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. 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 gelcells 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)
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
www.optimabatteries.com
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 crossmember = (Moment X Moment
of inertia of the crossmember ) / extreme distance from the neutral
axis of the crossmember.
