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Old 18 November 2019, 08:15   #1
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Country: Other
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Make: MKIII Zodiac GR 1986
Length: 4m +
Engine: Johnson 30hp 2/s
Join Date: Nov 2014
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Mariner 40hp 3cyl 2stroke Rev limiter, or why

I have problem about why my boat goes slow (relative)
Does anybody can show me how to remove the rev limiter on my mariner 40hp 3 cylinder. I had these props and test values below and i started to wonder how could it be...

1 - Old propeller from first owner, BLKMX 12 1/4 x 9" propeller, 27 mph, 5100 rpm
2 - China made aluminium 9 7/8x13" propeller, 31 mph, 5100 rpm
3 - Raised the motor one hole, same as 2nd try, but 32mph
All tests made in same conditions.
I keep 9"pitch as a spare and using 13"pitch al prop.
Motor seems to be powerful at midranges with 13" alu prop, actually It is fine even with 4- 5 people on board. But something wrong with full throttle speed, not acceptable. Thinking about getting a stainless 13" prop. But i wonder is there a problem about rev limiter or cdi
Any ideas, thanks...

Here is the motor and CDI
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Old 27 November 2019, 18:37   #2
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Country: UK - England
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Don't know whether this link will work for you.
http://www.kirschbaum.at/ki_prop/pdf...PM%20Chart.pdf

Lists the 40hp Mercury recommended WOT ranges.
40hp 2001-2002 2.0 gear ratio 4500-5500 rpm
40hp 2004-2005 1.83 gear ratio 5000-5500 rpm.

So if those are correct for your engine then likely there's nothing wrong with it & you're about there unless you want to try different prop pitches to increase your current max rpm.
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Old 02 December 2019, 02:16   #3
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WOT seems to be in range, normal.
I know i can't explain well. But it is like something is missing. At the mid rpm's it is very powerfull, raises my expectations.
Thanks Paintman.
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Old 02 December 2019, 04:07   #4
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Country: UK - Scotland
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Boat name: charlie girl
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A few things seem odd here firstly the prop sizes seem wrong are you sure you have the dimensions the right way round on both props? Usually diameter first then pitch. There seems to be too much difference in the sizes which makes me think the dimensions are reversed. I'd also check your tacho with a shop tach as I suspect you are hitting the rev limiter with both set ups & are actually under propped in both instances.
Switching from a 13 to a 9" should give a huge difference which it isnt so there is an issue somewhere
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Old 02 December 2019, 05:12   #5
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unfortunately it is 9" pitch... When i got the engine, I wasn't planning on buying a new propeller. But 9" is like a joke.
First rpm values recorded with an old tachometer, there may be some ghost digits on screen. But i remember at wot the engine was not screaming like 5500rpm. I have experience to tell that.

beamishken, that's what makes me think.
beamishken - "Switching from a 13 to a 9" should give a huge difference"

Or, the new china made propeller has a very thick material so is making resistance to turning. (shooting in dark)


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Old 02 December 2019, 05:23   #6
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It is almost impossible to check pitch but diameter is easy to check perhaps put the props side by side or on top of each other & take a picture even a poor prop should give a significant difference in performance if those numbers are correct
Also checking the tach reading with an optical tach to verify the readings would be useful
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Old 02 December 2019, 16:32   #7
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Quote:
Originally Posted by berat View Post
I have problem about why my boat goes slow (relative)
Does anybody can show me how to remove the rev limiter on my mariner 40hp 3 cylinder. I had these props and test values below and i started to wonder how could it be...

1 - Old propeller from first owner, BLKMX 12 1/4 x 9" propeller, 27 mph, 5100 rpm
2 - China made aluminium 9 7/8x13" propeller, 31 mph, 5100 rpm
3 - Raised the motor one hole, same as 2nd try, but 32mph
All tests made in same conditions.
I keep 9"pitch as a spare and using 13"pitch al prop.
Motor seems to be powerful at midranges with 13" alu prop, actually It is fine even with 4- 5 people on board. But something wrong with full throttle speed, not acceptable. Thinking about getting a stainless 13" prop. But i wonder is there a problem about rev limiter or cdi
Any ideas, thanks...

Here is the motor and CDI
It's hard to explain and to do the maths as its not exact science but I see it as the two props are similar both achieving 5100 rpm with 4 mph lower speed on the larger dia prop which will have more drag because of surface area, an inch of dia is roughly equal to 2 inches of pitch and an inch of pitch equal to 150-200 rpm lots of factors can change that.
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Old 03 December 2019, 03:33   #8
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Country: UK - Scotland
Town: north ayrshire
Boat name: charlie girl
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Length: 10m +
Engine: 40hp 2st 2x6lp 315
Join Date: Jul 2009
Posts: 2,271
Quote:
Originally Posted by berat View Post
unfortunately it is 9" pitch... When i got the engine, I wasn't planning on buying a new propeller. But 9" is like a joke.
First rpm values recorded with an old tachometer, there may be some ghost digits on screen. But i remember at wot the engine was not screaming like 5500rpm. I have experience to tell that.

beamishken, that's what makes me think.
beamishken - "Switching from a 13 to a 9" should give a huge difference"

Or, the new china made propeller has a very thick material so is making resistance to turning. (shooting in dark)


Attachment 131612
My thoughts are this, you are underpropped with both props & hitting the rev limiter , your tacho it under reading making you think your not at max rpm.
I think you need to go bigger in prop size. The Chinese prop is very odd being so small in the diameter. I'd look for a decent prop of around 11-12" dia & 14p and try that. I'd also check your tach reading with an optical tach. Alternatively you could see what rpm the engine makes with no load. With the engine warm take it up to max rpm for a few seconds & note the reading. It should record roughly 2-300 rpm over engines max rpm rating.
Dont worry the engine won't fly to bits by hitting the rev limiter for a few seconds with no load
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