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Thread: Prop Question

  1. #1
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    Default Prop Question

    Hay all

    I just bought an old Merlin mirage inboard 202 good nick just doesn't go very fast

    My mate has the exact same setup as me but his boat will do 80kmh at 5000rpm
    my boat will do 50kmh at 5000rpm

    Is this the prop that causes this or something that i have yet to learn about

    So if it is the prop were would i go to get a better one and how much should i be paying remembering this is just a fun weekender for towing a tube or two

    Thanksin advance

  2. #2
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    Your on the money mate.

    The prop is the gearbox & diff, as far as gearing goes in a midmount boat, if you take a look at the prop you have, you might find it dinted or bent.
    would be a good idea to take it to your local prop man, who you'd find in the ph book, and let him take a look.

    to have the prop balanced/tuned , for the correct application, makes a HUGE difference.

  3. #3
    Do you ever leave? TRIKHQ's Avatar
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    Precision props do a great job. Also the prop shop, both in Melbourne.

    Have a look at you mates prop there should be some numbers on it, record them and when you go to get prop checked out as suggested above ask them to compare.

  4. #4
    Senior Member Tinman's Avatar
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    80k at 5000 rpm for a 202 is a huge call.


    The pitch of the prop is basically your gearing. It is measured in inches and is the distance it will travel in one revolution.

    Generally the pitch is set to give your ideal maximum RPM of the engine to prevent over reving.

    On a ski boat it is common to pitch it down a bit to give better low end for pulling the skiers out of the water.
    Last edited by Tinman; 25-01-2009 at 11:10 AM.
    Cheers Tinman

    Jack of all trades master of none

  5. #5
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    Default Great

    Geez i didn't think i would get this much help

    So what kind of price should i pay and what should i expect to get realistically as far as speed goes

    As i said i dont want it to go like a rocket i just want to get around 50-55 without reving the ringer off the motor round 4000rpm would be nice

    Does this sound about right

    Thanks again

  6. #6
    Do you ever leave? TRIKHQ's Avatar
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    When I had my prop damaged it was about $120 to repair, (Precision props). To buy , mmm, $300 to ??? ,my guess.

    I would ask the repair specialist what the right prop is for you, you might get get low revs that you want but will need some good torque to get the boat moving from a slow start.

  7. #7
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    Expect to pay $400 + for a new prop, but i'd expect the prop repairer to be able to do something with your prop, personally ive seen repairers perform what i reckon was a miracle. (wishing i could post up a pic for you!)

    like some of the others have said before me, 50mph is a MASSIVE ask for a standard-ish holden 6.

    good luck

  8. #8
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    Default Engine

    Cool

    Ill fill you in on my engine "roughly"

    blue 202 yella terra heads port work heavy springs cant go much further on the heads roller roockers 500 holley New yesterday
    30thou bore dont have much details but it is a sick motor revs to 6000 no problem

    When i do the old holeshot take off the engine revs round to 6000 straight off the bat and pulls like a train

    i can only compare it to other boats i have driven my mates boat the same as mine and his old easyrider witch had a 50 or 60 outboard merc on it

    Now the outboard did 65kmh no probs so i would imagine my 202 could top that off the line no problems and i would expect having so much more torque it could go faster to

    "Dont forget to stop me if im wrong please i will not take offense im only new to boats"

    ill go to the local boat shop and ask what type off prop he recommends and ill run it through all the boaters here and just see what everyone thinks

  9. #9
    Senior Member Tinman's Avatar
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    Not too many people will be able to give you the perfect answer for what size prop you should be running. Obviously if the holden 6 revs to 6000rpm it is not stock. It is also a good chance that the prop is a bit small. What is the rev range of the motor. With a big cam it will have poor bottom end and therefor if you put a big prop on it you will have trouble getting out of the hole under load. So it may be a bit of a balancing act.

    What is the drive system? dog clutch, direct drive, gearbox?

    What revs and speed are you getting at wide open thottle?

    What size is the prop that is on it currently. It may be marked on it.
    L/R(left/right) Diameter X Pitch X Number of blades. eg. R11.5X15X3. You need to know this otherwise you will be just guessing.

    The first measurment is the diameter. The diameter needs to suit the size and weight of the boat. If it is too big there is no slippage and full load at low revs. Idealy a prop should slip about 10% at low revs. This is a bit like riding the clutch to take off. At full speed there should be no slippage. It may be that your prop is a small diameter to get the revs up into the power range of the motor quickly.

    I have not had anything to do with 6 cyliders in ski boats but I would think without having something pretty special in the way of a motor 60-65K would be about it. I would not be expecting too much more. Most stock 350's only do around the 80-85k.

    New prop $400-700 depending on brand and quality. The expensive ones still bend if you hit something. To get a prop straightened can vary greatly in the cost depending on the damage and who you take it to. The two mention earlier are quite expensive. There are places around that will straighten a prop for less than $100. They dont have the computer technology of the likes of precision props but but can do quite a resonable job.

    A prop can also be pitched up a bit by the prop repairers as well. How far they can go depends on the prop you give them and what has been done to it previously. Possibly up by 2" or so.
    Last edited by Tinman; 25-01-2009 at 11:13 AM.
    Cheers Tinman

    Jack of all trades master of none

  10. #10
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    Default

    just a quick note if the prop has almost any damage you will notice with shudder or vibration

  11. #11
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    Default Goes good

    Took it out this afternoon

    65kmh at 5750 roughly still had legs

    so my prop question is a bit pointless now

  12. #12
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    Hi TurboSteve............you've received good advice from the guys here.
    If you're spinning your prop to 5750...6000 rpm...for 65 kph..then that suggests that the prop is probably quite small( even as small as 11 to 12 inch pitch....and maybe 11" diameter)
    The fact that the donk 'flares' to 6000 rpm out of the hole, also suggests that the prop is small and does not have much 'cup' on the blades to assist with efficient thrust...( just like spinning the wheels running street tyres compared to slicks!)
    Find a good prop man(by asking around).........tell him how the prop's performing in your boat( rpm, speed ,etc)...and have him repair /repitch/cup it to your desired top rpm( lets say about 5000).............
    .........for a couple of hundred bucks ,it should be the best loot you'll ever spend on your outfit....
    ........it won't go much faster........ but it should pull far more efficiently through the entire rev range.........instead of 'ringing that li'l 202's neck' all the time........you'll be saving heaps in fuel consumed and maybe engine repair bills down the track a bit!!!!
    Hope this helps....
    Cheers Cactus

  13. #13
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    i think he needs a bigger prop.
    my boat cruises at 2-2500 pulling skiers.
    i dont rev mine further than 4500 and im flyin.
    why would u want to rev at 5000 all day.

  14. #14
    Senior Member cactus's Avatar
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    Quote Originally Posted by devilracer View Post
    i think he needs a bigger prop.
    my boat cruises at 2-2500 pulling skiers.
    i dont rev mine further than 4500 and im flyin.
    why would u want to rev at 5000 all day.

    .......he may need a bigger prop......or the prop repairer may be able to 'pitch up' his existing prop(depending on it's condition).......or tune his existing prop by 'cupping it',so that it's not revving so hard,to get the job done

    .....you must be swinging a pretty big prop............usually 2500 rpm with a 12x15" prop gives about 27mph/42kph in a centermount

    ....4500rpm with 12x15" would see you 'flyin' at around 48mph

    ....I agree.....why would u want to rev it at 5000 all day?............that's why I suggested having his prop tuned to a top operating rpm of 5000 so that he'll be towing skiiers at far less rpm most of the time.

    Cheers Cactus

  15. #15
    Senior Member Tinman's Avatar
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    I agree but depends on what torque his motor has down low. It might not pull to get it out of the hole. Remeber it does not have the torque that a V8 does and for the motor to do that kind of revs the cam is faily large. We still dont know what size the prop is that he is running.
    Cheers Tinman

    Jack of all trades master of none

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