Pros and cons of steel sailboats - Page 159 - SailNet Community
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post #1581 of 5317 Old 10-09-2013
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Re: Pros and cons of steel sailboats

"In a heavier boat, the percentage of increase in her displacement, by adding a given weight of personal effects and stores, will be less than it would be in a lighter boat, and The percentage of change will be far less in a heavier design. " Brent Swain

THIS IS TOTALLY WRONG! It shows what little grasp BS has of design elements.

Let me explain:
Say we have two 40' boats. One weighs 30,000 lbs. The other boat weighs 15,000 lbs. Half the weight of the other. Both boats have similar DWL's, say 33'. Both boats have similar beam, say 12'8". The heavier boat will have much greater hull epth and that's where the extra displ comes from.

But displacement has nothing to do with where a boat floats as it is loaded or unloaded. ZERO! The number we are looking for here is "pounds per inch immersion" or PPI. It's very easy to calculate. You take the square footage of your waterplane, the boat's footprint in the water, and multiply that by 64 and then divide by 12. For our example boats we will end up with a PPI around 1,300 lbs.

So, you put 1,300 lbs. of gear on either boat and it will sink an inch. Due to the flare of the hulls the waterplane will increase as the boat sinks and the PPI will go up gradually. But how a boat responds in flotation to loads being added has nothing to do with
displacement. This is often misunderstood by beginners or people not skilled in yacht design.

Two boats with generally similar DWLs and beam will have a similar waterplane area, give or take.
I could even easiyt draw examples where the heavier boat had less waterplane than the light boat. If this were the case then the heavy boat would sink more than the light boat for every pound loaded on. Consider this: You are only sinking the waterplane, not the boat that is already sunk. That volume is already "displaced".

"the heavier designed boat will be floating much closer to her original lines." Brent Swain

This is just nonsense. I'm sorry but anyone who claims to be a yacht designer should have a handle on PPI. I didn't make it up. Look on page 285 in Skene's. We might be falling back into that world where Brent thinks a boat can have a positive Rm of 182 degrees again. That still makes me chuckle.

If anyone has a problem understanding this let me know. I'll happily try to explain it in more clear terms if I need to.
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Last edited by bobperry; 10-09-2013 at 08:18 PM.
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post #1582 of 5317 Old 10-09-2013
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Re: Pros and cons of steel sailboats

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Originally Posted by bobperry View Post
Exactly Jeff.
I have no concerns about the build Betts has built a lot of alu boats. He voiced his concerns so I know he is aware of the difficulties involved and I think he will devise a way to deal with them. I can't see me standing there asking him to "Let me try". I have never welded anything and I choose not to start now. Maybe I'll take my guitar and sit and playt while Jim welds. I can sing wqelding songs. That's it.

"I've been working on the railroad,,,,,,,,,,,,,"
Bob. Welding is simply hand eye co-ordination , which your skill at drawing shows you have in huge supply. Try it ,you may like it.
The first 36 I built was for a top notch artist. Hand eye co-ordination a was a given. I lit the cutting torch and ,for the first time ever, he gave it try, a perfect cut the first try. He handed it to his girlfriend, also an artist, and again ,a perfect cut. The skipper did a lot of steel sculptures after that ,having picked up welding almost as quickly.
You could enjoy metal working so much you could get addicted . There are worse addictions . So go ahead, have a go at it. You may enjoy it immensely.

Brent Swain, Boat designer, Builder, and author of "Origami Metal Boatbuilding"
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post #1583 of 5317 Old 10-09-2013
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Re: Pros and cons of steel sailboats

Quote:
Originally Posted by bobperry View Post
"In a heavier boat, the percentage of increase in her displacement, by adding a given weight of personal effects and stores, will be less than it would be in a lighter boat, and The percentage of change will be far less in a heavier design. " Brent Swain

This is often misunderstood by beginners or people not skilled in yacht design.

If anyone has a problem understanding this let me know. I'll happily try to explain it in more clear terms if I need to.
Can you draw some simple pictures for me and Brent? It's a little complicated for guys like us.


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post #1584 of 5317 Old 10-09-2013
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Re: Pros and cons of steel sailboats

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Originally Posted by Classic30 View Post
Thankfully.. All you need is a Fein, boat nails and another piece of wood.

I can't think of anything in boat-repair much more difficult than patching a hole with plate steel. It looks simple enough to an onlooker, but I've seen even professional welders (cheap ones, I might add) stuff it up totally by not allowing for weld stress and heating.


EDIT: Thinking about this, in theory you might be able to cut wood with a plasma cutter. Dunno.. I'm not silly enough to try it. It's always easier to use the right tools for the job at hand.
interesting thought, plasma cutter for plastic. Lasers work well so why not one of those engineered for boat plastic
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post #1585 of 5317 Old 10-09-2013
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Re: Pros and cons of steel sailboats

I think BS stepped in it with that post. Don't imagine we'll see much of him tonight. I would hope he is reading a book on yacht design.

Yeah Smacks, I can draw a picture for you but not tonight.

If you have Douglas Phillips-Birt's book SAILING YACHT DESIGN it's on page 221.
If you have Steve Killing's book YACHT DESIGN EXPLAINED there is a discussion of waterplane on page 42.

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Last edited by bobperry; 10-09-2013 at 10:58 PM.
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Re: Pros and cons of steel sailboats

I ordered a couple. They wont get here for a week of so. One by Brewer and one by some guy named Perry.

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Re: Pros and cons of steel sailboats

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Originally Posted by bobperry View Post
Yeah Smacks, I can draw a picture for you but not tonight.
[[Psst - I actually understand your explanation. I just thought the pictures would help you know who - and I can take the heat as the "ignorant one". I don't mind taking a bullet for Team Steel as long as it helps you know who.]]
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post #1588 of 5317 Old 10-09-2013
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Re: Pros and cons of steel sailboats

I figured that Smackers.
I'm a bit torn. If there were others here who needed the drawing I would do it. But if there aren't I'm not keen on educating Brent.

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Re: Pros and cons of steel sailboats

Actually, I think you're right Bob. Just keep moving with the peenpod instead of drawing pictures (even though they're obviously needed in this case).

Showing how it's actually done with a final product is always the best way to teach.


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post #1590 of 5317 Old 10-10-2013
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Re: Pros and cons of steel sailboats

On other occasions and on other sites, I have had the discussion about relationship of design displacement to carrying capacity with Brent and others. As Bob has already pointed out, I too have tried to explain that the main factor in carrying capacity is the water plane of the boat, and then talked at length about some of the other factors that come into play as well. Its never an easy sell.

I completely agree with Bob that the key determinant in how much surplus carrying capacity (I am using this term to mean the difference between a boat with just enough gear to be sailed, and same boat fully loaded with as much as weight as it can reasonably carry and still sail reasonably well) a boat has is the amount of weight required to submerge the boat a given amount. And because that is determined by the water plane, its entirely possible that a lighter boat will have an equal surplus carrying capacity as a heavier boat in terms of the weight it can carry before it submerses to a point where performance and safety is unreasonably compromised.

The fact that a longer waterline boat will generally have a larger water plane relative to its displacement is one of the main reasons that I argue that in comparing two boats of equal displacement, the boat with the longer waterline of the two, will generally have a higher carrying capacity.

But, with all due respect, there are other factors in this discussion, some of which are especially relevant as they purtain to steel construction. I normally prefer to discuss this from the stand point of length to displacement, and to do so by comparing two equal displacement boats of different lengths. This is in contract to the normal way of discussing this issue is to refer to 'heavy' and 'light' boats, which in effect assumes that we have two equal length boats of differing displacement.

The reason I say that the conventional 'light vs heavy' terminology makes little sense is that when it comes to selecting an ocean crossing cruiser, it seems more appropriate to make a determination of the amount of surplus carrying capacity that is needed for the voyages being contemplated and to pick a boat with that carrying capacity. In that case, you are making a decision between a shorter boat with that capacity vs a longer boat with that capacity. And if that is the comparason, within reason, and most other factors being similar, the longer boat will be more seaworthy, offer better motion comfort, be more fuel efficient, have better greater performance, and should be easier to handle. Initial and maintenance costs should be similar since most opperating costs are proportionate to displacement (dockage perhaps being the exception).

But if this is being discussed using the conventional 'light vs heavy' terminology, the most significant of these secondary influences is sail area and the ability to stand up that sail area relative to displacement and this point is much more complex. Generally speaking, relative to a 'heavy' boat, a 'light' boat will have more stability relative to its drag and displacement and so will also be able to stand up to more sail area relative to its drag and displacement. But, on an absolute scale, the heavy boat will have more sail area. Even if we assume that the these two boats have the same carrying capacity since they have the same water plane, and a similar SA/D, the 'light' boat would experience a larger decrease in its SA/D when fully loaded than the 'heavy' boat, and that would in theory mean that the 'heavy' boat would experience less loss of performance, especially in lighter conditions. I use the term 'in theory' because the lighter boat would generally have more stability relative to its displacement and drag, and so may actually be able to physically carry more sail area than its SA/D would predict, and may not suffer as badly as the number might seem to predict.

But all that aside, coming back to the agrument that someone presented a while back that (paraphrased) "a steel boat is heavier, and a heavier boat is better for ocean cruising, therefore a steel boat is better for ocean cruising", I respectfully suggest that is a mistaken argument in many ways.

I can perhaps explain why I think this is a bogus argument by comparing the example of two boats with the exact same displacement, hull and rig design, only one is constructed in steel and one with a substantially lighter hull and deck.

Whatever that difference in the hull and deck weight, that weight savings can be better used in mix of ways. Some of that weight can be used to add more ballast weight, increasing stability and slowing roll rates, some of that weight can be used to increase the surplus carrying carrying capacity of the boat, and some of that weight can use to strategically beef up the structure. And so collectively, it would be easy to argue that that the boat with the lighter hull and deck weight would actually make the better offshore cruiser than the steel boat.

Respectfully,
Jeff
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