Outboard Motor Shaft Length & HP to Boat Calculating

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
bracket’s 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 & a deep Vee bottomed boat.


As you will see the 15″ standard shaft length is only an average

Height Measurement:

Recommended Shaft Length:

14” – 17.25”

15” Shaft (Short)

17.25” – 19.5”

Varies by application

19.5” – 22.5”

20” Shaft (Long)

22.5” – 27”

25” Shaft (Extra-Long)

 Copied & slightly modified from Nissan internet info page

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)

The following Graph is for informational purposes only !!

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