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Thread: Prop Blow Out

  1. #1
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    Prop Blow Out

    I recently dropped on a rebuilt 2.4L 175 to replace my 2L 150. It runs strong at the top end and I'm getting consistently 5-6 mph more top speed - in effect, I've gone from 70+ to 75+. 5900-6000 to 6300-6400 rpm. That extra speed has come with a price. I think I'm experiencing a partial prop blow out at top speed. It didn't do this with the 150. The set-up: No cone or lwpu, but have plugged the top three holes in the lower and the propshaft when the jack plate is raised all the way is about 2.5/3" below the pad, and I'm running a 27P Quicksilver Laser II. So, when I'm flying along at 75.7 mph sometimes the rpm will suddenly go up about 300 rpm, and the boat will suddenly cant to the port. The bow will turn to the left. The boat does not turn, but the boat will suddenly be going slightly sideways. Is that prop blow out? What can be done to alleviate it?

  2. #2
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    Change props or lower the engine

    I had a Laser prop on my SS2000 and had a somewhat similar experience. My revs increased more - I thought the hub had spun. Revs up, boat slowing down = not good. After the first time, I stopped and checked the prop. It was fine so I tried going fast again. Same deal.

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    Quote Originally Posted by David View Post
    Change props or lower the engine

    I had a Laser prop on my SS2000 and had a somewhat similar experience. My revs increased more - I thought the hub had spun. Revs up, boat slowing down = not good. After the first time, I stopped and checked the prop. It was fine so I tried going fast again. Same deal.
    I appreciate the response. While running around yesterday I did lower the engine and the blow out went away, but I also lost speed. Re. changing props, on this boat/set-up Iíve tried probably 15 speed props, and (maybe surprisingly) the Laser II is the fastest prop Iíve tried.

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    Are you running a torque tab?
    Living in the Freedom provided by Bud Conner and his fellow warriors.
    R.I.P. my Heathen Brother






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    I had a closed bow Baja 186 and had a similar issue, replaced the Laser II with a Tempest with a little added cup by Rich Booger and it solved about 90% of the problem, the rest was solved by bumping the jack down about 1/4-1/2". It was powered by a 2.0 L 150 Mercury

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    I had this issue as well on my first stream, also running laser2. In my case the prop would loose bite at high speed, high jack and the whole hull would drop down into the water. Very freaky. I was new to streams at the time and thought it was my fault raising motor too high. Switched to choppers and the problem literally disappeared and I picked up quite a bit of speed too. Tempest props are wicked if you get them worked. Ask for racing cut if your prop shop knows they make them work well, extra cup, some blade cutting and whatnot. Probably solve this issue I only had it with the laser. It did run well though, great mid range from that laser
    Hydrostream dreamin

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    Quote Originally Posted by transomstand View Post
    Are you running a torque tab?
    No. When the engine is up and the boat/bow is flying a torque tab would be doing anything anyway, right?

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    Seriously impressed with your speed!

    Torq tab is installed on the skeg. Most people that run surface drive, cut the one off of the cavitation plate to fit a bigger prop.
    Imo, your at the limit for that engine/depth, you need a lwp, and torq tamer to go faster safely.Click image for larger version. 

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    Last edited by OnPad; 09-17-2023 at 12:43 PM.

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    Quote Originally Posted by croSSed View Post
    No. When the engine is up and the boat/bow is flying a torque tab would be doing anything anyway, right?
    On the SKEG, not the stupid anode. I would never ask such an idiotic question.
    Living in the Freedom provided by Bud Conner and his fellow warriors.
    R.I.P. my Heathen Brother






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    Get a torque tab installed now! Then borrow or steal a big ear chopper prop (26" Pitch) and watch what happens. You will think the lake gods are your best friends. I hope you have a water pressure gauge because with that Chopper prop you will be able to really raise that motor up a lot. Just watch your water pressure.

    Best wishes, Carter Powell

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    To safely get your potential out of your hull and motor, it's probably time to invest in a LW pickup and torque tab. You'll be able to possibly run higher and spin a bow lifting prop safer. In the off season, box it up and ship it to someone who knows what there doing. You won't be disappointed.

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    If I have a coned lower done I want to get a different one. This is a 2:1 and I’d rather have a 1.87. I would have better selection of props. A 26P Chopper would probably turn to 7k. I’d like to keep it below 6k.
    Last edited by croSSed; 09-17-2023 at 08:05 PM.

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    Quote Originally Posted by croSSed View Post
    If I have a coned lower done I want to get a different one. This is a 2:1 and I’d rather have a 1.87. I would have better selection of props. A 26P Chopper would probably turn to 7k. I’d like to keep it below 6k.
    Or cone what you have and prop up. A 26p chopper with a 2to1 will probably work with your ski setup.
    Turning 7k is not that easy especially with a ski hull. Alot of drag boats run 2to1
    Last edited by Baker343; 09-17-2023 at 09:26 PM.

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    I agree on the torque tab on the skeg. That might solve the blowout.

    The Laser was the best of 15 props with the previous speed/ engine height. I don’t think Laser’s lift well at higher heights. That was my experience. The Laser might not be the best option now.

    If the torque tab or a prop change solves blowout, and your speed is limited by engine height, then a nose cone or a remote water pick up makes sense.

    Or if you are not blowing out, and are all the way up on the jackplate, maybe that’s it without somehow raising the motor further. 76 mph, 2.5/3” below the pad does not equal needs a nose cone to me. But often higher is faster and you can’t get higher without low water pick ups. My concern is that a cone with your current height could cost speed
    Last edited by David; 09-18-2023 at 11:52 AM.

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    Quote Originally Posted by croSSed View Post
    I recently dropped on a rebuilt 2.4L 175 to replace my 2L 150. It runs strong at the top end and I'm getting consistently 5-6 mph more top speed - in effect, I've gone from 70+ to 75+. 5900-6000 to 6300-6400 rpm. That extra speed has come with a price. I think I'm experiencing a partial prop blow out at top speed. It didn't do this with the 150. The set-up: No cone or lwpu, but have plugged the top three holes in the lower and the propshaft when the jack plate is raised all the way is about 2.5/3" below the pad, and I'm running a 27P Quicksilver Laser II. So, when I'm flying along at 75.7 mph sometimes the rpm will suddenly go up about 300 rpm, and the boat will suddenly cant to the port. The bow will turn to the left. The boat does not turn, but the boat will suddenly be going slightly sideways. Is that prop blow out? What can be done to alleviate it?
    Laser II props are great props. You can get very good speed out of them. But they have to stay buried in the water. They are not good for surface running. You are experiencing ventilation, not blowout. Switch to a Chopper, your ventilation problem should disappear. And add a few more inches of pitch. You are over revving already. However, my choppers always ran 3-4 mph faster than my Laser II. A standard mercury gearcase is good for about 80mph before blowout starts to happen. You are right on the cusp of that speed. A nose cone with LWP stops the blowout but slows you down a few mph. But then you will be able to jack it even higher to gain back the speed or even more. I had my 2.4 Bridgeport mounted directly on the back of my 21 Superboat with the propshaft dead level with the pad. At full speed and full trim, I never once experienced any kind of blowout with a Chopper.

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