Originally Posted by Hiracer
I never said steel
makes for good racing
boats. What I said is that in a cruising context steel
need not be meaningful slower than the next cruiser.
You did not twig to the context of my comment. If what you say is correct that increased displacement
can be made up by sail area then one has to wonder why open class race
boats are not built of nice strong steel (or heavily built of something else in an extravagent manner to ensure they will never break) and the weight penalty made up just by adding sail. That is not done and that for good reason.
It would be tiresome for me to get into a lengthy discussion of the reasons why this is so but anyone not knowing but interested should look out descriptions of Froude numbers, the effect of displacement
on wave making resistance at slow speeds and the way "displacement speed" (aka "hull speed") is decreased as boat
displacement increases. That will get one to finding that wave making resistance (which to windward is likely around 35-40% of total resistance) increases approximately linearly with displacement (so for two boats the same except for one being heavier than the other by 10%, total resistance (aka "drag") will increase by around 3.5-4%) and "displacement speed" is significantly reduced for a heavier boat
(as it cannot reach the same Froude number as lighter ones of same hull
If you take two sail boat hulls of same design and all else being equal (eg surface roughness on underwater surfaces the same) except one is heavier than the other then the heavier one will be slower at all speeds. For two such hulls the main difference will be in resistance in wave making - as the boat proceeds energy is taken away by these waves. Without even any analysis I think it is intuitive that a heavier displacement boat will "displace aside" more water
than a lighter one and so have more energy taken away from it in those waves.
I suspect that for vessels much under 40 foot it is possible to built a strong composite boat, for example, considerably less than half the weight, excluding ballast, of a best build in steel. I know of composite boats (one done multiple Cape Horns) over 50 foot that are lighter than our light for steel 40 foot steel boat (that comparison again less ballast, but including cruising fitout such as freezer
, good accomodation, etc). I leave out ballast in these as ballast, as opposed to the weight of the rest of the boat, is offsettable against sail area. It is easy to see that a small steel boat built as light as possible may suffer a resistance penalty of more than 30-40% over a lightly, but still strongly, built composite one if both carry a sail area appropriate to their ballast ie the small steel boat will be a "slug" compared to the composite one.
Now as you say, you can just add more sail but that will never increase the "displacement speed" as that is inversely proportional to displacement and the energy hump is too high to get over by adding sail for cruising boat designs. But a further fact is that if one can add sail to the heavy boat one to get speed up at slower speeds then one can add the same sail to the light one too and it will be faster again. In fact one can put more extra sail on the lighter boat because without ever giving any weight advantage to the heavier boat one can add displacement in the lighter boat's keel
as ballast enabling it to stand up to more sail than the heavier hulled boat can.
So the argument of adding sail to the heavy boat to beat weight is a spurious one and furthermore no matter what you do once hull speed
is approached the heavier boat, of the same hull
form, will always be slower no matter how much sail you put on it. The heavier boat can only be faster than a lighter one if the lighter one is disadvantaged in some way such as by less efficient hull lines, or smaller rig than such a boat could otherwise carry, or in less efficient crewing
Adding sail to defeat heavier displacement also introduces the disadvantages of carrying more sail than would otherwise be necessary on a cruising boat, especially one intended for blue ocean cruising.
There are though situations where the performance of two similar boats that are only differentiated by their displacements may wander away from the above - one situation that comes to mind is in heavy pitching conditions where the heavier boat will have a larger longitudinal mass moment of inertia than the lighter one and so the two boats will have different resonant frequencies in pitching. This may mean that in some pitching conditions the heavier boat gets an advantage and in others the lighter boat gets an advantage.
There are crew factors too, it may turn out that the distribution of mass in the heavier boat (by way of increasing longitudinal and roll mass moments of inertia) makes for lower accelerations in pitch
and roll than the lighter one. That is especially important in smaller vessels in heavy weather
and so may mean the heavier boat is faster in heavy conditions but that only from increased crew performance not from inherent boat speed.
But, overall, the lighter boat will always be faster and we know that as fact without even boring ourselves about all the above just by being aware of the quest for light displacement in open class race
boats, and where whatever discretionary displacement there is in the weight budget
goes into ballast in the keel
or crew on the rail.
Note I am not making any case that slow is bad for a cruising boat. I am just refuting the claim that my contention that because of the impossibility of building steel sail boats much under 40 foot that are not very heavy compared to what is possible in other constructions they will always be slugs in comparison, is wrong. I know from the great care that went into keeping the structure of our own 40 foot steel boat as light as reasonably possible that even at 40 foot you will come out with a boat that can only be described as very heavy compared to a composite one, for example, that equal care in weight has been taken.
In the end we mostly all love our boats but that should not make us blind to their deficiencies.
Off to my glass of wine now and no more from me on this refutation