LCB location in Displacement Powerboats

[Mat-C]

There's much info about on the best location of the LCB for planing hulls, but can anyone tell me where the LCB ought to be in a full displacement powerboat?

[Crag Cay]

Most sailing design books will have curves for LCB vrs projected speed. However ordinary sailing boats tend to compromise at about 52% aft Sta 0 as they need to operate over a great range of speeds and the penalties in higher drag are more acute at lower speeds.

But if your power boat is expected to run regularly nearer the top end of the displacement spectrum (S/L>1), you can afford to be more focused with your choice.

[Ad Hoc]

Do you mean like those typical fast displacement catamaran hull forms on fast ferries which regular operate at high froude numbers?

[Mat-C]

Thanks Crag Cay. I don't have access to a lot of sailboat texts... I'll have a hunt and see what I can find...

Ad Hoc - no I'm talking about your regular run-of-the-mill full displacement powerboat, designed to run up to a max of S/L^0.5 1.34

[Tad]

Ideal LCB moves aft as speed increases. See this....

http://www.tadroberts.ca/about/pdf/p...d-function.pdf

[Mat-C]

Thanks Tad. I obviously hadn't paid enough attention to the area curves graph when I've read your article previously. So essentially, a vessel designed to operate at an S/L^0.5 of between 1.0 and 1.3 should have an LCB at around 49 to 52% of the DWL aft of 0.
But surely there comes a point in reducing the S/L^0.5 when the LCB should stop moving forward. I guess it's somewhat of a moot point as there can't be too many boats that are designed to operate at less than around 0.8....

[Tad]

Usually other factors come into play.....huge ships operate at very low S/L....and they have parallel mid-sections for reasons of economy. LCB is at station 5 ...period. With moderate to small boats it's very easy (tiny power) to get up to S/L 1.0 or more.

[Ilan Voyager]

Hi Tad.

So it's you who wrote that article I've read in a Wooden Boat on 1997. My most sincere compliments, this article is the best I've read on the subject. Very clear and precise (nice boats also...).

[Tad]

Ilan.....

Thanks...hard to believe that was done a dozen years ago now! I also agree...some of them are nice boats, but some could have been better

[Ilan Voyager]

You're welcome Tad. Yes, already 12 years: I've found yesterday the Wooden Boat Mag in rather bad shape, it survived a few tropical storms and 2 hurricanes.

We think that we can do always better, it's that I call the "builder's or designer's syndrome".