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  1. #1
    Member
    Join Date
    Aug 2013
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    St. Louis, MO
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    4

    Jack Plates on HP170 or similar

    I recently decided to add a jack plate on my 2006 HP170 with a 90 HP 2 stroke Yamaha. I have only been getting about 37-38 MPH GPS and 4400 RPM with the motor mounted on the lowest holes. When I get to about 2/3 or 3/4 trim, I start to porpoise really bad. Rather than just raising the motor, I thought I would go ahead and get the jack plate. My prop is a 3 blade SS with 19-K engraved on it, but I don't know who its made by. I am assuming I should be getting closer to 43-45 MPH, but could be wrong.

    I recently purchased a CMC manual jack plate with a 6" set back. When I received it and looked closer, I realized that I may not be able to install it because of the existing screws that go through the transom. Two screws on each side would get covered by the jack plate. Anyone have any suggestions? Anyone else with a similar boat with a jack plate, and if so what kind?

    boat.jpg

    Thanks,

    Ed

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  3. Member
    Join Date
    Aug 2009
    Location
    San Antonio, TX
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    211
    #2
    http://www.bbcboards.net/g3-boats/68...ate-hp170.html

    Bubbatn put a 6" jp on his HP 170 also, http://www.bbcboards.net/g3-boats/68...tin-there.html , but I don't think that he ever posted any numbers. As I recall, it did help with both the speed and handling of the boat. The JP will could void your hull warranty if they can prove that's what caused the problem. It does seem that some have added them with no issues though. There are other ways of getting rid of the porpoising problem also. You could try a different prop or modifying the trim tabs to your hull. Most people are going to tell you to get rid of the hydrofoil too. Do a little searching before you make your decision.
    Last edited by Roberte; 05-01-2015 at 05:12 PM.
    2008 G3 Eagle 170
    Yamaha 90 2 Stroke (TX Boat)

    2007 G3 HP 170
    Yamaha 115 2 Stroke (MN Boat)

  4. Member
    Join Date
    Aug 2013
    Location
    St. Louis, MO
    Posts
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    #3
    Thanks Roberte. I am at least the second owner owner of the boat its a 2006, so I no longer have any warranty on the boat. I'll take a look at the post you referenced and will do some more research. I may end up selling the jack plate, but I don't know yet. I wasn't sure if the hydrofoil was impacting my performance since it has been on the boat since I got it.

  5. Member
    Join Date
    Aug 2009
    Location
    San Antonio, TX
    Posts
    211
    #4
    ifishmo2, I wouldn't worry too much about covering those two screws as they are there to connect the transom to the hull stringers. There are two choices to mount that plate:

    One is to come up with some custom spacers to account for the offset caused by the screw heads. You would need to look into some bulk aluminum bar stock that is the same length and width of your mounting points of your plate that is a little thicker than the screw head is tall. Probably ~1/8" - 3/6" thick should work. From there, I would drill the holes in the bar stock using the jackplate as a pattern and then relieve it to account for the screws. There should be plenty of material to keep it all one piece. This piece should work as a full length spacer that is sandwiched between your hull and the jackplate that can transfer the load along its entire length. Using aluminum should eliminate any bi-metal reaction you could get.

    Or the easy way which would be to relieve the jackplate itself to account for the two screw heads. You will have to double check you plate to make sure that there isn't any interference to the operation of the plate. Also you will need to look at the plate to make sure that the material removed wont affect the structural integrity of the plate, most likely this will not. If you do this, you will likely not be able to return the plate though.

    Either way you decide to go, you will get your plate mounted. If you decide to do this, let us know and keep us posted on your progress.

    Re-looking at your picture, I noticed you were in the top holes on your motor. I would raise that motor up at least 2 holes to see what kind of changes in performance you get.
    Last edited by Roberte; 05-05-2015 at 06:45 AM.
    2008 G3 Eagle 170
    Yamaha 90 2 Stroke (TX Boat)

    2007 G3 HP 170
    Yamaha 115 2 Stroke (MN Boat)

  6. Member
    Join Date
    Mar 2011
    Location
    dickson
    Posts
    65
    #5
    i would be interested in seeing the numbers.i have the same boat and want to put a jp on mine
    my top speed is 42 @ 5950 rpm and gets there fast but i can tell my hull is way dwn in the water so theres a lot of drag
    let'em go let'em grow

  7. Member
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    May 2017
    Location
    Clarksville TN
    Posts
    77
    #6
    Bump for any info. I just bought myself a 4 in jackplate for my HP170.

  8. Member
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    Aug 2013
    Location
    St. Louis, MO
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    #7
    I ended up installing it with the 1/4" aluminum stock like Roberte had suggested. I really haven't seen much of an improvement and my hole shot is actually worse now. I still get 38-40 GPS at about 4800-4900 RPMs. I think I may be over propped and might have been better off buying a new prop and raising the motor, before adding the jack plate. I am current running a yamaha SS prop, 19 pitch (with a K stamped on it).

    G3Ray and 8lug1, what props are you running?

    I've tried to attach two more pictures so I hope they work.

    Thanks!
    Attached Images Attached Images

  9. Member
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    May 2017
    Location
    Clarksville TN
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    77
    #8
    IfishMo2, I am running a factory 15P prop. I got 33 MPH. Did you have to to anything with the steering lines? I have a 4 in jackplate I want to throw on and I am worried about the cables.

  10. Member
    Join Date
    Jun 2004
    Location
    South Central Missouri
    Posts
    2,517
    #9
    I would be cautious of adding a jackplate to the HP series hulls.
    The transom design is not suited for additional stress!

  11. Member
    Join Date
    Aug 2013
    Location
    St. Louis, MO
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    4
    #10
    G3Ray, I did not have to do anything with the steering cable. I was concerned about it as well, but I was able to add the jack plate without any issues.