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There's a 'functional' problem with in-mast furlers as most discussions of the in-mast 'furlers' only dwell on the 'mechanicals' ... and there is a vastly more important consideration - SAIL SHAPE. The following will hopefully explain why in-mast reefing is of very LOW BENEFIT, (beyond 'ease of use' and disregarding entirely the reliability, etc. issues of the 'hardware') in controlling the all important sail SHAPE: ...
Sails are not 'flat sheets of cloth', they are quasi-spherical 3 dimensional shapes ... for a damn good reason: aerodynamics. Just about any sail has most of its camber or position of max. draft (POMD) at somewhere near mid-cord of the full-up sail; the 'approach or entry shapes' to the POMD will usually be found to begin @~15-20% of cord length. Most sails that are roller reefed are left FLAT, essentially NO GOOD for 'aerodynamic flow' and typically when beyond 30% 'roll-up' are no longer or greatly reduced 'aerodynamic shapes' .... and simply cannot be used effectively when beyond the 30% 'roll-up'.
Plus, when in conditions that require one to reef you usually (when in non-FLAT water) need a 'full drafted' sail for the POWER it supplies for punching into waves and steep chop; when roller reefing (beyond ~30%) all you're going to get is a distorted FLAT (the panel shape aft of the mid-cord is ~'FLAT') and that 'flatness' is for SPEED SAILING in FLAT WATER and definitely NOT the 'shape' you want for *PUNCHING THROUGH" and over waves and steep chop. If you roller-reef beyond that ~30% then all you're going to have aloft is something akin to an NON-aerodynamic 'flat sheet of plywood --- something with very much LESS than the 3 dimensional shape of a 'sail', although it may 'look like' a sail because its 'triangular'.
With a roller boom or slab reefed sail you can always 'move' the important POMB back and forth via halyard and/or cunningham tension .... try that with a in-mast furler and you will JAM the furler and without any change in POMD because it can be thus trapped deep within the 'roll' !!!!! therefore NO change in the POMB fore/aft location. With inmast roller reefing, you can adjust the outhaul until you have a spinnaker like shape or a flat as a sheet of plywood shape .... but NOTHING that you can do with an in-mast roller furler-reefer will change the position of the POMB!!!!!!!!!!! (the position of the POMB is the functional item that has the GREATEST effect on 'balancing the helm pressure' and much more effect than the amount of sail area exposed !!!!!!!!!!!!!!!!!)
Roller BOOM reefing, or better - slab reefing, will keep the POMD where it belongs, AND, you can power-up or power-down by changing the luff tension (via halyard or cunningham strain), etc. to change the location of Point of Max. Draft POMD to where its 'most effective' for the present wind and seastate conditions .... and can do this POMD relocation when single reefed, double reefed or 'deeper'.
So, If your cruising plans include venues wherein you might possibly be beating instead of ~100% of the time 'going downhill', then that FUNCTIONAL REQUIREMENT to have an aerodynamic shape aloft in high wind range conditions would almost entirely, ... or should, EXCLUDE an in-mast furler.
If however your intended venue is only for 'moderate' conditions that almost all of the time will only need a maximum 'single reef', then in-mast furling/reefing will be 'fine' .... (and with NO comments about 'reliability').
My emphasis here is SAFETY, not 'performance' as if you cant 'point' when you 'really NEED' to ... you're now vulnerable and 'dangerous'.
Last edited by RichH; 01-27-2011 at 08:59 PM.