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  1. #1
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    How are y’all getting solid hub props to bite?

    Recently picked up an old style Quicksilver 18p Chopper. I’m running it on a light 1548 semi v .080” hull with a 50 yamaha. 6” of setback with prop shaft ~ 1 inch below bottom. Prop vents horribly coming out the hole. IF I manage to get it planed off it runs pretty well, but I’m hitting rev limiter (will be deleted) at about 44mph. If my calculations are correct that’s about 19% slip.

    Question is, what can I do to cut down on the venting and slip? Will different thrust washers help? Can I relieve exhaust elsewhere? Also would cutting the weed cutters off help?

    Thanks!

  2. #2
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    well there is a few things that can be done to make it grip on the hole shot , some easy some not so easy.

    first one is the hardest,
    you need to make the exhaust come out above cav plate !! this will relieve the exhaust and stop most of it getting to the prop . this is why you see lots of
    outboards with snouts or with holes in the sides of lower unit just above the cav plate. these mainly run with over hub props when like this.

    2nd and often a good worker is to make or buy a blow out ring to help divert the exiting exhaust to go over the hub and not flare out straight over the blades.

    and you can also get the prop worked to have a roll on the leading edge, ive been told this works good, but would sacrifice top end a tiny bit.
    the prop getting worked is about the most expensive.

    depends how far your willing to go to get the desired effect.

    I have found over hubs to be crap for holeshot without doing to something to divert exhaust. they vent too much and cause bigger slip than
    if no exhaust in the area of the prop.
    Plus over hubs are meant to be run high, so would be noisy anyway once up on plane.

    pics below to show the things I mean

    1st is a plastic blow out ring made for omc and Yamaha, simply clips on the gearcase.,

    Click image for larger version. 

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    2nd is a Nice Home made blow out ring , this is attached to gearcase with 2 simple screws
    ( one each side of gearcase) due to ring having a lip to sit inside of gearcase..


    Click image for larger version. 

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    3rd is to show the holes above cav plate to relieve exhaust .
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  3. #3
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    Thanks phillnjack, just what I was looking for. I really would like to make this prop work. It runs far better and faster than any prop I’ve tried so far. Boat loves it once it’s on a plane, just hard to get it on a plane. I’m getting mixed reviews on drilling the lower to relieve exhaust. Some are doing it with good results, and some are saying it can lean out and burn pistons. I guess it all depends on how much you drill out, and how much you actually know what you’re doing.

  4. #4
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    Can't run a blowout ring with the weed cutters. Will not make much difference anyway. Boat setup and practice are your friends, takes some time to learn how to drive them out of the hole.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  5. #5
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    weed cutters should not make any difference to having a blow out ring on the gearcase or not..
    the type of ring I am talking about is not solid, its just a thin walled tube to extend the gearcase nearer to the prop.
    if making your own ring, you can custom make it so it is just 1/4 inch from the prop blades.

    no set up is going to make a over hub pull away good if its smothered with exhaust gases escaping and smothering the blades..
    remember this is not like the big motors where the prop is a lot nearer the gearcase,
    on my motor ( evinrude triple ) the prop is a long way from the gearcase and exhaust gases tend to spread out fast, unlike bigger motors.

    look at the pics below
    Both motors have the exact same OMC SRX props with exact same thrust washers and are both 56 cu in omc longshaft triples with the 2.42 gear ratio.
    Look how close to the prop the blow out ring is !! and this helps big time to stop gases escaping over the blades on take off.
    The wall thickness of the ring only needs to be 5 mm max and 2mm where it goes into the gearcase, this would leave plenty or room for weed cutters etc.
    on the red gearcase the ring is around 2 or 3 mm from the blades of the prop.

    p.s the pic of the ring and red gearcase is not my motor, its another members pic ..blue gearcase is mine.

    .
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by phillnjack; 12-08-2017 at 04:22 PM.

  6. #6
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    You could send the prop out to be worked. I have had over hub props rev like crazy and others just roll onto a plane.

  7. #7
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    Quote Originally Posted by David View Post
    You could send the prop out to be worked. I have had over hub props rev like crazy and others just roll onto a plane.
    There is a way to make them hook from a start, planting your foot down and waiting is not it. Choppers can be tricky but can be done.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  8. #8
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    the problem is needing the holeshot and top end, with the extended gearcase ring it lets the prop grab and not slip so much.if you have exhaust snouts or exhaust coming out the top of cav plate the slip numbers drop dramatically, from like 16% down to like 7 or 8%, thats huge reduction in slip and big boost in speed . But this is only IF the motor can still pull the prop with lower slip.

    Another way of making an over hub bite more at Holeshot , this type of thing turns it into a thru hub by slipping on a cup that a
    also has the thrust washer built into it .. now the exhaust is diverted and makes the prop a thru hub exhaust,

    !Click image for larger version. 

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    Here below is a over hub propeller with one of this type of cups fitted ..
    .
    .Click image for larger version. 

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    .
    These pics came from this site, although they can be found anywhere on the web too..

  9. #9
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    Thanks for the input guys. That’s my dilemma, I don’t wanna totally eliminate vent. Right now I’m calculating 19% slip at top speed. So motor turns it fine, hitting the rev limiter actually. Sounds like drilling the lower is a must, just unsure as to how much to still retain a little thru exhaust.

    Also, if y’alls theory of getting the prop closer to the gear casing eliminates vent. Is it okay to run w/o the thrust washer? I fitted it without just to see and the weed cutters cleared fine. I just didn’t know if that would cause any issues other than clearance. Both the Yamaha and mercury thrust washers are very thick and cause the prop to sit way back. I’ll try to grab a pic when I get back to the house.

  10. #10
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    You MUST run a thrust washer.19% is a lot of slip.

    Dave
    1980 Cougar 19 tunnel,90 2.4L Bridgeport EFI in middle of restoration.
    1988 BAJA Sunsport 186, 96 225 Pro Max
    79 12' Auminum, 95 Merc 9.9
    RIP Stu
    "So many idiots, so few bullets"

  11. #11
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    Quote Originally Posted by 97yamaha View Post
    Recently picked up an old style Quicksilver 18p Chopper. I’m running it on a light 1548 semi v .080” hull with a 50 yamaha. 6” of setback with prop shaft ~ 1 inch below bottom. Prop vents horribly coming out the hole. IF I manage to get it planed off it runs pretty well, but I’m hitting rev limiter (will be deleted) at about 44mph. If my calculations are correct that’s about 19% slip.

    Question is, what can I do to cut down on the venting and slip? Will different thrust washers help? Can I relieve exhaust elsewhere? Also would cutting the weed cutters off help?

    Thanks!
    Move the motor down till you lose speed then up in SMALL amounts till you get it back, i think your to high, its a flat bottom boat not a pad v so it wont respond the same with elevated motor heights and will need to find the sweet spot . honestly dont think a chopper is any better than a std quicksilver stainless on this rig, we had a 1648 mod v jon boat with magnum 40 merc doing 46 with a 19" std quicksilver stainless prop one hole up on transom. no setback no chopper no slip on holeshot no slip in turns ..... think your over engineering the program. my .02 cents.

  12. Likes Dave Strong, loop liked this post
  13. #12
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    We did debate welding a 4" wide pad to the bottom to see what would happen but never needed it as 46 mph at 4am in 35 deg weather to go duck hunt was fast enough and plenty cold enough for us.

  14. #13
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    I agree you may be too high. Height works on my SS2000. My old 16' Baja ran best 2-3" below the pad.

    there are limits to adjusting the throttle to get on a plane. With the Baja a 26 chopper really struggled to plane. I can avoid the 5000 rpm and wait business with other applications but not that one. Same boat, motor, engine height, a 27 Hoss copy just planed.

  15. #14
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    Have your prop shop put a heavy cup in it and run the cup almost around to where it looks like it's on the Leading Edge. I used to ski race with a 22 Chopper and I could snatch a water skier out of the hole on a Maharajah racing ski which was a hard ski to pull out. And you may have to lower your engine a little bit as well
    18 Talon/2.4 carb SOLD
    26 Deck Boat/250 Merc

  16. #15
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    Quote Originally Posted by 99fxst99 View Post
    Have your prop shop put a heavy cup in it and run the cup almost around to where it looks like it's on the Leading Edge. I used to ski race with a 22 Chopper and I could snatch a water skier out of the hole on a Maharajah racing ski which was a hard ski to pull out. And you may have to lower your engine a little bit as well
    BIG diff between a ski race boat and a 15' jon boat with a 50 hp tiller......lol
    just saying.

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