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 and 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


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.





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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Originated 01-27-2010, Last updated 12-15-2014
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