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It seems like I've heard some people call it a cavitation plate? I don't think it won't cause any significant problems. I'm sure you will get some better advice on it though.
The cavitation plate is the larger plate located just under the area you indicate. My Johnson has a similar protrubance, only smaller. It will be interesting to find out just what its purpose officially is.
Don't know what it is exactly called, but ovenman is correct about the lower one being the caviataion plate. Once on plane the cavitaion plate should ride at the surface of the water, making that fin pretty useless. Is that a 30" shaft outboard (XXL model)?
I don't know what the name is but I do know what it's for: It's to give the motor better leverage for bow positioning at sub planing speeds. I will be out of the water at planing speeds. I doubt seriously it will ever cause you a problem with the possible exception of increased corrosion at the broken unpainted surface. Corroison probably won't be an issue if you keep your sacrificial anodes in good condition.
By the way the big plate below is called an ANTI-cavitation plate and while it is often mounted best even with the bottom of the boat, many rigs run better and faster (including mine) with the anti-cavitation plate up to 1-2" higher than the bottom of the boat, and they run completley free of the water and high planing speeds.
I really don't kow if it will effect anthing but you must assume there is a reason for it being there. That said, I would carefully examin it to make sure there are no other cracks that could lead to greater damage. If it ckecks out OK, I would at least put a coat of paint on it. This will help seal up any micocracks that could be susceptible to the elements.
ding...ding...ding...ovenman, RingLeader, & canzes9 your all in error. The lower plate is an anti-ventilation plate...it helps the prevent air from being sucked into the prop which causes the prop to...ventilate & spin up in rpm. Cavitation happens more often on larger boats having engines with lots of torque and an over-pitched prop. Cavitation occurs due to the prop slipping & generates low pressure areas on the aft-surface of the prop..the water literally boils on the surface of the prop. This creates a lot of sheer & eventually eats away at the prop giving it a sort of pitted surface.
As for the upper plate that missing a chunk...not sure what it's called, but I'm sure it would also help to keep the prop from ventilating as you wind it up.
It won't hurt anything at all being as it is. If it were mine I'd take a grinder to it and clean it up a little bit before I painted it. If I was in a particularly jovial mood that day I would take the grinder to the other side and grind it to match the broken and now-smooted-out side, and then just paint the whole lower unit while I was at it. Then no one would ever know it had ever been broken.
Oh, ding, ding, ding, its not slippage that causes ventilation, its the other way around.
Here's what they are if it matters to anyone. Forward of the prop there is very low pressure and that can be so low it allows air to be pulled from above. The prop will not function anywhere near as effeciently in air as it would in virtually uncompressable water and so you get slippage. Caviation is a very different beast indeed. Extremely low pressure on the prop face will cause small bubbles to form (very much like boiling water) and it is the formation of those bubbles and their collapse that erodes props. If you have true cavitation, and its not a bit likely that you do, what You will see is the edges of your prop, particularly the forward facing side of the blade, will look like it has been sand blasted. This will be much more pronounced at the blade tips - becasue their speed through the water is faster, and consequently the pressure lower.
So it goes .....
Thom
"For every complex question, there's a simple answer. And it's wrong."
(--H. L. Mencken)
kevinA, you are indeed correct in your technical description of that plate located just over the prop. However, since 99% of the human poplace , including motor manufacturers and mechanics call it a "cavitation plate", I, in my blissful ignorance, shall continue to call it a "cavitation plate".
Kevin A got the closest. The lower plate is called the anti-cavitaion plate. The upper one is called the anti spray plate that is intended to do just what it says. The idea about bow leverage at sub-planing speeds or whatever the one guy said is pretty entertaining.
Thom,
"...it helps the prevent air from being sucked into the prop which causes the prop to...ventilate & spin up in rpm"
I'm not sure how this says "slippage causes ventilation"
Perhaps you referred to the my thought that slipping causes cavitation... I think this is correct as generating a very low pressure bubble on the prop surface requires alot of work(energy) & this energy has to come from somewhere.
I do like your grinding the other side to match option..might look kind of hip.
Ovenman,
You can call "Fred" if you like, though that wouldn't correct either.
zing pow,
I'm still in the anti-anti-cavitation plate camp
Sorry folks...I didn't intend to sound like a smart azz...three in a row that were certain struck me odd when I answered it the first time
I still call it a cavitation plate, simply because everyone else does. That doesn't make it correct. Surface piercing [ie cleaver style] props notwithstanding, ventilation is generally a bad thing. It only happens near the surface. Cavitation is related to the depth of the water, the torque on the prop, prop size, and its rpm. If your transmission could handle it, going from forward to reverse would easily cause cavitation. The deeper the water, the more forceful the reverse thrust would have to be before causing cavitation. Since most props on boats stay relatively close to the surface, the two items, although completely unrelated, are often confused.
It's kinda like Otis Redding getting it all wrong with Dock of the Bay. The dock is the body of water, not the pier, but "sitting on the pier or the bay....." just doesn't sound right. Is it summer yet? My thermometer read 7 deg F this am.....
i would agree that it will should not cause any problems, but how did it happen? If you hit something there is a chance that you may have a bad seal or two in the lower unit, might want to look before your next trip. Can also have something welded to that, good luck
What is this called? 
