Propeller dilemma (update 3/11)

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JerryS
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2011/12/28 21:50:23 (permalink)

Propeller dilemma (update 3/11)

I am hoping someone may have intimate knowledge of my prop problem. Little history: I bought a 16.5ft deep V boat in 1997 with a 40hp Johnson (for Erie) then added a 9.9 hp for the local state park lakes. Dry weight of boat only is approx 750 lbs.

I got tired of maintaining two motors and did not like the performance of the 9.9hp (5mph ugh!). This summer I sold both motors and bought a Yamaha 20hp.

Here is my dilemma. With a 9 1/4 x 8 prop, the boat will get on plane and will get to 19 mph (calm lake). At 19mph the motor exceeds maximum rpm and the motor starts knocking due to the internal rev limiter. Motor is rated 5000rpm-6000rpm and rev limiter hits at 6200rpm.
I went to the next larger pitch prop, 9 1/4 x 9. Normally 1 inch of pitch will only change the max rpm by approx 200. In my case the 9" pitch prop could not get the boat on plane so the rpms stayed below the power zone and my top speed was about 14 mph.

I am searching for a middle zone (16-17 mph) but not sure how to get there. I have two ideas to run past you:

There are 9 1/4 x 8 four blade after-market props. Since four blades would push more water, I thought this might lower the rpms slightly versus the 8" pitch 3 blade prop.

My other idea was to stay with the 8" pitch 3 blade prop and add weight (50-100 lbs?) to the bow of the boat.

Any thoughts? I have all winter to think about.

Jerry
post edited by JerryS - 2012/03/11 19:53:53
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    pghmarty
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    RE: Propeller dilemma 2011/12/28 22:17:07 (permalink)
    Try same size blade with more surface area/cupping or go to a 9 1/4 x9 stainless
    Buy used props on ebay then sell them til you get it right-should only loose postage


    #2
    JerryS
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    RE: Propeller dilemma 2011/12/28 22:50:59 (permalink)
    Marty,

    The Yamaha 8" prop already had a high degree of cup.

    I considered your stainless 9" recommedation but I didn't think it would gain enough rpms over the aluminum prop. I will research this option more.

    Thanks.
    #3
    pghmarty
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    RE: Propeller dilemma 2011/12/29 00:35:14 (permalink)
    I may of made a mistake above.
    Stainless may lower rpm because it won't flatten out like an aluminum.
    This makes a bigger difference with higher hp but a thinner prop could have the same results.
    9 1/4 x 8 SS could work
    There are so many variables that this can be a nightmare/expensive especially with a high HP performance boat prop.

    9 1/2" x 8 or 9"x9 might be other options
    Honda and Mercury props might also work-most have same spline

    post edited by pghmarty - 2011/12/29 00:47:19


    #4
    anzomcik
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    RE: Propeller dilemma 2011/12/29 06:02:09 (permalink)
    Does your 8" pitch prop have issues with getting your boat to plane out?

    It seems wierd to me that just one inch of pitch would make that profound difference.

    Ok check the 9 pitch prop, make sure you can read on the prop that it is infact a 9 pitch (lookin is free, new props are not free)

    Do you have your motor set up correctly? with tilt trim, motor height tuned in? Do you have a good balance of the weight in your boat? All of those help/hurt your preformance.

    On smaller outboards I have found that props tend not to follow the rule of thumbs when it comes to selectiong a prop. I have played around extensivily with my 25hp motor trying to drain every last drop of preformance out of it. And realised why the mfger puts a huge WOT range on the motor, thats because with smaller power things change very fast in RPM. But going from rev limiter to stuggleing to plane is very extreme, thats why i question the 9pitch, while your at it verify that you 8pitch is truely a 8 pitch.

    I made a guess that your outboard is newer, so I have a link i posted below. Props generally are ot returnable (it sucks) but you may need to buy several to figure this out. With a smaller motor personally i would stay away from SS, From what i read motors will not start to see the benefits of SS until the 50-70hp range (more torque, heavier loads) I think SS props are super cool, they look awesome and make you feel good inside... But i feel with your case it isnt worth the extra $.

    So with assuming your props are the pitch you think they are and the motor is set up correctly, If i were you the next prop i buy is a Solas Amita 4 in 8" pitch (on the page i put the link to). Changing brand of props can yield different results, but i think it is a safe starting point. the forth blade should help lower rpm, and provide better holeshot with more grip and hold better in corners. Solas aluminum props have the best quality and finish i have seen in a AL prop.

    If you still bump rpm limiter i would then go to a Amita 4 with 9" of pitch. One of those two props should be a good fit for you, and the prop that you dont use keep as a spare.
    http://boatpropellers.iboats.com/Yamaha-Outboard-Propellers/?pitch=8.00&chart=68&engine_id=810&cart_id=081266797
    #5
    leadmen
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    RE: Propeller dilemma 2011/12/29 08:24:41 (permalink)
    go to a bigger pitch the 9 is ok but the pitch will decrease or increase a example is a work prop on merq is 16x16 and factory was 15 x 17 1/2 this lowered the rpm but put the boat on plane fast and at lower rpm a ss prop will lower rpm but if you run the river i would be aware of that the blade will eat rocks and can damage the lower unit
    #6
    bulldog1
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    RE: Propeller dilemma 2011/12/29 10:01:02 (permalink)
    Go back to your original prop and adjust your throttle stop to keep it in the rpm range.
    #7
    JerryS
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    RE: Propeller dilemma 2011/12/29 22:26:05 (permalink)
    Anzomcik,
    I was amazed that one inch of pitch made that much difference also. Both props were labeled correctly. The 8" four blade Solas is the prop I was considering in my original post. This may be my best bet.

    Since it is a manual tilt motor, the trim adjustment is a pin with optional hole placement. It is currently placed in the second from the bottom hole which seems to be ideal for plane angle. Maybe I could try placing the pin in the bottom hole. This would push the bow down more which should lower the rpms on the 8" pitch prop.

    Another option that Marty got me thinking about is blade cup. Both the 8" and 9" pitch Yamaha props I used are marketed as "cupped blades". Yamaha also lists same size props that do not have the "cupped" designation (actually marketed as "weedless"). Maybe the 9 1/4 x 9 non-cupped prop would get me better rpm range.

    Bulldog,
    Since I currently have the original prop installed, I had considered that option. Although I worry that I need the full throttle range to get me on plane.

    Thanks to all for the ideas. You got me thinking about various experiments to perform this spring.

    Jerry
    post edited by JerryS - 2011/12/29 22:39:04
    #8
    jon_e_si
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    RE: Propeller dilemma 2011/12/29 22:37:23 (permalink)
    My brother and nephews go to a site called Scream and Fly - you might want to check their forums!

    http://www.screamandfly.com/content.php?35

    God does not take from your life, the time spent fishing!
    #9
    anzomcik
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    RE: Propeller dilemma 2011/12/30 06:14:25 (permalink)
    When adjusting the tilt trim you want the antivent plate thats on the motor to be parrallel to the bottom of the boat, or the setting that is closest to that. Also once you found that tilttrim setting idealy you would want the antivent plate then to be adjusted vertically. The rule of thumb for that is to have the antivent plate level with the bottom of your hull, but every boat is different, my boat runs the best with my antivent plate sitting 1 below the bottom of the hull.

    The height is easliy adjusted with some scraps of wood the width of the motor bracket cut to the height you want.

    Also you have the boat loaded correctly? (distrubiting the weight of the contents)
    #10
    hydrasports22
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    RE: Propeller dilemma 2011/12/31 20:37:59 (permalink)
    If you raise your motor 1" you will have less drag so the 9" prop should give you the right rpm's
    #11
    rmcmillen09
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    RE: Propeller dilemma 2012/01/01 13:18:02 (permalink)
    do they make a plane fin that fits your lower unit?  
    #12
    pghmarty
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    RE: Propeller dilemma 2012/01/01 13:43:10 (permalink)
    ORIGINAL: bulldog1

    Go back to your original prop and adjust your throttle stop to keep it in the rpm range.


    This might be the easiest fix


    #13
    JerryS
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    RE: Propeller dilemma 2012/01/01 16:25:18 (permalink)

    ORIGINAL: rmcmillen09

    do they make a plane fin that fits your lower unit?  


    I can't drill holes in a new motor yet!
    #14
    pghmarty
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    RE: Propeller dilemma 2012/01/01 16:35:08 (permalink)
    Use the small block planer tabs in the corners.
    All they are is a wedge that goes under boat



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    JerryS
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    RE: Propeller dilemma 2012/03/11 19:47:38 (permalink)
    Wanted to give you guys an update since the results were amazing!

    I purchased this prop: http://boatpropellers.iboats.com/Yamaha/20_HP_(2007Newer)_8.00-Pitch_Amita_3113-093-08_Propeller/810/8512/?cart_id=681322313

    When I went to install it I found the rear flange of the prop fit a little too tight in the lower motor housing. A couple minutes of sanding took care of this (Didn't even sand thru the paint).

    If you recall the original 9.25x8 three blade would over-rev and hit the rev limiter at 6200 rpms. This occurred at about 19mph. I tried a 9.25x9 three blade but was only able to get about 5200 rpms and 14.5 mph. I think the reason for the huge difference was the 9 pitch prop struggled to get the boat on plane.

    I tested my new prop on Wilhelm this morning. Here are my results.

    5500 rpm / 17.5 mph
    5800 rpm / 19 mph
    6100 rpm / 21 mph (this was max throttle)

    The biggest factor was this four blade prop got the boat on plane very, very fast. Planing with the three blade 8" prop was a gradual process.
    post edited by JerryS - 2012/03/11 20:26:12
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    Bogeyjoker
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    RE: Propeller dilemma 2012/03/11 20:46:32 (permalink)
    ORIGINAL: JerryS

    Wanted to give you guys an update since the results were amazing!

    I purchased this prop: http://boatpropellers.iboats.com/Yamaha/20_HP_(2007Newer)_8.00-Pitch_Amita_3113-093-08_Propeller/810/8512/?cart_id=681322313

    When I went to install it I found the rear flange of the prop fit a little too tight in the lower motor housing. A couple minutes of sanding took care of this (Didn't even sand thru the paint).

    If you recall the original 9.25x8 three blade would over-rev and hit the rev limiter at 6200 rpms. This occurred at about 19mph. I tried a 9.25x9 three blade but was only able to get about 5200 rpms and 14.5 mph. I think the reason for the huge difference was the 9 pitch prop struggled to get the boat on plane.

    I tested my new prop on Wilhelm this morning. Here are my results.

    5500 rpm / 17.5 mph
    5800 rpm / 19 mph
    6100 rpm / 21 mph (this was max throttle)

    The biggest factor was this four blade prop got the boat on plane very, very fast. Planing with the three blade 8" prop was a gradual process.

     
    Just out of curiosity...how much load did you have in your boat in achieving this performance?  One adult? Two?  Trolling motor batteries? How many?  The reason I ask is I'm in the same boat (pun intended) as far as having a 16' Lund with a 50 hp and 9.9.  I'm thinking of doing the same thing you've done, but I'm worried about performance with a 20hp.
    #17
    JerryS
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    RE: Propeller dilemma 2012/03/11 21:58:35 (permalink)
    Bogey,

    Todays test had three light people aboard,(175 lb, 120 lb and 100 lb). Since this was only a test trip, I did not have any fishing gear or trolling battery installed. I had a starting battery and 3 gallons of gas in a portable tank.

    One of the biggest weight reductions I made for this transition was to empty and remove the 24 gallon built in gas tank. Not only did this lighten the boat, but it also reduced the chance of gas going stale since these 20hp 4 strokes don't use much.

    As stated earlier my boat specs state a dry weight of 750 lbs. When I purchased the Yamaha 20hp at Pymy Boat Sales they stated they were averaging about 16.5 mph from their 16 footers weighing 800-900 lbs using the 9.25x8 three blade yamaha prop. I don't think they even try the after-market 4 blade props. Hopefully this info will help you determine where your Lund would fall depending on the boat's weight.

    I should note my speeds were measured with my fishfinder's paddlewheel so there may be some error. Comparing apples to apples, the same paddlewheel measured my 40 hp at 31 mph(5200rpms) with same boat load and half tank of gas (12 gal).

    I am tickled that I cut my horsepower in half but my speed was only cut by a third.

    Jerry

    post edited by JerryS - 2012/03/11 22:00:30
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    Bogeyjoker
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    RE: Propeller dilemma 2012/03/12 15:02:12 (permalink)
    Thanks for the info.
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    anzomcik
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    RE: Propeller dilemma 2012/03/13 12:52:48 (permalink)
    JerryS

    So are you pleased with the preformance of the four blade?
    #20
    JerryS
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    RE: Propeller dilemma 2012/03/13 22:49:33 (permalink)
    anzomcik,

    Beyond my expectations. I don't miss my 40hp anymore!

    This week I'm installing a new fishfinder. Hope it goes as well as the prop.

    Jerry
    #21
    Porktown
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    RE: Propeller dilemma 2012/03/23 12:24:34 (permalink)
    I've been contemplating the same for a little while. Although too cheap to put up the $2k+ for a 20. I have a 50HP, that hasn't been used in 3 years... Probably needs some work, although worked fine the last time used. And a 15 with 9.9 stickers. Was going to just try the smaller motor by itself, it could use a new prop too, a little dinged up from some rocks about 50 feet from the Jamestown boat launch. I'm getting about 10mph by myself now, and about 1mph less per person added. I'm guessing taking a 300lb weight off of the back should gain me 1-2 mph? Would love to be in the 15mph range, if taking the larger motor and new prop would do that... Any suggestions on prop sizes? It's a 94, Evinrude 15, guessing original prop - 3 blade. Boat is 16.5 (17) Grumman - side console, not sure of exact weight. Probably 100lbs of crap in storage that I could take off too...

    Anyone happen to want a 1988 (I think), Yamaha Pro 50 for cheap? I'd give it away, for help getting the 15 to run on it's own. I'm running a steering link, and not sure how to get it to steer the 15. I know the gas line to the external tank of the 50 needs replaced, and no longer have a starting battery...
    #22
    anzomcik
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    RE: Propeller dilemma 2012/03/23 12:48:11 (permalink)

    ORIGINAL: Porktown

    I've been contemplating the same for a little while. Although too cheap to put up the $2k+ for a 20. I have a 50HP, that hasn't been used in 3 years... Probably needs some work, although worked fine the last time used. And a 15 with 9.9 stickers. Was going to just try the smaller motor by itself, it could use a new prop too, a little dinged up from some rocks about 50 feet from the Jamestown boat launch. I'm getting about 10mph by myself now, and about 1mph less per person added. I'm guessing taking a 300lb weight off of the back should gain me 1-2 mph? Would love to be in the 15mph range, if taking the larger motor and new prop would do that... Any suggestions on prop sizes? It's a 94, Evinrude 15, guessing original prop - 3 blade. Boat is 16.5 (17) Grumman - side console, not sure of exact weight. Probably 100lbs of crap in storage that I could take off too...

    Anyone happen to want a 1988 (I think), Yamaha Pro 50 for cheap? I'd give it away, for help getting the 15 to run on it's own. I'm running a steering link, and not sure how to get it to steer the 15. I know the gas line to the external tank of the 50 needs replaced, and no longer have a starting battery...


    Pm sent.

    I think you are on the threshold of the power required to plane. You might be able to with weight reduction, but it could be spotty.

    I think we can get your sitution better, So we can talk more about it through PM.
    #23
    JerryS
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    RE: Propeller dilemma 2012/03/30 00:45:40 (permalink)
    Speed update.
    My new fish finder is also a GPS. When my paddlewheel measures max speed of 21 mph, my new GPS reads 19 mph.

    You would think the GPS is more accurate, but I question the GPS when trolling. I usually troll crankbaits between 2mph and 2.5 mph using the paddlewheel speedometer. At this speed the GPS reads 3.5mph to 4 mph, which I highly doubt.

    Any thoughts?

    Jerry
    #24
    Erie Mako
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    RE: Propeller dilemma 2012/03/30 05:59:19 (permalink)
    GPS is showing your speed over the ground in a bigger picture, the paddle wheel is showing your speed at that certain point and is also subject to water current and turbulence variations.

    By bigger picture, I mean that several satellites that are thousands of miles away are referencing your location on the entire face of the earth.

    On the internet, EVERYONE is entitled to their opinion!
    #25
    anzomcik
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    RE: Propeller dilemma 2012/03/30 06:05:09 (permalink)
    The paddle wheel when i ran one was always inaccurate, because water current weather going with or against will change how fast that little guy spins.

    2-2.5 mph should be just a hair above idle for your motor. (if that helps with the amount of throttle, just a reference). If you are questioning the accuracy borrow a hand held gps to double check, or even a gps from a car or smart phone, i dont think those will go to tenths of a mph but it will let you know which of the two numbers is the correct speed.
    #26
    JerryS
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    RE: Propeller dilemma 2012/03/30 20:20:37 (permalink)
    You are correct, the 2 - 2.5 mph is just above idle. This is why I cannot believe the 3.5-4 mph from the GPS.

    I will compare a handheld gps on my next trip. The high speed readings are a non-issue since I am happy with the top end performance. The trolling speed concerns me since I was hoping to not have to use both fishfinders at the same time.

    Jerry
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    jmat
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    RE: Propeller dilemma 2012/04/22 19:30:14 (permalink)
    Wow great post. I have a 2011 Alumacraft Classic 165 with a Yamaha 4 stroke and my boat can't get out of it's own way topping out at 7-9 mph based on how many people are in the boat, usually 2. I think I will look into this 4-blade prop. I would kill to get 15-16 mph out of this pig.
    #28
    JerryS
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    RE: Propeller dilemma 2012/04/23 22:52:55 (permalink)
    My Garmin handheld confirmed my fishfinder gps trolling speeds were accurate. It also confirmed paddlewheels, as others have stated, are not very accurate.

    At trolling speeds the paddlewheel was about 1.3 mph too slow. At full throttle the paddlewheel was about 2.3 mph too fast.

    I hope the others trying the Solas 4 blade prop see improved boat performance also.

    Jerry
    post edited by JerryS - 2012/04/23 22:53:37
    #29
    jmat
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    RE: Propeller dilemma 2012/04/29 14:40:47 (permalink)
    Jerry,

    I currently have a Yamaha F20 with a 9 1/4 X 8-J prop on a 16' 1000 lbs Alumacraft boat. I am currently getting about 8 mph out of it. And forget about getting on plane. My engine is running around 5500 rpm at wide open throttle. My engine range is rated for 5000-6000 rpm. What prop would you suggest?
    #30
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