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Discussion Starter #1
I purchased a Cat27 last summer and am hooked. She has an Atomic 4 that runs like a horse.

Last weekend as i was headed back into port (under power at top speed) the boat suddenly lost a lot of speed and the motor got loud, the boat also started to shake. I throttled back on the gas and ran it at almost idle; the shaking stopped and i made it back into port.

I had a mechanic inspect it and he found nothing noticeably wrong with it.

I took her out for a test run to see if the issue persists; it does, when in low speed it runs fine, as soon as i gas her up she starts to shake and although the motor speeds up, the boat itself does not.

Does this sound familiar to anyone?
 

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1975 Newport 28
1986 Hunter 31
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581 Posts
Atomic 4s are some notoriously smooth-running engines. If you're getting vibration on that scale it's something else than the engine.
 

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Chastened
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+1 on the fouled prop or shaft. Also inspect engine mounts.

Fixed prop or folding?
 

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Learning the HARD way...
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Prop, prop shaft (and cutlass bearing), engine mounts..
 

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Discussion Starter #7
The mounts we're fine and no, there is no shaking while in neutral... It's surprising (or maybe not) that my mechanic didn't mention anything about the prop. I'm going to jump in the water this afternoon to take a peek, i'll post a follow up... thank god its a hot day; 91 degrees in Seattle.
 

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When you say that the motor got loud, did it sound like the exhaust got louder or was it more of a thumping/vibration? If it's the former then there may well be something wrong with the engine itself. If the latter, like others have said, it's probably a fouled prop, bad motor mount, bad cutlass bearing, or some combination of those.


BTW, I would look for a new mechanic if I were you.
 

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Would bet you a beer that your prop has a lot of growth on it. When it starts cavitating, you are effectively losing your 'bite' on the water and spinning the prop in air bubbles as a result of the growth, which is why the engine is accelerating but your speed isn't. The cavitation is also the source of the vibration you're reporting. Usually, reducing throttle speed will help, but you really just need to clean your prop and she'll be good as new. Ignore it and you'll soon feel as though your prop is missing. This time of year, growth is quick to accumulate. Do it yourself or hire a diver -- git 'er done and get back out sailing. :)
 

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Discussion Starter #13
UPDATE:

I hired a scuba diver to scrub the bottom of the boat and take a peek at the prop. It turns out that one of the 2 blades had fallen off of the prop. The previous owner had claimed to have installed zincs but there was nothing! Luckily the shaft was fine and the missing blade didn't touch the boat as it exited. The scuba guy was great, best $45 i've ever spent.

I've since found a new prop and am having the same scuba guy install it, and a zinc, before the weekend.

If anyone in Everett needs a scuba guy, let me know and i'll share his contact info.

Thanks everyone for their tips and info!
 

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Discussion Starter #16
I thought i had given you all an update last year, but it appears i did not. The issue indeed was the prop, it threw a blade which, lucky for me, didn't harm the boat. But.. 4 weeks after replacing my prop the motor ate itself. I've converted her into an outboard and i love the extra space where the engine once was. Thank you all for the great tips!
 

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The A4 is probably the smoothest running marine engine I've encountered in more than 6 decades on the water. Glad you were able to solve the problem with low expense.

Gary :cool:
 
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