# Tritoon step hull design

Discussion in 'Boat Design' started by Cnhman2008, Dec 7, 2020.

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### Cnhman2008Junior Member

The 3 logs are 24 feet long and 25'' diameter the center log and transom are mounted 1' aft the outer logs. Not exactly sure how to calculate displacement.

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### Mr EfficiencySenior Member

The displacement is the weight of it.

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Ad Hoc asked about the "displacement" of your boat. He is asking how much the boat and all of its content weighs. The term displacement is to tell us how much water the boat pushes out of the way when it is standing still. Archimedes, that old Greek, taught us about that principle. A cubic foot of water weighs 64 pound if salt water and 62.4 pounds if fresh water. Therefor we know that if your boat weighs 6,400 pounds it will push 6400/64 = 100 cubic feet of water out of the way. We can also calculate how much lift that a particular planing pressure might affect your boats speed.

Ad Hoc and several other of our highly qualified members can do that kind of hydrodynamic stuff with authority. ( Full Disclosure; I am not one of those elite members)

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### Mr EfficiencySenior Member

Whilst weight is essential to any calculations, it is not the whole story by any means, two boats can have identical length and displacement, and their planing behavior like chalk and cheese, obviously this thing planes, for some reason he wants to get more speed, and to that end, barring the ability to jettison weight, or install more power, it becomes a matter of reducing resistance by altering the shape of it, and largely by reducing wetted surface area. That is the idea of stepped hulls, but that entails pretty radical surgery for something like this, you could tack on some "plank" extrusions without destroying the thing, and it would be easy enough to return the thing to its previous state, if the results were negative in some way, but steps would make it much more difficult to revert to the original condition, and you could be left with an orphan lemon.

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### ondarvrSenior Member

Have a shop make a composite pontoon to replace the center one.

Its much easier to build the exact hull shape for best performance out of composites.

You can also modify a composite hull rather easily for any adjustments required to fine tune it.

I'd hate to be on a tri toon in an 80 mph chine walk. I could see if the outer hulls came out of the water it could be interesting when one catches a wave.

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### Mr EfficiencySenior Member

I question why you would want to turn a pontoon boat into a speed machine, I think it is outside the intended use they are designed for. I have visions of those dragsters topped with a thoroughly unaerodynamic old crate of a body, more for novelty value.

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### Cnhman2008Junior Member

Around 3800-4000 lbs loaded
I run a fabrication and machine shop so fabricating a new hull is possible. Also familiar with composite construction I have made some auto parts hoods boots etc also have done some surfboards and stand up paddle boards so definitely not out of the question.

I love fabrication and hot rods it is definitely my passion and i just bought my first boat last year.So now to incorporate all of my passions into one!

I'll be honest I guess its the hot rodder in me that wants to have the biggest fastest meanest whatever. Single outboard speed record is my first goal. After that I'll be on to twins and or triples.

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### DCockeySenior Member

Speed record for what type of boat? What is the current record?

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### Cnhman2008Junior Member

Outboard pontoon 78 mph

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### Ad HocNaval Architect

So that is roughly 7.30m in my language.

And that is roughly 1.80 tonne.

So your length-displacement ratio (LD) is roughly 6.0... which is moderate.
Your Hp/tonne is over 260.

Thus on the face of it...the "numbers" suggest it may be possible. Since anything over 150-200 HP/tonne is a lot of bang for your buck!
The LD ratio, whilst 6 is sufficient, it would suggest you may have trim issue as the speed increases...hence maybe the question about steps, perhaps?

Whilst a flat horizontal surface may be "ideal", it would be more prudent to be a slight deadrise in them. As for steps, that is not so easy to state.
Steps are used for a multitude of reasons and their size location and number off, is not a simple rule of thumb or one liner....thus it may be a case of suck it and see which turns out best for you.

There ahs been some good R&D noting, as you have, about 25% fwd of the transom.... but as noted, this data, like others, is based on a very very different hull form.

Your main issues will be drag and dynamic stability.

I would, initially forget steps, add a slight vee, around 3-5degrees and just see how the boat feels with increasing speed. Take simple measurements and gauge what is going on and how any mitigation can be done. Since there is no "play book" for this. The data you create may well be the first, perhaps, in that sense. Thus you need a data set to make any firm decisions, which currently stands at zero!

So taking baby steps and making proper informed, and safe, decisions, with each incremental step. (excuse the pun none intended )
That will then provide you with quantitative data - which this is screaming out for - and any feel for the safety and stability with increasing speed.

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### Mr EfficiencySenior Member

78 mph is perfectly fast enough to kill yourself, frankly this isn't really a game for neophytes or armchair dilettantes, anything that is likely to break speed records, will entail risk to life and limb. Wanting to go a bit quicker than the standard pontoon boat, and wanting to break speed records, are two different things.

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### Cnhman2008Junior Member

I have to start somewhere right?

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Mr E, I hasten to agree that going really fast on water is fraught with risk. On the other hand you Aussies are by all accounts, speed mad. Your Guy with the Jet boat is the most maniacal of us all. He is hell bent to make more than 300MPH. How 'bout those wild men with their Aussie 18s?

Just Joking with you Mr. E. G'day mate

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### Mr EfficiencySenior Member

haha, I must be more timid than those speed freaks, Ken Warby did something remarkable way back, but I think he supplied the bravery, and some boffin supplied the brains, that allowed it to happen, without it ending in tears, but I do worry the family is still pushing their luck.

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### Cnhman2008Junior Member

HAHA! My supercharger came from down under.

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