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Old 15-05-2016, 15:29   #1
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Fat or Thin Hull for Offshore

My boat is 54' LOA with a beam of 12' - relatively narrow for the length. With the ever increasing beam of modern boats I was wondering if I have missed something? Others have told me that a narrower boat is actually better for heavy weather so I wondered if anybody here would have any argument for or against this?
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Old 15-05-2016, 16:15   #2
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Re: Fat or Thin Hull for Offshore

I think the modern trend for wide-assed boats has two motivators:

Firstly, in racing terms, these wide boats are easier to get up on the plan when sailing off the breeze.

Secondly, in floating-apartment terms, these allow even more capacious aft staterooms

If you are short-handed sailing and cruising, you probably don't need a planing hull shape. Getting a boat on the plane, even a wide-assed one, needs plenty of breeze and plenty of sail area - not an ideal combination for a short-handed cruiser, plus keeping a boat on its feet on the plane required a lot of concentration, good steering and constant sail trim.

As to the large stateroom thing - great on anchor, moderately useless at sea....
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Old 15-05-2016, 17:18   #3
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Re: Fat or Thin Hull for Offshore

It will all depend on what you ask the boat to do.

I like very beamy boats with plenty of beam well aft for off the wind (esp downwind) sailing. They roll less or not at all and I take rolling not too well at all so here my choice.

Upwind, I like long narrow boats best. They seem to just glide upwind without too much fuss nor trouble.

I think many modern boats are beamy as this is your only option to gain plenty of space down below. You will see many 1:3 small boats, but fewer 1:3 bigger boats. Nearly all boats get more slender as they get bigger.

So this is my personal take. Both wide and narrow are fine.

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Old 16-05-2016, 19:33   #4
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Re: Fat or Thin Hull for Offshore

Quote:
Originally Posted by BlackWind View Post
My boat is 54' LOA with a beam of 12' - relatively narrow for the length. With the ever increasing beam of modern boats I was wondering if I have missed something? Others have told me that a narrower boat is actually better for heavy weather so I wondered if anybody here would have any argument for or against this?
I don't know the answers, but I do have an observation (for what it's worth): I have never seen one of those beamy modern boats being sailed by just one person. I frequently see the narrower boats, like yours, sailed single handed.

Also, when the weather turns a bit nasty, I notice that the flashy new wide-beam boats stay at their moorings (with their frequent races and rallies and pursuits and so forth being postponed when I think conditions are perfect for heading out), while those of us with narrower boats go sailing and have lots of fun.

One thing I like about our Pacific Seacraft, which is more traditional in hull shape, is that it tracks like it's on rails. It can be steered with the sails. This sort of hull comes in mighty handy. Here's what ours looks like:

https://pacificseacraft37.com/overview/

Interestingly, someone recently said to me that my boat looks wide to him. Admittedly, he has a very traditional long-keeler that's very narrow by today's mass-market boat standards.
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Old 16-05-2016, 22:23   #5
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Re: Fat or Thin Hull for Offshore

The wider the beam, the greater the initial stability. Initial stability is resistance to initial heeling. A boat with higher initial stability will heel less initially for a given force applied abeam (like wind or a wave). This makes them faster (can carry more sail), but also less likely to self-right if they capsize. Think of a flat raft (wide beam) versus a kayak (narrow beam). The kayak is tippier, but easy to recover if you flip. The raft is less tippy, but if you flip it, you're pretty much cooked (this is why you occasionally find cat owners talking about putting access hatches on the underside of their hulls).

Wider beamed boats tend to have shallower hulls and higher centers of gravity (they don't have to, but this is the norm), which means they have less reserve stability. Reserve stability refers to the angle of heel a boat can reach before it capsizes. Narrower, deeper, better ballasted boats will have greater reserve stability, and be able to survive more extreme heeling angles without capsizing.

So, in general, I'd say a narrower boat is better for extreme conditions.

It will be more tender, but it's less likely to actually capsize, and better able to recover if it does.

The reason wider boats are more popular is speed and living space.

Consider the Capsize Screening Number, one metric of offshore seaworthiness:

Beam / cubic root of Displacement

Lower is better, 2.0 being the maximum (1.7 for high latitude sailing).

So the smaller the beam for a given displacement, the better.
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Old 17-05-2016, 05:12   #6
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Re: Fat or Thin Hull for Offshore

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Originally Posted by BlackWind View Post
My boat is 54' LOA with a beam of 12' - relatively narrow for the length. With the ever increasing beam of modern boats I was wondering if I have missed something? Others have told me that a narrower boat is actually better for heavy weather so I wondered if anybody here would have any argument for or against this?
I think I wouldn't like your boat, but if you do it just fine with me and that there is no reason to work at looking for reasons. I bet your boat is mostly a racer and the real reason it is 54' length and only 12' is to have a lighter weight to go with the waterline for speed. In heavy seas displacement/weight is your friend, so ...............
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Old 17-05-2016, 05:50   #7
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Re: Fat or Thin Hull for Offshore

The reason it is narrow is because after years of study and thought that architect thought this was the way it should be built . It will not necessarily be more tender than a fat boat . Many things will influence this , draft , ballast , many , many things come in to play . Fat boats are generally laid out as apartments ,many with short draft for convenience not safety . Skinny boats are easier to push ! Skinny boats require less power .
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Old 17-05-2016, 12:54   #8
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Re: Fat or Thin Hull for Offshore

There is a lot of nonsense in this thread mostly driven by people who have little experience sailing modern designs. I am going to use Kiss' answer to tear apart but it applied equally to all of them.

Quote:
Originally Posted by KISS View Post
The wider the beam, the greater the initial stability. very trueInitial stability is resistance to initial heeling. A boat with higher initial stability will heel less initially for a given force applied abeam (like wind or a wave). Not true, typically initial stability is the amount of force required to heel the boat by 10 degrees. A 'given force' could be of any magnitude. This makes them faster (can carry more sail), but also less likely to self-right if they capsize. again this is simply not true. Wide beam may be a factor in how well a boat will self right if flipped over, but there are a lot of others that play an equally large part. The depth of the keel, amount of ballast and where it carried, bridgedeck volume, etc. If you want to know anything about how a boat self rights then you need to see a stability graph.Think of a flat raft (wide beam) versus a kayak (narrow beam)Not if you build a semicircular enclosure over the top of the raft, the. It will self right immediatly. . The kayak is tippier, but easy to recover if you flip. The raft is less tippy, but if you flip it, you're pretty much cooked (this is why you occasionally find cat owners talking about putting access hatches on the underside of their hulls). cats are different than wide monohulls, in part because of their massive initial stability and lack of ballast.

Wider beamed boats tend to have shallower hulls and higher centers of gravity (they don't have to, but this is the norm), which means they have less reserve stability. Reserve stability refers to the angle of heel a boat can reach before it capsizes. no this is called the AVS - angle of vanishing stability, and is the point at which the CG (center of gravity) and CB (Center of Boyancy) are back in allignment, and any more heeling will push the boat upside down. Narrower, deeper, better ballasted boats will have greater reserve stability, and be able to survive more extreme heeling angles without capsizing. this is simply not true. As I mentioned before there are other things that play a part. The specific hull design, bridgedeck design, etc all play a part. SOME narrow boats have a high AVS, some do not, just like some wide boats have a high AVS while other do not.

So, in general, I'd say a narrower boat is better for extreme conditions. again, this is simply not true. It all comes down to the specific boats we are comparing.

It will be more tender, but it's less likely to actually capsize, and better able to recover if it does. nope. If you hold all other things to be equal a narrow boat will reach its AVS with far less force than a wide boat. Of course you can't hold all other things equally, but the wider the boat the more area there is under the stability curve. Which means it is harder to push over, regardless of the AVS number, and a soecific wide boat can have a higher AVS than a specific narrow boat.

The reason wider boats are more popular is speed and living space.

Consider the Capsize Screening Number, (CSN) one metric of offshore seaworthiness:

Beam / cubic root of Displacement

Lower is better, 2.0 being the maximum (1.7 for high latitude sailing).

So the smaller the beam for a given displacement, the better.The Capsize Sceening Number is a worthless bit of drivel. Because it only works if the two boats being compared have roughly the same CG and CB. If for instance you put the ballast in a bulb deep below the hull it will have zero effect on the CSN of the boat, but pound for pound lead slug deep below the hull has an outsized effect on the GZ curve. It would be functionally the same as comparing the speed of two cars on a race track by only comparing their weight. Sure weight is important, but what you need to know is the HP:weight ratio.
So what are the advantages of a narrow hull? it really depends on the given hull, but typically they are easier to drive to hull speed due to reduced wetted surface. And when very long and very narrow can increase the S/L ratio such that their hull speed is higher than you might expect. This is not always true however, very flat boats like the Pogo will lift the windward side of the hull and just leave a sliver of the chine in the water, allowing them very high S/L ratios when beating.

It is difficult to speak in broad truths because a wide slow cruiser like my Beneteau is not the same thing as a wide fast boat like a Pogo 40. They may both have wide transoms, but they are radically different boats. The same can be said for long narrow boats. The advantage of a long narrow boat is that it used to be the only way to go significantly faster than 1.34*sqrt(LWL) but that isn't true with modern designs.

As an example, below is the GZ curve of a modern Open 40, a very wide transom massively powerful boat. Note the high AVS, high area above zeroc, and small amount of area under zero. This indicates a boat that will be very difficult to flip in the first place, has to be inclined to a substantial angle to get there, and once inverted will be easy to right.
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Old 17-05-2016, 13:14   #9
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Re: Fat or Thin Hull for Offshore

Sorry I can't figure out how to post two pictures in one post from my iPad.

By comparison take a look at the GZ curve for a old 1/2 toner. Which in its day was a pretty reasonable off shore racer. Note the lower AVS, smaller area above zero, and larger are under zero. Indicating a boat that will get to its AVS with less force applied, reaches the AVS sooner, and then will have more difficulty righting itself by comparison.
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Old 17-05-2016, 14:10   #10
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Re: Fat or Thin Hull for Offshore

I'm not trading in my skinny boat. The feel of balance that comes from the weight of a keel and not the buoyancy of a beam, is just different, and to my sensibilities a nicer motion.
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Old 17-05-2016, 14:18   #11
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Re: Fat or Thin Hull for Offshore

Skinny old boats have rather nice AVS Curves also....

Here's one for the Contessa 32 which has a beam of 9.5':

https://www.google.com/search?q=cont...GbUg-xUrYMM%3A
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Old 17-05-2016, 14:27   #12
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Re: Fat or Thin Hull for Offshore

Quote:
Originally Posted by Stumble View Post
As an example, below is the GZ curve of a modern Open 40, a very wide transom massively powerful boat. Note the high AVS, high area above zeroc, and small amount of area under zero. This indicates a boat that will be very difficult to flip in the first place, has to be inclined to a substantial angle to get there, and once inverted will be easy to right.
I wouldn't call 130deg AVS high when 120 is considered as a minimum for offshore sailing. A bit better than bad perhaps..

ps. sorry it's closer to 127deg

Quote:
If you hold all other things to be equal a narrow boat will reach its AVS with far less force than a wide boat. Of course you can't hold all other things equally, but the wider the boat the more area there is under the stability curve. Which means it is harder to push over, regardless of the AVS number, and a soecific wide boat can have a higher AVS than a specific narrow boat.
Not true necessarily, while a a wider boat will have higher max righting lever GZ the AVS will be less. So the areas on the positive side are quite close to each other.


But as you point out there are different kind of narrow boats as there are wide range of wide boats. Knowing just the length and beam has no value at all.

BR Teddy
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Old 17-05-2016, 14:27   #13
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Re: Fat or Thin Hull for Offshore

I thought this was kind of interesting considering AVS:
Crash Test Boat - Capsize
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Old 17-05-2016, 14:49   #14
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Re: Fat or Thin Hull for Offshore

Quote:
There is a lot of nonsense in this thread...

Stumble (Greg),

Very good post, mate, well explained and pretty definitive. It addressed a lot of common misbeliefs that float around the sailing world.

Well done!

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Old 17-05-2016, 15:01   #15
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Re: Fat or Thin Hull for Offshore

Quote:
Originally Posted by Stumble View Post
Quote:
Originally Posted by KISS
Initial stability is resistance to initial heeling. A boat with higher initial stability will heel less initially for a given force applied abeam (like wind or a wave).
Not true, typically initial stability is the amount of force required to heel the boat by 10 degrees. A 'given force' could be of any magnitude.


Those are different ways of saying the same thing:

More initial stability means less healing for a given force.

Or

More initial stability means more force to generate a given angle of heel.

Same thing.

And, yes, this only applies to shallow angles of heel, beyond which reserve stability takes over.

Hence I used the phrase "initial heeling."

Quote:
Originally Posted by Stumble
Quote:
Originally Posted by KISS
This makes them faster (can carry more sail), but also less likely to self-right if they capsize.
again this is simply not true. Wide beam may be a factor in how well a boat will self right if flipped over


Not may be.

Is.

More beam means more initial stability upside down as well as right side up.


Quote:
Originally Posted by Stumble
but there are a lot of others that play an equally large part.

Of course, I never said anything to the contrary.

We're comparing wide to narrow beam, other things being equal.


Quote:
Originally Posted by Stumble View Post
Quote:
Originally Posted by KISS
Think of a flat raft (wide beam) versus a kayak (narrow beam)
Not if you build a semicircular enclosure over the top of the raft, the. It will self right immediatly.


Again, the comparison is of different beams all else being equal.

Put the same enclosure on a narrower hull and it will right even better.


Quote:
Originally Posted by Stumble
Quote:
Originally Posted by KISS
The kayak is tippier, but easy to recover if you flip. The raft is less tippy, but if you flip it, you're pretty much cooked (this is why you occasionally find cat owners talking about putting access hatches on the underside of their hulls).
cats are different than wide monohulls, in part because of their massive initial stability and lack of ballast.


Of course cats are different than wide beam monos.

I was just saying that both, having more initial stability than a narrow mono, are less likely to self-right.


Quote:
Originally Posted by Stumble
Quote:
Originally Posted by KISS
Wider beamed boats tend to have shallower hulls and higher centers of gravity (they don't have to, but this is the norm), which means they have less reserve stability. Reserve stability refers to the angle of heel a boat can reach before it capsizes.
no this is called the AVS - angle of vanishing stability, and is the point at which the CG (center of gravity) and CB (Center of Boyancy) are back in allignment, and any more heeling will push the boat upside down.


Yes, the angle at which the boat capsizes is the AVS. Reserve stability is the resistance to heeling at more extreme angles of heel. And the greater the reserve stability, the greater the AVS. So...not sure what your point is, unless it's just semantic.

Quote:
Originally Posted by Stumble
Quote:
Originally Posted by KISS
Narrower, deeper, better ballasted boats will have greater reserve stability, and be able to survive more extreme heeling angles without capsizing.
this is simply not true. As I mentioned before there are other things that play a part.


And I mentioned before, this is an apples to apples comparison.

Obviously beam is not the only factor in stability.

You aren't rebutting what I'm saying by pointing that out.


Quote:
Originally Posted by Stumble
Quote:
Originally Posted by KISS
It will be more tender, but it's less likely to actually capsize, and better able to recover if it does.
nope. If you hold all other things to be equal a narrow boat will reach its AVS with far less force than a wide boat. Of course you can't hold all other things equally, but the wider the boat the more area there is under the stability curve. Which means it is harder to push over, regardless of the AVS number


Holding all else (such as reserve stability) equal, that's true, but as I pointed out, beamier boats tend to have a higher center of gravity. They don't have to, but that's the way they're usually built. These boats have more initial stability, but less reserve stability. On balance, this tends to make them more likely to capsize (smaller area of positive stability).

Quote:
Originally Posted by Stumble
Quote:
Originally Posted by KISS
Consider the Capsize Screening Number, (CSN) one metric of offshore seaworthiness:

Beam / cubic root of Displacement

Lower is better, 2.0 being the maximum (1.7 for high latitude sailing).

So the smaller the beam for a given displacement, the better.
The Capsize Sceening Number is a worthless bit of drivel.


Yes, all those professional designers who use it must be morons...
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