Reverse-Engineering of Modern Cat hull shape

I can see design for planing lift in classes where foils are illegal(maybe), but it is widely known that a long narrow , low aspect lifting surface such as a cat hull is a very inefficient lifting surface.
For cat hulls in classes where lifting foils are legal it seems like it would be real important to design for the advantages foils give-not design the hull and then add foils!

 

Good point here, and it explains the square section on high speed catamarans (and tris) well.

The problem i see with cirrus R, is that races are generally won based upon the UPWIND performance, not the downwind leg. It would be quite easy to make any of these racing cats faster downwind at the expense of upwind performance and they would look quite different. Nobody does this because its the upwind performance they want to maximize.

Erwan, You need to forget about planning and semi displacement modes etc. The hulls of these cats are so slender (very low aspect ratio) that the normal text references to functions of Froude number do not apply. Just like wings, foils, sails or other lifting surfaces, the lower the aspect ratio -the lower the lift coefficient. Then you have the rocker in the hull, which in terms of lift is like negative camber and actually acts to suck the hull downwards, not lift it upwards. These hulls are not planning hulls, despite what you may 'feel'.

 

Just a bit of a relevant aside: the Moth class, before foils experimented a great deal with planing hulls, low wetted surface hulls and skinny hulls. They found that the improvement with a high L/B ratio hull(skinny) was superior to a low wetted surface hull -see sketch: ( Note that the advantage is particularly apparent with an 11' hull and less so as length increases)
click-

 

Sorry IDKFA I cannot read the link about Steve Killing document, but I am surprised by the conclusions regarding the brakedown between waves & friction drag.
In the Steve Killing document attached, on pages 4 one can read:

The hull shape was designed assuming that the boat
would almost always fly a hull and the full displacement of
750 lbs (boat and crew) would be supported by the leeward
hull. The top speed of a sailboat is normally limited by
wave drag as skin friction drag becomes a small percentage
of the total. However, in a narrow catamaran hull,
wavemaking drag is small, so even at high speeds reducing
wetted surface and therefore frictional drag is particularly
important.

The most important words seems to be "normally limited"

This remark supports GROPER argument that digging hard to reduce wave drag will not lead to a breakthrough, and I agree with this judgment.

Alos GROPER, I apologize for being a bit reckless in using semi-displacement/semi-planning terminology, and don't think is useful to start a semantic battle.

My purpose is just not to rule out, among other parameters, the possibility of a marginal dynamic lift.
If we fight for a few tenth of knots, a few hundred Newtons lift which is likely to decrease marginally wetted area and wave drag, at the cost of spray drag (the induced drag of the lifting force) can provide a tradeoff with a marginal but interesting decrease in global drag ? that is more a question than a statement??

Of course long narrow hulls do not plane like a 49er, but water-skiing shows that it is possible to get dynamic lift from a narrow long surface. Of course there are 100 HP+ to pull at the appropriate speed which is not the case for a sailing boat.

I agree with the statement than among others, the purpose of square hulls is to achieve a lower beam for the same displacement, under the assumption that transversal wave drag is proportional to beam^2 (AFAIK) ??

But why do not consider that in addition, square hull sections, with their flat bottom provide a better amortization of pitching motions? (That is still more a question than a statement)

Also, with regards to pitch amortization, if the basic approach is to consider waterplane area, and if an alternative to this, would be to consider the volume of water displaced with the pitching motion, So it would relevant to fill the underwater section to the maximum, as for the same alpha (°) of pitching, one will observe more water volume displaced compared to V or round hull section (Still a question).

Not to forget that square hull sections, with flat bottom and marked chines underwater, can provide some "grip" in the water sailing windward. But just like for dynamic lift of flat bottom, it is borderline esoteric.

Raising these points around square hull sections, aims primarily to suggest that there can be many explanations/reasons for the same design option, and if not relevant to run computer for a marginal but not granted improvment in drag as enlighted by GROPER comments. One should consider a global approach to achieve a better overall solution.

There is much more to be said about the advantage of thin hulls, I will come back latter on this point, as I have the privilege to talk sometime with Jacques VALER, the designer of the Hobie TIGER and also designer of successfull monohull JPK. But for the purpose of clarity it is probably not effective to address too much different issues in the same msg.

Thanks to continue brainstorming

Cheers All

Sorry cannot upload the Steve killing document, but it is easily available:
http://www.stevekilling.com/books.htm

EK

PS: I saw interesting post from Doug Halsey in my box, but cannot find it on the forum? I m messing somewhere?

Originally Posted by Doug Lord

Just a bit of a relevant aside: the Moth class, before foils experimented a great deal with planing hulls, low wetted surface hulls and skinny hulls. They found that the improvement with a high L/B ratio hull(skinny) was superior to a low wetted surface hull -see sketch: ( Note that the advantage is particularly apparent with an 11' hull and less so as length increases) ---End Quote---
For a multihull (where the section shape doesn't strongly affect the roll stability), a rectangular underwater section with the beam equal to half the depth can do even better than the square section shown. Not only would it have a larger value of L/B, but its wetted area penalty would be considerably less.

 

I tried a section with a beam of .71 and a depth of 1.42 to maintain the one sq.ft. sectional area in the sketch below and:

1) L/B(for an 11' hull)= 15.49

2) Wetted surface= 1.42+.71+1.42= 3.55

 

Apologies

I posted a comment on Doug Lord's earlier "relevant aside" yesterday, & later deleted it when I realized that I had it all wrong. All I should have said is that, among cross sections with flat bottoms (& unit area), the square does not have the smallest wetted length. The rectangle with beam equal to twice the depth has wetted length of about 2.8 (compared to 2.5 for the semicircle & 3.0 for the square). Allowing flare in the sides, or multiple chines, you can reduce the wetted length even more, but all of these shapes are wider than the square & thus would have smaller L/B values & larger wave-making drag.

I was doing several things at once & should have waited until I was thinking straight before posting to the forum. Sorry!

 

=====================
No apology required, Doug. Everybody has a bit of brainfade now and then-I know I sure do! But it still helped me to explore this a bit-so thanks!

 

No worries Mate, don't bother to apologize, any comments are welcome and even a wrong idea can trigger a relevant one.

Please, what means "Flare in the side" ?
Is it similar to "convex sides" in opposition to "flat sides"?

Cheers all

EK

 

I think that what gets called "sea keeping" in conventional ship design dominates catamaran hull design. This is all stuff that overwhelms any CFD program or analysis simply because the surface of the water cannot be reasonably modeled. As a result, one has to rely on experienced observers and making educated experiments to learn what does and doesn't work.
The term "wave piercing" is pretty much a misnomer here. I prefer the designation "high water plane inertia" to better describe these hull forms. They also have less fore and aft curvature, this goes with water planes t6hat are fuller at the ends. It is hard to tell whether the reduction in rocker is more significant to performance than the resistance to trim change. What we do know is that the hulls seem to handle very well, are easier to steer at speed and do not seem to need the high freeboard and stem height formerly regarded as essential.
So much of catamaran performance is based on pulling the apparent wind forward that it is hugely important to avoid down spikes in speed. So negotiating waves cleanly and with a minimum of drag is far more important than in any other craft. So it's the dynamics that are really important, and really hard to model in any meaningful way.
SHC

 

Yes erwan, the square section provides reduced pitching and reduced wave making resistance compared to round bilge section. There may be other advantages also. I dont think there is anything new here tho, most people are already using this type of hull for racing.

The high speeds of racing make immersed transoms more efficient at these higher speeds compared to a cruising type hull with with dry transoms, and thus more rocker and more negative lift. I have modelled this and confirmed it via Michlet and Godzilla predictions of slender high speed displacement hulls. At slow speeds, an eliptical waterplane or waterlines (kayak) is the most efficient. This changes at higher speeds, and id have to go back through my work to give you an idea of the Frounde number at which this occurs as i cant recall from memory. At high speeds, the transom can be the same sectional area as the fullest section of the hull, with no drag penalty.

 

I would be very wary of results for elliptical waterplanes at low Froude
numbers because the small longitudinal slope assumption is violated. It is
not as important at higher Fr because the wavelength is large compared to
the region over which the assumption is violated.

 

Thank you very much for posting Mr Clark, and even more for providing the relevant perspective and appropriate terminology.

As I was not familiar with the concept of "High Waterplane Inertia" I try to do some homework before posting, but I am not sure I get it in depth.

As far as I understood "high waterplane internia" seems to go along with hight prismatic, the other dimension of high prismatic being the fullness of the hull cross sections at the bow and stern?

In fact I am not sure which is the relevant dimension/unit for "waterplane moment of inertia" is it expressed in: feet ^3 ? or feet^4 ? I have still some homework to do.

On this forum, I found a link for user manual of a MaxSurf sea-keeping application, for those who are intereted there is something to chew.
http://www.boatdesign.net/forums/boat-design/x-bow-resistance-calculation-44182.html . I ll look at it asap.

Regarding THE POINT :"Negociating waves cleanly with a minimum of drag"

I wonder if an hull shape with above-mentionned modern design features, but with cross-sections showing a narrowing beam going above waterline (assuming there are no structural issues), could help to meet this objective.

This "narrowing" feature only for middle hull cross sections, in order to keep unchanged volume & waterplane at the bow and stern.

The idea would be to minimize "chop drag" , I mean the additional drag generated by the above water line part of the hull going through the chop, as far as this drag is proportional to beam^2.

I don't think it is a new idea, more or less it is the same "spirit" of what is called semi-SWATH hull shape( to be checked)

K2MAV, GROVER, I know it is always intimidating to post after C-Cat or CFD gurus, but don't be shy!

A great week-end to Everybody

Regards all

EK