Originally Posted by David M
With their higher speeds than monohulls the longitudinal hydrodynamic resistance on the hull is greater causing the sail to want to rake forward. Raked aft they are less prone to have the sails center of effort shift forward of the hulls center of lateral resistance causing lee helm.
At high speeds, imagine the wind
pushing the top of the mast forward and the hulls resistance pushing the base of the mast aft causing the bow to pitch
down, shifting the sails center of effort further forward.
You especially see a good amount of rake on lightweight racing catamarans whose hydrodynamic resistance on the hull compared to its sail area to displacement ratio is much higher. On larger cats not as much rake is needed because of the reduced sail area to displacement ratio and reduced righting moment for the amount of displacement.
I would argue that it is a lack of buoyancy in the bows that allow a cat to pitch bow down more than a keelboat. More an issue on beach cats than cruising cats. And while the CE is moving forward as you pitch bow down, at least on all the beach cats I have sailed the CLR moves forward much faster and farther due to more bow in the water
and less stern in resulting in weather
helm, not lee helm.
Top racing beach cat sailors change rake for the conditions. Hobie 16s with no daggerboard have a different reason. Boats with daggerboards will sail with a more upright mast in lighter conditions and will rake back for stronger winds. I haven't seen an explanation that I like yet but talk to any racing sailor and they will tell you that raking the mast aft will depower the rig and allow you to point higher, this is for cats, planing dinghies and keelboats. The beach cat sailors also like to point out that the weight of mast is further aft, but that seems a minor point to me.
The first cat I chartered, a Privilege
39 had a lot of rake, it did not look like other Privilege
39s. On one day we had a nice wind and on a broad reach we had to reef the main because the weather
helm was so bad the steering
system was binding and locking up. At first I thought we had the rudder
hard over, but a lull unloaded the gear
and then I could turn it farther.
I like the other explanation better that it was designed with the mast base forward and the mast raked to balance the boat.