Sunday, September 15, 2013

Sheeting Angle and Keels

rev. 9-18-2013

Cruising cats are not generally weatherly, and the PDQ 32 is no exception. But encouraged by the fact that some fine tuning (transom extensions clean bottom and a weight loss program) has made the speed polars quite attainable, I'm encouraged to move on.

The transoms helped. Less drag equates to higher speeds and higher speed, even a little, means higher lift on the foils. This is critical with low aspect keels, something I learned well in my years of sailing no-keel beach cats; they only point if you keep them moving.

New sails. My genoa sets perfectly (other than the sheeting angle, which we will get to). My main on the other hand, is a bag. But I still want to get a few years more and I'm patient. Jessica's just starting school and I still feel guilty about spending. It will pass. No question, a flatter and faster main will aid pointing, though I wouldn't expect any change off the wind, hence the low priority.

Sheeting angle. A few quick calculation show I use a self-tacker angle of 12 degrees but a genoa angle of 24 degrees. For comparison:
  • Very weatherly keel boats: 7-9 degrees.
  • Cruising monohulls: 9-12 degrees.
  • Dingies: 9-12 degrees.
  • Stiletto 27: 9 degrees.
  • Prindle 18: 12 degrees
While the PDQ doesn't have the best foils,clearly we can do better than 24, which is a reacher angle. But with beach cat rigging (3 shrouds, no back stay), I can't sheet tighter than the shroud.

I will probably add a short run of 1 1/4-inch track between the winches and just outboard (about 2 feet behind where the green tape is, the result of more test sailing) (both winches are the same size). Figuring the space will be tight and the lead may interfere with grinding if not tweaked just so. Most probably I will use the spinnaker sheet as the alternate jib sheet, which will require using a snatch block; I found a compact one from Garhauer that I like. I've got to be certain nothing interferes with normal genoa and spinnaker operation. I suspect a mock-up will be required, to get it right the first time. And more testing.

The track will be out of foot traffic and no new lines are needed. Unless the spinnaker is used, everything can be left in place. If the spinnaker is used, the line transfer should take only a minute, making use of a snatch block on the inner track.

I intend to replace the fixed aft block with another slider, so that both can be adjusted. The cross haul line in the photo is only to hold the snatch block in place during the photo and is not a part of the proposed system.

A side view shows that to get the same lead angle on the higher deck I can be somewhat forward, though any time you move inboard you move aft. The tighter the jib, the greater the risk of hooking the leach into the main up high.

If we were to bring the sheet inside the shrouds, I can go as close as 17 degrees before the cabin top gets in the way. Although 7 degrees sounds minor to the non-sailor, it equates to about 20 degrees of potential pointing, which is huge. We have tested this by barber hauling to an opposing winch (awkward--the line splits the cockpit) (would need to be reset on each tack) and after leeway found about 8-10 degrees course-over-ground improvement, and about an 30% improvement in VMG (velocity made good to windward), which is huge. Additionally is the promise of fewer tacks, which add to that improvement. Yup, gotta do it. We will need to add a short track just between and outboard of the winches with very low profile blocks (to get a good winch lead).

While we certainly must consider rough conditions (the below is based upon semi-protected water observations), the calculations below show the potential benefit of inboard leads. Also interesting is the relatively small change in VMG over a range of angles, thought that simplistic analysis ignores the reduction in the number of tacks required.

Keels. Of course, this will be further helped if the keels were optimized. PDQ designed the keels for balance on land, not in the water; nice when dried out, but way too far forward under sail. She loves to weather vane into irons when you're not looking. More area aft might help.

This is a more radical way to add area; a center board that would drop down when needed for leeway and balance. Certainly there is room in the under bunk storage, but I think the added wieght might cost more than the leeway, to say nothing of the complications in construction, operation, and mainatance. This has been done on both cats and monohulls.

PDQ keels are also not very efficient, being low aspect. Several sources suggest that about 5% of the sail area is good for this type and this sort of boat. With about 620 ft^2 of up wind sail area, that would suggest 31 ft^2 or 15.5 ft^2 on each side. Since the existing keels are more like 12.3 ft^2, we are seriously lacking in area. With the factory self-tacking jib, the up wind area is only about 506 ft^2 and the factory keels make perfect sense (12.6 ft^2 suggested). On flat water with a wide lead angle it also works with the large genoa, but when it gets rough, not so much. If I tighten the lead angle to 14 degrees, even more trouble.

The existing keels have a very blunt trailing edge, perhaps 5/8-inch wide. What if we extended this out to a fine edge? that would add about 1 ft^2 per side. What if we add a low angled fence keel for a short distance behind the keel, perhaps 30 inches long? Another 1.8 ft^2; not efficient because low aspect, but thin and very low drag. Both would be simple to do while out for painting and cost very little. No additional draft and nothing to snag on the bottom. I have a chain rode, so keel wraps are a non-issue. This would get the area back into the design pocket.

Note: I did fair the keel, 10/2015. Yup, really helped with balance and VMG. See Good Old Boat Magazine, 2016, for the story.


The plan? I think the inside genoa leads may appear within a few months--a little more testing and figuring. The new main and keel mods will wait for the next haul-out; I think they will make a fun 1-2 punch. While she will never be a reaching machine like my old Stiletto, I think she will be getting up and down the Bay a good bit quicker than stock, and smoother too; a very fast cruiser for her size. I'll have to calculate new speed polars!

Speed isn't everything. Efficiency means I can also reef earlier without loss, and that makes for easier, safer sailing. 

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