VP 2020D/MS10L-A gear ratio

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Plevier

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I have a VP 2020D engine with the MS10L-A gearbox. The forward ratio of this is definitely 2.35:1 (RH rotation by the way) - it's stamped on the ID plate. Published information in the engine manual and elsewhere is unclear and contradictory about what the ratio is in reverse - I've found figures of 2.72, 2.37, 2.35, even 2.26.
Anyone know for certain please?
(Object is to work out how to improve pitch matching of my KiwiProp which will only do 2000rpm going astern with WOT and feels unhappy. I'm going to make packing pieces or new stops to reduce the reverse pitch.)
 
You will find that your prop is deliberately coarse pitched in reverse. Pretty sure they are all set at the same pitch. Objective is to improve low speed performance and stopping power.
 
Tranona said:
You will find that your prop is deliberately coarse pitched in reverse. Pretty sure they are all set at the same pitch. Objective is to improve low speed performance and stopping power.
Click to expand...

Yes they are all set at the same pitch - about 24 degrees. I don't think this is any more satisfactory than if the forward pitch was always set at 24 regardless of engine power and gear ratio! One size fits all does not always work. It's marketing spin to justify a design compromise for simplicity. Even Kiwi admit that with some engines it's enough to stall them, and you should switch off engine driven fridge compressors or high output alternators before going astern.
Does a car have its reverse gear set to the same ratio as your top forward gear? No it will be more like first gear.
In many cases you can get away with it. However every time I leave my mooring I have to go astern up a narrow channel, sometimes with a strong current, for 100yds or so and the limited power delivery is very apparent. Also I have twin rudders and no propwash, I have to have waterspeed for manoeuvring, so I need quick bite which is better with less pitch, not more - the same as an aircraft uses fine pitch for take off and climb, coarse pitch for cruise.
I'm pleased with the prop but want to match it better to the boat.
As I said the engine feels unhappy and labouring too - it wasn't designed to run at full throttle opening against an excessive load that only lets it do 2000rpm instead of 3600.
 
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Troubadour said:
Yes they are all set at the same pitch - about 24 degrees. I don't think this is any more satisfactory than if the forward pitch was always set at 24 regardless of engine power and gear ratio! One size fits all does not always work. It's marketing spin to justify a design compromise for simplicity. Even Kiwi admit that with some engines it's enough to stall them, and you should switch off engine driven fridge compressors or high output alternators before going astern.
Does a car have its reverse gear set to the same ratio as your top forward gear? No it will be more like first gear.
In many cases you can get away with it. However every time I leave my mooring I have to go astern up a narrow channel, sometimes with a strong current, for 100yds or so and the limited power delivery is very apparent. Also I have twin rudders and no propwash, I have to have waterspeed for manoeuvring, so I need quick bite which is better with less pitch, not more - the same as an aircraft uses fine pitch for take off and climb, coarse pitch for cruise.
I'm pleased with the prop but want to match it better to the boat.
As I said the engine feels unhappy and labouring too - it wasn't designed to run at full throttle opening against an excessive load that only lets it do 2000rpm instead of 3600.
Click to expand...
Yes, if it is set finer you will get less reliable stopping power. If reverse performance is so important you may want to consider a different design of prop. A Featherstream for example will allow you to have separate pitch forward and reverse, but the preference is to have a coarser pitch in reverse to improve low speed thrust.

Pretty sure you can adjust the stops on your prop both ways, so why not experiment by just backing them off a bit in reverse and see what happens. Not sure that using a prop calculator will help as they are usually designed to establish the size (including pitch) to achieve hull speed at max revs - not strictly what you want in reverse.
 
No, the KiwiProp only has one reverse pitch, equal to the maximum forward pitch you can set. Sounds as though you haven't handled one, you're making wrong assumptions, which isn't awfully helpful.

From Kiwi website:
The pitch is not adjustable in reverse but automatically goes to the maximum available which will be ~ 23.5 deg.

My correct forward pitch is about 19 deg so this is a very big difference.
 
Sorry about that. Was only making suggestions. If you know it can't be adjusted then it is irrelevant what your reverse ratio is. Very few props are capable of having different pitch forward and reverse so it is always a compromise. I have a JF (predecesssor to the Featherstream) and the pitch is set much coarser in reverse (not dissimilar to the Kiwi difference) because it is fitted to a heavy long keel boat with a very small engine so we went for the largest diameter flattest pitch in forward and coarser pitch in reverse to improve low speed thrust, but like you not possible to achieve high revs in reverse Works in practice for what I want.
 
OK. The whole point is I have worked out how I can change the pitch but it means making bits to do it, it won't be variable, so I'm trying to work out how much to change it. That's why I need to know the reverse ratio. Many gearboxes have a higher reduction in reverse - that's why you can get away with some pitch increase - but I'm not sure mine does.
We'll have to disagree on when you want fine pitch and when you want coarse.
 
If its any help a good rule of thumb is that a 1" change in pitch will rersult in approx 300 change in revs. So reducing the pitch by 1" will increase the revs by 300 with a corresponding increase in power. You will of course have to convert this to degrees and then to the amount you change the stop by - however you are going to do that.
 
Kiwi always talk in terms of degrees not inches of pitch but in the mid range there seems to be a fairly close correspondence and a figure of 400rpm per degree has been mentioned although I can't find it now. As my forward is 19 deg for approaching 3600rpm and reverse is 23.5 deg and only does 2000 rpm, that is consistent IF my gear ratios are the same - which is the question i started with as there is conflicting info around.
 
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