Flat Pontoons

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.

 

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......

 

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.

 

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?

 

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?

 

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

 

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.

 

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

 

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?

 

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?

 

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.

 

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.

 

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?

 

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.

 

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?