To Strain or Not To Strain

Ed Schenck · Dec 5, 2010

So you are saying that anything that fits through the intake and hose gets in? Makes one wonder what you have been grinding up with your water pump? I suppose if you always motor in clean water then a strainer isn't necessary. Your seawater pump is probably all bronze so maybe it will last longer than with a rubber impeller. But I sure wouldn't take the chance.

njsail · Dec 5, 2010

As Ed suggests you might want to rename the post to grind stuff in your water pump or not. Unless your sailing in a bathtub there is a good chance eventually something will get ingested into your raw water system. If it enters the intake then it will pass into your raw water pump and your impeller with either chew it up, it will chew the impeller up, or spit it through. None of those choices are preferable. I would recommend some type of strainer prior to the water pump. Make sure when you install it to have it properly installed above the water line so when you open it it doesn't become a fountain.

Bill Roosa · Dec 5, 2010

another view

We have concrete pumps that "pump" gravel.
If the bronze thru-hull "strainer" is in good working order it will only allow small items (fish, flotsom, jetsom etc) to enter. The grill size is pretty small on the ones I've seen. Not really much different than having a full bore hole and a strainer. I had one just like it for 5 years and never had any issues.

kloudie1 · Dec 5, 2010

The heat exchanger tubes are probably the smallest passages in the seawater side. If the strainer openings are about half the size of the tube diameter, you'll probably be fine.. My strainer openings are about 1/6 the tube ID.. Ya probably don't want a lot of solids accumulation in the muffler also.

Steve Dion · Dec 5, 2010

Woody:

Just because it has not happened does not indicate that it will not happen. I think it would be prudent to put a strainer on both of these engines. Our marina has just started getting infested with some sort of sea grass (fresh water). We did not have a problem with this 3 years ago and now the owners are working most of the warm months keeping it at a manageable level.

Even with a strainer ther is no guarantee that you cannot suck up this stuff, but as you know these incidents normally happen at the worst time. Having your engine over-heat when you are trying to get back to your marina in a big blow is not my idea of a good time.

You can get strainers for a little over $100 or spend $1,000 but it may save you money in the long run.

My '85 Hunter 31 came with a strainer.

rpwillia · Dec 5, 2010

Get crap in my strainer all the time

Usually those little 1" pine needles that float in the water. Think about what those would do to your heat exchanger over time. I would put in the strainer.

Maine Sail · Dec 5, 2010

I personally detest external strainers unless they can be opened for cleaning. I generally prefer to up-size engine intakes to a minimum of 1" and then run a heavy duty Perko or Groco bronze strainer with smooth sweeping ells if needed. I have seen and dealt with far to many issues with external strainers and much prefer the ability to run a "snake" through my thru-hull if necessary from inside the vessel to clear a clogged intake..

This past fall I fixed a perplexing and multi-year over heating issue for a guy at the boat yard. The owner had spend a LOT of money replacing hoses, pumps, flushing the engine and replacing the t-stat and HX but he still over heated. He had hauled the boat early thinking he now had to replace or rebuild the engine. He was getting water flow but never measured the quantity against what it needed to be.

After chatting with him I walked up to his boat, took my screw driver and cleaned the junk and paint from his external strainer, then removed it can cleaned out the barnacles from inside it. His overheat problem was solved after many years. He is now flying sans external strainer and installed an internal one.

Steve Dion · Dec 5, 2010

MS:

"Detest"! I guess you really do not like those external strainers.

I would have to agree with you that they can/may be a problem. As for the paint, that should have been addressed by the owner or the yard before the vessel was relaunched.

Old Skool Neil · Dec 5, 2010

Last haulout I did exactly as Mainesail said with the exception of thruhull size. I went from a 1/2" with external strainer to a 3/4" open type. My raw water strainer is mounted slightly below the static waterline (no choice in my case) which provided a small benefit. After cleaning (thruhull valve closed) when air has been introduced into the strainer, simply cracking open the lid seal (valve now open) automatically purges the strainer.

Other benefits of losing the external strainer are you can get some bottom paint up inside the thruhull and if you let the paint go too long and get some growth in there, it can be rodded out from inside the boat. It's a wet process but no worse that inserting a knotmeter paddlewheel.

RichH · Dec 5, 2010

Totally agree with Maine. I Vehemently DETEST the strainer type through hulls - in 'growth areas' they become 'breeding baskets' for barnacles 'inside' the grill.

For most liquid systems to prevent blockage from large particles, etc. the 'rule of thumb' is that the 'strainer' retention size needs to be at least 1/4 smaller than the smallest 'orfice diameter equivalent' in the system to prevent 'bridging' of the particles. Bridging is where particles jam together mechanically and then block the flow.

Maine Sail · Dec 5, 2010

Old Skool Neil said:
Other benefits of losing the external strainer are you can get some bottom paint up inside the thruhull and if you let the paint go too long and get some growth in there, it can be rodded out from inside the boat. It's a wet process but no worse that inserting a knotmeter paddlewheel.

It does not have to be wet at all. Simply make your intake hose a little longer than it needs to be so that when you remove it from the strainer barb, with the seacock closed, you can lift it above the static waterline, open the seacock, and ream it out.. Very, very simple.

Maine Sail · Dec 5, 2010

Steve Dion said:
MS:

"Detest"! I guess you really do not like those external strainers.

Yep, after spending nearly three hours one day, in really, really cold water, even with a wet suit, trying to get eel grass out of one, I vowed to never, ever have one again.

They can be a safety hazard, there is little to no need for one, better straining options, and you can far more safely ream out your intake from INSIDE the vessel a huge safety margin when you don't have to physically get into the water. In one season here in Maine I was in the water three times with a coat hanger...:cussing:

Some reasons why I don't like them....

Beyond that many boat yards, some builders

, and DIY's install the scoops BACKWARDS on sailboats and many have actually caused their engines to become hydrolocked. If they face forward you risk physically scooping sea water up and over the siphon break then filling the exhaust and eventually the cylinders of your engine with sea water.

This strainer, like many out there, is another potentially expensive mishap waiting to happen. All it takes is some rough weather and a good surf down a wave to fill your engine with salt water. Facing them backwards can also add unnecessary vacuum on the raw water pump. Sailbaots should ideally have round, non-directional strainers, not scoops, or my favorite, none at all.

If you feel you absolutely must use one make sure it can be opened.

External strainers are NOT clog proof and if they can't open it can be nest to impossible to clear without removal of the external strainer screen. The worst ones are where the strainer is actually integral to the thru-hull fitting, and can't be opened or even removed without physically cutting it apart......

Stu Jackson · Dec 5, 2010

Even without the pictures (yikes!) it seems counter-intuitive to put something on the outside of your boat over a small opening (the intake) that REQUIRES continuous flow to cool your engine with something with even smaller holes.

Those things are junk and have no place on a boat.

And internal strainers, whether below or above the waterline, are a great improvement over nothing. In my arrangement (from the factory and/or PO) the strainer is right above the intake seacock, so I can't ream it out with a dowel from there. But I can, and have, blown out whatever was clogging by intake with a dinghy foot pump connected to the hose to the raw water pump. Works just fine.

FastOlson · Dec 5, 2010

Inside Strainer : Mo Beta

I sure do agree with Maine and Stu, to name only two...

Here's a link to a similar thread on the Ericson site, with pics and other related internal site links.
http://www.ericsonyachts.org/infoexchange/showthread.php?t=3251&referrerid=28

LB

Bad Obsession · Dec 5, 2010

I really agree with the statement that having an external strainer is a bad idea in growth areas. MS is right and he didn't need to post the pictures to prove it but, what the hell, it proves a VERY valid point.

Maine Sail · Dec 5, 2010

Woody,

Watch the flea markets and eBay and you can often find the good old Perko or Groco sea strainers. The Perko's will outlast your hull and can be very easily rebuilt when needed..

Rebuilding a Perko Sea Strainer (LINK)