Steyr repowering - Gear ratio question

[lore]

I did not know much about these engines untill last year when Bruce started to post information here. Few weeks ago I was in Venice (Italy) and I discovered that most taxis are powered with Steyr. People that are using them say only good things about these engines. Meanwhile I was lucky to find a very lightly used pair of Steyr 236 at a good price. So I have decided to repower my Bertram28 at the moment with gas Merc 260 on. My mechanic is suggesting me to get as transmission ZF45A with gear ratio 2:1 instead of 1.5:1. Now I am confused: these engines run in terms of RPM exactly like my gas Merc and now I have a gear ratio of 1.5:1. Why should I change the gear ratio ? Someone can explain to me.

[CaptPatrick]

Lore,

Your mechanic is correct in recommending the 2:1 ratio over the 1.5:1...

With the 2:1 you'll get the full advantage of the torque available from this engine & allow you to swing a larger prop. Bottom line: Much better performance...

Br,

Patrick

[lore]

Thanks, Capt

[Sean B]

I'm curious.... more of a hypothetical question than an actual planned project.

I've got gears with a ratio of 1.54:1 on my 3208T cats pushing my B33 around. The 3208 tops out at 28-2900 RPM. If I were to swap the gears out for ones that have a 2:1 ratio, and put on bigger and appropriately-sized props, should I expect an increase in engine performance from that change alone?

[CaptPatrick]

Sean,

You've already got the correct gear setup for your engines and boat....

Br,

Patrick

[Bruce]

Depends.

If you have the original 1-1/8 shafts and don't want to spend the money to upgrade, then you'll need the 1.5:1.

1-1/8 shafts are not really up to handling the larger propeller torque of the 2:1.

Those are high rpm as over 4200 engines. Conventional thinking changes some.

Sean,
You would see an increase in performance. Unfortunatly you won't be able to swing the prop needed. Not nearly enough room.

[Sean B]

Well thank you gentlemen, both, for your responses.

I shouldn't be trying to make up reasons for future up boat projects anyway... got plenty of them already that definitely make sense to do. But thanks for indulging my curiosity

[lore]

Bruce,

two years ago I upgraded the shafts from the original 28mm (1 -1/8) to 35mm (1-3/8 (?)) and also put stronger custom made struts. Probably I am OK for handling higher torque. But what size of propeller should I look for ? Do I have enough space for the prop needed ? Now if I am wright I have three-blade pro 17X19. Should I look for four-blade prop?