What propeller to fit 40 mm shaft to reach minimum speed

[oldharry]

Hi everyone, I removed the cable connected to the gearbox twin disk mg 506 and I tried to turn the valve gently and slowly till I make the shaft start turning slow than it it runs to normal speed. I didn't hear any harsh engagement or crunch in the Gearbox , too many told me that the hydraulic gearbox make that sound in the engagement, if that's the case so why the engagement is much smoother manually ? Many thanks

Not good. Like in a car you can control the speed by slipping the clutch - for about 5 minutes. Then the clutch fails. Same with your boat gearbox.

 

[Soulayman]

Not good. Like in a car you can control the speed by slipping the clutch - for about 5 minutes. Then the clutch fails. Same with your boat gearbox.

@oldharry , thank you for the information, if I change the ordinary coupling to a flexible coupling between the gearbox and prop shaft, will the engagement become smoother ? Thanks

 

[DownWest]

No, it just allows for slight mis-alignment or engine movement if on flexible mounts.

 

[Soulayman]

No, it just allows for slight mis-alignment or engine movement if on flexible mounts.

Thank you DownWest.

 

[PCUK]

The only sensible thing you can do is fit a gearbox with a trolling valve that allows the shaft to run slower than the engine.

 

[oldharry]

The only sensible thing you can do is fit a gearbox with a trolling valve that allows the shaft to run slower than the engine.

The sensible thing is to fit the correct propellor, like the rest of us.

 

[DownWest]

The sensible thing is to fit the correct propellor, like the rest of us.

Yes, except that it will have to be quite fine pitched to manage 1 kt at the lowest RPM that the engine will run at. But that is result of fitting a bigger engine for more speed, presumable to get out and back faster. Catching the markets?
Used to be done with a 'wing' engine on the main shaft, only used for the low speed stuff.

He could go back to his original engine?

 

[Soulayman]

The only sensible thing you can do is fit a gearbox with a trolling valve that allows the shaft to run slower than the engine.

@PCUK, my twin disc gearbox is 2:1 ratio. I Can change a different gearbox with 3:1 ratio. But that will be the hard way to do it as my shaft is at 8 degree , so I need to find a gearbox with 3:1 ratio that should fit my shaft. I measured my shaft inclination I find 7.8degree , but when I checked the manual of the gearbox twin disk mg 506, I find that should be 8 degree. There is 0.2 degree off. But the shaft and the gearbox now are perfectly aligned. But aligned in 7.8 degree, and not at 8dgree, can that 0.2 degree off cause a bump sound in the gearbox ? Many Thanks

 

[Tranona]

@PCUK, my twin disc gearbox is 2:1 ratio. I Can change a different gearbox with 3:1 ratio. But that will be the hard way to do it as my shaft is at 8 degree , so I need to find a gearbox with 3:1 ratio that should fit my shaft. I measured my shaft inclination I find 7.8degree , but when I checked the manual of the gearbox twin disk mg 506, I find that should be 8 degree. There is 0.2 degree off. But the shaft and the gearbox now are perfectly aligned. But aligned in 7.8 degree, and not at 8dgree, can that 0.2 degree off cause a bump sound in the gearbox ? Many Thanks

You really do seem to have limited understanding of boat propulsion. The last thing you want is a bigger reduction ratio. You will need something like a 26" diameter propeller with that slow running a shaft and the amount of excess power you have. The boat will be even more uncontrollable than it is now.

 

[Soulayman]

You really do seem to have limited understanding of boat propulsion. The last thing you want is a bigger reduction ratio. You will need something like a 26" diameter propeller with that slow running a shaft and the amount of excess power you have. The boat will be even more uncontrollable than it is now.

You are correct. I'm not sure about the diameter of my propeller I guess it's close to 26", it's bronze prop 14kg weight.

 

[Soulayman]

You are correct. I'm not sure about the diameter of my propeller I guess it's close to 26", it's bronze prop 14kg weight.

Here is a video of my boat at 200% video speed. The propeller is approximately 4 feet under water, deep V shape of the hull.

The Propeller size is around 25" diameter . It's a heavy wooden boat. 33feet long , 9.8 feet wide and 3.3 feet tall. Fiat engine 130 Hp . Reference Fiat 8062.02 , Gearbox : Twin disc mg 506 , 2:1 Ratio. Many Thanks

 

[oldharry]

Why have you not asked a local marine engineer for advice? Changing gearboxes will only make things worse as well as costing you a mint. The problem is the propellor is not matched to the engine and boat. End of.

 

[Soulayman]

Yes, I will check the propeller size and maybe replace it with new prop. Thanks

 

[DownWest]

It

Yes, I will check the propeller size and maybe replace it with new prop. Thanks

It is not so much the diameter of the prop, but the pitch. That is the twist of the blades and how much it (in theory) travels through the water for each revolution. A much finer pitch (less twist) will push the boat at a slower speed for the same revolutions. The problem then, is that when you increase the revolutions to go faster, the maximum revs of the engine will result in a lower speed and basically, will not be using it's power. Your problem is wanting to spend a lot of time at very low speed, while fishing. It is very difficult to match that to a big engine, as you have found.

 

[Soulayman]

Yes I think my prop pitch is wide, Its abvious. Thanks

 

[jamie N]

Soulayman, you've a fine looking boat, and your 'home' harbour looks warm. Where is that?

 

[Soulayman]

Tunisia here

 

[Soulayman]

Soulayman, you've a fine looking boat, and your 'home' harbour looks warm. Where is that?

Thanks

 

[scottie]

Thanks

In general Diameter is determined by availabilL HP whilst pitch affects speed

 

[Soulayman]

In general Diameter is determined by availabilL HP whilst pitch affects speed

Thank you