SX-MHP transom shield Exhaust bellows options?

Toypirate

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Aug 24, 2011
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My neighbor's 33 Larson Cabrio needs new U-joint bellows on both sides as they are cracking from dry rot, I'm gonna replace everything rubber in there as I go since it has been 8 years since they were done last.

Twin 7.4L GI PEFS and they get run fast all the time, He has mentioned some exhaust smell in the cockpit with the full cockpit canvas closed and the detector sounds off sometimes as i am told. So he runs with the front window open which remedies this but is kinda chilly in late october here on the upper mississippi.
I had installed bilge heaters so he can prolong his boating season which has brought up this entire thread. Can you imagine my dispare?

His pivot housings are original and I found the pivot housing Part#s to be the same for both the open and accordian style bellows. The old ones are open and I thought it better to replace them with the accordian style to better divert more exhaust further behind the boat.

My question is which exhaust bellows CAN or SHOULD I use?
I like the Volvo Penta SX-MDC transom shield (accordian) style with the small holes but would there be an exhaust pressure issue, major performance loss, backpressure, overheat, or cavitation?
How about the sierra units? The large slots would relieve some of this but might not reduce the exhaust intrusion into the cockpit by much when on plane.
Or should I keep with the original SX-MHP (open) design and tell him to keep the trims down, bundle up in october and by the way lets have those 16yrs old Co2 detectors checked out? I just don't think open is gonna do though.

There has to be a design flaw here and I'm thinking Larson used the wrong transom model number (off by one part) unless they intended the boat to be run with the canvas down.

Any Input would be appreciated guys and thank you for all.
 

dypcdiver

Lieutenant Junior Grade
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Nov 1, 2005
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1,016
I get fumes in my totally open cockpit when traveling at about 15 mph, the windscreen causes a low pressure zone behind it.
I have the standard convoluted type of bellows (Volvo OME)
 

Bondo

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He has mentioned some exhaust smell in the cockpit with the full cockpit canvas closed

Ayuh,.... That's known as the "Station wagon effect",..... The high canvas causes a low-pressure area behind it that draws in air from low 'n behind,...

Any of the bellows put the exhaust out through the lower drive, even the open bellows is glove fit into the drive when trimmed to normal runnin' levels,....
Runnin' down-wind makes it worse of course,...
 

Toypirate

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Aug 24, 2011
Messages
31
Ok, I get that it will most likely be unavoidable to eliminate the fumes entirely so the best I will hope for is an improvement maybe even long term as one bellows doesn't seem to seal as nice as it used to (probably due to the impeller failure last year). If I can muffle down the exhaust at any operating range that would be a plus as well. I'm also figuring since they fit like a glove when in good condition then performance should be a non issue and maybe listening to the engine with drives trimmed up will give telltail to there condition as well. I will go with the OEM closed bellows and hope for the best with the confidence you've given me, update to come next summer.

Thanks for the input guys.
 

Toypirate

Seaman Apprentice
Joined
Aug 24, 2011
Messages
31
Ok, I get that it will most likely be unavoidable to eliminate the fumes entirely so the best I will hope for is an improvement maybe even long term as one bellows doesn't seem to seal as nice as it used to (probably due to the impeller failure last year). If I can muffle down the exhaust at any operating range that would be a plus as well. I'm also figuring since they fit like a glove when in good condition then performance should be a non issue and maybe listening to the engine with drives trimmed up will give telltail to there condition as well. I will go with the OEM closed bellows and hope for the best with the confidence you've given me, update to come next summer.

Thanks for the input guys.
Exhaust bellows still going strong. I asked him how has the exhaust fumes been and he said what exhaust fumes so I think its perfect.
Ok, I get that it will most likely be unavoidable to eliminate the fumes entirely so the best I will hope for is an improvement maybe even long term as one bellows doesn't seem to seal as nice as it used to (probably due to the impeller failure last year). If I can muffle down the exhaust at any operating range that would be a plus as well. I'm also figuring since they fit like a glove when in good condition then performance should be a non issue and maybe listening to the engine with drives trimmed up will give telltail to there condition as well. I will go with the OEM closed bellows and hope for the best with the confidence you've given me, update to come next summer.

Thanks for the input guys

The boat has had lots of time put on it since I posted this...summers ago, not sure of the hours but he took it to Florida from Wisconsin since. Then used Cross Country Boat Transport to get it home. Not for failure but because the trip was over for time. Anyway the bellows are still in good shape and when I asked him about the exhaust fumes he said hardly ever and he only notices going slow sometimes, (probably with the wind) He cant really tell a sound difference but thats the big blocks for ya. I've installed nothing but the accordian style since and may never change.
 
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