gearbox ratio calculation

[Roberto Certain]

Thank you. I know that with the 120hp diesel twins I wont achieve the original boat speed. I Just want to move the boat at a low speed. Originally the volvo penta came with Stern drives but in Hot weather sterns get damaged very fast, so I want to change to a vdrive with a shaft and a propellor, what I dont know is what gear ratio and propellor size to use.

Or a conventional shaft if I have the space.

 

[rxcomposite]

Hot weather sterns get damaged very fast, so I want to change to a vdrive with a shaft and a propellor, what I dont know is what gear ratio and propellor size to use.

We built a lot of stern drive patrol boats with Volvo Penta engines. Volvo's are known for lightweight high speed diesels and as such, its use is for low to medium duty cycles. Overhauls, not necessarily failures is more frequent if you load it most of the time. This is called duty ratings. Engines with high duty ratings (loaded most of the time) are heavy, slow turning, and beefy. We reserve that for fishing boats. Better check what duty cycles you intend to use your boat. Most of the time, the engine choice is the problem, not the gear. There are 5 duty ratings for engines. Sometimes as much as 6.

If you are convinced about the V drive, the best advice you can get is from the supplier of the transmission. They are going to ask you about the ship particulars, type of hull, engine to be used and your intended use. They are usually spot on in giving advice.

 

[comfisherman]

With an existing hull is difficult because you're working within the constraints of adaptability we're starting from scratch and shooting for Optimum. This is the trade-off that so many boat projects always face.

I would mock up shaft dimensions and v drive configurations to see what size wheel you will be able to swing. Decide on diameter maximum and then you have two endpoints to work with.

 

[Adler]

Think about a solution .....
2 x Engines 120hp at 2400 rpm
2 x Gearboxes Ratio 1: 1.57
2 x Propellers Dia 470mm Pitch 470mm 4 blades GawnAEW Blabe Area Ratio : 0.70
2 x Propeller Shafts Dia:50mm Material: 316L

Cruise Speed 15 kts at 2170 rpm
Max. Speed 17+ kts at 2400rpm