Has anyone used or seen a mainsheet system with a 3:1 or 5:1 traditional system linked to an internal boom 4:1 system to give a two speed mainsheet?

This would enable coarse and downwind adjustment of 3:1/5:1 by the skipper and fine and upwind adjustment of 12:1/15:1 by the crew.

The only thing that I'm having trouble with is exiting the boom for the fine adjustment. Two options of a Harken ratchet flip-flip pulley under-slinging the boom or some plain through-blocks either side of the boom with a ratchet inside the boom.

I'm trying to cut down on the amount of rope floating around the tramp by reducing the traditional systems amount of purchase. The internal boom system will be sucked into the boom during tacks and downwinds and taut all other times.

Been there and tried it with little success, but for a not so obvious reason. My fine tune was led along the boom to a pair of blocks with a tail out to each stay. It works well until you are out on the wire and then want to move slightly further forward, particularly upwind. The decision then is which side of the stay do you want the tail to be on as for sure you may want to move slightly back or foward and the tail ends up dragging around the stay.

It may be Ok for a crew to deal with but I sail single handed and it was just too much hassle.

Since your crew would be doing the mainsheet upwind, why not go for a 8:1. This gives the crew more reaction possibilities in a big gust and since he/she has 2 hands to sheet in 8:1 is sufficient in my opnion.

On the other hand the skipper has 8:1 -> more finetuning downwind, because 3:1 seems a little difficult to trim the main to me.

There is no problem with playing the current 6:1 system, I'm trying to reduce the amount of rope loose on the tramp and reduce tangles. Somehow the crew ends up tied into the mainsheet, spinnaker sheet and halyard each time we tack.

Wow! Are you seriously using a 6:1 mainsheet?

haha, it isn't a full F16 rig, just a square headed mozzie rig, about 1sqm more than a standard mozzie main. 6:1 is fine for the crew but difficult when sailing 1-up.