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  1. #1
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    Default can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    Bought a used boat recently
    1982 16" fiberglass sportcraft open bow with 1990 Yamaha V4 115 2-stroke.
    The boat is a light (1400lbs loaded with 2 people in it) 16" v-hull/hybrid boat. It is rated for a max of 110hp.
    The propeller is a yamaha 13" X 21 pitch stainless. Max speed is 39mph alone (GPS tested). 37mph with 3 people. This boat defies logic and I'm trying to find the reason why it's so slow.

    I researched other similar boats and most of them are well over 40mph. Most of the them in the 45mph-47mph range and are able to hit 5300rpm+ with this size of propeller.

    What I've checked:
    -120psi compression on all cylinders.
    -new spark plugs well gapped
    -tried mounting the engine 1" higher to make the anti-cavitation plate line up with the bottom of the boat. This result in the same speed +-.5 mph with a lot slower acceleration due to prop venting. Also the boat can't turn hard now for the same reason. With the engine in the original position when the trim is down the boat handles very well.

    How the engine runs:
    never stalls seems to run really well, no hiccups or up and down rpm at WOT. Excellent acceleration out of the hole, shoot out and gets on plane very quickly. Hit 30 mph in around 5-6 seconds.
    But it just seems to not want to go over 4600rpm. Unless I overtrim the engine and lose a bit of mph. Trimming from bottom adds about 2.5mph.

    Next thing I need to check is Ignition timing and I ordered a 19 pitch aluminum propeller by turning point prop.

    Another note for the performance aspect is the boat has been repaired a few times and the bottom is kinda rough. A few patches not well sanded down and such. I don't know how bad this can affect top speed.

    Any other ideas or comments ?
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  2. #2
    Lieutenant Benny1963's Avatar
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    Exclamation Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    you shouldrun well over that.
    5500 or so.you may be over proped
    good luck benny 1963








    ,

  3. #3
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    Quote Originally Posted by Benny1963 View Post
    you shouldrun well over that.
    5500 or so.you may be over proped
    good luck benny 1963

    ,
    Maybe, but what I don't understand is this:

    Normally when a boat is over proped, adding weight slows the boat down a lot, especially in a light boat. I barely slow down adding 400lbs.
    Also the boat did not go faster by raising the engine 1". This doesn't make any sense. The engine shoots so fast to max RPM but then but hits a wall at around 4500rpm. You need to use a lot of trim to go up to 4600-4700rpm and even then most of it is slipping.

    Also a 21 pitch 13" prop isn't that big of a prop for a light 16' fiberglass boat ?
    Maybe I'm expecting too much out of the 115 yammie.

  4. #4

    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    I have a 16 foot 1977 Skeeter that I just added an 1993 Yammie 115. It instantly gets up to 50mph and has a Alum. 17pitch on it. RPM are more then you say, maybe your boat hull is slowing it down somehow?

  5. #5
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    Quote Originally Posted by 72Glastron View Post
    I have a 16 foot 1977 Skeeter that I just added an 1993 Yammie 115. It instantly gets up to 50mph and has a Alum. 17pitch on it. RPM are more then you say, maybe your boat hull is slowing it down somehow?
    What rpm are you turning with that prop @ 50 mph ?

    My fish finder says I do 47-50mph. But GPS says 37-39mph depending on conditions.

    My rpms do sound like they are @ 4500rpm. I have the yamaha digital gauge and it is set for the correct engine.

    I am starting to think that because the stern of the boat is shaped like a tri-hull in the back. When on a plane the boat has tri-hull type resistance causing low max speed. I should post a pic of the hull to figure out if it's one of the low efficiency one.

  6. #6
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    any other input ?
    I want to try something today.

    For checking ignition timing @ Wot with the boat out of the water I read somewhere about a trick ?
    I don't recall the exact procedure. Anyone knows how to do it ?
    I have a timing gun

    1990 Yamaha 115 2-stroke

  7. #7
    Supreme Mariner
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    go to a good dealership,rent their test wheel. if the motor will spin it your over propped,if it wont you have engine issues.

  8. #8
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    Quote Originally Posted by rodbolt View Post
    go to a good dealership,rent their test wheel. if the motor will spin it your over propped,if it wont you have engine issues.
    Good idea !! sweet. That takes away the boat as a factor.

  9. #9
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    yes it does.
    and on a boat with an unknown history its the only way to seperate hull/prop issues from engine issues.
    you would not belive the amount of new rigs we get that are way overpropped from the factory.
    used gets even worse sometimes with Ebay and such.

  10. #10
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    We will know soon with a new aluminum 19 pitch prop from turning point propellers.

    Checked timing advance today. Perfect at idle part throttle and WOT. 25btdc @ wot and 5 btdc @ idle.

    I checked something else today. I use portable tanks. And the primer bulb is pretty firm normally. However at WOT the bulb goes soft and there seems to be a vacuum created. I tried pumping it but it stays soft and sucked up.
    Don't know if this is normal. Going to 2/3 throttle the bulb pumps back up.
    Maybe I have a vent issue on the tanks ? Could this cause a lower than normal fuel pressure and lean out @ wot.

  11. #11
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    Default Re: can't go over 4600rpm @ WOT 115 yamaha 2-stroke

    I just tested the turning point prop on my yamaha 115

    What's nice is that I went from a stainless steel 21 pitch that did 39mph @ 4600rpm
    to an Aluminum 19 pitch that does 40mph @ 4900 rpm. The prop cost me 70$
    I do a lot of fishing on an old logging river. There aren't many logs around anymore but once in a while you see one floating by.

    When I raised the engine the 21 pitch yamaha ventilated quite a bit.
    I did not change engine height and now barely have any ventilation. Much better acceleration and handling. Also I can trim the engine so much more now and gain speed. Still 40mph is all I got trimmed out @ 4900rpm.

    What's strange is the 19 pitch prop runs 36.5mph @ 4600rpm with the engine trimmed down. Similar to the yamaha 21 pitch. It just allows me to trim the engine quite a bit more without ventilating.

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