Separate prop shaft coupling from gearbox

Soulayman said:
Hi to all, I reduced the output flange from 60mm to 40mm to fit my actual coupling prop shaft 40mm. And now I hear a bump everytime I put farward or reverse . Can that bump caused after reducing the output flange ? Many thanks to all
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Depends on how you reduced the internal diameter of the coupling.
 
Tranona said:
Depends on how you reduced the internal diameter of the coupling.
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@Tranona, I didn't reduce the internal diameter of the coupling, the actual coupling is already connected to 40mm shaft, but I change the exterior diameter of the output flange, because it was bigger than the coupling.
 
Soulayman said:
@Tranona, I didn't reduce the internal diameter of the coupling, the actual coupling is already connected to 40mm shaft, but I change the exterior diameter of the output flange, because it was bigger than the coupling.
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That was not clear. Did you have to redrill the holes for the coupling bolts? Normally the coupling and the flange are a pair and if you fit a different gearbox the half coupling on the shaft should be changed as well to match the one on the gearbox.
 
Tranona said:
That was not clear. Did you have to redrill the holes for the coupling bolts? Normally the coupling and the flange are a pair and if you fit a different gearbox the half coupling on the shaft should be changed as well to match the one on the gearbox.
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@Tranoma, I redrill the holes in the output flange of the twin disc gearbox, and I keep the coupling bolts. My old coupling shaft 40mm was connected to buck gearbox with ratio 2:1. I changed the engine as well as the gearbox to Fiat engine and twin disc gearbox 2:1 ratio. The output flange of the new gearbox twin disc is bigger than my old coupling, I think it was connected to 60mm shaft. so I make new holes in the output flange and I reduce the diameter to fit my actual coupling shaft 40mm.
 
mogmog2 said:
My Centaur gearbox situation (monoshift gearbox on MD2B) moved on a bit last night when I managed to undo the bolts holding the gearbox to the engine (a bit fiddly, but only took about half an hour: 8mm hex socket but driven by an 8mm ring spanner on it as there was no room for any ratchet/extension combo I could devise).

I had already undone the prop shaft coupling. I undid the stern gland nuts and tried a number of moderately gentle methods to slide it back: hit/tap with hammer the rotate, hit, rotate etc, drifting it back with a hammer & cold chisel, and striking a ring spanner laterally whilst it's on the coupling through-bolt head.

Pulling in the prop from outside the boat is a possibility, albeit unappealing as the mud is grim. And cold.

Does anyone have any tips for separating the coupling-with-shaft from the gearbox? Have I got to get brutal?
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ACF-50 may help. It does what it says on the tin.
 
Soulayman said:
@Tranoma, I redrill the holes in the output flange of the twin disc gearbox, and I keep the coupling bolts. My old coupling shaft 40mm was connected to buck gearbox with ratio 2:1. I changed the engine as well as the gearbox to Fiat engine and twin disc gearbox 2:1 ratio. The output flange of the new gearbox twin disc is bigger than my old coupling, I think it was connected to 60mm shaft. so I make new holes in the output flange and I reduce the diameter to fit my actual coupling shaft 40mm.
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Did you get the position of the new holes exactly right so that centre of the shaft is concentric with the output shaft of the gearbox? Too be honest it is really difficult to make suggestions about dealing with the issues on your engine installation when the whole concept of replacing an engine with nearly 3 times the power using the same stern gear is fundamentally flawed. As I said before you seem to have gone about this the wrong way. Powering a boat starts with the propeller in relation to the size/type of hull and speed required then fitting an engine and gearbox that has the right power and shaft speed to drive that propeller. If you want to raise speed with a displacement boat you are limited by wave resistance and simply adding more power will just cause the stern to dig in, the bow to rise and the engine to consume more fuel without any increase in speed. 130hp in your boat is a complete waste of time - the theoretical hull speed needs less than 40hp.
 
@tranoma, I built this new wooden boat in 2017, it's 33 feet long by 10 feet wide with deep v hull about 5 feet deep. Back in time my old engine buck 48hp was just fine dragging the boat with speed 5knt/h. I was happy with that speed unless I face heavy front wind and big waves there when my speed dropped to 3knt/h. Year after year my wooden boat become heavier, and my speed dropped to 4knt /h. I didn't want to invest money changing parts in the old engine but I bought a used Fiat engine 130hp. My boat speed become acceptable, up to 7knt/h. And I think I can reach the 8knt/h in full speed. I'm still afraid from the gearbox twin disc bump when I put forward or reverse. Many told me that hydraulic gearbox do that kind of bump. I didn't trust that a lot so I keep searching for the reason of that sound. I changed the gerabox oil. And the bump sound reduced a little. I searched about the oil viscosity of the twin disc gearbox and it said that I should use 5w 30oil . But I already put 15w40 oil inside the gearbox . Maybe that cause the bump and maybe something else. Many thanks
 
Soulayman said:
@tranoma, I built this new wooden boat in 2017, it's 33 feet long by 10 feet wide with deep v hull about 5 feet deep. Back in time my old engine buck 48hp was just fine dragging the boat with speed 5knt/h. I was happy with that speed unless I face heavy front wind and big waves there when my speed dropped to 3knt/h. Year after year my wooden boat become heavier, and my speed dropped to 4knt /h. I didn't want to invest money changing parts in the old engine but I bought a used Fiat engine 130hp. My boat speed become acceptable, up to 7knt/h. And I think I can reach the 8knt/h in full speed. I'm still afraid from the gearbox twin disc bump when I put forward or reverse. Many told me that hydraulic gearbox do that kind of bump. I didn't trust that a lot so I keep searching for the reason of that sound. I changed the gerabox oil. And the bump sound reduced a little. I searched about the oil viscosity of the twin disc gearbox and it said that I should use 5w 30oil . But I already put 15w40 oil inside the gearbox . Maybe that cause the bump and maybe something else. Many thanks
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Hi most twin disc box's use a single grade oil like sae 30 or 40 so make sure it's right. 15w40 definatly isn't!

A bump is difficult to diagnose, post a video?
 
Th
jwfrary said:
Hi most twin disc box's use a single grade oil like sae 30 or 40 so make sure it's right. 15w40 definatly isn't!

A bump is difficult to diagnose, post a video?
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Thank you jwfrary, I will post a video about the bump. I find this oil 5w30 in shop but I'm not sure. I think single grade oil should label just SAE 30. Many thanks
 

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Soulayman said:
Th
Thank you jwfrary, I will post a video about the bump. I find this oil 5w30 in shop but I'm not sure. I think single grade oil should label just SAE 30. Many thanks
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That's a Multi grade oil, which isn't the right thing at all, way to thin for this application.

A good auto store rather than a garage should be able to help. Straight 30 weight or sae 30 would be the phase to use.

Toolstation seem to sell it if your in a pinch.

Engine Oil 1L 4 Stroke | Toolstation

Its essentially classic motor oil, which you would have found in cars before the 70s ish.
 
jwfrary said:
That's a Multi grade oil, which isn't the right thing at all, way to thin for this application.

A good auto store rather than a garage should be able to help. Straight 30 weight or sae 30 would be the phase to use.

Toolstation seem to sell it if your in a pinch.

Engine Oil 1L 4 Stroke | Toolstation

Its essentially classic motor oil, which you would have found in cars before the 70s ish.
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Many thanks JWfrary. I will find that oil and replace the 15w 40.
 
jwfrary said:
Hi most twin disc box's use a single grade oil like sae 30 or 40 so make sure it's right. 15w40 definatly isn't!

A bump is difficult to diagnose, post a video?
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That's correct. In the twin disc manual I find a note . It's said to not use 15w40 oil viscosity. I changed the oil in my gearbox from 15w40 to SAE 40 . And now everything is fine.
 
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