# Efficiency: Hull interference vs length on catamarans

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jun 3, 2021.

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### Ad HocNaval Architect

Dustman

As you can see from the replies you already have those that do not understand the hydrodynamics of hulls creating misdirection and subversion based on little knowledge, merely to make a post to what end...who knows, as you have already
stated:

And then of course:

Only because they do not understand the question, yet continue to post, again, for what reason....who knows.

It is very clear and obvious you have going through a thought experiment to establish a variable, if any, that has an influence in what you are 'considering'.
As such, the mention of actual cruising speeds in reality or crystal ball talk, highlights their lack of comprehension of the question and subject matter at hand.

And this is now where you are deviating. The above is no longer a single binary question but a multi-varied one which has many elements to it. And a such, you are going into the "design" aspect of the question. This is nothing to do with your original question of:

As this is a simple case of L/D ratio. That's it.
The higher the l/d ratio the better the hull...and to be 100% clear, it is NOT greater speed as posters with no knowledge are stating.
The higher the l/d ratio the hull has less resistance for the given displacement, for that hull with increase in length. It is that simple, as also noted here, and by many other authors, as an example:

Although it is not clearly stated in your posts, one 'assumes' you are talking about a catamaran hull...not stated in your original question.

Firstly no idea where you get the 2 1 ratio from.
Secondly, this goes into your question of:

And the DESIGN of the vessel.

Design is greater than the sums of its individual parts. By that I mean, if you wish, as you started, by looking at one variable on a hull, for a hydrodynamic cause and effect, a single variable of a single attribute, of which there are many that became a finished design, then an increase in l/d ratio is better. It is always better. But when you are designing, your constraints are no longer a single variable. Thus what you end up with as a DESIGN may not fall into your preferred l/d ratio nor will it fall into the "optimal/efficient" l/d ratio or hull separation. Using your terms for understanding which is 'greater than' or 'more than'.

Because a successful design is one that meets the SOR. And it is clear your SOR is at variance with attempting to find/get a hull to sit in the 'sweet spot' of hydrodynamic resistance. It is a classic case of... never the twain shall meet. There is no design out there that hits every sweet spot for every attribute - it is impossible. But hitting the sweet spots of the SOR can be done, but with compromises of said other individual attributes. Unless of course you SOR is a single line entry that states, Hull = XX tonnes, and max beam of YY, what is the best length? Then it is simple.
But as soon as you introduce other variables, that premise no longer holds water, as there are other competing elements that come into play... that's called design!

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### Mr EfficiencySenior Member

The fact remains, that the real attraction of a displacement cat, at this size, is the ability to cruise happily at speeds that displacement monos, or planing boats, can't, that is in the mid-"teens", mph, so unless that is the target speed range, why dally with a catamaran, that requires two engines for a start. In any case, he hasn't got any guidance about the penalty of closer spaced cat hulls re resistance increase.

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### fallguySenior Member

...ceteris parabis @TANSL

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### fallguySenior Member

6-7 knots is a displacement hull, but you don't get to go faster

if you want to go faster; you need semi-planing/displacement hulls

@dustman

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### Ad HocNaval Architect

I beg to differ.
With an l/d ratio of 7.5, you can easily do higher Fns, speed is not relevant in that sense. The only dominating factor at higher speeds become the WSA and hence an increasing frictional resistance...but if it is accounted for in the Design, then it is not an issue.

Just look at endless high speed cat ferries, they are all displacement hulls.

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### fallguySenior Member

I was referring to the boats he presented, but I appreciate your reply. How can a displacement cat of length 30 feet achieve speeds of 15 kts?

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### Mr EfficiencySenior Member

cats that are displacement hulls can easily be driven to speeds well beyond the typical constraints of displacement monohulls, till frictional drag builds to the point you'd be better in a planing hull.

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### dustmanSenior Member

This is simply not true, there countless examples demonstrating this.

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### fallguySenior Member

Again, referring to the two options presented, I am not aware of a 30' displacement cat making 15 kts.

Glad to be wrong. Show me the boat.

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### dustmanSenior Member

Because the cat I am proposing will have hulls with a ratio better than 20:1

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### fallguySenior Member

A 30 foot cat to be at 20:1 would be 30/20 or 1.5' wide @ waterline or sort of knifelike...not sure I've ever heard of a knifelike displacement cat making 15 kts. Let me know what boat does. Glad to be wrong.

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### Mr EfficiencySenior Member

The 20:1 lends itself to a lovely L/D ratio resistance number, but stiffness in pitch will go AWOL

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### Ad HocNaval Architect

What you need to do, is look at the numbers in the sense of the hull.

So with a length of 24 feet = 7.31m in my language.
A displacement of 2000lbs = 907kg or 0.91 tonne in my language.
Thus the l/d ratio is 7.31/0.91^1/3 = 7.5
A speed of 15 knots = 7.72 m/s - Fn = 7.72/sqrt(9.81 x 7.31) = 0.91

Now referring to the graph posted above, you can see with increasing l/d ratio the resistance and notable, the hump resistance reduces.

So for a hull to go beyond Fn 0.5, the waterlines/buttocks need to change, in simple words, the aft end of the hull needs to prompt a flow of water aft and as horizontal as possible, to achieve the "clean break" at the transom.
A 'conventional shaped hull' will not do that..no matter its l/d ratio. The buttocks encourage the flow of water to go 'around' the hull and back up to the transom. As the water goes 'around' the deepest part of the hull and then up to the transom, the water is accelerating = lower pressure. This lower pressure 'sucks' down the stern of the hull - commonly termed 'squat'. The faster and faster you go, the greater and greater the squat. And coupled to this a massive increase in resistance.

This is why "conventional" hulls can't go beyond Fn 0.5. Their hull shape does not allow it.
It has nothing to do with the displacement.

So, if you now have a hull where the buttocks prompt a flow aft and cleanly, there is no longer the high negative pressure aft, as thew water flow breaks away cleanly. This can be seen by a "ventilated" transom. So when at speed the transom is clean and has no water on it. The flow of water is directly aft.

Thus, if the hull form, has buttock lines that are "conventional" in nature than it shall be limited in its speed as the increase in resistance to Fn = 0.5 and beyond is grossly exponential to the power of 7th.
Whereas, if the hull has Lines that prompt flow aft and break away cleanly..it can go beyond the prismatic hump with ease.

That's it in a nut shell.

Also noted HERE in a tad more detail.

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### fallguySenior Member

Is there a real world 30 foot cruising cat that makes 15 knots?

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### Mr EfficiencySenior Member

You mean a sailing cat ? There would not be any room in the hulls for accommodation for one optimized for speed.

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