Larson Flyer Hull Suction Help

Discussion in 'Boat Design' started by Niceorange, Sep 6, 2005.

  1. Niceorange
    Joined: Sep 2005
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    Location: New England

    Niceorange New Member

    Hi everyone, this is my first post, and I'm hoping someone may be able to help me with a hull design issue I have on my 16" Larson Flyer. Below the transom along the hull is a small step/recess where the drain plug is located. When coming to a plane, I believe this step is creating a strong suction which requires high speeds and a large amount of up trim to overcome. Typically this suction will drag on to around 35-40 MPH, and once the suction breaks free you can feel the bow drop and the boat raise up, and the acceleration is noticeable as the speeds quickly increase 5-10MPH. Before planning out you can see the water being thrown at the engine as it sprays off the drive housing similar to the spray a motor which is trimmed far to low does.

    I am trying to figure out what kind of solution I can come up with to help that water evacuate quickly. I've though about adding some type of strakes to channel air to the step to help break it free. Also, I though of using some sort of autobailer or even a hose running above water level through the drain to help suck air in.

    Any ideas or help would be very much appreciated. Here are photos of the area I am referring to.

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  2. KCook
    Joined: Jan 2003
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    Location: Arizona

    KCook Senior Member

    Keel pad plus transom notch, in combination. Not to unusual. If you think the transom notch is a problem, how about just filling it in? That could even be tried with some temporary material, in case this notch turns out to not be the problem.

    Kelly Cook
     
  3. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    While I think this notch is a really bad idea, it is probably not the whole cause of you problem. You can tape a well shaped block of foam in the notch and give that a try. If the performance improves, I'd fill it in and fit a pipe to a new drain at the aft end.
     
  4. BOATMIK
    Joined: Nov 2004
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    Location: Adelaide, South Australia

    BOATMIK Deeply flawed human being

    I suspect that it is not your problem - if there is a problem at all. Even at moderate speeds - above 15 knots you should be getting a clean breakaway from the edge. If the edge is heavily radiused the water could be swinging up into the stepped section - a radius of less than 1/4 in would be OK

    If it is a bigger radius use some white autofiller to make the edge where you want it to release a perfect sharp edged corner.

    I don't think a step like this will have a reallly significant effect.

    The sharp edge effect is well known with racing sailing dinghies which operate at much lower speeds and have much less power available.

    It might be a really simple way to make an improvement. IF THERE IS ANY TO GAIN - which may not be the case.

    If you are really worried about it fill it with foam with glass as the others suggest - then it would be great if you would post the result on this forum - whether positive negative or neutral.

    Best Regards
    Michael
    Web Page
     
  5. Niceorange
    Joined: Sep 2005
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    Niceorange New Member

    Thanks for the advice guys. I am pretty certain that the notch is the cause of the suction. We've had the boat for 5 years and it behaves the same way every time you drive it. Out of the hole it is good for about 35-40 until the suction goes away, which is not a gradual thing... It's immediate and drastic. Typically to clear the suction you need to give it a ton of trim, enough that once it breaks free and lays down the motor is too high up and needs to be trimmed down again. Once it's cleared, the suction is gone until you slow way down... basically until you stop. Also, when the suction is present the boat wants to porpoise, especially with weight in the stern. Again, I think the notch is to fault because of the way the rear is stuck down during the suction, and the way water is being projected slightly upward off of the center of the transom rather than having a nice smooth wake behind the boat (which occurs once the suction releases). I have a spare brass plug that I was thinking about drilling a hole through and tapping in a brass nipple, onto which I can attach a plastic tube and run it up into the motor well (for water to escape which is forced through during reverse. If the suction is like I believe it is, it should suck air through said tubing in a way that will release the water. I also think taking a closer look at the radius of the joint may be in order. I can’t recall how round it is currently, but I’ll look into it.

    The boat wont be in the water again until next June, so in the meantime I’ll keep looking into this. Thanks again!
     
  6. nevd
    Joined: Nov 2003
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    nevd Junior Member

    Step venting

    The simple rule is any step in a planing hull must be vented - some are naturally vented and in some cases venting is difficult.

    Trial venting through the plug hole should demonstrate the effect of the venting the step, but you need to ensure the pipe size is adequate, particularly if you plan to run the pipe some distance to be above water level at rest. The trial will show the cross section of vent needed and you should also look at your options in terms of how the permanent solution will be installed.
     
  7. Niceorange
    Joined: Sep 2005
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    Location: New England

    Niceorange New Member

    Does anyone know of a one way chackvalve (autobailer) that screws into the hull in place of the standard brass plug? I know autobailers can be problematic and get stuck open, but i didn't know if anyone made one that I could use to test venting.
     
  8. jacklake
    Joined: Oct 2007
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    Location: georgia

    jacklake New Member

    I have a solution to your problem. I know your post is quite old, but I have had 100% success with my 2000 Flyer suction problem. Solution: I placed a piece of tuct take along the bottom of the boat right at the transom. I let it hang over about 1/2 inch aft of the transom. This tape is horizontal. Next I mixed up a batch of Marine Tex (other materials could be used that are waterproof) and applied it all along the bottom of the transom flush with the bottom and up along the vertical transom about 1/4 to 3/8's of an inch. Basically the radiused area where the bottom of the transom meets the vertical transom was filled in and the bottom was extended about 1/8 to 1/4th inch to the rear. The transom also was covered vertically about the same distance and a little fillet resulted. Basically, instead of a radius of a quarter of an inch where the bottom meets the transom, there was a fillet extending backward aft of the transom. I have had problems with this boat since new. The only way for it to break the suction, keep water from spraying up onto the back of the boat, and to increase the speed of the boat up to 5 MPH was to hit a big wave going close to wide open. Now as soon as the boat starts to get up on plane, there is a noticable jump forward when the suction is broken. I can only thank those who wrote in possible suggestions and my son for finding it on your site for finally making my boat enjoyable and easy to use. Before it was hard to have three people ride in the 2000 Larson 16" Flyer without having the trim all the way back on the 115 Evinrude ito keep it from hopping. THANK YOU ALL FOR YOUR SUGGESTIONS. You cannot know how much they are appreciated!
     
  9. Niceorange
    Joined: Sep 2005
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    Niceorange New Member

    Sounds like a cool solution. I actually came up with one of my own. Basically I replaced the drain plug with a fitting I made up with a 3/8" nipple facing into the boat. Attached to that nipple I have tubing which runs up and into the motor well through the cable conduit. When the boat is sitting the water in the tube is at lake level, and in reverse it is forced up the tube and into the motor well, but at speed the suction causes air to be sucked into the tube and into the pocket, which breaks the suction. It's not instant, but it's far faster than it used to be. It will also break suction at much lower speeds. Like you said, before it would take full throttle and some chop to break it loose, now it will break below 30.
     
  10. juiceclark

    juiceclark Previous Member

    a simpleton's reply

    I had that problem with a dinghy once. After fooling around a bit, I found that the prop was just too far into the water - motor mounted too low. Trimming the engine just makes it worse as the angle pushes the transom down.

    When I put a 2"x2" wooden firring strip under the motor mount, it popped outa' the water like a shot. Just a simple thing I didn't want overlooked.
     
  11. tuckerman52
    Joined: Jun 2015
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    Location: Lafayette ,la

    tuckerman52 New Member

    My name is Rob Taylor. Just bought a 1997 Larson. 166 flyer...great shape. W a 90hp Johnson... I was noticing the same thing. Only turning bout 5100. Trimmed up at wot. All of a sudden.. Motor jumps bout 200rpm. And bout 3mph.. Only got it to do that. A couple of times.. But very noticeable.. Motor seems to be running good. And have. Sst. 17 pitch. S/s prop.. Should be turning bout. 5400. To. 5600. My son found this site... Been. Reading up... The filling in of that upset. Sounds kinda hard(for me). But was considering the vented. Plug. Would like to try a temporary. Fix?? And check out. Also going to check the height. Of. Motor. In relation to transom.. Have a "whale tail'. Was on there when I bought... Been told I might should take it off?? Any input. Would be greatly. Appreciated!!! Raised on the water. But been 20yrs since owned. Boat. Love this hull... But should be faster. Also noticed the porpising. W weight in back. Thanks for any help...
     
  12. xylotops
    Joined: Aug 2017
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    Location: Nassau County FL

    xylotops New Member

    I recently got a Flyer 166. Immediately noticed the problem and found this discussion. Got a little piece of 12mm thick "heavy" foam PVC board. Made two 6" x 6" wedges & two wedge strakes. Stuck them on with epoxy (Hysol 11C) in this pattern: upload_2017-8-29_16-39-58.png
    Now, at fairly low power she slides right onto plane from a 10mph plow and I hear the step slurp and gurgle a big breath of venting air WITH NO PORPOISING.

    The 6x6 wedges almost did the job but the strakes assure that water flowing into the step from its sides gets separated from the flat ceiling so there's an air break at their inboard edges.
     
  13. Marc Gressler
    Joined: May 2019
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    Location: Drayton Valley

    Marc Gressler New Member

    I like many of you have just inheited a Flyer 166 problem, thanks for the info. Would like to try to build your design, could you possibly supply more detail, do you put these pieces in front of the step or in the step?
    Thanks
     

  14. Corey Zoomer
    Joined: Sep 2019
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    Location: Ontario

    Corey Zoomer New Member

    I figured out how to break the suction for just a few bucks.


    It’s like a whole different boat now
     
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