# Would that catamaran ever sail?

Discussion in 'Boat Design' started by john5346, Apr 23, 2013.

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1. Joined: May 2010
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### JRDSenior Member

Its interesting to get a better view of the curved skegs or keels just below the waterline. It looks rather like they are expecting some degree of separation or "sub-optimal" flow at the kink and attempting to guide water flow around the corner nicely.

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### keroseneSenior Member

This to me proves that you are a troll or dense as a rock. Either way words are wasted on you.

If you want to learn, read book.

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### FMSSenior Member

"I think we have all preconceived notions of what a boat should be" is an accurate assessment.
Locating examples of different boat types which have flat panels and sharp angles is a fine exercise. Flat bottom fishing skiffs. Barges. Planing hulls. Drawing a conclusion from a small sample of photographs of different boat types without understanding each's SOR is incorrect.

The construction aspects have been thoroughly addressed earlier in this thread with words, animation, and photos.

Given the software skills demonstrated in this thread, a few days learning http://www.delftship.net/delftship/index.php/delftship/delftship-free and http://www.cyberiad.net/michlet.htm would provide the numerical reason "why not" for your specific hull.

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### john5346Junior Member

If you guys know the calculations that proves a curved hull is so much better, can you say it?

Write the calculations that it proves the drag resistance is that much lower.

"Although both frictional and wave-making resistance vary with Length and Speed, they do not vary in the same way. For over 100 years, it was realized that FRICTION varied with the wetted surface and the square of speed (SVĀ²) and mildly affected by length.
Although Froude did notice some irregularities in the results, the formula was used for over 50 years without much challenge and in fact, is still surprisingly good for boats under 50 ft."
http://smalltridesign.com/Trimaran-Articles/Boat-Resistance.html

Say the numbers, do the calculation, anything? Any calculation that proves a curved hull is so much better.

Yes, I should do it myself but you guys are saying you know that already, say the numbers? Thank you

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### Emerson WhiteJunior Member

You've been linked to the software that. for free, will run the numbers for you. Actually writing out the equations would take a non-trivial amount of time. If you are willing to pay someone a few hundred dollars to do it for you I'm sure that someone would oblige, but to appeal to someone as stubbornly wrong as you? Where is the profit in that? Even if they do it perfectly, chances are you don't understand 4 dimensional calculus and differential equations, and they have wasted hours of their life working hard to, by hand, analyze a hull that even a small child could see is very poorly designed.

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### DennisRBSenior Member

Remember this John?

You then rudely demand Leo should run the figures for you for free.

He has already helped you and everyone by developing this program and giving it out for FREE. But you still demand others do your calculations.

You have everything you need to get your own answers. No one else is going to bother as its a waste of time for them as they know the box will perform poorly.

You are seriously kidding yourself if you think someone with enough knowledge to write a fluid dynamics program like Leo does not know enough to tell you a pointed box will not perform well without proving it to you with calculations which you will not understand anyway.

Are you trying to get out of doing your own work for a school science project?

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### john5346Junior Member

You and most people say it is not a good design and that it will not perform very good.
How are you reaching that conclusion?

You can either reach it by experience, having built a model like that, used a boat like that or constructed something that resembles the design.

The other way is by simulating on software, some law in hydrodynamic or calculations. Since nobody made a boat just like that and if the calculations are so difficult, did you calculate it in your head before writing your conclusion? No? Then how do you know?

The.. "because I know" without any scientific proof is without sense.
How are you reaching your conclusions?
I didnt say it sails nice or bad. I didnt test it but how do you know it will sail that bad compared to a hull of the same size and proportions?
Thank you

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### troy2000Senior Member

Ummm.. if it were one person commenting on a single design, your comments sneering at them for saying 'because I know' might make sense. But you're demanding that genuine professionals with genuine projects on their calendar put them on hold, to mathematically prove something that has been empirically proven for thousands of years.

Common sense should tell you that if not a single culture in the entire history of mankind has ever gone with squared-off boxes and triangles for their boats, there might be a reason.... and it certainly isn't because they didn't know how to do flat surfaces and hard corners. Do you really believe the builders of the pyramids, the designers of the Parthenon and the men who engineered the Coliseum built boats incorporating curves only because they weren't smart enough or skilled enough to build flat boxes and triangles instead? Please.....

Here's a suggestion: why don't you just shut up and build your boat? Why are you pestering people here, demanding information and opinions that you've already decided to reject no matter how reasonable they are, or why they're offered?

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### PetrosSenior Member

John,

the "word" proof is simple. any fluid with a mass, such as water, takes engery to accelerate. anytime you acclerate a mass you consume engery. When you curve a fluid in a stream you are accelerating it. You can test this yourself with a garden hose, lay it on the lawn in a curve and suddenly turn on the water. the hose will move outward from the curve, IOW the mass of the water wants to stay in a straight line. On a boat with sharp corners the accelaration of that mass of water is higher, so it takes more energy to move at the same speed. On a sailboat that engergy comes out of the sails, for any given wind, with sharp corners or even small radious shapes, you will move slower becasue it takes more engery to push the hull thorugh the water.

On a flat front you are trying to accerate the whole mass of the water in front the flat plate up to hull speed. this takes a lot of engergy. with a shape pointy bow and large gentle cures on the hull, you are accelerating MUCH less mass of water at much lower acceleration than with a flat front. On a double ended hull like a canoe or kayak, you first accelerate the water out to the widest point, and than acceleration back down to the stern. So that is why longer thinner hulls have less drag, they require less energy (less acceleration) to push the water out of the way and than close it back down behind the boat.

this is a simple fact of physics. the force required is the mass of the water times the acceration. this is Newton's famous F=ma. this is the baisis of all CFD programs, but as you can see if you understand the basic pricniples than you can get a pretty good feel for what shapes have less drag than others.

That is all the proof you need, and if you do not want to do it for yourself, that is all you will get.

with the time spend here asking for proof you could have built several models and tried for yourself. One of my engineering professors used to say one simple test is worth a thousand expert opinions. Make a box out of a cardboard milk carton and tape, and make a similar size box with a canoe type shape with sharp prow and sterns, put a little weight in them and pull them across your bath tub or a pond with a string. You will feel the difference. You could have done this days ago. this is also kind of fun and useful. Try it now before you waste any more time on this forum, please.

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### DennisRBSenior Member

A cardboard milk or juice carton is almost the same shape as the proposed design aleady.

Fill this with weight so its half submerged and tow it behind a canoe or kayak using a luggage scale or similar.

Now build a crude curved developed flat panel model like this of the same displacement and repeat the result. (use old cardboard milk cartons and tape)

http://www.youtube.com/watch?v=1xHmvLBmnPE

Take note of which model is stronger and more resistant to flexing and crushing while you are at it. You should also make your own milk carton hull design to compare how hard it was to build both versions.

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### john5346Junior Member

I am not sure that common sense can be used to prove that. What was the last time you moved a rectangular shape through water? Any material and noticed a considerable drag to it only because it was rectangular? Anything you moved and thought..

"F*** too rectangular"

Exactly. I said the boat will have 17 feet, now imagine your normal sailboat with 17 feet too, loaded with equipments, supplies, motor and everything, does it contain more mass? Sure. Using the same sail area and the only thing in my boat are the very lightweight hulls, dont you think it will move faster because it is much thinner and with much less mass? The ballast alone of a small 17 foot sailboat is heavier than a lightweight catamaran. What do you think?

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### Emerson WhiteJunior Member

It's as if you brought us the design for a 15 watt incandescent bulb, and asked us to do the calculations to show you that it's going to give off less light than a bonfire built out of two tons of wood. Just because none of us wants to take the time to walk you through the math, or do the work for you, that doesn't mean that we don't know the answer already. And yes, actually know. This isn't some abstruse question of epistemology, "you haven't tried so you don't know" does not apply here. This is a well settled question, and you haven't brought us anything revolutionary.

ETA: Your square hulls will be much much heavier than round hulls of the same length.

More editing: It is true that your catamaran will probably have more of a righting moment than a 17' monohull, and it could even come out at a lower displacement than the hypothetical well made monohull, though even with the same sail plan poor design loses out every time. And what equipment and supplies do you think the average 17' monohull carries that you wont need to? It seems like you are cheating to posit that a monohull sailor would need food and bedding and heads and toys and gear while you would not due to your design. A better comparison would be between your poor design, unladen, and a sensible catamaran design, unladen.

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### john5346Junior Member

It is impossible the weight being even near a normal monohull.
Look at a Montgomery 17 http://www.montgomeryboats.com/mont17.php

It is almost 2000 lbs! That is without looking at carrying capacity numbers that you could fill the boat with things and it would still sail normally. Two very lightweight catamaran hulls using the same sail area would probably turn the boat and capsize it. The displacement would be so low that it would be necessary to add weight for it to sit better in the water.

F= m.a , right? Do you really believe the energy provided by the sails would move a 2000 lbs boat faster than a very lightweight hull only because it was curved?

Does that really make sense?

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### Emerson WhiteJunior Member

That boat is fully fitted out, and loaded with features that your boat will not have. That's not just the bare hull weight. But I was talking about comparing a round hulled catamaran to a square hulled catamaran. The fact is that round shapes handle the stress of waterpresure better than square shapes, so they need less reinforcement, so they weigh less. Also, if you read the description you will see that the boat in question is overbuilt and over engineered. Also, 1600 lbs isn't almost 2000 lbs, it's fully 400lbs less.

Try a fair comparison, and you will see that you are proposing a way to build a heavier boat, not a lighter one.

The planes on the front of your design rapidly accelerate water out to the sides, which causes the flow of water to delaminate at the corner, which causes you to accelerate an eddy of water in a circle behind the corner. Literally the impossible best case scenario is that your boat only accelerates the water that it displaces, but your liberal use of ill advised corners means that you will be accelerating lots of water lots of places that a more sensible design wouldn't bother to move.

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### john5346Junior Member

I agree with you. A properly designed hull of a catamaran with the same length would probably move through water much better. How much better? It is another question.

If you compare with that monohull with 2000 lbs, that is including 3 people, some supplies, it would move faster using the same sail area and that is good enough. The heavy displancement of the hull would probably influence drag more than a square geometry of the sailboat.

It makes absolutely no sense that 2000 lbs moves through water better only because it is curved, I doubt anyone would really take that position and believe that.

Thank you

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