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  1. #1
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    Gear case ratio relationship to prop efficiency

    Since it is possible to alter the gearcase ratio on a 2.5 Merc with different lower ends, is it preferable to maintain a certain maximum prop rpm to prevent prop blowout?

    My question relates to prop efficiency as prop rpm goes up since I assume once you hit a certain water speed over the prop surfaces you will hit a wall where an increase in prop rpm causes cavitation at the prop leading edges?

    I would like to discuss the effects in a submerged application as opposed to a surfacing application since the results would be different for each type prop.

    I realize most of this is theoretical and fluid dynamics related but obviously the manufacturer would recommend a gear ratio/gear case type for specific applications using hull speed and boat weight/HP rating to make a recommendation.

    The reason I ask is with a steeper ratio I could turn a shallow angle pitch prop at higher prop rpm than say a shorter ratio with a steeper pitch. Since there are so many related factors to consider is there a "laymans rule" for gearcase ratio selection?

    Thanks.

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    Your question can't be answered in a short statement. There are too many variables. Some engines need lower gearing to stay in their torque range, but some props need higher turning RPM's IE: Tip Speed, to be efficient so they respond better to higher gearing. Different engine heights change all that. Some props work better at depth while super cavitating props IE; as used on speedmaster style lower units, run at the surface. Take a cross section of a typical prop versus a true race prop. You'll find some similarities but I assure you the true race prop will look much like the cross section of a wing, even a round ear and not just a cleaver style. And yes these props push on the front of the blade and pull on the back. Attention to these details separate a good prop from a record setter. Not to mention the type of rake, whether the pitch is static or variable and a host of other things. I've been at this boat thing over 40 years. Props are like black magic and you won't learn that from a book. I spent some time at the high Perf divisions prop shop and learned a lot from Steve Konrad and Dennis Cavanaugh. They've both passed away now but they would tell you about the same thing. You can take 20 identical props right out of the box and run them on the same boat and one will be faster than all the others. Sometimes you can't see or measure the difference, you have to learn to feel it with your hands and fingers.
    www.eagleoneperformance.com


  3. #3
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    Sometimes you can't see or measure the difference, you have to learn to feel it with your hands and fingers. <!-- / message --><!-- sig -->
    Like a good woman...


    But seriously Eagle One your right as rain with evenly matched car, engines, tires, props even gears and such some just work better and what your referring to with props is truely an "art"...
    Last edited by Bartman39; 10-03-2009 at 11:55 PM.

  4. #4
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    Quote Originally Posted by Eagle One View Post
    Your question can't be answered in a short statement. There are too many variables. Props are like black magic and you won't learn that from a book. Sometimes you can't see or measure the difference, you have to learn to feel it with your hands and fingers.
    Nice Answer Noah! That statement is so good it deserves to be a on a sticky. Best gear ratio for what prop? Wow thats a great question to. Hard to add to what you said Eagle One. The only thing I know is as an example a trophy for instance though could be run above 7500 rpm really shouldn't be and this style prop also shouldn't be surfaced. The style is a semi surface type prop but more at home submerged as with most but not all rounder ear bigger barreled Types. The blades arnt designed for running half out half in as a true surface type prop as a cleaver for example. Hydromotive engineering makes a 4 blade that looks like a trophy type but is far from it in its ability to be surfaced and run at high rpm with out damaging the blades. The trailing edge of most surface type have a straighter back edge than the submersible type or even semi submersible. The rake is also a factor but not always the only tell tail as Eagle one said the variables are many. Cleavers traditionally have less rake than a Chopper style for instance but the cleaver is the style you'll see in most racing applications. The ET is a combination of a cleaver style and a chopper style and proven high rpm surfacing prop for drag application. The Yamaha Drags also have this going for them but they lean more to the chopper style that the ET. Both work great and one may work better on one hull than the other. Again Trial and error {Black Art} You'll hear guys say you cant try to many props. There is a prop out there for your hull,weight,power,rpm combination. The trick is finding it. A prop that cavitates to much and goes into super cavitation mode will actually start to remove blade area of your prop at the tips. The cup might be to great or insufficient blade area for your application and the suction creates a cavity on rotation so great the area for the next blade in the rotation has insufficient water to gain any pressure to make velocity. Not all cavitation is a bad thing. Some designs utilize this for spooling up the rpm,s before the thrust stage and can be like a sling shot effect as most drag boats use. Best thing to do is see whats working on other guys boats like yours and try the different props they are using. Make notes as to what prop you liked and why. When you get {the one}. You'll know it. Ive try ed at least 10 different props this year. I had the one. But it broke. I guess thats why it was the one. Thinned, labbed, cut, polished, and balanced to perfection. I think the thinned part is what made it so good but how good is a broken prop now!! So thinned isn't always a good thing unless you can afford to keep replacing them. If i was racing for money that would be a different story. I highly recommend the book Secrets Of Prop Design by Jim Russell. He is a member on S&F and lives hear in Canada close to you. User name Jimboat of Aeromarine Research. He has the answers to your type of questions. The book will help you out immensely in finding what works best for you and understanding whats going on under the motor.

  5. #5
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    Short and sweet, no there is no rule of thumb. Test, test and test some more.

    There are no short cuts other than starting where someone else with an identical rig is running faster than you.

  6. #6
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    Thanks for the advice gents,

    So if I understand correctly gear ratio selection is more a load dependant choice to make rather than a prop speed choice, in a nutshell more wetted area and weight the lower the ratio needed to get the motor operating into its powerband to carry the load. Really no different than a car, guess I was overthinking the whole thing.

    I know about the prop "magic" as I have two identical props (LaserII) and they both perform slightly differently with different slip rates etc. I will buy Jim's book, nothing like some interesting reading from a fellow Canuck to keep me warm at night.

    Is there a chart or list of different ratios available for the Merc lower end and which housing(s) they fit? How about a list of housings and their lengths and features (i.e. sportmaster, offshore, bass etc). There doesn't seem to be a single source for this kind of information.

  7. #7
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    Quote Originally Posted by 4JawChuck View Post
    I will buy Jim's book, nothing like some interesting reading from a fellow Canuck to keep me warm at night.

    Is there a chart or list of different ratios available for the Merc lower end and which housing(s) they fit? How about a list of housings and their lengths and features (i.e. sportmaster, offshore, bass etc). There doesn't seem to be a single source for this kind of information.
    That info some guys like Mark75 might know of the top of there heads the ratios for spacific gear cases. Its also in the manuals for each motor. That book is AWSOME I bring it work leave it by the computer at home and read it over and over. Theres so much usfull info there. Im heading out prop testing today as a mater of fact. Ill try differant gearcase hights at wot and mark the rpm @ what hight and top speeds. Ill also messure hole shot at those differant hights also with what works best in the midrange. I right it all down after each run. What boat motor gearcase you have BTW? Or are you looking to find something? Another good way to figure this out is to say what ya got and what works best for what you say you want to do with the boat. Guys with the same setup will respond. Good base line to start with.

  8. #8
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    My niave understanding of at least part of the equation is that props with pitch / diameter ratio of around 2.0:1 are most effective at turning rotational torque into forward thrust once speeds get to the 60+ range (for submerged props). that's 13 1/2 to 16 diameter props in the 27-32 inch pitch range. That's based primarily on the range of angle of attack required for the maximum pressure differential between the front and back of the blades (translates into forward thrust) in that range of speeds.
    With the caveat that a lot of factors come into play as mentioned above.
    ie. obviously different hulls need different amounts of bow / stern lift to carry the boat at it's fastest attitude.

    Ideally propped so max / peak horsepower is right at wot / max speed. In other words, I've understood that if I put gear ratio so that it hits that range of props at max rpm at wot, I'm in a good place to start.

    add about 17 other variables in and it's simple... right?
    Last edited by wca_tim; 10-04-2009 at 04:51 PM.

  9. #9
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    Ok here goes,

    The boat is a 16.5' Tuffy Rampage Sportsman, the hull design is a sponsoned vee. Weight once completed will be approx 1400 pds with a 2.5 Merc on the transom. Transom height is 20" but at my current 6" setback with the 90HP 20" leg Merc I have the motor pretty high which has me thinking a 15 inch mid might be a better choice and I am not averse to custom making a setback adapter to fit a 15" mid nice and low. Pics and details of the hull shape are available here in this thread;

    http://forums.screamandfly.com/forum...d.php?t=198104

    It is an unconventional setup for this kind of boat as it is not designed for this kind of power but the rebuild this winter will allow me to reinforce the transom/hull and re-rig for center console and hydraulic steering, the boat is in good condition and does not need a transom replacement.

    I am imagining a 2.0:1 ratio is what I need and I am not expecting the boat to exceed 70-80 mph, the current 90HP-2 stroke gives me 45 mph with a 22" Laser II at 5800rpm approx if that is any help.

    The motor would be a carbed 2.5L Merc as that is what I am shopping for, I would like to have the motor in hand before I start tearing the boat apart this winter so I have time to get it in shape and make any mods required to the transom. As you can imagine these motors are not very common up here and it looks like I will be importing from down south.

    Basically I want to get a handle on what all the options are for these motors so I know before I start making a commitment to buy.

  10. #10
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    Quote Originally Posted by 4JawChuck View Post
    Thanks for the advice gents,

    Is there a chart or list of different ratios available for the Merc lower end and which housing(s) they fit? How about a list of housings and their lengths and features (i.e. sportmaster, offshore, bass etc). There doesn't seem to be a single source for this kind of information.
    Standard Merc ratios are 1.62, 1.75, 1.87 and 2.0 for most OB applications. There are others out there on both ends of that but I think the question you asked just got answered.

  11. #11
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    i agree with test, test, test..... i have ran so many props, and have so many, i cant remember how many i have........ anymore I modify the ones i buy to be as close to my best ones.... beat on them, cut them, whatever it takes. have one guy who works some for me too....... the formula you ask is going to be different on every boat, motor, gearcase, gears, prop, set up....... i have been tinkering with my ss for 6 years... i think i have the fastest set up to date on there right now. it is a new set up too..

    get out the wallet and start testing... usually you can sell props for what you have in them on here.. may loose shipping, but you can test it.....


    i think the most important think to a racer is prop... it is to me...... most have light fast hulls, close to the same hp in the same class. close in weight.... props are all a little different...... most great racers would not take a 1000 bucks for there best 700 prop.....
    a couple of mph, and rpms over the rest is priceless, and maybe the difference between first and second, or last.
    93 xr 2001 drag with a 15" drag
    92 xb 2002 comp with a 15" drag
    R.I.P. Phil "rpm racing" Watch over the rest of us my friend.

  12. #12
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    Know a few guys that have played with ratios, its like props some work on some boats some don't. For me half the fun of this sport is the never ending need for a little more, but can be hard on the wallet at times.
    Dave

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    I could see how moving to a stiffer ratio would be beneficial as speeds go up beyond say 90 mph, it would allow you to stay within the common prop pitches and avoid cavitiation due to excessive pitch.

    When I was a kid I built a inboard tunnel hull with .45ci running 90% nitro and with the pitches required for best top speed you had to throw the boat into the water or it wouldn't move because of the steep prop...of course it idled at 5000 rpm also so...

    This boat will need load carrying ability because its still a fishing boat and the point of this exercise is to have some punch under the foot throttle not have some dog out of the hole. Looks like I will stick with a 2.0:1 ratio for the grunt.

    Thanks for the help folks.

  14. #14
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    Cavitation is a function of blade tip speed rather than pitch. You can have cavitation with no pitch if the tip speed is sufficient

  15. #15
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    Quote Originally Posted by 4JawChuck View Post
    I could see how moving to a stiffer ratio would be beneficial as speeds go up beyond say 90 mph, it would allow you to stay within the common prop pitches and avoid cavitiation due to excessive pitch.

    When I was a kid I built a inboard tunnel hull with .45ci running 90% nitro and with the pitches required for best top speed you had to throw the boat into the water or it wouldn't move because of the steep prop...of course it idled at 5000 rpm also so...

    This boat will need load carrying ability because its still a fishing boat and the point of this exercise is to have some punch under the foot throttle not have some dog out of the hole. Looks like I will stick with a 2.0:1 ratio for the grunt.

    Thanks for the help folks.
    A few years ago Bass&Walleye did a test with one of thier test boats, they had 4 gearcases with 1.87:1,2:1,1.75:1,1.62:1 the results were real intresting. I can try and find it if your interested?
    Dave

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