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.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 10-10-2013 at 09:58 AM.