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Thread: Porpoising

  1. #1
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    Unhappy Porpoising

    I have a 1987 Stratos VT180 Ski (VT Rocket) and just replaced the Mariner 150 that it came with a 200 HP EFI Merc. Pushing it with a 24 pitch trophy 4 blade SS prop. I mounted the Merc on the same hole pattern and vertical position on the 4" jackplate, but the boat porpoises almost continually at most any speed short of wide open in anything but totally smooth water. The center of the prop shaft appears to be about 3" beneath the bottom of the keel. I tried jacking the motor up higher on the jackplate but ran out of adjustment room. At that point the center of the prop shaft was just about level with the bottom of the keel.

    Any suggestions?

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    You need a low water pick up lower unit, an over the hub prop and raise the prop shaft to 1 inch above the pad. More HP will also help but your problems seem to be a partial throttle.
    Last edited by us1; 08-03-2009 at 05:06 PM.

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    Trimming in further helps. There may be a minimum trim limit pin in your mounting. Take it out.

    A boat that has the center of gravity far back, set up for high speed, will tend to porpoise at lower speeds. The solution is to trim the engine in and force the bow down, or move some weight forward.

    I have a 360 lb fishing buddy. If we have any fish in the swimming pools, I have to make him sit forward or the boat is squirley as hell.

    Sometimes I take an 80 year old preacher fishing. I put 150 lbs of sandbags in the bow so I can get on plane without killing him.

    But when I'm alone, or have a reasonable fishing partner, the sand is gone, and it goes like a scalded dog.

    hope it helps
    John
    To fish or not to fish? What a STUPID question.

  4. #4
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    How much more does the 200 weigh? Sounds like it's just a weight issue. I've seen guys put a whale tail on the motor and it works just fine.








    Silhouette II Racing

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    They should weigh roughly the same...maybe 30 to 40 lbs difference at best.

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    As far as I'm concerned, a whale tail is a potentially costly fix (in damage) for a setup problem.

    With that kind of power, you should be able to run reasonably slow, and yet trim it out and fly it pretty well.

    Did you check for the lower trim limit pin?

    You can just rebolt it one hole higher on the jack plate and get the range of adjustment you need. Just for starters, try to set it up so when it's trimmed out about 2/3 of the way, with stock water pickup, you begin to lose water pressure. Then move a little weight around. You might be surprised.

    Is there any chance you have a gob of water in the foam under the rear deck?

    hope it helps
    John
    To fish or not to fish? What a STUPID question.

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    John,

    I will check for the lower limit trim pin. Not sure what it looks like on a 1999 200 EFI Merc, but I will find it. I also will raise up the motor on the jack plate as you suggested and see if that gives me the additional adjustment I need to smooth out the ride. Thanks for the assistance.

    Rick

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    They also make some alum. pcs. that go on the transom or jack plate to increase the tuck under for the motor. This makes the boat plane faster and run slower w/o porpoising. They are about 1"at the top and taper to 1/8" on the lower. Made a big difference on my rig. 18' cajun with 200 hp.....

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    Quote Originally Posted by XstreamVking View Post
    They also make some alum. pcs. that go on the transom or jack plate to increase the tuck under for the motor. This makes the boat plane faster and run slower w/o porpoising. They are about 1"at the top and taper to 1/8" on the lower. Made a big difference on my rig. 18' cajun with 200 hp.....
    They are called transom wedges, but they won't help his problem if it is porposing at pretty much all speeds except wide open. I would assume that pretty much all speeds except wide open would mean he has ran at different trim angles , so this is why a say they won't help his problem.

    Hydro
    SPEED KILLS! RUN AN OMC,LIVE FOREVER!
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  10. #10
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    Quote Originally Posted by hydroholic View Post
    They are called transom wedges, but they won't help his problem if it is porposing at pretty much all speeds except wide open. I would assume that pretty much all speeds except wide open would mean he has ran at different trim angles , so this is why a say they won't help his problem.

    Hydro
    That's why I told him to look for the idiot-pin, i mean trim limit pin.

    John
    To fish or not to fish? What a STUPID question.

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    2009z - Porpoising is pretty common...any vee hull (or tunnel hull) can be susceptible to porpoising, depending on design and setup. Flatter bottom vees are more prone to porpoising than steeper deadrise vee hulls, but there are several contributors to the occurance and any vee hull can find the problem caused by dynamic instability.

    The "bouncing" or porpoising comes from a rapid change in the location of the center of Lift as the boat accelerates. The relocation of static weights is one way of dampening the rate of change of the CofL...so it's not always obvious whether to move weight fore or aft in order to cause the "dampening". The solution can be calculated, but we use boat performance software for that. It's not too difficult for you to find out through testing, whether moving weight fore or aft will help your particular problem.

    The resolution to a porpoising problem with a hull design is most always addressed by causing the boat to run with less trim. There are many different ways of achieving this.

    Porpoising is a function of the lift generated by your hull, the deadrise of your running surfaces, and the trim angle that is needed to get that lift. The onset of porpoising is, in part, influenced by the weight balance in the boat. Altering the deadweights in your boat (fuel, payloads, etc) can affect the speed and trim angle that porpoising will initiate. If a boat is porpoising at a given speed and load, lowering the trim angle will reduce or eliminate the porpoising. There are several ways to get there, but the bottom line is to reduce trim at the velocity of porpoising onset. (Higher deadrise hulls are less suseptible to porpoising.) Even if the hull design is operating in the "Porpoising Regime" through a full range of velocities, reducing trim in some way will improve or resolve the problem.

    Porpoising can be predicted on any hull design. (the new version of the TBDP software now does this "porpoising analysis" for tunnel hulls, vee hulls and vee-pad hulls).

    For your hull design and setup, the speed and trim angle at which you will see the onset of porpoising can be determined. Changing trim angle, changing motor height, changing propellers, moving weight foreward, will all help to reduce porpoising at a given velocity. Changing design variables such as running surfaces design, deadrise, engine setback, etc. can help maintain the hull in stable planing regime, avoiding porpoise regime.

    Trim tabs (or more extreme whale fins) will also improve a porpoising problem, but these will also affect overall performance of the hull. Adding transom wedges can often help too, since they will allow for more negative trim travel, if it's required. Raising the prop shaft higher will shift CofG foreward and reduce trim angle. More HP also reduces trim angle. Reducing your trim angle in some way will always help a porpoising problem.

    Consideration of your design and setup arrangement can predict whether you will see porpoising and at what speed you will experience porpoising with the TBDP Porpoising Regime Analysis for Vee Hulls, Vee-Pad hulls, Tunnel boats, Power catamarans and Mod-VP type hulls.

    Check out this thread... there is more discussion there on porpoising.

    Also, have a look at this article on "Hump Zone/Why does my Boat Porpoise?". It may be of some help to understand the problem.
    Last edited by Jimboat; 11-19-2010 at 10:33 AM. Reason: updated link reference

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    I can only go from my before and after... W/o wedges it bounced out of control. With wedges you could trim it in enough to stop the porpoise at mid range. and after building up some speed trim up and yer flying..... Just my 2 cts.....

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    I spoke with 2009z and the wedges seemed to have helped. Good call ExtreamVking!

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    one piece of the puzzle not mentioned here is the hull,check if it has a rocker in the bottom,no amount of adjusting will completely eliminate porpoising at the speed matching the rocker balance point.

  15. #15
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    Quote Originally Posted by j_martin View Post
    I have a 360 lb fishing buddy. If we have any fish in the swimming pools, I have to make him sit forward or the boat is squirley as hell.
    John
    Boy, you must be a hell of fisherman, that a 360 lb. guy balances your fish load!

    just jerkin'g yer chain. off to read this thread now.

    -peter


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