Mast Position

[spaldi01]

I am currently trying to design an IOM but I am having problems identifying the mast position and would appreciate any advise.

[Chairman]

I am currently trying to design an IOM but I am having problems identifying the mast position and would appreciate any advise.

Hi Spaldi

I guess you are asking about two things. One is the "lead" of the rig centre of effort over the hull centre of effort in order to give you a balanced boat. No one knows the exact answer! If you take the mast stepping position as "proxy" for the rig CE, and the leading edge of the fin as proxy for the hull CE, most IOMs seem to have a gap of about 20 or 30 mm between these two positions. Best thing to do is wander down to your local club where they sail IOMs and take some measurements, and then give yourself ample room to vary the mast step in your resulting build. Some sailors like to be able to move the whole rig fore and aft up to, say, 25 mm each way, while others rake the rig forward (up to around -2 degrees) and aft (up to around 7 degrees).

A second question is where to place the rig and fin "package" along the centreline. Some boats have their rig and fin well aft, at around 58% or even 60% of length, and others have them more central, at around 52% or 55%. (Keep in mind that the jib boom counterweight must not project over the bows -- this limits you to around 50% of length at least in No.1 rig). If you think no one really knows the "lead" of rig over hull CE, for sure very few people *know* the effect of aft or forward placement of the rig and fin package on a given hull. Again, taking the fin LE as proxy of the location of the rig and fin package, measure this on a variety of designs along with their position of maximum waterline beam.

It is the rig and fin package which is placed somewhere along the centreline, while the bulb stays where it needs to be, more or less under the hull centre of buoyancy. This makes sure that the hull floats on its lines, or at least makes sure you have the amount of transom immersion you want. If your rig and fin package is forward, and your hull centre of buoyancy is aft, you will end up with something like an "L" keel. If your rig and fin package is more or less located over the hull centre of buoyancy, you'll have a "T" keel. And the other extreme is a "reverse L" or prognathous keel. Many designers think that a "T" keel is best, especially if you do not have a particularly stiff fin, so that means you'll end up positioning the rig and fin package more or less at the hull CoB.