What prop direction ?

[Liambennett]

Hi All,
I’m getting a new prop made for my boat.
It’s powered by a Cummins VT555m
The boat never had a prop on it when I took the project over.
Would it be a left or right hand prop ?

 

[SteveK]

It is the transmission that will determine this.
Get make/model which will tell you and go from there

 

[tiltrider1]

If you have twins, it will be both. If its a single then Post 2 has your answer.

 

[Liambennett]

From the spec sheet that Cummins supplied it’s says it’s a RH drive
The transmission is a MG502RHA twin disc
So I’m thinking a RH prop ?

 

[Caballero II]

From the spec sheet that Cummins supplied it’s says it’s a RH drive
The transmission is a MG502RHA twin disc
So I’m thinking a RH prop ?

I'm thinking that tranny number is telling you it's Right-Handed (based on the RH in the serial number), but do double check. To my knowledge, it's the tranny that determines the prop rotation. The engine will be one or the other, but the tranny will decide which way the prop is turning.

 

[SteveK]

From the spec sheet that Cummins supplied it’s says it’s a RH drive
The transmission is a MG502RHA twin disc
So I’m thinking a RH prop ?

Search MG502RH (without the A) one on eBay for $3950 US
Wonder what the 'A' stands for?

 

[twistedtree]

A lot of transmissions can happily be run either way, and in fact that's how they operate in twin engine boats. One gear is in "forward" and the other "reverse" in normal operation, then they switch when backing down.


And the same is true for single engine boats. When my boat moves forward, the gear is in it's counter-rotation (rotation relative to the engine's rotation) mode.


I woudl begine by figuring out how you boat is set up today. It will be a bit tedious, but start with the engine rotation. That is likely counterclockwise, facing the flywheel. The look at which position the gear selector is in when your controls are in forward and reverse. The gear manual will tell you which selector position gives shaft rotation that matches the engine rotation, and which is counter rotation. Now you know how your boat is set up today. If you don't like the answer, it's probably very simple to swap it around. Assuming your gear is OK running in either direction, all you need to do is reverse the controls, either at the engine or in the shift control itself. If you typically dock on one particular side, it's convenient to have reverse propwalk that pulls in the stern.

 

[Cigatoo]

Before you do anything be sure you know what reduction ratio you have. Don’t go by what is marked on the gear. Get an inexpensive hand held photo tachometer and measure it yourself. Run the engine in gear at 1,000 rpm. Measure the rpm at the engine and transmission output at the same time / rpm and do the math. I have seen more than one gear where the ratio has been changed and / or does not match what is on the tag. You will also know if you need a left or right hand wheel. In the rare instance that the tag is wrong you will have to pull the prop and pay a prop shop to re work it.

 

[Baker]

Search MG502RH (without the A) one on eBay for $3950 US
Wonder what the 'A' stands for?

Maybe for the American market?? I only say that because Yanmar does that. There is a 4LH and then there is a 4 LHA. The LH was completely assembled in Japan and then shipped wherever. With the LHA, the short block was shipped to America and then all of the “accessories” were added here.

 

[Nick F]

Standard engine rotation is CCW when viewed from stern towards bow. If the transmission does not change the direction, then the prop will also be rotating CCW when viewed from the stern. This is called LH rotation and the prop will be a LH prop.

Boats with single engine generally have LH rotation when going forward. This pushes the stern a little to port when going forward. The preferred docking side is to starboard, since going astern will suck the stern towards the dock.

In my opinion:
If you have a choice, operate the boat with the transmission NOT changing the direction of rotation. There will be less load on the transmission internals and the transmission will probably be slightly more efficient.

 

[Bmarler]

It is important to know what the transmission is designed and rated for. Some are rated for full loading in forward only. Reverse may have reduced duty cycle.

 

[kapnd]

Dave Gerr reccomends that the prop walk should be to the helm side of the boat, so the helmsman can clearly see the dock, and more easily assist with line handling.
I’ve known people who changed the rotation to accommodate docking in their home slip.

 

[denverd0n]

Given the cost of a new prop, I would want to be absolutely, 100% sure. So, I would put it in gear, turn the engine over once or twice, and see what direction the shaft rotates.

 

[Rufus]

Given the cost of a new prop, I would want to be absolutely, 100% sure. So, I would put it in gear, turn the engine over once or twice, and see what direction the shaft rotates.

A Twin Disc MG502 is hydraulically shifted. The hydraulic pump in the transmission is driven by the engine. The clutches in the transmission won't shift without the engine actually running. The design allows the same transmission to be used with either rotation by simply redirecting oil flow.

BTW, I have an MG502 RH for sale in the parts section for $999.99. I've kept it as a spare for some 20 years. Rediscovered when cleaning out the garage after selling the boat.

Twin Disk https://twindisc.com/

 

[C lectric]

I have a V555M with a T.D 506 gearbox. It is a single engine installation.

Stock rotation on this is L.H. which means looking at the prop from the boats aft end the prop is spinning in an anticlockwise rotation.

 

[Mako]

Dave Gerr reccomends that the prop walk should be to the helm side of the boat, so the helmsman can clearly see the dock, and more easily assist with line handling.
I’ve known people who changed the rotation to accommodate docking in their home slip.

I haven't read that in Gerr books. Interesting. But are you referring to being in forward gear or reverse?

Personally my preference is opposite to what many state here. It is intuitive for me, and reflex, to have a RH prop in forward and to prefer to dock to port. In reverse my arse walks into dock. I'm considering a semi-assymetric deckhouse build and would make it tight to starboard, with the exterior walkway to port. Dare to be different, Ha

 

[Sailor of Fortune]

A Twin Disc MG502 is hydraulically shifted. The hydraulic pump in the transmission is driven by the engine. The clutches in the transmission won't shift without the engine actually running. The design allows the same transmission to be used with either rotation by simply redirecting oil flow.

BTW, I have an MG502 RH for sale in the parts section for $999.99. I've kept it as a spare for some 20 years. Rediscovered when cleaning out the garage after selling the boat.

Twin Disk https://twindisc.com/

Correct! 502 TD can work as either rotation desired

 

[kapnd]

I haven't read that in Gerr books. Interesting. But are you referring to being in forward gear or reverse?

Personally my preference is opposite to what many state here. It is intuitive for me, and reflex, to have a RH prop in forward and to prefer to dock to port. In reverse my arse walks into dock. I'm considering a semi-assymetric deckhouse build and would make it tight to starboard, with the exterior walkway to port. Dare to be different, Ha

Prop walk is always more pronounced in reverse, where there is no rudder in the thrust path.