Overprop. What to do?

[Philip_Oceanis 361]

Following a change of gear box and getting a new propeller my engine now can achieve maximum rpm of 3050. It’s a Volvo Penta MD2030D engine with rated maximum 3200-3600rpm. So, basically now I am rather over-prop. Am I damaging the engine provided I NEVER run it continuously above, let’s say 2800rpm? By the way my cruising rpm with this prop is now around 2200rpm so there’s plenty of power left in case I need to use more power.
The propeller’s size is 16X9 (3 blade). If I wanted to increase max rpm to 3200-3300 so I am within Volvo’s limits what do you recommend to do? Decrease pitch or diameter?
Thanks

 

[Winkfish]

I would decrease the pitch. You probably won't do damage to the engine if you don't run it at maximum continuously, but it's probably not moving your boat as efficiently as it could.

 

[Philip_Oceanis 361]

but it's probably not moving your boat as efficiently as it could.

Before I had a different gear box and properly sized prop (I could achieve max 3500rpm). 2500rpm with previous combination achieve same speed and same fuel consumption as current 2200rpm. So, I really don't know (cannot see) if efficiency has decreased.

 

[Winkfish]

Yeah, it looks like efficiency actually increased. Well, maybe the new gear box and the new prop "changed" the recommended specs. You're now out of my comfort zone. A call to Volvo might be in order, giving them the new specs of the gear box and prop versus the old gear box and prop.

 

[tommays]

The problem comes from the fact that a basic diesel has mechanical fuel injection and pumps in fuel based on the throttle setting NOT what RPM the motor is making

If you cant reach full RPM it will always be pumping in extra fuel at all RPM settings

 

[tkanzler]

Have a prop shop adjust the pitch down a little bit. Let them know the gear box ratio so they know how fast the prop is spinning. But in the end, you have too much pitch. I would not reduce the diameter.

 

[Philip_Oceanis 361]

Yeah, it looks like efficiency actually increased..

No it hasn't. It seems that the 'new' lower rpm load the engine the same as the old 2500. The shaft makes the same rotations because gear ratio was 2,35:1 and now 2.05:1.

 

[Philip_Oceanis 361]

I agree with reducing pitch because a large diameter is always welcome. But 16X8 does it make sense? I mean is there any limitation in diameter vs pitch?

 

[Gunni]

The problem comes from the fact that a basic diesel has mechanical fuel injection and pumps in fuel based on the throttle setting NOT what RPM the motor is making

If you cant reach full RPM it will always be pumping in extra fuel at all RPM settings

And with that excess fuel, carbon-buildup in the cylinders, oil contamination, and mixing elbow clogging. The instructor at my Yanmar class went on at length about an over-pitched prop causing engine problems.

 

[Ross]

I would have a prop shop reduce the pitch. At 1000 rpm one inch of pitch equals a theoretical one knot so a 9 inch prop would give a theoretical 9 knots much of which is lost as slip. an 8 inch pitch will give you a better dead slow speed and some times that is more important.

 

[kloudie1]

Yup, a prop shop can reduce the pitch a little , maybe 1" or so to start with.. The engine would be affected only if you try to run it at max a lot.. If you see black smoke, that is the clue to back off until it does not smoke .. The "throttle really adjusts the governor and nothing else.. it gives the governor a speed set point. The governor adjusts the amount of fuel up or down to get to the speed set point.. If the engine is over-propped, the engine can't make enough power to reach the rpm set point so it continually adds more fuel than the engine can burn using the air that is available to burn the fuel. The result is black smoke. As soon as you back off the "throttle" to the point where there is no more black smoke, then all the fuel that the governor is providing (by adjusting the rack in the injection pump) is being burned and the engine is happy.. The only negative to operating that way is that you will be "giving away" some horsepower by not being able to achieve full RPM.. on my Yanmar, that RPM difference would be about 3-4 horsepower. Your engine is probably similar.. Ya won't hurt it if you don't let it smoke, but you will not have full power available if you need it. Your fuel burn rate at a set boat speed will be slightly better being overpropped by this small amount. No rush, but do get a prop shop to re-pitch the wheel ..

 

[Philip_Oceanis 361]

If the engine is over-propped, the engine can't make enough power to reach the rpm set point so it continually adds more fuel than the engine can burn using the air that is available to burn the fuel. .

So is this the reason why it smells if i run it hard (2800+)? I did smell something like fuel a few times i run it hard but was never brave enough to run it hard for long to see if black smoke will come out of the exhaust.

 

[Steve Dion]

Philip:

Why don't you give these guys a call. It is Killian Props in Alameda, CA. 510/614-0596

They do some magic with the prop tips. It takes a little off the diameter but allows the prop to bite better. They refer to this as a Non-Slip-Tip.

 

[Philip_Oceanis 361]

I have contacted the propeller manufacturer again and they claim that the prop size is correct and my engine would have been able to achieve maximum rpm of around 3200-3250. So, could you let me know what could prevent the engine from not achieving above 3050 rpm?
Exclude and dirt on prop/hull because even on the day the boat was back in the water with new antifoul and a brand new shining prop max rpm was again 3050.
Thanks

 

[gord331victoria]

Hi Philip


I've had this happen twice to me:

Prop:
#1. have the prop verified to be built to specs. (when I installed my new campell sailer, it was not buit correctly and only after a $65 repitch from a good prop shop did I come to proper RPMS).

Injection fuel flow:
#2. have a good mechanic review all your engine areas for alignement, blocked exhaust manifold or pipe, and finally adjust your injection fuel flow. on the Yanmar 3GM this is the nut with the seizing wire on the from meant only to be adjeusted by a mechanic to blance fuel with RPM, mine was running too rich after getting my injector tips replaced.

I was not happy when I had these problems, but I got them sorted eventualy.

Good luck!
good luck