What is a better gear ratio?

[biggdave92]

1.85:1
2.09:1
2.15:1

Assuming all motors maxed at 6k rpm's and were the same cubic inch's. Please explain your answer if you can. thanks!

[david bryan]

efficiency of the propeller increases with the rpm that one reason the unlimited boats turn the prop 10.000 thousand

[biggdave92]

so you are saying that a higher gear is better?

[david bryan]

ron hill but a thee blade cleaver on a inboard awhile back with a 36 % over drive they set a record a d lake

[FrenchPhil]

Inshore race boat gears are close to 1:1, they use smaller diameter/pitch props because of light boats/loads. As loads increase so does gear ratio & prop diameter/pitch. So if you just want speed you'd go with the 1.85. I you want to carry more weight go with 2:1 +


so you are saying that a higher gear is better?

[david bryan]

i would agree

[david bryan]

but sk s run 12 to 18% over drive whith lots of hp

[Mark75H]

One thing that is consistent thru all these boats is that bigger boats use bigger props and smaller lighter boats use smaller props.

From this is seems to follow that the first concern is correct diameter. Once you have sufficient diameter for your boat, your gear ratio should run that diameter prop at the correct tip speed for the type of prop. Special "super cavitating" props will be geared for speed above blade tip cavitation speed, more conventional props will need to be run at tip speed below cavitation inducing speed.

Gear ratio choice relates to boat weight and optimum prop diameter for that weight.

[JohnsonM50]

Im not sure you could get the answer without some benchmark from testing. Even then the direction to go might not indicate how far to go. Holding the Rs to 6 g's might be the most elusive part unless you were holding back to it by throttle & of course every props different.

[zul8tr]

1.85:1
2.09:1
2.15:1

Assuming all motors maxed at 6k rpm's and were the same cubic inch's. Please explain your answer if you can. thanks!

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There is no real "better", it depends on the use of the boat/motor. All selections of prop diameter, pitch, gear ratio, horsepower and torque available at various rpm are a compromise for the use intended. High speed boats like hydros use surface piercing small diameter and large pitch props, and if there is great horse power an overdrive gear is used for prop rpm to exceed engine rpm. Generally if all else remains the same, as the gear ratio is numerically increased the prop pitch usually can increase. But there are other factors that come into play along with pitch there is: blade diameter, blade shape, blade area, blade face contour, hub diameter, rake, number of blades, etc.

In the final analysis the prop geometric parameters, style and gear ratio has to be selected to absorb the available engine horse power and torque to achieve the desired rpm, speed and performance.

Generally, all else remaining the same, as gear ratio gets numerically larger (i.e. prop rpm reduces relative to engine rpm) the greater the pitch needed to prevent reving beyond the desired rpm.

[JohnsonM50]

On second look & reading Ill add that the ratios listed have about a 12% spread. Thats likely to make less difference than a pitch change. All the other prop factors count but pitch being the supposed measure. I doubt changing gears would do much & prop changes are easier. 12% wont be likely to relate directly to MPH. An all out racing crew of a boat engineered to near perfection would find gear ratios more important than a guy tryin to get more performance.. :cool: