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Thread: alright, now I got a speed issue.

  1. #1
    Member rimfirematt's Avatar
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    Default alright, now I got a speed issue.

    According to GPS, my boat does 10.3 MPH with 3 guys, all the gear for camping ect. THats with the 35 horse wide open. Now we ran my 6 horse kicker wide open, same exact payload and it pushes the boat at 5 MPH.

    What is going on with that? Youd think that 30 extra horse power would get you going better than an extra 5 mph.

    I took the boat out with just me and a little gear and it was alot sportier ( I forgot the GPS) but I bet I was going 20.

    Do I maybe got the wrong prop on?

    The engine ran great, just didnt seem to want to push the boat.

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    Default what boat?

    You didn't mention the boat... What make, model and size?

    I'm no expert but sounds like too little motor with too much boat... The problem is amplified w/ the extra weight...

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    Member rimfirematt's Avatar
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    Boat is a 16 Ft Aluminum. Made by Gregor. Its a Modified V-Hull, with a pretty flat bottom.

    Yeah I think the Motor is a little small, but, What is confusing is that the 6 horse can push it to 5 MPH but the 35 can barely get it to go 5 MPH faster. I mean were talking 6 times the motor.

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    Member Akgramps's Avatar
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    I doubt the boat is getting on step w/3guys and gear. Even though you have 6x hp over the 6hp kicker. It takes alot more power w/ more weight to get up on step.
    When you tested it by yourself did it sound like the motor was achieving full rpm? If it continued to slowly accelerate you could be overproped.
    If it hit 20 and plateaued out it may need some help in the form of a tune up. Have you checked the compresion on the motor. The cylinders should be within 10% and probably not much less than 90 PSI.
    Hope this helps and you get it squared away.
    I had a 16 foot jon w a 35 hp and it would hit 22mph w/3 people, but that was a really light hull, your boats looks considerably heavier, did you get the wet foam or figure out the weight problem?
    Some of the older motors were rated at the powerhead so you would lose some at the prop shaft.

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    Member AKBighorn's Avatar
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    I don't know exactly what your boat is but my old 15' Klamath had no issues with 4 people, gear and fish going up the Kenia with an old Johnson 25HP motor. My brother had a yamaha 35 on his Klamath same size and it was an absolute rocket. There is definetly more to your story because you shouldn't be having any issues getting on step or past the 20mph mark.

    As far as the difference between the 2 motors and speed. If your not getting on step then your not going to go much faster just because you have more HP. The 6hp motor doesn't have enough power to reach planing speed but it sounds like your 35 doesn't either. I have seen boats with large motors have extreme difficulty getting on step despite the amount of HP they had. Come to find out they had spun the rubber hub inside the prop and the HP wasn't being transfered to the prop as it was slipping. If you have an old motor and an old prop and the motor is in good running condition I would be suspect of the same as I just described. If you fire up the motor and give it full throttle and it is obviouse that the motor is hitting high rpms then you need to replace your prop. Just my .02

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    Moderator Paul H's Avatar
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    It takes a signifigant amount of power to get a planing boat on plane, and not much power to push it in displacement mode. That is what you are seeing with your boat. A good rule of thumb is that you should have no more than a total of 40#/hp. With your 35 that means your boat, engine, fuel, gear and crew should be no more than 1400 pounds. My boat with family gear et all is well over 3000#, and it's 8 horse kicker pushes it 5 mph.

    It sounds like you were simply overloading that 35. If the hull is rated for it, I'd imagine a 50 horse would have been pushing you 20 mph.

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    Default I'll bet it's the prop

    I used to have a 1982 Klamath 18ft open w/ Evenrude 35hp prop. That gave me approx 27 net HP at the prop itself. With 3 people and gear, it would go 18-20mph light load, and 15 mph if really heavy. 2 people and light gear and it would go 25 mph at full throttle all day long.

    I put a 50hp 4-stroke Yamaha on it, and it would travel at 28 mph empty, and 25mph no matter how much weight you put in it. ( even overloaded ). Remember HP is " capacity to do work ". Increasing HP doesn't exponentially increase speed--- it will travel faster with heavier loads across a wider range of load factors. But---- I think someone was on the right track a bit earlier--- I think you are either propped wrong, or have the internal slip mechanism within the prop itself engaging, and eating your thrust. Those motor-props your size don't use a shear-pin, they have an internal slip-clutch to protect your lower unit. Once they've engaged, the prop is toast. They aren't something you can reset.
    That 35HP should be pushing your boat somewhere near the above speeds I've described.

    I'd check and reprop--- always nice to have a backup, anyways...

  8. #8
    Member rimfirematt's Avatar
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    Thanks for the help guys. I got a brand new prop with the motor. Well looks pretty new anyway. Ill put it on and see what happens. I actually dug the prop in the ground and put a big chip in it on my way back to the dock. Susitna Lake is wild. All of sudden you can go from 80 feet to nothing out in the middle of the lake. I couldnt believe it. Learned about depth alarm after that.

    So any guesses as to what my 16 ft welded aluminum weighs with just motor and boat?


  9. #9

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    Mines a 16 welded and comes in around 995 dry w/a 4-stroke 40.

    http://www.trackerboats.com/boat/?boat=2843

    Pretty much seems that you are under propped for weight (passengers). I swapped to an adjustable prop for big water and I can dial in rpms to suit my needs...changed everything!


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    Member russiarulez's Avatar
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    You probably have too high of a pitch on the prop, or it's slipping as was described above. I used to have a 16' searunner and had two different props for the 40 Yamaha. One was a 10 pitch and the other was like 13 (If I remember correctly). I would put the smaller pitch on when I had lots of people/gear and it would get me up on step, but going empty it would overrev the engine actually. And I used to put a higher pitch prop when I was light and needed to go fast/long distance.

  11. #11
    Member rimfirematt's Avatar
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    AK2AZ, what was that adjustable prop you got?

  12. #12

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    Quote Originally Posted by rimfirematt View Post
    AK2AZ, what was that adjustable prop you got?
    I can`t for the life of me remember the name but I did get it at West Marine...they sell replacement blades as well in the event of a strike. It`s easy to adjust and doesn`t require taking it off.

    I went from 30 seconds to plane down to about 7-8 seconds with the adjustments made for the weight. It takes a bit of experimenting but once you take notes life will be good. BTW it`s a 4 blade so you may not see super high speeds but it will pull anything out of the hole and stay on step at much lower speeds.

  13. #13
    Moderator Paul H's Avatar
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    Likely a pro pulse or pirahna prop.

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    Default Linky


  15. #15
    Member rimfirematt's Avatar
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    Is there a way to tell if my prop has one of the slip-clutches? I still havent taken off the one on my boat, but the spare one I got just looks like it has solid splines.

    I went down to west marine and they dont have that pro pulse prop in stock. So Ill try out this other prop of mine first before I order.

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    Default Do you have remote trim/tilt?

    I don't remember if you can visually tell or not if the internal slip-clutch has deployed. I did want to mention, however, that when I bought my new Yamaha 50HP 4- stroke at the time, it had a totally lame prop size and pitch on it , and my boat behaved sluggishly. When I brought it up to the dealer, they flat out told me that was a crappy prop for my boat hull/motor model combination. They gave me a proper matched prop for my boat type , and when I switched them out, it was like night & day.

    One issue I forgot to ask about, and hasn't come up yet is: do you have remote trim/tilt adjustment ability with your throttle mechanism? Typically you keep the motor all the way down to get out of the hole faster, and when you begin to come up on step, you trim the motor up to raise the bow untill you settle in with the water line washing the hull approx 2/3 to the rear of the hull , and/or the bow stops porposing. Without trim capability you could end up plowing water with the nose always too far down . That eats speed and gas consumption efficiency.

    Have you concidered adding a hydrofoil to you motor's cavitation plate?
    I'm a very big fan of these. They are not a magic pill, but I've ran prop motors both with and without, from 25Hp up thru 115HP, and I've always experienced noticeably positive results using them: quicker to get on step, much easier ( and less trim ) to trim it just right, and the boat will stay on step at lower rpms, once you've acheived on-step running.

    Had to smile a bit at your comment about the ding on your prop from Lake Susitna/Lake Louise. I used to be on a first-name basis with the guys at the Prop shop till I learned all the danger spots.

  17. #17
    Member rimfirematt's Avatar
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    Yeah it has power trim. But it of course wont work without getting on step. I do have one of the dolphins on the motor right now. Ill bet prop is just the wrong one. Im grounded till weekend after this one. Ill post back the results.

  18. #18

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    Quote Originally Posted by AK2AZ View Post

    Looks like a sweet prop, only wish they had a 22 to 28 pitch! max of 21 is WAY under propped for my boat

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    Sounds like a weight problem to me - 35hp for that boat is prob too small. If the boat has floor boards you should make sure that that boards are not waterlogged and that the floatation under them is not either. How'd she run with 2 peeps and no gear?

    a lower pitch'd prop might help - but if your not getting out of the hole - its either too much weight, or motor problems.

    is this a new motor? Older? 2 stoker?

  20. #20
    Member rimfirematt's Avatar
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    I checked for waterlogging. The boat runs on step at about 20 with two guys. Its that 3rd person that bogges it down. The motor sounds great, I dont think there is a problem there.

    With all the 16-18 footers im seeing with 35's Im pretty confident that the motor size is okay, yeah it could and should be more, but I think that once I get a lower pitch prop on there it will be alright.

    But also, I really dont plan on having 3 people in all the time neither. Its doable but still pretty cozy. 2 is way better.

    The motor is a early to mid 80's 2 stroke merc.

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