# Interceptors - How to calculate dynamic MCTC

Discussion in 'Boat Design' started by gmd.na, Jul 15, 2013.

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### gmd.naNew Member

Hi all,

I've got a quick question - we've got a work boat that is in need of trim correctors due to the wide range of weights required to be carried, and I've got to work out how much force is required to be generated by interceptors at the transom in order to reduce the trim at 20 knots by a couple of degrees in order to help the boat make the required speed. We don't have the time or money to do a CFD analysis of the problem, so we need to work it out using first principles.

So far, I've used an estimate using the static MCTC at the running angle of trim to work out how much is required to push the bow down - but my question is: will this MCTC value change significantly as the boat speeds up and in what way will it go. Ie. does it require more force at speed to drive the bows down than it does when stationary?

Cheers,
Jon

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### Mr EfficiencySenior Member

The fact people can "hang ten" on the old Malibu tells you the answer to that. You get a lot of lift at the leading edge when underway at speed in a planing vessel. If you lift the stern, the leading edge moves forward, and with it the centre of dynamic lift. Needless to say, all boats are different, and it may be you get a lower trim angle, but no extra speed, due to increased surface drag and the drag generated by tabs/interceptors. But you would get a better ride most likely.

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### gmd.naNew Member

Thanks - I've had a bit more of a think through of the problem since previous post. Basically the answer should be "the MCTC will increase substantially when under way" and I have sized some adjustable interceptor plates to suit. Essentially I worked the question along the lines of "how does GML change with speed" by considering a shift in G due to the dynamic lift (estimated using savitsky's method) along with the effective displacement and effective length and then threw them all together to get a number of approx 2.5 times the static MCTC.

To put it short - it's as you said, if people can hang 10 on a malibu, it will take a lot of force to change the trim significantly when planing.

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### Mr EfficiencySenior Member

What size and type of boat is it ?

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### gmd.naNew Member

24m work boat.

Sorry I can't go into much more detail as it's commercial-in-confidence.

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### Mr EfficiencySenior Member

That is a large vessel to be doing 20 knots, and the effects of dynamic lift are much less than for a small planing boat, your ratio of weight to bottom area is way higher. So what I said earlier is less applicable. But it also means that interceptors would have less effect than on a smaller vessel, at least in the range of acceptable drag increase.

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