Mark wrote: "Beware of pushing the balance too far..."

I think Mark is right.

And while on the subject, it bears repeating: there is a trap when calculating mast balance for a SJR, which can lead to under-estimating the balance, which in turn can lead to the issue Mark has identified.

The entire area of the sail profile must be considered (including the slot). It is not correct to simply consider the ratio (area of jibs)/(area of mains + area of jibs).

A simple and conservative (safer) estimate of mast balance is simply to look at a lower batten, or chord. The slot is ignored. The ratio of chord ahead of the mast centre line divided by the total chord length, is the ratio to use for SJR. This will give a slight over-estimate of the actual mast balance.

I think it is best if this ratio does not exceed 33%.

Roger wrote:  "... I have to work out details for a mast and will be looking at suggestions for the use of aluminum tubing... "

The ideal is a tapered aluminium tube (i.e. lamp post) if you can get one - we can't get them in the country where I live. I have made a tapered aluminium mast from three different diameter aluminium tubes with over-lapping joins. I notice Johan did something similar (see this month's Boat of the Month). The joins need to be faired, to smooth out the step-change in diameter - and the joins need to resist the rotational forces, and the downward telescoping forces, both from the halyard. Johan fixed his with rivets, but I don't like drilling holes in a mast so I did mine differently.

Another alternative, which is not unusual here, is a basic aluminium tube mast with a light, tapered, wooden topmast extension, with sufficient bury, and a polyurethane-type glue.

Actually, for a small boat like yours, there is probably no serious reason why you can't just use a single straight tube - as was done on Little Tri in those video clips - if you can find one long enough - the downside being a bit more windage and weight aloft - and the aesthetics.

The most important issue is deciding the right scantlings (diameter, wall thickness and grade of aluminium).

Perhaps someone with a comparable sized multihull can give you the details of their mast, so you have a comparison to work with.

To keep the mast in or near to its original position, using a single junk sail, you will need a sail with a "mast-balance" greater than is normally found on a junk rig.

I can’t quite read the numbers on the sail plan, but it looks as though the mast-balance of your current rig is about 35%. If that is so, then you might just about be able to get away with keeping the mast where it is, if you were to use the junk rig with the highest mast balance possible: the “split junk rig”.  Because your current bermudan rig is fractional, and with a non-overlapping jib, it is possible that its current mast position will be the same as, or near to, the right mast position for a split junk rig.

To be on the safe side, I would make the sail with 33% balance, though 35% is thought to be possible. 

If you feel the need for jibs, then this is the sail I would consider.

This is Slieve McGalliard's proven Amiina Mkll "split junk rig" sail design, 33% balance.

At 35% balance I think you could use the original mast position. I believe a 35% balance has been done, and it works for model boats, but it is right on the limit of what is safe for a lug sail. [Edit: I would consider 35% to be experimental and as already stated, I feel more comfortable with 33%]

At 33% mast balance, on the above rough drawing, it looks better to me if the mast centreline goes just a few inches ahead of where the original mast was placed, as shown, if that is possible. However, given that you can adjust your weight distribution, and also adjust the centreboard, it is probably near enough to plonk the above sail plan mast right where the old mast was, I think you would be OK to do that. (I would rather do that, and carry perhaps  a little bit of weather helm, than have an over-balanced rig. With my little boat, with its Amiina Mkll rig, I can easily manage helm balance by shifting my weight and adjusting the centreboard, as conditions vary).

I think you would get away with it.

[I should also add, the above drawing is "eyeball" only. On second looks, the 33% rig may well put the mast right where the old one was. Do an accurate scale drawing, and some calculations.]

Anything else in the way of a junk rig, I think, will need the mast to be moved forward.

If you want to know how a little trimaran can go with a split junk rig, take a look at these two video clips here and here

You won't need a spinnaker with a rig like this, but I would keep the working sail area of your current rig. With junk rig you can easily and almost instantly reef down the sail area when necessary.

I would like to convert the trimaran to a junk rig. It is a Haines Hunter Tramp designed by Ian Farrier with folding amas. This version is a "Super Tramp" which carries way more sail than I want.

I am told that this tri sails poorly with no jib. Can a junk sail be designed to take part of the place of a jib, that is with part of the sail forward of the mast.

I attach a sail plan and and am wondering if the existing location of the mast could be used or would it need to be moved forward.