New engine intake thru-hull mounted wrong?!? - SailNet Community

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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #1  
Old 05-18-2009
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New engine intake thru-hull mounted wrong?!?

My engine was overheating last year, the input strainer thru-hull was corroded and not letting much water in. So I had the boatyard replace said intake thru-hull today, used to be 1/2" scoop (clamshell) bronze, I upped it to 3/4" marelon (also scoop or clamshell). The OLD thru-hull was mounted with the slots facing BACKWARDS. I assumed the yard would install new 3/4" the same way, but of course they put it in with slots facing FORWARD this time. They're telling me this is how it's supposed to be, and there won't be any issues. Is this right?

I'm worried that with a bigger thru-hull that is now facing forward, the scoop will ram more water in than the 'ol volvo can handle (1980 MD7A 13hp by the way) possibly I dunno flooding the engine with saltwater or something ... ?? Dunno.

Oh and they drilled for the scoop holes & bedded it with 5200 so it's going to be a big deal to change.
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Last edited by FishFinder; 05-18-2009 at 07:02 PM.
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Old 05-18-2009
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Given the speed your boat sails at, I seriously doubt that it will "ram" enough water into the through-hull to cause your engine problems. Yes, IIRC, this is pretty typical on slower boats.
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Old 05-18-2009
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Maybe not a big deal

I haven't seen a lot of thru-hull strainers, but most that I have seen are pointed with the grate aft, but I have seen them like your new one is positioned also.

Having the grate in either direction is not the controlling factor in the amount of water that comes through the grate. Basically the weight of the boat pushing down on the water, the % of open area in the grate, and the size of the hole in the thru-hull dictates the flow of water through the hull.

The thing that controls the amount of water flow through the motor is the water pump, not the thru-hull size or it's orientation. The bigger the water pump and/or the faster it turns, the more flow through the motor. So as long as you are using a water pump that is properly sized for your motor, you should have no issues.

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Last edited by DrB; 05-18-2009 at 07:27 PM.
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Old 05-18-2009
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Ok thanks dawg and dr. Good to know it's not a big deal so I won't raise a big stink. Just wish the guy would have asked me before he switched the orientation.
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Old 05-18-2009
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MIne way originally pointed aft. When I changed it out a couple of years ago, I pointed it forward. Didn't seem to make any difference.
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Old 05-18-2009
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Same here. I prefer the idea of it mounted aft, because things like plastic bags will slide off it easier when the engine's off.

But as I have had this happen exactly once in my sailing life, even I doubt the size of the test sample.
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Old 05-18-2009
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Quote:
Originally Posted by JimsCAL View Post
MIne way originally pointed aft. When I changed it out a couple of years ago, I pointed it forward. Didn't seem to make any difference.
Very good to know.

Want to add that this is the first time I've had the boatyard work on my boat. Up until now I've done everything myself, and I was surprised how difficult it was for me to watch the work going on ... made me very nervous. Guess I care more about my boat than I realized. Paying $82/hr for the labor (and seeing 3 mechanics standing around looking at my thru-hull) wasn't helping either ...
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Last edited by FishFinder; 05-18-2009 at 08:17 PM.
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Old 05-18-2009
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hehe.

FWIW I turned a strainer to face forward on purpose on my boat - this may make it slightly more succeptible to debris, but on the other hand it probably improves the water intake somewhat.
2 years later, over a 1000 miles of motoring - and no issues so far.
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Old 05-18-2009
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The reason for it

The reason it is intended to face aft on sailboats is so not to flood the engine by forcing water through the raw water intake. With a low enough anti-siphon loop and a good turn of speed down a wave it is entirely possible to "scoop" enough water to force it up over the anti-siphon and begin filling the wet exhaust.

I am very surprised your boat yard would 1) install it this way and 2) after messing up still insist it was correct..?

Hardly seems worth the $82.00 per hour.. Personally I am a non-believer in external grates and much prefer a 1" or larger intake that can be snaked from inside the boat if necessary..
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Old 05-19-2009
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The use of 5200 is probably the more disturbing part. They went for the easy way out. You may want to complain to the yard management about improper installation and improper materials being used. Maybe they'll cut you a deal, although it won't be anywhere near enough to cover the cost of repair when you need to remove the thru-hull and 5200.
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