Mast Rake On Catamarans

Hi all,
i am both monohull and catamaran sailor.
In monohulls i sail on a TP 52. This boat has an hydraulic arm to trim mast rake with diferent windspeeds. The higher we are in the range the more we go further forward with the mast. We have always beeing told that new balance is required taking out some load from the rudder. My experience in my A Cat is exactly the opposite. When the wind increases i move the rake back and the boats seems to be better balanced with less load in the rudder, i have always thought that is the sail twist induced but for sure there is something more!
Can anyone explain me why are there such a difereneces on the efect of mast rake betwen monohulls and multuhulls??
Regards

 

The major component of weather yaw on a mono is heel angle. At greater heel angles the boat has a greater yaw moment so greater rudder angle is required. On a mono the offset between the sail effort and the keel/rudder effort is the sail effort forward of the foil effort (causing the boat to turn off the wind), heel angle balances this force to get the right rudder loading.

On a cat, another axis comes into play. The rig is on the centreline (in general) and the boats do not heel as much as monos. The drive from the sail on a cat is to windward of the total drag. This acts to turn the boat off the wind. When sailing on one hull all the water drag is at the leeward hull, and the "lee helm" is great, raking the mast aft in a breeze brings the rudder load into balance.

What you say makes perfect sense.

If I have this wrong, someone will be sure to correct me!

 

I have raked my SW24 mast well beyond the conventional wisdom for the class and it works well. The boat points better and is faster uphill IMO, no class racing here so thats harder to tell. Gusts could produce surprising leehelm with the mast upright, thats now much less of an issue... any loss of downhill speed will be made up for with bigger extras (on way ). The experience of small cat sailors seems to be the same, one design classes like the H16 tend to run as much rake as is available to them. There is a theory about the rake also helping to produce lift and making the boat quite a deal faster through this effect... I can't argue that one but have read about it.

Anyway, the so called "excessive" rake that I run has not produced any weather helm at all, the boat is still neutral in most conditions and the lee helm is a lot more controllable when it hits.

If you have a cat my size you can try it quite simply by putting a big shackle in the forestay and winding the caps right down... I added 76mm to the forestay and took 50mm out of the caps.

http://www.boatdesign.net/forums/showthread.php?t=16525

 

Hobie 16s rake the mast aft to load up the rudders so they act like daggerboards resisting leeway as well as steering. We took out most of the resulting weather helm by redrilling the rudder blade bolt holes so we could rake the rudders forward. Obviously there are better ways to build a cat to go to weather!

 

I have sailed Ok dinghies and Finns for almost 40 years but because of difficulties hiking I have now joined the catamaran brigade. Most of the really old people in my club sail cats.
I bought an old Prindle 16 off Ebay for a song and I am sailing this cat rigged singlehanded. I have the mast rake set at the normal position and I presumed raking it further aft would aid tacking. The long term cat sailors in the club disagree. The "handbook" for setting up the boat suggests the rudder blades set well forward. Now for an "outsider" to cat sailing that sounds like poor initial design. Why weren't the rudders built with more area forward to start with? The fact that you set your Hobie blades further forward makes me think that a new set of blades might be worth constructing to suit a Prindle 16 cat rigged. Not interested in class rules, just fun.

Any suggestions greatly appreciated.

 

My boat tacks better with the rake. FWIW. The conventional wisdom in SW24's is to run it more upright... but they have never tried any different. Try it and see what happens, I was happy with the result on my boat, mebe it will work 4 U in a Prindle ?!

http://www.nornet.on.ca/~doverw/tuning.htm

 

I think I will just try a a lot of aft rake and find out for myself. Theory is nice but the reality is sometimes quite different.
Thanks for the link too.