Displacement vs Length
(The long answer between spells of shoveling snow)
This is not a simple set of questions to answer. I think your first question ("If a truer measure of a boats size is its displacement and not its length why do so many talk about the length of boat that someone could handle?") perhaps does not have only one right answer but I can venture a couple possible explanations. In the past, say before the 1960''s, when the range of choices in boats were comparatively similar in design and weight, it made some sense to talk about the size of a boat by its length and to assume that displacement would be somewhat proportionate to the length. That is less of a reasonable assumption today as there can be huge variations in displacement between boats of similar lengths on deck. If I would have to venture a guess at the answer to this fist question I would therefore say I think that sailors who have not considered this analytically or who don''t have the experience to understand this issue habitually refer to size (and therefore ease or difficulty in handling a boat) as being synonymous with length.
As to the second part of your question, ("Given two boats of equal length and similar rigging one being steel with heavy displacement and one FG would someone who could handle the FG boat not necessarily be able to handle the steel boat?") the answer is that all things being proportionately equal, the lighter boat should be easier to handle in all conditions and all points of sail and if the lighter boat was at the limit of what that individual could physically handle the heavier boat would be beyond the limits of what that person could safely handle.
I will try to explain this opinion by using the two examples that you have cited of a heavier displacement steel hull vs. a lighter displacement fiberglass hull. If we took two boats of equal strength, with similar rigs and hull forms, depending in the materials and methods of construction, the hull of the fiberglass boat would typically be 1/3 to 1/2 the weight of the equal strength steel hull. Since the hull of a boat typically represents roughly 1/4 of the weight of the fully loaded boat, so if that was all the only difference in weight between a fiberglass and steel boat the steel boat would only weigh perhaps 8.2% to 12.5% more than the glass boat, which is not all of that big a deal. But there is more to this than these numbers imply.
To begin with, loadings on the various parts of a boat vary pretty widely. From decades of testing, designers know that the highest possible loads come at the bow and are most risky at or near the waterline in a collision with fixed or nearly fixed object of small contact area. The lowest loaded areas with the least risk of down flooding are the topsides in the aft quarter. The recommended typical loads used for design purposes between these two areas stipulate a bow loading that is 4 times the loading on the topsides aft. In small steel boats, changes in plate thickness are not really recommended but in fiberglass it is easy to vary the laminate specification in different areas of the boat. This can result in as much as a 20 % reduction in weight over a hull designed to a constant set of loadings taking our hull weight difference into a range of roughly 10% to 15%.
Even that is not all that large a difference in weight. But weight breeds weight: adding weight has a way of adding more a lot more weight. So in other words, 15% increase in displacement typically ripples through the whole design. If a boat is 15% heavier it requires 10% more sail area to maintain an equal SA/D. 10% more sail area is a big increase. It means a taller mast with higher loadings. The taller mast is usually enough to go to a larger mast section. The combination of a taller mast and higher loadings often means stepping up one rigging wire size. So now you have a boat with a larger sail area and a higher center of effort meaning more heeling moment, and with its heavier mast and rigging a higher center of gravity. Combined this means a very significant increase in ballast weight. At this point the process becomes reiterative. More ballast means more sail area and more sail area means more ballast until you reach an equilibrium with a steel boat that has now picked up another 10% to 15% in rigging and ballast weight and which is now 20% to 30% heavier than it''s a fiberglass boat of equivalent strength carrying roughly 12% to 20% more sail area.
But it does not stop here. If we are to keep things relatively equal, the heavier boat would require 20% to 30% more horse power which in the worst case means a bump up in engine size or in the best case, a more powerful propeller with its greater drag which when added to the greater drag of the higher displacement hull will result in a significant increase in fuel consumption. If we are going to keep thing equal the fuel tank size will go up. And with the weight of the increased engine size and fuel tankage, we would go back through another reiterative process of more sail area and more ballast so the weight increase means more weight and more sail area than would implied by simply adding in the sheer weight of the fuel and larger engine.
Here is where we really get to the rub. We now have a boat that has much higher rigging loads, much higher ballast loads, and with its increased loads, much higher impact loads than the lighter fiberglass boat. In theory we should go though another reiterative process to increase the strength of the various components to withstand these greater loads. This would again trigger a reevaluation of sail area, rigging, ballast, engine, propeller, and rudder size, fuel tank capacity, and scantlings or more to the point a further increase in weight.
Which finally gets us to the answer to your second question. In the end, we end up with a steel boat that weighs somewhere between 30% to 50% more than the weight of a fiberglass boat capable of withstanding the same speed impacts and which carries perhaps as much as 20% to 30% more sail area. (Of course in designing both steel and glass boats all kinds of compromises are made so the "all things being equal" model rarely exists) This increased sail area, displacement, and ballast means substantial increases in line loads and rigging loads. It means a boat, which is harder to fend off or to manhandle. It potentially means greater steering loads and so on. Where this really shows up is in heavy weather sailing. In heavy air you need to be able to maintain enough speed to maintain control and windward ability to claw off a lee shore. The more easily driven lighter boat can maintain the necessary speed and windward ability with a substantially smaller amount of sail area as compared to the sail area required to overcome the cumulatively higher drag of the heavier boat. It is here, when the loads are at their highest, that people cannot physically handle the heavier boat while they might still be capable of handling the lighter boat. Even if they can handle the loads while they are fresh, the higher loads of the heavier boat will grind them down more quickly.
Regards,
Jeff
