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Old 01 May 2021, 15:06   #1
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Mariner skeg broken

So I almost bought a rib!
Went for a test drive, guy selling the boat takes control approaching the slip and the engine tips a rock, resulting in the skeg braking off. Thereís probably enough meat left to fit a skeg guard rather than replacing bottom half of engine.
So my questions are
What do you think of skeg guards
Is it a good idea rather than replacing bottom unit
What other damage could of happened
Thanks
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Old 01 May 2021, 15:57   #2
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Not a great day for the guy damaging his own engine prior to a sale.

The skeg acts like fixed rudder, a bit like the trim tab under the anti-ventilation plate, however it's other function is to protect the propeller, so does need to be replaced.

If the propeller is aluminium and it's taken a knock too, then you'll want to investigate further. If it had a stainless propeller, and it's damaged then there is very likely lower unit damage, on the prop shaft and the drive shaft, all depends on the RPM at the time.

There's a few videos on YouTube on fixing broken skeg, and potentially removing the entire lower unit, bearings and gears as you'll cook the oil seals when the alloy has to be repaired. If there's enough meat on the skeg, and if it's an older engine then a skeg guard is a good compromise I suppose.

You can drain the lower unit gear oil to check for swarf which will indicate possible internal damage. I'd also want to see the engine go in and out of gear easily without any crunch or friction. Dropping the lower unit off the engine will reveal the condition of the splined driveshaft, but I'd probably only do that if the propeller has taken a knock.
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Old 01 May 2021, 16:04   #3
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Iíve a 225 opti. Bust the skeg approx 4 years ago and fit a stainless skeg guard as a temp measure. Itís been that good itís still on Sonia now perm. Itís unlikely Iíll ever get the skeg fixed now
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Old 01 May 2021, 17:07   #4
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Personally I'd have it welded easy job and looks better than a skeg guard which looks like a bodge imho.
They also increase corrosion being stainless, I removed one that had been on a few years and the corrosion under it was unbelievable the whole area looked like it had been eaten by woodworm
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Old 02 May 2021, 15:16   #5
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Thanks for all your replies.
I still think I will buy with a bit of€ to allow me replace the bottom unit with a used unit. Could of happened to anyone to be honest. It was only a tip.
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Old 02 May 2021, 15:22   #6
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Gear change

Thank you all once again for your thoughts and help.
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Old 06 May 2021, 09:15   #7
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My old Formula 292 SR1 - a 29' twin engine thing with a pair of big block V8s had the skegs missing - it had maybe 2.5" of skeg left on the bottom of the bravo drives.

I put a pair of skeggards on it and they worked very well.

I have to admit in the end I did remove them - but only because as a twin engine rig, it was quite well balanced anyway and I gained a few mph top end by not having them on. That's not a criticism of the skeggards I had fitted - I just liked the few extra mph no skegs gave me, albeit the butt was a little less well planted and you could feel it wiggle once we really started going for it. On a single engine boat, they would have been a necessity.

Long story short, they're decent.
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Old 06 May 2021, 16:49   #8
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I have decided to get a new secondhand lower unit, mainly because I found a donor engine.
Thanks for all your help and comments.
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Old 10 May 2021, 22:15   #9
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Ive seen some very neat welded skegs you wouldn't know were very damaged.
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Old 11 May 2021, 01:45   #10
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Check out this amazing(!!) product which can be used to fix skegs:

Make sure you don't mistakenly buy the Chinese counterfeit rods.
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Old 11 May 2021, 17:35   #11
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Quote:
Originally Posted by Limecc View Post
Check out this amazing(!!) product which can be used to fix skegs:

Make sure you don't mistakenly buy the Chinese counterfeit rods.
So have you used it ?
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Old 11 May 2021, 17:45   #12
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So have you used it ?
Not yet. Was tipped off about their existence by a friend who saw the video and purchased some EBay rods to try. Unfortunately they were made in China and he got an 'equivalent' product, hence my warning. His were like comparing Chinese 'super six' MMA electrodes to quality ESAB rods. No comparison, not worth their weight in scrap. I see the real Durafix are on Amazon, £18/5 rods, £7/1 rod.

While on the welding subject, ESAB and Magmaweld make a similar breakthrough product that allows hardened tool or silver steel to be welded to mild steel with no cracking or loss of strength at the heat affected zone. Simply amazing! '309' dissimilar electrodes. I wouldn't have believed it had I not tried them.
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Old 12 May 2021, 02:52   #13
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On a tender or small portable, fair enough, which may include the OPs boat.

But on anything high performance, I wouldn't recommend using one of those rods. Losing a skeg at high speed on a single engine rig could be not so different to losing the jesus bolt on the steering and cause you to spin out.

The loads on the skeg can be astronomical on a high performance boat - weld it properly, or replace.

(Written this so that in a couple of years time when someone searches for skeg repair on their opti 225 powered 70mph rig they don't go brazing a new skeg on)
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Old 12 May 2021, 02:55   #14
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On a tender or small portable, fair enough, which may include the OPs boat.

But on anything high performance, I wouldn't recommend using one of those rods. Losing a skeg at high speed on a single engine rig could be not so different to losing the jesus bolt on the steering and cause you to spin out.

The loads on the skeg can be astronomical on a high performance boat - weld it properly, or replace.
They are the aluminium equivalent of a brazing rod.
Have you any experience of either?

Respectfully I know the answer because that scenario is not even a remote possibility. And where do high shear loads on a skeg exist? Only when it strikes a rock.
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Old 12 May 2021, 03:01   #15
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No, but I know quite a lot about engineering & hydrodynamics, having a degree in it and having been on the safety & technical committee for the powerboat racing lot a few years back.

I'm afraid your understanding of the hydrodynamics isn't quite right, the skeg is not just there for hitting rocks.

The skeg fulfils a couple of roles:
  • To keep the back end of the boat at the back - it's like the fin on a surfboard or the rudder on a yacht.
  • Try sailing a dinghy without a rudder - the whole thing goes completely unstable, even if you can balance the rig momentarily to go in a straight line.
  • Even when you're rowing a sailing dinghy - you lift the centreboard, but always keep the rudder down, otherwise it just goes everywhere.
  • And arguably more importantly, it offsets the paddlewheel effect of the prop. So if you lose a skeg on a single engine rig at high speed, the prop is instantly paddlewheeling the boat round. At high speeds and high horsepower, this load is really large - high performance rigs tend to lift the motor higher and run the prop in a surface piercing scenario. Either way, it's the bottom half of the prop that provides most of the thrust.
  • You can see this by the fact that when you're on the plane, all the steering loads are one way.
So you lose the skeg at high speed, the boats may want to swap ends ever so quick.

Of course, have to caveat all this and say "may". If you chose to, you could setup a skegless rig with a locked motor and external rudder - that's been done. Or some of the twin engine race boats removed their skegs completely - but you'll always find people were usually scared of those boats! I could name a few from history.

Or on a twin engine setup you have a lot more balance so tend to need less skeg. However, even my formula 292 I reference above, with it's "mini" skegs from the damage they had, it was OK, but when I started to really open the taps, the back end was ever so squirrely as the boat rolled slightly in even a beam sea. It didn't do this with the skeggards on.

I've also got an OK workshop, including a decent TIG welder, and have a bunch of brazing rods in the stores. Horses for courses, brazing is great for lots of jobs and will even be fine for low performance stuff on a skeg. I'm just saying don't do it on high performance stuff.

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Old 12 May 2021, 03:07   #16
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No, but I know quite a lot about engineering & hydrodynamics, having a degree in it and having been on the safety & technical committee for the powerboat racing lot a few years back.

I'm afraid your understanding of the hydrodynamics is incorrect, the skeg is not just there for hitting rocks.
(hold on - adding some more detail.
I'm impressed with your qualifications but no matter. You never tested the strength of a brazed joint.

Often stronger than the base metal, there's no heat affected zone so the crystalline structure of the aluminium won't change. Also no need to be concerned about joining dissimilar alloys.

https://www.thomasnet.com/insights/welding-vs-brazing/

I never heard of anyone losing a skeg without it hitting something.
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Old 12 May 2021, 03:26   #17
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Oh. Here we go. I'm not trying to argue with you, I'm just trying to keep people safe. See ya round, buddy.
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Old 12 May 2021, 03:43   #18
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Oh. Here we go. I'm not trying to argue with you, I'm just trying to keep people safe. See ya round, buddy.
Yes sorry. I'm not looking to start an argument or offend. stupid me.
Need to have a realistic view here though.
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Old 12 May 2021, 05:40   #19
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Mariner skeg broken

Well, speaking as neither a welding or hydrodynamics expert, but as a 30+ year boater. Iíve lost the odd skeg or 2 in my time, none of which resulted in a catastrophic loss of control, in fact with one of them, I didnít realise Iíd lost it until we pulled the boat out. All were repaired by welding on replacement blanks & shaping to suit. As far as I know, the respective engines are still doing sterling service. It might be different in high powered boat racing circles, but I think a well executed repair is nowt to worry about for Ribnobburs.
Just my opinion as usual.
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Old 12 May 2021, 06:09   #20
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This is the point. With a 200hp+ stepped hull thing, it'd be very risky. A Pascoe SR7 perhaps.

But we have to remember context - some ribnobba searching in 5 years reads this and off they go bodging their skeg back on with a badly executed bit of aluminium brazing because they left it a couple of weeks to repair and it's oxidised.

That's why I qualified it in the beginning with smallish stuff - yeah, all fine. If it works for you, it works. But even a lightweight 19' hull with a 150hp motor. Nope!

And somewhere in the middle is a grey area. You choose where the line is.
I chose not to say don't repair under any circumstances because I think even the high performance stuff here isn't at the level John (raceman) is referencing...but you can see his thoughts, clearly...

https://www.screamandfly.com/showthr...roken-CLE-skeg

"As far as welding skegs back on for use on a low perf boat, the problem is, sometimes they get sold to somebody with a high perf boat later on. Since runnin' em on a 100+ MPH boat (actually slower than that) is life threatening, I think they oughta be scrapped when broken."

"Raceman" is a very well known, prolific, and experienced, racer.

Where do we decide the threshold between low performance and high performance is? Always, lets err on the side of caution and play safe. The same goes with the Jesus Bolt on the other thread.

Did you know that at 70mph (a 21' non stepped hull with a 200 can do this), water pressure is 70psi? So if your arse hits the water after being ejected at 70mph because the skeg's fallen off and the boat hooks, that's 70psi of pressure your innerds will be taking...?

Just trying to keep you lot safe.

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It might be different in high powered boat racing circles
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