposedly does 30mph or so, 5k with a 130 on the back.

So I there's round pontoons about the same weight that do 30+. Flat or shallow vee bottom HB's about the same weight or heavier that do 30+. If I could just get 20mph out of a 150 at 6000lbs with 2 flat pontoons I'd be happy.

I guess the one question is are they able to do those speeds because they get up on plane? I still don't exactly know what defines "plane". When a pontoon boat is going 30+ is it considered to be planing? (even though it doesn't have a "planing hull"?)

[narwhal]

We rented a pontoon houseboat on the Rideau Waterway several years ago; it was an older version of this http://www.threebuoys.com/boatsales.html , about 42' long, and while it was to underpowered for its weight to plane (120hp outdrive), its performance was acceptable. Our criticisms of the vessel were not due to its pontoon design.

I've seen some pontoon deck boats that have have aluminum angle (3x3?)welded longitudinally to the round pontoons to give more bearing to support planing.

[WoodenPontoon]

I've seen some pontoon deck boats that have have aluminum angle (3x3?)welded longitudinally to the round pontoons to give more bearing to support planing.

Those 3x3 angles are called lifting strakes and are designed explictly for that purpose. Most of the newer pontoons with larger engines have these.

[Stefan H]

Topspeed 15,5 knots !!
Size 20 x 6 meter
2 x 160 diesel engines
Glassfiber construction

Weight is unknown...

[Stefan H]

I guess the one question is are they able to do those speeds because they get up on plane? I still don't exactly know what defines "plane". When a pontoon boat is going 30+ is it considered to be planing? (even though it doesn't have a "planing hull"?)

With enough power any hullshape will plane but...

Have you considered making longer pontoons to maximise the displacement speed?

[jdworld]

Topspeed 15,5 knots !!
Size 20 x 6 meter
2 x 160 diesel engines
Glassfiber construction

Weight is unknown...


haha - that is hilarious! That's taking the word House - Boat to a whole new level. Interesting. Weird, but interesting. A wakeboarders dream boat.

[jdworld]

hmmmm.....i wonder if they ever get any water up over that bow?

[PAR]

It depends on who you'd ask, some here might suggest she's immersed occasionally.

[nightfisher]

I have a friend who owns a 1989 Misty Harbor (they only made the flat pontoon design for a couple years.)

It is an 18 footer wih a 90 yamaha on it. I runs and handles like a boat -- not a pontoon boat. It goes about 30-35mph with two people on it loaded with gear. Every once in a while there is a little spray coming over the front...but this could have been fixed easily by increasing the deck a few inches further forward.

All the best.
Greg
www.ugliboats.com

[ddrdan]

Hey JDworld,
You came to the right place for info. The Pro's on here were an immense help in my design and build process on a build almost similar to yours. PAR is a boat design God!!:):)

I'm a true novice but here's some tips from my mistakes:
Trailered or not? Makes a big difference in the design.

Your power requirements are not just weight related with a house structure on top. It's a dam sail with its own mind!!! lol And as aerodynamic as a tractor trailer. lol

I centered my outboard. I now wish I had put jack plates on both hulls and split the power.

Up the scale on your model. I started small and found it acted like a base ball in water. I found for weight loading tests you might as well just use total load centered to review how the design acts.

Loading the model with specific location weight did not give me true effects on the design. I don't know why, maybe i was doing something wrong, it just didn't work for me. Luckily I built the toons with a temporary deck, launched the rig, and located weight (5 gal buckets - water 8#/gal) on the deck for fixed weight items. I center loaded the super structure (houseboat) weight evenly down the length of each hull as it was going to be a uniform load port to starboard. That method gave me a much better idea of their effects.

I used some of the internal space in the toons for components like batteries, water tanks, etc. No fuel, trust me, you don't want to go there. I integrated the cross members supporting the deck into the hull bulkheads at longer spans and laterally framed between them for deck support. It reduced the deck framing weight by 8%. But I had to increased those bulkhead sizes. It 'almost' turned it into a catamaran style hull.

For the houseboat walls I used 1/4" ply and sandwiched 1.5" rigid insulation between. Fiberglassed the exterior. I fabricated a 2x2 T&G wood insert in the joints of the wall panels I built. I used luan on the interior so you can paint or stain. Another tip, after some research, I found 1/4" sub flooring is almost equal to marine ply and cheaper. And using that lightweight sandwich panel reduced my wall framing requirements.

I also added a 2 step HySuCat foil style chine to the inner sides of the hulls. Although my hulls are flat sides and bottom, not convex like the picture. A pro from another boat forum convinced me it would help. I wasn't wanting it up on plane so I'm not sure if it really did help, but it looks cool?:D
http://www.multihullsmag.com/magazine/articles/cat%20hulls/J1.jpg

Good luck!