Importance of D/L ratio

[TJ D]

Originally Posted by estarzinger

As an aside . . . I believe (even) today, heavy-ish boats still rack up more southern ocean miles . . . They just don't get as much publicity as the ultralights.

We have four friends doing single handed solos right now and one is light and three are heavy. The Antarctic/South Georgia charter fleet is about 90% heavy-ish and 10% light.

Evens, you are likely correct on this. Still, my point is that the light boats, even singlehanded, are out there in some of the most rugged stuff and doing quite well. I actually doubt that the folks on the lighter boats are better sailors than the ones that prefer the heavy boats down in that region. They're all at the top of the sport if they're down there and succeeding for any length of time.

[GordMay]

Greetings and welcome aboard the CF, Cale.

[GordMay]

FWIW:

Carl Adler developed this online Sail Calculator for comparing sailboats and its database has grown over a number of years to almost 3000 boats.

Carl's Sail Calculator v3.55 ➥ Sail Calculator Pro v3.54 - 2800+ boats

Multihulls ➥ Multihull Dynamics, Inc. - Home

[rognvald]

The D/L ratio of a boat is a truly deceptive concept since displacement and length are not the only important numbers in how a boat sails. Factors such as sail area, weight aloft, underbody shape, type of keel/rudder, bow/stern shape, point of sail and particular sea conditions that truly dictate performance on a particular boat. Numbers only give you a general idea of how the boat might sail, however, the true litmus test is to sail a boat in all conditions. This is not always possible, ergo: the numbers. So, if you're interested in a particular boat, try to find a way to charter or sail with someone who owns that boat since sailing characteristics are so individual and oftentimes the numbers are misleading. What is one man's wine is another's vinegar. My personal experience is that I prefer boats with a lighter displacement and am not bothered by the quicker motion than one would find on heavier displacement boats since the tradeoffs of less sail area, faster tacking, greater SOG, and surfing abilities far outweigh that of the perceived comfort of a heavier vessel. Boats are like beautiful women:they're in the eye of the beholder. Good luck and safe sailing.

[Polux]

Originally Posted by barnakiel

....
BTW At the further extremes of the DL you may notice some boats will opt for different heavy weather tactics - the low DL boats may run sooner and do so in relative safety. High DL will likely go upwind longer and then have tougher time when running before the storm, etc. But this is not, I think, what you are after.

Cheers,
b.

I don't know if you are right about that. I would say that it will depend of the boat and there is many types of light boats but it seems to me that you start running when you have no ability to keep the boat sailing upwind, very close to the wind or if the waves are just too big and breaking. You go running when you have not the power to make speed to control the boat.

What determines the need for running is power or the lack of it to keep going upwind and that has not to do with the boat being lighter but with stiffness that equals power.

Performance modern boats are more powerful than old heavier boats and can keep going upwind while it is too much for a less powerful boat, being them light or heavy.

The reason I felt the need to another boat regarding my previous Bavaria 36 was just that: On very demanding conditions, against the wind and waves the boat had just not enough power to go on and I had to open up course to get less heel and more power. My actual boat is muck more powerful and in situations were the Bavaria would be almost stopped, it can
make 5.5K.

That has also to do with performance boats having much finer entries, slamming less and losing less to drag.

Due to that superior power (on the light boats that have it) they can be sailed much longer than less powerful sailboats that have to run sooner, unable to be sailed in any other course. Not all old heavy boats were low power boats dough.

On this video on min 1.17 and over you have an old designed cruiser racer, heavier and less powerful than a racer that appears on the same video at the minute 5.50. Compare the movements ad the sail ability upwind and you will see that even if the conditions at min 5.50 seems to have gone worse, the lighter and more powerful boat does not only pitch and heel a lot less as it does more headway.

https://www.youtube.com/watch?v=3urvpv-y8sM

These are pretty nasty conditions to keep going upwind, not only in what regards the head wind but the nasty waves. Even the older boat that I mentioned is a powerful boat, a cruiser-racer. These condition would have stopped on his tracks considerably less powerful boats, being them heavy or light.

[rognvald]

Great example and video, Polux!

[Polux]

Originally Posted by GordMay

FWIW:

Carl Adler developed this online Sail Calculator for comparing sailboats and its database has grown over a number of years to almost 3000 boats.

Carl's Sail Calculator v3.55 ➥ Sail Calculator Pro v3.54 - 2800+ boats

Multihulls ➥ Multihull Dynamics, Inc. - Home

GordMay, it is difficult to disagree with you with that avatar an all your nice manners.

I had one of those calculators many years ago and I through it out long ago. It really does not serve any purpose except maybe regarding comparing most common types of cruisers that existed when that calculator was created, or better, when the different ratings that served to access a boat and that are on its basis, were created.

We are far away from the 70's early 80's and most of the things the calculator indicates, regarding comfort, stability or even the definition of the type of boat make today no sense and does not provide valid information regarding the vast amount of different boats on the market that are all very different from the typical boat from the 70's.

[thomm225]

Originally Posted by Polux

On this video on min 1.17 and over you have an old designed cruiser racer, heavier and less powerful than a racer that appears on the same video at the minute 5.50. Compare the movements ad the sail ability upwind and you will see that even if the conditions at min 5.50 seems to have gone worse, the lighter and more powerful boat does not only pitch and heel a lot less as it does more headway.

These are pretty nasty conditions to keep going upwind, not only in what regards the head wind but the nasty waves. Even the older boat that I mentioned is a powerful boat, a cruiser-racer. These condition would have stopped on his tracks considerably less powerful boats, being them heavy or light.

Actually it appears the sea state has eased just a bit at 5:50 (much less white water) and that boat seems to have fallen off the wind a tad for more power and easier sailing. Quite a different camera angle etc as well.

I'll check it again later on a better monitor but that's the way it appears now.

But you are going to go for the newer design regardless. I get it.

[donradcliffe]

Injecting a little science into sea sickness, take a look at Figure 6 in

http://www.cehipar.es/__files/users/...ciones/185.pdf

Its a bit technical, but has a good discussion on the Motion Sickness Incidence index (MSI is the percentage of subjects who vomit), which is dependent on the frequency of the motion, peaking at 1 radian/sec or a period of about 6 seconds.

The MSI is also a strong function of the acceleration experienced. The figurecovers accelerations of 1 to 5 m/sec2. At 5 m/sec2 and 1 rad/sec, 90% of the subjects lose their lunch.

In real life, lets look at 15 knots of wind in deep water and fully developed waves, say 2 meters in height with a period of 8 seconds. The wave velocity would be about 1.5 times the period, or 12 m/sec, or 23 knots. Say we are close-hauled at 6- 7 knots, so the frequency of wave encounters goes up to one every 6 seconds--right about the worst frequency for sea sickness. Assuming a sinusoidal wave, the acceleration of the wave face will be 1 m/sec, with a MSI of 30%--sounds about right as the base case.

In a crab crusher, you will be going slower and the heavy boat will not match the acceleration of the wave face, so the MSI should drop a bit.

If you decided to sleep in that sexy forepeak berth that sold you on the boat at the boat show, your acceleration and MSI are going to more than double, because you have to add the pitching of the boat and your distance from its pivot point.

Now turn around and go downwind. The frequency of wave encounters drops to once every 11 seconds for the crab crusher at 6 knots, and once every 13 seconds for a lighter boat at 9 knots. Not only are you outside the peak MSI frequency at 2 rad/sec, the acceleration has dropped by a factor of 4 (square of the frequency change) to 0.25 m/sec2. The MSI is under 10% and the admiral is much happier.

Now this is a very simplified version of what really happens. In reality the waves aren't sinusoidal, and you have to deal with the accelerations from rolling, as well as pitching. But you can start to understand the relative effects.

The good news for all is part B of figure 6, which shows the MSI versus time. It peaks between 1 and 8 hours, and drops to near zero after 48 hours. For most people, your body will adapt to the motion if you can survive the first 2 days.

[Polux]

Originally Posted by thomm225

Actually it appears the sea state has eased just a bit at 5:50 and that boat seems to have fallen off the wind a tad for more power and easier sailing. Quite a different camera angle etc as well.

I'll check it again later on a better monitor but that's the way it appears now.

But you are going to go for the newer design regardless. I get it.

That has nothing to do with personal preferences. In fact both boats are rather old, the difference is that one is obviously more powerful than the other, one is a cruiser-racer the other is a lighter race boat.

[thomm225]

Originally Posted by Polux

That has nothing to do with personal preferences. In fact both boats are rather old, the difference is that one is obviously more powerful than the other, one is a cruiser-racer the other is a lighter race boat.

I still believe the video is inconclusive.

Plus by that camera angle after 5:50 the sea state seems to have fallen off a bit from say 3:25. Plus that boat after 5:50 appears larger than many of the others and appears to have come off the wind a bit more than some for more power.

[Don C L]

I may have missed it, but in Adlard Coles' book "Heavy Weather Sailing" Olin Stephens discusses the importance of D/L in a couple of graphs. His recommendation is avoid the extremes so between 300 and 400, approximately, being optimal. As far as a boat's motion that feels favorable and "sea-kindly" I'd recommend getting a ride in an old school, narrow beam, deeply ballasted boat that is NOT carrying a heavy load fore or aft (that might encourage hobby-horsing) for a test of comfort. The Pearson Rhodes 41/Rhodes Reliant/Bounty II is an example of a design I'd check. I have a narrow beam Sparkman and Stephens design that has a very nice feel to it (for me anyway.) I crewed on a 65 foot lightweight flat bottom Frers many moons ago and it is true, downwind there's nothing better, upwind, nothing worse. That might be a bit harsh, but my bunk was up in the bow. I have a friend with a wider beam and higher freeboard (but long keel like mine) and it gets pushed around like a cork compared to my boat.

[Cadence]

Originally Posted by ontherocks83

Sorry I'm not familiar with GM?

GM is the difference between the center of gravity and the center of buoyancy.
Basically if a boat is stiff or not. It has more involved than just displacement. Figuring both is not a simple task. In short the lower the weight is the lower the center of gravity is the fuller the hull the higher the center of buoyancy is.
That is an over simplification. For both your calculus would have to be better than mine.

[barnakiel]

Originally Posted by Polux

I don't know if you are right about that. I would say that it will depend of the boat and there is many types of light boats but it seems to me that you start running when you have no ability to keep the boat sailing upwind, very close to the wind or if the waves are just too big and breaking. You go running when you have not the power to make speed to control the boat.

(...)

Due to that superior power (on the light boats that have it) they can be sailed much longer than less powerful sailboats that have to run sooner, unable to be sailed in any other course. Not all old heavy boats were low power boats dough.

(...)

Yep. Your take is more in depth. I was just touching the surface.

I was sorta pointing my finger at the assumption that lower DL boats may have better/easier downwind ride in heavy going, as compared to boats with higher DL numbers.

I think you rightly pointed yours at the fact that many LD boats can in fact sail upwind long after some HD boats were forced to put their tail under their belly and show their heavy bum to the waves.

As you can read in the upper part of that post, I generally disregard DS since it says nothing about all other factors that add and detract from how a boat sails. To attempt judgements, we would need two otherwise identical boats with different DL sailing in the same conditions. In my book, this never (hardly ever?) happens. Hence the discussion of DL as a factor in how it contributes to any specific boat's comparative properties is an exercise in vain.

I think we may be on the same wagon when it comes to our likes and dislikes: I think I have seen both heavy and light boats sailed very well thru a very broad range of conditions. I never knew their DL though ...

b.

[Island Time O25]

Originally Posted by donradcliffe

The good news for all is part B of figure 6, which shows the MSI versus time. It peaks between 1 and 8 hours, and drops to near zero after 48 hours. For most people, your body will adapt to the motion if you can survive the first 2 days.

Thanks for pointing out the scientific underpinning of the salty dogs' tales. On all of my longish open ocean trips 99% of the time everyone more or less adopted after 2 days, most after just 1 day. The problem with explaining that it'll pass to those who are initially severely affected is that the very first day, being usually the toughest sticks in their minds and they never want to set a foot again on a boat going out of the harbor.