Cruising with a big rig

[StuM]

Originally Posted by UNCIVILIZED

Here's one source of PHRF ratings, & US Sailing (or similar) has a much more extensive list. The ratings vary a little bit from location to location, based on average local wind strengths, & how well models of a particular design do in that area in terms of true speed & winning races. And you'll find plenty of other PHRF rating charts for various locales if you do a search. Plus I have a copy of the big, extensive version put out by US Sailing which I found online.
PHRF New England - Handicapping - Base Handicaps

We use the Schell formula to help establish start handicaps for new boats. It gives a pretty good approximation to PHRF numbers with just a few easy measurements: I,J,P,E LWL,Draft and Displacement.

R' = 610-8.36*(SA/Disp^.333)+0.0000511*(SA^2)-55*(P/(J+E)) -30.8*(LWL^.5)-602*(DR^2/SA)



where SA= .5*(I*J)+.5*(P*E)

[Paul L]

Originally Posted by StuM

We use the Schell formula to help establish start handicaps for new boats. It gives a pretty good approximation to PHRF numbers with just a few easy measurements: I,J,P,E LWL,Draft and Displacement.

R' = 610-8.36*(SA/Disp^.333)+0.0000511*(SA^2)-55*(P/(J+E)) -30.8*(LWL^.5)-602*(DR^2/SA)




where SA= .5*(I*J)+.5*(P*E)

Are the units like PHRF, essentially seconds per mile?

[Dockhead]

I am going to express a somewhat contrary view to most of what has been posted above.

Every sail plan has a wind range in which it's effective. Above or below and it's not effective. It is especially hard sailing a rig above its effective wind range -- you can't "just reef" - reefed sails don't work as well, especially headsails, and especially if you're trying to get upwind. It is tough sailing in strong conditions with a rig which is too big -- more drag, more heeling, sails reefed to ineffective shapes, etc., etc.

If you are able to change to smaller headsails, that can extend the wind range of a bigger rig, but there are limits to that. Besides that, taller rigs have more windage, which does not go away with smaller sails.

On top of that, cruisers don't like the very deep keels which you need, to make a bigger rig effective. The worst combination of all is a too big rig with too shallow keel.

So you should choose the rig based on where you sail and what kind of wind conditions you care most about. In the lower latitudes but out of the trade winds, where you have way more days with too little wind than too much, then a bigger rig can make sense. But choose a boat which has a keel which corresponds (say a Savona 45 with 7 1/2" of draft and a big bulb -- now that boat can carry some sail).

In higher latitudes with a lot of windy days, you will be better off with a more compact, lower windage rig. Get your speed from low D/L, not from power of large SA/D, which you cannot use above its wind range, and which will start to work against you. Contrary to SA/D, which is a double-edged sword, D/L (and low wetted surface) is all good. The deepest bulb keel you can accept is also all good -- allowing you to carry more and more sail and giving you better hydrodynamics to boot, with no downside except access to shallow water.


Where I sail, a boat you have to reef in 15 true would be practically unusable. 20 knots true is an average day up here. YMMV. My boat is fairly light for a cruiser (188 D/L) and has SA/D of 16.5. The sail plan is a little big for where I sail. But I suffer from a different disadvantage -- I have in-mast furling, so the rig is taller (75 feet) than it would otherwise need to be, and the mast is fatter. With a slender mast and full batten main I could lose 5 or 6 feet of mast height, and the boat could carry more sail.

But with reasonably light D/L, you can get the boat moving very well indeed with this amount of sail, or less. In our average weather up here, I am blowing right past racing boats with reefs in, all the time. Admittedly these are boats with shorter waterlines . But once you have a roll in your headsail, you can kiss performance goodbye, especially upwind. Bigger is not better -- what is better is the right size of rig for the wind range you sail in.

[StuM]

Originally Posted by Paul L

Are the units like PHRF, essentially seconds per mile?

Yes. And generally it's close to published PHRF numbers for the same vessel. Essentially, Schell derived his formal to come up with a simplified system to approximate them.

If you want to convert to a "time on time" "time correction factor", there is a simple fomula. TCF=A/(B+R')

where B is a variable which you can change based on predominant conditions.
You usually see the formula in ts simplest form with B = 550, but that can be tweaked based on prevailing conditions. It's common to use:
Heavy Air or all off the wind = 480,
Average conditions = 550
Light or all Windward= 600

Changing B affects the percentage spread over the fleet.
(With a B of 480, our fleet spread between fastest and slowest boat is 21%. With a B of 600, the spread is only 17%)

A is a number used to modify the range of TCFs for a given fleet. 650 is the usual number to use when B=550. But if you want your fleet's average or highest TCF to be different, then you can tweak the A to make the fastest boat or the middle boat (or whatever) be on 1.0000 - it won't make any difference to the percentage spread between boats.
(With an A of 650, we have 3 boats that are rated over 1.000. With an A of 700, our fastest boat is on 0.9395)