SA/D on cutter?

[massnspace]

Looking at a possible boat to buy. Old 80s fiberglass beast. Not impressed with the sail area to displacement, it is around 14.

But….this is a cutter rig, so 2 foresails.

Does SAILBOAT DATA use both foresails when calculating the number? If not, can I “add” the size of the staysail to this? And does a staysail work to add to the power in light winds, or is it just for heavy winds?

David
Seattle

[Montanan]

Cutter Rigged Sailboats [GUIDE] Advantages, Sailing, Options & Features

Snipet:


"And a cutter rig offers increased sail options–it offers increased sail area in light winds and easy and efficient ways to decrease sail area in heavy weather. In heavy weather, a cutter will drop or furl her larger headsail – usually a yankee or a genoa. That leaves just the smaller inner staysail."

https://mycruiserlife.com/cutter-rigged/#:~:text=And%20a%20cutter%20rig%20offers%20increas ed%20sail%20options%E2%80%93it,genoa.%20That%20lea ves%20just%20the%20smaller%20inner%20staysail.

Bon voyages.

[Don C L]

I believe the SA is calculated with all the sails. But, though 14 is low, I have seen boats that were well-known "bluewater boats" around 14 also. In light air they'll need a big headsail. Which boat is it?

[tenchiki]

The standard way of calculating sail area is to sum mailsail (+ mizzen if exists) + 100% fore triangle (ie: I*J/2). So the calculation doesn't use any specific jib (or staysail) sail(s) on the boat.

[Bowdrie]

On every set of sail plans for cutters that I've looked at the designer used the staysail in calculating the SA/D ratio.
Interestingly, the old S&S Swan 43s, (a sloop,) with the standard rig had a SA/D ratio of ~18, that would have been with a jib of ~100%, what would be referred to as "all plain sail", or "working sail".
Even your old doubled-ended Alajuela 38, (a cutter,) at ~14 tons has a SA/D of ~16 with the standard height rig.
All sails create drag, what counts is that they have the best shape the sailmaker can give them so that they have a better "Lift over drag" for the wind strength they are designed for.
"Sometimes" a cutter using its combination of jib and staysail can be advantageous over just a jib, (or a sloop with its only headsail).
But much depends on several factors, a few are wind strength, height of total center-of-effort of the headsail(s) above waterline, and importantly, how they are cut/rigged/sheeted so as to achieve clean airflow back over the mainsail.
We today tend to see and use staysails as a "heavy air" option, and they are, but in many older designs they were considered as a good working sail, some were even sized to lap the mast, (Genoa Staysail).

[TrentePieds]

TrentePieds is "permanently reefed" having an SA/D of 13. Yet in winter weather here in the benign Salish Sea I have to reef. On the other hand, on August afternoons in the same waters I wish my SA/D was 26 :-)!

OBVIOUSLY your "normal" operating conditions is the determinant of what is a desirable SA/D for you!

Spurious accuracy is the sailorman's bane! Absolutes are misleading, and a desire for them is an indication that more insight is desirable.

TrentePieds

[rls8r]

Many folks pay a lot of attention to a boat's calculated ratios ... but I don't. There really are too many things (i.e., windage, provisions for cruising, form stability vs keel stability, etc.) that affect the performance of a boat for me to have any reliance on a few ratios. I suppose I'm just not that much of a technical boaty person.

I have an old '80s fiberglass beast with a SA/D of 14.84. It moves right along on a beam or broad reach. It's upwind performance isn't that of a racer - but I don't race (and remember, gentlemen never sail to weather). I get hull speed (or close to it) on many points of sail across a wide range of wind speeds. As I understand things, a boat with a SA/D <16 is considered to be 'underpowered.' That's fine with me. I'm satisfied with the boat's performance and I think being 'underpowered' makes it somewhat of an easier boat to sail.

I have a removable "inner forestay" and two staysails; one for light winds and one for heavy winds. So, the boat can become somewhat of a 'cutter' rig (with a mizzen sail to boot). I sometimes set the light-air staysail with the 130% genoa and it gives me a little more oomph in light airs. It's a pain to tack, so I'll only use both sails on long and long-ish reaches. I think that the staysail improves the "slot effect" on the genoa and main. Perhaps that's just wishful thinking. I have no idea what the SA/D is for that configuration.

In heavier winds I'll furl the genoa, reef the main and set the heavy-weather staysail. This arrangement gives me better balance than I'd get by 'reefing' the genoa since my boat essentially becomes a sloop with its center of effort remaining closer to where it's 'supposed' to be. I have no idea what the SA/D is for that configuration either.