Lcb ?????

[josch]

Hi I am studying naval architecture and there is one question that nobody of our teachers can really answer: what is the right position for the LCB of a sailing yacht operating in Froude numbers between 0,4-0,45.

I have seen drawings of fast yachts where the LCB was at -13% (seen from Lpp/2), but the delft series for example say -3,4% is the best position.......
My teacher said towards that question there are no rules in yacht design everything is possible, but what is good????? -3,4% to -13% that is a big field...........................................

Is that true that there is no advise according to that question???
I would like to learn more about that field of hull design who could help???

I would be glad about answers

josch

[nico]

Taking Delft series as basis, LCB "optimum" position is highly dependent of your slenderness ratio (LWL/(Displacement)^(1/3))
I have posted a spreadsheet on the forum http://www.boatdesign.net/forums/attachment.php?attachmentid=403
might be of some help.

[DaveB]

Would reserve bouyancy/flare come into it? Seems like newer boats have plumb stems while some from the delft series might have more angled bows giving them more reserve bouyancy and allowing the lcb to be further forward?

Am I out to lunch?

Dave

[nico]

Lcb will be the same for a boat at all speed and heel angle (except for planing mode etc) since Lcg doesnot move :) (except for small yacht and ballasted one). Flare etc will influence Cp, a Cp will change when heel, speed is changing. Must have some influence on the "optimum".

[SailDesign]

nico,
I don't know who is teching there now, but they obviously forgot this handout :)
Circa 1979, so hopefully they won't mind it's publication. And hopefully you can read it.
The provenance of the graph is uncertain, but the teacher knew his onions.

The "reduced slamming" part is very telling on resistance in real situations (as opposed to the tank), and the "increased wetness" is very true...

Steve

[SailDesign]

Second attachment dropped out, so here it is again.
In the meantime, I discovered that the graph is from "Sailing Yacht Design" by Phillips-Birt, page 63. Worth checking out since there is discussion in his text also.

Steve

[nico]

The "optimum" Delft lcb is for calm water, there is much more than only optimising for calm water performance.