MacGregorSailors.com

Glad to see the site backup…. Missed it dearly!!

So, here is something for the OB motor experts to chew on.





I noticed the cracks shown in the pictures a week ago. I am quite certain they were not there (or at least visible) when I changed the gear oil in the lower unit last October. The boat has been in my yard ever since. When I changed the gear oil there was no evidence of water. Even if there was water in the case, it would have been drained when I changed the oil.

The crack on the right side is a lot worse and is located 9-12 mm from the edge. The crack on the left side is closer to the edge (4-8 mm).

I am really puzzled about this. The location of the crack is not in an area where water could have been trapped. In fact the inside is exposed for 13.5 mm.

The motor has now 6 seasons on it. I have used the boat for 4 seasons and the first 2 seasons, the boat got little use as it was a demo of the Mac dealer.

During the last season, I noticed a small amount of oil seeping past the prop shaft seal (the boat is kept on the trailer). I was not too worried about it because the amount was so small. I checked the oil level regularly. There always seemed to be sufficient oil in the unit.

I have had some minor accidents, like a line wrapped around the prop etc. During my second season my cheap PVC dinghy got sucked into the prop while anchoring. It broke one blade. I’ve also had some fishing line wrapped around the prop (not last year) but not to the extent that it was wrapped around the prop shaft putting outward pressure on the casing. I also never hit the bottom or a rock with the OB.

I have considered the possibility of bent shaft. This would cause stresses on the outer bearing holder which could in turn cause the cracks in the casing. However, I would think that the motor would have suffered from excessive vibrations. Also, spinning the prop does not show anything out of alignment, at least not with the naked eye.

I did get a line wrapped around the prop early this pas summer. I cannnot remember that it was wrapped around the cast just forward of the prop. The line was quite tight. Could it have been so tight that it skweezed the cast, causing it to crack? Perhaps.

I showed the pictures to a local Honda Marine dealer. He had never seen this before and had no clue what might have caused it. However, he quoted me a whopping $2500 to replace the lower unit. Defender quoted me $1100, a lot better.

So, I am planning to replace the lower end before the season starts. I really would like to know though what caused this so I can prevent this from happening.

Any thoughts?

Harry

Is the skeg still absolutely straight? (measured??)

are there any marks on the inside of the casting?

I had a bravo outdrive with a similar crack, due primarily to lots of full (and I mean full speed) heavily weighted full turns. But this also cracked and bent the skeg.

The back of the prop and shaft was flexing from the side loads.

Good luck

Harry van der Meer wrote: ... I did get a line wrapped around the prop early this pas summer. I cannnot remember that it was wrapped around the cast just forward of the prop. The line was quite tight. Could it have been so tight that it skweezed the cast, causing it to crack?

Hmm -- well a Honda 50 won't exert that much side pressure while cutting an arc behind a Mac, but it might indeed if a line entangles and pulls up tight, without breaking. I had no idea, a complete mystery, until your comment Darren.
But then reading Harry's story a second time ... it seems plausible, eh?

Harry - Im not an engineer, but cracking that structure in compression would be highly improbable, and I would guess almost impossible with a dock line.

Chip H is probably the best guy to put an educated eye on this situation...

My theory would be an impact which gave you a small crack, followed by propagation into the long crack you now see. I would drill the ends for stoppers, then fill in with JB weld or try the braize rods on the market - if its aluminum PM me and I can send you some.

cracking that structure in compression would be highly improbable, and I would guess almost impossible with a dock line.

I agree with this. I am unaware of any impact. There is no other damage.

Harry - I wonder if that crack doesnt go all the way through on both sides - ie is it really two cracks or one? That would give credence to my impact theory, and self validate my paltry existence today.....

When you go to drill those crack stoppers, the hardest part is finding the actual end of the crack, which can be quite a bit further developed than the 'visual' end of the crack....strong mag glass can help here....

I don't think I am going to play around with this. For 1100 bucks I do not want to risk a major failure out in the open water.

I would like to know what caused this though.

..

I would try the honda dealer first. See if this is just your motor or a whole lot of out drive castings. It looks like the crack is caused by a weak spot in the casting.

As soon asI saw that picture I looked out my window and made sure my motor was is the full down position. A few weeks ago we had a rainstorm change into freezing ice and if the motor was in the up position I can see water trapped around the prop. Sorry to hear of your problem. Maybe you could try to locate a used part. I go through quite a few parts on my dirt bike and find used parts for less than half price on E-bay or locate a want-ad of someone junking a bike selling parts. Good Luck.

I'd guess that those are stress cracks started at the factory installation of the shaft.

Unless you can get a heck of a steal on a lower unit replacement casing, I'd dissassembe that unit, take it a prop or other aluminum welding shop and have them weld the cracks inside and out, then grind them down to original specs. Then maybe, on the outside weld a thin band of metal around the entire unit for added strength. A couple of hundred bucks at most. Not pretty maybe, but boat bucks still in your bank account are very pretty.

I agree, looks like an oem imperfection in the casting which eventually propagated. Maybe you could check with the manufacturer?

With the cracks on both sides out near the rim of the housing the first thing that I think of is someone had the lower unit chocked in a vise since the cracks are right at the part of the casting where the reinforcing ribs do not extend, and they are at the widest part of the bell shape. The last 3/4" or so is pretty thin there and could have been overstressed by a bench vise. I can't imagine any operating conditions that would exert the outer to inner obliques force needed to crack both sides.

Wooden blocks may have prevented toolmarks from the vise from ending up on the lower unit, but aluminum is soft.

Since the deeper part of the cup is such a heavy casting I doubt the damage is more than cosmetic, but I'd be concerned how welding heat would affect the other lower unit parts.
If it's only cosmetic I venture a guess that a fill-in aluminum engine block crack filler could fill the voids as long as the rest of the lower is OK, but I'll yield to the more experienced metallurgists here.
Me, (not sailing miles offshore mind you), I'd patch it and run it, keeping an eye out for an affordable lower.

someone had the lower unit chocked in a vise

Argonaut, you may be on to something. I changed the water pump a year ago and had the lower unit in a B&D work mate. I just had an other look at the motor and it is not impossible that I did not pay enough attention to have the blades of the work mate tapered the same way as the lower unit. It would explain the cracks very well.

I showed the pictures to the manager of our machine shop. He also suggested that this was very easy to repair, especially now it looks like this was not induced by a faulty cast or other mechanical defect.

Once again this community of sailing enthusiasts from all over the world is of tremendous help. I feel a lot better now. Thanks much!

Follow-up to my earlier post. The cracks are actually in the aluminum enamel. With some course sand paper I removed the enamel. No cracks are visible in the aluminum casting. Some sanding and a little paint and she's ready to go again. Big relief.