How to choose the correct transmision gear ratio

[survivor2]

Which is the criteria when choosing the correct gear ratio in a repowering?
I?d like to know technical principles when choosing a gear ratio of transmision for a boat,
I have my particular project, but I?d like to know how theory works for all cases,
Is the same criteria for sterndrives and v-drives transmisions???
My boat is a copy of the crestliner 1986 cuddy 20ft V hull , I have an OMC 5,7l 1986 v8, 260hp and I guess is around 340 lb-ft of torque. It came with an OMC stringer sterndrive that is broken (im not going to repare it).
What about transmission gear ratio if I replace my actual gas engine (lets say 4500-4800 wot rpm) with a SAME TORQUE diesel engine (4000-4200 wot rpm) ???
I see towing sports boats uses 1:1 - 1,2:1 ratios (wakeboarding boats) I also see different gear ratios for mercruiser sterndrives for example, but y don?t know it?s relationship with engine RPMs torque, boat usage, etc.
I know prop choosing is very important, I understand its basic priciples, so I would prefer to leave it out of this question, I know everything is related, but I need to understand why choosing an specific gear ratio in any case.

 

[Bondo]

Re: How to choose the correct transmision gear ratio

Ayuh,.... Welcome Aboard,.....

Generally speakin', Props only come in a rather narrow choice of Pitch,....

Ya pick a transmission ratio that puts the motors peak torque band, into the proper rpm band for the prop to be most efficient,...

 

[Scott Danforth]

Re: How to choose the correct transmision gear ratio

to add to that, the 350's generally had close to a 1.5:1 ratio in a single s prop outdrive

 

[mpsyamaha]

Re: How to choose the correct transmision gear ratio

from what ive seen that sounds right... somewhere around a 1.5:1 ratio for a setup like that.

 

[survivor2]

Re: How to choose the correct transmision gear ratio

to add to that, the 350's generally had close to a 1.5:1 ratio in a single s prop outdrive

thanks capt, i got it
what would change in a non single prop application?

 

[survivor2]

Re: How to choose the correct transmision gear ratio

Ayuh,.... Welcome Aboard,.....

Generally speakin', Props only come in a rather narrow choice of Pitch,....

Ya pick a transmission ratio that puts the motors peak torque band, into the proper rpm band for the prop to be most efficient,...

THANKS !!! thas accurate information aboard!!!

i think im understanding how it works, more i know, more i like marine mechanics....

now, were can i get information about engines power curves? (yes that "confidential" information as mercruiser sais)
and more important, wich is the criteria (or information) related to proper rpm band for props?

 

[Bondo]

Re: How to choose the correct transmision gear ratio

THANKS !!! thas accurate information aboard!!!

i think im understanding how it works, more i know, more i like marine mechanics....

now, were can i get information about engines power curves? (yes that "confidential" information as mercruiser sais)
and more important, wich is the criteria (or information) related to proper rpm band for props?

Ayuh,.... What motor, 'n What drive,..??

Yer questions are gettin' way to Vague,....

Criteria for pickin' the gear ratio is still the torque curve, 'n peak torque,...