Paul,
Wow!! that is the clearance? Here is the formula information I used at random for you boat. Errors may well be in attendance as I am estimating much.
Will keep you and the forum posted. My biggest concern is the requirement of raising the cabin floor 1 1/2 inches to accommodate the height. With duplicate boat having the bigger Perkins anticipated installing the floor must have been heighten at construction. This is said as the built in fiberglass exhaust tube installed is only the 2" allocated for the smaller present engine in our boat. Fortunately there is substantial headroom to allow the floor rebuild. We will have to run the exhaust out the side due to lack of being able to access the bulkhead to install a 3 inch exhaust required. Always challenges to a engine change as you may find with the Yanmar conversion. Good luck back to you.
Al
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Data Input
Waterline length in feet: 30 feet
Beam at the waterline in feet: 11 feet
Hull draft in feet (excluding keel): 3.6 feet
Vessel weight in pounds: 14500 lbs
Engine Horsepower: 85 HP
Number of engines: 1
Total Engine Horsepower: 85 HP
Engine R.P.M. (max): 3000 RPM
Gear Ratio: 2.5:1
Shaft R.P.M. (max): 1200 RPM
Number of shaft bearings (per shaft): 1
Desired speed in Knots: 8 knots
Horsepower Calculations
This will calculate the maximum horsepower and torque available at the prop(s).
Total available horsepower at the engine(s): 85 HP
Total available torque ft/lbs at the engine(s): 149 ft/lbs
Horsepower loss of 3% per gearbox: - 2.5 HP
Horsepower loss of 1.5% per shaft bearing: - 1.3 HP
Total horsepower available at the propeller(s): 81.2 HP
Total torque ft/lbs available at the propeller(s): 355 ft/lbs
Speed & Power Calculations
Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 7.34 Knots
Minimum horsepower required at propeller(s) for Hull speed: 31.6 HP
Calculations based on desired speed and available HP
HP required at propeller(s) for desired 8 knots speed: 37 HP
Estimated maximum speed with existing 85 horsepower:
This is the speed we will use for the propeller size. 10.34 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.
Propeller Size
Number of blades Diameter (inches) Pitch (inches)
2 Blade 22.9 X 15.6
3 Blade 21.8 X 15.5
4 Blade 20.5 X 15.2
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.
