Go Directly To The Sail Calculator Here
What Carl’s Sail Calculator Does:
Physicist and sailor Carl Adler developed this online Sail Calculator for comparing sailboats and its database has grown over a number of years to almost 3000 boats. It should be one of the first places you go on the Web if you want to know the vital statistics about a sailboat, including Length Overall (LOA), Length on the Waterline (LWL), Displacement and Sail Area.
The Sail Calculator will also give you valuable performance numbers for any vessel in its database or any numbers you enter, including the Displacement / LWL ratio, Theoretical Limiting Hull Speed, Sail Area / Displacement ratio, Length to Beam ratio, Motion Comfort value, Capsize Screening value, sailing category and Pounds per inch immersion value.
Naval architects use these values when they design a new boat, and from them you can determine a conventional displacement hull boat’s purpose and predict its performance. Note that planing hulls, catamarans and hydrofoil vessels are not defined in the same way. Here’s what the performance numbers mean:
Displacement/LWL ratio – Heavy boats (D/L above about 300) will carry big loads but require plenty of power to drive. Light boats (D/L below about 150) are generally quicker and more responsive but are affected by loading. Most boats have moderate displacement and they compromise the conflicting virtues of the extreme designs. Contemporary racing boats often have D/L ratios well below 100.
Hull Speed – A conventional hull, which moves through the water rather than rising atop it and planing across the surface, is limited in speed by length of the waves it produces; long waves travel faster. This wave length can be calculated and the top speed of the hull predicted. Long boats make long waves.
Sail Area / Displacement ratio – The SA/D ratio is like the power/weight ratio of an automobile. A high SA/D ratio (> about 18) indicates a powerful rig, while a low ratio indicates a more docile boat.
Length / Beam ratio – A long, narrow hull with limited interior space is easier to drive than a short, fat one with plentiful capacity. Compare L/B ratios to gain insight into the purpose of the boat.
Motion Comfort value – Not as widely used as the previous numbers, the Motion Comfort value tries to predict whether a boat has a quick, motion through the waves or a slow, easy motion. Note that some people get more seasick with a slowly rolling motion than a quick, jerky one. Your mileage will vary.
Capsize Screening number – Developed after the Fastnet Race tragedy, the Capsize Screening number is a quick way to judge if a boat is seaworthy. Values below 2.0 are desirable for offshore yachts. Do not put too much faith in the exact number, as it is an approximation only.
Pounds / square inch Immersion – When you load a boat, it sinks deeper into the water. This Immersion value indicates the weight carrying capacity of a vessel.
There is also a Prop Sizing section which will calculate the optimum propeller to use on any displacement-hull boat, based on noted naval architect Dave Gerr’s formulas.
I just saw this and am sorry for the long delay in approving and answering. I don’t know what happened to the notification that the site is supposed to send me.
This is a great question and there’s no good answer. Nigel and I have worked together many times over the years at SAIL Magazine and this is a perpetual discussion point.
Fact is, the only way to get the actual displacement of a specific boat is to haul it out and weigh it. Even two boats of the same design will not weigh the same and all boats gain weight with age due to water absorption, added gear and other factors. As boat reviewers we have to rely on the number reported by the builder or the designer and those vary all over the place. By the time a boat is built, it nearly always displaces more than the designer specified, so that’s one variable.
Ideally, we could get an accurate half-load displacement from each builder but most will not provide that because it produces less attractive numbers for their marketing. Hallberg-Rassy is an exception and I applaud them for it. If you can get that number, use it instead of the “Light Ship” (empty, no optional equipment, perhaps partially rigged) value usually reported.
The smaller the boat, the more inaccurate the number. In an extreme example, take a 12-foot racing dinghy weighing 150 pounds but carrying a crew of two who weigh 250 pounds. A “Light Ship” number there is meaningless except for carrying the boat to its trailer.
Welcome to the wonderful world of marketing vs. engineering!
— Tom
Calvin,
I finally got around to approving your post on my Sail Calculator regarding an error in the listed sail area of the CT-41. Sorry for the delay.
I don’t see that boat listed. I do have a CT-42 Mermaid with a sail area of 793 sq. ft. Is this the same boat?
If the CT-41 is new to the Sail Calculator and you’d like it listed, just send me the data (fill out the form on the page) and I’ll add it on the next update.
Thanks for your participation in this crowd-sourced site.
When researching information on the CT-41 in the sail calculator, I found that the sail are is listed as 100. Sailboat data shows a sail area of 792.00 ft². Thanks for the awesome job of keeping this calculator alive.
You can use the Sail Calculator to look up sailboat dimensions in a large database of boats or enter your own boat’s dimensions and compare it to the others. In addition, for any boat you can calculate a set of values that will help you measure how it will perform Hull speed can be expressed as a simple mathematical formula 1.34 X the square root of the length of the waterline (HS = 1.34 x ?LWL). For instance, if a cruising sailboat has a waterline length of 36 feet, she should be able to sail 1.34 x 6, or approximately eight knots.
https://slope-unblocked.org/
Hi Tom, many thanks for the calculator – it’s fantastic!
One small question: for comparison’s sake, does the calculator have a standard basis for calculating displacement? Nigel Calder in his Handbook for Cruisers points out that most manufacturers list a very low value for displacement (ie empty tanks, minimal equipment, sometimes without even sails) and that in practice displacement is considerably higher.
Browsing through the sail calculator, (for instance) a Hallberg Rassy 352 is listed with its manufacturers listed displacement (14,770 lbs) while the Hallberg Rassy 312 is listed with a realistic half-load displacement (12,782 instead of the manufacturer’s listed 10,803 lbs). This in turn affects various other values such as motion comfort & capsize ratio. Which value should be considered more accurate?
I corrected it. Thanks!
S2 7.3 specs from factory brochure (visible at boatbrochure DOT com SLASH products SLASH s2-7-3-meter-brochure the free preview is pretty legible)
LWL is 18.5 not 18.73
Beam is 8.0 not 8.5
Displ is 3250 not 3373
S.A. is 255, not 261
Thank you so much for your work maintaining this web page; it is tremendously valuable resource that I refer to often!
Tom,
The Colgate 26 has a sail area different from that published on your calculator. It’s listed as 338 SF per https://www.colgate26.com/specifications/
Thanks
Chris,
Thanks for the note; I’m glad you find the Sail Calculator useful.
I’ll change the value on the database on the next update, since your source is probably more accurate. I rarely know what the sources are when a user submits data, so there are definitely errors in there.
It’s possible that one of the numbers is based on the 100% foretriangle measurement and the other is with a larger jib, which could be either the working jib or a Genoa. I get this question from time to time and probably should add something to the description about it (www.tomdove/blog/sail-calculator/).
–Tom
Hi Tom, thanks for Carl’s calculator alive. I have a Tayana 48 DS and from their website, I get a different sail area.
1316 sq ft vs 1048.
Regards,
Chris
Hi Tom, Looking at your specs the Marieholm 26 literature does not match what is posted. There were 3 versions of this boat built with the Marieholm being the middle one. The Folkboat website shows this: loa 25.83, lwl 19.83, beam 7.17, s.a. 280 sq. ft., draft 4′, disp. 4740, ballast 2750. The 1st model was built from wood, the last (3rd) model is called the Nordic Folkboat built from fiberglas but made with lapstrack design to look like wood. It was heavier in weight than the other 2 with less s.a.. Google “Folkboats Around the World” and the info is there on the main page. Hope this helps. Like to see values once new info is inserted. Wish I could figure it myself but not sure how to. Thank you, Sam
Thanks for catching that. I’ll correct it on the next update.
— Tom
The Goletta Oceanica De Biot 39 is missing a decimal point in the LWL so it is throwing off all of the calculations.
David,
There’s no simple answer to that. If you enjoy sailing the boat, it’s a good one.
When you put the numbers into SailCalculator, it will return some basic information that can be very useful, but note that small, lightweight boats like the American 23 are sensitive to loading. The working displacement is actually the “Light Ship” displacement plus the weight of the average crew. Try adding the weights of you and your crew in SailCalculator and see how that affects the performance numbers.
The people I have known who have the American 23 like it. It looks like a nice, stable daysailer.
Enjoy!
— Tom
I have American sailboat 23 ft. sailboat with a displacement 3500 lbs my keel is a 1000 lbs with a beam 7ft and 11 inch just wondering how good is this boat for sailing thanks
Mark,
Very interesting. I can see why the Length/Beam ratio at the waterline would be the defining characteristic for hull speed. That can be an evasive number, I think. Multis with very narrow hulls will sink deeper into the water quickly as the boat is loaded, so the LWL/BWL could change dramatically. It seems that you’d have to be careful about specifying the displacement that produces a specific LWL/BWL ratio, don’t you think? Is there an issue of one hull being submerged more than the other when the boat is under sail? This seems especially important in trimarans, which often have one hull flying and the other deeply submerged, but a long, narrow cat would have some of the same response to a breeze.
Keep me posted on your thoughts. I think you’ve hit on a key element here.
— Tom
Hello, I’m a mechanical engineer and experienced multihull sailor that has long thought multihulls need a better performance parameter for comparison so sales guys can’t hoodwink people! I have some graduate school education from Dr. Marshall Tulin (UCSB) who has published many works regarding high-speed displacement mode for long slender hulls for naval/military applications and I think this work is very applicable to sailing multihulls. The critical parameter as far as hull drag for catamarans is really L/B at the waterline since other parameters as far as hull form go (prismatic coefficient) are generally within a narrow range. It has the benefit of implying displacement and waterline length as well, since a heavy boat must be either fat, or long to carry the displacement. As a result, I’ve been working on a parameter that includes both sail area and L/B at the waterline for performance comparisons. The trouble is Schionning is one of the few designers that cites L/B in all of his designs but it would be an easy measurement to take dockside, when the true displacement isn’t known.
Steve,
I’ve never seen that formula but would love to have it. The speed of a multihull is largely a factor of the hull shapes, and most multis are not limited by the “Displacement Hull Speed” that determines the maximum speed of most monohulls. The hulls are generally long and narrow and do not create the speed-limiting waves. There are exceptions, and I think any formula that predicts the speed of a cat or tri would have to incorporate prismatic coefficient (“sharpness”). Most boats are not speed-limited by their sail area.
I’m looking for a formula that predicts potential performance of a cruising catamaran, in teh same way that SA/D does for monohulls. I saw teh formula years ago – it uses sail area and the second power (i.e., the square) of a factor, but I don’t recall anything else. Can you help me?
-Steve
Charlie,
Thank you for the compliment. I enjoy running the site and meeting so many people who love sailing. Good luck on your boat search; there are many good deals on used, mid-size cruising boats available now in the U.S. because builders flooded the market with 40-footers a few years ago. Now that so many Baby Boomers have finished their lifetime sailing adventures, the boats are for sale.
I’ve just been introduced to your site by a good friend from the US. Im looking for a retirement live aboard that can take me around the world. He gave me a potted history and speaks very highly of Carl Alder in this site in general. What a great tool. I’ll be flying to the states to view some boats that otherwise wouldn’t have even been on my Radar. Thank you Tom, Thank you Carl (Thank you Harvey).
Thanks for keeping Carl’s program alive, Tom. I sent him hundreds of small boat specs over the years and found quite a few errors from other inputs that Carl tried to correct. Between his poor vision, a lot of incorrect input (especially the difference between LOD versus LOA for most people) and the vague info from boat builders it was a long process.
People should have supported him with far more donations, he was a good guy.
Les Hall, San Antonio
Thanks for catching that, Paul. Yes, that would be a mighty powerful boat.
It appears that the displacement should have been 14,500, so I corrected that. The SA/D ratio still looks a bit high, but I don’t know what the submitter used as a source.
Enjoy the site and please send any other corrections you see.
— Tom
Cavalier 37, LWL=30, Sail Area to Displacement=2314.05
Cant possibly be correct
Great calculator, thanks for keeping it available.
Cheers