Cat hull resistance change with heel?

[Richard Woods]

Richard, I never said velocity increased 40%, but only wetted area decreased, and accordingly friction drag.

I know you didn't, that was bjn

RW

 

[Stumble]

My point was that a simple model of flying a cat hull needs to define this more closely. Is a floating hull considered floating when <5% of the boats displacement is being carried by that hull? Or does it need to be 50% of the displacement?

Because at no time does a cat go from 50% to 0% in a moment, there is a process that occurred to lift that hull, and at every step along the way the hull resistance will change.


As for my experience with A-Cats, yes I have some experience with them. I own and race one after all.

 

[patzefran]

Capt Rubin, I am glad you are an A cat fellow. I agree the hull lift progressively under some wind treshold, but above 8/9 kt this take few seconds , so this is unsignificant on VPP performance predicts. My only goal was to compare two hulls flat (which never happen, I agree !) to fully lifted windward hull. On my VPP I just interpolate between the two extreme drag (two hulls, one hull), depending on heel angle. This is only a mathematical artefact I agree, but I don't think the overall performance results are wrong. I can only observe they agree quite good with my numerous gps records. I used to sail a BIM V1 , and you ?

PG

 

[Stumble]

Capt Rubin, I am glad you are an A cat fellow. I agree the hull lift progressively under some wind treshold, but above 8/9 kt this take few seconds , so this is unsignificant on VPP performance predicts. My only goal was to compare two hulls flat (which never happen, I agree !) to fully lifted windward hull. On my VPP I just interpolate between the two extreme drag (two hulls, one hull), depending on heel angle. This is only a mathematical artefact I agree, but I don't think the overall performance results are wrong. I can only observe they agree quite good with my numerous gps records. I used to sail a BIM V1 , and you ?

PG

I have an Evo XT. It's from a small shop here in the US called BlueStreak Boatworks.

At least on an A-Cat I don't think you can keep both hulls in the water at 8-9kn of wind, at least not with both hulls evenly loaded (or close). But I was thinking about this and it crossed my mind that it may be testable with a tank. Drop one hull in the tank at a given speed and measure the resistance, then drop both hulls in and measure the speed at the same resistance.

If you assume (and I think you can, but maybe wrong) that the resistance is equivilant to driving force, speed at the same resistance would allow you to quantify the difference.

 

[Richard Woods]

If you mean a towing tank that would be a huge, very long tank, especially given the speed you'd want to go. For a full size test its better to tow the Acat with a spring balance. But a bit tricky doing just one hull!

RW

 

[DennisRB]

I'm still here. Thanks guys. Interesting discussion. I am trying to learn Michlet, not an easy task.

I think I have finally got my head around a few things here. But seems there is some disagreement within the responses regarding if the drag goes up, down or stays the same with heel.

Adhoc, who I know is seldom wrong on these sorts of issues says there is no big difference (if I have understood correctly). So Adhoc, I presume you meant the only real difference occurs thanks to things dragging through the water such as, outboard pods, bridgedecks and beams, and big differences in transom immersion?

Others have said there is a large difference (40%) thanks to reduced wetted area. I can understand there would be a reduction in wetted area. The total displacement stays the same, so it does seem intuitive that the total residuary resistance stays very similar, but I thought spreading that displacement over either one or 2 hulls of the same shape would also have some effect, in the same way as changing BT does.

I was hoping a commonly known file existed plotting the various forms of drag as heel increases. I think I have seen this before but could not find it again.

 

[DennisRB]

Here is an interesting article by Shuttleworth. It only mentions wetted surface area, but it shows the difference between a cat and a tri, with the tri starting off with lower WSA which increases until a certain point when the main hull is starts to unload. The cat has a lot more WSA to begin with but it always reduces with heel. It would be very interesting to see these same plots with residuary resistance and total resistance included to establish a trend.

http://www.shuttleworthdesign.com/BritFertalk.html

 

[bjn]

I know you didn't, that was bjn

RW

I said that the friction resistance of the hulls becomes less by a factor of the square root of two.

And I also tried to say that it's not digital. It's very analog. The resistance will decrease just by heeling a bit. Others have added that one extra advantage of full flying, is that boards and rudders will be less (or not at all) in the water, which decreases friction further.

Frictional drag is proportional to speed squared. If lifting a hull in 10 knots, speed should increase a little less than 2 knots (increase by the factor of 2^(1/4)), if you only consider the frictional drag of the hull. But there are many other factors in the equation apart from hull drag.

 

[MichaelRoberts]

Good discussion all.

I am building a cruising cat of my own design, a Rhino picture is attached.

Related to the changing wetted surface and prismatic ratios is another possibly rather dangerous problem which does not seem to be talked about much.

The problem is this: a torque that turns the boat downwind arises when the cat heels a few degrees and the resistance of the lee hull becomes becomes greater than that of the windward hull.

This turning force is generated because the driving force on the rig is in the centre and the extra drag on the lee hull is a few meters to lee. The boat wants to head down wind when heeled by a gust!

This tendency to turn down wind is opposite to a monohull which likes to round up when heeled because the centre of effort moves to lee side of the centre of center of lateral resistance.

So it becomes very difficult to get the balance between centre of effort and centre of lateral resistance. In my case I had to put the centre boards a bit forward of the mast.

Be grateful for any comments.

Thanks

Michael

 

[DennisRB]

Have you ever actually noticed this lee helm though? In my experience weather helm has still always been required to keep any cats straight that I have sailed. Even with only the jib up my cat still needs a few degrees of weather helm to maintain course. There must be more to it.

Nice looking boat. Going by helm feel, board location and rudder angles of boats I have sailed on, I would guess you will have a lot of weather helm with those boards so far forward. If you came up with that location you have to ask why are they further back on existing designs which are pretty similar to yours? My boards are behind the mast I wish they were even further back, especially with the square top main which made weather helm even worst than before. Take a look at kurt huges's site, his boards are really far back.

 

[MichaelRoberts]

Does a heeled cat want to turn down wind?

Thanks for your prompt response and kind remarks DennisRB.

The boat is not finished yet, that's why I'm still wondering. Your comments encourage me to increase the rudder size to give a bit more lateral resistance aft.

The reason my centre boards are fwd is because there is not much lateral resistance in the forebody ... the bow just graces the waterline ... unlike some cats like Lagoons which have deep bows. My idea is the long, low buoyancy bows will pierce short chop waves like a surf board instead of pitching up and stopping the boat.

Suppose if I've screwed up I'll just have to add more jib and genoa.

I did some sums on this torque and it ain't nothin. At 15 knots the resistance per hull is about 4,000 N, if the lee hull sinks a bit its resistance could easily be 1,000 N more than the windward one.

Multiply 1,000 N by the distance from centre of effort (half the beam), that's 3.5 m, and you get a torque of 3,500 Nm. Possibly significant?

Looking forward to your responses gentlemen

Michael

 

[Ad Hoc]

... (if I have understood correctly). So Adhoc, I presume you meant the only real difference occurs thanks to things dragging through the water such as, outboard pods, bridgedecks and beams, and big differences in transom immersion?

Others have said there is a large difference (40%) thanks to reduced wetted area. I can understand there would be a reduction in wetted area. The total displacement stays the same, so it does seem intuitive that the total residuary resistance stays very similar, but I thought spreading that displacement over either one or 2 hulls of the same shape would also have some effect, in the same way as changing BT does.

Sorry for delay, been travelling.

There is next to no difference, save for, as you correctly noted appendages which are now dragging through the water and the varying transom depth ratio.

The change in WSA...ok...how much change?...until a value is attributed to this, one cannot say how much it really effects the final resistance. Since if the LD ratio is low, you wont notice any real change, thanks to the higher residuary. If the LD ratio is high, then changes to the WSA may be more dominant. But for that to occur it must be a radical change in hull shape from non heeled to heeled. But that would suggest, for example, floating and heeled at an angle of say 60-90 degrees, for a "typical" hull that has a flat deck.

Since if the hull heels say 10 degrees, the change hull shape in the now immersed 10 degree hull side ( say port) to that of the unsubmerged side of + 10 degrees (stbd) would not be that much, thus the change in WSA on the heeled immersed side would not alter the WSA total much at all either, minimal. The change in WSA would have to be dramatic to radically change the frictional resistance. Thus appendages would be the biggest influence, and the transom/depth ratio.

So, if the weight does not change (no external load applied) the residuary remains the same, since the waterline length is, ostensibly unchanged. The WSA change is minimal at best as the length of submerged hull side (athwartships) is nearly the same as the unsubmerged side, thus the frictional change is marginal at best. Therefore unless there is a radical change in heel that produces a significant change in the shape and hence WSA, by a large factor, there shall be minimal change, if any to the frictional too. Especially with a low LD ratio hull. Ergo the total resistance, everything else being the same, experiences little if any change. Unless one wishes to cite changes in decimal places shown on a computer program as "evidence" of a change!!. Since any small change is change positive or negative, from the datum, but is it of any real importance or significance? er...nope!

So draw waterlines at 5 or 10 or 20 degrees on your hull. Measure the difference in athwartship hull length...how much difference overall from the non heeled is it?...and there is your answer

 

[waikikin]

Have you ever actually noticed this lee helm though? In my experience weather helm has still always been required to keep any cats straight that I have sailed. Even with only the jib up my cat still needs a few degrees of weather helm to maintain course. There must be more to it.

Nice looking boat. Going by helm feel, board location and rudder angles of boats I have sailed on, I would guess you will have a lot of weather helm with those boards so far forward. If you came up with that location you have to ask why are they further back on existing designs which are pretty similar to yours? My boards are behind the mast I wish they were even further back, especially with the square top main which made weather helm even worst than before. Take a look at kurt huges's site, his boards are really far back.

Great points, typically multis will develop some forward trim when pressed harder.. maybe this brings the clr forward.

Jeff

 

[Richard Woods]

I have also been travelling (by catamaran!)

If I was designing my first catamaran, especially as an amateur, I would copy what others do. The vast majority have their CLR aft of the mast. I think Lock Crowther was the only one to have forward of the mast foils. The Crowther I sailed set up like that had a masthead rig. Crowther also fitted too small rudders and foils. They never did well in the UK

Wharram put forward foils on his Pahis but they generated massive weather helm, almost uncontrollable unless you were hull flying (I know, I sailed the prototype 35 a lot and yes, hull flying on a Wharram!)

I would go for a 4% lead and make a deep forefoot. Otherwise you'll slam badly and the bows will drift off coming out of waves going to windward. I have never had any lee helm problems even with the lee hull well under water and the main hull well up. Its different on a trimaran though

Of course hull flying occurs at a lesser angle with a shallow wide spacing catamaran than a narrow deep one like a wharram or Catalac. Maybe 5deg for a racey boat, 15-20 deg for a Catalac

sorry bjn I misunderstood what you said

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[MichaelRoberts]

CLR and shallow forebody

Thanks for your comments Richard. Your scenario of side slipping shallow bows sounds scarey. Oh well, too late now, the hulls are built and waiting to meet their bridge deck.

In spite of the boards being in front of the mast beam, the CLR of my design is well behind the mast and there is the usual slight lead ahead of centre of effort. The more subtle problem is how to actually determine CLR and centre of effort. I have simply used the centroids of hull and sail areas. But this does not really account for hydro and aerodynamic effects.

So how do real designers estimate the centre of effort of a sail plan?

One other reason I have a long ramp of a forebody is to ensure the hydro lift under the bows does not go to zero even when the boat pitches forward a few degrees. I theorise loss of lift under the forebody causes longitudinal oscillation aka hobby horsing.