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

[dustman]

I've decided to with a cat-ketch rig on a 30' ultralight catamaran. How far back from the center of lateral resistance should I place my center of effort for the sails? What I have read so far seems to suggest approximately 20% aft of CLR. Thoughts? Feel free to expand on the subject.



Also hoping to get people's opinions on how far apart to place the sails. The way I see it, the farther apart I place the sails the more "leverage" they will have around the CLR, as well as having less potential to interfere with each other in the air stream. Thoughts?

[a64pilot]

And I thought a head sail and a main worked together to form a slot? Pretty sure my Staysail does, so I assume a Jib does as well?

[dustman]

Originally Posted by a64pilot

And I thought a head sail and a main worked together to form a slot? Pretty sure my Staysail does, so I assume a Jib does as well?

Are you indicating that I should bring the sails closer together?


If you are saying I should have a headsail... I'm keeping everything as simple, reliable, safe, and easy to handle as possible. From all my research, a cat-ketch with freestanding masts is what I settled on. I'm willing to give up a little efficiency and pointing ability to meet my criteria.

[a64pilot]

I hadn’t assumed a Cat Ketch, I had assumed a sloop. I must have skipped over that part.
I have a friend that had a Cat Ketch, he said everyone loved to make fun of it, but that it actually sailed very well, and he’s a racer, so I assume it must have.
He was an Aeronautical Engineer at my last job, and is sort of responsible for getting me into a cruising boat.
His was a Herresshof Cat Ketch so of course a mono.

I’m the furthest thing from a “real” sailor, so I won’t comment on design, cause I’m not qualified to do so.
You will I believe have an unusual boat when your done.

I’d assume that you may want to look at the Herresshof design though for ideas?

[Montanan]

https://www.youtube.com/watch?v=iFle9YZCzLw

Quite fast.

[psk125]

Generally people want their boats to have a touch of weather helm - with the CE aft of the CLR - as the OP has indicated. This is because having the boat turn into the wind ends up with it pointing into oncoming waves, which is a stable position for the hull. Hulls that are sideways to the waves tend to get flipped over. Turning the boat into the wind also causes the sails to luff, reducing the heeling they cause and helping to keep the boat upright. Having the CE forward of the CLE works going downwind, and will help keep the boat pointed downwind, but if you are trying to work your way off a lee shore that is NOT what you want.
Read Markaj’s book on aero and hydrodynamics. https://www.amazon.com/Aero-Hydrodyn.../dp/0877429936

[Don C L]

yes except that as a boat heels the CLR changes and aggravates weather helm, Now with a catamaran that is a different ball game. I thought this was a nice intro to it: https://www.jordanyachts.com/4023
That 20% figure you mention sounds really extreme to me especially I'd imagine when talking about a catamaran but I could be wrong.
And I'd still like to hear from cat ketch owners as to how much they lose in pointing ability, especially in a catamaran. I've seen cat boats, monohulls, Nonsuches, that seemed to me to point really well up on the SF bay many years ago. I WANTED to sail on one, but haven't yet had the chance!

I think the more you research Polynesian catamarans, the more inspiration you'll find... I'm thinking you'll find it's best to keep the masts as close together as possible.

[Don C L]

Originally Posted by Montanan

Quite fast.

Wow! Quite an endorsement for Freedoms.

[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.

On my cat ketch monohull the Center of Effort is 7% of the Design Water Line aft of the Center of Lateral Resistance. There is a balanced foil rudder which carries very light weather helm just sufficient to push the boat head to wind when the tiller is unmanned. Shims are used to individually tweak the main and mizzen mast rake to achieve perfect balance.

For comparison this is a 26 foot DWL boat with a design displacement of 3500 lbs. The mizzen is 130 sqft and the main is 216 sqft which readily gets the boat to hull speed.

The foot of the main sail is about 1.8 feet ahead of the mizzen mast. Close hauled there is definite interplay between the air running off the main and the power in the mizzen, just like between the jib and main on a sloop. Play with this separation carefully.

Thomas Firth Jones built a biplane cat ketch catamaran, which he wrote about in his book Multihull Voyaging. Here the masts were side by side one over each hull. Without the main - mizzen interplay above this boat was described as incredibly slow on the wind.

BTW your cat ketch cat is going to be blazing fast off the wind running wing and wing with the airflow reversed across the sail area (leaches become luffs). Make sure the sheets are long enough to get the sails well forward of the masts, then slowly sheet in and watch the speed climb.

[Don C L]

Brent, how well does your boat point upwind?

[dustman]

Originally Posted by psk125

Generally people want their boats to have a touch of weather helm - with the CE aft of the CLR - as the OP has indicated. This is because having the boat turn into the wind ends up with it pointing into oncoming waves, which is a stable position for the hull. Hulls that are sideways to the waves tend to get flipped over. Turning the boat into the wind also causes the sails to luff, reducing the heeling they cause and helping to keep the boat upright. Having the CE forward of the CLE works going downwind, and will help keep the boat pointed downwind, but if you are trying to work your way off a lee shore that is NOT what you want.
Read Markaj’s book on aero and hydrodynamics. https://www.amazon.com/Aero-Hydrodyn.../dp/0877429936

On page 540 of that book as we speak(type).



Seems like when going downwind having the full main and partially reefed mizzen would help keep the boat stable. I'll probably have a spinnaker onboard for light air downwind work.

[Sparx]

Originally Posted by Don C L

Brent, how well does your boat point upwind?

Frankly I am pretty happy with it. It maybe won't go as high as a tightly strung racing sloop, but it is right in there with an average cruiser. Short tacking is a breeze and the boat is quite nimble tacking up a narrow channel. Reaching and running it runs down most similar DWL boats.

[Don C L]

Originally Posted by Sparx

Frankly I am pretty happy with it. It maybe won't go as high as a tightly strung racing sloop, but it is right in there with an average cruiser. Short tacking is a breeze and the boat is quite nimble tacking up a narrow channel. Reaching and running it runs down most similar DWL boats.

My hypothesis is that with a catamaran, being faster, it will point even higher. I look forward to hearing more on it all.

[Sparx]

Originally Posted by Don C L

My hypothesis is that with a catamaran, being faster, it will point even higher. I look forward to hearing more on it all.

I too would like to follow this through to see the finished results.

I found a definite learning curve moving from a sloop to a cat ketch. I had to keep my head in the rig until I understood it. Now it all is second nature and the boat is a joy to sail.

[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...)