Tritoon step hull design

[Cnhman2008]

Hello,

I have been tossing around the idea of adding ventilated steps to my 24' tritoon and have a few questions i haven't been able to answer through personal research. The goal here simply is speed and efficiency. I know its a pontoon but it has a 475hp outboard and i would like to try to set the record for fastest single engine.

1) how big do I make the step? I read somewhere that 1 to 1.5 degrees from the front of the hull should be sufficient and the step should be 25% of the lwl forward the stern. That would make the step 3.14 deep and 5 feet forward the stern.

2) Should the step angle back down like the red line or straight back like the green one?

 

[Cnhman2008]

 

[Cnhman2008]

A view from the stern should the step go straight back.

 

[Mr Efficiency]

I can see how just taking a "green" slice off it, gives the sectional shape pictured, but how do you arrive at the "red line" slope ? If these are cylindrical tubes, it is not the same game as a vee-hull with steps. Off the top of my head, I would say it more likely that adding runner planks to these pontoons would be a lot easier, and get it out of the water more.

 

[Cnhman2008]

I can see how just taking a "green" slice off it, gives the sectional shape pictured, but how do you arrive at the "red line" slope ? If these are cylindrical tubes, it is not the same game as a vee-hull with steps. Off the top of my head, I would say it more likely that adding runner planks to these pontoons would be a lot easier, and get it out of the water more.

I’m not sure where the red line idea came from.
Runner planks? Like lift strakes?

 

[Mr Efficiency]

Like this, plus side strakes.

 

[Cnhman2008]

I considered something similar. Like giving it a pad. But i thought it would increase my wetted area possibly creating more drag. That’s when I figured a ventilated step to a flat pad would not only reduce my wetted area but also add air lubrication.

 

[Mr Efficiency]

It will reduce your wetted area, by creating more lift, raising your tubes higher in the water, I really don't think a cylindrical tube lends itself to steps. Of course, you will firm the ride up, as a consequence. The placement of the side strakes would be a matter of nice judgement.

 

[Mr Efficiency]

Pontoon boats are rather less common in my part of the world, but I would wonder why a runner plank isn't de rigeur, it seems a very obvious way to improve the planing performance.

 

[Cnhman2008]

It will reduce your wetted area, by creating more lift, raising your tubes higher in the water, I really don't think a cylindrical tube lends itself to steps. Of course, you will firm the ride up, as a consequence. The placement of the side strakes would be a matter of nice judgement.

The fastest single outboard is currently running a step hull. To my knowledge it is custom non production. At wot the side logs are barely touching the water they act similar to sponsons.

 

[messabout]

round configurations of the pontoons are appropriate for displacement operation. The round planform is a miserably inefficient shape for higher speed or planing performance. Adding a step does nothing much to improve the lift characteristics of the round sections.

A loose description of lift force goes something like this.........(Rho x V^2 x A x Cl)/2 Rho is the the mass density of the medium in which the lifting surface is operating. In this case water. Mass density involves the Slug unit. enough to say that for water use 1.9 for rho. V is velocity in feet per second...Square that number...........A is the area of the lifting surface in square feet. Cl is the coefficient of lift for the supporting surface. In aircraft wings and other lifting surfaces the angle of incidence plays in to the actual value of the coefficient. Too much angle and the stall regime is entered and the whole deal is off. For a strake attached to the pontoon lets use the coefficient as one. That makes it simple but not necessarily an honest assessment.
That gets us into the nitty gritty as follows.

The round section does not present much of a lifting surface because the dynamic force of the water tends to wrap around the round hull surface toward lower pressure at the water surface. The water is looking for a way to relieve the pressure. A flat surface allows the pressure to escape at the edges but it is massively more supportive than a round or considerably more supportive than a typical vee bottom. That is not all the complexity but that is enough for now.

If you put a half acre of horizontal strakes on the pontoons then you will almost surely go faster than the guy with the stepped hull unless his strakes are bigger than yours or that your boat is heavier than his. It all boils down to a given amount of lift per pound of boat. If going fast is your aim then lots of flat horizontal surface will be the way to go. Steps? Try the flat strakes first. You do know that the strakes on deep vee boats contribute to lift. They are not there just for spray knockers or structural reasons.

While exploring around on the web, I find that there are some guys who are making their tritoon go near 100 MPH. Of course they are using monstrous horse power to achieve that. I will wager that there are some other reasons too....like more efficient lifting surfaces.

 

[Mr Efficiency]

There are a few "planing" catamarans with rounded hulls not that different to pontoons, though from what I have seen, they were also skating along on the tunnel flat to some degree, but a common issue was a tendency to porpoise, as differences in trim angle did not sharply alter the lift characteristics, as they do with a hard chine boat. You can see something similar, but to a lesser degree, with chine hulls that have no "flat" at the chine.

 

[Cnhman2008]

You guys know way more than I do.
So here is my current strake positions from the manufacturer. And proposed flat area in red.
If speed is my main goal I would only need to extend them to the forward boundary of the planing surface.
My though is to make these from composite material first so I can attach them to the existing hull and trial them before redesigning entire new hulls.
Obviously flat pads with 90 degree sides are not ideal for turning stability at speed. Maybe add some strakes? Thoughts?

 

[messabout]

The vertical parts of the pads will help the boat "hold on" when turning at speed. The boat will probably skid less than it might when using the round sections alone. Experimenting with the box like appendages is a good idea that will not permanently change the original boat if the boxes are made removable.

You need to close the the front end of the boxes so that water will not enter the boxes violently when at speed. Front end transition walls will not be geometrically difficult. The transition can be a long slender vee shape coming to a point at the lowest part of the round section.

 

[Ad Hoc]

What is the length of your hull..and what is the displacement?