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Results 1 to 19 of 19
  1. #1
    Join Date
    Feb 2010
    Location
    Saint Cloud, MN, US
    Posts
    7

    Default "I have an 18' Quantum bas

    "I have an 18' Quantum bass & ski boat with a 150hp black max 1996. I am reving 6400RPM's@50MPH.My current prop is an aluminum 21"pitch.I would like to get closer to 60 MPH and not rev so high and still be able to jump out of the water.What would be the best prop to acheive this?"

  2. #2
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    8,380

    Default "To start with you are definat

    "To start with you are definately "under-propped" you have a max rpms of 5500 with that particular motor - you are lucky that have not damaged the motor.

    And to answer your question you need to do some math [img][/img]

    Assuming your tach is reading correctly you need to move up about 5" in pitch to at least 26.

    However, simply trying to achieve greater speed is a little more complex than simply changing your pitch.

    The pitch increase to 26 would (technically) give you 5 more inches of forward motion for every revolution of the prop.

    However, since you would be bringing the rpms of the motor down (to where they should be, about 5400), you would get 1000 less revs at the crank which (after the gears) would translate into about 535 less prop revolutions per minute.

    To figure (roughly) what kind of speed difference you can expect (if slippage stays the same) we can do the math.

    Currently - 50 mph @ 6400 rpm with 21 pitch.

    To calculate your "slippage" we divide your rpms by your gears to determine prop shaft rpms = 3422 - then multiply that by the theoretical distance the prop should travel in an hour (21" (pitch) x 3422 (rpms) x 60 (minutes to get inches per hour) divided by 12 to turn it into feet then divided by 5280 (feet in a mile) to figure out your "pre-slip" speed - which in this case equals 68.05 mph.

    Since you are only seeing 50 mph that means you are "slipping" 18 mph or 26.5%

    If all stays equal, a 26 pitch should result in (26" x 2887rpm x 60 div 12 div 5280) 71.08 mph reduced by the 26.5% slip for a (possible) top end speed of 52 mph.

    So while you can probably save your motor from an early grave by properly "propping" it, you are unlikely to achieve the 60 mph you are looking for...."
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

  3. #3
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    8,380

    Default "Although my response was some

    "Although my response was somewhat "long winded" I really should clarify (before someone else points out my terminology error).

    When I stated 26.5% "prop slip", that percentage also includes a number of other factors which ultimately reduce potential speed (and can be easily addressed).

    Those would include the condition of the hull, the total displacement weight and how well the motor is trimmed.

    Some of that overall (slip) or loss can be mitigated by -

    running as light as possible - remove un-necessary "stuff" - only carry enough fuel for your particular outing (with enough in case of emergency), leave the wife/kids/buddies on the dock [img][/img]

    install a jack plate to trim the motor as perfectly as possible

    install trim tabs to compensate for minor hull deficiency/deformaty or unbalance

    Those could increase your performance by upto 5% (or about 2 more mph).

    Ultimately, at the end of the day to achieve the results you are looking for you need to significantly increase your horsepower.

    (rule of thumb) a 50% increase in horsepower will result in a 15% performance increase.

    So if you take that 150 off and strap a 225 on the back (plus make the other changes) you should see 60 mph on your rig..."
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

  4. #4
    Join Date
    Feb 2010
    Location
    Saint Cloud, MN, US
    Posts
    7

    Default Thanks Graham for all the info

    Thanks Graham for all the info. I guess i'll get a higher pitch prop and have to live with the outcome because the boat is only rated for a 150 HP motor

  5. #5
    Join Date
    Feb 2010
    Location
    Saint Cloud, MN, US
    Posts
    7

    Default "Graham, after thinking about

    "Graham, after thinking about the percentage of slippage I am having, would it be better for me to lower my motor farther down in the water? I have one hole left on the mount to lower it. Another thing, when i'm under full power the motor has to be trimmed all the way down or the boat will start porpusing. Once I try to trim it up the front end starts bouncing up and down"

  6. #6
    Join Date
    Feb 2010
    Location
    Maunaloa, Molokai HI,
    Posts
    21

    Default "Graham is right. If you could

    "Graham is right. If you could get someone to let you try dfferent props. so to try 22,24,26 pitch prop.This will get the right pitch for your boat. every boat is differant.That motor should run 55 to 58 RPMs at full throttle to reach max speed.
    Spend the extra dallor and go stainless,aluminum tends to flex.Also the larger your pitch the slower your hole shot is going to be.(trim all the way down, then throttle all the way down and back of once shes on plain)
    The higher your motor is mounted will also help your speed. this is how a jack plate works.It lifts the boat higher out of the water at full speed thus having less friction on the boat.So lowering your motor may slow you down.To high could cause cavitation.
    I have had four differant bass boats and driven many differant ones. I could set the throttle half open and start trimming up and make the boat porpus all of the them would do this.I simply put the hammer down and drive through it.find the right prop, this will help. Note: 50 to 55 mph is not a bad speed for that boat. good luck"

  7. #7
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    8,380

    Default "Unfortunately there is no eas

    "Unfortunately there is no easy calculation to determine how to (mount or adjust) the motor for perfect trim (or as perfect as possible).

    There are a couple of general "rules of thumb" to get you started, but after that it's alot of trial and error.

    In general, the higher a motor is mounted the higher the potential top end speed. Of course you don't want to mount it so high that the water intakes break the surface and you suck air into the cooling system.

    Merc has done many tests on that exact subject and for your particular model they have found that in the pursuit of 60 mph, mounting the motor with the cavitation plate (upwards) of 2.5" above the bottom edge of the transom is the max they recommend.

    In fact, in a "normal" installation on a rig capable of achieving 60 mph, they recommend that it be mounted 1.5" (high) instead of the normal (rule of thumb) that would say to mount the cav plate in line with the bottom edge of the transom.

    Since transoms are normally "stock sizes" (15, 20, 25 or 30 inches) that is why I suggested a jack plate (which you often see on bass and performance boats).

    That allows you to "fiddle" with the mounting height (even the basic plates often have a 4-5" vertical adjustment) to find the "sweet spot" where your rig will give you it's top speed.

    Additionally, the jack plate has some "set-back" which in effect mounts the motor 4, 6, 12 inches (or whatever) further back off the transom.

    That allows you a some additional wiggle room in how high you can adjust the motor.

    I have never found any hard studies but the general concensus seems to be about an extra 1/2" higher (over recommended heights) for every 4" of setback.

    However, all these adjustments are purely in the "pursuit of top speed".

    That extreme extra height does have the effect of reducing your initial hole shot because until the prop really "bites" the water and pulls the transom down your prop is pretty much breaking the surface and not getting a good grip on the water - so you will experience a "lag" before she really gets going.

    Without a jack plate you can fiddle with the trim "angle" which can still help (if it's set less than optimal - again, trial and error), but you must be able to control/adjust the mounting height to make a significant difference....

    (significant in this case would be a 2 or 3% performance change which could be considered HUGE without repowering a rig - there is very little you can do to make a very noticable change if all is rigged/fitted properly)"
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

  8. #8
    Join Date
    Feb 2010
    Location
    Saint Cloud, MN, US
    Posts
    7

    Default Thanks guys for all the info.

    Thanks guys for all the info. Now all I need is the ice off the lakes so I can do A little trial and error with different pitch props.

  9. #9
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    8,380

    Default "I hear you there - we have 18

    "I hear you there - we have 18" of ice up this way.

    The lake trout fishing is still good, it just sucks having to drill 50 holes in a row if you want to do some "trolling" [img][/img]"
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

  10. #10
    Join Date
    Feb 2009
    Location
    perth, wa, Australia
    Posts
    956

    Default "Mark what type of prop are yo

    "Mark what type of prop are you running it is achievable to hit 60mph mark with your engine with the right prop. You should have between 10 and 15 percent slip, 26 percent would be aceptable on a cabin cruiser not a ski boat"

  11. #11
    Join Date
    Feb 2010
    Location
    Saint Cloud, MN, US
    Posts
    7

    Default "It is a stock aluminum prop,

    "It is a stock aluminum prop, 21'x14.25 I beleive"

  12. #12
    Join Date
    Feb 2009
    Location
    perth, wa, Australia
    Posts
    956

    Default If you try a laser11 prop with

    If you try a laser11 prop with 24" pitch you will be the right prop for you and how did you check your speed? because speedo's are not accurate u need a gps

  13. #13
    Join Date
    Feb 2010
    Location
    Saint Cloud, MN, US
    Posts
    7

    Default "Yes, the speedometer is about

    "Yes, the speedometer is about 6-7 MPH off. The GPS reads 49 MPH and the speedometer reads 55 MPH. With a 24" pitch prop will I still be able to get the boat out of the water?"

  14. #14
    Join Date
    Feb 2009
    Location
    perth, wa, Australia
    Posts
    956

    Default "Yes no prob, you can get a ve

    "Yes no prob, you can get a ventilated prop which will give you good accelleration out of the hole

    I would use the 21p for skiing"

  15. #15
    Join Date
    Feb 2010
    Location
    Saint Cloud, MN, US
    Posts
    7

    Default "Jamie, what is a ventilated p

    "Jamie, what is a ventilated prop? Is that the ones with the holes in them?"

  16. #16
    Join Date
    May 2009
    Location
    Marine Engine
    Posts
    564

    Default A vented prop allows exhaust g

    A vented prop allows exhaust gas to escape in front of the blades so when you first power up becouse of the air and turbulence the motor picks up rpms faster than if in water only. ( like poping your clutch ) giving a better chance for a hole shot.
    after roughly 1000 rpm the exhaust gas and water pressure keep the gas from exiting those holes

  17. #17
    Join Date
    Jun 2009
    Location
    New Plymouth,Taranaki,New Zealand
    Posts
    282

    Default "Or you could try a Pro-Pulse

    "Or you could try a Pro-Pulse prop,which is fully adjustable.four allen screws and you loosen the hub and adjust the 4 blades to get the right pitch.ive used them on both my boats,and they work out well,easy adjust for different applications,replaceable blades.about $380 NZ for the bigger ones.[Im not an agent]See their website."

  18. #18
    Join Date
    Feb 2010
    Location
    Maunaloa, Molokai HI,
    Posts
    21

    Default "Mark I beleive your boat w

    "Mark
    I beleive your boat will do just find out of the hole with a 24 pitch with proper throttle contral.Also gain speed top end. I have used the vented props and they do help,but not a lot of differance.Jamie is right: if you know your going out to ski or with the family than drop down in pitch. if your out for speed and long travel step up your pitch. I traveled the country fishing bass boats and carried 3 props. And a simple formula: big lake, bad weather,Means low pitch anywhere( going to be climbing waves all day). Two: Mid country, max pitch prop if good weather. Three: southern states with lower sea levels will make your engine run lien it will fly. So I would carry a mid prop if to make a long hard run. To keep the RPMS in check. I beleive you will find the 24 laser to be a good all round prop. Good luck"

  19. #19
    Join Date
    May 2007
    Location
    Inverary, Ontario, Canada - The Great White North Eh!
    Posts
    8,380

    Default "As Ian suggests, a ProPulse i

    "As Ian suggests, a ProPulse is worth looking into if they produce something in the (pitch range) you require.

    I run one (4 blade version) on my 140 Evinrude.

    Mine is adjustable from 16 to 20 pitch and for my application it works well.

    They are however, a composite construction (plastic of some sort) which many people don't trust.

    They do have the added benefit of being totally modular and every component can be replaced/repaired "on board" should the need arise.

    So if you toast a single blade you can replace it in a few minutes with a minimal collection of tools.

    It's not a prop that I would have bought myself, but I got it for nothing and decided to give it a try and so far it has worked well.

    I personally favour Solas' line of composite aluminum props (Alcup models in particular) which are somewhere in between regular aluminum props and stainless in terms of durability and flex but often sell for less than either..."
    Graham

    A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it :)

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