lcb cp midship

[metin_mehel]

Hello,
I have some questions for small sailing catamarans:
1-What are the optimised values of cp and lcb? In what conditions the lcb should be located behind midship (to aft side)? In some resources say
lcb position 6% lwl.. What is the reference and the direction? Is it to aft?
2-The default value of midship is Loa/2 in delftship program. What will be the best location of midship?
3-Is it possible modelling a catamaran by free edition (delftship)?
4-Is it possible to place at least three control point on a vector (delftship)?
Thank you very much.

[tspeer]

This may not be very helpful because by "small catamaran" I take it you mean a beach cat. But in "Sailing Yacht Design - Practice" (edited by Claughton, Wellicome & Shenoi) Alexander Simonis presents a case study for the design of a cruising cat. He presents a table of the design parameters for 17 of his charter cat designs. The LCB for his designs varied from 53.1% to 56.43%, and the center of flotation varied from 53.53% to 58.42% of the waterline length. In contrast to Shuttleworth, he places the LCF a little behind the LCB. In the paper he discusses the evolution of these parameters, and their relation to the rig center of effort, as his designs evolved over time. He says of his latest design "It is considered that the fine entry combined with the LCF positioned as far back as possible would reduce the pitching motion as long as the LCB moved rapidly foward when the bows went down. This is also partly why the bows have a slight angle forward instead of being plumb as seen on earlier cruising designs." For this design (the Moorings 4500), the LCB is at 55.23%, the LCF at 56.36%, the CE at 55.95%, and the center of lateral resistance (CLR) at 56.03%.

I think it depends on what behavior you want from the boat in different conditions. You may want to have everything farther back on a beach cat, so the bows have more leverage with regard to avoiding pitchpoling.

[metin_mehel]

Are these percentages of 'lwl' starting from 'FP' ore 'AP' ?
In other words the referance point is 'FP' .
And could you please tell me what will be appropriate cp value for each condition?
Thank you very much for your assistance

[tspeer]

FP.

He presents a table of 17 different designs. Cp ranged from 0.534 to 0.598.

Again, these numbers are for large charter cats, not beach cats.

[metin_mehel]

How about the hull speed (or froude number) while the cp and lcb values are between the range?
Thank you very much

[rayaldridge]

"It is considered that the fine entry combined with the LCF positioned as far back as possible would reduce the pitching motion as long as the LCB moved rapidly foward when the bows went down. This is also partly why the bows have a slight angle forward instead of being plumb as seen on earlier cruising designs."

Tom, this interests me, because I've never been enthusiastic about plumb bows. Both of my small cat designs have angled bows, because it seems like cheap insurance against pitchpoling, though you get less waterline length in a given overall length. How do you feel about plumb bows?

[terhohalme]

I have found plumb bows more much comfortable than angled bows. In immersion, plumb bows has buoyancy much earlier than angeled bows and they don't sink as deep. Also plumb bow boats have bigger longitudinal second moment of inertia, which reduse their piching. Here is the example:

[Questioner]

Only have a few numbers for A-Class

ASG3
LOA 5486.4mm
LCB 2946.78, 0, -66.13mm (53.71%)
LCF 3010.02, 0, 0mm (54.86%)
COV 3043.49, 0, 83.79mm (55.47%)

Marstrom
LOA 5486.4mm
LCB 2996.80, 0, -65.41mm (54.62%)
LCF 3060.73, 0, 0mm (55.78%)
COV 2761.02, , 97.73mm (50.32%)

COV is Center of Volume for the entire hull

IMO sailed well, it appears that the Marstrom is the better boat of the 2 right now

[metin_mehel]

thank you very much for these all good information. pitching prevention is so exciting.