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Old 09-06-2011, 03:13 PM
anthem anthem is offline
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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!
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Old 09-06-2011, 03:43 PM
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Richard Woods Richard Woods is offline
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All the boats you checked are essentially the same. Long narrow with very long overhangs.

Try a couple of more modern hull shapes (unless what you are designing is long low and narrow) and you'll probably find they have Cps around .56. In general higher Cps are "better" at higher speeds and on heavier hulls. A fine hull can have a lower Cp. Except that a high Cp hull tends to pitch less.

I don't use Delftship so cannot say if the hydrostatics will be accurate

The best yacht design book to explain things fully is Rolf Elliason's Principles of Yacht Design. If you read that you'll know you get the real facts, whereas you can never be quite sure of what you read online

Richard Woods of Woods Designs
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Old 09-06-2011, 04:05 PM
NoEyeDeer NoEyeDeer is offline
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Two things to check. First is that if you have drawn the whole boat including the keel, then Delftship will obviously be including the volume of the keel in the calculations. Since that is concentrated in the middle of the boat it will make for a lower calculated prismatic. Really the calculation should be done without the keel.

The other thing to note is that Delftship will default to using the section at 0.5 DWL for calculations, and this may not be the largest section on some hulls. In those cases, you'll get an artificially high calculated prismatic.
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Old 09-06-2011, 05:18 PM
Gary Baigent Gary Baigent is offline
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I've had similar low figures with my foiler Seditious Sid, in Freeship, this is a design a lot finer than 30 Square Metre, Etchells or Soling,
foiler figures:
7.62 LOA x 8 m BOA,
main hull WL beam - 0.350m
block coefficient - 0.4390
vert. prismatic coefficient - 0.54
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Old 09-06-2011, 10:14 PM
jehardiman jehardiman is offline
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Originally Posted by NoEyeDeer View Post
Two things to check. First is that if you have drawn the whole boat including the keel, then Delftship will obviously be including the volume of the keel in the calculations. Since that is concentrated in the middle of the boat it will make for a lower calculated prismatic. Really the calculation should be done without the keel.
I second this, you have to be very careful where you place the apparent baseline to compare different hull types. Better to look at volume distribution than prismatic coefficient.
A vessel is nothing but a bunch of opinions and compromises held together by the faith of the builders and engineers that they did it correctly. Therefor the only thing a Naval Architect has to sell is his experiences.
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Old 09-08-2011, 09:42 PM
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philSweet philSweet is offline
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You should be able to dig up quite a bit on the Swede 55 "Vortex". It is well known boat.
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