Rules of Thumb

Rules of Thumb

Engine
* When sizing engines for "displacement" hulls, a rule of thumb formula is 1.5 x (per ton), to arrive at HP required to move a vessel to theoretical hull speed. Therefore for a 3k displacement, 1.5 ton x 1.5 = 2.25 hp.

Water Pressure
* 1 atm (atmosphere)  is about 15 psi   (pounds per square inch)
* Every 33 feet of water above equals 1 atm.

Displacement
* Sea water weighs about 64 pounds per cubic foot.
* Air volume decreases in proportion to the increase in atm.  From the surface to 33 feet of from 1 atm to 2 atm's the air volume is cut in half, so that 4 cubic feet at the surface becomes 2 cubic feet at 33 feet below the surface.   From 33 feet to 66 feet it is cut in half again and becomes 1 cubic feet.  Therefore changes in displacement for soft ballast is much more dramatic closer to the surface.

Amp Hours
The Amp Hour rating tells you how much amperage is available when discharged evenly over a 20 hour period. The amp hour rating is cumulative, so in order to know how many constant amps the battery will output for 20 hours, you have to divide the amp hour rating by 20. Example: If a battery has an amp hour rating of 75, dividing by 20 = 3.75. Such a battery can carry a 3.75 amp load for 20 hours before dropping to 10.5 volts. (10.5 volts is the fully discharged level, at which point the battery needs to be recharged.) A battery with an amp hour rating of 55 will carry a 2.75 amp load for 20 hours before dropping to 10.5 volts.

Trolling Motor Amp Hours
Calculate the Ampere draw based on the pound thrust of the motor:
(Pound Thrust / Motor Voltage x 12 = Ampere Draw)
Example:
- 55 Pound Thrust / 12 volt x 12 = 55 Amp Draw
- 55 Pound Thrust / 24 volt x 12 = 27.5 Amp Draw
- 55 Pound Thrust / 36 volt x 12 = 18.3 Amp Draw
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Typical AMP loads: