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Old 21-08-2008, 22:42   #1
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Displacement and Vanishing Stability

I'm looking to better understand the ratios that can be determined from this site http://www.sailingusa.info/design_wi...haracteristics

I've been calculating ratios for various boats and seeing which ratios are better or worse, but I don't understand how much they matter, for instance if a boat is a "moderately heavy cruiser" versus a "heavy cruiser", does that make the "heavy cruiser" more ocean worthy?

By ocean worthy, I mean more able to tolerate worse conditions, therefore conditions that are beyond my capabilities are less likely to occur.

I understand vanishing stability rather well, such as if it rolls over 130degrees from verticle it won't right itself but rather come to rest capsized.

If that were the angle of vanishing stability that is...

I don't understand how the motion comfort calculations relate to your experience, so maybe you could please calculate it for your own boats and share the number and your level of discomfort in what conditions...

This situation became an issue when I'm looking at the difference between a boat in the Great Lakes for less than half the price of a boat off LA.

The difference seems to be that only a fool would take the boat on the great lakes out to sea, even though it has plenty of room, is only 2 feet shorter than the other boat, and the lay-out is just as sufficient between the two boats.

So that is to say; the stability of the one in the Great Lakes to me seems such that it would be less comfortable in the ocean, and less stable in the same conditions as the other boat, making the other boat, though more expensive, much more safe of a purchase for my livelihood.

So...any comments, and opinions, experiences concerning the matter would be greatly appreciated.

Thanks,
Brandon
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Old 22-08-2008, 00:05   #2
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Hello Brandon,

It's not just stability. A heavy cruiser will carry more of the essential for long passages. More water, more fuel, larger anchor, more chain and etc. And carrying that load better means she'll ride higher and make better headway. This makes her a safer boat.

The question then becomes, how much stuff do you plan to carry? A boat for a week cruise to the Bahamas needs much less displacement than one for meant for forty days on the Pacific.
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Old 22-08-2008, 01:45   #3
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I would say I want the "40 days on the pacific". If only to just be prepared for things that come-up, a ship that can do a long haul should also have been designed to fair better in heavier seas.

At least in my logic...logic is not always the case in designs I suppose.

So what figures would I be looking for when finding a boat to sail for long journeys?

Like...around the 45'-55' range.

I've found a 47' boat with 40,000 pounds displacement, and that is my one boat example, the other is a 45' with 26880 pounds displacement.

To me it seems the later boat which is on the great lakes, is meant to stay on lakes and such or within site of land, even if it is an ocean boat. It is so light it seems it would be easier to capsize, and a rougher ride in heavier seas.

Also you mentioned the consumables, yes, that lighter boat does have greatly reduced storage space, it only has 25gallons of fuel space, that's rediculously small for what I'm looking at...since I am advised to have enough fuel to cover at least 1,000 or more miles by motor.

So what are good rules of thumb when looking to buy a boat of that size. Is 40,000 pounds a more ideal weight? Is it too heavy? Is there such a thing as too heavy and there's some "goldilocks" range of weight per LOA?

How much does Beam really factor into the stability and handling of the boat in storm conditions?

Naturally...I want a boat that will help me survive reasonable situations and concerns further out to sea...
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Old 22-08-2008, 04:20   #4
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These newer mono's, wide across the stern for stiffness, do give a less pleasant ride for mono men, but are faster. If you go blue cruising you will meet 40 to 60ft waves and screaming winds. Weather windows are likely to close as you approach mid point.
Paasge planning to stay within a day of safe harbour or lee of an island reduces the risk but you are taking a bigger risk as the boat gets smaller.
Fuel. A storm lasts three days. That's how much fuel you need. ALWAYS. Not when you set off but when you are a day from arriving. Food and water for half as long again as the trip will last allowing for a planned diversion to restock or the watermaker fails.
Tonnage. A 6 ton Cat is ocean going. Some are. Loaded with six weeks of water, food and fuel that 40 days becomes 50 days. In a mono it's not so bad but you don't have so much space to store stuff. Better to plan the trip in legs of 10 to 20 days. Reasonably fresh food, no chance of scurvy, better sailing performance.
Often a good passage planner (VPP2 is one, try the free demo) will show good places to plan for stop off's, emergency diversions and what they'll cost in days on the trip.
Gerry Cans of fuel and water are not a problem, and some advantages. The small fuel tank is not an issue if there a good place to store smelly diesel. But re-filling that tank in a storm from 5 gallon cans is going to be difficult.
AND Always the most dangerous thing is not being flexible. If the wind is against you change your plan, go with the wind and do the planned trip next month/year.
AS the wind increases sail fast towards the safeR quarter, forget the rumb line, forget the plan. Check the options. Preferably before you set off. C R U I S E .
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Old 22-08-2008, 05:18   #5
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What are the makes of the boats you are considering?

Quote:
Originally Posted by FreeMason View Post
I'm looking to better understand the ratios that can be determined from this site http://www.sailingusa.info/design_wi...haracteristics

I've been calculating ratios for various boats and seeing which ratios are better or worse, but I don't understand how much they matter, for instance if a boat is a "moderately heavy cruiser" versus a "heavy cruiser", does that make the "heavy cruiser" more ocean worthy?

By ocean worthy, I mean more able to tolerate worse conditions, therefore conditions that are beyond my capabilities are less likely to occur.

I understand vanishing stability rather well, such as if it rolls over 130degrees from verticle it won't right itself but rather come to rest capsized.

If that were the angle of vanishing stability that is...

I don't understand how the motion comfort calculations relate to your experience, so maybe you could please calculate it for your own boats and share the number and your level of discomfort in what conditions...

This situation became an issue when I'm looking at the difference between a boat in the Great Lakes for less than half the price of a boat off LA.

The difference seems to be that only a fool would take the boat on the great lakes out to sea, even though it has plenty of room, is only 2 feet shorter than the other boat, and the lay-out is just as sufficient between the two boats.

So that is to say; the stability of the one in the Great Lakes to me seems such that it would be less comfortable in the ocean, and less stable in the same conditions as the other boat, making the other boat, though more expensive, much more safe of a purchase for my livelihood.

So...any comments, and opinions, experiences concerning the matter would be greatly appreciated.

Thanks,
Brandon
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Old 22-08-2008, 05:45   #6
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Originally Posted by FreeMason View Post
By ocean worthy, I mean more able to tolerate worse conditions, therefore conditions that are beyond my capabilities are less likely to occur... I don't understand how the motion comfort calculations relate to your experience...

Brandon: folks (from rank amateurs to skilled naval architects) have been trying to come up with the magic formula for at least a couple of centuries… as of yet, the science still has a huge “art” component in it – or so is my impression… Some decades ago I started the Westlawn course (in part) asking similar questions to yours… since then, I may have gleaned some insight (my Admiral would probably characterize it as an old-goat’s opinion… nothing more), but even after writing several dozen primitive spreadsheets I’ see no magic formula… capsize ratios, vanishing stability, D/L or whatever… all are valid, all are limited by their inbuilt application … For instance, the afore-mentioned vanishing stability calculation(s) rarely take into account the cabin shape – the vertical CG placement (as verses the vertical center of buoyancy) can be vastly different when comparing a tall-heavily crowned cabin as verses a flush-decked race boat… these numbers are rarely known, with the possible exception of some naval and coast-guard rescue boats where based on the bouyancy the superstructure-shape provides they (can) calculate the ability of a vessel to self-right…

Bottom line – for us out here in the unwashed proletariat, the general numbers available to us provide ammunition for semi-learned discussion, but will only provide hints (albeit pretty good ones on many cases) of what the various formulas purport to provide…

Two books that might give you additional vectors for your answer – or at least further food for thought -- are: Seaworthiness: The Forgotten Factor by Marchaj and Seaworthy Offshore Sailboat: by John Vigor… anything by Marchaj is worth reading, although it can be heavy-sledding at times because besides being a passionate sailor (and sail-plane pilot) he is also a fairly heavy-duty mathematician (but he has a knack for being able to dumb it down for stumblebums like me). Vigor’s little book takes a practical, more pragmatic approach to the question – light on theory, wonderfully heavy on application. What I like about his approach is he views the boat as a system – rather than dissecting the boat in various (mathematical) attributes, he focuses on how it all acts together; and, one thing I really like is at the end of each chapter he asks the question of “how does this all work when the vessel is inverted…” Whether discussing ports, joinery, the placement of navigation tools or the all-important hull integrity, he always returns to the question of what happens when the boat capsizes… Both Marchaj and Vigor are on my ever-expanding list of “must-reads.” If left to one book, however, it would be… Moitessier’s: A Sea Vagabond's World: Boats and Sails Distant Shores Islands and Lagoons. There’s more practical wisdom on blue-water preparedness from vessel to navigation, in the book than almost any I’ve seen…

Although they have several different factors to consider, I see a systematic approach in more experienced multi-hull designers as well – discussions of escape hatches, watertight bulkheads are not uncommon and signal (I think) designers and owners who are thinking blue-water (and then probably trying never to get into those conditions… And I think therein lies the key to your question – looking at the overall vessel… Can a Great Lakes vessel tackle the ocean – of course… should any Lakes boat tackle the ocean (or visa versa), entirely different question… there are certainly empirically derived formulas (screening and comfort numbers, for instance…) that will get one in the ball-part to answer both the seaworthiness and sea-kindliness questions, but at the end of the day, they still require a fair amount of art…

Thanks for an intriguing question… this thread has the potential to run for some time…
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Old 22-08-2008, 13:06   #7
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The conditions of the Lakes boat gets even better, hand-built by someone from 1984-1994, the survey though seems fairly favorable, with only a few "no-nos" but no structural problems. No-nos such as no epoxy underneath the anti-fouling paint.

I think the boat is light, that is my concern, I don't think it has sufficient displacement, for anything...for holding tanks which are small compared to heavier cruisers, for comfort in larger swells, for safety in heavier seas.

How much this is true is, I agree, an artform, because positioning the boat when in a condition, and not getting into certain conditions to begin with all are great factors in this, but given how it should behave on paper against another boat of heavier displacement and ballast, I feel that this one is subpar for seaworthiness.

It's a shame, because it's so cheap, but it must be so cheap for a reason....

I haven't the experience to tell, however, if that is the actual case, and maybe this boat is well built, and would take on the seas rather well if not even faster...

Because as I read also, you want some level of speed...speed means less time in the open ocean and thus less time for things to go wrong in a dangerous place.
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Old 22-08-2008, 15:38   #8
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Look at a beachball. It would take only a tiny ballast ratio to make it totally self righting. A raft could have a 70% ballast ratio and still stay upside down if inverted. So take one look at a boat's midsection and compare it to a beach ball or a raft. The more it resembles a beachball ( trunk cabin with a high cabintop camber , wheelhouse , etc) and the less it resembles a raft( flat flush deck, no deck structures with buoyancy, excessive beam, etc) the better it's self righting ability. The air in my modest sized wheelhouse has the same effect on ultimate stability as adding 3,000 lbs of ballast to her keel.
Excesss beam brings it closer to the raft shape, and further from the beachball shape , drastically reducing it's ultimate stability. A slight reduction in beam drastically improves ultimate stability.
When people began demanding more interior space, designers did it with greater beam . This resulted in lack of ultimate stability. So the next step was to convince people that the best they can hope for is a boat that capsizes at 120 degrees. Many older designs had positive stability very close to 180 degrees. An article in Cruising World ,Dec 1989 by Richard McCurdy points this out clearly, altho the examples he gives don't take into account deck shapes, a big mistake.
Wide aft sections and lean bows drastically reduce directional stabilty, making a boat tiring to steer and hard to control downwind. My first boat had this problem and I had to drasticaly reduce sail to get her to steer straight, drastically reducing speed downwind.
It's OK in racing boats with large crews, and twin rudders ,but a big mistake for shorthanded cruising boats.
My first boat was a 36 footer, too big for me. After crossing the Pacific in her ,I began to envy people in smaller boats. After ten years in my second boat, a 29 footer I've cruised 23 years in my current 31 footer, just the right size for my needs.
Choosing to big a boat has killed many a cruising dream.
Brent Swain
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Old 22-08-2008, 15:49   #9
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I aim for a big boat because I have a lot of good friends to sail with, I aim for a cheaper end maybe older big boat, only because I don't want financial entanglements between families.

If everyone I were dealing with were family, I'd just opt to co-sign and have everyone pay monthly rents, ensuring certain aspects that I find myself limited to in my current goals.

On top of friends I have family I'd want to live-aboard at times as well...so it's just unreasonable for me to go for a smaller boat because it's not just me that'd be going, at any time I'd expect at least 1 or 2 other people on board.

I really am coming to think the mid-40s and at most, the mid 50s (though I haven't really seen anything above 50 feet that has caught my eye persay) is the range I'm looking for...

When it comes to the parameters, like as you say, a stiff stern makes hard for steering and management before the wind and under-crewed...these things are advice I definitely would love from those with experience.

I'm not against hard labor, I doubt most boats require much strength but when it comes to strength and stamina myself and my closest friend and bodybuilding partner have quite a bit of it...

So that's an aspect of my physical condition that probably is more limiting for some individuals, just throwing that out there. Plus I'm coming into my mid-20s and my closest friend is going on 30 in a couple years...still pretty youthful so we have time to learn about a lot of factors in a boat before selecting the actual thing...

My mental image of what I want is beginning to mature the more I get surveys and the more I shop around online looking at what's out there and what the surveys have to say about them.

And this thread is the result of discovering that I can further mature my understanding of my "future" boat, by figuring out if it is built leaning more to what I want to do (global cruising) or more toward other agendas such as island cruising and coastal cruising.

Which leads me to ask...is "offshore" cruising the same thing as coastal? Or does that mean further out to sea, like between continents?
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Old 22-08-2008, 15:51   #10
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Basically, to clear up some of my personal position (in hopes it helps some give further advice); As good as friends may be and as reliable, I refuse to have entangling financial obligations with them, that would be like a business partnership and especially with a liability like a boat, too much can go wrong where I may find myself over-extended.

It's best if I can afford the boat myself without quitting my day job, then everything my friends and family are willing to chip in is just gravy to pay off any financing or continue with upgrades//maintanence...

I would think that reasonable and financially sound policy for something like a boat.
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Old 22-08-2008, 16:23   #11
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Quote:
I aim for a big boat because I have a lot of good friends to sail with, I aim for a cheaper end maybe older big boat, only because I don't want financial entanglements between families.
You see already there are ratios and some of them are relative to each other, but in the end it appears it is more about the "relatives". There are several ways to approach the solution and yet it still can be a good choice in the end. Try an alternate approach:

Basic rules I think include big enough to haul all your stuff, the smallest large boat that works, and something that works with the money you have and mindful of the money it will require over a reasonable amount of time. Those factors rule out so many boats already.

Elimination is a valid approach if the pool of choices at the end are quite small. Non blue water boats vibrate in 25 knots of wind with some sea state some vibrate even in 20 knots. Most all the production boats fall into the list. Might drive you nuts or get you killed either way is not desirable. These boats are not suitable to your purpose even if well suited to other purposes. If you look into build quality you usually can separate out all the boats that are fundamentally unsuitable. If you then throw in basic needs such as tankage and gear attached to the boat you already have eliminated a great many more boats. If you then take your budget and ratio of "relatives" to friends into consideration you suddenly find very few boats. Now eliminate all the boats not for sale currently. Out of all these remaining your task is to pick the best one. I doubt the ratios will matter much. Find three good boats and the Admiral will tell you which one is the one. The Ratio of the Admiralty is more important than all the rest.

Computing up the perfect boat generally fails slowly or quickly depending on how deep you get into the numbers and if you really want to get a boat or just act like you do. Actually sailing is more valuable than the numerical computational exercise.
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Old 22-08-2008, 17:03   #12
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Heavy will carry more?

[quote=starbolin;196639]Hello Brandon,

"A heavy cruiser will carry more of the essential for long passages"

I believe you mean: "A sturdy boat will carry more..." However, for instance, Two boats of an identical hull shape, one with an 1/8 thick hull and one with a 1" thick hull can be loaded with the same gear... less the weight the 1" thick hull adds to the heavier boat. I have had some pretty heavy boats, they require more sail area to move, pass through the waves lower and wetter etc. I was motor sailing down the Chesapeake once on a real snotty day in my Passport 47 (which weighed in at over 40k lbs) and was making poor progress due to the steep short wave action, I noticed a boat overtaking me from behind. It didnt take long for him to pass me and continue out of sight. Bow what a bummer to watch. The boat was a fairly lightweight Hunter or Catalina style., I cant remember now as it was 10 years ago. When I sold the Passport 47, I moved everything from one slip directly to the adjacent slip into a 42 foot catamaran. The Passport came up about 6" on the waterline and the cat only went down about 3". (the passport was so far down in the water that it started to get blisters above the boot stripe) To each his own, but in a storm I would rather be in a sturdy lighter boat riding the crests than a heavy one plowing through and inundated with water. Just my $.02.
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Old 23-08-2008, 12:48   #13
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Well Cheechako, that goes against other things I've heard, not to say it's wrong it makes total sense, any other comments on that? It might make for rougher sailing, but you won't be sitting in a snotty condition so long heh.
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Old 23-08-2008, 15:13   #14
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There is an excellant article in this months edition of Good Old Boat written by Ted Brewer on this subject. It would be worth going by your local news stand or-perish the thought-West Marine, and buying a copy.

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Old 23-08-2008, 16:44   #15
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Having friends to help you sail is wonderful .Having to depend on other people to allow you to sail is a different matter. Make sure you can sail alone comfortably when all of those people who offered to help you sail her don't show up. Talk is cheap.They usually don't show up. I was wishfully naive about that in my early 20's.
Thin hulled boats don't do so well in night collisions with all the floating debris out there, especially when they hit it faster.

Good friends are god's compensation for family members." Mark Twain.
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