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Old 12 October 2013, 15:09   #1
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merc,mariner,tohatsu 20 4 stroke prop size

Hi
I have been considering a prop size change, my mariner 20 4 stroke came with a 9.25 x 10 prop which delivers 20 mph on my 4.0m SIB exactly what mercury quotes for this prop,has any tried an 8.25 x 11 or 11.5 with this set up? mercury state a possible 26mph with an 11.5?
GOOD IDEA OR A WASTE OF MONEY???
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Old 12 October 2013, 22:08   #2
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It will only go as fast as the horsepower will allow. The power must allow it to operate in the RPM range specified for the motor.

Just for giggles, try the following formula for theoretical speed. 10-13% slip covers most hulls:

RPM x lower unit ratio x prop pitch divided by 1056 = theoretical speed in MPH less 10-13% slip. My rig has a 11% loss of speed due to slip for instance.

If you assume ample power, you will find the formula pretty dependable. However, the power of the motor will dictate just how fast the boat will go. If you select too much pitch, the motor will not reach proper RPM and will be developing less than rated hp. You need at least one test to see what the motor is capable of with your present prop.

I would not buy another prop without seeing what the present one is doing.
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Old 12 October 2013, 23:30   #3
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You must install an induction tach and check max wot rpm readings with you alone wigth current prop before playing with different prop pitch to start with. Wild guessing is costly and can ruin your engine in a very short term.

Happy Boating
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Old 13 October 2013, 06:15   #4
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I would add that your present prop seems to be pretty close to ideal, but only a tach would prove that. As Loco points out, don't waste money until you know the facts.
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Old 13 October 2013, 13:07   #5
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Hi went out today with an induction tacho and in choppy water reached 6020rpm and 26 mph peak on gps and running against the tide 23mph , quite pleased with that for a honwave 4 ,
I think the rev limiter is set at 6000 as seemed to sit solid at 6020rpm
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Old 13 October 2013, 13:13   #6
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Ps tacho from eBay is brilliant at 12 delivered , does run hours and service reminder too,
Steel developments do a 9.25x11 bit less pitch than the mercury 11.5
WHAT DO YOU GUYS THINK??? :-)
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Old 13 October 2013, 14:29   #7
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If you are turning 6000+ RPM, you need a prop with more pitch, not less.
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Old 13 October 2013, 14:46   #8
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hi
i agree i currently have a 9.5 x 10
mercury go from 10p to 11.5p
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Old 13 October 2013, 17:10   #9
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The rule of thumb is a change of 150 to 200 RPM per inch change when changing pitch. A 1 1/2 inch change from 10 inch to 11.5 inch would drop your RPM from 225 to 300 assuming the same efficiency in both props. What is the operating range published for your motor?

The closer you come to the middle of the range, the more HP the motor will develop. The 11 1/2 inch looks doable, as many new motor specs are for a 1,000 RPM range.
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Old 13 October 2013, 17:30   #10
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I will look up the rpm range
I wonder if the rpm is hitting the limiter therefore there may be some more slack to be had
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