Gearbox Ratios

[Jellyfish84]

hi, just wondering if someone can lead me in the right direction as to gearbox ratios to suit a boat and motor i already have i am considering 1:1. i will be using it primarily as a leasure boat with hopefully 80-100km/h top speed.
The motor has
Maximum Power: 127 kW @ 4800 RPM
Maximum Torque: 293 Nm @ 3600 RPM
The boat i a planning hull 19 foot that will weigh 850kg max. beam at transom is 2m with 20 degree deadrise. i currently have a 13inch 17 pitch prop which i would like to use for this application if possible.
cheers
eric

[Jango]

If your total weight with Driver and half tank of fuel is 850 Kg max, Max speed calculates to 72 km/hr (39 Knots) with 127 KW (170 hp). Correct prop with 1:1 ratio is a 11 x 11, turning 4800 rpm w. 11% slip.
In order to use a 13x17 prop, gear box ratio will need to be about 1.5:1. Max speed will remain 39 Knots @ 4800 engine rpm.

[Jellyfish84]

thanks
when i said max weight i meant with people, an esky full and fuel tank of full. so it wouldnt be that wieght for long. just wondering how a 12" with 17"pitch would perform
cheers

[Jango]

A 12x17 prop will not work very well with the 1.5 gear. You need the 13 dia with the slower turning prop. You will have increased slippage and less speed.
With a 1:1 gear, even with a 600 kg max, your engine does not have enough power to turn a 17 pitch, Prop will bog engine down, developing less power, rpm and speed.

[Jellyfish84]

jango
did you work these figures out on a three bladed prop or something else.
cheers

[Jellyfish84]

also what size prop shaft would you recommend (will 1" be enough). i will have about 800mm of unsuported shaft between the stuffing box and the skeg.

[Jango]

Three Bladed, 55% D.A.R minimum. You probably can use a 12x17, 4 bladed, 63% D.A.R. min with the 1.5 gear also.
1" shaft should work fine.