Blasphemy? External Strainer installation?

[Ironworker]

So, I'm going with external strainers on my boat and internal strainers as well (belt and suspenders). The issue is this article by Capt. Pat. I have a lot of respect for a man I never knew but please help me understand this article.

https://bertram31.com/proj/tips/strainers.htm

The article recommends toeing it the external strainers to maximize water flow over the strainer. I'm having a hard time understanding the hydraulics. It seems to me that toeing in the strainers would definitely increase hull drag. While I think I understand the concept for flow over the deep V hull it would seem the chines would direct the water flow parallel to the keel.

My thinking is with a planing boat the external strainers should be parallel to the keel not toe'd in. Anyone have an opinion either way? I'd appreciate your thoughts.

[Tony Meola]

Rick

Not sure how you figure that out without borrowing a tank from a place like Steven's Institute or similar place, and testing it out. I have the internal strainers and after this year with all the grass I kept running through I am adding the external strainers.

Had them when we had the gas engines and they were lined up straight as you would think is accurate. I know the diesel shop did not recommend them as he felt they would cause the engines to starve for water. I would have to go back to him and ask him why, I just took his word for it since he is pretty good.

It is paossible that he did not like the idea of angling the strainers since they would not sit flat to the hull and had to be forced to meet the hull.

[Snipe]

I installed external strainers and so far don’t seem to have any issues. I did angle them slightly as per Pats article. If there is any issues with flow you can slightly oversize the holes in the screen right below the through hull. I am running Cummins 370’s with 165 deg thermostats and ran at 178 deg while cruising on a 100 deg day

[scenarioL113]

Rick, Is it possible that the hull on the 31 is more of a semi-displacement hull or deep-vee and not so much the same as a planing hull? Capt Pat in the article mentions it as a deep-vee with his own handwritten diagram showing the flow of water.

[Carl]

My take on what Capt Pat was saying is the Deep- V slices through the water pushing it BOTH Outwards and Back along the bottom. Think of the water coming out from the bottom midship being pushed out and back. The idea of the toe-in is to take advantage of that outwards angle pushing the flowing water directly into the strainer.

Is a couple of degrees going to make a difference? I'm not sure how much it helps to push water into the strainer nor how much turbulence it creates having the strainer angled.

I put mine on many moons before joining this forum and they are in line with the hull. I also had them off a couple years that I was not going into the Great South Bay as where I travel otherwise does not have Eel Grass. But I did pull a bag once that had to be pulled out of the scoop with big slots. I put the Eel Grass strainers back on the following year and pulled out the big Grocco strainer inside the boat. I have picked up bags or something with the eel Grass strainer. But stopping and a short reverse has always seemed to clear the intake.



Tony- I have heard two schools of thought on the Eel grass strainers.

First thought is - the small holes restrict water flow more than the standard scoops with large slots.

The second school says the large face of the Eel Grass Strainer with many multiple holes, angled into the flowing water pushes more water into the intakes, making it easier on the pumps.


Having had my boat with Eel Grass Strainer, without them and back to with them I can say the motors never seemed to lack water, impellor life didn't seem to change nor did my performance. But that was with gas. With the diesel...they seem to flow just fine WOT needle does not rise.

[Amberjack]

I’ve had ‘’em for 5 years, like ‘em. I especially like being able to seemy through hull valves now the old Grocos are gone. Engines run at temp, water flow is not an issue.

I did toe mine in per Capt Patrick’s illustration. That guy had an eerie understanding of Bertram 31s. The strainers will not sit flush to the hull if toed in so rather than torque them we fabricated a slight glass build up on the high corner and everything is nice and tight.

Rick, rather than creating drag I think toeing in conforms to water flow. B31’s displace an enormous amount of water at speed and it can’t all go straight back, some of it must be forced up and out. Be mounting the external-strainers in line I think you could be creating drag.

[Ironworker]

You guys have similar thoughts that I have. First I'm using oversized external strainers which will accommodate a 3" intake. My plan is to put both a 2" intake and a 1 1/4" intake (for various pumps) under each external strainer. These big Cummins draw a lot of water. I am thinking about increasing the screen size in my external strainers.

We have a lot sargassum weed where this boat will be running, which is the reason for both the exterior and internal strainers.

[Carl]

The only thing I liked about having the internal strainers is I could see the water flow by lifting the hatch. Nice feature on the first in water start of the season.

[Rawleigh]

I have both external and internal. I had the internal ones in place and I figured they may help catch sand, etc, but I have never had to clean the internal ones.

[Carl]

I had the internal strainers when I got the boat then added the external after as I was going in some areas with heavy seaweed.
Later I believe it was Capt Patrick that said either or, not both and that was around the time I added the crash valves. The valve n hose took up alot of space and decided to pull the strainers out. As you said, very little if anything did I pull out of the strainers.

I never had a problem running both either. But they do take up alot of real estate under the deck.

[pschauss]

After finding tracking down my overheating problem to a clogged strainer, I plan to replace my external strainers with something that I can clean out more easily.

Any thoughts on sizing of the Groco APHS strainers? My through-hulls are 1 1/4" but my current strainers 13" x 6 1/2" which is roughly the same size of the Groco 3000 (3" through-hull) strainer. Would the 3000-size be overkill for me?

I see that Buck-Algonquin makes a similar strainer without the access door. Do I really need the access door since the screens on the Buck-Algonquin strainers are removable? It looks like I could get a larger screen area for a similar sized footprint with these.

[Ironworker]

Peter,

My installation includes the Groco External strainers for the 3" thru hulls with the external door. I'm installing a 2" and 1 1/4' thru hull under each external strainer. I have a Tee installed above the thru hull so I can rod out any sea creatures that decide to take up residence in my thru hulls. The trick is to get the top of the Tee above the waterline so that you can open it without flooding the boat.

I'm installing both a 2" and 1 1/4" on board strainer as well. I'm likely going to change the screens on the external strainers to a little larger hole mesh.

[Rawleigh]

The external strainer is really sized by the size of the external mushroom head on your thru hull. It must fit completely inside the strainer. The trap door is, in my opinion necessary, at least on the Groco configuration because you cannot get a straight shot into the thru hull with a cleaning tool by merely removing the screen. I cannot remember how the sizing runs because I put mine on 20 years ago.