A second raw water intake?

I Have a KK 42 with the standard FL 135. I had a highly respected long time mechanic aboard recently and amongst other suggestions, he suggested I install another thru hull with a second raw water intake that would be "T-ed" into the line seaward of the raw water strainer for the main engine. His rational is you can never get a vacuum and a severe failure of the impeller as a result of a plating bag or kelp clogging a single inlet. I wonder if I could put in a T off of the genset rat water intake but am uncertain if this might compromise operating the genny at the same time as the main. Thoughts?

Jim

Jim, Sent from my iPad using Trawler

My KK42 had 2 inlets, 2 seacocks, 2 strainers mainifolded for main intake. Not sure if it is/was necessary. The 120 Lehman only needs a 1" according to ADC. With grated scoop underway has anyone clogged a thru hull? Not like sitting at dock w/ A/C running. (off topic: do you have the drawings for your paravane installation?)

I don't have the drawings for the paravanes system but I do like them. I believe Krogen has drawings...talk to Tom Button and see if he has them. Larry (Hobo) had his installed in Anacortes, I believe and he probably has drawings. Someone on the KK forum may also have them.

Jim, Sent from my iPad using Trawler

JDCAVE,
I've never had kelp clogging and suspect it's rare. And I don't even have a strainer over my intake. If I already had experienced clogging I may think otherwise but I doubt it. I wouldn't worry about it.

I have some insurance .. coolant wise though. We have a "Murphy Switch" that monitors the coolant level in the system and sounds an alarm when it drops some .. far less than it would take to get your coolant high temp alarm to sound when damage may already have taken place. Saved us already once. The switch itself is all mechanical. See pic of ours mounted to the exhaust manifold. When the alarm sounds the colant is still at normal temperatures and you can still loose some more coolant before temps start to rise.

The white/orange thing is the switch. When there is coolant in the system you can see it through the window. And it shows the coolant level in the manifold that it's attached to but you can install it kinda wherever.

Sealife said:

My KK42 had 2 inlets, 2 seacocks, 2 strainers mainifolded for main intake. Not sure if it is/was necessary. The 120 Lehman only needs a 1" according to ADC. With grated scoop underway has anyone clogged a thru hull? Not like sitting at dock w/ A/C running. (off topic: do you have the drawings for your paravane installation?)

Click to expand...

YES!
North end of Vancouver Island. Plugged with jellyfish.
Was shut down for 3 days before they disintegrated enough for me
to blow(turnred, blue?, cross-eyed etc. ) them clear.
Was scheduled for tidal grid on day 4. Cancelled!
Ted

I left scoop strainers off this time and after a 2000 mile ICW trip...not once did I pick up enough stuff to fill my inside strainer. During the summer, 2-3 times the strainer filled and seacock filled with reed grass...but the good news is I just pull the hose and ram a dowel down to clear everything.

On the assistance boat I run where we slide into shallows all the time...I took out a transducer a few years back and instead of filling the hole...I put in a seacock and left a standpipe to above the water line. Now when I plug the other scoop...I just switch intakes...it's saved the day many a time.

JD. I think your first question has been addressed. The second was whether tee'ing into your generator's raw water supply would have negative results. There is a possibility that your main engine's raw water pump might reduce the amount of water reaching the generator - especially if the engine's thru-hull had a blockage of some kind. Be a shame to save the main engine at the expense of the generator!

I do have twin raw water intakes. One on each side of the keel, close to it. The outside also has a grill facing forward. The seacocks are joined inside the boat with a Tee , isolation valves and a big loop of hose bewteen the two.

I have been thankfull to have the setup. Many times I have run through clouds of jellyfish, lots of kelp and when there was lots of logging here all kinds of wood trash that clogged the toilet inlet but never the engine intakes.

My hull was a commercial fish boat hull built for pleasure though so this was standard for the builder. I strongly suspect that one reason for no trash is the intake is really close to the keel. I have seen boats where that is not the case.

On the other hand most vessels for pleasure never have a problem unless an unusual circumstance crops up.



I cannot say though that if I only had the one intake that I would have had problems but the only thing I have ever found in the strainer was about the size of a woodbug in 4,000+ hrs.

Folks in areas with jellies may find 2 intakes for the Air Cond to save many hours of work at O Dark 30.

FF said:

Folks in areas with jellies may find 2 intakes for the Air Cond to save many hours of work at O Dark 30.

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This is 100% correct.

I have special screens (they're called either Barnegat Bay or Great South Bay strainers) over the scoop-strainers and they do an excellent job. However, we happened to be in the area where very large red jellies invaded the area for some time period.

While being anchored for a week in a raft of several boats we all had intake clogged with jellies. Usually, it's not too bad, but as you can imagine, with screens it's harder for them to get in and it's even harder to get them out of the trapped sections. One night I couldn't clean the intake but I needed to fire up my generator to recharge the batteries, so I had no choice but "borrow" A/C intake (thank God it's the same size). While leaving things as-is overnight did the trick. The clogged intake freed itself overnight, so all I had to do is put everything back in place.

However, this situation really got me thinking that additional intake might not be a bad idea.

Going back to the OP, I don't think that it's the best approach having both intakes constantly in service. In situations like I had, I'm almost positive that both intakes would have very high risk of getting clogged, so you'd be left without a plan B. On the other hand, if the 2nd intake remains on "stand-by" (with seacock closed) with no suction to pull in on the jellies or other underwater living things, then when a primary intake gets clogged you can simply switch to the stand-by intake while the primary gets cleaned.

The real answer is a sea chest with adequate screening....but 2, oversized intakes might do both...eliminate the chances of sucking a bag into one and have such reduced flow that both would not become clogged by sea critters or even floating weed. I upped my intakes to 1.5 inches and the engine has never inhaled much even after several thousand miles...but the a/c still seems to gag itself.

FF said:

Folks in areas with jellies may find 2 intakes for the Air Cond to save many hours of work at O Dark 30.

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Been there, done that. This year on the Chesapeake we had a bad run of the little sea nettles jelly fish. Twice in the wee hours of the morning they packed my AC suck from hull suction to the strainer. I used the shop vac to blow the line out through the hull suction.

Personally, I wouldn't add a second hole in the boat for an additional raw water intake for the main engine. If the intake has a screened, forward facing scoop that is sized correctly you should be OK from our experience. If you're worried about losing flow, add a flow alarm.

https://www.aqualarm.net/comerus/store/comersus_viewItem.asp?idProduct=34

If there is a way to tap a second through hull I agree adding one seems excessive..but being able to continue quickly with a single engine and a clogged intake is pretty paramount in my book.

That's why I left the screen/scoop strainer off...so I can always pull the hose and ramrod the through hull...again form NJ to FL and Return....all ICW and I never picked up enough to clog my interior strainer and cause an overheat.

Well see how it goes this year...leaving in 3 weeks....

The first winter I owned my boat I removed the old thru hull (1 inch) and went with a much larger 1 1/2 inch thru hull and strainer. I have never clogged mine as the oversized (for a Ford Lehman) strainer has lots of capacity.

Stupid question probably, but could an air hose be connected between the strainer and the seacock so clogs could be cleared from inside the boat using compressed air? Blowing the obstruction out into the water? Or would the water pressure too much for the air?

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cardude01 said:

Stupid question probably, but could an air hose be connected between the strainer and the seacock so clogs could be cleared from inside the boat using compressed air? Blowing the obstruction out into the water? Or would the water pressure too much for the air?

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Actually, this is a good question. My clogs were so bad that I had to use my friend's high pressure water pump with a hose trying to blow the stuff back into the water via seacock. The clog was so bad that it didn't work the last time (wasn't enough pressure), but it worked previously. For this reason, I now have small air compressor on board. Haven't tried it yet, thank goodness didn't needed. But at least I'm better prepared.

Compressed air would do it. Water exerts 2.31 pounds per foot of elevation.
If your through hull was 3 feet below the surface, 7psi would be equal the force of the water. 10 or 15 psi would clean it out, if the junk is not too stuck, if you had enough cfm moving through the through hull.

Actually, water is about 0.43 psi/foot of depth.

Whichever - compressed air (usually 100-120psi) should easily blow it out. Make sure you don't pressure up your exhaust hose etc. with the compressed air.

Cardude01,

Great idea! Adding it to my list of tricks, and it's a great excuse to buy more gadgets!

AusCan, thanks.
I should of said 2.3' below water line equals 1 psi.

cardude01 said:

Stupid question probably, but could an air hose be connected between the strainer and the seacock so clogs could be cleared from inside the boat using compressed air? Blowing the obstruction out into the water? Or would the water pressure too much for the air?

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It's not a stupid question, but it could well be a dangerous thing to try. For this to work, the raw-water pump would have to resist the air pressure until the clog clears - the alternative being that a vane in the pump would deflect and let the air out through the heat exchanger(s) and into the muffler. If it did work, you could be subjecting a (Perko?) strainer to anywhere between 0 and 120 psi of shop air. Can you spell shrapnel!