pontoon houseboat questions

Hi,
I am planning to retire to a houseboat, build it myself..but
I will stay in protected waters, intercoastal waterway, rarely under power, no need for speed or planing. 10 knots would seem more than fast enough. I have a source for 24 foot 20" alum pontoons and structural steel building studs. i am envisioning a 16' x 34' boat w/ a 12'x22' cabin. Using 5 pontoons, 1 center and 2 staggered on the sides (so they creat a 34' length). I calculated them for 50% floatation to support 8,000 lbs. Using the steel studs for the walls and ceiling. I expect to be able to maintain the 4 tons of weight as I don't have much stuff to take with me. My questions are:
1. How do I calculate the size of motor, prop & pitch and rpm's required to push a 5 tube pontoon boat that weights 4 tons at 5 and 10 knots at lowest rpm?
2.Would 2 small outboards be better than one large as far as cheap and slow movement goes?
3.With the staggered tubes in mind, is it better to put the outside tube towards the stern to allow for better turning?
4.Are my floatation calcs accurate? pi x rad squared (a) 3.14 x 100 = 314"area (b) 24' x 12" = 288 linear " (c) 314 x 288 = 90432 cubic " (d)1728 cu" per cu ft (e) 90432 / 1728 = 52.3 cu ft (f) 52.3 /2 = 26 cu ft @ 50% capacity
(g) 26 cu ft x 60 lbs = 1560 displacment per tube (h) 5 tubes x 1560 = 7,800
5.Gee, I better find out how much those tubes weight....
Any other suggestions?
Thank you

 

Structural steel studs will not last very long in a boat, especially the ICW or other saltwater areas.

You can calculate some of the basics, but you have quite a bit of understanding to absorb, before you should attempt a boat on as grand a scale, as your vision currently suggests. For less then the price of one of your pontoons, you could have a design, with the engineering already worked out for you.

 

It would be a good idea to stay away from designing the boat around the pontoons in any case. Rather, decide on the hull type and material based on intended use. A barge type hull will be far better than pontoons for many reasons. The tophamper will be reduced, lowering the center of gravity, the reserve bouyancy will be far greater, maintainence will diminish with less exposed hull, you'll have a higher payload, and probably higher fuel efficiency.
The pontoon design you mentioned would also be quite noisy at night I think.
Sixteen feet is pretty wide too. You might get a flat-bottomed barge type hull down to 11 feet or so on a 34 ft length. This will help with efficiency too, saving fuel. Yet it will be more stable than pontoons because pontoons begin to lose bouyancy as the water reaches the upper half of the tubes. Not so for a purpose-built hull, which might have not twice, but five times the reserve bouyancy to rely on.

Alan

 

Alan has pointed out just a few of the "bits' of understanding" I mentioned above. Most of us start of with a displacement target, which is a general figure based on rough weight calculations, in an "everything and everyone aboard" situation (of course with a reasonable reserve element tossed in). This displacement goal permits you to evaluate hull(s) volume, elemental volumetric efficiency and basic scantlings for the structure. Then you "widdle" up or down to suit your desires and/or requirements.

 

Thanks Par & Alan,
I saw a post yesterday w/another member who has a cheap barge to sell. I don't have enough money and expect to do as much as posible my self. A good design though is well worth the effort. I am very interested in having a design from an experienced seaman who understands that when I start I have no turning back and only one shot. "Always jump with both feet" .Par please send me an email and I will fully describe what my vision is. If you feel comfortable with my explanation, please reply with an estimate for how much a design would cost. Thank you for your efforts to keep me afloat.

 

That would be me I still have the hull.
24' X 20" pontoons are "light weights", designed for weekend pleasure pontoon boats. Not even close to the displacement needed for "living" on a houseboat. Remember, living on water is much the same as on land. The day by day necessities are almost identical. PLUS, some added systems. This requires a large displacement platform.