Efficiency: Hull interference vs length on catamarans

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

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dustmanSenior Member

Say you have 2 boats of the same displacement(2000lb) and width, one 30'x12', the other 24'x12'. The first boat having finer but longer hulls displacing the same amount of water as the shorter boat. One would assume the longer hulls have more wetted surface area, reduced wave making at speed, and would experience more wave interference than the shorter hulls which have the 2 to 1 ratio.

Which boat would be more efficient at, say, 8 kts and 15 kts?

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If both the same displacement the longer hull will have a superior length-displacement ratio.
As such its residuary resistance shall be lower....as such the longer will always be more efficient.

At 8 knots both are in the hump region...not ideal.

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TANSLSenior Member

As a well-known member of this forum likes to say, first define "efficient". Without that, it is not possible to give a professional or serious answer.

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In this context if one understands the question the definition is not necessary.
In this context (only for this question) means - more than.
Thus what ever "efficiency" one is calculating or assume, its actual value is not relevant, what is.. it shall be more than the other hull.

Perhaps Google Nuances would help better than Google Translate ... just perhaps?

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DejaySenior Newbie

You could test this out by exporting some basic hull shapes in obj and testing them in prelimina.com. It takes a bit of time to figure out but the tools you'd need are free. If you do, let us know what you find!

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TANSLSenior Member

Nobody talks about the "actual value" of anything, perhaps, just maybe, it is a question of finding out among the many qualities that are required of a ship's hull, which one is trying to prioritize in this case, only in this case. Perhaps, just perhaps, someone who knows a little about ship design understands, only on this occasion, that the design of a ship is a compromise between a large number of variables and, without resorting to either Google translator or Google Nuances, perhaps a professional, just perhaps, is capable of understanding that it is necessary to specify well what is intended to be achieved with a specific design.

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dustmanSenior Member

Just maybe I'm trying to get down to the fundamental principles. In this case, is the increased interaction between the hulls outweighed by the decreased wave making resistance in longer hulls, generally speaking, assuming beam and displacement is held constant. I think it is obvious that I'm trying to understand how to decrease power consumption(increase efficiency), and how certain variables come into play.

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dustmanSenior Member

Thanks for the helpful reply. I'm getting the drift from this and your past posts on other threads that length-displacement ratio is the dominant determiner of efficiency at speed.

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TANSLSenior Member

With all due respect to you, but I wonder if variables such as the wetted surface, the separation between hulls, the block coefficient, the shapes of the hull, etc., etc. they don't concern you. Simply with the length-displacement ratio you can already judge the hull's efficiency in relation to energy consumption. I, and forgive me for not agreeing, I do not see it clearly enough.

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dustmanSenior Member

They do concern me, but at this time I am considering the relationship of two variables. For what I am designing I have a constraint on maximum beam, thus hull separation. The question is; would it be beneficial in terms of efficiency to stretch the hulls from 24' to 30', even though that would mean departing from optimal hull separation, which I gather is approximately 2 to 1. Since I'd like to maintain decent efficiency up to 15 kts, it seems like it would be of significant benefit to increase the l/d ratio. The very idea of deviating from the optimal hull spacing seems to be off putting to many on these forums, but I have an inkling that resistance from hull interference at such high l/d ratio is insignificant because wave making is so low in the first place.

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TANSLSenior Member

There is an aphorism that says, "for the same displacement, the greater length represents greater speed, the greater beam represents greater stability." Increasing the length-displacement ratio, of course it is beneficial, but we do not know anything else. This "magical" relationship may have nothing to do with the interaction between hulls, which depends more on the separation between them and on how the water cuts the bow. That is why I think that only with this relationship you will not be able to improve the "efficiency" of your hulls or know to what extent it improves. But let's see it from another point of view that is also not very useful in practical terms, "lowering the length-displacement relationship is not convenient", or yes? Perhaps, just perhaps, it is. But you with the maybe, just maybe, you're going nowhere. That is why you would need, in my opinion, a more professional answer based on a study from a series of data that you have not given us. Maybe what you need is to decrease the block coefficient, who knows? but that's always good for some aspect of a hull's effectiveness.
Since you want an answer: provide more information and maybe, just maybe, someone will be able to give you a professional answer, without looking at the crystal ball. Ad Hoc has made us used to asking for facts, not assumptions. Maybe, just maybe, in this case, just in this case, you too prefer facts over assumptions.

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Mr EfficiencySenior Member

I don't think it makes much sense to be fussing over the resistance of a light displacement cat at 6 or 8 knots versus 12 or 15 knots, if you anticipate spending a lot of time cruising at 6 knots, you are just wasting the speed potential of the hull form, and might as well have a displacement mono, that will be cheaper to push along at such dawdling speeds, and especially with a single engine. I think you have to bite the bullet, nominate your "normal" desired cruise speed, and work backward from that decision. Sure it would be nice to have a boat that slips through the water with minimal drag at all speeds, but you can't easily tick all boxes. In any case, the main drag component at any speed you care to nominate, will likely be skin friction, you just accept that going faster will cost more, but pick a cruise speed short of where it starts to hurt too much. These things can't reduce their wetted area, and we know that frictional increases at near enough to the square of the speed. Trying to turn them into speed machine is a bad idea, but so too is crawling along at walking speeds, their selling point is the ability to cruise at higher speed than displacement monos, but not to rival fast planing boats.

Last edited: Jun 3, 2021
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dustmanSenior Member

My normal desired cruise speed would be around 10 knots, at times when casually exploring probably 6 or 7, but would like to have the option to cruise at 15 knots without the need for a big, heavy, expensive outboard. It seems that I should optimize for higher speed.

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dustmanSenior Member

Tell me what information you need, I'll do my best to provide.

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Mr EfficiencySenior Member

I think it gets down to what the "typical" cruise speed wanted is, not the occasional diversion from it, that way too you can choose the best engine size, you don't want a boat with too little power that cramps the ability to go faster, come the day you need it, or one with big engines that will rarely be used at their optimum rpm, because you go slow most of the time.

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