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Old 17-02-2016, 08:21   #16
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Re: Importance of D/L ratio

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Originally Posted by ontherocks83 View Post
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
But the size of the boats waterline is a very important item regarding comfort and one that is not so dependent of other factors (to be important) as D/L.

I am not talking about a difference of some cm but a considerable difference is very important (like between 35 and 45ft). You have just to look at some movies and see how small boats bob around in waves will bigger boats have a much more stable movement.

Regarding that movement (pitch) D/L is only important regarding how fast or slower that movement is and many times heavier slower boats have a more pronounced movement.

As a rule, bigger boats are more comfortable in a seaway but the type of movement is different between really lighter and heavier boats. The best is to try both types of boats and see if that is really as important as you think it is. Some have even a preference, in what regards movements, to the less ample but faster movements of a lighter boat.

Regarding the ballast ratios you post they are pretty much meaningless regarding rightening moment since they have to be looked in conjunction with draft and the type of keel. Also the type of hull is important, regarding having more or less form stability.

In what regards your original quetion is dificult to reply with more pratical knowledge than

TJ D and I agree with what he says:

Quote:
Originally Posted by TJ D View Post
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.
I would just refine the last part regarding B/D, saying that it is not only B/D but that in conjunction with the draft and the type of keel and I think that what he means: the boat power given by the RM that comes from ballast that joins the one that comes from form stability.

The amount of it is what is important for the amount of sail a sailboat can carry.

Regarding comfort and sail area, a faster boat can be more uncomfortable at speed but you can sail slower on a fast boat (but not faster on a slow boat).
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Old 17-02-2016, 08:24   #17
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Re: Importance of D/L ratio

First . . . . the big picture . . . . . being at sea in big weather in a small boat (and ALL of the ones we are talking about are small) . . . . is uncomfortable. Does not matter the design, or measurements . . .they are all uncomfortable (compared to a nice chair in a nice living room ashore). There are degrees of discomfort and factors that effect those degrees . . . .but you first fundamentally have to realize - comfort - not.

Second, seasickness is caused by different factors and effects different people very differently. For me it is not at all boat size related - it is stress and smell (diesel smell - bad!) related. For my wife it is related to two very particular conditions - heat and a very slow long roll (like downwind in the tropics on a heavy boat without enough sail area up); and really severe slamming - she's generally fine going upwind, but not in conditions where we are slamming violently (which is quite rare, I can only remember 3 times). In the first of those two conditions a heavy boat is (generally) worse for her and in the second the heavy boat is (generally) better.

Third, boat design is a decently complex subject, and D/L is too simple a ratio to mean much. Just for instance two boats can have the same identical D/L but one with most of the weight in a keel bulb will move differently than the other with most of the weight in a heavy interior.

Fourth, moment of inertial is a key engineering/design factor and more relevant than D/L - and moment of inertia is driven by the fourth power of length - so size is a huge driver. So long as you accomplish equal strength and reliability, and roughly similar shape and weight distribution, bigger length will be the winner in less discomfort (and also in heavy weather stability). But bigger does also come with other trade-offs - notably higher costs to the 3rd or 4th power and higher loads.
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Old 17-02-2016, 08:25   #18
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I don't want to get too far into the weeds on design, but the comment on light, strong, cheap is dead on.

The only way that we got on board with purchasing a boat with such a light D/L is the fact that she's entirely carbon composite and the hull and deck panels were FEA and destructively tested prior to construction, and we got to review the data prior to making the call to proceed with the purchase. This was the gist of my original comment about the boat probably needing to be of some kind of composite construction to be very lightweight but still strong enough to be sailed by mere mortals in the real world.

As I said before, it's a pretty big caveat to have to overcome if one chooses a light boat for long distance cruising. I would also likely pass on many (all?) of the mass-produced lightweight boats for offshore work, certainly offshore work outside tropical routes.

Unless you get quite lucky and find such a boat (and there are some out there), a more conservative D/L would be more appropriate.

TJ
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Old 17-02-2016, 08:27   #19
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Re: Importance of D/L ratio

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And you start speaking of D/L and end up talking about SA/D?
If you read through the thread you would see that I was listing all of the information I could find at this time just to put as much info out as possible. Also highlighting that propotionally the sail areas are very similar so that it would avoid the topic of over powered small boat or underpowered big boat and so on.

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Originally Posted by ontherocks83 View Post

As far as numbers all I can find so far is weight, ballast ratio, and sail ratio.
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Old 17-02-2016, 08:35   #20
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Re: Importance of D/L ratio

Slightly OT example:

Marina neighbour's steel Bruce Roberts (39-ish I think), similar size to my boat but maybe a little lighter, both multi-chine designs.
With a flat bottom plate my bilge is probably somewhat slacker, rocker about the same, ballast ratio probably lower.

He was berthed 2 pontoons downwind from me in a big gale a couple of years ago, both boats were head to wind.

My mast and everyone else's except his (taller) were swaying plenty degrees, his hardly moved. Looked like the hull was nailed to the bottom of the pond or ashore on sticks.
No idea why such a difference.

Point is I think a person would need 5 years' serious study of NA to arrive at a confident judgement based on the numbers.
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Old 17-02-2016, 08:42   #21
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Re: Importance of D/L ratio

IMHO, all other things equal, the higher DL boat could have a better (easier) ride.

Then again think about it - real life examples of boats that are identical except for their DL???

None. Except we twist the concept and take two identical boats and either modify them (e.g. add ballast to one) or, again, twist the concept more and simply overload one (now the overloaded one can claim higher DL, no?)

So, frankly, forget about it. In real life, as soon as you swap DL you likely swapped boats and there is no comparison between apples and oranges. It is very, very difficult to run a strong multivariate analysis of things like different boats' behaviour in rough going.

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.
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Old 17-02-2016, 09:01   #22
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Re: Importance of D/L ratio

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Originally Posted by TJ D View Post
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
This is insightful, TJ. Thanks
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Old 17-02-2016, 09:30   #23
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Re: Importance of D/L ratio

Hello! I am newly registered at this great forum, although I have been following it as a reader-only for a longer period. I find most topics interesting. As a retired merchant navy captain I like to read everything that has to do with boats, sea and weather. One remark though; one of the first subjects at nautical college, long time ago, was to use clear communication at sea: "The Standard Marine Vocabulary". The amount of abbreviations being used in this forum is stunning. Valid abbreviations or self invented abbreviations, it doesn't matter, to me it looks like a similar phenomenon as the newborne smartphone language. Most abbreviations I manage to understand. However be warned that many incidents have happened at sea due to miscommunication! Iow, it is advisable not to practice these abbreviations at sea!
About this topic; it would be good to know the correct stability conditions of your boat. Both static- and dynamic stability conditions will tell you exactly how seaworthy your boat is, apart from construction strength but that is a different subject. Keep up the good work!
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Old 17-02-2016, 09:37   #24
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Re: Importance of D/L ratio

Sure thing. It's an interesting topic.

The reality probably is that any well-found boat, whatever the construction, D/L (insert your favorite metric here) will probably be just fine in the conditions that any of us are likely to encounter as long as they're managed well when it turns awful out.

If it gets really, really, awful, I don't really care what you're sailing on anyway. A bit of bad luck, being in the wrong place when that particular wave decides to go vertical and break, and all of these discussions don't mean a thing. There is a good measure luck involved in the outcome when it gets most fierce out there.


About 53 weeks ago, I had the pleasure of having a 40 or 50' freak wave smash it's way through my pilothouse window on a 2000 ton boat which is designed for service in that kind of weather. Any illusions I might have held about one sailboat design over another were firmly shattered after that episode. Actually, it was shattered well before that...

Probably the best advice is to sail what you have (within reason), make sure she's structurally and mechanically up to snuff. Mind the weather, mind the season, and all will be well as long as one keeps their head on straight when things take a turn south.
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Old 17-02-2016, 09:38   #25
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Re: Importance of D/L ratio

[QUOTE=Cale;2048479]
Quote:
Hello! I am newly registered at this great forum, although I have been following it as a reader-only for a longer period. I find most topics interesting. As a retired merchant navy captain I like to read everything that has to do with boats, sea and weather. One remark though; one of the first subjects at nautical college, long time ago, was to use clear communication at sea: "The Standard Marine Vocabulary". The amount of abbreviations being used in this forum is stunning. Valid abbreviations or self invented abbreviations, it doesn't matter, to me it looks like a similar phenomenon as the newborne smartphone language. Most abbreviations I manage to understand. However be warned that many incidents have happened at sea due to miscommunication! Iow, it is advisable not to practice these abbreviations at sea!
Welcome to the internet!


Quote:
About this topic; it would be good to know the correct stability conditions of your boat. Both static- and dynamic stability conditions will tell you exactly how seaworthy your boat is, apart from construction strength but that is a different subject. Keep up the good work!
I would love this info but it is not posted on their web site. If you know where else to find it I would love to search
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Old 17-02-2016, 09:49   #26
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Re: Importance of D/L ratio

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Originally Posted by Cale View Post
Hello! I am newly registered at this great forum, although I have been following it as a reader-only for a longer period. I find most topics interesting. As a retired merchant navy captain I like to read everything that has to do with boats, sea and weather. One remark though; one of the first subjects at nautical college, long time ago, was to use clear communication at sea: "The Standard Marine Vocabulary". The amount of abbreviations being used in this forum is stunning. Valid abbreviations or self invented abbreviations, it doesn't matter, to me it looks like a similar phenomenon as the newborne smartphone language. Most abbreviations I manage to understand. However be warned that many incidents have happened at sea due to miscommunication! Iow, it is advisable not to practice these abbreviations at sea!
About this topic; it would be good to know the correct stability conditions of your boat. Both static- and dynamic stability conditions will tell you exactly how seaworthy your boat is, apart from construction strength but that is a different subject. Keep up the good work!

Hi Cale,

I think that proper terminology is being used on this particular thread. I can help with a few if you're not following.

D/L-Displacement to length ratio. This is not a simple calculation, and I can't tell you how it's arrived at off the top of my head.

SA/D Sail area/Displacement

B/D- Ballast/displacement.

FEA-Finite element analysis

DDW- Dead downwind

RM-righting moment (you know this one)

I think that's all we covered in this thread. As far as the rest, Google is your friend!

TJ
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Old 17-02-2016, 09:59   #27
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Re: Importance of D/L ratio

Stability testing is part of the MCA coding of vessels in the UK - got the numbers for mine somewhere.

Wolfson Unit is a UK research establishment, some info here:

http://www.wumtia.soton.ac.uk/sites/...SCYC1991BD.pdf
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Old 17-02-2016, 10:03   #28
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Re: Importance of D/L ratio

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Besides, which boats are really racking up the miles in the southern ocean? Ultralights. They do need a different approach, though.
J
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.
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Old 17-02-2016, 10:06   #29
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Re: Importance of D/L ratio

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Originally Posted by ontherocks83 View Post
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
The GM may be what you are looking for?
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Old 17-02-2016, 10:07   #30
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Re: Importance of D/L ratio

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The GM may be what you are looking for?
Sorry I'm not familiar with GM?
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