LCG Centered or more aft?

It seems to me that most designers try to keep the LCG centered (relatively speaking) and keep the main weights of the vessel as centered as possible. But some are on purpose moving the LCG more aft to increase speed potential.

But what ahppens to the heaving and pitching moments when the LCG is moved further aft?
I would imagine that they would increase. but is there any way to relate the speed potential to moving the LCG and at the same time see what happens to the "seekindliness".

Since I know I'm a newbie here and we have some real pros on theese boards, I have to excuse myself if I used the wrong terminology in anyway.

Regards

Erik

 

Yes.
Both of these articles discuss the questions you have, however, most relate more to the lwl/beam ratio in discussing stability and comfort factors. Yet the lwl and beam both are a part of the base factors of the LCB and CG. I hope I put that right. Anyway check out these articles.

http://www.tedbrewer.com/yachtdesign.html
http://www.johnsboatstuff.com/Articles/estimati.htm

 

If Larsson is to be believed LCB wants to be about 53.4% aft in most cases. If a vessel has a low displ/length ratio and is light in the ends with a carbon rig and so forth, it needs less reserve bouyancy forward, and making it very straight and fine, as Bruce Farr does, can reduce the bow's tendancy to initiate pitching.

An interesting question, for which I don't have a good answer, is what difference the relative locations of LCB and LCF make. I've heard that Brendan Dobroth has published on this question, but I don't know where or what he said.

 

Suggest you also look at my thread on Delft Series for sailing yacht hulls. If you set up this spreadsheet you can optimise parameters for different Froude numbers, the optimum LCB location agrees with Larsson (not surprising really) of @ -53.3%.

As I pointed out, the more recent version of the Delft Series allows for LCF adjustment, though I am not convonced as to its reliability. I have however read articles by designers such as Farr that suggest that LCF - LCB separation is important.

In non prismatic hull forms such as sailboats, there maybe a natural constraint on LCF location, being the need to have a fair (non bulbous) waterline aft.