Ideal displacement for a certain length?

[GTom]

I understand, that heavy boats are more stable (require more energy to shake) in a storm but also need more sail power, otherwise they don't move an inch below force 4.

What is the ideal size/displacement ratio? Or go with the heaviest vessel and compensate with canvas? (See Flicka 20, 20 feet and 2.5 tons with standard SA/Displ =12)

[UNCIVILIZED]

This question, in various forms, gets asked quite a bit. So below is a cut & paste excerpt from another post I answered on this topic. Hope it helps.

This may sound harsh, & the truth can be. But the fact is, that probably 90-95% of the cruising boats out there are under-rigged, or under canvased for light airs.So that until this is optimized, you’re stuck with turning on the engine when the breeze drops to somewhere below 10kts.

Specifically, most boats just aren’t designed for light air. Their rigs are simply too small. Especially when you consider that in order to do well in winds of under 10kts, or more importantly, under 5kts. You need an SADR of 20+, & 25+ is optimal, inclusive of jib overlap & mainsail roach. And for a multihull you want a Bruce Number of 1.3 or better, as suggested by Chris White IIRC.

For example, on my Ranger 33 (tall rig), I could sail quite well in winds down to 6-7kts. Often doing 3-4kts, & I could keep her moving in breezes down to about 3kts.This with an SADR of about 22 when the extra area of the 130% was factored in.And on my 2-tonner, I’d do 2/3 of wind speed or better down to 5kts of breeze.Only getting stuck when there was no wind, with an SADR of 25 +/-.And with my Searunner, I’d hit the above performance numbers when she was unladen, & her Bruce Number was then at right about 1.3.

These figures take into account the weight of gear onboard, jib overlap, & mainsail roach. And this is for upwind sail only.You need proportionally more for downwind work, especially as the AWS is lower.Though the downwind numbers are easy to figure, based on rig dimensions & displacement.The caveat being that if you’ve got as much wetted surface area as a tennis court, you’ll have more trouble when it’s light.Even with heaps of sail.Oh, & don’t drag your ass, er, transom It’s slow.

To achieve such numbers you can use a Code 0 designed for upwind work. Meaning one which has a highly tensioned luff with an anti-torsion cable built into the sail (& a 2:1 halyard). Or add a large hank on jib or reacher to a Solent Stay, to be used when a 130% on the furler is just too small.And off the wind, spinnakers & drifters are the tools of choice.With specialized sails like a windseeker working best, when there’s truly no wind, but with a sea running which knocks the wind out of normal sails.It’s to be set board flat, along with a similarly shaped main.

As if you hit the 20 figure, you’ll likely sail at about 2/3 of wind speed down to 10kts or so of breeze, & ½ of wind speed down to 5kts. Only stopping in even less wind, or if the breeze is light but there’s a sea running that kills the boat’s momentum.

If you can hit the 25 figure you’ll sail at 2/3 of wind speed down to 4-5kts of breeze, & then only get stopped in such light air if again, there’s a sea running. For a multihull you want a Bruce Number of 1.3 or better, as suggested by noted designer Chris White, & borne out by my 31’ Searunner.

[StuM]

If there were an "ideal length/displacement ratio", all boats would be built to it.

[GTom]

Uncivilized, thanks for your thoughts! SADR 20+ is indeed very rare even among racing boats. One could of course add a bigger genova but the rigging might become the limiting factor after a while.

I am after a stable (and heavy) seaworthy boat, looking at Hallberg Rassy-s, Malo-s, Rivals & Co. - these unfortunately around SADR 15 or even 12(!). I don't know if it is a viable option to beef up the rig (e.g. adding a bowsprit) and add more canvas.

Originally Posted by StuM

If there were an "ideal length/displacement ratio", all boats would be built to it.

[sailorboy1]

It has always seemed to me that the SA/disp for most cruisers is 15-18. If lot more it is a racer and a lot less it is a motorsailor. Other than that I don't think there is an optimum number. I feel you are better off getting a boat that you like for other reasons and not worrying about the SA/disp because that really is waste of time.

[danielamartindm]

Most production boats need the capacity for greater sail area than they are capable of flying for the sake of better light air performance. The answer to your question really touches upon the issue of mission statement: where will you do the bulk of your sailing?

[denverd0n]

As StuM said, there is no "ideal." There is a wide continuum. Where you want to be on that continuum depends on a range of very personal criteria that only you can determine. All boat designs are a large collection of compromises, and you will have to decide what you are willing to compromise on and what you are not.

[UNCIVILIZED]

Originally Posted by danielamartindm

Most production boats need the capacity for greater sail area than they are capable of flying for the sake of better light air performance. The answer to your question really touches upon the issue of mission statement: where will you do the bulk of your sailing?

Sadly, the above is often true. Though if one is willing to invest the time & money, anymore, it's not so hard to change this. Since if you add a roachy, full battened main, & a big jib, or a Code 0 designed for upwind work, then getting the SADR up around 20 or a bit more is possible. Though by no means did I necessarily say that it's cheap.

An SADR of 20 really isn't an extreme number, especially when you consider that a huge percentage of "cruising" takes place in areas, & during seasons where winds of 10kts or less are the norm. Something frequently noted in magazine articles, & blogs, etc. So if you want to sail, & sail well in such breezes, you have to plan for such from the outset. And I'm a big fan of sailing instead of motoring, so a boat that goes well in light air is important to me.

The question is, what's your threshold for turning your engine on/minimum boat speed? And what do the polars for your boat/candidate boat say about her performance in various wind speeds & angles? Those things pretty much constitute the bottom line.

[GTom]

Converting a SADR=12 boat to SADR 20 is quite extreme in my books). I'd be worried a bit about stability...

[KP44]

Not sure what you mean by compensate with more canvass, but I guess that is what we do. I carry an asymmetrical spinnaker for light air sailing on my 45', 18 ton cutter. Most newer boats are lighter, probably less sea-kindly, and no faster.

As far as under-canvassed, my boat came with an 18 foot boom. But the foot of the main is about 16.5' because any more mainsail created weatherhelm. The boat is easily powered by the genoa and balanced with the mail. The stays'l gives some options when reefed or heavier weather.
Perfect tradewind sailing is a single reef in the main and the staysail. I have both headsails cut with high clew, almost a Yankee, so I can see under them.

[GTom]

I meant taller rig, larger main and Genova. Spi is not much use at close hauled angles

[TrentePieds]

GTom:

It is a fact of life in the modern world that sailboats - "cruisers"- are "permanently reefed". There are obviously good reasons for it being so, but there are also good reasons that it SHOULD NOT be so!

A cruising boat needs to be able to get out from under her own feet. Particularly when going to weather. That is NOT necessarily so for racing boats, particularly not for "class boats".

The pitifully small SADRs that are now common, are common due to the evolution of the rules by which "class boats" are measured. Measuring rules strive to create a "level playing field" rather than superior sailing qualities.

Go back to the "J" boats: Thomas Lipton's "Shamrock" had a SADR of 45! Lipton was able to afford both the boat and a professional crew capable of handling such a boat. As the "common man" became able to afford engaging in rich men's hobbies, particularly in the 1970s, the design desiderata changed, because every Tom, Dick and Harry had to be a sailor, and the harbours filled with lubbers. And where is the fun in that? Unless you race your boat? In consequence we wound up with "cruiser/racers" with diminutive SADRs and underwater profiles that belong to 16 foot dinghies - not to grown-up cruising boats. The design desiderata for decent cruising boats are VERY different from those for the sort of floating holiday cottages the French are fond of building for German families to rent in the Aegean.

Now, my poor little TrentePieds has a SADR of 12.5. I don't complain because MyBeloved just LOVES being in 'er. But sailing? Not a hope! If I stuck a six foot sprit on 'er, and a four foot bumpkin and lengthened her mast by 15 feet I could get a SADR of 25. But given that her underwater profile is what it is, let alone her lines, that would not be safe. And any serious sailing would scare MyBeloved to death anyway ;-)!

There is no reason that you can't take a permanently reefed boat anywhere you want to go. As long as you stay off lee shores it is probably safer for most people to sail in such boats rather than in ones with more canvass. I think the craze for roller reefing speaks to that. Reefing betimes is obviously crucial to safety in a boat with a SADR of 45!

Having said all that, a type that many like as a cruising boat, the Colin Archer "Redningsskoite", has a SADR of 10!

And having said that also, the conclusion has to be that there is no "ideal" SADR. There are boats designed to achieve specific purposes in specific waters. And their SADR reflect what they were meant to do. To cruise in the Aegean or where the coconuts grow is not the same as to cruise on the west coast of Norway. Or in the Salish Sea or poking about Tierra del Fuego as one of our members does :-)

TP

[StuM]

Interesting that the OP asked specifically about length/displacement ratio (more commonly referred to as a non-dimensional Displacement Length Ratio or DLR number) but everyone is talking about sail area/displacement ratio.

To bring it back on topic and to clarify:

DLR = (Displacement in long tons) / (LWL in feet) ^ 3

Commonly accepted DLR numbers are:
Ultralight: less than 90
Light: 90-180
Moderate: 180-270
Heavy: 270-360
Ultraheavy: 360+

Where you settle in that continuum is dependent on where and how you intend to use the boat and how you value performance v comfort.

Once you have determined your preferred DLR range, you then need to determine how much sail to carry.

[KP44]

Gtom,
Here's a prolific designer, Ted Brewer, who talks about comfort as much as speed. Unless you're racing, which has little to do with live-a-board sailing, comfort is part of your safety. If you're beat up and/or exhausted, you're not safe.

Ted Brewer Yacht Design

[elandra65]

There is a lot more to length/displacement Vs seaworthiness than just the numbers. Look at the "Spray" and compare to Dashews yachts that are considered ultralight but extremely seaworthy. I have owned both ends of the spectrum and i would consider they were all seaworthy. Dont get hung up on numbers. The "ideal" cruising yacht has evolved over the years and will continue to evolve. You may/will find though that a modern performance yacht will be faster, easier and more satisfying to own than a heavy tub. The ability to make fast passages adds to safety esp with reliable longer range forecasts and it is really nice to go past other boats!

A boat is only as seaworthy as the weakest part I have seen large new Halberg Rassys using the cheapest Lewmar standard portlights. Makes you wonder where else they cut costs!

Andrew