S.A./Displacement confusion

[shmorrell]

Hi all - I’m spending a lot of time checking out boats in the 38 ish range for a purchase this year. I want something that will be reasonably comfortable offshore (ie heavy) but still have decent sailing performance.

The spec S.A./D seems to indicate whether a boat carries, or is designed to carry enough sail. Sailboatdata says:

SA/Disp.:
A sail area/displacement ratio below 16 would be considered under powered; 16 to 20 would indicate reasonably good performance;
above 20 suggests relatively high performance.

These values are derived by adding up the sail area of the fore triangle and main triangle and plugging the number into a formula. But many, if not most, boats carry headsails that are much bigger than the fore triangle. A 150% genoa obviously has a lot more square footage than a 100% jib. And an asym or code 0 will have even more.

Is there something I’m missing ? Why would an architect design a boat to be underpowered? ( the Endeavour 37 has a SA/ D of 12.5)

Thanks!

[thomm225]

I don't think it's as simple as that. You also need to consider LWL, Disp, Beam, etc to figure speed/performance

Below 16 SA/Disp might be right for some boats and not necessarily mean they are under powered.

It could mean they have the right amount of sail for what the boat was built for

Example: The Rustler 36 which was used by quite a few sailors in the recent Golden Globe Round the World Race (2018) many times had it's main reefed because it was OVERPOWERED in the conditions

It's SA/Disp is 11.43

It all depends on the boat, how it's built and what it's built for. The Rustler has a beam of 11' and disp of 16,805. LWL 26.92

One of the Rustler 36's won that race ......

There were a few other boats in that race with SA/Disp above 16 that had slower hull speeds than the Rustler.

One was the Endurance 35 with SA/Disp of 16.8. Disp 18, 607, Beam 11", LWL 26.64

[psk125]

Thomm is right, there are lots of variables involved in determining whether a boat is undercanvassed or overcanvassed. Look at the IMOCA 60's that are finishing the Vendée Globe race after going around the world. Their beamy designs with canting keels and specialized rigging allow them to have huge sailplans that would be recipes for disaster on a regular monohull. Having the right amount of sail for the conditions that the boat sails in is the prime criterion. A boat built to sail around in SoCal needs a lot of sail area because the wind is generally light. A boat that is kept in SFO Bay can manage quite well with smaller sails most of the time. Since it's easier to reef a sail than it is to increase the height of the mast to hoist a bigger one, most people go for the most sail area they can get. This enables them to keep sailing in light air, while people with less sail area end up motoring. If you're planning to cruise across oceans you may not want a tall, spindly mast that might have expensive parts break in a remote spot on the globe. On the other hand, you don't want to get hit by a hurricane en route because it took you an extra week to get there. Every boat is a balancing act. You need to find the balance that works for you.

[wingssail]

Originally Posted by shmorrell

Hi all - I’m spending a lot of time checking out boats in the 38 ish range for a purchase this year. I want something that will be reasonably comfortable offshore (ie heavy) but still have decent sailing performance.

The spec S.A./D seems to indicate whether a boat carries, or is designed to carry enough sail. Sailboatdata says:

SA/Disp.:
A sail area/displacement ratio below 16 would be considered under powered; 16 to 20 would indicate reasonably good performance;
above 20 suggests relatively high performance.

These values are derived by adding up the sail area of the fore triangle and main triangle and plugging the number into a formula. But many, if not most, boats carry headsails that are much bigger than the fore triangle. A 150% genoa obviously has a lot more square footage than a 100% jib. And an asym or code 0 will have even more.

Is there something I’m missing ? Why would an architect design a boat to be underpowered? ( the Endeavour 37 has a SA/ D of 12.5)

Thanks!

Yes, there is more to sailing performance than the SA/Disp. BUT, it is a good "apples to apples" comparison. What you need depends on where you plan to do most of your sailing, (and don't overlook that ALL circumnavigators spend a huge amount of time in light air) and what kind of sailing boat you are looking at. A boat with a bigger potential sail area which can reduce area in a breeze but put up a big sail in the light stuff has more flexibility.

Why would a designer design a boat with a small sail area? Maybe the maket he is designing for values something else besides sailing performance. Or maybe the boat is tender and can't carry more sail area.

Finally, some of us really care about how the boat feels when sailing. We like a boat that comes alive in 7-10 knots of wind, and is fun all along the wind range, and one which can sail in a moderate breeze with very small sails, because even with a reefed main and a working jib the boat can perform. I sail all the time because it feels good.

So pay attention to those numbers, don't look for reasons to discard them. Among other things to look at they are good indicators.

[wingssail]

Originally Posted by thomm225

I don't think it's as simple as that. You also need to consider LWL, Disp, Beam, etc to figure speed/performance

Below 16 SA/Disp might be right for some boats and not necessarily mean they are under powered.

It could mean they have the right amount of sail for what the boat was built for

Example: The Rustler 36 which was used by quite a few sailors in the recent Golden Globe Round the World Race (2018) many times had it's main reefed because it was OVERPOWERED in the conditions

It's SA/Disp is 11.43

It all depends on the boat, how it's built and what it's built for. The Rustler has a beam of 11' and disp of 16,805. LWL 26.92

One of the Rustler 36's won that race ......

There were a few other boats in that race with SA/Disp above 16 that had slower hull speeds than the Rustler.

One was the Endurance 35 with SA/Disp of 16.8. Disp 18, 607, Beam 11", LWL 26.64

Thomm, regarding the Rustler, we've had this conversatation before. It was used by several competitors because the rules resticted their choice of boats. The Rustler qualified, and they were available and cheap. It is not surprising that a Rustler won when so many of the entrants were using them.

That doesn't mean that they are a good design nor do I think much of Endurance 35 (I think thse are subjective opinions, and of course others will disagree).

A boat with a more modern hull design could probably have carried more sail area longer and any modern racer cruiser would have certainly crushed the Rustlers.

Yes, Rustler 36's with a low SA/DISP of 11.43 had to be reefed in the Southern Ocean. That doesn't mean that is an adequate SA/DISP. It means that this poor design can't carry any more sail area than that.

In any light winds that Rustler would be sluggish. In heavy winds it would be hard to impossible to handle, and the proof is that so many had equipment failures and retirements.

Stop using the Rustler 36 as an example of good sailboat design or as a model of what SA/DISP should be.

If you want a good example of a design that fits the "old fashioned" era, look at a Swan 36 designed in 1967. It has a SA/DISP of 14.93

[thomm225]

Originally Posted by wingssail

Thomm, regarding the Rustler, we've had this conversatation before. It was used by several competitors because the rules resticted their choice of boats. The Rustler qualified, and they were available and cheap. It is not surprising that a Rustler won when so many of the entrants were using them.

That doesn't mean that they are a good design nor do I think much of Endurance 35 (I think thse are subjective opinions, and of course others will disagree).

A boat with a more modern hull design could probably have carried more sail area longer and any modern racer cruiser would have certainly crushed the Rustlers.

Yes, Rustler 36's with a low SA/DISP of 11.43 had to be reefed in the Southern Ocean. That doesn't mean that is an adequate SA/DISP. It means that this poor design can't carry any more sail area than that.

In any light winds that Rustler would be sluggish. In heavy winds it would be hard to impossible to handle, and the proof is that so many had equipment failures and retirements.

Stop using the Rustler 36 as an example of good sailboat design or as a model of what SA/DISP should be.

If you want a good example of a design that fits the "old fashioned" era, look at a Swan 36 designed in 1967. It has a SA/DISP of 14.93

Yep, I was just using the Rustler comparison to the other boats in the Golden Globe race because it was a good example of a boat with a low SA/Disp Ratio that was actually faster than a boat with a quite a bit higher SA/Disp Ratio.

On the other end of the spectrum, my racing boats (4 beach cats) had SA/Disp Ratios of about 76.

When you got much about 18 knots, you were starting to get on the edge of being out of control.

Races weren't called until the wind stayed about 21 knots for I believe 5 minutes.

So if you are singlehanding a Beach Cat with a spinnaker and coming into the downwind mark in traffic at around 20 knots it can get quite dicey at times especially on a normal buoy racing WLWL course. Even with crew with Main, jib, and spinnaker it is a tough rounding especially if you are in heavy traffic.

And btw, you don't reef sails in beach cat racing......you use thicker battens, derotate the mast, flatten sail, travel out, feather the jib if your boat has one, etc

Now on the lower end, my Bristol 27 with it's SA/Disp of 14.28 is just starting to come alive at 18 knots and isn't to hard to control at least downwind in 30 knots......

[Don C L]

My boat is about as tender as they come and it was designed with about a ratio of 16. Still things have to get up to about 18 knots upwind before I reef, but I have to reef earlier and more often that heavier boats with lower ratios I 'spect if I want to stay at about 20 degrees of heel. I see it like 6 speed transmissions for smaller engines. I can kinda predict how a ratio of 14 will sail, it's a little too sluggish for my tastes. My friend's Luders 30 has a waterline of 22' and weighs about 10,000 lbs and has a ratio of 14 or so. I have 22.5' wl and weigh about a ton less. Once there is a really good breeze we're pretty well matched, but in a little lighter air, I can sail circles around him.
I agree with Fred that it provides a good apples to apples basis for comparison.
My own boat came out later with a ton more ballast, presumably to stiffen it up for better upwind work and to give it a little more range (in terms of wind speed before reefing.) The later version had a SA/D of about 15.

[shmorrell]

Thanks to all for the informative replies.

[sandy stone]

There is one other aspect of the SA/D ratio to consider (leaving aside the fact that the displacement of a boat is a pretty fuzzy number).

Up until maybe 15-20 years ago, most boats were designed with inboard shrouds and were expected to carry overlapping genoas. As you note, this resulted in sail area unmeasured by the simple formula. Lately the trend has been towards shrouds out to the gunwales, and non-overlapping, or even self-tending, jibs. This is understandable, since the smaller jibs are much easier to tack.

So, all sail carried by the boat is now accounted for. To make up for the "lost" sail area, designers specified slightly larger rigs on the newer boats. For that reason, on an older boat, a SA/D of 18 or so was getting to be fairly racy, but on a boat with a nonoverlapping jib you might be looking at a SA/D of 22 or so to get similar performance. I can't comment on older CCA-era boats.

Just something to keep in mind when comparing numbers.

[Jim Cate]

^^^^^^

True, but one can surely carry an overlapping genoa on the modern boat as well, useful at all points of sail except close hauled. Or more likely these days some sort of code zero sail, even larger than typical genoas.

Jim

[wingssail]

Originally Posted by Jim Cate

^^^^^^

True, but one can surely carry an overlapping genoa on the modern boat as well, useful at all points of sail except close hauled. Or more likely these days some sort of code zero sail, even larger than typical genoas.

Jim

Well, Jim, Close hauled is of critical importance for a racing boat, and useful on a cruising boat.

We sail virtually all the time against modern boats with wide shroud bases (out to the side of the boat) and non overlapping jibs while we have inboard shrouds and 153% genoas. In lighter air, when we can use our genoa they have no answer. Even with the most advanced code zero sail, such as a North 3DI raw on its own furler, they cannot match us to windward in point or speed. Believe me, it is a constant source of frustration to our biggest rival in the fleet, sailed by a national champion with the North sailmaker on board, that even when they rate 39 seconds a mile faster and have SA./DISP of 23.38 to our 20.97 we still beat them to windward in under 12 knots of wind.

Non-overlapping headsails makes it easier for them to manage the boat, and roller furling even more so, but they are not as fast in light air. Once we have to go to our No.3 jib it all changes. Off the wind the modern boats are faster unless it is heavy air and we can sail close to dead down wind.

For boats not racing the ability to go to windward helps on upwind passages. VMG is improved.

So Sandy Stone's point about the "unmeasured sail area" for boats with overlapping headsails is quite true.

But what this really points out, and to the topic of the thread, more SA./DISP only benefits you if you can carry all that sail. Once the wind gets up, it does you no good, even harm perhaps.

[Jim Cate]

Well, Jim, Close hauled is of critical importance for a racing boat, and useful on a cruising boat.

Well, of course, Fred! I should think that you would know that I agree with this.

My comment was directed at Sandy's comparison of the meaningfulness of SAisp numbers in cruising boats of varying designs, not round the buoys racing in competitive boats with pro crews.

Your successes in Wings are inspiring, but not really representative of typical cruising endeavors.

Jim

[Elzaar]

I'm sure there are many angles to this and much more experience here than I can offer, but I had the same conundrum when trying to figure out which 38'sh sailboat was right for me. I eventually settled on using PHRF ratings for my spreadsheet (in addition to SA/Disp). I felt PHRF likely gives the most consistent "Apples to Apples" comparison in that it considers all points of sail and the averaged ingenuity of many sailors sailing different boats. That is, afterall, why PHRF exists. I also liked that it can give some idea how a boat can be expected to perform in a specific region. As mentioned, when my particular boat was designed (1982) no one had ever even imagined a Cruising Code Zero. Even the concept of an asymmetric spinnaker was fairly new.

For my friends who are accomplished sailors, willing to wring every last 1/10th of a knot out of their boat even if it means significant discomfort, SA/D probably means more. For me, I wanted to be able to tell if one boat was significantly slower than another *in general*. PHRF accomplished that.

One of the top boats I considered was a Passport 40. It has a SADR of 15.4 and a reasonable PHRF at 141. The boat I bought was a Wauquiez Hood 38. Its SADR is 14, but its PHRF is pretty consistently rated around 130.

With my upgrade to a Code Zero last year, Im researching *reducing* my genoa size, not increasing it.

Just my .02 cents.

Good luck with the search. Buy the boat that makes you feel good just to look at it.

[wingssail]

Originally Posted by Elzaar

I'm sure there are many angles to this and much more experience here than I can offer, but I had the same conundrum when trying to figure out which 38'sh sailboat was right for me. I eventually settled on using PHRF ratings for my spreadsheet (in addition to SA/Disp). I felt PHRF likely gives the most consistent "Apples to Apples" comparison in that it considers all points of sail and the averaged ingenuity of many sailors sailing different boats. That is, afterall, why PHRF exists. I also liked that it can give some idea how a boat can be expected to perform in a specific region. As mentioned, when my particular boat was designed (1982) no one had ever even imagined a Cruising Code Zero. Even the concept of an asymmetric spinnaker was fairly new.

For my friends who are accomplished sailors, willing to wring every last 1/10th of a knot out of their boat even if it means significant discomfort, SA/D probably means more. For me, I wanted to be able to tell if one boat was significantly slower than another *in general*. PHRF accomplished that.

One of the top boats I considered was a Passport 40. It has a SADR of 15.4 and a reasonable PHRF at 141. The boat I bought was a Wauquiez Hood 38. Its SADR is 14, but its PHRF is pretty consistently rated around 130.

With my upgrade to a Code Zero last year, Im researching *reducing* my genoa size, not increasing it.

Just my .02 cents.

Good luck with the search. Buy the boat that makes you feel good just to look at it.

This is a very interesting comparison. The Hood 38 is a fine boat. So is the Passport 40 which is lighter and has more sail area, but the the Hood rates faster under PHRF.

Maybe the sleeker hull shape and the centerboard gives it an advantage. Or maybe there are just more keen sailors racing Hood 38 boats (in previous years).

One thing to think about when considering a code zero for your boat: If you are expecting it to work as well as a big genoa on close reaching then you might find it disappointing. The genoa is very effective upwind and on close reaches, even beam reaches. At wind angles farther off the wind than a beam reach then an assymetrical spinnaker will work better than a code zero.

Where the code zero really shines is on boats which cannot carry a genoa because of the rig design, and even then the sail has a limited range of wind angles where it really shines.

[Elzaar]

In answer to Wingsail: All fair points. I suspect it is at least partly the lifting CB, and somewhat hull shape. The Hood picks up at least a 1/2 knot downwind when the board is lifted. Hood was also an avid racer and specialized in CB hulls, so I'm guessing some of that is also designed into hull form. The Passport has the modified fin keel which may not do some things as well as a 10+ foot center board. Those are discussions for Bob Perry and Ted Hood. Which gets us back to the point of the thread - SADR is only an indicator of potential, not a promise. That's why I suggest PHRF is a better general speed indicator.

I'm certainly no expert sailmaker, but I think the Cruising code zero is cut closer to being a very big genoa. My sailmaker advised - and my experience has been - that in light winds my boat points very well even with the CZ. As winds pick up, my pointing ability goes down. Which is why I'm considering something closer to a blade jib or likely a 110 yankee. My genoa is too big (for me) by 16 or so knots true wind, leaving me a not great shaped, partially furled genoa when I could be just getting the best out of something smaller. I wanted to improve performance on my boat in light winds - a CCZ seemed the best solution. Besides,, they look cool and I love having two different sails ready to unfurl. All that said, a brand new genoa might have reduced heel and maintained shape into higher winds, but then it won't do some things the CZ will. It's all compromises and preferences. I plan to focus on seeing what my removeable inner forestay staysail can add this year - which adds another unrepresented piece to the SADR measurement. All this explanation to underscore for the OP that there is much more involved in maximizing boat speed for conditions than just comparing SADR numbers.

You may well be right that more serious racing types race the Hood than the Passport, but since there is no easy way for me to factor that in, I just have to assume the numbers are the numbers. Mostly though, I wanted to be sure a given boat wasn't substantially slower ... 10 points in PHRF wasn't significantly different to me, but if I looked only at SADR, 14 vs 15.4 and the arbitrary assignment of terms like "slow" and "moderate" would have been significant - and misleading.

In my own search, I tried to quantify performance looking at numbers that I could understand individually, but found I could understand less so in the aggregate. You can stare at eggs, and you can stare at sugar and flour, and you can understand water, but how they mix to create a complex whole is what makes cake. Certainly, a big part of that is the almost undefinable skill of the baker. PHRF is closer to tasting the cake after someone else has made it many times.

Which is also why I want to know what people like those on this forum think about blade jibs, genoas, and code zeros. And soon, staysails.