Mariner 40hp 3cyl 2stroke Rev limiter, or why

[berat]

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

[paintman]

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.

[berat]

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.

[beamishken]

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

[berat]

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)

[beamishken]

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

[jeffstevens763@g]

Originally Posted by berat

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.

[beamishken]

Originally Posted by berat

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

[berat]

the boat season is over, the planning season has begun
i can't wait to try
i will post the progress soon...if we survive the covid19

[Fenlander]

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

I have no experience with your engine but have lots of prop experience 25hp and under.

First not sure which prop is in your image post #5 but unless the camera angle is deceptive that looks like a low pitch high thrust prop for lower speeds that may not best suit your boat.

The other thing is you can't trust a pitch vs speed/revs comparison between the cheap Chinese props and a maker's OE prop. You are correct often their blades are thicker with blunt leading edges and other small factors like poor design of the central boss where the exhaust gas flows through.

In my experience the OE Mercury/Mariner props are nicely made.

Last year I found one 9" Chinese prop effectively had the pitch of a 10.5" OE prop and another Chinese 9" ran as a 10" OE.

[phantom 19]

I agreed regarding not trusting the pitch - I was running a 10 5/8 x12” genuine Yamaha Alloy Prop, over revved by approx 2-300rpm. I purchased a brand new reputable make stainless prop 10 3/4x12” pitch thinking that it would be spot on, instead I lost 1000rpm. I sent the prop away along with the Yamaha to prop shop and the supposed 12” pitch stainless was actually 14” when measured.

[berat]

Thank you all for your comments. I can test the boat alone at last. But it seems that just my boat is heavy.
I'm in a mood like "32mph with 40hp is ok" as long as we can travel to covid free bays for now.
Had a temporary power reduction this morning, like %30 is gone, may be something blocked one of the carburators. But it recovered spontaneously. I hope it is not because of the electronic parts.

[berat]

One more thing. When slowing down from cruising about 3500rpm to idle some terrible sound, like getting out of lean gears, for about 3 seconds. But only slowing down. Gears, impeller what can it sign to.
I would be grateful for any input.