Raw Water Intake Upgrade
The raw water intake system for the old Perkins consisted of a 1 1/4" thru hull with integrated exterior “clam shell” type slotted strainer, sea cock, and elbow that went to a clear plastic, internal basket strainer.
I needed to upgrade to 2" for everything to meet the water specs for the Cummins 305. This engine pushes almost 60 gallons a minute. I thought this part would be pretty straight forward, BUT with almost all things in the boating world everyone has their opinion.
The split in opinion comes from an external vs. internal strainer debate. When I first started this part, I just assumed I would replicate what I had but just larger. However, when I ordered some parts I got an external screen type strainer which I did not expect. This was a result of the person helping me with the parts discussed my situation with one of the "old salts" in the supply chain. This person recommended the exterior screen strainer because of the large water intake needs of the engine; so that’s what I got. (as opposed to a clam shell type, slotted strainer). Well, I took a look at it and was not real inclined to go with it at first. I then consulted my personal "Yoda" on this project, Tom Drew and he was a strong supporter of the exterior strainer camp. I must add there were other people whose opinions I respected, that did not think highly of this arrangement.
I next spoke with Cummins and to be honest, they don't answer questions directly; it's just not what they do. Generally speaking they answer your question by regurgitating technical specifications of the engine. So when I asked, "what do you recommend; a clam shell slotted or screen type exterior strainer?" the answer I got was, "well you can't exceed 5 bars of mercury". I finally did get them to say, if you use the external type screen strainer, you cannot use an internal type basket strainer in addition to it; it’s to restrictive.
Well, that got me actually considering the screen. In many years of use with the old Perkins, outfitted with the slotted clam shell exterior unit, and internal basket strainer I may have removed a grand total of a shot glass full of eel grass from the basket. The basket strainer I would need for this engine would cost over $400 (not that $400 really means much at this point) but it is 16” high, and BIG. Here’s an interesting take on this debate; http://www.bertram31.com/proj/tips/strainers.htm This is a good article to read if you do decide to go this route; it gives some good installation tips. The two biggest being the angle of the strainer relative to the keel of the boat, and what type strainer. They make one that opens up for cleaning. I did not get that kind and really wish I had.
They also give a tip on this product; http://wolfwire.com/tefgel/frameset.htm called Tef Gel and I used it and it seemed to work pretty well.
So, much to even my own amazement, I’m going to give the external screen system a shot, and skip the internal basket strainer. I promise to report back on this, whether this experiment ends in success, or in me dead drifting across open water, headed for a pile of rocks, while I’m swimming under the boat with a brush trying to scrape barnacles off the screen.
Here is the replacement of the old, and installation of the new;
Getting the sea cock off wasn't to bad, but I did need to heat it with a torch. However, the through hull nut was a little sticky and when I got to this point here it made the entire through hull spin as I pulled on the wrench.
I was by myself that day and had no one to hold it down below so I put some pressure on it with a jack and that worked pretty well.
Victory! The tightening nut is off!
Here it is, I'm home free.
The integrated slotted exterior strainer, just like the old one is what I planned on installing.
But the more I thought about the savings in space I began to think about the exterior screen.
First up I had to replace the thru hull, and with that a backing plate that the exterior screen would bolt thru. The sheet below is the backing plate which was a material called "G12", it is some sort of fiberglass material. I suppose ply wood would work too.
Here is the thru hull, sea cock and hose connector. The products I used were from Groco; http://www.groco.net/
Heres the old hole, I have to make it bigger;
Once I did that, I cleaned up the area where the external strainer would go.
Here it is installed; BIG TIP! I had a very hard time getting the locking nut tight because there is nothing stopping the entire unit from spinning. As you can imagine this is all going on while caulk is starting to get every where as its being squeezed out, and then getting on your hands, and your tools. The trick to it is there are two small bumps on the inside of the thru hull, on the out side end. They make a special tool (which I didn't have) that goes up into the thru hull, from the out side that acts like a big wrench. So you should end up with one guy on the inside turning a wrench on the nut, and one guy on the out side holding the special wrench to keep the thru hull from spinning. I improvised and made something up with a pipe.
Here is the external screen installed;
Here is the raw water hose for the engine hooked up to the raw water sea cock. You can see the absence of the basket strainer.
Addendum;
The first summer we used the boat with the new engine we didn't get in the water until mid July. So, at the end of the season we had just about 50 hours running time. I can report no problems at all from not having the internal basket strainer. Also, the screen came out looking pretty clean; no barnacles or blockage at all. 50 hours probaly is not enough for a definitive test but so far I'm happy with my decision.