davierobb
Well-Known Member
Why do some gear boxes spin the shaft/propeller faster in reverse, compared to forward?
My hurth reduces at 2.75 forward and 1.86 reverse.
My hurth reduces at 2.75 forward and 1.86 reverse.
Why do they do that (Gearbox Ratios)
- Thread starter davierobb
- Start date
Dalliance
Well-Known Member
I'm guessing but I suspect that it's because the pitch of regular fixed props is less efficient in reverse so the prop is rotated faster to increase output.
david_bagshaw
Well-Known Member
some times if it is an epicyclic box the reverse is taken from the other side of the annular gear.
commom in borgwarner boxes.
commom in borgwarner boxes.
Oen
Well-Known Member
...and that's why some mobos have contra-rotating props of different pitch, as BW didn't make pairs of gearboxes at the same ratios...
EG port engine 1.91:1, starboard 2.11:1, thus same prop revs, different engine revs (wrong!) or same engine revs, same torque (almost), different prop speeds, and perhaps, just perhaps, a reduction in prop-related vibration...
EG port engine 1.91:1, starboard 2.11:1, thus same prop revs, different engine revs (wrong!) or same engine revs, same torque (almost), different prop speeds, and perhaps, just perhaps, a reduction in prop-related vibration...
Cornishman
Well-Known Member
Could it be the same as for cars? Most have reverse as the lowest gear in the box, or am I being just a bit too simple? /forums/images/graemlins/confused.gif
burgundyben
Well-Known Member
I think david bagshaw is correct.
On BW boxes they do run slightly different ratios fwds and reverse, however, they can be run either way, so fwds could be with a CW engine and RH prop or could be CCW with a LH prop.
Not sure any mobo's with contra rotating props have different pitch.
think that its one engine running the other way, certainly the case with perkins.
On BW boxes they do run slightly different ratios fwds and reverse, however, they can be run either way, so fwds could be with a CW engine and RH prop or could be CCW with a LH prop.
Not sure any mobo's with contra rotating props have different pitch.
think that its one engine running the other way, certainly the case with perkins.
alb40
Well-Known Member
Yep, on my twin perkins installation, one engine turns clockwise, the other counterclockwise.
The BW boxes turn as per engine rotation in FWD
The BW boxes turn as per engine rotation in FWD
Oen
Well-Known Member
[ QUOTE ]
Not sure any mobo's with contra rotating props have different pitch.
[/ QUOTE ]
Sorry, Ben, they do! I owned one and I know of others...
Not sure any mobo's with contra rotating props have different pitch.
[/ QUOTE ]
Sorry, Ben, they do! I owned one and I know of others...
burgundyben
Well-Known Member
It had crossed my mind as a possible way to get my boat contra rotating, run one gearbox in reverse with the prop handed the other way.
the thing is...whilst it is perfectly possible, its not without problems, first the issue of the slightly different ratio and second, most important, the BW gearboxes are not supposed to be run at full power in reverse, the manual says only run at max 1000 rpm in reverse, indeed if you look inside the box the route the power takes through the box is less engineered in reverse.
Certainly with BW velvetdirve boxes I would not do it. It may well be a cheap and easy way to get contra rotating props without having one engine run the otherway which is the best solution I think.
the thing is...whilst it is perfectly possible, its not without problems, first the issue of the slightly different ratio and second, most important, the BW gearboxes are not supposed to be run at full power in reverse, the manual says only run at max 1000 rpm in reverse, indeed if you look inside the box the route the power takes through the box is less engineered in reverse.
Certainly with BW velvetdirve boxes I would not do it. It may well be a cheap and easy way to get contra rotating props without having one engine run the otherway which is the best solution I think.
