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Seawater cooling

Discussion in 'Technical Discussion' started by DAVECAD2.0, Apr 8, 2010.

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  1. DAVECAD2.0, Apr 8, 2010
    #1
    DAVECAD2.0

    DAVECAD2.0 New Member

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    Crystal River, Florida
    Santo Cruz, 1980, 2667, Single 351W, V-Drive, once thru cooling. Would love to know the original set up of the cooling system. From thru hull intake location to engine driven sea water pump. When I purchased mine, the thru hull was located forward of the engine left of the V-Drive with no seacock? Impossible to get access. I moved the thru hull and added a seacock aft of the engine just inside the port engine stringer. Located the strainer low on the port fuel tank stringer. This does not work as the Sherwood G21 mounted engine pump only has about 0.5' of suction head. (Won't suck)
    The engine pump sucks water thru the hull strainer, seacock, V-Drive cooler, and tranny cooler. The only things on the positive or discharge side of the pump are the exhaust manifolds, and engine jacket feed.
    Obviously this isn't correct. Ideally all components to be cooled should be on the positive or discharge side of the pump. I'm about the fourth owner with as many re-powers. So, I'm sure this set up has been altered. Hard to believe Carver would have done this goofy setup from the factory.
    Thinking of losing the engine driven pump and installing a Jabco (or similar) 12V pump with at least 2' of suction head capability just after the strainer. If anyone could help with pic's of similar models, or diagram of Carvers original set up that would be huge. Sanity check.

    Thanks, Dave
  2. K1W1, Apr 8, 2010
    #2
    K1W1

    K1W1 Senior Member

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    Hi,

    I do not have personal experience of your particular boat but do have a bit of experience with flexible impeller type pumps.

    A Pump with a 1" Port should have a lot more than .5' (6"?) suction.

    I am assuming here that the pump is in good condition with no damage to a nice flexible Impeller.

    Are you finding that it won't pump when you are at the dock or underway?

    Is it possible to take a rubber hose from just after the inlet strainer straight to the pump inlet and another one on the pump outlet to see what sort of flow you get when you run the engine for a quick test? Make sure all connections are tight. By quick I mean about 30 secs max

    If there is a good flow can you re connect the inlet side but leave the outlet open to the extra hose, repeat the quick run and see what the flow is like now. If it is less then maybe the coolers downstream from the pump inlet are restricted somewhat and need cleaning. If the flow is the same then the problem isn't your pumps lack of suction performance.

    If you are only having this problem underway you need to have a cup or scoop on the bottom of the boat to force water into the inlet when you are moving along - this is not an uncommon problem. Doing this will actually give you a positive suction pressure when you really get moving forwards.
  3. DAVECAD2.0, Apr 12, 2010
    #3
    DAVECAD2.0

    DAVECAD2.0 New Member

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    K1W1:
    Thanks for your reply.
    The Thru Hull (scoop type) and seacock are placed as low in the keel as possible just above the V to obtain a round thru hull hole. A short hose to the strainer inlet, also mounted as low as possible, and then to the V-Drive cooler inlet, tranny inlet, and then pump suction. The strainer inlet is approx. 6" above the hull inlet. Yes, I had performed a complete system flow check via garden hose. The engine pump is newly rebuilt. Testing every component individually and together. Everything is clear. When idling dock side using shore water cooling via a garden hose I have adequate cooling. When using normal eninge pump cooling. Nada. Unless the strainer inlet is placed below the engine driven pump inlet it won't draw. (suck). However, due to the low engine mounting, the strainer has to be placed on its side in the bilge, unmounted, which renders it non fucntioning from a filtering standpoint. Its a Perko filter with vertical basket removal. Laying on its side unmounted in the bilge the system works. This places the filter inlet below the engine pump suction inlet. This is how I discovered the very low NPSH of the engine pump. With the vertical constraints of the engine mounting and pump inlet, the filter cannot be mounted in a secure manner.
    Was hoping another Santa Cruz owner could shed some light, and maybe some photos. Thats my dream anyway.
    Thanks K.
  4. K1W1, Apr 12, 2010
    #4
    K1W1

    K1W1 Senior Member

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    Hi,

    Am I understanding that the system works fine with the strainer left out of the system?
  5. DAVECAD2.0, Apr 12, 2010
    #5
    DAVECAD2.0

    DAVECAD2.0 New Member

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    No. I never left the strainer out of the system. Just unmounted (hoses connected) and relocated it.
  6. K1W1, Apr 12, 2010
    #6
    K1W1

    K1W1 Senior Member

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    Hi,

    If your pump is working properly it sounds like you must be drawing air somewhere in that circuit if simply re positioning the strainer makes it all work.

    I did a search for the performance specs on your Sherwood Pump but can't find anything that gives suction and discharge head numbers.

    A 1" Jabsco can self prime from 2.4m which I am sure would be plenty for your application. I would have thought that your Sherwood with similar size ports had a similar performance.

    If you are going to change the pump go with the Jabsco (http://www.ittjabsco.com/products/m...destal_pump_52580/iid_2026-item2026/index.htm) and drive it from the engine , I would shy away from a 12V Cooling Pump in that application.

    Here is the PDF on the Jabsco Pump : http://www.ittjabsco.com/files/52580_43000_0805.pdf
  7. DAVECAD2.0, Apr 12, 2010
    #7
    DAVECAD2.0

    DAVECAD2.0 New Member

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    K1W1:

    All new hoses and fittings (except strainer housing) double clamped to the sea. Bristol fashion and Coast Guard required.
    You've found the same data I did, or didn't.
    No pump curves for the Sherwood pumps.
    As a Mechanical Engineer, I can tell you they exist. You can't design, or spec. a hydraulic circuit without those data. Sherwood won't share.
    But, through actual field testing, the empirical data reveals that the NPSH of this pump is only 0.5 ft/hd.
    These pumps are supplied as standard equipment on all PCM Ford engines. 1" inlet 1" outlet rubber blade impeller. Crusader (GM based) and original engine to this boat, uses a similar engine pump. Not sure which brand.
    Thought about another brand of engine mounted pump as the output varies with engine RPM.
    But, then there’s the mounting issue. Custom mounts etc. That’s why I thought about an electric.

    Thanks, Dave
  8. K1W1, Apr 12, 2010
    #8
    K1W1

    K1W1 Senior Member

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    Hi,

    I am sure the discharge volume of a Sherwood Flexible impeller pump vary with engine RPM just like a Jabsco one will.

    Some how you will have to find out the output value of a new Sherwood pump at what RPM and then find the appropriate Jabsco one.

    How is your current pump mounted?

    If the current one is belt driven it should not be that big a deal to get a pedestal Jabsco one to fit it.

    What about one of these? http://www.discountmarinesupplies.c...CO_18840_0010_PLEASURECRAFT_COOLING_PUMP.html
  9. DAVECAD2.0, Apr 12, 2010
    #9
    DAVECAD2.0

    DAVECAD2.0 New Member

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    K1W1:

    Spec.'s look good.

    Flow rate: Nominal 40 Litres/min (8.9 gallons/min) at 1500rpm
    Self-priming from dry up to 2.4m (7.8ft)

    Dave
  10. jhall767, Apr 12, 2010
    #10
    jhall767

    jhall767 Senior Member

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    Dave you may have the wrong impeller in the pump. I had this happen to me. Pump and replacement impellers were supplied by the marina. Next year after doing my regular maintenance I had problems with getting the pump primed. Changed gasket again, New gasket on sea water strainer, etc etc etc. Turns out there are two impellers that are almost identical but about 1/64" difference in height. After fighting pump suction problems for over a month I figured out I had the wrong impeller. There was no way to tell from observing the installation. Only by placing the two impellers on a hard flat surface side by side could you tell the difference. There was just enough gap to lose suction. A worn pump will also have no suction lift.

    John
  11. DAVECAD2.0, Apr 12, 2010
    #11
    DAVECAD2.0

    DAVECAD2.0 New Member

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    Thanks John.

    But, I bought the repair kit for the Sherwood G21 pump. I did check and they told me there is only one repair kit to fit this pump. But, as you say the bronze case could be worn. I reused this pump from the old engine and rebuilt it. This pump could be a couple of engines old. I'm going to try the Jabco replacement that "W" found. Letcha all know.

    Dave.