yet another prismatic coefficient question!
Hi, newbie here, hopefully someone can help!?!?
I'm using 'Delftship' hull modelling software to design what might be described as a modern interpretation of a 30 square metre. the particular hull I am designing is 11.5m long, 2.1m beam, 0.4m draft (excluding foils) and 1800kgs displacement. One of the features of this software is that it can calculate a number of key hydrostatic properties more or less instantly, including prismatic coefficient.
I have read at great length just about everything I can find on the web about prismatic and block coefficients, and have no trouble understanding the concept- but when I take my new found knowledge back to the design, it stops making sense. The problem is this- all the conventional wisdom I can find seems to suggest that a reasonable prismatic coefficient for a sailing monohull is likely to be somewhere between .5 and .6. According to Delftship, the hull I have drawn has a Cp of 0.41- which, by what I have researched on the subject, would indicate it to likely be a fairly poor performer.
So, to prove to myself that I wasn't going completely mad, I imported the lines plan of an existing 1925 rule 30 square metre into delftship, and modelled it as accurately as I could in the software, then used the hydrostatics function to calculate the Cp- according to this the 30 square has a Cp of 0.36.
Then, to prove I wasn't going completely mad again, I used Delftship to model a Soling hull, and then do the same calculation- it thinks the Soling has a Cp of 0.41.
Lastly I did the same thing with a lines plan procured from the net for an Etchells, and If one doesn't split hairs over the last two decimal places, the Cp is, according to the software, more or less the same as the Soling- which didn't surprise me, as they are proportionally extremely similar.
So, if 0.4 or thereabouts would generally be considered an extremely low Cp for a sailing monohull, why does it repeatedly appear for some of the most efficient and well-mannered sailing yacht designs to have appeared in the last century?
Is it a peculiarity with hulls which have a pronounced deadrise from the centreline and very little radius to the bilge below the waterline? (something all of the above have in common)
Or does it sound like an anomaly of some sort in the software throwing out erroneous numbers?
Any help with this would be much appreciated!