Sea water scoop fitted the wrong way around!

Zagato

Zagato

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I have realised my water scoop has been fitted the wrong way round on my Beta sixteen HP! Apparently on motor boats they point forward but on yachts they should point back. The Beta manual says "do not fit it the wrong way around as water will be forced through the pump into the exhaust system whilst the vessel is sailing. This is very dangerous as the exhaust will eventually fill and sea water will back up into the engine through the exhaust valve. Catastrphic failure will result as the engine is restarted!" My engine which is new to me has only done 50 hrs, it is 5 years old. The seller motored it for over an hour without me on board to lift it out when I bought it, it started fine so hopefully no damage was done in those 50 previous hours! Asking the audience if anyone has had a similar experience? ... anyone know a mechanic willing to have a look at the whole engine installation which is in Billingshurst it is out of the water at the moment. Happy to go with Cutlers who seem to be the only people in the Chi area nowadays but pricey when you add in travel time. Maybe wait until the spring and ask them to look at it then when it is near Chi for launch...
 
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First thing don’t worry. If the engine is working, then the dreadful things threatened in the manual have not come to pass.
Secondly, the pressure effect will only (possibly) happen with the engine off and the boat under sail. When the engine is being used to move the boat, then the exhaust will be moving the water through the pipework and nothing will get anywhere near the exhaust valves. You can stop the fairly minimal chance of anything untoward happening for the time being by turning off the raw water inlet valve when you turn the engine off to sail: just remember to open the valve again before starting the engine....
Then reaffix the scoop when you next haul out. Or remove the scoop and don’t bother with it.
 
Thanks Duncan, she is out of the water at present... she has a high level strainer system with visual inspection glass mounted above the water line so that may have saved the engine...
 
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Zagato said:
I have realised my water scoop has been fitted the wrong way round on my Beta sixteen HP! Apparently on motor boats they point forward but on yachts they should point back. The Beta manual says "do not fit it the wrong way around as water will be forced through the pump into the exhaust system whilst the vessel is sailing. This is very dangerous as the exhaust will eventually fill and sea water will back up into the engine through the exhaust valve. Catastrphic failure will result as the engine is restarted!" My engine which is new to me has only done 50 hrs, it is 5 years old. The seller motored it for over an hour without me on board to lift it out when I bought it, it started fine so hopefully no damage was done in those 50 previous hours! Asking the audience if anyone has had a similar experience? ... anyone know a mechanic willing to have a look at the whole engine installation which is in Billingshurst. My first contact charges dealership car prices including travel, ouch, so just need someone less than £75 per hour.Thanks although I think I will need to sort it myself. I don,t have any cylinder head pressure gadgets etc.
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My gut feeling is that you don’t have a significant problem. I would doubt that the dynamic pressure created by the forward facing scoop would be enough to overcome the height of the vented loop. My other thought is that if you have the ubiquitous flexible vane type of seawater pump that will act as a very effective obstruction to any unwanted water flow. I would agree however that it is usually worth complying with a manufacturer‘s recommendation. So if you will have the opportunity to lift your vessel out of the water in the not too distant future it’s not a huge task to remove, rotate and reseat the scoop. Incidentally, when removing it there will probably be a small screw which keeps the scoop oriented whilst tightening its nut. That screw may have been hidden by coats of antifoul.
Mike
 
mikegunn said:
My gut feeling is that you don’t have a significant problem. I would doubt that the dynamic pressure created by the forward facing scoop would be enough to overcome the height of the vented loop. My other thought is that if you have the ubiquitous flexible vane type of seawater pump that will act as a very effective obstruction to any unwanted water flow. I would agree however that it is usually worth complying with a manufacturer‘s recommendation. So if you will have the opportunity to lift your vessel out of the water in the not too distant future it’s not a huge task to remove, rotate and reseat the scoop. Incidentally, when removing it there will probably be a small screw which keeps the scoop oriented whilst tightening its nut. That screw may have been hidden by coats of antifoul.
Mike
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And it's considered to be best practice to close the raw water seacock when not using the engine.
 
I cannot imagine any typical cruising sailing yacht going fast enough to generate much water pressure from a fowrawd facing inlet grille/scoop. Could happen with a fast powerboat though or a really fast extreme racing yacht.
 
jwilson said:
I cannot imagine any typical cruising sailing yacht going fast enough to generate much water pressure from a fowrawd facing inlet grille/scoop.
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I don't have any figures but I'm inclined to agree.

Personally, rather than turning the scoop around, I would replace it with a plain skin fitting. The typical grilled scoops can block with fouling, and only seem like they'd be helpful against a very specific kind of debris - small enough to fit into the skin fitting, but not small enough to pass through the pipe into the inboard strainer - and I'm not convinced there are very many pieces floating around that answer that description. If anything does get in, a grilled scoop will ensure you can't blow or rod it back out again.

If the scoop is tightly and securely fitted, leaving it in situ and cutting off the protruding parts will probably be quicker and easier than either turning it around or replacing it...

Pete
 
Thanks for the comments I was more concerned initially that the scoop would have blocked too easily. Coppercoat has left barnacles and green slime weed but nothing else. The scoop looks to be fitted over a skin fitting /intake hole with a screw. Pete all sounds plausible... I wonder if anyone has sucked in a jelly fish yet, or had one block the intake. There are thick shoals of them in Itchenor Reach sometimes...
 
pvb said:
Is it? Do you mean every time you stop the engine, or just when you're leaving the boat for a period?
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I go on board, all seacocks are opened, they stay that way till we go home. Likewise the fridge, on when we go onboard, off when we go hone
 
If you wish to find out what the pressure on the raw pump vanes are due to the speed of a boat when the scoop faces forward you and apply the Bernoulli Equation as used in pitot tube velocity calculations.

You will also need to consider the static pressure it the raw water pump is significullary above or below the waterline.

Pitot Tubes
 
Zagato said:
Thanks for the comments I was more concerned initially that the scoop would have blocked too easily. Coppercoat has left barnacles and green slime weed but nothing else. The scoop looks to be fitted over a skin fitting /intake hole with a screw. Pete all sounds plausible... I wonder if anyone has sucked in a jelly fish yet, or had one block the intake. There are thick shoals of them in Itchenor Reach sometimes...
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remove the strainer scoop & fit a straight through skin fitting
 
Zagato said:
Thanks for the comments I was more concerned initially that the scoop would have blocked too easily. Coppercoat has left barnacles and green slime weed but nothing else. The scoop looks to be fitted over a skin fitting /intake hole with a screw. Pete all sounds plausible... I wonder if anyone has sucked in a jelly fish yet, or had one block the intake. There are thick shoals of them in Itchenor Reach sometimes...
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I have sucked jelly fish in, albeit not in UK waters, and only when running the engine to charge batteries, rather than for propulsion. It was also somewhere that seemed to be a jelly fish "bloom", I have never seen so many again. There were thousands of them, about 2-3 inches across and in many case they were hanging in the water like chains, with 8 to12 in a vertical line in the water. Any marine biologists know what was going on?
 
jwilson said:
I cannot imagine any typical cruising sailing yacht going fast enough to generate much water pressure from a fowrawd facing inlet grille/scoop.
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A fluid dynamicist writes ...

The highest pressure you can get that way is the stagnation pressure, caused by bringing a moving fluid to a stop. It's 1/2 density x velocity^2, so for a yacht doing 10 kt (5 m/s) in water of density 1000 kg/m^3, the stagnation pressure is 12.5 kPa. Since atmospheric pressure is 101 kPa, that means the highest pressure the scoop will generate be 1/8 of at atmosphere.

Since that's a head of 4 feet of water, it would probably get around a vented loop but I really can't see it forcing any significant amount past a vane impellor.

Furthermore, stagnation pressure is proportional to velocity^2 so at 5 kt it would be a head of only 1 foot, which on my boat is less than the change of depth of the seawater inlet when I tack.
 
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