Engine RPM

[Webone]

I am operating a 46 hp 4 cyl. Yanmar diesel on my 40 Oday (1060 hrs). I was running about 7.2 knots at 2500 rpm (max no load rpm 2900). Had to replace the tranny after which I was getting 7.2 knots at 2200 rpm with max loaded rpm about 2500, this has since gone to 7.2 knots at 2000 rpm, max 2200, and then 7.2 knots at 1800 rpm with max loaded rpm at 2000. Does not make any sense to me that I should be maintaining speed while losing rpm. Have a 3 blade self trimming prop. What concerns are there for this behavior???

 

[Ross]

It would seem that the pitch on the prop is self changing and is now too large.

 

[JohnShannon]

Either your prop pitch is changing, your gear ratio is changing or your tach is faulty, you now motor only downwind, or the hull has gotten cleaner.

I suspect your tach is jacked.
Maybe when you changed the tranny you got a one time change in gear ratio which could explain the initial step, but wouldn't explain the steady drop in full speed RPM after that.

Does the motor sound different at full speed compared with before? What do your ears tell you?

If your prop pitch had changed you would start overloading the motor, as the propeller would be demanding more power at lower RPM while the motor power capability is falling with RPM.

Really I bet installing the trans a sender got bumped and you most likely have a bad connection sender to tach.

 

[Steve Dion]

I am betting that your transmission ratio changed. You will need to have your prop matched to the transmission.

You should be able to get 3400-3600 rpm at WOT if everything is setup properly and your bottom is clean.

 

[Ross]

The only varible ratio transmission I know of is a "Link-Belt" and those are manually adjustable. Also I am not sure what is meant by a "self trimming" prop.

 

[Steve Dion]

Here is a link. Find you engine and you will see the number of options for transmission ratios.

http://www.yanmarhelp.com/i_proptbl2.htm

 

[Tim R.]

Here is a link. Find you engine and you will see the number of options for transmission ratios.

http://www.yanmarhelp.com/i_proptbl2.htm

True, trannys have different ratios but it does not dynamically change. They come that way from the factory or you need to get into it proactively to change the ratio. So this could be the case when he swapped the tranny the first time but it continued to change after that.

 

[redhead78]

Web, it may be the tach, I did a repower this winter, and covered everything, prop, shaft, engine, all sized by the people who sold me the engine. Everything so far is good, I think the pitch is off just a little after testing per their instructions. My tach has an hour meter in the center in LED, and everything works except after 85 hrs it is showing all different hrs in the thousands. My point to you here ,and I will get to is, when we finally determined it was the tach that needed to be swapped out, we first had to get the correct one, out of 2 possible, then he needed my engine # so he could properly calibrate the tach to this engine and trans. This setup is a beta 35, and I was talking to a guy named Stanley who seemed to really know his sh*t.....So maybe the tach has to be calibrated to your new trans......Red

 

[Ross]

Think about it, In your car if you have a tach and an automatic transmission the tach reads the engine speed not the transmission speed. The speedometer usually reads drive shaft speed and as such must be matched to the rearend ratio.

 

[alexco38]

Tech for diesel engine

The tech on a diesel engine doesn't read the tranny. On gas cars, the speedometer reads the tanny while the tech reads the engine ignition impulse.

Since a diesel engine doesn't have ignition, the tech signal on a diesel engine came from the pre-rectified (A.C.) alternator output. The tech usually has a selector switch on the back to match the number of poles on the alternator and the pulley ratio.

If your engine "sounds" the same but the tech shows varying rpm, may be you have a problem with your alternator. It could be the regulator (likely) or diodes (less likely).

 

[Tim R.]

I agree with Ross. Tach reading(engine rpms) has nothing to do with tranny output ratio.

 

[druid]

On a diesel engine in particular, you NEED to get ahold of a hand tach and confirm what your panel tach is saying. If your max loaded RPM is indeed 2000, you NEED to take some pitch out! Even 2500 is too low for that engine.

Ebay has some great deals on hand tachs.

druid

 

[sailaway78]

Not the tranny

Think about it guys. If the new tranny had a different ratio, then the tach/speed readings might change. But once the new tranny is installed, the ration is not going to change. What ever the new ratio is, it isn't going to change. I would first look at the tach itself. I recently had a tach that started acting funny, reading all over the place. Some checking revealed that the sending unit to the tach was bad. The sending unit on the Yanmars screws into the bell housing, port side, and counts flywheel teeth. Check this first. Is much cheaper than the tach. I guess that a ratio change in the tranny could possible effect the prop pitch, but I would certainly give the tach a once over first.

 

[Ross]

Re: Not the tranny

N&E wrote,
"The sending unit on the Yanmars screws into the bell housing, port side, and counts flywheel teeth."
Is the flywheel losing its teeth?

 

[zeehag]

my yanmar had a tach replacement--it failed. there was a thing in the tranny they needed to modify for my engine and tranny so that it would work--the sender goes into the tranny on mine also. sending units for these fail. i had to have mine replaced after less than a year...it has worked well since then--had it 5 yrs....or 6...

ross--it will have to gum its oil soon..like mine----

 

[Randy6414]

On my Universal M-18 the engine rpm is read from the alternator. A slipping alternator belt will cause a low rpm reading on the tach. If your belt is "progressively slipping" your rpm reading could be falling while engine speed remains the same.

If your tach reads from the alternator, check the belt tension first. It could be an easy fix.

Randy

 

[Webone]

Either your prop pitch is changing, your gear ratio is changing or your tach is faulty, you now motor only downwind, or the hull has gotten cleaner.

I suspect your tach is jacked.
Maybe when you changed the tranny you got a one time change in gear ratio which could explain the initial step, but wouldn't explain the steady drop in full speed RPM after that.

Does the motor sound different at full speed compared with before? What do your ears tell you?

If your prop pitch had changed you would start overloading the motor, as the propeller would be demanding more power at lower RPM while the motor power capability is falling with RPM.

Really I bet installing the trans a sender got bumped and you most likely have a bad connection sender to tach.

Sounds pretty much the same as before. Have noticed a slightly loosened alt. belt. Great replys. Can I adjust the prop pitch?

 

[Ross]

Sounds pretty much the same as before. Have noticed a slightly loosened alt. belt. Great replys. Can I adjust the prop pitch?

You said in your first post that you have a self trimming prop. Please tell us more about this prop and someone will tell you if the pitch is adjustable.

 

[JohnShannon]

Knock yourself out, your ear tach is telling you that the tach instrument is wrong. The alternator belt is loose which could explain the tach shift and the fact that it is getting progressively greater. If you want to fool with the pitch what are you hoping to achieve? It would seem that if you want your tach to read correctly you should tighten your alternator belt and calibrate your tach as others have explained. This should cost you very little money and has a good chance of fixing the problem. Fooling with the prop sounds potentially expensive and unlikely to fix the funny tach reading