gear box ration choice

[hinemoa]

hi , am getting to stage where i need to select final drive ratios for my 16m cat, powered by twin toyota 2h diesels, power rating 118hp at 3300rpm, but would like to keep them turning at aprox 2500rpm, which should be aprox 86hp...
prop size is 18x18 lh and rh props and shaft diameter is 1 3/4 stainless shafts with rudder steer.....
any info or views i will definetely be happy to hear

thanks.....

[baeckmo]

Without going into detail analysis of the resistance of your hulls > possible performance > optimum propeller, the following is just a rough target.

For 86 hp and a standard three blade prop with dia 18", the screw should rotate something like 1610 rpm; ie for ~2500 engine rpm's you need a reduction gear ratio of 1.55:1. All provided your hull is able to reach ~18 to 19 kn with this power.

If not, the pitch has to be adjusted; the diameter remains fixed. A resonable hull shape for this performance could have a total displacement of something like 5.5 m3. Is your vessel outside this envelope, you should reconsider the prop sizing.

[hinemoa]

great thanks... hulls are aprox 600 wide in water depth area and 400-500 mm draft and are reasonbly veed, thank you for your advise, this will give me an idea of what to look for as i was looking at 1-1 or 2-1 but as you have stated i need to be inbetween...

[FAST FRED]

IF there are no other boats built , that you could find out the performance.

Boat speed vs GPH of fuel burn at cruise you can do better than a guesstimate.

Launch the boat with out engines or props and get a fellow with a big fish killer to tow you at various speeds.

Measure the line pull at 3k on up to how ever fast you think you would like to power . How thick is your wallet?

The resistance will allow a good prop / gear reduction , along with the engine Mfg spec sheet.to be optimized for the cruise speed you can pay for.

[hinemoa]

hi ok, that sound difficult as there are not really any large boats on the lake, it was previously powered by 120hp caterpillar engines, but i dont know if there is a difference in real 120hp industrial engines or more domestic like i am looking at the hino 2h diesel out of a truck as from what i have understood there is a difference in a marine 120 hp and automotive/truck 120hp, i know there is a huge price difference as the cat engines are around 40+k per engine

No, there is no such difference.

120 hp is 120 hp.

But there is a hughe difference in the output of any diesel engine! You can have the same engine producing 120 or 450 hp, depending on the purpose she is used for.
It is common tht a marine engine is available in ratings from M1 to M4, where M stands for marine use and the 1 for unrestricted continuous output over 24 hrs. The M2 would have some 20% higher output, but that cannot be used continously, just for say, 1hr out of 6.
And so on.
The same engine having 120hp in M1 rating can have 380hp in a truck.

And you should be aware that the more threads you start on the same topic, the less likely you get all the information and attention you search for!

Regards
Richard