By

Dave Kautz

Owner of P-15 #1632 "Tilly Lucy"

Dave Kautz sailing Tilly Lucy on San Francisco Bay

The mast on a P-15 is a pretty flexible piece and does bend in the appropriate ways just with leech tension. The long batten may even help, acting as kind of a gin pole.

From what I've read, having the leading edge of the jib taut and straight is of importance when going upwind. When going downwind, having the forestay go "soft", as it tends to do on it's own, is appropriate since a rounder, fuller sail shape is desired when off the wind.

My observation is that the forestay does little other than hold the mast up when the jib is down. Even with 1 to 1 1/2 inches of "preload" on the rig on the Tilly Lucy, if I pull the jib's luff nice and tight with the jib halyard, there's little to no tension left on the forestay. If I'm paying attention to performance (i.e. there's another P-15 sailing nearby) I'll tighten the jib and main halyards when going upwind and loosen them a little when off the wind - you gotta love those "mastknockers"! I've convinced myself that I can feel the difference. Also, tightening the mainsail luff moves the draft forward when the wind comes up. This should help reduce weather helm and the hydrodynamic drag associated with it.

Vang sheeting doesn't appear to be practical on the P-15, due to the small section of the boom. You just can't put much tension on the vang without pulling the boom out of column more than I, at least, am comfortable with. Going close hauled I typically have no tension on the vang at all, the tell-tales indicating the sail wants all the twist it can get - keep in mind that the mainsheet arrangement is different between the 15 and 19, the bridle on the 15 acts a little like a traveler. As one gets near to close hauled the mainsheet's pull is approaching vertical.

Since it's the leech pulling on the top of the mast rather than a backstay, the mast top yields in the direction that it's being pulled. That is to say it bends both back and to the side in line with the leech. The mast is supported aloft at only the one point where the shrouds and forestay attach (there being no spreaders). I surmise that this point acts as a pivot and that the forces leveraging from above will result in a deflection of the mast in the same plane below that point. That should be in approximately the same plane in which the boom lies - and it too is pressing forward on the mast further encouraging flexure in that direction. The arc developed should pull material out of the belly of the sail, flattening it, while the reduced chord length should help the leech spill wind. The effect is not dramatic since much of these forces are balanced as the luff of the sail is pulling in the opposite direction, but in a masthead rig those luff forces are opposed only by the rigidity of the mast, spreaders and shrouds, resulting in the need for heavier components and loads on the boat. Fractional rigs are really neat this way!

Regarding headstay tension - there's only so much that one can do given the flexibility and limited adjustment of the rig. But, the std. jib is very small at 23-sq. ft. and the loads are proportional to size. The easier-to-manage mainsail does the lion's share of the work on the mighty P-15. You know Hunter is pushing their big yachts with a backstay-less rig, large roachy main and small jib touting the ease of operation like they invented it! We'll have to wait and see how bad the headstay sag is when I get my new lapper, but sag may help explain the observations that the P-15 can't point as well with the genoa as it can with the smaller jib.

I think that the "give" in the system comes partly from flexure of the mast and boom but also in a large part from the boat yielding to compression loads from the mast. On my boat at least, the leeward shroud hangs slack even in a moderate breeze. In fact, one can give a firm tug on a shroud and see a small amount of movement at the cabin top. The compression post steps on the bunks and I believe there are some bulkheads under there to transfer loads to the hull but the arrangement is not visible from above so I don't know exactly how it works. The other loads to be resolved are coming from the chainplate (U-bolt) and the bow and I don't think we can assume that the structure between these points is perfectly rigid, given the shape and materials used.