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  1. #21
    Join Date
    May 2007
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    539

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    With regard to the prop and the engine position, basicly two style of props. the cleaver or flat back edge (surface running) and the other rounded sweeping blade. As you lift the engine higher you will need more of a cleaver style propeller. Helps reduce the cavitation.

    On my ski boat, (hydrostream viking) I run what is called a high five by merc, it is a 5 blade cleaver style prop. the only way I could get the boat out of the hole. A guy actually told me to lower the engine when I was trying to prop it for skiing. tried about 20 props before it we found the right one. For speed, run a high pitch long sweeping blade. The best one I had, before it threw a blade, had a second gear when the boat got up out of the water. The more blades the more the prop will hold the water. with the 5 on it, backing off the throttle is a kin to slamming on the brakes.

    For what you are building, for stability, you might want to push a little water. You will not have much aerodynamic lift or aerodynamic stability (downforce), the control of the boat will be primarily the contact of the sponses and the water. (my guess) You will have to play with it to get comfortable.

    If you are going to let anyone drive it, keep that in mind when you prop it and balance it. Some folks just dont have much sense when it comes to go fast toys.

    As you get up to higher speeds, turning you could catch a wave and end up swimming. Remember two strokes are compression transfer engines, ie if you pump water through the engine, you blow out the side of the block. We always had alcohol in a squirt bottle to get the water out of the engines, when we dunked them.

    Ran the kneel down hulls for a couple of years and a 45 tunnel.

    Looks like a fun project, keep us posted.
    Tom

    edit: you might want to add an engine kill pull off cord. My ski boat actually has one, if you get bounced off it would prevent a run away.
    Last edited by trstek; 05-29-2007 at 07:27 PM.

  2. #22
    Join Date
    May 2007
    Posts
    100

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    That kill switch will be a must.

  3. #23
    Join Date
    Oct 2006
    Location
    DFW area
    Posts
    180

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    Quote Originally Posted by Doug Doty View Post
    That kill switch will be a must.

    Oh yeah,,,,,
    Depending on how your steering works, a 'no feedback' system that locks itself down when you stop turning the wheel, or the regular kind where you can push on the engine and make the wheel spin, a boat with no driver will do one of two things....

    The no feedback system; it keeps going however the steering wheel was left/set was when the driver got ejected.

    The regular kind: the prop torque will push the engine over to one side, and the boat will keep going in circles trying to run you over until it runs out of gas.

    I snap my kill switch lanyard onto my life jacket first, even before I start the engine.......
    every time.

    Something you might want on it is a spring loaded 'hotfoot'. It uses the throttle cable that normally is in your shifter box, and puts the fuel control down on the floor like a car. Plus, it goes back to idle when ya let off of it. They used to be only used for racing, but nowdays most high powered bass boats have them too.
    If you keep the normal/original shifter, you have F & W like a regular boat, and you don't lose the neutral safety switch.

    .
    Last edited by Winger Ed.; 05-30-2007 at 09:18 PM.
    "Gone are the days of wooden ships, and Iron men.
    I doubt we'll see either of their likes again".

    Circa 1920.
    Author:
    Unknown US Coast Guard unit Commander.

  4. #24
    Join Date
    May 2007
    Posts
    539

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    Looking at the craig boat, looks like the steering is done in the center with a lever front / back motion to turn the engine? And the throttle is on the right side.

    I am interested to hear how it does in the water. Seems counter intuitive to push the lever forward and back to steer.

  5. #25
    Join Date
    May 2007
    Posts
    100

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    I am equally curious to try it out, I tracked down the maker of the steering system and they have one now with a much higher ratio now. The one in the video's of the craig cat require an almost verticle to horizontal movement to turn fully one direction.

    I got the tubing bent and the attachment stands all made up for the front of the boat this week. I will weld them tonight and start on the rear this eve. It will be more time consuming as I don't have a complete plan yet and it will include the motor mount.
    Last edited by Doug Doty; 05-31-2007 at 05:13 AM.

  6. #26
    Join Date
    Oct 2006
    Location
    DFW area
    Posts
    180

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    Quote Originally Posted by trstek View Post
    . Seems counter intuitive to push the lever forward and back to steer.

    I've heard it takes a little getting used to, but once ya do-- its fine. Years ago, it seemed all the small, narrow bass boats with a 40-55 hp engine on them had that lever steering system, and the driver's station was mounted real far forward from most other boats. To driver them, you sat right up there with the trolling motor controls.

    Is my memory getting fuzzy, or do airboats use that lever system to turn thier rudders? I seem to remember seeing a steering lever beside the driver, but not in front like a tiller.


    .
    "Gone are the days of wooden ships, and Iron men.
    I doubt we'll see either of their likes again".

    Circa 1920.
    Author:
    Unknown US Coast Guard unit Commander.

  7. #27
    Join Date
    May 2007
    Posts
    539

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    How is the boat coming along?

  8. #28
    Join Date
    May 2007
    Posts
    100

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    I have the cross tie bars made but need some finishing on the motor mount befoer I can cut out the deck material.

    I had some surgery 5 weeks ago and am having a real tough time with the healing up side of things and can only be on my feet part of the day without swelling making me miserable so there is no after work progress taking place. I am slowed down for a while here. If I could have 1 good long weekend I would be close to done. I am going for a second opinion tomorrow morning so we'll see how that goes. All for now.

  9. #29
    Join Date
    Mar 2007
    Location
    Deltaville, VA
    Posts
    2,239

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    Doug,

    Been following the boat building project. Great work there.

    Just a couple of comments, based on racing boats for many years.

    The flat, sponson hulls will create suction at speed. On a hydroplane sponson, they are shaped so as to create lift. You may find that you have to add a step to the sponson to break the drag effect.

    With the 90 degree angle on the outer edge of the sponson, the boat will be "very" tender in a high speed turn. This leads to what we call hooking.

    With a 40 HP engine, I would think she'll run like a "scalded ape". Just be careful in the turns.

    There are several manual and hydraulic "jack plates" out there which may give you some ideas about an engine mounting system.

    Don't leave the dock without an operational "kill switch". I was the one who pulled Hank Bowman (former sportswriter for Popular Boating) out of the water during the '66 Orange Bowl Regatta (Marine Stadium-Miami). He was driving an unlimited inboard, I was driving an unlimited outboard (9 Hr Regatta). He got thrown out and run over by his own boat. The vision of 3 prop walks up his left side stay with me to this day.

    Great work on the boat. From the looks of some of your other "toys", I'm sure it will come out great.

    Keep the photos coming.

  10. #30
    Join Date
    May 2007
    Posts
    539

    Default

    Doug,

    Don't over do it, you can always work on the toys later.

    I was just wondering about the project. Sounds like that lever steering is common.

    Sundown added some good points.

    The tunnel hulls use a 90 deg inside turning edge. The 2 cylender 45 formula lights were pulling about 2.5g in the turns with the v6 formula merc boats pulling 4 - 5g in the turns. Those boats run full 5 pt harnesses and enclosed canopy.

    The tunnels used a angled bottom instead of a flat like you are running, like Sun said the hydro's use a step, they are an air entrapment hull design.

    The prop selection and engine height are the key to setting how you want the boat to run, speed and stability.

    The rule of thumb is after 60mph the water is like hitting pavement, scrapes, bruses etc. One of the guys I used to barefoot with in my misspent youth had a doctor tell him each fall was the equivalent to a pretty good car wreck, as far as the forces put on the body. that was about 35 - 40mph. when you stuck a toe, were lucky to close your mouth before hitting the water.

    Water is nothing but fun, but, worthy of a high level of respect.

    Hope you are feeling better soon.
    Tom

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