Hull Design Question

Discussion in 'Boat Design' started by ddrdan, May 4, 2009.

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ddrdanJunior Member

I know there is probably a plethora full of variations but what I'm looking for is simple max displacement shapes. I have no marine design experience but I am a mechanical engineer. Consider very low draft, under 0.5 feet.

1. I would like to know what outrigger hull shape provides the best flotation. Square, round, chamfered, tunnel ..etc.
2. Do certain rigger hull shapes resist in a lateral direction if the load is applied at angles to the flat water plane or do they just slide?
3. Is it better to apply the load to the rigger from directly above or at an angle? I'm not concerned with the ability to plane or even its use under way.

I'm trying to understand the resistance when a hull pushes down on the water. Is there a resource which tells me how and which direction (opposing angle of resistance) are applied to the hull? More or less I want to design resistance in a certain lateral direction. Or does water have no lateral resistance to a hull in such a small draft?

Shapes I've been considering. Can you rate them in both catagories, floatation and angular resistance. (have any other shapes that are better let me know)

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There are many books and papers that can give you a 'heads up' on the basics of resistance. Once you have read them they will answer most of your questions.

You'll also find that your question is also back to front.

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ddrdanJunior Member

Youâ€™re a college professor, aren't you?

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alan whiteSenior Member

The best shape for flotation question would imply that some variable like surface area was included, which you don't mention. Given the same surface area, the round float has the most flotation.

The load direction has nothing to do with the point of attachment. It has to do with the dynamic interaction of the hulls.

Applying the load from above maintains a distance to the water surface to avoid contact with waves and consequent slowing of the boat.

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ddrdanJunior Member

Thanks for the flotation note. I was hoping it would be round.

I guess the second part of my question is hard for me to explain because of my lack of marine design knowledge. I'm so use to designing with the resistance of solid ground that I'm being blinded.

I'm trying to find waters resistance to a shape when pushed againsts its profile. Like pushing a boat sideways thru the water. What shape would give the most resistance? I assumed a flat and vertical? Then I thought an angled concave shape would discharge water in oposition and create further resistance or maybe even lift????. I know equal and opposite reactions on land, I need to learn the same for water.

My intent is to have floataion with a blade extended below it, and attached, that would provide resistance when the flotation is pushed sideways. I'm not trying to float and glide thru the water. I want to float and opose the water.

Maybe it's an answer I wouldn't understand even if you did answer it?? I read up on propellers, as my design intent is wave generated propulsion. I probably need to inquire deeper into the dynamics.

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timothy22Junior Member

My commenting here is clearly a case of fools rushing in, because there are folks here much more experienced than I in these matters, but I can't resist. Part of the answers you seek depend on which variable you specify. For example, for a given wetted surface area, the round section will offer best flotation, up to its maximum beam. for a given draft or immersed depth, your section 4 will float more. You are looking for two things, the volume of the shape below the waterline, and the waterplane area of the hull (the area of the shape bounded by the waterline of the hull) The lateral resistance will be maximum when you are trying to push a flat plate through the water. the resistance will vary with how fast you are trying to push. Here shape 4 is also the best. Anything you can do to resist the flow around the shape is good. Think of a foundation for a building built on sand. You need a broad shallow shape to resist settling. (The trick of driving pilings into the sand to support the building by friction won't work in water)

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jehardimanSenior Member

ddrdan;

If you are looking for wave generated propulsion, you are asking the wrong questions because hull form is actualy the tail of the dog. You need to understand how wave energy potential is mainfested first, so get a copy of Oceanographical Engineering by Wiegel. Once you've got that then you can use strip theory to analyize any hull shape to get the phaseing necessary to extract energy from the wave, and there are several methods to do that. And on a side note, energy extraction from the wave is accomplished by vertical plane motions, not horizontal, because any side motion is all lost to damping as there is no restoring force unless it is input by other means.

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ddrdanJunior Member

Thanks Tim, Thats the basics of what I was looking for.

What???? Marine designers can't use sky hooks?

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ddrdanJunior Member

Thanks Jehardiman,
All of the wave propulsion design I've found is sub-surface. I'm using the energy from the suface lift of the wave and converting it to horizontal energy on the fall. A scissor type lift and fall. I'm using the gravitational effect of the wave on the vessel. Granted this may not provide massive propusion, or 0 in calm seas, but I'm at the basics of the concept right now.

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jehardimanSenior Member

Then what you want is a large waterplane area float of minimum mass working relative to a small waterplane area base of large mass (conceptually similiar to a Sterling engine pistons). Most methods that attempt to use buoyancy instead of celerity are not very efficient because the hull that the float works against is also heaving with the wave. FWIW, the best float power generator I've seen so far is a spar buoy surrounded by float disk. The two are linked (mechanicaly or magneticly) to produce power.

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alan whiteSenior Member

In answer, I think, to your second question, a knife-like vertical hull shape (the opposite of a round section) is likely the most resistant to sideways travel. But maybe I misunderstood your question.
In reading the other posts, it seems a form of propulsion based on wave energy is possible as long as direction can be maintained, though that direction may be limited to a predominantly downwind direction (though faster than simple downwind drift). Probably that directional limit could be widened and even reversed by some means though not as efficiently.

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