Power and speed of a HUGE pontoon boat

Dennis,
So, if that's the stern and bow view of the pontoon (you didn't say),
then it may take a lot to make 15 knots.
The stern should look like the bow, it doesn't.
I'd settle for a couple of 75 - 125's and enjoy the 8 - 10 knot ride.
Or, a couple of 25's and see what she'll do.
It's a party barge, no? So enjoy the ride.
Why rush?

 

With our Tiara, we are the mothership for our kids and friends to raft their ski boats too. Already, by the time we get back to the Marina 9-9:30 pm) they are all back on their hydrohoists and gone. Any slower and my deckhand/wife may mutiny.

How do you calculate hull speed on hulls this narrow? And could lifting strakes be added?

 

Corrosion might be a factor here, inside and out.

 

I think that one of the main sources of drag re the cat in your photos will be those deeply immersed transoms - they will cause a lot of turbulence and form drag. The deep forefoot at the bow does not help either - some rocker at each end would be better.

 

I don't think the transom is a problem, unless the cruise speed is low, then it will cause excess drag. But the weight is the key, if wanting a lively cruise of around 15 knots.

 

How thick is the aluminum, do you know?
Relevant to weight and level of concern around corrosion.

Hull speed... 47' loa by ~28" that's about 20:1, nice.
So, there is no hull speed to speak of.

But those vertical transoms suck... literally.
If they were shaped like the bows it would rule in displacement mode,
which is all it's ever going to do without HUGE horsepower.

By the sounds of it, 225's at 650 pounds each, plus fuel and tanks.
$40k for the engines and who knows for the midship mounted jumbo fuel tanks.

Not sure those pontoons are going to meet your needs... economically.
How about a couple inboards in the pontoons?!

 

You're not going to have anywhere near the weight of the previous houseboat waterline. I wouldn't be surprised if you reach your goal with some modest power.

 

If he keeps the weight down, yes. Gotta say that alloy worries me, corrosion wise.

 

Can a surveyor check for corrosion? This has been fresh water all its life and appears to have about a bazzilion coats of paint on it.
My though was to add outboards to the rear of each pontoon. Would it be better to mount the outboards inboard of the pontoons and modify the ends of the pontoons to taper them in and bring the bottom up?

 

It really is all about the final weight, including passengers and their stuff.

I drove a 28 foot pontoon boat commercially for five years.
It had a Honda 225hp on it.
Empty, it would do 22 - 24 knots WOT.
With 12 passengers about 12 knots at 3600 RPM.
Three, 24" round pontoons with similar ends to what you have.
All aluminum construction and only 10 feet wide.

Yes, a survey would reveal the level of corrosion and a number of other things.
This would be the first step anyway.
The owner should pay for half the survey whether you buy or not.

"Would it be better to mount the outboards inboard of the pontoons and modify the ends of the pontoons to taper them in and bring the bottom up?"
Yes, but I'd only use one o/b. Cheaper, lighter, faster, better.
A sound box could go over it for added comfort on deck.
Put a little o/b on the front if you want a bow thruster.
The tiny ones spin 360 and can even be used as a break but you need a crew member to operate it.
They have their own built in fuel tank.
I have one on the bow of my houseboat, brilliant.
But you won't have the windage so not necessary IMO.

 

Now I'm starting to think I might be better off with new pontoons. No corrosion issues and I could order exactly what I want.
I would probably go with 40' with a 34-36' deck and a 13' beam. A little smaller/lighter but less expensive to dock and still huge.
I would still go with twins because I might not always have someone to man the "forward engine", although the idea is interesting. Maybe if there was some small motor that could be controlled remotely. Electric steering?
I don't really want round pontoons because I think v-bottom ones will lift some especially with the right bow shape.
I also read on boatdesign about a possible lifting foil between the hulls? Something near the longitudinal CG? Is this worth pursuing? Is this similar to the "Waveglide" third pontoon I've read about?
Or should I even be thinking about planing? Can a properly designed boat like this get near a 15 knot CRUISE without running at WOT in displacement mode?

 

The beauty of long slim catamaran hulls is that they don't need to plane to run at that speed, you can get a wide range of effective cruise speed, unlike most boats, but that 15 knots is well within it.

 

Yes, in fact likely only in planing mode.
Dr Tom Speer might have something to say about catamarans planing... they don't.
Especially if you're 20:1 l/b ratio.
What's happening here is a lot of guess work and that is not a productive way to look at boat design.
Before you spend a bunch of time and money guessing,
figure out what you want this vessel to be able to do,
what you want to do with it and how the vessel will do it.
It either will, or it won't, there's no guess work.
And, you'll get what you payed for, nothing more and nothing less, just what you want.
Providing you were right in the first place about what you want.

The chances that your existing pontoons and deck are going to fit what you really want/need/expect are slim.
I have a feeling a production pontoon boat is going to meet your needs.
Certainly worth looking into.

Oh, and you don't need twin screw or a bow thruster for maneuverability, even dockside.
You've got directable thrust in an outboard and very little windage as it's a flat deck.
Even in a gale I docked the pontoon boat in off dock winds by simply T-boning the dock,
tying off a slack bow line and powering into it with the wheel hard over.

 

That is simply incorrect, 15 knots is no problem, and no dynamic lift required, the world is awash with high speed cat ferries these days, and they rely on the reduced wave making properties of slender hulls, to do it.

 

Mr E, you may want to reread my post.
You got it backwards.
Read the question and then read my answer, a couple of times, until it sinks in.
Okay?