Excellent article on downwind sail trim for Cat Ketches

[rattus]

Michael Storer, designer of the Goat Island Skiff, has an excellent blog post on trimming for downwind sailing with a cat ketch with explanations and footage from the Texas 200 to demonstrate. Fascinating.

http://www.storerboatplans.com/wp/design/rig/sails/sailing-unstayed-cat-ketches-and-cat-yawls-safely-and-efficiently-downwind-in-strong-winds/

His is a more unbalanced sail plan (mizzen is quite a bit smaller than the main) but I believe the sail positioning tips would also apply to CS designs.

If a picture is worth a thousand words, perhaps a video is worth a thousand pictures! ;-)

Mike

[John Turpin]

That was very interesting, but I'm not sure that I understand why sheeting in/out changed the heeling angle like that. Which heeling scenario would be more adventageous? Heeling toward the wind?

[MisterMoon]

I personally find a leeward heel more comfortable. I think the point is that if you are by the lee and the leeward heel becomes excessive, pulling the sheet in flattens the boat unlike other points of sail where easing the sheet gives the same effect.

[MisterMoon]

I personally find a leeward heel more comfortable. I think the point is that if you are by the lee and the leeward heel becomes excessive, pulling the sheet in flattens the boat unlike other points of sail where easing the sheet gives the same effect.

[boatmik]

HI,

Thanks for putting the link up to the article. It is meant to be suggestive rather than "the way to do it"

The first part talks about which way to goosewing - this is applicable to the B an B rigs which have a bigger mizzen than most. It also applies to the Cat rigs

The sailing with the most powerful sail - partly because of position and partly because of area) by the lee gives you a way of controlling heel in a controlled way. It is not really to have weather heel or lee heel, but to be able to understand the reasons for heel and to take it under control.

Generally boats are fastest pretty level. But with this method you can "dial in" the amount of heel that you want or use a sudden trim to damp rolling.

There are limits to it too, such as having a long enough mainsheet to let the boom forward of the mast and reverting to the normal methods when you want to reduce power as the wind gets stronger.

The other advantage is for speed sailing that the front sail is at maximum power much more of the time. Something quick boats like the B and Bs and the Goat will benefit from more than others.

HOpe this helps explain it a little more.

Best wishes and good sailing.

Michael Storer

[John Turpin]

Thanks, Michael. Can you explain the physics of why the sail angle affected the heeling like it did? In the video's scenario, did sheeting in the main simply depower the sail and cause the boat to heel less or did the new sail angle somehow create some power toward windward and "pull" the boat to a more flat orientation? What is actually happening there? Or is it just magic?

Thanks!

[Peter HK]

Thanks, Michael. Can you explain the physics of why the sail angle affected the heeling like it did? In the video's scenario, did sheeting in the main simply depower the sail and cause the boat to heel less or did the new sail angle somehow create some power toward windward and "pull" the boat to a more flat orientation? What is actually happening there? Or is it just magic?

Thanks!

John

I think this image from the article is the explanation you want.

As you can see the force of the sail (blue arrow) is not directly forward but a little to the side when fully eased and a little the other way when trimmed in. The force can be considered as a combination of a forward component and a small sideways component. As this force is acting through the CE of the sail, nearly halfway up the mast, these sideways forces will heel the boat one way, or the other, depending on trim.

HTH

Cheers

Peter HK

[Michael Collins]

Hi Michael,

Thanks for the article and video. I accidentally learned that sailing by the lee in a cat ketch was stable through many hours in Everglades Challenges. However, I do not remember noticing that you could adjust the heel so dramatically with sail trim. I look forward to trying this next time I am on a cat ketch.

Great job,

Michael

aka Greybeard

[boatmik]

Howdy Michael.

It is also applicable to Cat rigged boats with unstayed masts. It originally came from Finn, Europe and Laser class racing boats.

Peter HK - thanks for putting the diagram up. It shows the different direction of the forces as the boom angle changes.

You have to be a little bit beware of such diagrams because they show general principles ... not something measured and scientific. But you can see the change in heel in the video on my website. It is clear and immediate as a real effect.

MIK

[Ray Frechette Jr]

There are limits to it too, such as having a long enough mainsheet to let the boom forward of the mast and reverting to the normal methods when you want to reduce power as the wind gets stronger.

The other advantage is for speed sailing that the front sail is at maximum power much more of the time. Something quick boats like the B and Bs and the Goat will benefit from more than others.

HOpe this helps explain it a little more.

Best wishes and good sailing.

Michael Storer

As specced the CS boats have enough sheet length to allow sails well forward of beam.

Also when sailing downwind with a mizzen staysail it is advisable to foot off a bit and not go directly downwind. Gybe main over to windward deck forward of beam and cleat off staysail on windward deck and fly that on leeward side. Granted the Mizzen blankets the staysail a bit but you are still sailing faster than without staysail.

[Hirilonde]

With my limited couple hours sailing Ken's CS I can't claim any significant knowledge about sailing a cat ketch. But most boats aren't worth sailing dead down wind at all. They are faster, and for planing boats much faster tacking down wind (gybing back and forth) between broad reaches. Even though the course is longer, the added speed more than makes up for it. For moderate winds most boats have an optimum TWA (towards wind angle) of between 135º - 150º Has Graham ever released the polars for any of his boats and/or has anyone ever tried to figure out just what TWA is best for sailing when trying to make headway down wind?

[Peter HK]

has anyone ever tried to figure out just what TWA is best for sailing when trying to make headway down wind?

I once went to the trouble of doing a set of figures for a polar diagram for my CS 17, armed with a compass and GPS. The limitations are obvious as this was only one set of figures in about 6-8 knots of breeze which was mostly steady but clearly there would have been some fluctuation in wind strength as I was changing courses. To get any real accuracy this would need to be done a few more times and averaged. It wouldn't necessarily apply in stronger breezes. Nevertheless the figures showed an optimal run angle of 150 degrees- at that angle I was sailing 19% faster than square on a course 15% longer. At 140 degrees I was sailing 30% faster than dead square on a course that was 30% longer i.e. no net gain. 160-170 degrees was no faster than square.

Best VMG to windward was at 50 degrees.

Cheers

Peter HK

[Hirilonde]

Thanks Peter, that is the kind of answer I was hoping to get. Much more research would be needed to really establish proper polars. And stronger winds would likely show different TWAs, probably more like 145, maybe even less to be more efficient. This would become more evident as the boat started to plane. But your results give a good place to start when trying to sail down wind efficiently.

Now I just have to build my own cat ketch so I can do my own research

[Joe Anderson]

I have been enjoying this thread.

Sailing by the lee

Controlling heal angle

TWA

VMG

Polar somethings

And you are right Dave to have a boat to try them out with. Get familiar with the techniques and your boat. And then decide what approach suits your sailing attitude and the situation at hand.

Thanks guys.