Prop Sizes and Gear Ratios

I see a large number of discussions on bulletin boards and web sites on the relationship between prop sizes and gear ratios. Since everyone seems to have an opinion, I thought I would throw my thoughts in as well.

The weight and length of the boat, the horsepower and torque of the engine, the ratio of the drive (or transmission) and the size of the prop all work together to affect the boat's performance. In the discussion below I am ignoring slip (amount of prop inefficiency around 10%), the power losses by the gears themselves (around 3-5%), and the hull shape.

A prop's two dimensions each affect performance in different ways. For every inch of change in prop diameter the RPM varies by about 500 RPMs (up or down) and every inch of pitch changes the RPM by approximately 150 to 200 RPM. So if you have a prop that measures 14"x19" and then go to one that is 15"x17" instead, your RPMs will go down by approximately 100 RPMs (moving from 14" to 15" will decrease the RPMS by 500 and changing pitch from 19" to 17" will increase the RPMS by about 400 RPMs). The effective top speed won't change much as these two props are about the same, but the increase of diameter will help the engine get up on plane quicker (hole shot). With dual props however, pitch changes are measured as a function of both props. Therefore, the change is approximately 70% of single props: 280-350 RPMs or 105-140 RPMs respectively per prop.

The diameter of a prop decides how much water you will disturb while the pitch determines how quickly you push the water displaced out of the way. A prop's pitch is similar to an airplane wing as well, with high and low pressure sides. So a prop with a large diameter prop with no pitch would not displace any water until pitch is introduced. The boat moves because of a combination of the differing pressures and Newton's Law that every motion has an equal and opposite reaction.

So if you have a barge or a sailboat (neither are going to get on plane), then you want the biggest diameter you can swing with the flattest pitch. This allows the prop to take advantage of the torque the engine develops. For these types of boats the important thing is to have as much power (thrust) available for use, rather than trying to develop speed.

If you have too big of a prop you won't reach maximum RPMS. This relationship is the same at whatever RPM you are operating at, but is easiest to see at full RPM where the torque, horsepower and fuel consumption efficiency curves cross and your engine is working harder than it should to reach any RPM. It is inefficient and in the long run (very long run) can damage your engine. You'll have a terrific hole shot, but won't be able to run at full RPM-- like driving a car uphill in fourth gear when you should shift down to third.

If you have too small of a prop it means that you can go over your maximum RPMS and that your engine over-revs to get to a specific speed. It also means that you may have trouble getting out of the hole or it may take forever to get up to plane and you won't be able to go very quickly as you will have the prop turning so quickly that cavatation develops. Cavatation destroys the wing lifting component. (You can actually get the water boiling on the low pressure side.) It is like driving down a hill in third gear when you should be shifting up to fourth gear.

It is all a question of torque. For the math lovers among you, the formula is usually stated as follows T(torque) = (5252 x Engine HP)/RPM. The lower the RPM and the higher the horsepower, the greater the torque that is developed. In other words, slower turning props deliver more thrust as they receive more thrust for the same horsepower.

A Volvo engine with 210 HP creates 368 foot/pound at 2400 RPM. If you had a 1:1 gear the torque at the prop shaft would be 368 foot/pounds at 2400 RPM. If instead there was a 2:1 gear (or drive), the prop shaft would turn at 1200 RPM and the torque on the shaft would be 919 foot/pounds.

If you have a high ratio gear like a 2.0 it means the engine has to turn two times to every one turn of the prop shaft. So you would need a more aggressive prop (larger diameter and bigger pitch) to take advantage of the reduction and corresponding increase in torque. But having an aggressive large prop assumes you have a drive (or gear) that can handle this amount of torque and enough horsepower to turn a big prop. All of the variables need to be balanced (HP, ratio, RPM, prop size etc.) This is why tug boats have relatively large horsepower engines, big ratios like 5:1, and huge props. My father served on Liberty Ships during WWII. They had 2,500 HP triple expansion steam engines that used 18-foot low-pitched props that turned at a maximum of 76 RPMs.

Conversely, if you have a really low ratio like a 1:1 then the engine would not have any reduction, so the shaft would turn really quickly but with only the torque developed by the engine. It is similar to how you can stop a little 1/2HP electric motor going 5000 RPM with your fingers, but you can't stop your 1/2 HP table saw electric motor turning only 3500 RPM with a piece of oak. With electric motors it is a function of the windings and current and RPM, with diesel engines it is Cubic Inches(HP), gear reduction and RPMs.

As a rule of thumb, the larger HP engines have either a lower gear ratio (1.68:1) for lighter, speedier pleasure boats and higher gear ratios (3:1 or higher) for heavier work boats (tugboats).

Smaller HP engines have either no ratio (1:1) for higher speed small light little boats (where torque is not as important) or higher ratios (2.15:1) for heavier pleasure boats where the lower horsepower can be somewhat overcome with the increase in torque.

And finally, prop-sizing can partially mitigate incorrect gear ratios, but only partially. We've had customers who had drives with too high a ratio (2.08 vs. 1.78) but compensate by using more aggressive props. Their performance was fine but the high torque put tremendous pressure in the drives until they prematurely failed. On the other hand we have had customers who have had drives at too low a ratio (1.68:1 vs 1.95) which meant that they couldn't get a prop big enough to optimize performance, even if they had enough horsepower to swing it.

I would love to hear from you if you have any comments on this issue or just need a part. We are here to help.

Sincerely,

J. D. Neeson
President, Marine Parts Express
My email: jdneeson "at" marinepartsexpress.com
(yes, just put the @ in place of "at", don't you just hate those spammers?)