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Outboard Motor Shaft Length:
How do I measure?
If mounting directly to a boat transom:
Measure
from the top of the transom down to the bottom of the hull
or keel (lowest point). Measure the area
of the transom directly where the engine will be mounted.
This measurement is primarily designed for single motor propulsion.
A deep Vee hull will result in a different measurement than a flatter bottom hull. As you will see in the chart
below there is nothing set in stone.
Each motor (depending on the hull design) may have to be fitted (raised or
lowered) for optimum performance.
Tip: If you are using a auxiliary motor on a shallow Vee hull
where the main power is a jet in a river where you may be fishing in shallower and changing water, then the shortest shaft (even with
the anti-cavitation plate slightly above the hull bottom) may save the prop from a bad case of rock rash. See diagram below.
If using an auxiliary mounting bracket: With the bracket in the down
position, measure from the top of the brackets mounting surface down to the bottom of
the hull or keel (lowest point
where engine will be mounted).
Tip: For most auxiliary applications, it is possible to use a longer shaft
length than what is called for, provided there is sufficient clearance when the engine is
in the up position. This can be
especially useful in heavy seas.
If measuring an existing outboard: Measure from the inside top
of mounting bracket down to anti-cavitation plate above propeller (this is the largest plate/fin
directly above the
propeller). See diagram below.
You will notice a variable distance in the listings below. Take into consideration the degree of Vee in the bottom Yes, the anti-cavitation plate is where you measure to, but the actual usable numbers may well vary between a flat bottomed boat and a deep Vee bottomed boat.
| As you will see the 15" standard shaft length is only an average | ||||||||||
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Copied & slightly modified from Nissan internet info page
One method of making this height fit, is to add a Jack Plate as seen in the photo below
| Here someone cut down the transom 2 1/2", so adding a Jack Plate was a simple method of getting a correct adjustment without added expense. |
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Formula for
calculating boat horsepower
for small boat owners
There is a very simple formula for calculating the maximum safe horsepower your bat can handle. The Outboard Industry Association (OIA) has recommended this formula. The following information was in a magazine dated 1965.
(1) Multiply the overall length of your boat (in feet) by it's widest beam at the stern (excluding any Fins and or Shear)
(2) Locate that value on the left hand vertical column of the chart below.
(3) Move horizontally across the chart until you hit the graph curve.
***Note: if your numbers fall between the readings, take the one to the right
-the higher value. ***
Overall Length (in Feet) X Overall Stern Width (in Feet)
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The following Graph is for informational purposes only !! |
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