Two hulls, which has least drag?

I've just started another thread on "planing" that might be of interest. Your
prof's definition is similar to my starting point.

And it is "planing". Only people with auto-correct use "planning"

 

Doh!!...

Sussed it...

 

Yeah, some planning is required before writing about planing.

The plan should include the item "uncheck the automatic spelling correction feature".

 

Hey Mikko, relative to that topic - I think you'd be interested to see this thread too: http://www.boatdesign.net/forums/hy...-simulation-cray-xe6-supercomputer-45247.html
Cheers

 

Mikko - That is a most generous offer and I will certainly bear it in mind if I ever get to the stage of making models for tank testing. I would say that when I started this thread I hoped that the experts would simply tell me the answers so saving me the considerable work of model making!

Ad Hoc has produced a graph that shows that varying b/t whilst keeping length and displacement the same has little effect on total resistance, at least in calm water. I have only had time to skim through the Southampton University paper from which this graph is extracted (I was pleased to find that the paper is freely available from the internet), I do need to study it in more detail. I note that the hull forms used in the tank testing on which this paper is based were more typical of power catamarans with immersed transoms than sailing catamarans that are usually designed to float level with non immersed transoms. Whether that is significant, I would not know at this stage. Also, although the graph shows little difference in total resistance between the three b/t values, the small differences that are indicated are consistent over a range of Froude number and are probably quite large enough to make the difference between first and second place in many yacht races!

Groper then told us that he has run Michelet for hulls of same length and displacement but varying waterline beam, presumably the narrower beam hulls having greater depth. He found that minimum total resistance was at around 25:1 length to beam ratio, lower length to beam ratio and hence greater b/t presumably giving significantly greater total resistance. This seems at odds with the graph from Southampton University as referred to by AdHoc which indicates little variation in total resistance with varying b/t. Having said that, since we dont know the displacement/L^3 values for the hulls that Groper modelled with Michelet we dont actually know the b/t values. The 25:1 length to waterline beam that Groper found to be optimum does seen to be quite a high ratio, higher I would have thought than most multihulls afloat, including America's Cup boats?

Increasing b/t at same displacement and length will certainly increase hydrostatic pitch stiffness for small pitch angles and would also seem likely to increase pitch damping. One might imagine this would have some effect on total resistance in rough water, but we dont really seem to be at the stage that we can say whether such effect is significant or even which way it will go. It may perhaps be rather dependent on wave encounter frequency. One point is that greater hydrostatic pitch stiffness will presumably help to resist pitchpoling of a sailing multihull.

 

The americas cup multis and other racing cats do have a L/B ratio up around 20-25:1...

The b/t ratio of the hulls i was modelling were between 1 and 1.5 because the length displacement ratio were on the favorable side of multihull performance, not fat overweight boats to which optimizing the resistance is clearly of little importance in a design SOR.

 

That’s for you to decide. Since any change in the principal dimensions, geometry only, shall influence other factors of the design. No one aspect is mutually excusive of others, they all affect each other. Changing B/T shall of course alter the structure, but is it altered significantly to maintain that very very small window?...does it alter the statical stability curve that requires a little extra top side beam??..does it alter the WPA enough the alter the natural periods of motion which negates the positives.. is the eqpt which is ‘heavy’ being seated a tad higher..is that effecting any other parameter...This is design per se.

And of course, if one wishes to be more pedantic, …the motivation of the crew, was poor today because they had a late night…the weather made us feel a bit off colour as we just landed after a long haul flight etc…is thus also thrown into this mix as a variable..and so on.

A boat is not made up of a series of "best" absolutes, it is far greater than the sum of its individual parts. How you put them all together, and achieve your SOR is …ahhh..that’s the fun part. Tank test results…is just the beginning of the process, not the be all.

 

They do have hard chines aft from which the water separates cleanly when they are traveling at high speeds. Cut off how the aft several feet of the hull, add a transom to fill in the hole, and the the resulting hull shape is that of a narrow "planing" sailing dinghy.

 

When I was an IC sailor, my boat would do something that I and others confidently described as planeing (my spelling). Water would break cleanly from the aft surfaces despite the pointy end. The chines aft were sharp on most of the various designs that were competitive. An almost universal characteristic is a very straight, flat run. Some with vee bottoms, like most of the Whitmans, transitioned to near flat at the exit. The boats in what we may have erroneously called planeing mode do have a slight positive AoA. Fox, Marchaj, Nethercott, Proctor, Beaver, Whitman and the rest would all argue that their IC would plane.

These boats are neither fish nor fowl. They will plane off wind and sometimes upwind. When the skipper is at planks end he can sometimes see the centerboard. The boat will be heeled and it is then not planeing but still going like a bat. In those cases it is essentially a unimaran, long and very skinny. So maybe it is both a ULDB and a planeing boat. I agree with Ad Hoc on the one hand but not on the other.

OK so that diatribe does practically nothing for establishing an adequate definition of planeing. Leo has a nearby thread that is examining that definition in detail.

 

Looks like plan(e)ing to me! And there's not even much wind there,

 

from messabout

When I was an IC sailor, my boat would do something that I and others confidently described as planeing (my spelling). Water would break cleanly from the aft surfaces despite the pointy end. The chines aft were sharp on most of the various designs that were competitive. An almost universal characteristic is a very straight, flat run. Some with vee bottoms, like most of the Whitmans, transitioned to near flat at the exit. The boats in what we may have erroneously called planeing mode do have a slight positive AoA. Fox, Marchaj, Nethercott, Proctor, Beaver, Whitman and the rest would all argue that their IC would plane.
______________________________

Planing in a sailboat and a powerboat are different. The thrust of a sailboat is way up high and forward while a powerboat thrust is low and aft. So a sailboat gets most of its lift in the mid part of the hull bottom while lift in a powerboat is further aft. This is just a general statement and things change for different boats and hull shapes. For instance, some sailboats that will readily plane under sail just become wallowing dogs under too much power as the aft rocker tries to sink the stern. Sailboats designed specifically for speed with flat aft bottom sections perform more like powerboats and vice-versa.

Some have designed powerboats with "mid planing" hulls and they apparently perform pretty well. I might put Calkin's Bartender in this category although others may disagree.

 

Cats may have a flat bottom for reasons other than planing. For example, with a flat bottom, the windward hull is clear of the water at a lower heel angle. This can make a significant difference because best performance is typically found with the windward hull just kissing the wave tops.

 

IIRC, that is exactly how the Finn came about. The hull shape is literally a sailing canoe with the stern cut off.

 

well??

so like all these things that get started is the any simple answers to long and skimmy or broad and same length .
whats the outcome of all this airing of veiws .
Me im with board not skinny .

 

what did you mean with pitch stiffness and pitch damping? and is this the right question? if you sail into a wave what is the question?

A: how fast the hull will reakt against the pitch move generated from the wave,

your hull with higher b/t will reakt faster and stronger, so its stiffness and damping is increased, as you said

or

B:how much change of trim will the same wave generate to the hull

IMO a smaler hull with lower b/t will react less to a wave and at the end his change in trim is lower although his stiffness and dumping is degreased



for a given length and displacment, the underwater surface is minimal with half circle spant ---->(b/t = 2)

so if you change the spant in direktion to lower b/t the surface with his flow-drag will increase, but the bow-wave resistant (and also pitch reaktion against outside waves) will degrease because of smaler hull, smaler entery angle

so the optimum seems found as the minus for the hull-surface drag is equel to the plus for the wave drag

until both changes with different exponentials over the speed (fn) there will be a different output for different speeds....

the optimum can also be with deaper than wide hulls b/t < 1,

but for a sailing catamaran with different speeds over a high range I tend to the value of groper and would start with b/t=1, and a nearly eliptic spant-form......