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  #1551  
Old 10-08-2013
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Re: Pros and cons of steel sailboats

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Originally Posted by bfloyd4445 View Post
Jrff, one mistake, weigh decreases carrying capacity. A vessel of a given volume will displace x amount of water. any weight added will decrease its carrying capacity or payload. A Tupperware wood AL boat being lighter will have a greater payload than a steel boat of the same size.That's why I stated that under 50 feet steel was more of a liability than asset unless you are a habitual reef banger.
that sai8d I still like steel vessels of any size
I think that you and I are trying to say roughly the same thing. I was saying it in the negative by pointing out that in and of itself, increasing the displacement of a boat does it automatically add to carrying capacity.

You are pointing out in the positive, a heavier boat does not have greater carrying capacity if that added weight is in the form of a heavy hull and deck structure.

Jeff
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Re: Pros and cons of steel sailboats

"any weight added will decrease its carrying capacity or payload. A Tupperware wood AL boat being lighter will have a greater payload than a steel boat of the same size."

I can't figure out what you are trying to say here. Not sure what displ has to do with "payload" carrying ability. Unless you are talking exclusivley about volume. I consider carrying capacity to be primarily a function of waterplane area and the lbs. per inch immers. This has zero to do with displ. The caveat of course being volume to stow this payload and that can be a problem with lighter displ.
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Re: Pros and cons of steel sailboats

This stock Nordic 40 just completed a single handed circumnavigation.
So much for "plastic marina queens".
And it did it while looking very good. A capable, handsome yacht.
Generalizations are so easy and so innacurate.
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  #1554  
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Re: Pros and cons of steel sailboats

Quote:
Originally Posted by Jeff_H View Post
In and of itself simply adding displacement does nothing good for a boat, especially offshore. Weight does not automatically add to stability, weight does not inherently add to motion comfort, it does not inherently increase strength, nor does it automatically add to carrying capacity. Excess weight, especially in the wrong places hurts all of these things, plus it makes a boat harder to handly and hurts performance. But that's a topic for another thread.

Respectfully,
Jeff
Jeff, here you are saying just adding displacement doesn't improve. In the sense that this is not always improves.
But on the other hand, (mass) moment of inertia is
Pros and cons of steel sailboats-moi-discrete.jpg

or
Pros and cons of steel sailboats-moi-contin.jpg

The mass moment of inertia is a factor in the efforts to change a body (here boat) movement (around an axis). The higher MoI the more energy is required. Thus a boat with higher MoI will not be so easy to excite in movements as rollings etc - and then percieved as more stable (downside is then when the boat actually has been exited, then it take longer time to dampen the movements).

As the MoI is increasing with mass and size, both correlated to displacement, then one would generally expect higher displacement - higher stability (within limits).

Marchaj reasoned along these lines in his book "Seaworthiness -the forgotten factor". However, some of the conclusions in his book seems exaggerated. As said above, increased MoI has its drawbacks.

Comments?

/J

Last edited by Jaramaz; 10-08-2013 at 01:12 PM.
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  #1555  
Old 10-08-2013
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Re: Pros and cons of steel sailboats

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Originally Posted by bobperry View Post
Jabe:
Welcome to the debate.
"Steel sailboats are better for offshore - more displacement.

I'd like to know exactly how you figure that.
you forgot to add" and less payload".
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  #1556  
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Re: Pros and cons of steel sailboats

Quote:
Originally Posted by Jaramaz View Post
Jeff, here you are saying just adding displacement doesn't improve. In the sense that this is not always improves.
But on the other hand, (mass) moment of inertia is
Attachment 15894

or
Attachment 15895

The mass moment of inertia is a factor in the efforts to change a body (here boat) movement (around an axis). The higher MoI the more energy is required. Thus a boat with higher MoI will not be so easy to excite in movements as rollings etc - and then percieved as more stable (downside is then when the boat actually has been exited, then it take longer time to dampen the movements).

As the MoI is increasing with mass and size, both correlated to displacement, then one would generally expect higher displacement - higher stability (within limits).

Marchaj reasoned along these lines in his book "Seaworthiness -the forgotten factor". However, some of the conclusions in his book seems exaggerated. As said above, increased MoI has its drawbacks.

Comments?

/J
I think this is converging two separate ideas. You are correct that larger roll and pitch moments of inertia will slow roll and pitch acceleration rates. That comes at the price of larger roll and pitch angles. You are also correct that adding mass can add to the boat's roll and pitch moment of inertia. This is where the discussion gets more complex. The roll and pitch moment of inertia results from the collective moments of inertia of each mass based on its mass and that mass's distance (r) to the instantaneous roll and pitch axis.

By and large, since r is squared (or cubed depending on the calculation) it is changes in the distance to the instantaneous roll and pitch axis that has a greater impact on the the roll and pitch moment of inertia rather than the simple weight itself.

In other words, if we looked at two boats of equal mass, one which had all of its mass concentrated near the rotational axis and the other spread out into a bulb at the keel and a heavier mast, obviously the boat with the bulb and heavy mast would accellerate more slowly. In reality, even if we greatly added more weight near the rotational axis of the boat with concentrated weight, the accelleration rate would not be slowed to the point of matching the boat with the disbursed masses.

Going back to my earlier point, from the standpoint of optimizing motion comfort, the main factors are weight and buoyancy distribution and dampening, with overall displacement only playing a comparatively small role.

In terms of stability, "As the MoI is increasing with mass and size, both correlated to displacement, then one would generally expect higher displacement = higher stability."

I respectfully suggest that this is just plain wrong. Its true that if greater mass were added in a manner which did not change the center of gravity, or changed the center of gravity in a manner which is beneficial, such as added to a keel bulb, then adding weight would add stability.
But if that weight were added in the form of a heavy deck structure or mast, then that weight would decrease stability.

Which is why I said, "In and of itself simply adding displacement does nothing good for a boat, .... Weight does not automatically add to stability, weight does not inherently add to motion comfort.... Excess weight, especially in the wrong places hurts all of these things...."

As to Marchaj's book, "Seaworthiness -the Forgotten Factor", from everything that I know, Marchaj's basic science still remains correct and consistent with what we know today. If there is a shortcoming to the book, it is that much of the research was based on the boats that were available over 30 years ago when it was first written. In those thirty years, there has been a lot more research. And to one degree or another the lessons pioneered by Marchaj and later theorcists have been applied to improve the seaworthiness and motion comfort of the better sea going designs. So while the science is right, and the conclusions are correct for the type forms that existed at the time, some of his conclusions were too broadly applied and did not consider the subsequent design solutions intended to address the concerns that he raises in his book.

Respectfully,
Jeff
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Last edited by Jeff_H; 10-08-2013 at 05:03 PM. Reason: Typos and clarity
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  #1557  
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Re: Pros and cons of steel sailboats

Disregarding factors of material, design. moments etc, advantage of steel over wood is obvious when you consider inside volume.My planking is 2 inches, frames 4 inches, deck beams 6 inches, carlins 6x6 shearclamp 10 by 10 all in a hull that could be better used if the same shape were steel or even FG.This volume wouldn't have to be full of stuff but would make it easier to swing a cat.Since captains aren't allowed to do that any more, I wouldn't change a thing.
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  #1558  
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Re: Pros and cons of steel sailboats

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Originally Posted by Capt Len View Post
Disregarding factors of material, design. moments etc, advantage of steel over wood is obvious when you consider inside volume.My planking is 2 inches, frames 4 inches, deck beams 6 inches, carlins 6x6 shearclamp 10 by 10 all in a hull that could be better used if the same shape were steel or even FG.This volume wouldn't have to be full of stuff but would make it easier to swing a cat.Since captains aren't allowed to do that any more, I wouldn't change a thing.
As contrast, the western red cedar strip planking of my new Perry 62'er is one inch thick and the West System sheathing (set in tri-axial 24 ounce Vectorply) adds only a fraction of an inch, so no more than 1-1/16 total. She has basically no other protruding structural elements as the deck and entire interior (bulkheads, berth-flats, etc) are foam core composite and designed to be structural. Therefore she is very open down below.

Granted steel would be thinner, but then you would have insulation, etc to add to the steel and a steel vessel would not be near as light.

I have read that various lab tests show that wood/composite can be stiffer and stronger than steel. (And I believe that wood/composite is a better insulator for temperature and noise.)

I have nothing against steel as a vessel material (or any other material for that matter), each material has a valid place in boat building. (I have 316SS keel floors to distribute the keel loads via some G-10 girders throughout the hull.)

This argument of one material being better than another makes no sense to me, each material has pros and cons, the best vessels use many different materials in different applications to accomplish the desired mission. As all missions are different, each might take a different approach and use different materials.

My vessel is built of wood (western red cedar, fir, cherry, ash), West System Epoxy, carbon fiber, solid G-10, titanium, several brands of dense and light weight foam core, VectorPly in several different weights, 316 SS, Aqua-mat 22, anodized aluminum, sheathed mild steel, lead, Dacron, Spectra, and I am sure I have forgotten to list other items used in her build.

All boat building materials have their place IMHO, even ferro-cement.

Cheers!
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  #1559  
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Re: Pros and cons of steel sailboats

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Originally Posted by kimbottles View Post
As contrast, the western red cedar strip planking of my new Perry 62'er is one inch thick and the West System sheathing (set in tri-axial 24 ounce Vectorply) adds only a fraction of an inch, so no more than 1-1/16 total. She has basically no other protruding structural elements as the deck and entire interior (bulkheads, berth-flats, etc) are foam core composite and designed to be structural. Therefore she is very open down below.

Granted steel would be thinner, but then you would have insulation, etc to add to the steel and a steel vessel would not be near as light.

I have read that various lab tests show that wood/composite can be stiffer and stronger than steel. (And I believe that wood/composite is a better insulator for temperature and noise.)

I have nothing against steel as a vessel material (or any other material for that matter), each material has a valid place in boat building. (I have 316SS keel floors to distribute the keel loads via some G-10 girders throughout the hull.)

This argument of one material being better than another makes no sense to me, each material has pros and cons, the best vessels use many different materials in different applications to accomplish the desired mission. As all missions are different, each might take a different approach and use different materials.

My vessel is built of wood (western red cedar, fir, cherry, ash), West System Epoxy, carbon fiber, solid G-10, titanium, several brands of dense and light weight foam core, VectorPly in several different weights, 316 SS, Aqua-mat 22, anodized aluminum, sheathed mild steel, lead, Dacron, Spectra, and I am sure I have forgotten to list other items used in her build.

All boat building materials have their place IMHO, even ferro-cement.

Cheers!
you cant take a cutting torch to wood, cut out a piece then weld in another like you can with steel. Aluminum can be handled this way but it takes more skill and annealing is a problem. fibreglass is pretty easy to repair and properly laid up very very strong. I prefer wood for many reason but no longer own any wood boats because of the upkeep
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  #1560  
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Re: Pros and cons of steel sailboats

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Originally Posted by bfloyd4445 View Post
you cant take a cutting torch to wood, cut out a piece then weld in another like you can with steel. Aluminum can be handled this way but it takes more skill and annealing is a problem. fibreglass is pretty easy to repair and properly laid up very very strong. I prefer wood for many reason but no longer own any wood boats because of the upkeep
Wood/composite is easy to repair if you have the necessary skill. There are several schools in the USA and elsewhere that teach these skill.

One example: Northwest School of Wooden Boat Building | Port Hadlock, Washington
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