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Old 17-02-2016, 06:18   #1
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Importance of D/L ratio

How important is D/L ratio to heavy weather handling ability?

More specifically if you compare a smaller boat with a higher D/L ratio to a larger boat with a lower D/L ratio which one would handle better as weather increased?

I'll use some specifics.

If you have a 35 foot sloop with a 240 D/L ratio and a 45 foot sloop with a 193 D/L ratio which would handle better in rough / rougher weather?

Now I know there are a lot of variables here but I'm just trying to get a better understanding of handling. Assuming sail plans are proportionally equal would you have to reef the 45 footer before the 35? Would the larger boat handle better in seas but not wind or vice versa?

My last boat a Catalina 30 had 291 D/L ratio if that helps with giving a comparison to handling.

As usual thank you in advance
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Old 17-02-2016, 06:54   #2
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Re: Importance of D/L ratio

Hi, I think that I can speak to this one pretty well.

My previous 3 boats had D/L's of somewhere between 250 and 4-500. I do not know what they were exactly, but these were all full-keeled, heavy cruisers. One CT, one Cabo Rico, and the heavyweight was a 43' Roberts design built in steel.

The current boat is a carbon fiber sloop, with a D/L of 72.

I've sailed each of these boats in the neighborhood of 10,000 offshore miles. Some a bit more, some a bit less, but a good cross section.

I think that in many ways the ballast/displacement ratio and hull shape is more significant regarding the ability to carry sail.

My conclusion more generally is that the ultralight is a better offshore boat. This is for a couple of reasons.

First, when sailing off the wind, the speed of the lighter boat is a tremendous asset. Where we would get corkscrewed around and have control difficulties on the heavy boats in big seas, the ultralight will simply surf out ahead of a breaking sea, under control and with the autopilot handling things.

Second, upwind sailing in hard weather is necessarily a slow proposition for everyone, so we don't really find the light displacement to be a liability. We de-power like everyone else and try not to shake the hell out of everything and everyone.

We have not been in 'survival' conditions on this boat, or any of our other boats, for that matter.

I do think that in a worst case scenario, the nod has to go to the heavier boat, simply because it's easier to go passive with one of those types of boats after you've fought the good fight and have nothing left to give.

I don't think that we could heave to very well in F10+ conditons. We would find ourselves having to be more involved in the management of the boat. This would entail running off with our series drogue if sea room permitted. If not, we would be in a very depowered fore-reaching situation. We have used this technique in 30 knots just to take a break and found it works well. I think it would be effective in stronger conditions as well.

My thinking on going to the ultralight really was that after all these years we've never encountered survival conditions, and I was starting to lose patience with sailing slow, heavy boats just for the sake of some possible additional safety in a storm which we are not likely to encounter on traditional routes.

Besides, which boats are really racking up the miles in the southern ocean? Ultralights. They do need a different approach, though.

For my part, I think that lighter is the way to go, provided the construction is of a strength adequate for offshore service. This is hard to find without going to composites. That's really a big caveat, and it does tend to disqualify a lot of the lighter boats for me personally. No offence to any owners of these boats! I'm just talking about my choice here.

I hope this was at least of some use.

Good sailing, TJ
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Old 17-02-2016, 06:54   #3
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Re: Importance of D/L ratio

First, and most important, you have to remember that this is just one element, among many, of the boat's design. The answer really depends on a whole lot of other factors.

That said, a very broad generalization would be that a higher D/L ratio will have a more stable ride in rough water.

In your example, the difference between a D/L of 193 and 240 really isn't all that great, so I would want to know a whole lot more than just the LOA and D/L to make a choice. The difference in this case is, in my opinion, on the borderline of being enough to matter. People who get hung up on D/L differences of 15-20 are just completely missing the point of these kinds of ratios.

An example of a really meaningful difference in D/L would be something like the J-133 at 153 and the Bristol 43.3 at 319. These boats have a very similar LOA but for heavy weather I would choose the Bristol every single time.

Edited to add: TJ D's answer made me think of one other important issue--your sailing style and what you consider comfortable. In reality that is probably a more important variable than anything else.
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Old 17-02-2016, 06:59   #4
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Re: Importance of D/L ratio

There are many here better qualified than me to answer but maybe a search of boatdesign.net might be better still?

My view would be that concentrating on DLR as a predictor of "seaworthiness" isn't going to help much, eg. why not ballast ratio or righting moment?

Fully crewed modern racing boats carry on sailing long after most heavier cruisers will have chucked the drogue overboard - horses for courses really.

I'd guess that my 10 ton steel 11.5m will be around 400. Pretty useless in determining actual sailing characteristics except to say it won't win many races.
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Old 17-02-2016, 07:05   #5
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Re: Importance of D/L ratio

Also don't dismiss the D/L's relative - the motion comfort index. I have done some long distance sailing on a friend's 46 footer which weighs only a few thousand pounds more than my 36 footer. As a result it is significantly more tender in rough weather than my boat. Sure it'll get you where you're going faster but unless you're outrunning some monster storm I'd rather get there a bit later with all my lunches passing through their proper hole that to arrive quicker but in a sorry state.
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Old 17-02-2016, 07:10   #6
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Re: Importance of D/L ratio

Ted Brewer Presents A Primer on Yacht Design - The Numbers
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Old 17-02-2016, 07:26   #7
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Re: Importance of D/L ratio

Thank you all for the replies so far.

Let me put a little bit of a finer point on this. The most important thing as of now is comfort underway. Particularly for the wife. She enjoys sailing but more to relax and be "comfortable" while sailing not to get beat up. The size of boat is not nearly as important to her as the ride is. Tighter quarters are ok, she's easy in that manner (don't tell her I said that)

As far as numbers all I can find so far is weight, ballast ratio, and sail ratio.

The 35: is 14,000 lbs, 41% ballast ratio, sail area to disp is 17
the 45" is 24,000 lbs and 33% ballast ratio, sail are to disp 19
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Old 17-02-2016, 07:28   #8
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Re: Importance of D/L ratio

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Originally Posted by Island Time O25 View Post
Also don't dismiss the D/L's relative - the motion comfort index. I have done some long distance sailing on a friend's 46 footer which weighs only a few thousand pounds more than my 36 footer. As a result it is significantly more tender in rough weather than my boat. Sure it'll get you where you're going faster but unless you're outrunning some monster storm I'd rather get there a bit later with all my lunches passing through their proper hole that to arrive quicker but in a sorry state.

This is an interesting point. My wife struggles with seasickness. We were pleasantly surprised to learn that the quicker motion of our light boat is actually far easier on her than the long, heavy wallow of the traditional steel boat. I find the motion to be quite agreeable too. To each their own, of course, but having been on both sides of it a good deal, I actually prefer the quicker, lighter motion. We also don't ever sail DDW thanks to the speed gain of reaching, so that is a great boon on the ride too.

TJ
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Old 17-02-2016, 07:40   #9
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Re: Importance of D/L ratio

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Originally Posted by TJ D View Post
This is an interesting point. My wife struggles with seasickness. We were pleasantly surprised to learn that the quicker motion of our light boat is actually far easier on her than the long, heavy wallow of the traditional steel boat. I find the motion to be quite agreeable too. To each their own,

TJ
Of course you have to go muddle things up with the exact opposite cause of sickness compared to island time, that makes figuring this out harder not easier, Thanks
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Old 17-02-2016, 07:44   #10
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Re: Importance of D/L ratio

Quote:
Originally Posted by TJ D View Post
Besides, which boats are really racking up the miles in the southern ocean? Ultralights. They do need a different approach, though.

For my part, I think that lighter is the way to go, provided the construction is of a strength adequate for offshore service. This is hard to find without going to composites. That's really a big caveat, and it does tend to disqualify a lot of the lighter boats for me personally. No offence to any owners of these boats! I'm just talking about my choice here.

I hope this was at least of some use.

Good sailing, TJ
It's true that it's ultralights that dominate offshore racing including races in the southern ocean, but these boats are helmed by professional sailors 24/7 and that's nothing like the situation on the typical cruising sailboat. Even IF the experience level and sailing skill of the cruisers approaches that of professional racers (that doesn't include most of us), on a boat with a shorthanded crew, they will quickly become exhausted in extreme conditions if they must actively helm the boat. Also, while continuing to sail fast in extreme conditions, dynamic forces become very high so you are more likely to break things that you will then have to deal with in those same extreme conditions. I understand the desire for speed while sailing, it's more fun to go fast after all. But I also think it's important that a cruising sailboat that will often be sailed shorthanded have a D/L ratio and underbody configuration that allows it to heave to in order to lessen forces on the rig and to allow the crew to remain rested enough so they can respond appropriately if/when conditions require them to.

As far as the OP's choice goes, both of those D/L ratios aren't that far apart and if the underbodys were similar, I'd go with the longer boat. 193 isn't a horribly light D/L and by the time you load it down with all your cruising gear, it's actual D/L will be well over 200.
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Old 17-02-2016, 07:46   #11
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Re: Importance of D/L ratio

Of course, my pleasure!

Maybe take her out on a nasty day on one of your marina neighbor's race boats and see what she thinks of it, and then do the same on a heavyweight.

With my wife, the difference was immediately apparent which boat suited her particular inner ear situation the best. Perhaps you will also be so lucky.

TJ
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Old 17-02-2016, 07:46   #12
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Re: Importance of D/L ratio

And just to throw in another monkey wrench...

Almost everyone (at least everyone who owns a catamaran) will tell you that you never get seasick on them, because they don't roll the way a monohull does. Well, I beg to differ. My wife and I chartered a catamaran a while back and both found the motion to be much more discomforting to the stomach than most any of the monohulls that we've been on. Now, of course, that could be in part because we have much more experience on monohulls. But mainly, I think, it just goes to prove that seasickness is a very, VERY personal thing. What upsets one desperately may be extraordinarily comfortable to another.
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Old 17-02-2016, 07:55   #13
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Re: Importance of D/L ratio

Quote:
Originally Posted by TJ D View Post
This is an interesting point. My wife struggles with seasickness. We were pleasantly surprised to learn that the quicker motion of our light boat is actually far easier on her than the long, heavy wallow of the traditional steel boat. I find the motion to be quite agreeable too. To each their own, of course, but having been on both sides of it a good deal, I actually prefer the quicker, lighter motion. We also don't ever sail DDW thanks to the speed gain of reaching, so that is a great boon on the ride too.

TJ
I guess a lot of it is truly personal. Myself, I find it much more difficult to adjust to sideways motion than your regular up and down surfing. But on that friend's boat I can feel it's "corkiness" even when at anchor as it reacts to other boats' wakes. We sometime actually make comparisons by watching different boats at the mooring field react differently to a boat passing by. Some are bobbing up and downs and sideways, others serenely sit undisturbed, like a duck atop of a wave. IMO this is very indicative of these boats' comparative comfort.
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Old 17-02-2016, 08:00   #14
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Re: Importance of D/L ratio

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It's true that it's ultralights that dominate offshore racing including races in the southern ocean, but these boats are helmed by professional sailors 24/7 and that's nothing like the situation on the typical cruising sailboat. Even IF the experience level and sailing skill of the cruisers approaches that of professional racers (that doesn't include most of us), on a boat with a shorthanded crew, they will quickly become exhausted in extreme conditions if they must actively helm the boat. Also, while continuing to sail fast in extreme conditions, dynamic forces become very high so you are more likely to break things that you will then have to deal with in those same extreme conditions. I understand the desire for speed while sailing, it's more fun to go fast after all. But I also think it's important that a cruising sailboat that will often be sailed shorthanded have a D/L ratio and underbody configuration that allows it to heave to in order to lessen forces on the rig and to allow the crew to remain rested enough so they can respond appropriately if/when conditions require them to.

As far as the OP's choice goes, both of those D/L ratios aren't that far apart and if the underbodys were similar, I'd go with the longer boat. 193 isn't a horribly light D/L and by the time you load it down with all your cruising gear, it's actual D/L will be well over 200.


I agree with this thinking, mostly.


First, remember that many of the long distance/RTW races are singlehanded.

There is a middle ground, however. It has not been our experience that the boat needs full time attention. I believe that we could successfully go, let's call it, 'semi-passive'. Certainly, keeping going fast while someone is working their butts off hand steering is not our only option in storm conditions.

No, we probably won't be able to back the jib, sheet down the main and just go wait it out below, but I also do not expect to have to be terribly involved in a running off under drogue or fore-reaching situation.

Fast is fun, but there comes a time to throttle back and look after the boat, and that can certainly be done on the more aggressive designs, if they are set up right.

After 3 heavy boats, going to the light extreme was of some concern for me, but we've put a lot of miles on the light boat, some of it in highish latitudes and a couple of F8 blows, and we have found the management of the boat to be actually easier than the heavy boats.

I will admit that I was as incredulous at this discovery as anybody. I had been indoctrinated from childhood that the only true bluewater boats were heavy, with small rigs. It's been a revelation to find out otherwise.

If it starts blowing 60, I might miss the old steel beast! Time will tell. For now, I'm a changed man when it comes to my approach.

I'm not trying to sell anybody on anything, just my POV in relation to the OP.

Good sailing, TJ
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Old 17-02-2016, 08:03   #15
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Re: Importance of D/L ratio

I think you've gotten some good answers.

My experience is more or less consistent with the above. D/L ratio increases stability and comfort. But length and total mass do the same, and probably more so (that is, in a non-linear fashion).

Besides that, the ability to make speed is beneficial in heavy weather. Running off at 10 knots -- which will be very controlled and stable on many boats with waterline lengths of around 50 feet -- immensely reduces the energy of wave encounters, compared to running off at 5 or 6 knots in a smaller vessel.


So I guess I would summarize as follows -- for a given length of vessel, one with a higher D/L ratio will be more comfortable, safer, and more seaworthy, but a longer vessel with less D/L will be even more comfortable, safer, and more seaworthy, and to boot will be much faster.

Obviously "less" has to be quantified and if you go too far you may get a net loss of comfort and seaworthiness, but note the post above about a boat with D/L less than 100.

I went from a heavyish 37 foot boat with D/L of about 350 to a lightish 54' boat with D/L of less than 200, and the larger, lighter boat is incomparably better in heavy weather, although the smaller boat was renowned for its seaworthiness. You SAIL in the weather, rather than bobbing around surviving it.

Whatever the size of the boat, one crucial thing for heavy weather is STRENGTH. The forces go up exponentially with wind speed and wave height. Like bicycles and aeroplanes, boats can be either light, strong, or cheap -- pick any two. To make a large, light boat which is suitable for heavy offshore conditions, costs money, more than many are willing to pay. I don't want to bring up the whole inflammatory theme again, but there are some large boats which are ultra light and ultra cheap (Hansa 57 for example) which must be brilliant in good weather, but I would not want to be offshore in a storm in. Not because the shape or anything is necessarily wrong, but look at the chainplates, for example.
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