# Prismatic coefficient discussion

Discussion in 'Sailboats' started by AdrienJousset, Dec 31, 2010.

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OK... I think I have got the answer to my question. I do apreciate the time you have spent. We do things a certain way, we have learned to do things a certain way, but have not necesseraly wondered why we were doing them that way. Anyway... thanks.
I am however not convinced by the compressible different parametres of the water and the air and therefore the non interaction of the keel volume on the hydrodynamism of the hull. I think I will probably spend a bit more time studying this theory... and if not coming out with a detailed answer I will probably learn something else...

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### JoakimSenior Member

It's not about compressibility. Yes aircrafts are flying too close to Mach=1, thus you need to take compressibility into account while designing an aircraft. But the real difference is the free surface and the optimization of Cp is all about reducing residual drag and most of the residual drag comes from the free surface, which aircrafts don't have. A submarine would be very similar to an aircraft, but surface piercing hull is not.

I don't quite understand all the discussion about Cp, since there are methods to calculate the residual drag of the fins. Why not use Delft Series to compare different designs with different keels? At least the ones I use take keel and rudder volume into account in the residual drag. Surely this gives more information than just comparing Cp.

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### Perm StressSenior Member

Modern flat-and-wide hulls could lift top of the keel above the water from ~15-20 degrees. When I think it is necessary to account for it, I modify Aspect Ratio of the keel in my calculations for surface-piercing instead of end-plated foil. I leave the hull untouched.
However, at what immersion still use full-end-plate Aspect Ratio, and how to handle intermediate values at very small immersions, is a guess... I mean, when considering extreme situations, with keel top well clear of the water surface, this approach could show usable, but not accurate results (for example when one need to asses the balance of sailboat at large angles of heel). As a toll for general computation -it is nearly worthless, because of lack of systematic published data.

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To Joakim and Eric... I think the discussion is going a bit away from my original thread. The main topic of my thread was the volume distribution of the hull including the appendages. I remembered Farr mentioning it in one of his design. Ypster brought an interesting article about the area curve rule. So far in yacht design, the coefficient telling us how the volume was distributed, is the prismatic coefficient. Therefore I was wondering why we do not take into account the appendages in Cp calculations. Now I have my answer...(comparing apples with apples is probably the less "blablabla..." explanations).

Although the main topic was "would it be interesting to reduce the picks in the area curve due to the appendages to reduce the overall drag?" making it smooth and showing no abrupt changes in cross section along its length. I know this is a valid theory on high speed aircraft, would it not be also valid for boats. Ok I understood the problem of the compressibility of the air, the free surface effect, the shock waves, etc... I also understand that being far from each other the effect of the appendages on the hull volume distribution should not be taken into account... that's why I mentioned taking a part of this volume.

The original sentence of Farr Design Office is:
"Note in the profile how the hull flattens out right around the base of the keel fin. The Farr office does this in order to attenuate the “bump” in the longitudinal distribution of volume caused by the additional volume of the keel. Jet fighters do this too. ..."
http://www.sailingmagazine.net/boats/3-perry-on-design/309-rosebud

I know this is not the most conventional way of thinking in the highly conventional way of designing boat... Although it is only on attempt to think forward and to discuss a little more an idea brought by one of the greatest naval architect of all time...

I understand that there is no comparison element, no much published data, etc...

Thank you for teaching me how to calculate the Cp and LCB...also remembering how to find the drag generated by the keel... this is, I think, a little far from the interest of my discussion and shows an interesting consideration...

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### AlikSenior Member

Given low wave making of keel I see no point to consider it together with hull at speed range where wave-making drag is prevailing. Yes, maybe designers do it and they have some basis for that other than intuition and feelings (and marketing!). There is no published research available on this particular effect, but would be know to see such.

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### JoakimSenior Member

If you would change the way you calculate Cp, what would you do with the "new" Cp? Use it to optimize Cp for desired speed range? Use it in the Delft Series resistance prediction? Both of these would fail, since the original data is done with Cp not including appendages. What else do you use Cp for?

Does the latter come from Perry or Farr Design? Jet fighters have quite different "appendages", are mostly designed for Mach > 1, do not have free surface etc. I don't think they are any reference for boat design, but may make a nice reference for marketing BS.

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### yipsterdesigner

would be nice to hear JuanK in papers Alik mentioned as CSYS-11 (1993) proceedings

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### dougfrolichSenior Member

I think that one reason for the dimple of the JaunK canting keel boats is because the keels cant to 55degrees and the fin gets very close to the hull, flow over the upperside of the fin needs some "clearance" from the hull.

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### Perm StressSenior Member

Main reason for dimple is to raise the rotation axis of the keel up. This way, for the same draught from WL, keel pendulum is longer => with same 55 degrees of swing, ballast is offset to the side further.

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### fcfcSenior Member

There was a paper on the net about hull bulb influence. Sorry, I do not remember author, nor title. But basically from memory , it said once hull-bulb distance is greater than 4 * bulb diameter, bulb influence on hull is very weak.

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### sorenfdkYacht Designer

Hey - don't you guys read Professional Boatbuilder? You should

Here's a quote from an article about Juan K. in the Oct./Nov. 2010 issue:

"It's a feature we have been doing for many years now. Particularly efficient in canting-keel boats, it's just a depression or hollow that you iclude in the section between the appendage and the hull that changes the pressure field at that section; and as a consequence of that, the drag of the intersection is reduced. You have to know how to do it, or you can increase drag."

This hollow is just a relatively small dent in the bottom of the hull. It has nothing to do with the prismatic coefficient or the area-rule.

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### MikeJohnsSenior Member

I'd like to see their data and the reality of a practical results. You generally see some small improvement in drag at some Reynolds number under ideal conditions.

Most of these sorts of improvements are so minute that they are almost swamped by the noise even from the ideal smooth water tank testing.
Once they get in a seaway the flow field around the hull and appendages of a vessel experiencing heave pitch yaw sway and leeway effects tends to make such claims rather dubious in application.

Aircraft, particularly supersonic aircraft gain a huge drag reduction from the application of the transonic area rule, but they are operating in a compressible flow that's not applicable to boats.

Last edited: Jan 14, 2011
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### sorenfdkYacht Designer

I'd be very surprised if there is any measurable effect!

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### yipsterdesigner

thanks and oeps, must have raced to part 2 of Peters on fast powerboats but reading JuanK now
here
some 3d keelflow animations, trying http://www.tecplot.com/ as http://www.flow3d.com/ has no trial

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### daiquiriEngineering and Design

As others have said before, I'd like to see some numerical data in support of this claim.

So, they make a hollow or flattened area in the hull around the keel in order to reduce drag. But what drag component due to keel are they acting on?
If the intent is to reduce the wave drag of the heeled hull at the water-air interface, then the hollow must have been designed in such way that the pressure is increased at the suction side of the keel-hull junction. In that way, the pressure disturbance due to the keel would become smaller, and would contribute less to the creation of wave drag.
But by doing so, both the lift of the keel and its hydrodynamic aspect ratio would be decreased. It means that the keel would have to operate at a somewhat higher angle of attack to give the same lift, which would translate into a higher induced drag (or drag due to lift). Besides, a hollow in a hull would also produce an additional drag due to the flow disuniformity or vorticity, and perhaps a premature boundary layer separation.

So which effect prevails here? In absence of more data, I see it as a blanket-too-short situation - leaving either your feet or your shoulders exposed to the cold.

Sorry if it's off-topic.

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