Stern gear sinks boat ??

[clyst]

In a recent TV program a 12mtr river cruiser with twin outboardlegs was reported to have sunk . The reason being that the river was in flood and she was moored stern to the flow and water ingressed the boat through the outboard legs . The moral of the situation was dont moor stern to the flow .

Being a "raggie" and not familiar with outboard legs can anyone explain what could have actually caused the boat to sink .

 

[rib]

one thing that can and did happen in the past was the exhaust from the engine up and thru the leg ,where it sat the water level either side transom was equal,and the ali pipe used to rot at the rubber gaiter in board,so with a flow of water up the exhaust leaking at the rubber gaiter part water would enter hull aft end would sink slowly increasing water flow speeding everything up,hope that makes sense

 

[MapisM]

Being a "raggie" and not familiar with outboard legs can anyone explain what could have actually caused the boat to sink .

Ermm... Water, I suppose?
Jokes aside, that isn't as unusual as you might think.
Some O/B boats have a rather low transom, and when the current flows against a flat and low transom, water can form a sort of wave against the transom, and reach its top.
Possibly not a lot of water gets inside at the beginning, but the more water enters, the lower the boat sits in the water, so the process gets progressively faster.
Of course, this shouldn't happen with boats which are designed to be self - emptying (i.e., boats whose cockpit floor is higher than the waterline), but not all MoBos are.
And when they aren't, even rain can sink them!
IIRC, being self-emptying is not required for CE marks up to C, whilst all B (and obviously A) boats must be self-emptying.
Otoh, I'm a bit skeptic on the water ingress through the exhaust.

 

[rib]

check with volvo then thay brought out a nylon tube to fit in the swan neck at the gaiter,and i have seen 4 rot there on friends mb,all be it thay were kept at cobbs quay.and my own boats were checked every year,only say what i know or have seen dont do storys

 

[MapisM]

Well, I neither said nor thought that you were telling porkies!
I'm just skeptic that this can be a problem with OUTBOARDS, which I assumed is what we're talking about.
Otoh, you're probably thinking of outdrives with inboard engines, as I understand?

 

[Ramage]

More info here;

http://www.hsse.co.uk/story.php?codenum=21

 

[rib]

mapism-------yes sorry i think there was a bit of cross wires there,i was indeed thinking out drives as that was how i read it,and not a great fan of volvos

 

[clyst]

Sorry it appears that I misled you lot . The boat was a 12mtr cruiser with twin inboards driving legs . It appears she went down overnight fortunately whilst the owner was ashore .

 

[MapisM]

and not a great fan of volvos

LOL, neither am I, tbh!
Anyway, in the webpage Ramage linked in the meantime, they say that "the pressure at the back forced water into the seals", therefore the OP possibly meant outdrives.
If so, you're actually the one who got it right...

Edit: ops, I obviously didn't see the last post from clyst while writing my reply!

 

[clyst]

More info here;

http://www.hsse.co.uk/story.php?codenum=21

Thanks . That appears to be the one .

 

[rib]

then i still might be correct.if in deed the ali exhaust had rotted at or near that point,and the thames was in good flood water being driven back up the exhaust and leaking from said part in to engine bay,as it got lower water would be ingress faster.BUT it could also be a number of other things,just i have experianced this one possible way nearly happening

 

[rib]

messages all out of synic,happy days not got hump with anyone no points scored either have a good xmas all

 

[Cookee]

More info here;

http://www.hsse.co.uk/story.php?codenum=21

IMHO total rubbish! If that were true boats would be sinking all over the place! There is back pressure when an engine is running as the exhaust exits underwater, the only explanation as someone said above is that the water came over the transom. Having said that the boat would sink when going astern - poor design or it was a bit low in the water to start with.

 

[Elessar]

In a recent TV program a 12mtr river cruiser with twin outboardlegs was reported to have sunk . The reason being that the river was in flood and she was moored stern to the flow and water ingressed the boat through the outboard legs . The moral of the situation was dont moor stern to the flow .

Being a "raggie" and not familiar with outboard legs can anyone explain what could have actually caused the boat to sink .

to get back to your question.......

Fist they are out drive legs, otherwise known as stern drive legs. Out boards is when the engine is outboard too, as in a clip on tender engine.

The exhaust on sterndrives often exit through the centre of the prop.

The exhaust pipe on the inside of the boat comes up to around about the waterline.

The exhaust pipe out of the engine up high and often goes up higher through an anti siphon loop. This comes down to withing about 1/2" of the exhaust bolted onto the transom, which goes out through the leg.
They are joined with rubber hose and jubilee clips.

See below, I've drawn an approximate waterline on.

It's just possible for the water flow to raise the water in the exhaust enough to leak through a faulty seal.

If the seal isn't faulty flow wouldn't get the water up high enough to do damage and anyway it would fill the engine up and ruin it, not sink the boat.

The rubber bellows that the prop shaft goes through is underwater and the boat will sink if that fails. That would have nothing to do with the water flow though.

There could be other reasons that flow sunk it unrelated to the outdrive as has been mentioned.

 

[rib]

thank you eleesr for your diagram/photo,and if the single/twin engine was a tad on the large size for the boat ?.any way could have been hit by something as well etc

 

[rafiki_]

To answer Mr Raggies original question, there really is nothing about sterndrives which would cause the boat to sink. Unless there was a serious malfunction on the raw water side, but this is not likely to have been caused by mooring stern-to the current. Most likely poor or no maintenance.

 

[Elessar]

thank you eleesr for your diagram/photo,and if the single/twin engine was a tad on the large size for the boat ?.any way could have been hit by something as well etc

well yes the waterline would be higher. But even then no reason for it to sink without a failure, as rafiki says. Sterndrives aren't inherantly unsafe, ie make the boat sink if the water moves!

 

[rib]

agree.what im GUESSING at is. the boat was stern to the flood/stream per article.if the swan neck exhaust/raw water ali pipe joined by rubber and clips had indeed corroded and therefore leaking at that low level would water not have found its way and in to engine bay etc,i have seen many motor boats sitting a bit to low with their big engins/tender etc.it is only a guess based on personal experiance back in the mid 80,s when i ran a sealine at cobbs quay and i was not the only one to suffer this corrosion luckly i had a good local volvo engineer who alerted me to the problem be for any thing bad happened to me of course a bigger problem if your under way miles off shore.also belive its part of the service to check it but peeps who do their own dont allways know /or bothered.volvo did have a white nylon pipe that you could insert to repair as a kit in those days,any way it was just a guess for the po bets its something else completly now so much has been said on mybehalf hope po wont be put off asking any more questions in future.regards