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Thread: Here's a really stupid question!

  1. #11
    AeroMarine Research Jimboat's Avatar
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    Quote Originally Posted by Fastjeff57 View Post
    How high does a lower unit (prop) have to be above the keel to generate a rooster tail?
    here is an article on 'Rooster Tails'

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    First of all, thanks very much for taking the time to reply to my question.

    I thought that, since I've never seen a rooster tail on my boat, that might indicate something that needs to be done about it. I now see that a rooster tail is not needed, especially since I boat in really rough water for a 12 footer. (Was out yesterday and got clobbered by large wakes of boaters coming back in late in the day. Mornings for me from now on!)

    Okay, so I'm up to 42 mph at 5,800 rpm with the latest modification to my homemade prop. The cav plate is 2 inches above the bottom (deep Vee. 20 degree deadrise hull with a 10 inch pad. Love to post some photos but...) The 9 by 19 inch chopper bites well in all cases, planes off swell, holds in tight turns, etc. And it holds up to about 10 degrees trimmed out past vertical. I have another 3/4 inch left in the homemade jack plate, so I'll be trying that next, along with re-installing my 'hot rod' motor; an older Merc triple with external reeds and three carbs. Its power band runs strong up to 6,500 rpm, where the stock triple (I'm running now) 'falls off a cliff' above 5,500 rpm. (Merc spec say it has a 4,800 to 5.300 rpm operating range.) Should be interesting!

    Again, thanks for all the help.

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

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    that extras 3/4 inch might get you another 1mph maybe more, worth a try.
    when in the rough stuff the rooster ona small boat would be a waste if its a big rooster, and when running in the rough I always keep motor down.

    ideal thing to do is get a real cheapo endoscope camera and attach that to the back of the boat point toward the cav plate. these are fully water proof and around $15 for a cheap one.
    they are brilliant for looking inside the motors and up exhausts etc and great for watching the cave plate on the phone.
    you can keep the videos they take and watch em back later.
    This is great to use as you can then see if the plate is riding on the top of the water at full throttle while your in your normal driving position .
    The closer the prop to the surface will be best for top speed with your prop, and being as you made it , you know how to change any part of it too.

    would like to see this boat run, as 42mph is not hanging around at all !!!!

  4. #14
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    I DO have a camera and been taking video forward and aft all along. (I post it here but...) At 1 3/4 inch up the cav plate is clearly above the stream coming back, and I'm now at 2 inch and will shoot for 2 3/4 inch (max possible without modifications).

    Dying to try the 'hot rod' motor as it makes at least 15 more hp at a thousand plus more revs (which should be perfect with the present prop).

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington
    Likes filthy phill liked this post

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    Update: Built several props this summer, and got the my dink boat up to 42 mph. (I know: big deal, but that's 6 mph faster than before.) Raising the cav plate from a 1/2 inch above the keel to 2 inches up was primarily responsible. By the end of summer I got my homemade props to hold with the cav plate just under 4 inches above the keel. The temp gage went up so fast I actually saw it move, so more work needs to be done on my water inlet system.

    Jeff

    PS: My respect for prop gurus like Ron Hill knows no boundaries!
    "We live at the bottom of an ocean of air." - General Marvage Slatington

  6. #16
    Administrator Ron Hill's Avatar
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    Default O. F. Christner Always Told Me He Was A Welder

    PS: My respect for prop gurus like Ron Hill knows no boundaries!


    Thanks, Jeff but, I basically call myself a BLACKSMITH. Over the years many of my best props were props we just beat on and and twisted and beat on them some more.

    Today, my son, Chad, has some interesting equipment to measure propellers and reproduce them like "POPCORN".

    Test, test and more testing. You mentioned water temperature. Over the years, I would hook up a long water hose from the "PEE HOLE" and have the water run on my hand, usually my throttle hand in kneel down boats. When the water started getting hot, I'd let off the throttle and slow down....

    PS

    I loved O.F. Christner, I always thought of him as more than a welder.


  7. #17
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    ..."Test, test and more testing."

    That's surely what I have been doing, Ron; and relearning what guys like you already know. I too am basically a blacksmith, and damn proud of it! In the old days, those guys could make practically anything the towns people needed.

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

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    Blacksmiths were and are very clever people who did and do wonderful work. and I salute the blacksmith's of the world.
    To be a blacksmith you have been gifted from birth to work with metal, you can only tech those who can take it in, and taking it in is not as easy as some might think.

    But you must not forget about us who have mastered the art of bodging up stuff, I call myself a bodger as I can bodge anything to get me home.
    In the fine art of bodgeneering I would definitely come under a master bodge artist, from now on I will call myself a Bodgesmith just to make it sound a bit
    more up market.
    We in bodgecraft use a close tolerance of around 1/4inch for our very intricate stuff and about 1/2inch for normal everyday work.
    Anything under 1/8th tolerance is a master craftsman to us and all the talk of 1,000ths we think is pure mythology as our rulers dont even have marks
    smaller than 1/8th so we know it is just fantasy for normal people to work like this in reality
    Those who claim to realy work in the 1,000ths are obviously wizards, witches, or beings from a distant galaxy and should not even talk to the like of us in the damp
    garden sheds of the world.
    These people do not understand that the oily , greasy pile of scrap and rubbish they see in our corners of the sheds is actually our master pieces that took blood, sweat
    and lots of bandages and cups of tea to get them looking like this..
    When I see props made by the likes of ron and chad hill I drool at just how good they look, to me they look more like a prize trophy rather than something to get wet
    and mucky.
    But these master pieces only work when put under water so its obviously something to do with black magic or sorcery of some kind, maybe even Voodoo
    Obviously he is one of the above three mentioned and a Wizard.

    Now to fastjeff57
    His home made prop work is great, he has made his own and done some great work and got good results, this has to be applauded by us normal mortals.
    Not only has he "had a go" at making a prop, its running very good and he is now getting great results with fine tuning his set up.
    I take my hat off to him ( all us bodgers wear beanie hats as we are not yet up to peak cap standards). and I love to see home done stuff working good.

    And putting a powerhead off one motor onto another totally different mid -section and then making a custom prop to fit this is just incredible so he also must
    be a top ranking understudy to a wizard or sorcerer !!!

    This talk of testing is just to fool us into thinking he is not sure what he is doing, we are not fooled, we know that you know what your doing.
    you have already done what to us is the impossible task of making scrap metal into a piece of working marine art, you sir are a genius just fine tuning a great
    piece of work.

    All of us at Bodge school salute you, we cannot take our hats off again as its freezing out here in the cold damp sheds of doom, danger and high voltage.
    We just keep looking for your next results of " testing " and we are not disappointed in what we are seeing so far.

    Keep up the good work.

  9. #19
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    Thanks a bunch for your most considerate comments.

    Read your post above on your own prop travails and am at a loss as to what I'D do next in your place.


    I'm sure somebody here will give you good advice.

    Jeff

    PS: In my "machine shop", the most sophisticated tool is a cheapo Harbor Freight drill press.
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    A harbour freight drill press is 100% better than no drill press.

    with your boat and motor you will get to speed you desire and then some, I think its brilliant that your doing so well with it.
    and no way I could ever make my own props, I could make one and make it look good, but no way would I want to trust my welding ha ha .

    props are weird in that they dont look much different to each other and yet perform so differently.
    I have seen props that are crap and yet a tiny little bit taken off in a certain place and they work great.
    recently I tried a brand new prop I got off ebay cheap, its just an alloy 12 3/5 x 23 alloy prop nothing special, tried it and total disaster, was churning up terrible at slow speed
    and when running on plane was corkscrewing the boat.
    I took it off and looked at it and noticed the leading edge was extremely thick, so I sharpened ut like a razor ( not realy good for alloy props) but it transformed it into a
    decent usable prop that handles ok. atleast your home made prop is working very good.
    I like to see home made stuff going as planned.

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