Rake on multihulls

[cjs]

In clases like tornado and Formula 18 the trend seem to be to sail with more and more rake to improve the upwind performance. Does anyone have any physical explanation to why that gives better performance.

One reason could be better coarse stability when the preasure moving backwards compared to the centre of gravity. (like the dart arrow).

My opinion is that a the aft movement of centre of preasure from the sails should go hand in hand with more aft positioning of the centre board but on the tornado the centreboard possition is fix.

[Eric Sponberg]

Raking the mast aft moves the center of effort aft, and this, in general, creates more weather helm and makes the boat harder to balance. That is, with more weather helm, the tiller angle is higher to compensate, and that creates more work for the crew and more drag from the rudder which is counter to what you want for high speed. So, one could surmize that the original designs of these boats had too much lee helm, and raking the mast corrects a bad situation. I somehow think this might not be the case.

A second reason may be that the sailors themselves detect an increase in speed with a raked mast. This would be due, most likely, to a decrease in induced drag caused by the mast rake. I don't know of this is true or not, and perhaps some of the more experienced aerodynamicists on this forum could chime in on this point.

A third reason for the rake may be that at higher wind speeds with the full mainsail up, boats naturally gain weather helm due to increasing heel, even on a multihull. As the crew reefs the sail on a raked mast, the center of effort moves forward to counteract the building weather helm. This keeps the boat in balance with the correct amount of weather helm in the high-speed, reefed condition. This is the reason I most favor.

Eric

[cjs]

Could one other reason be to help the bow from digging in to much (without having the transom creating to much turbulence which it will do when you will help the bow from digging in by moving the crew weight aft?)

[Eric Sponberg]

Possibly. It is all a matter of aerodynamic and hydrodynamic balance. With less induced drag (if that is in fact what is happening) there is less side force. With less side force there is less heel for a given speed, or more speed for a given heel. With less heel, there is less submergence of the leeward hull, and depending on the shape of the hull and the location of the LCB and LCF, the bow may very well be submerging first as it does heel. So it is conceivable that an aft raked mast could be beneficial in this regard.

Presumably you are an experienced sailor in the F18 (more than me, anyway) and you undoubtedly sail with other experienced sailors. They might be able to shed some light on the behavior of the boats with different rakes in the mast. Certainly, the behavior that you experience is all a result of balancing all the forces acting on the boat.

Eric

[PI Design]

I was always taught that cats develop lee helm when they heel as the windward board lifts out the water, providing less lift. The leeward board acts further aft the the windward board, so that when the windward board lifts less the overall CLR is further aft, resulting in lee helm. I did point out to my lecturer that this is not something I had ever observed in practise, but that was the theory.

Maybe F18/Tornado mast are being raked more aft as the sails get more efficient. As the lift vector from the sails acts more in a forward direction (ie less drag), either the dagger/centreboards need to be moved forward or the rig needs to be moved aft (by raking) to maintain balance.

Anyway, aft rake looks cool.

[messabout]

Aside from balancing helm, those sailors may be trying to gain a
bit of an edge by using excess rake. There will be a vertical lift component which may raise the hulls with respect to the water. It works for sail board sailors, but they use an extreme amount of rake to make it happen. Racing sailors will try anything.

Other more ordinary reasons for rake...It helps keep the boom end out of the water when heeled. Also when the sheet is eased, say out to a broad reach, the CE of the sail moves forward with respect to the boat. Lots of rake will help keep the balance in those conditions. The likes of a Tornado, when there is a good breeze, do not sheet out much because the apparent wind moves forward. In light air, the advantage may be worthwhile as the boat does not go so fast and the sheets will be eased more.