Quote:
Originally Posted by MidLandOne 
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's just about what I said, only with fewer words. I mentioned the ease of escaping hull speed, but you are right that hull speed for light vessels is slightly higher than for heavy boats.
But, if you compare hull speed between two equally sized vessels, one with a D/L ratio of 180 (about as low as you can reasonably get in a cruiser under 40') compared to one with a D/L of 300, you will find the difference in hull speed to be essentially meaningless. It's a small fraction of a knot.
Quote:
Originally Posted by MidLandOne 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. |
Hull speed is hull speed. Light boats have a higher hull speeds, but the increase in hull speed is so slight that in the cruising context it's meaningless. Heavy boats need big sails to reach hull speed. Big deal.
As I said the first time, light boats accelerate faster and can exceed hull speed easier. Both of these performance attributes are important in racing. Neither is a major benefit in the cruising context.
And as I previously stated and you have pointed ignored, displacement is a major aid to carrying a heavy loads, something cruisers should very much be concerned about, certainly more concerned that the incredibly small incremental increases in speed you are referencing.
Trying to argue that what's good for racing is good for cruising doesn't strike me as the most effective argument. Racers are not load bearing vessels. If you look at the people actually cruising with sub-40' boats, you will be hard pressed to find many such boats that are akin to racers, for the simple reason that sub-40' racing sailboats can't handle the load. Most cruisers under 40' are mid-weight boats. A properly designed steel boat can keep with those boats just fine, as those of us with first hand experience can attest.
The real blow to your argument, which I will deliver at a later time, is to take two boats the same size, same SA/D ratio, but different displacements. After you load them with 4,000 pounds of gear, the lighter boat will have a resulting SA/D ratio that is significantly lower than the heavy boat. Meaning, the lighter boat will be slower, as a matter of physics, simply because the lighter boat has less power from its sails compared to the heavy boat. The addition of the cruising load adversely decreases SA/D much more for the light boat than the heavy boat. Which goes a long way to explain why you don't actually see many light sub-40' boats out there cruising. After you load them up, they are too slow.