Gearbox Replacement and ratio query

[BlueSailor]

Hi All,

The gearbox fitted to our Nanni engine (Nanni 4.340 TDI) has failed. The gearbox is a Twin Disc TM545A ratio 2:0. The gearbox is now obsolete. The gearbox failure has been caused by the key shearing. To repair the gearbox requires new Input Shaft, Clutch Housing, Bearing & Key as well as various seals. Some of these parts are apparently on a 12 week leadtime which is not acceptable to us.

A UK supplier has found 2 TM545A gearboxes in stock but they have ratio of either 1.5:1 or 2.4:1. I have no expertise in this area and my questions are:

1. What are the implications if I fit either the higher or lower gearbox to my engine?
2. Would it be feasible to strip either of these gearboxes and take the required parts. The gears on my current gearbox are OK, so we are just talking about the input end of the gearbox.
3. Any other bright ideas

Thanks in advance,

Will

 

[scottie]

You would likely require major surgery to or replacement of existing propeller to use either but 1.5:1 could be easier for the shafts
MIT used to be go to people for twidisc

 

[DownWest]

Don't know those boxes, but worth digging up the parts diagrams and numbers and see if the parts you need are common to your box. Otherwise, for a quick fix, I would fit the 2.4:1...

Edit: Scottie went for the 1.5:1 while I was typing (bit slow...) but I was thinking the 2.4 is less of a % difference. Also, as a quick fix while you wait for fresh bits for your existing one. (If available..)

 

[scottie]

Don't know those boxes, but worth digging up the parts diagrams and numbers and see if the parts you need are common to your box. Otherwise, for a quick fix, I would fit the 2.4:1...

Edit: Scottie went for the 1.5:1 while I was typing (bit slow...) but I was thinking the 2.4 is less of a % difference. Also, as a quick fix while you wait for fresh bits for your existing one. (If available..)

Going up in ratio increases loads I believe

 

[DownWest]

Going up in ratio increases loads I believe

I was thinking the opposite, as in the prop is running slower with the higher ration, as most boxes are reduction, not running the prop at higher speed than the engine.

Also, just curious that it needs a new 'clutch' housing?

 

[PCUK]

If you use either of the above boxes you will need a new prop'. I'd go for the cannibalise option.

 

[DownWest]

Going back to the OP, If you fit the higher ratio box, your engine runs faster for the same prop revs. Assuming that it was well matched to start with(?) then you will likely have a lower cruising speed. But, maybe, not far off the fuel consumption/ knot.

 

[BlueSailor]

Thank you all for the input so far. I am in touch with MIT and they have found these 2 gearboxes for me. I've attached a couple of photos showing the sheared key and the 2 halves very much locked into their respective housing. Definitely thinking cannibalisation is the way to go. Just need to see if anything else gets found in the USA. Just to complicate matters we are in Antigua and need to depart before hurricane season, so we have 6 weeks to solve this.

 

[Boater Sam]

I would bite the bullet and fit a new box from PRM

 

[DownWest]

Thank you all for the input so far. I am in touch with MIT and they have found these 2 gearboxes for me. I've attached a couple of photos showing the sheared key and the 2 halves very much locked into their respective housing. Definitely thinking cannibalisation is the way to go. Just need to see if anything else gets found in the USA. Just to complicate matters we are in Antigua and need to depart before hurricane season, so we have 6 weeks to solve this.

Think I would bet that the input shaft the same on the boxes.

There are ways to build up the shafts and re-machine them, but the catch maybe the damage to the gear's internal surface, as that is part of the reduction ratio.

 

[Tranona]

You don't say what your boat is. This is the most important piece of information to get an idea what difference a change in reduction ratio might bring. The designer will have specified that ratio for that engine as the optimum for the propeller required to drive the boat. The engine is rated at 130hp at 2600rpm, so your current shaft speed is 1300rpm, with 1.5:1 it is 1733 rpm and 2.5:1 1040 rpm. both substantially different in terms of the propeller they will drive. The former will be smaller size and the latter larger. Generally smaller sizes are used on lighter easily driven boats and larger propellers for heavier boats. Generally speaking a large diameter finer pitch propeller is more efficient because of the larger blade area, particularly in adverse conditions. However diameter is often constrained by clearance so a compromise of 1300 shaft speed is very common. If the engine were different , particularly in terms of maximum rated revs then a different ratio box might be used to get the shaft speed right. For example my boats engine is max 3600rpm and is commonly used in many 30-35' sailboats with a 2:1 (1800 shaft speed) and a 15" diameter propeller. My boat is long keel heavy displacement and is best with a 17" propeller so I fitted a 2.5:1 reduction to reduce the shaft speed to 1440 rpm needed for that diameter propeller.

If you post details of your boat it is possible to calculate what the optimum propeller is and the impact on propeller size and performance of the alternative ratios. However my gut feeling is not to change ratios, particularly if you have an expensive propeller.

 

[BlueSailor]

Thank you Tranona. My boat is an old Oyster 55. Heavy displacement (26T) and shaft drive. The prop is a 3 Blade Max-prop that has been set at 26o RH.

 

[Tranona]

That is generous power you have compared with the original which I think would have been a rather mean 4 236 Perkins. Not overpowered - you need 120hp to get hull speed.

I would guess that you have a 22" or so diameter 3 blade and set to a pitch equivalent to13 or 14". Calculations from vicprop (commonly used programme) suggests for 1.5:1 a 19*10 and a 2.5:1 a 25*16.

All suggests staying with the 2:1 either with a modified Twin Disc or changing gearbox to another make. I am not familiar with the internals of hydraulic boxes or whether it is possible to fit your existing reduction set into a new box in place of the existing , but that does seem the simplest solution. PRM do a suitable hydraulic box the PRM 500 but probably would not be a straight swap as size and output height are likely to be different.

Have you tried AR Peachment. the Nanni distributor in the UK? peachment.co.uk/discontinued/4-340tdi/

 

[BlueSailor]

Many thanks. Yes I am in regular contact with Peachment and MIT. I think that it is all pointing towards buying one of the 'wrong' ones and taking out the parts that we need. Seems an awful waste.

 

[arcot]

Hi All,

The gearbox fitted to our Nanni engine (Nanni 4.340 TDI) has failed. The gearbox is a Twin Disc TM545A ratio 2:0. The gearbox is now obsolete. The gearbox failure has been caused by the key shearing. To repair the gearbox requires new Input Shaft, Clutch Housing, Bearing & Key as well as various seals. Some of these parts are apparently on a 12 week leadtime which is not acceptable to us.

A UK supplier has found 2 TM545A gearboxes in stock but they have ratio of either 1.5:1 or 2.4:1. I have no expertise in this area and my questions are:

1. What are the implications if I fit either the higher or lower gearbox to my engine?
2. Would it be feasible to strip either of these gearboxes and take the required parts. The gears on my current gearbox are OK, so we are just talking about the input end of the gearbox.
3. Any other bright ideas

Thanks in advance,

Will

Larger ratio such as 2.4. 2.6 equals slower turning prop.....so you're able to use a larger prop.

 

[BlueSailor]

Thank you all again. We finally found the exact gearbox languishing in the Nanni warehouse, same model, size and ratio. Unused, but it had just sat on the shelf for many years - maybe 20! Managed to get it shipped to Jolly Harbour and it is now fitted. All appears good. Valent goes backwards and forwards again, so big step forward. We finally left a few days ago and had a successful 8 hr motor to Nevis. Everything crossed that the issue is now resolved.