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Thread: Here's a really stupid question!

  1. #1
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    Default Here's a really stupid question!

    How high does a lower unit (prop) have to be above the keel to generate a rooster tail?

    I'm running a Mark 55/ Merc 400 lower unit with a three cylinder Merc on it and have never seen a rooster tail. The cavitation plate is 2 inches above the pad, and the prop is a homemade chopper of 17 pitch (that is not cavitating at this height). All I see is two spumes from the leading edge of the lower unit below the cav plate.

    Jeff

    PS: I'd post some photos but... What's with this deal? Was always able to post them before.
    "We live at the bottom of an ocean of air." - General Marvage Slatington

  2. #2
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    well the cav plate is far too low to cause a rooster unless you were trimmed out realy crazy.

    lets say the prop is 12 inch diameter for example and a 1 inch space above the prop to cav plate.
    1/2 of the 12 is 6 plus the 1 is 7, so realy the cave plate needs to be about 6 inches above the plate to get the rooster going.
    to surface properly and get a real good one going the cave plate needs to be 7 inch above the pad atleast.

    the rooster is only realy made when some of the prop is out the water, but with its centre line on the surface it gives the full effect. but could be slowing you down ???

    sometimes a rooster is just wasting power if a bit too high, ideal with a surface prop would be with top of rooster no higher than top of motor to get best thrust.

    lots of theory's about the rooster tail, but unless running on the surface you dont want to see it behind your boat.


    yes the forum is playing up big time again, could not make a new thread last night or post any pics.

    No such thing as a silly question, they are all valid...
    I normally ask Daft questions, and that is a different matter
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  3. #3
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    well the cav plate is far too low to cause a rooster unless you were trimmed out realy crazy.

    lets say the prop is 12 inch diameter for example and a 1 inch space above the prop to cav plate.
    1/2 of the 12 is 6 plus the 1 is 7, so realy the cave plate needs to be about 6 inches above the plate to get the rooster going.
    to surface properly and get a real good one going the cave plate needs to be 7 inch above the pad atleast.

    the rooster is only realy made when some of the prop is out the water, but with its centre line on the surface it gives the full effect. but could be slowing you down ???

    sometimes a rooster is just wasting power if a bit too high, ideal with a surface prop would be with top of rooster no higher than top of motor to get best thrust.

    lots of theory's about the rooster tail, but unless running on the surface you dont want to see it behind your boat.


    yes the forum is playing up big time again, could not make a new thread last night or post any pics.
    and not allowed to post pics today neither.
    someone has definitely altered the settings...........

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    Thanks, Phil. My prop is 9 inch OD, Gonna measure the tip to cav plate distance.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    most people when running suface type props cleavers and what not, seem to run with centre line of the prop up as near as possible to the surface when running.
    I know it sounds crazy, what that is the whole point of the surface type props.
    some are ok with just say 1/3 of a blade out the water, some like to be completely out the water, thats where the rooster comes in, unless you have a jet with a diverter.
    you also get a rooster with a conventional type propeller, but that normally only happens when over trimmed and running it higher that it is designed for.

    put your motor with cav plate about 4 inches above the pad, might gain a lot more than you think, just need to see how it handles in the corners !! that can often be
    the case of people running surface props too low , they dont like the corners on surface ...

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    Four inches up! Yikes! I've struggled like mad to get my homemade props to grab at half that height.

    Forgot to add.... My boating area (just down the road) is fairly rough most of the time. Running a 12 foot Deep Vee with a 10 inch pad can be rather 'thrilling' in a serious chop. I need for the boat to plane right off--now!--if I get knocked off plane by a wake. So far the prop digs right in and goes--something I doubt would happen at 4 inch up. Agreed?

    Jeff

    PS:: Thanks a bunch for your advise. It's all in my Keeper File.
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    just go up 1 inch at a time until you find the right place for it to be a good all rounder.
    no point in having fast top end if it wont go in the rough water that you use..

    top speed is not always going to win a race, if it dont come out the corners it aint going to win..
    there is a real good race on you tube with a nice 56 motor racing a sst60.. the sst60 is a lot faster on the straights, but the 56 loses it on the turns.
    makes for a realy good race and they are equal in amount of time per lap.

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    Team Member zul8tr's Avatar
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    Default Rooster tails

    Jeff If set up right with correct prop on a good designed hull that can stand the engine lifted high at full speed the rooster tail will not be that high since most of the rearward water from the prop is mostly straight back. The higher the tail the more vertical thrust wasted. Ex. on my hydro at full speed with 6 degree rake 2 blade prop the rooster is maybe 18" high. I use a 7" diameter prop and run the shaft center 3/4" below bottom thus 2-3/4" of blade out of water. Also with a very efficient boat set up correct and engine high with a surface piercing prop the water going backward off the prop is a little bit greater speed than the boat speed going forward it can never be equal since slip is present. Pete
    " Three may keep a secret if two of them are dead" Ben Franklin
    " ------- well Doctor what have we got a Republic or Monarchy? A Republic he replies if you can keep it"
    Benjamin Franklin, 1787 Constitutional Convention, as recorded by signer James McHenry's in his diary at the Library of Congress

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    sounds about right

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    Team Member Master Oil Racing Team's Avatar
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    When you are racing, and I am talking closed course such as 1 mile or a little larger or smaller, you are going to set up for your best time around the course. Filthy phill is right about winning. Most checkered flags are between halfway on the front straight to about 3/4ths down the front straight. You have to be in front after exiting from the final pin rather than being the fastest and ahead coming up to the next turn bouy. With this in mind, the testing and setups on outboards with surfacing props the result is a roostertail. In order to get competitive, the motor has to be lifted to reduce drag. The higher the motor is raised, the less the drag is, but there is a point where you will lose water pickup. Then you will fry your motor. Before that happens, there is a point where you will lose control as Phil mentioned. You could jack the motor so high you could swap ends in a turn. We always ran the center of the lower unit about and inch to 1 1/4 below the bottom of the boat. We ran 6 1/2 to 7 1/4 diameter props, and there wasn't much of a cavitation plate. It was bolted on to the lower unit and was about three inches wide and five or six inches long.

    So far I have given kind of an explanation of how we set up and raced our engines. Now I will say that we never tried to make a roostertail per se. All the boat racers did their setups to win, and made adjustments according to the size or the course, the wind, etc. Roostertails were just a result of our setups. I have seen on our lake a number of jet boats that raise their nozzle just to make a roostertail. The can look impressive, but lose speed because the jet of water is not pushing the boat forward, but burying the transom. Jet skis do the same. Some people with jet skis tap into the water discharge to send a jet of spray into the air. All of that is cool if that's what they want to do, but just to make a roostertail doesn't mean you are going fast.

    Can't give any advice on your boat though. You need to jack the motor up for sure to get a roostertail, but with a V bottom and small pad, you are going to have to do some serious testing, and make sure you always are hooked up to your kill switch.


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