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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 ExpressVisit our website for more information on determining your prop size.
Comments? Questions? Suggestions for topics for our blog or newsletter? Send them to
info@marinepartsexpress.com.
Marine Parts Express is a division of Water Resources, Inc., a privately held Maine Corporation.
For all your marine engine parts needs, call us toll free at 877.621.2628, or outside the U.S. at 207.882.6165.Related
December 11, 2009 / JD Neeson / 23
Categories: Boating and Fishing, JD Neeson
Tags: gear ratios, Marine Parts Express, prop sizes
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NICE ADDITION TO YOUR SITE J.D.
Which size and weight prop I can use to the marine gearbox it have 3.095:1ratio? and the engine which have 280hp
Hi JD
What fun this “Rutters of the Sea” piece is! I had no idea such books existed. Your mother does a fine job describing it.
Do you know I use the same blog format you do, only my publicist’s husband added my book cover and different typeface to the heading.
I’m wanting to purchase a 4 blade SS prop 18 p
but don’t know if my 125 outboard optimax Merc can handle it.
We need more thrust out of the hole for slolam sking. We have a 3 blade 18p now and it’s not enough. The boat is a 2010 16ft Checkmate might be the type (pulsare)
I will pass your request onto our parts department and you will hear from them via private email message today. Thank you!
Noreen
in addition to this question: the 4 blade 16p prop is for 150- 350hp, so this is the reason the quetion
Okey dokey. I will include this message with your earlier request. Thanks!
Noreen
i made a mistake on the pitch, its an 18p four blade stainless steel prop..
also a spelling and sentence correction, this is the reason for the question. sorry bout that.
Alright–not a problem. I’ll give the info to the parts department. Once you hear from them, you should respond to their email since it will give you a direct link to the parts. Thanks! ~Noreen
I have a 23 ton steel motor sailer – a Spray hull shape. I am getting a 75 hp engine with a 3:1 gear ratio. I have a 24 inch 17 inch pitch prop which needs replacing and have been recommended a 24 inch 19 inch pitch prop. The old gear ratio was 2:1. The old engine was a Ford 72 HP (circa 1960s), the new one a Beta 75 HP. How will the performance compare?
need help replacing gears and would like to use existing props need help figuring proper gear ratio to install…?
Hello. I am looking to turn a pocket bike motor 49cc. Into a small dinghy motor.your article was very informative, and I was hoping you could fill me in further with my custom project. The pocket bike has a6 tooth drive gear, and a 68 tooth rear sprocket. Which based on your article is not going to work. The motor is 3-4hp and Im going to make my own propeller or of a sheet of galvanized steel. I was thinking about 4-5inches and minimal pitch with about a3:1gear ratio. Does this make sense to you? Or could you give me advice on advancing to the next step please. Thanks in advance
Kaeli, I will forward this question to our gear ratio guru and have him take a look at your problem. Thanks for posting!
Thanks for a brilliant and clear explanation. We are in Uruguay with a propeller shot by galvanic corrosion. The link to the anode wasn’t reconnected after our engine was changed and we didn’t realise. We haven’t been satisfied with the performance but we now conclude the gear ratio is wrong, rather than the prop.
As rule: The larger props are able to “hold” the boat to the water better and allow more precise tracking and handling with less pitching and rolling, and less effort at the steering wheel, as a result.
This is a measurement referring to the total diameter of the propeller as measured by drawing an imaginary circle around the blade tips. Increasing or decreasing diameter has a dramatic effect on handling and ease of driving, Boat propellers with smaller diameter blades don’t usually perform as well as larger-diameter props when the water gets rough.
more info: http://www.craigmarine.info/online_shopping/controls_boat_parts_and_steering/Boat_Propellers.php
manufacturers: http://www.craigmarine.info/accessories/controls_steering/Propeller_respected_manufactures.htm
Thank you very much, its the most comprehensive article I have read.
I have a Jenneau sunkiss 47′ sailing boat with a 20-30 propeller and i’m buying a 57HP yanmar (3,000 RPM) engine, which gear ratio should I use?
Hello,
Thank you for your comment. I’ve relayed your question to JD Neeson and he needs to know the weight of the boat and what is the model # of your engine. And by 20-30, do you mean 20″ diameter and 30 degree pitch?
Thanks,
Brian
hello i have a mason clipper 24ft about 3ton in weight very heavy can only do 8 knots at 2,200rpm,have a 15×15 prop has a nissan ld28 diesel with a volvo 280 not sure what ratio yet but i think it may have the wrong ratio set up,i have tried many props ,but cant get more speed,the hp is 90hp?
I have a 22.5 1985 Baja I repowered the boat with 383 HP 504 @6300 rpm trq is 474@4900 rpm the prop I was running was a mirage quick silver 21p three blade prop boat top speed was 65 mph @5200 rpm ..boat weight not sure mabie under 5000 lbs . so looking for more top end speed and the right prop for this boat. The out drive is se106 1:47-1 any ideals for help would be nice.really don’t won’t to by the wrong prop. Thank you mike s
The prop size is 14.5
I have a mercruiser engine making 183 hp for an 18.5 foot cris craft scorpion. I recently had to replace the stern drive with one that was meant for more hp with a gear ratio of 1.5:1 from 1.84:1. The current prop is a 14.5 x 15 and it is not reaching top RPM and is very slow to plane. I was thinking of reducing pitch and wanted to know how low to go, is. 14.5 x 13 to radical? What size would you recommend?
Hello , I have a Sessa S26 Weighing around 2500 kg 24.8 foot long with a volvo penta 5.7gxi 320HP v8 the gear ratio is a 1.78 DPS-A Drive spinning F2 props which if not mistaken are the smallest stainless steel props volvo makes.I feel that the engine is slightly loaded and would like to know if it is possible to change gears on the upper unit to a lower gear ratio spinning a slightly bigger prop would help .Also if this can be done will it have a better fuel consumption
You have the best article on this subject i have seen yet. I am re-powering a 30 ft Aquasport express. The motors i now have is 260 HP volvo Penta engines, 350 CID. I am changing them to 383 CID’s with 405 HP. I have Volvo Penta MS5A drives with a gear ratioo of 2.4 to 1 ratio. The Props are 17 X 18.5 pitch. At this time with the engines i have the boat wont come on plane and my WOT rpms is at 4300, 4600 is recommended i think the engines are old and not putting out 260 HP. My question is with the new engines and change in HP and torgue if the drives should be changed to a lesser ratio and the props should be changed to a 4 blade 18X24 pitch. I see you deal in velvet drives if i was to change to them what would i havve to change on the engine {bell housing, fly wheel etc.} to make the velvet drives work? Thank you for your time and advice.