1:5 to 1.. or 1 to 1:5 .. You make the call.....

[PipeDreamsMarine]

Two members of this site where having a conversation, they were talking about inboard boats. Speed versus horsepower always plays a part in any inboard boat. During this back and forth banter, an in past was struck upon. Here is the issue. With a 1:1 gear the prop spins at the same speed as the motor, so if the engine is turning 2500rpm the prop is spinning close to that same rpm, thus the 1:1 ratio. Both parties agree upon that fact. Now if you have a 1:5 to 1 ratio…I say that for every 1:5 turns of your motor the prop will make one turn, thus increasing the torque of the motor because of the gained advantage of the 1:5 ratio. So you can turn a prop with more pitch/dia and increase your speed, because a slower turning prop is more efficient then one spinning faster. . Now the other person claims it is the other way around that for every 1 turn of the motor your prop turns 1:5 times. This makes the prop spin faster and makes the boat go faster. This is done by turning a bigger wheel, faster in their eyes. So we decided to ask the masses on their thoughts about this matter. Please note that the names have been with held to protect their pride and the loser will not be known to the masses so that public humiliation will be kept to a minimum.

[3rdday]

Please! I am curious to know also , strange I had that thought also this morning. To turn it 1:1.5 wouldn't that require a more power full motor than normal like a 454 or a diesel/ turbo diesel ?
To take it a step further; on an I/O would you need a beefier I/0 for a powerful motor or diesel?
My purpose is not to derail disrupt the above question but to get additional answers on same subject.

[Bushwacker]

A marine transmission with a 1.5 to 1 ratio is often called a reduction gear, because that's what it does - it REDUCES the speed of the prop by 50% and increases torque by same amount, allowing you to turn a larger diameter prop. Big slow turning props are generally more efficient and produce more thrust, except for light, very high speed (over 50 mph) boats. The Suzuki 4-stroke O/B's put an extra gear between crank and driveshaft to get more gear reduction and turn a bigger prop without increasing size of the bullet in lower unit.

Years ago British Seagull made a 5 hp outboard for use on sailboats. It spun a 5-bladed prop that looked like a windmill, about 10" diameter with a hub that must have been about 5" in diameter; must have had about 5:1 reduction gear in it. My neighbor used one on the dinghy for his 42' Wheeler sedan cruiser, which displaced about 20 tons. He could TOW that big Wheeler with that old Seagull!!

[Fr. Frank]

A marine transmission with a 1.5 to 1 ratio is often called a reduction gear

This is correct. The ratio is stated in terms of input shaft rpm to output shaft rpm. 1.5:1 is 1.5 revolutions of the input shaft for every 1 revolution of the output shaft.

[nestorpr]

I agree with Don, Bushwacker & Fr. Frank, the first figure is the input or engine revolutions and the second is the output or prop shaft revolutions. It's a standard throughout the industry, be it marine, automotive, etc.

[PipeDreamsMarine]

I agree with Don, Bushwacker & Fr. Frank,

Please no names.. were are trying to protect the misinformed ...

[3rdday]

Is there a special outdrive for diesel engines? Beefier gears and different ratios since diesels turn lower rpms in thier optimum ranges? Can anyone reccomend a diesel and outdrive setup for a 2800 lb 22ft. craft? Prefer turbo diesel.

[Finster]

Please no names..

OK fine I had it backwards. It happens.