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  1. #1

    Default 25HP Yamaha - new prop size/material

    Here's my setup:

    • 2008 2-stroke 25HP Yamaha with about 25 hours on it ("new-old" engine). Just fully broke in and had a couple tanks of 100:1 fuel through it. So far a great little engine.
    • 15' Gregor "semi-v." Lots of small bangs and dents in the bottom, but overall a fairly straight, lightweight boat.
    • Normal load is me (180), dog (90), 6g fuel (40), G27 battery (50?), anchor/rode (20?), junk (30?), poles (20?), etc. Say 500 pounds, more or less.
    • Stock prop - 9 7/8 x 11 1/4 aluminum white Yamaha thing, just getting the paint worn off the tips.
    • "Whale tail" which completely fixed a cavitation issue.
    • TinyTach, when the damned thing works. Around 5700RPM wide-open trimmed up.
    • Right around 25MPH on the GPS - a bit more when everything's perfect, a bit less if it's rough, etc.
    • Gets on plane more or less instantly, depending on trim.
    • Tossing another person/more beer/extra gas/dead fish in doesn't make a lot of difference. I've had it plane out and run around 20MPH with 5 adults and a big pile of crab gear.
    • Haven't measured fuel consumption, but it's not burning much - certainly less than the worn-out Johnson on the same boat was.


    So, everything's perfect, right? Well - maybe. The problem is, I don't think I'm loading the engine up, and I think I might be able to turn more prop. Maybe. I hope. For example tonight, coming in, with everything trimmed up, I'm making 27MPH on the GPS at full throttle, turning 5700 RPM. If I back the throttle of to 75% (there's a nifty little "power curve" diagram thingee on my tiller), I lose about 1MPH and 50 RPM. You can hear the difference, sort of, maybe. At 50% throttle you can definitely hear a change, but I'm still turning around 5500 RPM and running 25MPH. 25% power is around 4500RPM, and still on plane and stable somewhere in the high teens.

    I'm actually pretty happy with the performance I'm getting, but I need a spare prop and if possible I'd like to wring a bit more speed out of her. I make a long run every now and then, and 5MPH would mean less pounding around and more dead fish. I do NOT want some ridiculously-oversized prop that won't get me on plane in a good chop or makes me burn twice as much gas for 1MPH or anything, but I think I can turn more prop.

    I'm leaning towards a stainless prop, but that's yet another variable that I'm not sure how to control.

    http://www.mercurymarine.com/propellers/prop-selector says I can turn a 15-pitch. I doubt that.

    What say ye?

  2. #2
    Petty Officer 1st Class Johny25's Avatar
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    Default Re: 25HP Yamaha - new prop size/material

    Quote Originally Posted by dustymc View Post
    Here's my setup:

    • 2008 2-stroke 25HP Yamaha with about 25 hours on it ("new-old" engine). Just fully broke in and had a couple tanks of 100:1 fuel through it. So far a great little engine.
    • 15' Gregor "semi-v." Lots of small bangs and dents in the bottom, but overall a fairly straight, lightweight boat.
    • Normal load is me (180), dog (90), 6g fuel (40), G27 battery (50?), anchor/rode (20?), junk (30?), poles (20?), etc. Say 500 pounds, more or less.
    • Stock prop - 9 7/8 x 11 1/4 aluminum white Yamaha thing, just getting the paint worn off the tips.
    • "Whale tail" which completely fixed a cavitation issue.
    • TinyTach, when the damned thing works. Around 5700RPM wide-open trimmed up.
    • Right around 25MPH on the GPS - a bit more when everything's perfect, a bit less if it's rough, etc.
    • Gets on plane more or less instantly, depending on trim.
    • Tossing another person/more beer/extra gas/dead fish in doesn't make a lot of difference. I've had it plane out and run around 20MPH with 5 adults and a big pile of crab gear.
    • Haven't measured fuel consumption, but it's not burning much - certainly less than the worn-out Johnson on the same boat was.


    So, everything's perfect, right? Well - maybe. The problem is, I don't think I'm loading the engine up, and I think I might be able to turn more prop. Maybe. I hope. For example tonight, coming in, with everything trimmed up, I'm making 27MPH on the GPS at full throttle, turning 5700 RPM. If I back the throttle of to 75% (there's a nifty little "power curve" diagram thingee on my tiller), I lose about 1MPH and 50 RPM. You can hear the difference, sort of, maybe. At 50% throttle you can definitely hear a change, but I'm still turning around 5500 RPM and running 25MPH. 25% power is around 4500RPM, and still on plane and stable somewhere in the high teens.

    I'm actually pretty happy with the performance I'm getting, but I need a spare prop and if possible I'd like to wring a bit more speed out of her. I make a long run every now and then, and 5MPH would mean less pounding around and more dead fish. I do NOT want some ridiculously-oversized prop that won't get me on plane in a good chop or makes me burn twice as much gas for 1MPH or anything, but I think I can turn more prop.

    I'm leaning towards a stainless prop, but that's yet another variable that I'm not sure how to control.

    http://www.mercurymarine.com/propellers/prop-selector says I can turn a 15-pitch. I doubt that.

    What say ye?
    Sorry if this doesn't come out right, I am new here and do not know how to reply or post or anything yet?

    Ok what is your Yamaha RPM supposed to be at WOT? If you are over the recommended WOT RPM range then I would go up in pitch. Your not pushing much weight so the motor shouldn't have an issue hitting WOT RPM. I don't know Yamahas that well so I don't know what they run RPM range?

    I wouldn't go to a 15 pitch though, maybe a 13?
    Last edited by Johny25; April 10th, 2012 at 03:47 AM.

  3. #3

    Default Re: 25HP Yamaha - new prop size/material

    Yamaha recommends 5-6000 RPM WOT.

    Based on absolutely nothing, I was kind of leaning towards a 13 pitch too, or perhaps a 12 in Stainless.

    http://goo.gl/s7zOg or http://goo.gl/fdCmS or something else?

  4. #4
    Petty Officer 1st Class Johny25's Avatar
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    Default Re: 25HP Yamaha - new prop size/material

    Ok so you are within the RPM range already. I am not sure you will get any more top end out of it with the 13 or 12 pitch but your holeshot should improve I believe. For every 1 rotation you will move 13 inches on a 13 pitch and 12 for the 12.....etc. Your also going up a little in diameter which equates to pushing more water also. The stainless prop will also help in my opinion. That stilleto looks nice! You will probably drop to 5500 RPM but maintain same top end. Motor doesn't have to rev so high (less wear IMO).

    I just ordered a 10 1/8x 11 pitch stainless from Cabelas for my Johnson 25hp. Got a good price like the one on the solas you posted a link to.

    I am interested in hearing what happens though if you buy one of them. I am sure someone on here has more info than I do though.

  5. #5
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    Default Re: 25HP Yamaha - new prop size/material

    Heres the thing Your total weight boat,motor, you,your "little" dog and gear is about 765lbs.
    I think your are doing fantastic! Prop calculator says your at a very acceptable 7.5%slip.
    Years ago I hade a 18ft Crestliner utility with an old tiller Gale 25
    It would plane with 6 adults and minumum gear.I feel the big relatively light boat helped get up on the water with that load.
    I digress,I did notice that in the iboats prop store it appears the most popular prop size is a 12" And it includes 3 ss props.
    Its very difficult to tell what the results will be going to ss.Prop calculator says your present slip is about 7.5% a decent number.
    The ss prop should lower that number some(thats good).Prop calculator says 28.2 mph at 5500 rpm 6% slip.
    Not much but it could be a bit more but a 5 mph increase isn't likely.
    the Solas Amita ss is I think $179. Solas props have a good reputation.
    You could also look for a Turning Point Rascal Which comes close to ss performance at al prices.
    Perfornance results depend on how well the boat attitude reacts to the prop change.
    calcutor only knows a 12" prop can do this much at this rpm and slip.
    Its not out of reason that motor could turn the same 5700 resulting in about 29 mph.

  6. #6
    Supreme Mariner Silvertip's Avatar
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    Default Re: 25HP Yamaha - new prop size/material

    Quote Originally Posted by Johny25 View Post
    Sorry if this doesn't come out right, I am new here and do not know how to reply or post or anything yet?

    Ok what is your Yamaha RPM supposed to be at WOT? If you are over the recommended WOT RPM range then I would go up in pitch. Your not pushing much weight so the motor shouldn't have an issue hitting WOT RPM. I don't know Yamahas that well so I don't know what they run RPM range?

    I wouldn't go to a 15 pitch though, maybe a 13? I believe low pitch number for top speed and high pitch for more push in the hole shot or for heavier loads to push.
    You have that wrong: High pitch numbers pull down engine rpm since the prop is taking a bigger bite of water. speed will increase "theoretically" only provided the engine has enough power to twist it. If the engine is already below its recommended WOT rpm increasing pitch makes the situation worse and speed will likely DECREASE. Low pitch numbers allow the engine to rev higher and therefore have better hole shot with the net result of overreving at WOT.

  7. #7
    Supreme Mariner Silvertip's Avatar
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    Default Re: 25HP Yamaha - new prop size/material

    I think you need to get your Tiny Tach set up correctly. There is no way you are running 4500 rpm at 25% throttle or 5000 RPM at 50% throttle as you indicate. At 25% throttle, running 25 mph (nearly 92% of your WOT speed) which you say is 27 MPH is simply not possible. You do not get 92% of your top speed on any boat at 25% throttle. Do you have the correct tiny tach? Does your engine use "wasted spark" meaning does the plug fire at both the top and bottom of the stroke? Your engine is a two stroke so you need a tach for that application. All of this makes a difference. And keep the sensor wire away from other plug wires.

  8. #8

    Default Re: 25HP Yamaha - new prop size/material

    Quote Originally Posted by Silvertip View Post
    I think you need to get your Tiny Tach set up correctly. There is no way you are running 4500 rpm at 25% throttle or 5000 RPM at 50% throttle as you indicate. Running nearly 75% of your WOT speed at 25% throttle is simply not possible. Do you have the correct tiny tach? Does your engine use "wasted spark" meaning does the plug fire at both the top and bottom of the stroke? Your engine is a two stroke so you need a tach for that application. All of this makes a difference. And keep the sensor wire away from other plug wires.
    My engine "overloaded" the TT, causing EVERYTHING to come on. I spent some time on the phone with them (basically put it on the bottom plug and cut some sensor wire off, after poking around in there with a pocket knife to reset the thing), so I'm pretty sure it's working as well as it can. It's a little delicate, but I don't see any reason to think it's inaccurate.

    You're more than welcome to come along for a ride. I don't have a fuel flow meter of manifold vacuum gauge or anything of the sort, but when the twist-grip thingee is on the 3rd of it's 4 lines, not much is different compared to wide open. I'll try to get a crappy phone video of it, but I'm really not making **** up. That's why I think I'm not turning enough prop. Not quite the same, but I've spent a lot of time trying to wring more performance out of airplanes - I don't think I'm completely off-base here.

    I am impressed with the big-boat-smallish-engine thing. It's about twice the boat of a 14, but trailers the same. I can beach-launch it with my little Pathfinder, drag it around when the tide goes out, etc.

    I'm really not sure how my holeshot could improve much - depending on trim, it pops up in a boatlength or so.

    I am really very happy with the setup, but I need a spare prop anyway, and if I can get a bit more out of it...

  9. #9
    Petty Officer 1st Class Johny25's Avatar
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    Default Re: 25HP Yamaha - new prop size/material

    Quote Originally Posted by Silvertip View Post
    You have that wrong: High pitch numbers pull down engine rpm since the prop is taking a bigger bite of water. speed will increase "theoretically" only provided the engine has enough power to twist it. If the engine is already below its recommended WOT rpm increasing pitch makes the situation worse and speed will likely DECREASE. Low pitch numbers allow the engine to rev higher and therefore have better hole shot with the net result of overreving at WOT.
    Sorry Silvertip but if you read what I said it clearly states that I said "if you are OVER your engine RPM recommendation then go up in pitch" hence the RPM will drop down.

  10. #10
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    Default Re: 25HP Yamaha - new prop size/material

    Dusty,I wouldn't put much stock in your tiller markings.They are more a reference point
    than an indication of where your throttle actually is.
    Silvertip; His wot numbers he gave us works out to about 7% slip at 27 mph.
    I don't think the tiller markings have much value in relation to actual throttle position.
    25 @ 5500 works out to 11% slip, 20 @ 4500 about 13% its pretty normal for slip to go up as
    prop speed is reduced.

  11. #11
    Supreme Mariner Silvertip's Avatar
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    Default Re: 25HP Yamaha - new prop size/material

    Quote Originally Posted by Johny25 View Post
    Sorry Silvertip but if you read what I said it clearly states that I said "if you are OVER your engine RPM recommendation then go up in pitch" hence the RPM will drop down.
    Here is the last line from your post: " I believe low pitch number for top speed and high pitch for more push in the hole shot or for heavier loads to push."

    Low pitch numbers do not provide the highest top speed and high pitch numbers do not provide optimum hole shot. You had it right initially but then contradicted yourself. Lower pitch can only increase top speed if the engine is severely over propped. Conversely, higher pitch cannot increase hole shot unless the engine severely underpropped -- to the point where blow out occurs on holeshot. To put this in perspective a boat that is heavily loaded requires a prop with "less" pitch than the same identical boat with a very light load. Both boats should be propped to put their WOT rpm at the top of the manufacturers recommended rpm band. In that example, the lightly loaded boat will be faster than the heavily loaded one because it can swing a prop with more pitch. The heavy boat needs less pitch to get out of the hole and to reach the same WOT rpm.

  12. #12

    Default Re: 25HP Yamaha - new prop size/material

    Crappy phone video it is! I hope I win an Oscar or something for this....

    I'll narrate, because - well, it's needed...

    http://www.flickr.com/photos/dustymc/6849979603/

    I'm running upriver, into maybe 4K of current and a couple MPH breeze. The first shot is showing WOT, 25MPH on the GPS, 5740 on the TinyTach. Then I ease off the throttle to the 25% mark, and let it stabilize at 4420 on the TT and 16 on the GPS. Then there's a dramatic sweeping shot of gimpydog, who's serving as ballast and dropped bait disposal.

    When not "filming" and wondering just when I was going to find something hard, I was actually able to get the 25% throttle position to stabilize around 20 MPH by backing off the throttle REALLY slowly. Come off like I did in the vid and it kinda half-falls off plane.

    Notice how nothing really changes through about the first half of the throttle-twisting action.

    I still think I need to be turning more prop....

  13. #13
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    Default Re: 25HP Yamaha - new prop size/material

    The numbers dealing with wind and current aren't much help.
    If you want to do another test a 2 way run perhaps where the current is reduced.
    I don't think further test is needed until you try a new prop.
    Did you read my post that mentions the 12" props.
    I thought you would see that trying a 12" prop seems reasonable.
    Your motor is allready slightly below the max rating.If you went with the same brand and material
    logic says you will further reduce rpm how much isn't an exact number.
    If we say 5500 and same 7% slip we get 27.9 mph.
    A ss prop may produce more speed due to better slip and a favorable affect on the boat.
    Unfortunately sometimes a ss will lower rpm too much with poor results.
    More later.
    Usually a ss prop selected carefully will give across the board improvement.
    I looked up some test reports 2 Glass Caribbean Skiffs 15' and 16' 25 yammy. about 1100 lbs test
    weight 11 1/4 al props both ran about 5900 rpm and 27 to 29 mph.
    2 Alumacraft 15' and 16' about 1100lb test weight 11 1/4"al prop each turned about 5500 at 24mph.
    Note that the al boats are signifigantly slower and your boat though at about 800 lbs it is aluminum. I've noticed in all the tests I've checked that usually al boats of roughly the same weight or lighter are slower than the glass boats.I assume it is the somewhat more primative bottom shape usually full of rivets.I think we can see that your somewhat lighter boat could do well with a 12" prop.
    And it appears on a good day your doing pretty well. at 27 mph/5700 now.
    check out the Yamaha performance bulletins.

  14. #14

    Default Re: 25HP Yamaha - new prop size/material

    Quote Originally Posted by steelespike View Post
    The numbers dealing with wind and current aren't much help.
    If you want to do another test a 2 way run perhaps where the current is reduced.
    I don't think further test is needed until you try a new prop.
    Did you read my post that mentions the 12" props.
    I thought you would get that trying a 12" prop seems reasonable.
    Your motor is allready slightly below the max rating.If you went with the same brand and material
    logic says you will further reduce rpm how much isn't an exact number.
    If we say 5500 and same 7% slip we get 27.9 mph.
    A ss prop may produce more speed due to better slip and a favorable affect on the boat.
    Unfortunately sometimes a ss will lower rpm too much with poor results.
    More later.
    Please don't take this the wrong way - I really appreciate all of ya'll taking the time to offer advice - , but...

    That wasn't a test. That was just a response to Mr. "There is no way you are running 4500 rpm at 25% throttle" up there, who's obviously never tried to operate anything under-propped and fully-instrumented and outright called me a liar. Why the hell would I come ask for help if I wasn't going to be honest about my setup? I'm trying to AVOID buying a crappy prop, to AVOID blindly throwing $250 (or $60 for that matter) at my screen in the hopes that it'll work for me.

    Yes, I read your post, but I'm not sure you're understanding my specific question, possibly because I haven't sufficiently communicated it. I really did read everything I could find online before I decided to ask for specific advice for my specific situation here, hoping someone had experienced something similar and found a solution. This isn't my first boat or first prop, it's just the first one I've owned that's acted this way.

    (I'm down to a 32' sailboat, this Gregor, a dingy with a 2hp, and a dingy with a 4hp at the moment, if that helps my street cred.)

    That is, even though I can't quite make max WOT recommendations, I can get pretty close and stay there way below the maximum power output my engine is capable of. Why is that?

  15. #15
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    Default Re: 25HP Yamaha - new prop size/material

    You may be slightly under propped. But again your tiller markings have no bearing on actual throttle position,
    You should be only looking at the rpm and speed.Actual time to get on plane, speed and rpm that it falls off plane.
    Your initial runs seemed pretty normal with the rpm, speed and slip.
    When you say under propped it means not enough pitch and motor may be overeving.
    Is that what you meant? Or did you mean too much prop so you are under max rpm?
    imo I don't think you are under propped , your certainly not under performing. when compared to the test reports
    and you are well within the rpm range. And your prop size is exactly what they are running on tests of 4 boats some that are very different from one another.I think your looking for a fix that doesn't exist.
    Note in the test reports they don't refer to throttle position but only rpm.

  16. #16

    Default Re: 25HP Yamaha - new prop size/material

    I _should_ be looking at EGT or fuel flow or some indication of power, but I don't have those data. I'll agree that throttle position is a poor proxy of that, but so is RPM. (Less so in the context of speed, generally, but here I'm not so sure as I think I'm not loading my engine properly.)

    I may indeed be trying to do something that can't be done. Won't be the first time....

    THanks!

  17. #17
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    Default Re: 25HP Yamaha - new prop size/material

    Rpm is an excellent reference and you can check if it is reasonable by using your gps speed, prop pitch and gear ratio,rpm and slip.
    Slip can be derived by using the 4 known specs speed,pitch,gear ratio and rpm.
    Slip usually improves(lower percent)as prop speed increases.as the prop slows slip goes up.
    Slip results are not linier and depend on the prop design and boat caricturistics.
    Thus a 500 rpm change can result in different slip numbers with different prop design,boat design etc.
    Marine engines use about 9% of the horsepower rating in gallons per hour@ wot.Thus a 100 hp would use about 9gph.
    @ wot.On here we use 10% for in your head figuring.Your 25 will use about 2.5 gph at wot and thats whether its pushing a runabout or a house boat. Basically you improve mpg by improving wot speed or the fastest cruising speed while letting the motor operate freely within its rated rpm range. Marine engines are working hard like they are pushing the boat uphill all the time
    if run with too much pitch it is like your making the motor do the hill in over drive. Not good for the motor or mpg.
    Your setup is ideal You stated your max rpm rating is 6000. 5700 is very good reving freely yet obviously working well with 7% slip. I think you can see that if you increase speed by 1 mph at wide open throttle your not going to save much fuel over a one hour run. And you need to do that speed change without forcing you to open the throttle more to maintain cruising speed or slowing hole shot both of which would increase gallons per hour under those cercumstances.

  18. #18
    Admiral smokeonthewater's Avatar
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    Default Re: 25HP Yamaha - new prop size/material

    Quote Originally Posted by dustymc View Post
    ...........That is, even though I can't quite make max WOT recommendations, I can get pretty close and stay there way below the maximum power output my engine is capable of. Why is that?
    Because the MARKS on your tiller are WORTHLESS..... Nobody is calling you a liar.... You are being told that there is another explanation for what you are perceiving. YAMAHA has set the engine up to react much more to throttle in the first half of the range than to the second half..... Just the fact that you said you can barely hear a difference supports this.

    You are assuming that you make 25% power on the first mark, 50% on the second, 75% on the third and 100% on the fourth. It MAY POSSIBLY be far more accurate to assume 50% on the first, 75% on the second, 95% on the third and 100% on the fourth.

    Steelspike and Silvertip ARE in your corner and you will do quite well to listen to what they have to say.... Please don't look for an insult where there is none.
    Kevin

    Quote Originally Posted by Tinnie
    But, where are the reevets?
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