Gearbox Ratios

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

 

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.

 

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

 

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.

 

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

 

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.

 

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.