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Rules of Thumb



Rules of Thumb

Rules of Thumb

* 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.

* 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.  However if you discharge batteries below their recommended depth of discharge or DOD the you will damage the battery.  Lead acid batteries commonly have a DOD of 50% and gel-cells are 80% so if you are using 55 amp from a 55 amp at 20 hours battery, then don't do that for more than 10 hours if the DOD is 50%.  You can pull more than 55 amps from that same battery but the available amps drop significantly and you will reach the recommended DOD much faster. 

Trolling Motor Amp Hours
Calculate the Ampere draw based on the pound thrust of the motor:
(Pound Thrust / Motor Voltage x 12 = Ampere Draw)
- 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  

Typical AMP loads:

Engineering Stuff

Moment = force X distance ( Newton X meter) ( for the US systems (Lbs X foot)

force = mass X acceleration

Weight = mass X gravity.  In the US systems Weight = Lbs (force)

Maximum shear due to bending on the cross-member = (Moment X Moment of inertia of the cross-member ) / extreme distance from the neutral axis of the cross-member.