Monday, May 19, 2014

Speed Polar, PDQ 32/34

Several years ago, after I adapted to sailing my new-to-me PDQ 32 fairly well, I set about developing a speed polar diagram, to help me better understand how she could be sailed to best advantage. I'm a cruiser, not a race, but I like to get where I'm headed. As an engineer, I can't relax on a boat that is not well-trimmed. No, I'm not a trim and grind racer--I'll set things and let the autopilot keep her steady. But I like things set properly.

A speed polar will never be right and never be finished. Most properly, there would be a family of graphs, one for every combination of load, sea state, and bottom condition. This is just sort of average for me, with a little growth on the bottom, a tender on the davits, family, a mix of new and old sails, and fullish tanks. If you're a little slower, perhaps there is a sail adjustment that will perk things up. If you're a little faster, good! If you here of a speed several knots faster than what is here... I doubt it.

And if all of this is confusion and frustration, turn the page and don't bloody worry about it. There are days when I can't hit these numbers--perhaps a foul tide or sloppy wave conditions--and it just ain't worth stressing over.

Speed and course over ground vs. true wind speed. Where the curve ends the heeling force is too great for my comfort; sails will stretch, sheets must be out of the self-tailers, and someone experienced stationed on the winch.

Apparent wind speed at true wind direction vs course. This is the partner to the above graph. I generally reef when the apparent wind is over 21 knots up-wind, and 10-15 knots off the wind, depending on the sails and waves. I'll reef earlier if I simply don't feel like working so hard, am single handed, or if the weather seems changeable.

What is the Portsmouth Yardstick rating? While the US Sailing figure is based on limited information, it is rated at 72.5, which is about the same as the Stiletto and common beach cats. How could it be so close? The rating does not count reaching speeds, only windward/leeward, and the PDQ is pretty solid going to weather. How much have I shaved? I suspect the revised Yardstick is now  under 70, based upon side-by-side trials and the polar. Who'd of thunk?

And for those of you with PDQ 36s, I pulled this from a 1991 Multihulls test. I can't vouch for the circumstances, but I'm pretty sure she was factory prepped, fresh paint, empty tanks and no junk in the lockers, no dingy and hard top. Cheating, in other words. A little more weatherly (the jib is in closer), a little faster in heavy winds, and probably just about identical everywhere else when carrying a load.I also clipped the PDQ 32/34 curves where I think cruisers will reef. They did not. For the non-racer, pressing canvass that hard is just asking for stretch.

For comparison, the speed polar of a typical 41-foot cruising monohull (NTP 41). A different shape curve, weatherly in lighter winds, but slow in comparison in a breeze. An 8-9 knot speed limit; that just when it starts getting pleasant!

And just for those of you that care about measurements and PHRF ratings:

I - 40.33
J - 12.75
P - 35
E - 13.75

"P" is the luff length of the mainsail, measured along the aft face of the mast from the top of the boom to the highest point that the mainsail can be hoisted or black band.
"E" is the foot length of the mainsail, measured along the boom from the aft face of the mast to the outermost point on the boom to which the main can be pulled or to the black band.
"I" is measured along the front of mast from the genoa halyard to the main deck. The main deck is where the deck would be if there were no deckhouse.
"J" is the base of the foretriangle measured along the deck from the headstay pin to the front of the mast.
"JSP" is the length of the spinnaker pole or the distance from the forward end of the bowsprit (fully extended) to the front face of the mast.
"ISP" is measured from the highest spinnaker halyard to the deck.
"PY" and "EY" are, respectively the luff length and foot length of the mizzen of a yawl or ketch measured in the same way as for the mainsail.
"IY" is the measurement from the staysail halyard to the deck.
"JY" is the measurement from the staysail stay to the front face of the mast.
"LP" is the shortest distance between the clew and the luff of the genoa.

No comments:

Post a Comment