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Shipmates,
The wires are pretty slow lately, so perhaps I will toss in one of those Boating 101 facts not everyone may know. There are two basic hull types: planing hulls that can ride over the water, and displacement hulls that cut through the water. If you have a planing hull, more power can get you more speed. But if you have a displacement hull, like submarines do, your speed (on the surface) is limited by your length. Once you reach the maximum speed for the length of your hull, adding power does not increase speed. As a rule of thumb, anything over 4 HP per ton is a waste of resources. I believe German VII C's had 4 HP/ton.
Top speed is 1.34 times the square root of the waterline (not the part of the boat above the water in the bow or stern). So a boat with a 25-foot waterline and a 4 HP motor will max out at 6.7 knots on the surface. Double the HP to 8, and the maximum speed on the surface is still 6.7 knots. That is why racing shells are so long and thin...to get the extra speed.
I have seen folks over the years make extremely optimistic estimates as to how fast their boat will go on the surface. Even with the fine lines of a WW II era German U-Boat (which few psubs ever have), the speed is limited by the length. If you want to transit back and fourth to your dive site at a stunning 10 knots, you will need a 55 foot boat. Fortunately, most of us are not in a rush.
Fact for the day.
Doug Farrow
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