Originally Posted by djmarchand
That is a somewhat bassackwards way of asking the question. Usually you would say "I have x hp engines at y rpm, how fast can I go?". But I can live with the way you asked it.
You will need roughly two 350 hp engines running at 3,000 rpm to push a 37', 20,000 lb (guess) slow planning hull (as defined by boatdiesel's prop calculator) to a top speed of 25 kts.
To hit 9 kts at wot you will need roughly two engines producing 50 hp each at 2,200 rpm.
I assume that you have much closer to a pair of 350 hp engines than 50 hp engines, so something is terribly wrong and it isn't the props. The props are roughly what you would expect for twin 350 hp engines.
So from the data you describe those big engines won't turn more than 2,200 rpm and that means you are probably getting huge amounts of black smoke out. Also you are really hurting those engines by running them that way.
My guess is that the bottom and props are really fouled, but I see that your pictures don't show that. Then if the prop and bottom are like the picture, then other possibilities are that the engine is severely restricted in its exhaust or its intake like a plugged exhaust mixer or an extremely fouled air cooler or a stuck turbo.
Thanks for looking up that info.
Pics are from 2014, and the barnacles are back in 2016.
Engines are OEM IH 392 with 260HP, gas not diesel.
Port engine will push to 3000 rpm, other to 2500 rpm and only 8.5 knot.
I am not too surprised at a 350 hp rating, but these engines factory were only 260 hp.
The calculator I linked suggested 500 hp for 18 knots.
Waterline length in feet: 35 feet
Beam at the waterline in feet: 12 feet
Hull draft in feet (excluding keel): 3 feet
Vessel weight in pounds: 20000 lbs
Engine Horsepower: 260 HP
Number of engines: 2
Total Engine Horsepower: 520 HP
Engine R.P.M. (max): 4000 RPM
Gear Ratio: 2.57:1
Shaft R.P.M. (max): 1556 RPM
Number of shaft bearings (per shaft): 1
Desired speed in Knots: 25 knots
This will calculate the maximum horsepower and torque available at the prop(s).
Total available horsepower at the engine(s): 520 HP
Total available torque ft/lbs at the engine(s): 683 ft/lbs
Horsepower loss of 3% per gearbox: - 15.6 HP
Horsepower loss of 1.5% per shaft bearing: - 7.8 HP
Total horsepower available at the propeller(s): 496.6 HP
Total torque ft/lbs available at the propeller(s): 1676 ft/lbs
Speed & Power Calculations
Basic displacement speed and horsepower required
Displacement hull speed (1.34 X sqrt of waterline length): 7.93 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 25 knots speed: 1244 HP
Estimated maximum speed with existing 520 horsepower:
This is the speed we will use for the propeller size. 18.36 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)
2 Blade 24.5 X 19.5
3 Blade 23.3 X 19.3
4 Blade 21.9 X 18.9
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.
Website and all contents copyright © Victoria Propeller Ltd. 2015, all rights reserved.