CD31 Quiz Question

[CruiseAlong]

The CD31 has a total working sail area of 504 sq-ft. Of that the main is 227.5 sq-ft and the combined stay sail and working jib area is 276.5 sq-ft.

Quiz: Is the staysail area 105.8 sq-ft and the working jib area 170.7 sq-ft?

Thanks, Dana

[Cathy Monaghan]

Since the "working" sail area includes the area of the mainsail plus the area of the foretriangle -- equal to a 100% JIB (not the staysail plus the yankee), it's hard to say what the size for each headsail would be since either can be pretty much any size. The combination of the two sails is usually greater than 100%. Anyway, let's see what people come up with?

Cathy
CD32 Realization, #3
Rahway, NJ
Raritan Bay

[CruiseAlong]

The reason I give these numbers for the stay sail and working jib areas is....when one measures the sum of the areas of the stay sail and jib sail from the CD31 "line drawing" with a planimeter from the CD31 line drawing, one gets a sum of 276.5 sq ft. I am inclined to believe that the center of effort for each sail is then combined to an equivalent fore triangle composite center of effort. That in turn is combined with the main sail center of effort and area. This gives a total boat sail center of effort just forward of the mast for a real nice sail balance to the boat. It is just that I can not find confirmation of any working fore sail areas for the CD31 confirming this. Yet it would affect the balance of the boat and its handling. I would suspect that larger jib sail areas combined with the staysail would account for additional lee helm.
Dana

[Jim Walsh]

I sent my working sails to Sailcare last winter and their measurements are as follows; my mainsail is 221 sq.ft., my staysail is 107 sq.ft. and my yankee is 280 sq.ft.. This makes my working sail area 608 sq.ft.. I also have a 135% which is cut with the foot much lower than the yankee and seems huge. I have never measured it.

[Cathy Monaghan]

Total working sail area equals the area of the mainsail plus the area of the 100% foretriangle. The staysail is not part of the equation. It never is.

Here are the formulas.

Mainsail = (P x E) / 2
100% Foretriangle = ( I x J ) / 2
Then add those two results together to get the working sail area.

For the CD31:
Mainsail = (35 x 13) / 2 = 227.5
100% Foretriangle = (41 x 13.5) / 2 = 276.75
Working sail area = 504.25

You'll find the numbers for P, E, I and J in the owners manual for the CD31. Your boat most probably carries more than the working sail area, and she should. When you bring your sails to the sailmaker this Fall for cleaning and/or inspection, ask him to measure them for you.

Your yankee or genoa can be any size from 80 to 150% or more and will most likely have a high-cut foot since the CD31 is a double-headsail sloop (cutter). Most cutters carry a yankee between 80 and 120%. We have a 110% yankee on our CD32.

The staysail, if on a boom (it is on the Cape Dory), can't be any larger than the triangle formed between the inner forestay and the boom but the boom can be any height/angle from the deck, and the head of the sail may or may not reach the top of the stay, so that sail can be a variety of sizes as well.

Though there are a few sloops that carry a 100% jib, especially racing boats that carry a whole inventory of sails, on a cruising cutter with a yankee/staysail combination there's really no need since dousing either sail reduces sail area pretty drastically.

Cathy
CD32 Realization, #3
Rahway, NJ
Raritan Bay

[The Patriot]

... I am inclined to believe that the center of effort for each sail is then combined to an equivalent fore triangle composite center of effort. That in turn is combined with the main sail center of effort and area. This gives a total boat sail center of effort just forward of the mast for a real nice sail balance to the boat ...

The CE in a well-designed sailing boat is never forward of the mast. It is typically located abaft the mast and also aft of the CLR (center of lateral resistance). The longitudinal distance between these two design parameters is what determines helm, either weather or lee. A proper design yields a yacht with at least some weather helm, but never lee helm. For a full explanation see Ross's "Sail Power" or any recognized text. For a quick explanation, see:

http://www.sailingusa.info/sailboat_balance.htm

[wstringer]

Most line drawings I have seen show the CE leading the CLR a little. The reason, as I understand it, is that as the boat heels, the CE moves laterally to leeward of the center of resistance (to forward movement) and creates weather helm. This is part of the reason weather helm increases in stronger winds.

Pax,

Warren

[CruiseAlong]

I believe that I may have unintentionally lead people to believe that the total center of sail effort should be in front of the mast to be correct. I was only trying to point out its position to the relationship of what we can all "see" visually. In truth the mast can be ahead or behind the center of effort based on the design of the hull and sail plan. . Designers move the mast postion to achieve a relationship of the center of lateral underbody resistance to the sail plan center of effort.

But back to the quiz question. (Does one believe that all combinations of the sum of the foretriangle sails lead to the same total foretriangle center of effort as long as the total area sum=276.5 sq feet? Or does the total foretriangle sail center of effort move as the type of cut, sail area, etc. change for each sail although the total sail area remains the same? Or does the sailmaker need to know alot more about the boat than just the E and I value...such as the distance between the two forestays, lenght of the stay boom, etc. But we can simply ask the question...
What is the sail areas of the working staysail and jib of the Cape Dory CD31 owners out there? AKA what did Carl and Cape dory have in mind. What did other CD31s buy?

[John Vigor]

The CE in a well-designed sailing boat is never forward of the mast. It is typically located abaft the mast and also aft of the CLR (center of lateral resistance). The longitudinal distance between these two design parameters is what determines helm, either weather or lee. A proper design yields a yacht with at least some weather helm, but never lee helm. For a full explanation see Ross's "Sail Power" or any recognized text. For a quick explanation, see:

http://www.sailingusa.info/sailboat_balance.htm

No. The design CLR of a normal cruising hull lies AFT of the CE by 2 to 8 percent of the load waterline length.

Placing the designed CE ahead of the CLR like this would, logically, cause harmful lee helm; but when the boat has way on, the fore part of the keel is moving in water less disturbed than the after part, so the CLR moves forward. Just how much it moves is often an informed guess that taxes the skill and experience of the designer.

Of course, both CE and CLR are just handy reference points from which to begin more detailed calculations. Neither is scientifically accurate.

John V.

[The Patriot]

The CE in a well-designed sailing boat is never forward of the mast. It is typically located abaft the mast and also aft of the CLR (center of lateral resistance ...

No. The design CLR of a normal cruising hull lies AFT of the CE by 2 to 8 percent of the load waterline length.

Placing the designed CE ahead of the CLR like this would, logically, cause harmful lee helm; but when the boat has way on, the fore part of the keel is moving in water less disturbed than the after part, so the CLR moves forward. Just how much it moves is often an informed guess that taxes the skill and experience of the designer ...

I bow to your expertise, of course, but it must be recognized in the current discussion that we (i.e., you and I in particular) have not fully entered into a discussion of static versus dynamic conditions and the accompanying calculations. The static case as a model is what was under examination in the original posts to this thread. Allowing for the dynamics of sailing conditions gets us into a much more complicated analysis. Of course one could also take the position that the end result in actual practice is the only important point, and that in a dynamic situation the CE always lags the CLR, no matter how one calculates the initial static design. The problem then disintegrates into a circular discussion of little benefit to anyone.

For the sake of simplicity my comments refer only to the discussion one encounters in a reference work such as Wallace Ross's "Sail Power" and similar established works. In fact the discussion given in Chapman's (93rd Ed) restricts itself to the static case for simplicity, which is perhaps the prudent choice in a first approximation. That seems to me sufficient depth for the purposes of this discussion but then, "different ships, different long splices."