Gearing

[Anthony]

Hey guys.
I have a question that I require advise on. I own a 27' Luhrs Alura weighing in at about 6000lbs with a Yanmar 4LHADTP 200hp diesel with less than 100hrs. My current gear is a 1.5:1 ZF on a 1.25 shaft turning a 18x14 3 blade ACME prop. My current WOT RPM is 3400 at about 20knots.
I am thinking about changing my gear to a 2.0:1 or even a 2.5:1 and reworking my prop. At the end of the day I'm looking for better top end performance with out overworking the motor or changing shaft. What do the experts think?
Happy new year to all.

Anthony

 

[Keelboater]

You will have to change the shaft. How big can you go on the prop diameter?

 

[pugsley]

200 hp is 200 hp, very little will change with a gear change.

 

[Keelboater]

200 hp is 200 hp, very little will change with a gear change.

Output torque from the 2.5:1 gearbox is increased significantly from the 1.5:1 box. If you want to use that torque, a bigger shaft is required to turn a larger prop. The HP does not change.

 

[pugsley]

Output torque from the 2.5:1 gearbox is increased significantly from the 1.5:1 box. If you want to use that torque, a bigger shaft is required to turn a larger prop. The HP does not change.

i agree, 2.5:1 would be a much bigger wheel

 

[captainlarry84]

Hey guys.
I have a question that I require advise on. I own a 27' Luhrs Alura weighing in at about 6000lbs with a Yanmar 4LHADTP 200hp diesel with less than 100hrs. My current gear is a 1.5:1 ZF on a 1.25 shaft turning a 18x14 3 blade ACME prop. My current WOT RPM is 3400 at about 20knots.
I am thinking about changing my gear to a 2.0:1 or even a 2.5:1 and reworking my prop. At the end of the day I'm looking for better top end performance with out overworking the motor or changing shaft. What do the experts think?
Happy new year to all.

Anthony

I would do nothing. You are there. If you went with a 2:0 or 2.5 gear you will need a 1.5 shaft. I doubt that your current strut or bearing housing could take that machined increase, so the price goes up.
With a 2:0 gear you would swing a 18 X 18 & with the 2.5 and 18 X 20.
I do not know if the bottom of your vessel can feed enough water to these wheels. Let see some pictures of the bottom.
Also was the boat a repower or factory install. Luhrs makes a lot of boats so if it was factory I am sure they gave a lot of though to the correct gear. You are doing very well @ 20 knots & 200 ponies.

 

[Anthony]

Thanks guys. What I failed to tell you all was that this is a repower from a gas to diesel. There is no keel on these boats and are planning hulls. She is single screw and runs really strong. I guess what you all are saying is LEAVE GOOD ENOUGH ALONE.

Thanks guys and happy new year to all.

Anthony