# LB ratio & powercat fuel efficiency

Discussion in 'Boat Design' started by Mcarthur, Feb 14, 2021.

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### Mr EfficiencySenior Member

The OP hasn't even mentioned the size of the boat, if he bothers to reply, he might ! It isn't irrelevant to the discussion

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### McarthurJunior Member

The boat length is irrelevant unless you can support that there is something in the relationship that breaks down at a certain length (high or low). Otherwise why would we have ready reckoning formulae that have no length cutoff (ie. they don’t say “not applicable below 10’ or above 200’)?

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### McarthurJunior Member

Ad Hoc is of course right - have to scale the displacement and WSA for greater/lesser LB ratio.

And thank you for associating directly that my factor of fuel consumption (my measure) is more accurately discussed by talking resistance. I was wondering (perhaps not loud enough ) whether resistance through the water caused by wave making, rather than direct drag from the hull-water interface, was able to explain the relationship. Ie. keep the hulls of two vessels identical in respect of hull drag (WSA), but change the BWL from A:1 to B:1 (increase of X%) and lose a wave making resistance (and thus gain consumption) of Y%, creating an X-Y formula that could be linear, log, or other polynomial, but likely not exponential.

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### DCockeySenior Member

The use of the term "hull drag" appears to be causing some confusion. The commonly used proceedure for going from scale tow tank test results to full size involves splitting the resistance into two components. One component is refered to as (viscous) frictional resistance which depends on Reynold's number and wetted surface area. This component is usually assumed to be independent of the shape of the hull(s). The other component is refered to as residuary resistance and includes wave making resistance as well as other, usually less signficant effects. This component depends on Froude number and the shape of the hull(s) including hull spacing for multihull vessels.

In multihull vessels the resistance is affected by effect of the flow around each hull on the adjacent hulls. The primary form of this effect is usually associated with wave making, and depends on the spacing to length ratio of the hulls, Froude number, shapes of the hulls and displacement. This has been studied extensively, both experimentally and analytically. There is no simple, universal curve for the effects of spacing to length ration on resistance.

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### Mr EfficiencySenior Member

You deigned to re-appear ! You appear to be a pedant, but just to entertain you for a second, there is more to boats than formulae, it may be desirable to have wider hulls at a certain size range, if they are to be used for accommodation, even if it comes at the expense of greater resistance. And it may not be desirable to have too slim hulls, if you need to carry a little bit of extra weight, as it will increase wetted area, and resistance, more than less slim hulls. And I won't even mention pitching motions of very slender hulls, that would likely be a bigger problem in a smaller boat. Nice to see you came back, I don't think I'll bother !

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And what "relationship" is that?

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### McarthurJunior Member

Thanks David - that's the response I thought but you have written well: there's no simple, universal curve. Damn ...

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### McarthurJunior Member

I entirely agree with you. For the purposes of my question, if I were interested in other aspects I would have mentioned them

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