Lower rpm when going in reverse - perkins/max-prop

[LifesBetterWhenYoureBeating]

On my newly aquired boat I've just noticed that when going in reverse the engine (Perkins 4.108) can't reach more than 1200rpm, but I'm pretty sure it went past 1500rpm before. Also, in neutral or when going forward it can reach 2000rpm.


I have a max-prop and I've already checked that it is not fouled and can feather, so I'm out of ideas as to what could be causing this behaviour. Any suggestions on what to look next?

[boatman61]

Does it do this when the clutch is disengaged as well.???

[LifesBetterWhenYoureBeating]

Originally Posted by boatman61

Does it do this when the clutch is disengaged as well.???

Nope, in neutral it goes much higher

[tkeithlu]

So, you've got a source of drag (on the engine) that is dependent on your being in reverse. Hmmm. Unless you missed something, it's not in the operation of the max-prop.

No unusual noise/vibration? It's running smoothly, just not at 1500 RPM?

I'm inclined to think that there are justifiable causes of this difference. Propeller blades shaped to be efficient in forward gear, by far the most usual, will not be as efficient in reverse. More drag. Same goes for gears in the system, including the tranny. That's why your car whines in reverse. So, absent barnacles in the prop gears or a hanging up blade, I'm not convinced that there's a problem. I could be very wrong, but you have not convinced me yet.

If that's not a satisfactory answer, do some more testing and see if you can tell us more. Good luck with it.

[NautiNarwhal]

I belive tkeithlu is correct.
Maxprop actually reverses the blade so you are using the front of the blade. The trailing edge is the same in forward and reverse. It is not just folding prop. Your reverse pitch does not necessarily match your forward pitch either. I know you can give them specifications and pick your forward and reverse pitches.
I don't think you have an issue.

[Jim Cate]

^^^

NOt sure about Max props, but on the feathering prop we once had one could adjust the pitch to different values in forward and reverse. Could be so with MP as well. Having a much steeper pitch in reverse could explain your issue.

Jim

[Finalee]

Is the boat untied running free during this test?

[valhalla360]

Originally Posted by gsalgado

On my newly aquired boat I've just noticed that when going in reverse the engine (Perkins 4.108) can't reach more than 1200rpm, but I'm pretty sure it went past 1500rpm before. Also, in neutral or when going forward it can reach 2000rpm.

I have a max-prop and I've already checked that it is not fouled and can feather, so I'm out of ideas as to what could be causing this behaviour. Any suggestions on what to look next?

Is the 2000rpm the maximum in forward & neutral?

Poking around for 5min on the internet, it looks like it should be capable of turning around 3600rpm. So it may not be just a reverse issue.

[fstbttms]

Originally Posted by Jim Cate

NOt sure about Max props, but on the feathering prop we once had one could adjust the pitch to different values in forward and reverse. Could be so with MP as well.

This is not the case with Max Prop.

[GordMay]

The Perkins 4.108 Engine has a Rated Engine Speed of 3000-3600 RPM, so, maxing out at 2000 RPM, you may be over-propped.
I also expect that your marine transmission [reduction gear] might have different drive ratios for clockwise & counter-clockwise rotation [Fwd-Rev].
I only offer these ideas, that someone, more qualified than I, might comment, or refute/discount them.

[FPNC]

Depending on your transmission, it will very possibly have different gear ratio in reverse. Could be higher could be lower.

Also, it could be you need a throttle cable adjustment. Always check max RPM values with the throttle cable disconnected and engine throttle arm moved by hand to make sure you get maximum throw.

[Bill O]

Originally Posted by gsalgado

On my newly aquired boat I've just noticed that when going in reverse the engine (Perkins 4.108) can't reach more than 1200rpm, but I'm pretty sure it went past 1500rpm before. Also, in neutral or when going forward it can reach 2000rpm.

I have a max-prop and I've already checked that it is not fouled and can feather, so I'm out of ideas as to what could be causing this behaviour. Any suggestions on what to look next?

If my memory is correct, the 4.108 is a continuous 3,000 rpm and an intermittent at 3,500 rpm motor. You should be able to get to those rpms in neutral. Depending how the prop pitch was set up you may/may not be able to get to them in gear.

The issues you are seeing could be related to bad tach info. Is the tach/rpm driven off the alternator? (my 4.236 is) Since you don't indicate you can get a rpm reading above 2K, would check the alt. belt tightness or replace the belt if glazed/worn. Does the belt fit in the pulleys properly? If sitting too low in the groove, that can cause slippage.
Also would check the tach stud wire off the back of the alternator to see if it needs service.

Gord and FPNC are correct, there are different gear ratios for fwd and reverse. There should be a plate on the trans indicating the model and gear ratios for forward and reverse. Be good to write these down somewhere for future reference if you want to do pitch adjustments/calculations for the MP.

[LifesBetterWhenYoureBeating]

Originally Posted by Finalee

Is the boat untied running free during this test?

No, I was at anchor, running in reverse to check that it was holding properly. Haven't had a chance to test it when running free yet. But shouldn't it reach the same RPMS regardless of weather it's tied of not?

[LifesBetterWhenYoureBeating]

Another thing I've noticed is black smoke on the exhaust. Also important to note is that I'm pretty sure a month ago it'd tech 2000 RPMS in reverse without any black smoke. I say I'm pretty sure because the boat is new to me and although I remember pushing it to high RPMS after dropping the anchor I'm not 100% sure of the exact number

[fstbttms]

Originally Posted by gsalgado

But shouldn't it reach the same RPMS regardless of weather it's tied of not?

No. Tests of this type should be done while the boat is moving in calm water.