Power and speed of a HUGE pontoon boat

I'm looking at a set of 50' aluminum pontoons and 16' wide platform that at one time was the base for a houseboat. The pontoons are rectangular in shape and appear to be approx 28" wide and have about 15 degrees of deadrise at the transom. Currently there is no power but it was an i/o in a center engine pod. The pod is missing.

I would like to use this as a pontoon boat with just a small basic helm. Just like an island I can move where I want for the day.

How can I figure out if a hull like this can be made to CRUISE at 15 knots with twin outboards? And if so, with how much HP?

Thanks, Dennis

 

How much will it weigh when you've fitted it out, without engines?

 

Yes, the weight is the main impediment.

 

Where the boat is now, there isn't a way to weigh it. I'm guesstimating on the following....

I looked at aluminum weights on line and came up with approx 1.25 lbs sq ft for the skin. Then erring on what I thought would be the heavy side using 50' x 4 sides and 3' of width for each side and a fifth "side" for weight the internal bracing, etc, I came up with 938 lbs per pontoon. Seems really light.

I haven't counted the 4" channels but they appear to be on 12" centers which would be 51. So by my calcs and using 2.35 lbs ft for the channel, the deck without plywood, would weigh approx. 2200 lbs.

Am I way off with my guesstimate here? Approx 4200 lbs.

And if I'm close and add another 1600 lbs of plywood deck and 3000 lbs of equipment, fuel, etc, (no "house", this will be an island for day use only) for a total weight approx 9000 lbs without power.

I would like to run twin outboards for maneuverability. Is it reasonable to want to cruise at 15 knots?

 

If it is only going to gross 9000 lbs, that is very light for 50 feet. Sounds way light.

 

That's what I'm thinking. Although I do see that the houseboat (Masterfab) weighed 18,000 lbs complete. So the "house" walks and roof with all of the furniture and appliances, heaters, swamp cooler, etc, would have weighed another 9k.

 

Ya, probably double that once everything is on.
And then there's people and their stuff.
15 knots is going to mean two 150 - 225 HP.
That's 800 - 1300 lbs in motors.

I was on a 60' x 40', two level, open barge last weekend with 20 people.
We were making 2 knots with a single 9.9Hp outboard in calm seas, no wind...

What is the over-all shape of your pontoons?
Pictures?

 

I just looked at a new 24' Suntracker online and it weighs 2200 with a 115 hp outboard so maybe my guesstimate is not far off. The new 24 is half the length and half the width so multiply by 4 and you come up with 8800 and that's with engines.
I'll try to take a pic of the hull bottom and get some better dimensions.

 

Speed and weight are the two largest variables.
Why 15 knots? Distance to travel is large?
If you could drop the speed you'd save weight in engines and fuel and spend a lot less.

It's also the shape of the pontoons, especially at speed.
Are they round in section (profile)?
Flat ended or tapered?
18 degrees [EDIT should read 15 degrees] deadrise: from the horizontal in side view or end view?

 

My current boat, a 1990 Tiara 3600 Open, cruises comfortably at about 15 knots. This puts our favorite anchorages less than an hour from our Marina.
The hull is not a round section pontoon. It has flat sides and an approx. 15 degree deadrise bottom at the transom.

 

Oops, sorry, my mistake 15 degrees, not 18.
Bottom flat then?

 

For-what-it's-worth, my houseboat is 25 feet long and 10 feet wide and weighs 12 000 sailing.
Did 5 knots when it was 8000 pounds with an 8Hp 4-stroke.
7 knots with a 25Hp Merc Bigfoot at 12 000 pounds.

We need more info. Pictures can be very helpful if they show enough.
I'm still wondering what the bow of these pontoons look like, square?

 

Not the best pics but hopefully enough until I can get back to the boat.

 

Going on the marine growth, and your dimensions, the displacement does appear to have been in the 20,000 lbs region. Assuming that was without the central pod. What is noticeable, is the lack of clearance.

 

These hulls are narrow and deeply immersed, hence they will be 'displacement' type hulls - they certainly will not be planing hulls.
However with a hull beam of 2'4" and an overall length of 50', your hull L/B ratio is greater than 20; your average displacement boat has an L/B ratio typically around 3.
This L/B ratio defines the maximum speed that an 'average' displacement hull can achieve - typically around 1.4 x the square root of the waterline length.
However with your skinny hulls this rule does not apply!
We built a 50' x 16' ally power cat where the hulls have an L/B ratio of approx 15, with the waterline beam being just over 3'. However these hulls were optimised for less resistance at slower speeds, with a lot of rocker on the hull bottom, as the initial engines installed were just a pair of 70 hp outboards. She still managed to achieve 16 knots at full speed with these engines and approx 8 people on board. She now has a pair of 150 hp outboards, and with these she achieved 25 knots at full speed.
Here is a photo of her for reference.