Fishboat
09-12-2003, 09:04 PM
I read an article in the recent Good Old Boat magazine where Ted Brewer floated a formula to describe a boat's comfort at sea. I've seen something similar in a formula for "heave" where heave = displacement/water plane area.
The problem here is obtaining/determining the water plane area for a number of boats that you're comparing & considering for purchase. Brewer stated that the WPA is roughly equivalent to [0.65 * (length @ water line, LWL) * (beam @ water line, BWL)], where the beam(B) is used to approximate the BWL. To account for hull overhang the LWL is further broken down into weighted components of LOA + LWL. He further weights the beam (B) in the final formula to account for wider boats increased form stability & associated acceleration.
Brewer's "Comfort Ratio" formula in its final from becomes:
Comfort Ratio = Displacement(lbs)/ [0.65 * (0.3LOA + 0.7LWL) * B^1.333]
Where higher CR's are more desirable.
I'm familiar with a number of the typical ratios that describe hulls and their performance. They can help in sorting through various boats, but this comfort ratio seems to add quite a bit of clarity to the sorting task. In my rather simple mind the CR has pretty much replaced the D/L ratio as the CR is not only considers the D/L, but also the overall comfort(motion) of the boat in a seaway.
Brewer gave a number of example boats & their CRs in the article. I have calculated out another 30-40 CRs for other boats & it's interesting to see how some boats have quite high D/L ratios (300+) while actually having (according to their CRs) quite uncomfortable motion.
From what I've seen there are a good number of folks here that are quite skilled in the design area. Is the 'comfort ratio' as useful as it seems to be?
The problem here is obtaining/determining the water plane area for a number of boats that you're comparing & considering for purchase. Brewer stated that the WPA is roughly equivalent to [0.65 * (length @ water line, LWL) * (beam @ water line, BWL)], where the beam(B) is used to approximate the BWL. To account for hull overhang the LWL is further broken down into weighted components of LOA + LWL. He further weights the beam (B) in the final formula to account for wider boats increased form stability & associated acceleration.
Brewer's "Comfort Ratio" formula in its final from becomes:
Comfort Ratio = Displacement(lbs)/ [0.65 * (0.3LOA + 0.7LWL) * B^1.333]
Where higher CR's are more desirable.
I'm familiar with a number of the typical ratios that describe hulls and their performance. They can help in sorting through various boats, but this comfort ratio seems to add quite a bit of clarity to the sorting task. In my rather simple mind the CR has pretty much replaced the D/L ratio as the CR is not only considers the D/L, but also the overall comfort(motion) of the boat in a seaway.
Brewer gave a number of example boats & their CRs in the article. I have calculated out another 30-40 CRs for other boats & it's interesting to see how some boats have quite high D/L ratios (300+) while actually having (according to their CRs) quite uncomfortable motion.
From what I've seen there are a good number of folks here that are quite skilled in the design area. Is the 'comfort ratio' as useful as it seems to be?