Sterndrive - corrosive exhaust?

Konrad says an advantage of their drive is "No corrosive exhaust through drive". Has corrosion by exhaust ever been a problem with a Mercruiser Alpha or Bravo drive?

 

Don't know. Down here those drives corrode apart from the outside long before exhaust would hurt anything! Seriously, they're talking about the small amounts of SO2 that are present in exhaust gasses, that when mixed with water form sulfuric acid. Outboards all have the same exhaust arrangement, and don't seem to suffer for it. I guess they are also talking about a raw water cooled engine, where the exhaust is mixed with sea water and exits through the drive and prop hub. Again, same as an outboard has for decades.

 

The big thing to look at is: Is the boat tied up in a marina or taken out of the water .
If the boat lives on a trailer, then it does not matter.
If the boat is tied up in a marina, you are going to need good annodes. Where the exhaust is will not make much difference to corrosion.

 

in the past i had raffled tails cosmeticly lifted, dont recall how they did that, do remember it was chiep and looked like new again.

 

I had a 7.4l 1989 Mercruiser Bravo 1 eat up 2 upper and lower housings from the inside out over a 6 year period.
Repowered to a 7.4 Volvo DP, going on the 8th season, NO corrosion at all.
In the spring a good cleaning, light sanding, and a coat of Volvo drive paint.
Boat is in the salt water in NH May - Nov.

 

There's a few main factors affecting corrosion, and exhaust gas isn't one of them.
What the drive's made of, and what it's touching, is the big one. Look up the galvanic series table and mark all the metals in your drive and hull. The ones that are near the anodic end of the series will rust like hell. Good drives are made of alloys near the cathodic end of the series, and have big fat exposed anodes of magnesium or zinc.
Saltwater is far more corrosive than fresh as it forms an electrolyte, allowing a galvanic cell to form between parts of the drive. Gear designed for saltwater can be used anywhere; freshwater gear won't survive in the ocean.
Corrosion is an electrochemical process. Acidity caused by exhaust gas is not a substantial factor in corrosion.
Konrad's drive is a heavy-duty unit designed for commercial applications. The engines it usually uses put out too much exhaust for through-hub to be efficient, hence it does not run the exhaust out the drive unit.

 

I'm sure Conrad makes a good product so this is no criticism...
What you are reading is a marketing tag line. You should read it this way:
"We do not have enough surface area in the drive to use it for exhaust discharge without exceeding back pressure limits so we need to discharge through the transom.."

Karl

 

thats what i was thinking.marketing will make any stupid little thing look like a positive selling point.What makes conrad so good is they got big ol gears inside and a ton of oil they get them big tough gears in there by not having any shifting going on inside of the foot you shift with a transmition behind the motor where it should be.

 

Power Tech. I noted the transmission where it belongs. Hamilton Jet Drives tech. liked my idea of using a trans with his drive. Said the efficicency would be very high. AAAAND. I would have a power reverse to blow any weeds or fish out the inlet so I would not have to go over the side.

 

damn good idea it geuse it would not get you reverse. you would use the clamshell for that still.but it shure as heck would blow the crap out of it when you got cloged.they need to start doing that.That would be good for rescue boats and the millatary for sure or any body. :idea:

 

yeah... I've seen a few Volvo-powered diesel jet units that have a F-N-R tranny in there as well as the scoop. It's a wonder it's not a more popular idea. Such an incredibly useful addition... all jet pumps could use the weed-clearing ability at least! (I suppose it could conceivably help push you back off a sandbar or rock, too ;-) )

 

The corrosion problem is in diesel installations. The exhaust gases combine with water to produce sulfuric acid.