Please can anybody explain what happens (in terms of rake, mast bend, turning moment etc) when you move mast foot forward or aft? Is there an accepted position relative to the bow/transom? are merlins 'sensitive' to the mast foot position or does it not make much difference?

Thank you

David,

I have a note that says for an NSM2, the optimum distance from the transom to the rear face of the mast is 2370mm. I think this came off Seventh Heaven. As to your question, I believe that moving the mast forward results in a reduction in weather helm while the reverse is true if you move it back. I'm sure others will have different opinions

While most people measure the mast foot position from the transom, the important element is the relationship of the centre of effort relative the centre of lateral resistance. The centre of lateral resistance generally is somewhere along the centreboard. So you would need to look at the position of the centreboard also, again as with the mast foot position measure from the transom. However this does not take into account the angle you set the centreboard, or the rack of the rig.

While most people measure the mast foot position from the transom, the important element is the relationship of the centre of effort relative the centre of lateral resistance. The centre of lateral resistance generally is somewhere along the centreboard. So you would need to look at the position of the centreboard also, again as with the mast foot position measure from the transom. However this does not take into account the angle you set the centreboard, or the rack of the rig.

thanks Chris and David, So I have a 22ft 6" rig which I guess means the centre of effort of the main is further forward and higher than the conventional rig.  So i guess my mast foot should be further back than 'normal' ?

I honestly don't know. When I bought Gigolo many years ago, the jib tack had been moved back when a spinnaker chute was fitted. On that basis, the mast heel should have moved back, I guess but apart from setting back the fairleads, legend has it that no other changes were made. It was a stunningly fast boat..

I'm sure Laurie would tell you...It's bound to depend on design a bit.

I measured a few different boats of different ages and found the jib tack to be fairly constant, but the mast foot dimension did move around a bit (obviously not much on recent winders)- I moved mine by 10mm or so, working on the idea possibly in ignorance!) that raking the mast would make more than 10mm difference and that wasn't too disaterous, or that not sailing the boat flat would have a more profound effect - I was also told at the time that the rudder to CB distance also has a part to play in the feel of the helm - obvious really I suppose. All said and done, it still heads up gently and doesn't have any bad habbits. I supposed at the time that moving the mast further back would allow a wide slot between the jib and forward would close it.

The remarkably consistent figure for CT variants is 52 inches from the jib tack. This applied across a range of builders.

Chris, is that 52 inches to the front of the mast?

Yes.

And i beg everyone's pardon it's 53 inches not 52!!

Dv, suggest you stick with your normal tactic.
ie. throw the rig up with any old rake, postion & pre-bend and go sailing.......

The Dave Fowler measurment,ie Heaven Sent, is to the sail track and not the foot of the mast.