Why starboard engine uses more fuel than port engine?

[Alex F]

I’ve seen some discussions on the subject and I was noticing similar behavior on my boat, but last week while cruising on a long distance trip I had confirmed that my starboard engine uses about 1GPH more than my port engine. Here are some numbers I’ve noted:

Running in the ocean with head wind and 2-3’ chaps 21mph/18kts @3400rpm, using 12.5gph on starboard engine and 11.2gph on port engine.

Since the difference is not huge, in the past I thought that this is due to generator using strbd side, but at times I though that with the rate of 0.5GPH (I have 5E Kohler) it shouldn’t use so much. So, roughly it comes down to about 10gal difference on the full tanks.

Is this simply a sign that my strbrd engine needs some tune up or is there something else that I’m not aware of?

Thanks,
Alex.

 

[skolbe]

Doesn't the generator pull off the port side.

 

[Hampton]

Both engines run in forward direction. One transmission runs in reverse, so, they have different gear ratios. This gives you counter-rotating props, but at slightly different RPMs when the engine RPMs are matched. Your props come from the factory mis-matched by .5" pitch, same diameter, in an attempt to match the engines' performances. The port prop has more pitch. I had mine tuned to even more pitch than the factory provided. Mine are now mis-matched by about 1.25" pitch, port higher. Now, they both run at identical engine RPM and fuel flows.

 

[rcknecht]

so what you are saying is that we should run the port engine at a slightly higher RPM than the starboard if our prop pitch is not quite right????

 

[Hampton]

I didn't say that, but, that would match the sides more equally. Problem is, SC records how long each engine spends in each RPM zone. I would rather put more gas in one tank than try to explain why the RPMs are mis-matched during survey at re-sale. If that is no concern, yes, I would match the fuel flows, if all else is equal. Your boat will, overall, run truer and therefore more efficiently.

 

[Alex F]

Thanks for the explanation John.

I had my props tuned this spring to Class 1. One was up to specs and the other slightly off, so it was tuned. I have:

LH - 18.997 x 19 x 3
RH - 18.499 x 18.5 x 3

I thought that size difference was to compensate the counter rotating props/transm, but I guess the builder left a design gap for fuel usage.

Is this the case for most V-Drives boats or just selective models?

 

[Pakmule]

Is this why I see the engine sync gauge get buried to the starboard when I go on plane by pushing the throttles forward together?

 

[Hampton]

Thanks for the explanation John.

I thought that size difference was to compensate the counter rotating props/transm, but I guess the builder left a design gap for fuel usage.

Is this the case for most V-Drives boats or just selective models?

That is what I said. The last post I made above was to answer a specific question about engines that were not matched by proper prop tuning.

In the past, one engine ran backwards, so, the transmissions were both in forward with the same gear ratio and matched props which spun opposite directions. I don't know about other transmission designs. I believe it is at least a Hurth issue, if not with many similar transmissions. The way they are engineered, they cannot get the same gear ratio in forward as reverse (at least not without additional parts, weight, and re-engineering...).

 

[Southpaw II]

Both engines run in forward direction. One transmission runs in reverse, so, they have different gear ratios. This gives you counter-rotating props, but at slightly different RPMs when the engine RPMs are matched. Your props come from the factory mis-matched by .5" pitch, same diameter, in an attempt to match the engines' performances. The port prop has more pitch. I had mine tuned to even more pitch than the factory provided. Mine are now mis-matched by about 1.25" pitch, port higher. Now, they both run at identical engine RPM and fuel flows.

So....if I understand correctly......if you run both engines at the same rpm, and the sync gauge shows them to be as perfectly matched as possible....the port prop is turning slightly slower because of the different gear reduction. Sea Ray compensates for this by spec'ing the port prop with slightly more pitch, to provide more thrust, to make the boat run true. If this is true, the port engine should be under more load, and consume more fuel. did I get it right, Hampton?

 

[carterchapman]

If this is true, the port engine should be under more load, and consume more fuel. did I get it right, Hampton?

My 340's port fuel flow is about 6-8% higher than the stbd (reading it off the SC gauges). Even with stbd tank being used for genny fuel, I still put more fuel in the port tank than the stbd. I match my RPMs in cruise.

 

[Alex F]

Doesn't the generator pull off the port side.

I think that fact that more fuel was used out of the starboard side made me think that genny is using strbd tank, but I just checked the manual and according to diagram on page 74 genny is actually using port side tank. Thanks for pointing this out. Now I'm convinced that genny uses very little fuel compare to the difference in the way counter rotating props/trans performing on the strbd side.

 

[Hampton]

So....if I understand correctly......if you run both engines at the same rpm, and the sync gauge shows them to be as perfectly matched as possible....the port prop is turning slightly slower because of the different gear reduction. Sea Ray compensates for this by spec'ing the port prop with slightly more pitch, to provide more thrust, to make the boat run true. If this is true, the port engine should be under more load, and consume more fuel. did I get it right, Hampton?

Not quite. They pitch the port more to provide more thrust than you would get with it spinning slower, but only to match the thrust of the stbd side. So, same thrust will average out to same fuel flow if the props are equally efficient in the water. The engines will turn the same RPM and create the same thrust, port and stbd. The gearing in between is different.

 

[Southpaw II]

Not quite. They pitch the port more to provide more thrust than you would get with it spinning slower, but only to match the thrust of the stbd side. So, same thrust will average out to same fuel flow if the props are equally efficient in the water. The engines will turn the same RPM and create the same thrust, port and stbd. The gearing in between is different.

Understood.......but, I would assume the differences in gear ratio and pitch would create a different engine load. Maybe they net out.

I would equate this to having 2 cars with engines running the same speed. The transmissions/differentials have a slightly different ratio, but different tire sizes are used to even out the inaccuracy of the speedodemters.

 

[Southpaw II]

Not quite. They pitch the port more to provide more thrust than you would get with it spinning slower, but only to match the thrust of the stbd side. So, same thrust will average out to same fuel flow if the props are equally efficient in the water. The engines will turn the same RPM and create the same thrust, port and stbd. The gearing in between is different.

Understood.......but, I would assume the differences in gear ratio and pitch would create a different engine load. Maybe they net out.

I would equate this to having 2 cars with engines running the same speed. The transmissions/differentials have a slightly different ratio, but different tire sizes are used to even out the inaccuracy of the speedodemters. One is going to be more efficient than the other.

 

[Hampton]

Understood.......but, I would assume the differences in gear ratio and pitch would create a different engine load. Maybe they net out.

They should net out. The transmissions might realize different loads, but remember, these differences are fairly minute.

 

[fwebster]

Guys, the engine load is the same.

There is about a 2% difference in transmission ratios, port to stbd, which is handled by the odd pitch combination on the wheels. Therefore, the engines run the same speed, put out the same hp/torque, the transmission makes fewer turns on one side, but that prop has more pitch which moves the boat further thru the water, so the net effect is 0.

A one gph differential is normal.......it is a different engine, has different wear, different clearances, different ECM, different injectors, etc. This may be a case of information overload......SmartCrap gives us actual fuel burn, its different, so we worry.

If we didn't know actual burn rate, would we even question a 1 gph differential?

 

[tobnpr]

Thanks, Frank. My brain was beginning to hurt from all that nonsense.

 

[Alex F]

Good point Frank.

....So, same thrust will average out to same fuel flow if the props are equally efficient in the water.....

John,
I took my props to top guys in our region and they said that props were tuned to the best performance Class 1 with report full of numbers for each parameter. So, I thought I did everything make my props as efficient as possible. So, I think Frank's point summarized the other side of the issue. Both engines run fine, so since the difference is minor I'll leave it alone and will monitor to make sure that the fuel birn stay in the same range and the difference doesn't go over the current numbers.

 

[Ray K]

When I saw this thread and the responses I was glad to see others have noticed the problem with the different gear ratios in the ZF Hurth v-drives. Most boaters that I've asked are totally surprised about this. The ratios are different? I must say until a recent failure of my starboard transmission I never even new the ratio of mine or that the ratios would be different. When I was collecting the model number and ratio I realized the ratios were different. As soon as I discovered this it started explaining a lot on how the boat performs.

I know a few of you here have figured this out and how to compensate. I've been trying to calculate the right changes to make to the port prop and have been working in an excel spreedsheet for a few days now calculating theoretical movement, engine rpm, shaft rpm and prop slip etc.

Hampton, what ratio is your ZF Hurth 63 and what diameter/props did you start out with and what are they now? How did the RPMs change on each engine from before to after. I'm surprised you made such a large change in pitch.

My 370 (sorry not a Sea Ray, Cruisers Yachts) with the 420hp 8.1 Volvos has ZF 63 IV transmissions wtih a ratio of 2.5:1 (S2.48, P2.53) and I have 22 x 23 4 blades with no cupping (michigan wheel). At WOT this amounts to the engine RPM to be out about 100rpm to achive the same prop shaft rotation. At cruise when engines were synced starboard always used more fuel, throttle advanced is more and has a slight pull to port. Started running the gennie off the port tank to compensate a bit. During the repair of the transmission while the boat is out of the water I'm going to have the props checked and then make a change to the port one as it is exceeding WOT limit by about 100rpm. Currently WOT on port is 5100 and starboard 4900. Once I have the thrust equal on both sides I'll have to figure out where the other 100rpm is being lost. Based on the info I've calculated I should increase the pitch on the port prop by 1/2" to equal the thrust out. If I do more than that I'm afraid and be transferring the problem to the port side. I also don't want to compenstate too much as I need to see why the starboard engine rpm should be a bit higher at WOT currently.

Any other examples rpm, pitch etc from anyone else that has been able to even things out would be great as I don't want to pull the boat out mid season just to tweak a prop.

 

[320Bob]

Alex,
My 320 factory 3 blade props had 1/2" more pitch on the port side. I replaced them with 18x18 4 blades and now my starboard side runs 130 rpm slower at WOT than port. Both the prop shop and I suspected this might happen but they did not have an 18x17.5 RH prop in stock so they said they would reduce the pitch on the RH prop to compensate if necessary. Before grounding, my port side was running more than 100 rpm less at WOT with the factory props so with that data we decided to go with equal pitched 4 blades initially to see how they preformed. I suspect that something was either binding on the port side or the factory port prop may have been slightly bent by the PO causing it to run slower. So FrankW's point about the slight transmission ratio difference is correct or I would not see the difference with my new shafts, couplers, cutlass bearings and props.