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Old 12-20-2012, 09:58 PM   #157
Art
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City: SF Bay Area
Country: USA
Vessel Model: Tollycraft 34' Tri Cabin
Join Date: Feb 2011
Posts: 8,917
Keeping all things equal; at an established/specific “through water speed” (10 knots at 2,200 rpm in the following examples) the friction of a boat’s hull surface against the water touching it is only equal to the surface area of said hull touching the water, nothing more and nothing less. In following examples a boat is at 90 degree angle to the opposite direction 5 knot currents, water surface is considered always flat, and there is no wind.

1. Traveling Against 5 Knot Current: If at 2,200 rpm the boat’s “through water speed” is 10 knots then that boat is traveling at 5 knots “over land speed”
2. No Current: If at 2,200 rpm the boat’s “through water speed” is 10 knots then that boat is traveling at 10 knots “over land speed”
3. Traveling With 5 Knot Current: If at 2,200 rpm the boat’s “through water speed” is 10 knots then that boat is traveling at 15 knots “over land speed”

Friction component of hull against water alters not and therefore does not change the stated 10 knot “through water speed” no matter what direction, or no direction, the water mass may be accomplishing. Only the “over land speed” will alter as per the current’s direction, or no direction.
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