Fryewe, it's not 13X9 it is 13X11.
Thanks everyone for the input.
I stumbled upon this https://www.vicprop.com/displacement...tion=calculate
which tells me that to replace 15X9 with 13" diameter it would need to have a pitch of between 12 and 13. I am close with 11 (just as I thought) but not exactly. It would be OK to experiment
if I just be using the boat for days sails
etc. But since I am planning to take her to FL I'd rather be exact with this. So I will update any new info or calculations.
Waterline length in feet: 22.75 feet
Beam at the waterline in feet: 8 feet
in feet (excluding keel): 1.5 feet
Vessel weight in pounds: 7450 lbs
Engine Horsepower: 14 HP
Number of engines: 1
Total Engine Horsepower: 14 HP
Engine R.P.M. (max): 3000 RPM
Shaft R.P.M. (max): 1402 RPM
Number of shaft bearings (per shaft): 2
Desired speed in Knots: 6.5 knots
This will calculate the maximum horsepower and torque available at the prop(s).
Total available horsepower at the engine(s): 14 HP
Total available torque ft/lbs at the engine(s): 25 ft/lbs
Horsepower loss of 3% per gearbox
: - 0.4 HP
Horsepower loss of 1.5% per shaft bearing: - 0.4 HP
Total horsepower available at the propeller(s): 13.2 HP
Total torque ft/lbs available at the propeller(s): 49 ft/lbs
Speed & Power Calculations
speed and horsepower required
Displacement hull speed
(1.34 X sqrt of waterline length): 6.39 Knots
Minimum horsepower required at propeller(s) for Hull speed
: 16.3 HP
Calculations based on desired speed and available HP
HP required at propeller(s) for desired 6.5 knots speed: 16 HP
Estimated maximum speed with existing 14 horsepower:
This is the speed we will use for the propeller size. 6.12 Knots
At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM. This gives the engine some reserve power to allow for variable loading in the vessel.
Number of blades Diameter (inches) Pitch (inches)
14.5 X 8.7
13.8 X 8.6
4 Blade 13.0 X 8.4
The propeller sizes shown above do not contain calculations for cavitation or blade loading.
If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule
of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.