Center of lateral resistance vs center of effort, where to place sails on cat-ketch?

[roland stockham]

I am also wondering how you are going to get the forward mast stepped far enough forward on a cat. I suspect it would need to be well forward of the bridge deck. I have seen a cat rigged with cat sails on each hull, it had a bridge between the mast as well but it was 'in passing' so I have no details except to think it has bee done.
I would also say that unless you are very good with CAD you probably need to make at least one of the mast basses movable by at least 5-10% so you can tune the rig by trial and error. With such a unique setup there may be no other way to find the optimum mast positions. Sounds like an interesting project.

[Door Sail]

Talk to Chris White.

[Don C L]

Originally Posted by rhubstuff

Hmmm...A cat ketch has the mainmast right forward in the bow. I'm trying to envision a catamaran with a cat ketch rig. Which hull is the main going to be stepped in? Or one in each hull? Or somehow between them? (Do not try this!) In my experience owning two cat ketches (Herreshoff 18 and Sea Pearl 28), I think you are a bit too concerned with CLR vs. CE. What happens with this rig is the you can vary the CE a great deal at will, with sail trim, from CE way forward to CE way aft. In fact, this is exactly how you will trim the boat to sail herself for hours with no one at the helm. On my SP 28, with the main full and drawing, the mizzen likes to carry just a bit of a luff about midway up, then she's balanced and happy for miles. I would simply use a diagram of the boat and draw in what looks right. (Seaman's eye and all that. If it looks good, it will be.) Make the mizzen the same height as the main, or a bit shorter, and put a huge roach in the mains'l, less roach in the mizzen. You will need full battens rigged on cars at the mast track, probably four of them on each sail. Refer to the Sea Pearl 28 drawing in <sailboatdata.com>. Good luck, Sir. (I'm still stuck on where the main will be stepped...)

Good point!! Here I am saying catamaran but thinking proa (for unstayed masts) and a Polynesian rig for catamarans... although I’d be interested in seeing the engineering necessary for unstayed masts on a catamaran.

[zstine]

I think the OP may have misunderstood the 20% figure he was reading...? The CE should be much closer to the CLR and I would suggest, in lieu of detailed analytical models, that he reverse engineer an existing design. The center of lift on a foil is often assumed near 20% cord length. Maybe that's the figure the OP was referencing?


I also second the structural concerns retrofitting a cat-ketch on a catamaran. Most cats have little structure where the mast step would go... This cat-ketch mod is a huge undertaking!

[rhubstuff]

I would suggest that the cat ketch rig is most suitable for a mono-hull or maybe for a tri, but not at all for a catamaran. I cannot imagine how it would be done. And the thing about split rigs (ketches, yawls, schooners) is that the CE is, for all practical purposes, variable way beyond what you need for balance. You use sail trim to balance the boat. On my Sea Pearl 28, for example, I can move the CE up to about 12 FEET (!) aft of the CLR by raising the mizzen alone. (I actually do this every time I sail; raise the mizzen, sheet her in hard, and the boat sits head-to-wind calmly while I fuss with the main, Then, back the mizzen to port or starboard, and off we go.) Or I can move the CE about 6 FEET (!) forward of the CLR by raising just the main. When sailing, I can ease either main or mizzen sheet and instantly change the CE/CLR relationship any way I want to. I bought my present SP 28 three years ago, and after having sailed and skippered on sloops, schooners, yawls, ketches, and a square rigger, I was amazed at how easily a cat ketch handles. In fact, like a good schooner, if you trim her right, you only have to touch the helm to tack or gibe. So don't over-stress about the CE/CLR thing. Its all about trim.

[dustman]

Originally Posted by rhubstuff

Hmmm...A cat ketch has the mainmast right forward in the bow. I'm trying to envision a catamaran with a cat ketch rig. Which hull is the main going to be stepped in? Or one in each hull? Or somehow between them? (Do not try this!) In my experience owning two cat ketches (Herreshoff 18 and Sea Pearl 28), I think you are a bit too concerned with CLR vs. CE. What happens with this rig is the you can vary the CE a great deal at will, with sail trim, from CE way forward to CE way aft. In fact, this is exactly how you will trim the boat to sail herself for hours with no one at the helm. On my SP 28, with the main full and drawing, the mizzen likes to carry just a bit of a luff about midway up, then she's balanced and happy for miles. I would simply use a diagram of the boat and draw in what looks right. (Seaman's eye and all that. If it looks good, it will be.) Make the mizzen the same height as the main, or a bit shorter, and put a huge roach in the mains'l, less roach in the mizzen. You will need full battens rigged on cars at the mast track, probably four of them on each sail. Refer to the Sea Pearl 28 drawing in <sailboatdata.com>. Good luck, Sir. (I'm still stuck on where the main will be stepped...)

The masts will be integrated into the fore and aft crossmembers. Here is a picture of a quick drawing I just did. Don't mind the proportions, they are not even close to scale.


As far as CLR vs CE, my goal is have to have the boat be well balanced(with very slight weather helm) while carrying full main and mizzen, then make minor adjustments in trim to balance her, reef one or the other only when necessary.



I'm still working on sail/rigging design. Trying to come up with the simplest, and simplest to handle rig possible, while maintaining reasonable performance on all points of sail. I'm not going to obsess myself with pointing as high as possible, like the majority of people on here seem to be obsessed with.

[dustman]

Originally Posted by roland stockham

I am also wondering how you are going to get the forward mast stepped far enough forward on a cat. I suspect it would need to be well forward of the bridge deck. I have seen a cat rigged with cat sails on each hull, it had a bridge between the mast as well but it was 'in passing' so I have no details except to think it has bee done.
I would also say that unless you are very good with CAD you probably need to make at least one of the mast basses movable by at least 5-10% so you can tune the rig by trial and error. With such a unique setup there may be no other way to find the optimum mast positions. Sounds like an interesting project.

The front crossmember to which the mast will be mounted, will meet the hulls toward the bows. But keep in mind the boat will be very light with relatively small sails for a waterline of the 30' length, giving me ample room to play with sail placement in the design process. Ill be building a smaller version of this to test out various factors/characteristics before building the final product. I know there are scaling issues, but will get me closer.

[dustman]

Originally Posted by zstine

I think the OP may have misunderstood the 20% figure he was reading...? The CE should be much closer to the CLR and I would suggest, in lieu of detailed analytical models, that he reverse engineer an existing design. The center of lift on a foil is often assumed near 20% cord length. Maybe that's the figure the OP was referencing?


I also second the structural concerns retrofitting a cat-ketch on a catamaran. Most cats have little structure where the mast step would go... This cat-ketch mod is a huge undertaking!

That figure doesn't sound right to me either, so I am doing much research and even building models before implementing at full scale. 5-10% sounds like a much more reasonable figure to me.



The frame of this catamaran will be extremely overbuilt for the 120-150ft2 of sail I will be carrying divided between two masts with a low center of effort. I am much more concerned with wave impacts and long term metal fatigue.

[dustman]

Keep in mind, I won't start building this project for probably 2 years. There will be ongoing research on my part between now and then. I will certainly be showing my plans to a structural engineer before finalizing my design.

[Sparx]

Seems a simple engineering solution to run a beam down the center spanning between for and aft cross beams. Then mount the masts on tabernacles riding on this beam. The tabernacles include shims to adjust mast rake, plus the tabernacles are bolted on in a way that allows the tabernacles to be shifted fore or aft on this beam. Between moving the tabernacles and tweaking shims the mast placement is dialed in with experience, starting with an engineering best guess.

There is a sweet spot for the main mast location where there is sufficient buoyancy to carry the press of the main and not bury the lee bow. I suspect it is farther aft than on a monohull cat ketch. For this reason it might be useful to put some fullness in the bows for secondary buoyancy forward. I keep referring mentally to the Wharram Tiki design (which carries a jib of course).

I would not skimp on sail area, intending to build in sufficient power to press through the waves. I might err on the high side, planning to reef early if need be. But I like a fast ride.

[carlosproa]

Originally Posted by rhubstuff

Hmmm...A cat ketch has the mainmast right forward in the bow. I'm trying to envision a catamaran with a cat ketch rig. Which hull is the main going to be stepped in? Or one in each hull? Or somehow between them? (Do not try this!) In my experience owning two cat ketches (Herreshoff 18 and Sea Pearl 28), I think you are a bit too concerned with CLR vs. CE. What happens with this rig is the you can vary the CE a great deal at will, with sail trim, from CE way forward to CE way aft. In fact, this is exactly how you will trim the boat to sail herself for hours with no one at the helm. On my SP 28, with the main full and drawing, the mizzen likes to carry just a bit of a luff about midway up, then she's balanced and happy for miles. I would simply use a diagram of the boat and draw in what looks right. (Seaman's eye and all that. If it looks good, it will be.) Make the mizzen the same height as the main, or a bit shorter, and put a huge roach in the mains'l, less roach in the mizzen. You will need full battens rigged on cars at the mast track, probably four of them on each sail. Refer to the Sea Pearl 28 drawing in <sailboatdata.com>. Good luck, Sir. (I'm still stuck on where the main will be stepped...)

You can take a look a K-Designs DUO800S for a biplane rig. Bernd Khoeler has also a smaller design. He is active in his group and may have advice. There is a good explanation on how to play with the sheets to create a slot.

https://duckworksmagazine.com/14/des.../#.XjuJEiROnYU

[Ken Fry]

Originally Posted by Don C L

Wow! Quite an endorsement for Freedoms.

Interesting. It would have to be carrying a tremendous lee helm.

[valhalla360]

Originally Posted by Sparx

A cat ketch cat is a great idea. I am surprised I don't see more of these out on the water. The two masts allow the boat to carry a powerful sail plan with the center of effort relatively close to the deck. Free standing masts will bend off in the gusts allowing the boat to sail powerfully in the wind with low risk off capsize in the gusts. Further the mizzen is easily tweaked to keep the rudders neutral for great speed.

Problem on a cat is how to support a free standing mast between the hulls. Particularly on smaller cats.

Free standing masts on Monos are typically keel stepped. That gives a strong arm below the deck to resist the winds effort to bend the mast over. That arm is often 5-7ft long.

On a small cat, getting even 3ft can be difficult which means you really would have to beef up the deck and bottom of the bridge deck to resist the forces. It can be done but creates other complications.

Cats do lend themselves to some interesting sail configurations. If I were building I would look at an A-frame with a crab claw sail. That keeps things simple and the center of effort low. Also makes dropping the rig to get under bridges very much viable to do on the fly. It would need a bit of reinforcement across the deck as the A-frame would put the deck in tension but that is a much simpler problem to solve.

[Jim Cate]

Originally Posted by Ken Fry

Interesting. It would have to be carrying a tremendous lee helm.

If the photographers boat is hard on the wind the Freedom is pointing very well, which makes one doubt much lee helm.

One does wonder what sort of boat is involved, w hat point of sail and how hard they were trying. Looks pretty embarrassing!

But FWIW, I did some singlehand racing out of SF years back, and a Freedom of similar size was no match for my Yankee 30, upwind or down. I was kinda surprised, for her skipper had talked a really good race ahead of time!

Jim

[Ken Fry]

Originally Posted by dustman

Thoughts?

Yes, I used to have those.

Oh wait, here comes one or two: with your quite small sail plan you will not have much helm, one way or the other, to deal with. So if you are off, you can easily adjust it out with sail areas, or even a little easing or hardening.

I like to calculate these things according to the amount of helm I want in an ideal condition. (But my last couple boats have had hard wings, where more precision is required than for a typical boat.) The rudder and daggerboard will have their centers of effort at 25% of chord. The center of effort of each sail will be in a pretty similar location -- 25 percent behind the leading edge. Determine how much balance you want around the rudder pivot. I'd think that none (pivot at or near leading edge) would be good, again for the small sail plan, light boat. I'm making guesses that all these things are not yet finalized. If, because you are crazy, you want no feel at all, then pivot the rudder at 25%. 15-20% gives very light feel -- probably too light for your boat.

Pick a sail force, equally distributed between the two sail centers, that represents a condition that feels good for you, and for which you can say: in that condition I want a tiller force requirement of one pound... or 2, or one half. It's personal. Some people like to sail with a slight lee helm. I don't. Most people do not. It is "safer" to have some weather helm, and in a cat the helm changes less due to heal.

In english units, the formula for lift is: (sq ft area) x .0012 x (apparent windspeed in fps, squared) x (lift coefficient, for which you can assume .8)

Then it is just a matter of calculating the various leverages.

On the other hand, if you just put the combined centers at 5% behind the 25% chord point on the daggerboard, you'll be fine.

Remember:
As you bear off, you'll gain some weather helm from the lateral movement in the centers of effort.

Assumptions: daggerboards near the hull CLR, then the daggerboards can be thought to predominate. Daggerboards and rudders with standardish airfoil shapes.

How obsessive do you want to get?:
In a perfect world, you would play with daggerboard angles of attack rudder size, etc to make them both operate at an angle of attack (re the onflowing water) of 4 degrees. Also, you will not want the entire boat to be angled at 4 degrees, because the hulls are extremely inefficient at creating lift to windward. Very narrow hulls, which yours will be, are even worse for being dragged slightly sideways through the water than fat hulls.

For daggerboard and rudder lift, in the lift formula substitute .97 for the .0012, to account for the different density of water, and use boatspeed in fps instead of windspeed. 9 knots might be a nice speed. (or you can calculate this more accurately by looking at resistance, the vector of sail lift aimed forward.

If, by the way, the daggerboards are asymmetrically angled, then you need to be sure to lift the windward one.

Many, if not most, boats are not designed through this process. Rules of thumb are used, and if you want, you can just scale up pieces from a beach cat, or use a Stiletto 27 as a guide, etc.

For these various moments, scale effect has no effect. So a model will balance like the full sized boat. That's not perfectly true, but its true enough.

This might be a pic of my Windrocket. It should give you pause about following any of my advice: