Model Boat journal
Lester Gilbert on boat balance
The boat is balanced by either moving the sail plan forward to reduce weather helm/increase lee helm, or moving the sail plan back to reduce lee helm/increase weather helm. Depending upon the mast step you have in your hull, you'll do this by raking the mast (pivoting it on its heel), or moving it fore and aft in a mast slot.
For any given hull/rig combination, Graham Bantock feels the degree of weather helm is dictated by three basic factors:
* mast rake/mast position
* relative twist in main and jib
* relative sheeting angle of main and jib
with the minor factors:
* relative camber of main and jib
* relative shape of main and jib.
He goes on to say:
There is a fairly small range of successful pairings of twist and sheeting angle. The sheeting angle ranges are covered by the range 8 to 15 degrees for the jib, and 2 to 8 degrees for the main. Keep the twist in the sails so they look much the same from astern. Use mast rake/mast position and relative sheet angle to tune the balance. Use sail camber primarily as a throttle - fairly flat in very light airs, fuller as wind speed increases up to the point where you need to keep the boat more upright for best speed and pointing when you flatten sharply. Camber can also be used to adjust the helm of the boat. Experiment.
As far as I can tell, my Ikon and Italiko tune up just like any other IOM, the major issue being to get their balance right -- ie, appropriate weather helm. So far, I've found both work best for me with a touch of weather helm, that is, when they reliably round up into the wind rather than sailing "neutrally" with my finger off the rudder stick. If you have your rudder trim calibrated, I'd suggest going for that amount of mast rake which gives you about 2 or 3 degrees of weather helm as the "ideal".
As an aside, some sailors feel that mast rake does other things for you, perhaps improving the lift characteristics if the mast is raked well back to give some sort of "sweepback". As far as I can see, mast rake in an IOM is exclusively concerned with boat balance and helm, and has negligible other effects on any other characteristics of the boat.
There is an aspect of helm that has puzzled me for quite a while, and that is the phenomenon called "snap" weather helm. This happens when an otherwise well-behaved boat luffs violently into wind when a puff hits, and I had this problem with my Ikon. When I started sailing my Italiko and until recently, I never encountered it, and began thinking that snap weather helm might be a characteristic of hull shape. I realised I was wrong about hull shape when my Italiko snapped back at me, and I spent a little time investigating. It now seems to me that I had inadequate rig tune.