i have a beneteau Oceanis400 1993 with a Perkins prima 50 and 17in max prop (3 blade feathering). At 2800 rpm, which is high, I can only motor at 5.8knots in flat water. I've adjusted the pitch to the highest recommended by the manufacturer. The engine gets to 2800 easily. Any ideas?
Propellor, speed
- Thread starter dpearce
- Start date
Probably 3000 rpms. The manufacturer recommends 2200 for long engine life. At that rate I'd only get to 5knots. I'm wondering if I need a new larger prop or engine rebuild. It has 3000 hrs on it
DP
VicProp has a handy calculator to give you some raw data about prop size. It is a place to start. I ran the numbers I estimated for your boat and got the following data printout.
Here is the link...
https://www.vicprop.com/displacement_size.php?action=calculate
Data Input
Waterline length in feet: 40 feet
Beam at the waterline in feet: 12.83 feet
Hull draft in feet (excluding keel): 5.42 feet
Vessel weight in pounds: 20000 lbs
Engine Horsepower: 50 HP
Number of engines: 1
Total Engine Horsepower: 50 HP
Engine R.P.M. (max): 3000 RPM
Gear Ratio: 2.42:1
Shaft R.P.M. (max): 1240 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): 50 HP
Total available torque ft/lbs at the engine(s): 88 ft/lbs
Horsepower loss of 3% per gearbox: - 1.5 HP
Horsepower loss of 1.5% per shaft bearing: - 0.8 HP
Total horsepower available at the propeller(s): 47.8 HP
Total torque ft/lbs available at the propeller(s): 202 ft/lbs
Speed & Power Calculations
Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 8.47 Knots
Minimum horsepower required at propeller(s) for Hull speed: 43.6 HP
Calculations based on desired speed and available HP
HP required at propeller(s) for desired 8 knots speed: 33 HP
Estimated maximum speed with existing 50 horsepower:
This is the speed we will use for the propeller size. 8.98 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 20.2 X 13.4
3 Blade 19.2 X 13.3
4 Blade 18.1 X 13.0
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.
VicProp has a handy calculator to give you some raw data about prop size. It is a place to start. I ran the numbers I estimated for your boat and got the following data printout.
Here is the link...
https://www.vicprop.com/displacement_size.php?action=calculate
Data Input
Waterline length in feet: 40 feet
Beam at the waterline in feet: 12.83 feet
Hull draft in feet (excluding keel): 5.42 feet
Vessel weight in pounds: 20000 lbs
Engine Horsepower: 50 HP
Number of engines: 1
Total Engine Horsepower: 50 HP
Engine R.P.M. (max): 3000 RPM
Gear Ratio: 2.42:1
Shaft R.P.M. (max): 1240 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): 50 HP
Total available torque ft/lbs at the engine(s): 88 ft/lbs
Horsepower loss of 3% per gearbox: - 1.5 HP
Horsepower loss of 1.5% per shaft bearing: - 0.8 HP
Total horsepower available at the propeller(s): 47.8 HP
Total torque ft/lbs available at the propeller(s): 202 ft/lbs
Speed & Power Calculations
Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 8.47 Knots
Minimum horsepower required at propeller(s) for Hull speed: 43.6 HP
Calculations based on desired speed and available HP
HP required at propeller(s) for desired 8 knots speed: 33 HP
Estimated maximum speed with existing 50 horsepower:
This is the speed we will use for the propeller size. 8.98 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 20.2 X 13.4
3 Blade 19.2 X 13.3
4 Blade 18.1 X 13.0
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.
The motor is turning fast as it can so the issue is the prop isn't turning that rotation into thrust, or you have huge drag. Is the bottom clean? If it is you need more prop.
Yes, the engine is rated for 3,000 RPM so easily reaching 2,800 is not that far off the mark. You could have your tachometer verified to insure a correct reading. The other thing is the method being used to measure speed. A GPS measures speed over the bottom while a knot meter measure speed over water. A GPS reading is affected by wind and currents direction and intensity. To get closer to actual speed 2 way runs 180 degrees apart to average out the readings. The knot meter gives the boat speed you may be looking for but these gauges foul easily and readings can be unreliable. Another factor is excessive drag, what is the condition of the bottom? A freshly cleaned or painted bottom can give a surge of speed to any boat. The prop could be a little over pitched but verify RPM readings and actual speed over the water before making any changes.