rondds
Well-Known Member
- Oct 3, 2006
- 8,859
- Boat Info
- 2001 380DA
- Engines
- Merc 8.1s (2008)...Hurth ZF 63 V-drives...WB 7.0 BCGD (2013), Garmin 8208 & 740 MFDs, GMR 24xHD dome
So I'm reading this article in Power&Motoryacht on a new Island Packet Trawler and the author presents this formula for HULL SPEED, which is 1.34 x the square root of LWL. He then says that, by calculation, hull speed for this boat (with a LWL of 34.75') should be 7.9 knots. He then marvels at the fact that he clocks a WOT speed of almost 11 knots, stating how the amazing hull design outsmarts physics by creating a 38% increase in speed over the theoretical 7.9 knots. So then I go to the "Numbers" page and notice that at 5.9 knots, this boat makes 34MPG (yes that is MILES PER GALLON), at 7.3 knots, it makes 16 MPG, at 11 knots (WOT), it makes 2.6MPG. Still impressive; but, by definition, isn't HULL SPEED the most efficient speed for a displacement hull? And if my understanding of this is correct, isn't comparing HULL SPEED to WOT like comparing apples and oranges?
As a follow up question, does the formula only apply to vessels with a displacement hull? OR, does every hull design, even a deep-vee, when not on plane, exhibit characteristics of a displacement hull?
If you plug my boat's numbers into this formula, hull speed for me is 7.8 knots. I can assure you that at that speed, I am not at my most efficient. My definition of HULL SPEED (going off of Floscan and GPS readings) would be more like 5 knots. Is that because I do not have a displacement hull? Or is the formula just for general guidelines purposes? Please don't make me go find my copy of Chapmans.
As a follow up question, does the formula only apply to vessels with a displacement hull? OR, does every hull design, even a deep-vee, when not on plane, exhibit characteristics of a displacement hull?
If you plug my boat's numbers into this formula, hull speed for me is 7.8 knots. I can assure you that at that speed, I am not at my most efficient. My definition of HULL SPEED (going off of Floscan and GPS readings) would be more like 5 knots. Is that because I do not have a displacement hull? Or is the formula just for general guidelines purposes? Please don't make me go find my copy of Chapmans.
Last edited: