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| Originally Posted by CaptPKilbride I had always thought that the major difference between the two was in their resistance curves?
I percieved that a semi-displacement hull would have the sharp transom, allowing water to flow cleanly off the transom, and with more and more horsepower the hull would go faster to a certain point. This point being faster than a displacement hull with its theoretical hull speed limitation (1.44 x square root of LWL). Once the semei-displacement hull reached this "sweet Spot" in it's resistance curve, it takes exponentially more horsepower to produce speed gains given the same displacement.
And conversely, I thought that true planing hulls would produce more linear speed increases with increased horsepower (again given the same displacement)
Am I on the right track? Or was my perception off base.
By the way, Mr. Blount, I love what your company has done with the hull design and performance of the Rybovich sportfishing yachts !! |
Thanks for your kind words about the Rybovich sportfishing boats.
You are on the right path. Hulls designed in a speed range where the vessel's weight is, for the most part, supported by buoyancy will have the aft end of it's hull lines buttocks curving upward toward the water's surface at the transom. This minimizes hull resistance below hull speed. If you try to power a displacement hull to higher than hull speed, the resistance increases rapidly.
As semi-displacement, semi-planing or planing speeds become a design requirement, the aft hull buttock shape must become straight, possibly with some hook (only for hump speeds) in order for the hull to begin to rise up relative to the surface of the water (not squat).
Using the square root of LWL x a constant multiplier (you used 1.44) is a representative way to describe relative boat speed for displacement vessels, but other factors become important for describing comparative speeds of different size boats when dynamic lift provides a significant component of support for the boat's weight.
For a relative example, Destriero at 60 knots has V/(square root of LWL) = 4.28 which would be the same as a Rybowich with LWL = 60 ft making a speed of 33 knots. A Rybovich of this size operates near 40 knots which would be V/(square root of LWL) = 5.16. In a relative hydrodynamic dimensionless speed comparison the sportfishing boat at 40 knots requires higher speed hull form technology than did the much larger Destriero design operating at 60 knots.