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Old 09-01-2009, 10:35   #76
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Originally Posted by MidLandOne View Post
I know Hiracer will vehemently disagree with me again on all that as he has often claimed in other threads that displacement can be made up for by piling on more sail area. But as he has told me in another thread that he does not think Froude numbers are important in sail boat design (which is the same thing as saying that wavemaking resistance is not important ) I personally would not take much notice.
I never said that. This is the straw man argument, putting words in my mouth.

I said that SA/D is more important that Froude numbers. Big difference.
You can have fantastic Froude numbers but without suffice SA/D you will be slow, light or heavy displacement.

My boat had a D/L ratio of about 300. I regularly out sail lighter boats with better Froude numbers because they have inferior SA/D numbers--particularly after we have both been loaded with cruising stores. There is no better proof than that.

Of course, when I go up against unloaded lightweight boats with bigger SA/D numbers, I get my ass handed to me.

* * *

To put a finer point on it, between two boats of the same size, displacement, similar underwater configuration, and the same SA/D, the lighter boat will be incrementally faster because of less wavemaking and less surface area resistance. But the difference is incremental and the heavy boat by virtue of having more stability can handle more SA/D.

On the water displacment is the enemy of speed, but with proper design displacement is not the huge liability that some like to hype. Further, in the context of cruising, displacement offers some important attributes: better motion, more space for storage, stronger build, more stability, and better able to carry cruising stores without negatively impacting SA/D as much.

In fact, in the smaller boat context (sub-37 feet, or so) this latter attribute takes on such a prime consideration that it is possible that the heavy displacement boat actually becomes faster in all around sailing conditions after burdening it with cruising loads, simply because the SA/D of the lighter boat has been so seriously compromised. Likewise the issue of safety via the verticle increase of CG of the light boat after loading with cruising stores.

The point being that the needs of the cruisers are vastly different than that of the go-fast crowd. Cruisers need to carry stuff, lots of stuff. That reality fact how to go about getting safety and speed.

Nor am I saying going heavy is the only way to cruise, particularly in the larger boat sizes. I am saying that those who say all heavy cruising boats are slow are wrong. The more correct statement is that some heavy cruising boats are slow, primarily from (1) low SA/D and (2) lots more surface area underwater (full keel in light airs). Get a heavy cruiser under 40' with lots of SA/D and and fin keel and you have yourself a boat that in all likelihood that can maintain a turn of speed even after being heavily loaded and has a nice motion to boot.
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Old 09-01-2009, 10:44   #77
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Froude numbers:
Holy cow! How have I gotten by all these 41 years in the business?
I don't use them. Never have. Never heard of them being used by any sailboat designer I have worked with. We know what they are but I don't apply them to the design process. I rely on SA/D and SA/WS.
I think Hiracer is right, in the end SA/D is the important number.
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Old 09-01-2009, 11:15   #78
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Bob,

So, does this mean I owe you a drink next weekend?
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Old 09-01-2009, 11:30   #79
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Originally Posted by Hiracer View Post
....
Nor am I saying going heavy is the only way to cruise, particularly in the larger boat sizes. I am saying that those who say all heavy cruising boats are slow are wrong. The more correct statement is that some heavy cruising boats are slow, primarily from (1) low SA/D and (2) lots more surface area underwater (full keel in light airs). Get a heavy cruiser under 40' with lots of SA/D and and fin keel and you have yourself a boat that in all likelihood that can maintain a turn of speed even after being heavily loaded and has a nice motion to boot.
I agree with what you say above, even though you know I'm on the side of lighter cruisers. You're on a relatively small, by today's cruising standards, steel boat. It happens to have much better B/D and SA/D than you would expect from this type boat. Many of the boats that are hyped as great heavy cruisers just do not have same type ratios - so yes, they are slow. As you've stated before, it is much easier to get a good SA/D number on a lighter boat. And those sails will be smaller and easier to handle. You reframed the question on heavy boats by going to the SA/D ratio. So what is a light-weight cruising boat? Is an Alberg 35 at 12,000 lbs a classic cut-away-keel medium weight cruising boat, while a J/37 at 13,000 lbs a light weight cruiser?

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Old 09-01-2009, 11:43   #80
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Paul:
FWIW: When referring to cruising boats:
I use a D/L of 250 as being the "middle" of the displ range. A V 40 is about 260.
I use anything over 325 as heavy. Most fullish keel boats have D/L's over 300.
I use anything under 160 as "light" and anything under 100 as ULDB.
But there is noithing sacred in this breakdown. It just works for me.

Your Alberg 35 with it's short DWL ( I guess 26.25') has a D/L of 296, kind of heavy for it's DWL.
Your J/37, with 32' of DWL, has a D/L of 177, kind of light.
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Old 09-01-2009, 11:44   #81
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Hiracer:
No question about that.
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Old 09-01-2009, 12:22   #82
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Originally Posted by Paul L View Post
So what is a light-weight cruising boat? Is an Alberg 35 at 12,000 lbs a classic cut-away-keel medium weight cruising boat, while a J/37 at 13,000 lbs a light weight cruiser?
D/L is a dimensionless number that most use to compare displacement. I've noticed that it's not the best representation because it's overly sensitive to LWL. Your comparison of the Alberg 35 to your boat is a good example of that. Your boat has a light D/L number while the Alberg is going to be much higher because of its much shorter LWL, even though the two boats are not radically different in size or displacement.

But I don't have anything better to offer than D/L ratios, so that's what I use.

What is a lightweight D/L ratio is a matter of opinion. Further, it's easier to get low D/L ratios with bigger boats so in the end I don't think D/L is a truly dimensionless number.

Bottom line, I can't give a great answer to your question because I don't think there is a perfectly tight definition of what is a light or heavy boat. It's a judgment call and there is room for disagreement.

But in the end, all cruisers face pretty much the same issues. The most important question is not whether a boat is light or heavy. It's how well does she sail, how does she handle loads, how does she feel, how safe is she, is she well built? There are lots of ways to make a good cruiser.

My beef is with those who say that all heavy cruisers are unsuitably slow. This is demonstrably false. Heavy cruisers can offer a decent turn of speed if designed properly, albeit I admit that many are de facto slow--too slow for my taste.

And, of course, my experience of starting out on a medium displacement 33' and then moving to a heavy displacement (by anybody's definition) 36' has caused me to appreciate the huge difference in motion of the heavy boat. I recognize that you and others may react to the motion differently, but to me it's a wonderful thing.

As a consquence, when discussing sailboat 'performance,' I have concluded that motion comfort is the most expensive performance attribute to be found, simply because heavy boats cost more--at least sold new they do. But motion comfort has become a very important criteria to me, all of which was completely unseen and unexpected until I actually started using a heavy boat. As I say, I went to steel for the strength and got completely blindsided by the comfortable motion. But now I'll never go back.

To each his own.

This is a bit ironic, because when I started looking at cruisers, my first instinct was multihulls. I gave up on that because my pocketbook could not afford one. The dollar-to-payload ratio was too high. In fact, I've gone the exact opposite direction because with my limited pocketbook I've figured out that the best dollar-to-payload ratio is a smallish heavy mono. Steel is disfavored in this country, so I was able to get a five year old boat fairly well outfitted for pretty cheap. She's a wondeful boat that was listed and left unsold for two whole years, a crime IMO. (Of course, the price dropped by 60% over that period, which is how I came into the picture.)

Should I ever win the lottery, I might go lightweight: a big lightweight trimaran. I would have to try on the motion first.
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Old 09-01-2009, 12:38   #83
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Originally Posted by bob perry View Post
Hiracer:
No question about that.
Can't wait to tell my wife that I owe Robert Perry a drink.
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Old 09-01-2009, 12:45   #84
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If you want to read some interesting background on this argument go back to the L. Francis Herreshoff book COMMON SENSE IN YACHT DESIGN. In that book, if memory serves, there is an fireside chat between Mr. Heavy Displacement and Mr. Light Displacement. That was written a few years ago and still the debate rages.

My best advice is to avoid generalities when discussing boat types and performance.
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Old 09-01-2009, 12:58   #85
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Originally Posted by bob perry View Post
If you want to read some interesting background on this argument go back to the L. Francis Herreshoff book COMMON SENSE IN YACHT DESIGN. In that book, if memory serves, there is an fireside chat between Mr. Heavy Displacement and Mr. Light Displacement. That was written a few years ago and still the debate rages.

My best advice is to avoid generalities when discussing boat types and performance.
Very good piece of advice Bob....
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Old 09-01-2009, 13:03   #86
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Originally Posted by bob perry View Post
If you want to read some interesting background on this argument go back to the L. Francis Herreshoff book COMMON SENSE IN YACHT DESIGN. In that book, if memory serves, there is an fireside chat between Mr. Heavy Displacement and Mr. Light Displacement. That was written a few years ago and still the debate rages.

My best advice is to avoid generalities when discussing boat types and performance.
I will pick up a copy at a used bookstore when the opportunity presents itself.

It was Dave Gerr's noodling book that first turned my head in the direction of heavy displacement, even before my first boat.

Message boards are boring without raging debates.

I noticed that you kind of side-stepped the debate in your recent book taking the agnostic approach, aka, the-customer-is-usually-right position.
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Old 09-01-2009, 13:39   #87
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With 7 years experience, I would have thought you'd seen an awful lot of boats out motoring -- often in conditions that could be sailed. Even in the Caribbean where the XMAS winds blow consistently, you still see a lot of motoring. If you set a rule like you have, then you will reduce your motoring a lot. Most boats don't and most cruisers do a lot of motoring. It is much easier to get a light, not overloaded boat moving in light airs than a heavy overloaded boat. For me, that is more important point than how comfortable I will be when it gets rough out. I have found that when it is rough, it is not comfortable - you are just measuring levels of discomfort.

Paul L
I know that this is digressing a bit but I just went back through this thread and this post struck me as an important one to address.

The Caribbean tends to draw a lot of new cruisers and charter boats. Most passages in the Caribbean are short in nature and I think that some tend to go like hell to the next port before sunset, no matter what.

You're right, normally the wind blows pretty good there, especially in the windward isles. I saw many boats pounding into wind and seas, with their engine running and just the main up, trying to drive the boat faster to make it to port.

In the SoPac and Indian Ocean, where passages are 3-10 days in duration, I seldom saw an engine running on a sailboat, unless they were entering an anchorage. If it was a matter of making landfall during daylight (which is wise), one would simply push the boat a little harder (under sail) or stand-off until daylight. I think that motoring had little to do with boat speed or hull displacement.

I think that most full time cruisers have little interest in "speed" when choosing a boat for cruising. I think that the main focus is on comfort and safety. It was not very often that I saw a spinnaker used back when I was cruising. I used one a lot. Whenever the wind came just behind the beam and it was 15kts or below, I drug that thing out. It wasn't as much about going fast as it was about having fun. Although....I must admit, I blew out a spinnaker or 2 while heading up the Great Barrier Reef with a bunch of cruising boats that enjoyed pushing it. There again, we were day-sailing and anchoring every night. Pushing it was a matter of getting in before nightfall. I didn't see any of us motoring. I think that is something that you see in the Caribbean more than anywhere else.

Sorry about the digression here.
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Old 09-01-2009, 16:09   #88
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Originally Posted by bob perry View Post
Froude numbers:
Holy cow! How have I gotten by all these 41 years in the business?
I don't use them. Never have. Never heard of them being used by any sailboat designer I have worked with. We know what they are but I don't apply them to the design process. I rely on SA/D and SA/WS.
I think Hiracer is right, in the end SA/D is the important number.
Bob, thanks for your reply.

I made no mention of Froude numbers being used in the day to day design of a boat nor any claim that they were so used. I know that routine design is pretty much emperical. The references I made to them were entirely during the matter of endeavouring to explain the "behind the scenes things" as to why some boats are faster than another when it comes to wavemaking resistance, the place of wavemaking resistance in the overall resistance picture and effect of displacement on those. My intention was that my post did not come across as just based on opinion, but included a lead for anyone who had a serious interest into further research to test the veracity of what I was saying. Froude is the key to looking further and if they did so then I doubt whether anyone will find that the thrust of what I said is incorrect.

Four matters for me to ask of you -

Are you saying that if one has two hulls of the same lines but one of greater displacement than another, then they will both have the same displacement speed, and that wavemaking resistance will not increase faster for the heavy boat than it will for the light boat.

Are you saying that the wavemaking resistance of both will be the same?

Are you saying that wavemaking resistance is not a significant part of total resistance at other than very low speeds.

For SA/D - which is the better approach when designing a fast performance cruising sail boat 1) Less concern for displacement as can increase SA/D by adding SA, or 2) Designing a light strong boat (say using exotics, etc) then determining SA to get a desired SA/D? ( I realise there are compromises on both sides here).


In my thread I did not say that SA/D was not important but only made the point that for two hulls the same but one heavier than the other, the lighter one will be the faster one all else being equal except for the ability to pile sail onto each - that is if one is allowed in design to pile sail onto the heavy one during design one then can do the same to the lighter one and the lighter one will be the faster all round boat (some conditions excepted). I would not have thought that claim is able to be disputed or why else would we be seeking to design light boats when looking for speed. Nor would I consider it able to be disputed that there is a practical limit as to how much sail one can put on any boat, that for both physical and boat management reasons, especially for boat management if looking at fast cruising sail boats.

I am intrigued by the idea that one can make a heavy cruising boat as fast as a light one just by putting more sail on it and the inference by some that this is the way to go - that was the claim I was addressing. Given the benefits of comfort many claim for displacement and the need for speed by some, I have to wonder why we are not seeing fleets of 25 tonne comfy 45 footers with clouds of sail enabling them to sail at hull speed in 8 knots of wind cruising the world. I personally think the answer is obvious from both design and boat management points of view.

Out here I am not very familiar with your designs so I can only make the following observation by judging some photos of a couple of nice boats on your website by eye as to the design intention behind them. Icon (especially) and perhaps the Far Harbour 39 look to be intended as performance cruisers and if so I trust that you are not telling me that an equally effective design strategy for them would have been to design heavy hulls with low modulus materials but just cram on more sail to make up for it (what about steel for the 39 footer - that would make a nice strong boat, some nice 6mm plate for the hull to make it real strong cos we can just make up for it with more sail if we want a fast boat ?)?

Again, thanks for your response which I appreciate Bob. While I can understand the overt glee of some others as to your support for them I hope that on further consideration you find that you may have jumped in a little too quickly without giving thought as to what was really in in my thread.


By the way, my own boat is a 40 foot steel sloop, while it was built for me with care to minimise displacement (insomuch as it can be in that construction and sized vessel) and with some sort of hope for improved hydrodynamic performance over the usual run of the mill steel boats as it is fin keeled and composite spade ruddered it is clear that I did not place a big emphasis on speed for my own needs. I own no performance fast cruising boat myself so all the above is not blinkered by some sort of allegience to my own boat, a trap easy enough to fall into.

I'll leave it at that, long past time for a beer and I suspect you'll need one too if you manage to get through reading my post .

John
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Old 09-01-2009, 16:42   #89
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I am intrigued by the idea that one can make a heavy cruising boat as fast as a light one just by putting more sail on it and the inference by some that this is the way to go - that was the claim I was addressing. .
Weight is a penalty regarding boat speed, no doubt about it. I've never said otherwise.

In the context of small boat cruising, what I've said is that with proper design it is not the huge impediment that many hype it to be, and that displacement can return other benefits which may, or may not, be of value to the owner.

Quote:
Originally Posted by MidLandOne View Post
Given the benefits of comfort many claim for displacement and the need for speed by some, I have to wonder why we are not seeing fleets of 25 tonne comfy 45 footers with clouds of sail enabling them to sail at hull speed in 8 knots of wind cruising the world. I personally think the answer is obvious from both design and boat management points of view.
Expense and difficulty operating come to mind.

It's quite easy to take any idea to an illogical extreme in order to invalidate it. I specifically was addressing the smallish cruiser because that's where the ablilty to maintain speed under heavy loads is most difficult.
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Old 09-01-2009, 16:47   #90
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you ask a lot. I'll give you cryptic answers to save some time. I am not cranky. No matter what they say. If you by my book I'm sure you will find answers to your questions.

Question No 1:
No I am not. Both boats will have the same hull speed. The lighter boat will require less energy to get to it's hull speed. Just think in terms of aux size required.

Question no. 2:
No I am not.

Question no. 3:
I am saying the opposite. Wave making resistance is not important at low speed, say below half of hull speed. At those low speeds it's about wetted surface. As you approach hull speed the wave making is the primary resistance.

There is no clear answer for your next question. You are breaking down the process too much I think. For me. the design takes shape as a complete entity with all the ratios in balance and that balance depends upon the character I am after in that particular deign. For todays fast boats you go for a light hull with a huge rig. Cruising boats need displacement for a number of reasons. Speed is not one.

Lighter boats do not have less sail carrying ability. Tender boats have less sail carrying ability. There is far more to that than simple displacement. A light boat with lots of draft and a big bulb can be very, very stiff.

I never, ever said a heavy boat can be as fast as a light boat if you pile enough sail on the heavy boat. It's not that simple. Nobody trying to design a fast boat today goes for heavy displacement. Just the opposite.

I do not even know what you are asking in the next paragraph. I will say this. For me steel is to heavy for a fast boat. When I say "fast" I mean fast by today's standards. If I design in steel I am going to have to first float the material then I would worry about the shape. A "light" steel boat would be heavier than a similar composite boat where my hands are free in terms of material weights. I do not want my hands tied by heavy materials. Unless, the client requests steel. Then I can do it but the steel boat will be heavier than a similar composite boat.

I sort of think you are approaching this in little bites of this and that. I know from experience that boats with D/L's between 200 and 300 work best with SA/D's between 17 and 21. If you put a big rig on a heavy boat you will end up with a towering rig that will make the boat tender. It's just not practical.

You are certainly entitled to allegiance to your own boat. I think that is a good thing.
Congrats on having your boat built for you. Your biggest mistake was not having ME design it. But I forgive you. But you MUST open yourself to the idea that perhaps in a wide range of conditions there is a cruising boat that has better all round performance. Now the real trick for cruisers is to define "performance". That's Pandora's box.

I started sailing in 1961. I have sailed more boats than I could possibly count. I pretty much love them all. I have sailed some pigs in my time, unhappy boats that were just not going to do my bidding regardless of the techniques I used. These are generally boats with severe design flaws. But mostly I judge each boat according to it's genetic proclivities. I don't fault a heavy underigged boat for being slow. I just look for ways to make it go. I want to sail each boat sympathetically to it's genetic proclivities. That way I can enjoy each boat for what it is.

Class dissmissed.
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