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Old 01 April 2014, 16:09   #1
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Prop slip numbers are impossible

I have a 2 stroke 90hp Tohatsu purchased brand new last year. The manual states it has a gear reduction ratio of 13:26.

The boat is a 5.6m aluminium hull rib which is pretty light.

The propellor is a Patriot 17p.

My gps speedo shows 39 knots or 45 mph and the tacho is showing about 5050rpm.

According to the prop slip calculators I have minus 11% slip which is obviously an error.

Logic says that either the tacho is out, the gps is out (confirmed with phone gps), the gear ratio is wrong or the prop pitch isn't 17p.

Anything I missed? Any suggestions on where to start?
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Old 01 April 2014, 16:31   #2
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ratio is wrong 13:26 that is a step up ratio not a reduction ratio, hence your -prop slip figure

If the figures were just the wrong way round it would just be a 2:1 ratio & expressed as such

where did you get those numbers from?
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Old 01 April 2014, 16:59   #3
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The throttle range for a Tohatsu 90hp is 5,000-5,500rpm at wide open throttle (WOT). If you're hitting a maximum speed of 39 knots (plenty in my book if boat is as light as you say) then you may be able to squeeze a little more speed out of it.
http://www.tohatsu.co.jp/en/boat/pro...0A2_130411.pdf

Turn to page 87 of the online manual. Says it can take up to the maximum of 21 pitch. It's all about compromise as a bigger pitch propeller will probably result in poorer holeshot. I'd have thought the 17 pitch was about right.
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Old 01 April 2014, 17:07   #4
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ratio is wrong 13:26 that is a step up ratio not a reduction ratio, hence your -prop slip figure If the figures were just the wrong way round it would just be a 2:1 ratio & expressed as such where did you get those numbers from?
The ratio was from my manual. Using 0.5 instead of 2 as the gear ratio I get prop slip of 72%.
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Old 01 April 2014, 17:14   #5
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Old 01 April 2014, 17:29   #6
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The throttle range for a Tohatsu 90hp is 5,000-5,500rpm at wide open throttle (WOT). If you're hitting a maximum speed of 39 knots (plenty in my book if boat is as light as you say) then you may be able to squeeze a little more speed out of it. http://www.tohatsu.co.jp/en/boat/pro...0A2_130411.pdf Turn to page 87 of the online manual. Says it can take up to the maximum of 21 pitch. It's all about compromise as a bigger pitch propeller will probably result in poorer holeshot. I'd have thought the 17 pitch was about right.
I am happy with the speed and the overall performance. This is just about trying to understand what is going on. If I input 21p as the propellor, it all works giving 8% prop slip. It is almost as if my prop is labelled incorrectly.
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Old 01 April 2014, 17:39   #7
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The ratio was from my manual. Using 0.5 instead of 2 as the gear ratio I get prop slip of 72%.
Your prop is travelling:
5050 rpm divided by reduction of 2 = 2525 rpm at the prop shaft
2525 rpm * 17 in/rev divided by 12 in/ft = 3577 ft/min (no slip accounted for here yet)

Your boat is travelling:
39 nm/hr divided by 60 min/hr = 0.65 nm/min
0.65 nm/min * 6076 ft/nm = 3949 ft/min

Slip would be about negative 10%, i.e. your boat is travelling faster than a 17" prop can possibly drive it. So it appears that one of your inputs its incorrect.

I don't really know where to start, but I would start with confirming the pin settings in the tach are correct. Then test on a second tach. Then double check the prop pitch (not just what's stamped on it). Lastly look into the gear ratio.
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Old 01 April 2014, 18:15   #8
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Originally Posted by captnjack View Post
Your prop is travelling: 5050 rpm divided by reduction of 2 = 2525 rpm at the prop shaft 2525 rpm * 17 in/rev divided by 12 in/ft = 3577 ft/min (no slip accounted for here yet) Your boat is travelling: 39 nm/hr divided by 60 min/hr = 0.65 nm/min 0.65 nm/min * 6076 ft/nm = 3949 ft/min Slip would be about negative 10%, i.e. your boat is travelling faster than a 17" prop can possibly drive it. So it appears that one of your inputs its incorrect. I don't really know where to start, but I would start with confirming the pin settings in the tach are correct. Then test on a second tach. Then double check the prop pitch (not just what's stamped on it). Lastly look into the gear ratio.
I have no idea how to check the real pitch of the propellor. It is a possibility as it is a second hand prop from a race boat and may have been re pitched.

The tacho theory sounds plausible. If it is set to 4 pole instead of 6, the engine is actually spinning at about 7500 rpm and the prop slip is 24%.

I will not be home in daylight hours today so I will have to wait for tomorrow to check.
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Old 02 April 2014, 07:26   #9
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Tacho set to 6 pole so correct according to the manual.

I guess it comes down to the prop being modified or the tacho is inaccurate.
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Old 02 April 2014, 08:15   #10
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two things you can easily check:- mark the flywheel & prop whilst in gear turn the flywheel 2 full turns & see where the prop is to confirm the ratio

find out where the rev limiter cuts in take engine up to rev limiter briefly this will give an idea of how accurate the tach is or use a shop tach to check dash tach at high rpm
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