Exhaust flow through Alpha One Sterndrive

[zkurtb]

Not sure if this belongs in this forum or the Engines forum.

Have a 1987 Wellcraft 19' runabout with Mercruiser 5.7 260 and Alpha-one. Just picked it up last fall. So catching up on the maintenance and learning it's quirks...

Checking while trailered, running at idle with the muffs on, should I see water coming out the propeller exhaust? There is water coming out the exhaust ports under the sterndrive at the transom. But just dry exhaust gas out the prop.

I only ran it for a minute, then shut her down as I don't want to overheat the plastic (pump housing/etc) inside the lower unit if it's not getting cooling water.

I just replaced the impeller (and housing), and Water Pocket Cover (because it was semi-melted - actually the impeller housing also showed signs of excessive exhaust heat as well).

I checked the water flow into the engine at the goose neck junction from the impeller pump hose, it is fine. I then checked water flow thru the exhaust risers/elbows, it is good. And since I see water out the upper transom exhaust ports, I know water is in the exhaust as it should be. So why don't I see any water coming out the propeller exhaust?

 

[mkast]

Re: Exhaust flow through Alpha One Sterndrive

There is water coming out the exhaust ports under the sterndrive at the transom.

Normal.

 

[ziggy]

Re: Exhaust flow through Alpha One Sterndrive

are the exhaust bellows connected to the the drive and the gimble housing? sometimes they fall off, or could have holes in it.
agree, ya should have water coming out of the prop hub when things are proper.

mines a mc-1, pre-alpha.

this would be the norm for my rig. water exiting the idle relief holes, both port and starboard + out the prop hub, and out the pee hole in the ft. of the bullet. my port side idle relief hole generally has a mist, while starboard seems to spit more water. i'd think your alpha1 would be similar.

 

[zkurtb]

Re: Exhaust flow through Alpha One Sterndrive

Mkmast - Yes, I thought this was normal - mentioned it to show there is water in the exhaust, so why not also out the prop?

Ziggy - yes, the bellows are connected. When replacing the impeller I also pulled the upper unit to check those, and grease the u-joints, check the gimbal, etc.

 

[zkurtb]

Re: Exhaust flow through Alpha One Sterndrive

Ziggy - just had a closer look at your picture - Do you get water flow out the prop with the engine off? I didn't, and first thought this was bad, until I realized the impeller fins act as a valve between the pump's inlet and outlet ports, so water can't (shouldn't) flow unless the impeller is spinning (i.e. when the engine is running).

 

[Bondo]

Re: Exhaust flow through Alpha One Sterndrive

Ziggy - just had a closer look at your picture - Do you get water flow out the prop with the engine off? I didn't, and first thought this was bad, until I realized the impeller fins act as a valve between the pump's inlet and outlet ports, so water can't (shouldn't) flow unless the impeller is spinning (i.e. when the engine is running).

Ayuh,... Have ya taken a look at the Shutters while you've been workin' on it,..??

My guess is,...
When the other stuff got burnt up, the shutters did too...
Now some of 'em has fallen down, blockin' the flow yer lookin' for...

 

[Don S]

Re: Exhaust flow through Alpha One Sterndrive

Mkmast - Yes, I thought this was normal - mentioned it to show there is water in the exhaust, so why not also out the prop?

Water follows the path of least resistance. If the water all runs out the idle relief ports, there won't be any left to run out the prop.
If the drive is in the trimmed up fully position, the prop is probably higher than the exhaust bellows, and water doesn't run uphill.

 

[ziggy]

Re: Exhaust flow through Alpha One Sterndrive

Do you get water flow out the prop with the engine off?

nope. the water just squirts out the side of the muffs. when started, the water is sucked into the drive and most of the water stops coming out the side of the muffs. no water exits anywhere other than around the muffs when the engine isn't running.

how about maybe the flappers fell off during the overheat and have lodged into the exhaust cavity trying to exit the drive. but they got stuck inside the exhaust passage. maybe? don't know if the flappers would seal well enough in the exhaust passage to stop water flow though. just a thought..

 

[zkurtb]

Re: Exhaust flow through Alpha One Sterndrive

Bond-O/Ziggy, are the shutters and flappers you both mention the same thing? Are these the ones in the exhaust pipes just aft of the elbow/risers? If so, I did check on the starboard, it's ok. Haven't looked at the port.

Don - this is happening with the drive trimmed fully down.