Overpropped?

[penfold]

That means the problem may well lie somewhere other than the prop; lumpy antifouling will be very draggy but it would not cost you 1000 rpm. Is the engine control lever being moved to its full extent?

 

[Tranona]

Now that I have cleaned up the prop I see to my embarrassment that the size is actually 13 X 11 not 15 X 11 !? So that seems to be about the right size ... But on the principle that I still need a rev gain of 600 rpm plus, does that mean I should be looking for a 13 X 9 inch propellor ?

Re pitching my old one will cost around 140 quid. I have seen a new (old stock) 13 X 9 for 200 quid, which would mean I would still have my old prop if I'd made some gross miscalculation - or if the problem lay elsewhere. What do you think ?

No, Not if you have a 2.72 reduction. You can't turn the prop fast enough. Maximum shaft speed is 1323rpm compared with 1800 for a 2:1 reduction, A 13*8 would be about right for 2:1 - and a 13*11 would indeed be too big. However the Hurth box does not have a 2:1 option (at least not currently) but 1.79 for A (forward) and 1.86 for reverse. You have 1.86 reverse whereas the 2.72 forward seems to come with a 2.15 reverse which makes more sense.

The question now - what is the actual ratio in forwards. as you see it is absolutely critical you get this right. If it is 1.79 then the correct prop is a 12*7. Basic principle, the faster the shaft speed the smaller the propeller.

 

[jonathanhsm]

All i know is what is on the grubby plaque on the gearbox ... see attached pic ...
It says HBW100-3RP A=2.72 B=1.83
Many thanks.

 

[Tranona]

Appreciate that, just that I can't find any reference to that combination of A&B. ".05 as A is common for older boxes with that B which does make sense. Might be worth a call to ZF
aftermarket.zf.com/en/aftermarket-portal/services-and-support/partner-finder/#/result/detail/c804e0f2-8c52-ea11-a815-000d3a1abe5c

 

[jonathanhsm]

thanks ... but I'm not really clear what I'm asking them..... am I asking if the gearbox with that combination existed? Surely the plaque isn't wrong ?

And if the plaque is right ie a 2.72 reduction going forwards ... what would you calculate is the right size prop. Diameter would have to be 13 as I'm not sure a bigger one would fit.

 

[Tranona]

Back to the original - with that reduction you need a 15" prop. Crudely if you add the diameter and pitch you get "size" as in the load it puts on the engine. so the 15*10 is 25. Go down to 13" diameter and you need 12" pitch.

Essentially whoever installed the engine did not understand what they were doing and specified the wrong gearbox - or more likely just bought what was on offer.

You can physically check the reduction by turning the engine manually in forward and observing the turns of the output flange or propeller. One turn of the engine should give 0.5 turns of the output for 2:1 or 0.37 for 2.72..