----- Original Message -----
Sent: Tuesday, December 21, 2004 2:57
PM
Subject: [PSUBS-MAILIST] Calculating Hull
Speeds
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