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Conference unifix::sailing

Title:SAILING
Notice:Please read Note 2.* before participating in this conference
Moderator:UNIFIX::BERENS
Created:Wed Jul 01 1992
Last Modified:Mon Jun 02 1997
Last Successful Update:Fri Jun 06 1997
Number of topics:2299
Total number of notes:20724

738.0. "Bilge Pumps -- How Big?" by MSCSSE::BERENS (Alan Berens) Sun Jan 31 1988 18:01

In discussions of bilge pumps, it is usually assumed that high 
capacity pumps are better -- after all, sinking is not much fun. 
However, I wondered, how much water actually comes in through a hole 
in the hull?

After a little research I discovered that

	V = 7.22 * C * A * D**0.5

where V (gallons per minute) is the volume of water entering the boat
      C is a coefficient between 0 and 1 (typically about 0.6)
      A (square inches) is the area of the hole
      D (inches) is the distance from the hole to the surface of the 
        water (D**0.5 means the square root of D)

Note that the farther below the surface the hole is, the greater the 
rate at which water comes through the hole. As the boat fills with 
water, the rate decreases. 

     diameter of hole (in inches)
       1    2    3
 D
12    20   79  177      with C = 1.0 (which is worst case)
24    28  111  250
36    34  136  306
 
The frightening aspect of this is that so much water will come into the 
boat through a rather small hole that no bilge pump can prevent sinking. 
And, worse, sinking can be very rapid.

The largest through hull fitting is usually the head outlet and usually
has a 1.50 inch seacock. If the hose comes off the seacock, some 22
gallons of water per minute will come into the boat (assuming C = 0.8,
the diameter of the seacock opening is 1 inch, and the through hull
fitting is 24 inches under water). It takes a large bilge pump with
large, short hoses to pump 22 gallons per minute over the side. 

Another implication of these flow rates is that it is important to be
able to close all openings in the hull (ie, dorade vents, engine air
intake, cabin heater exhaust pipe, etc) in weather bad enough that
capsizing is at least a remote possibility. 

So, how big should the bilge pump be? As big as possible. 

T.RTitleUserPersonal
Name
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738.1Emperical verification!SSMP03::SAVIERSSun Jan 31 1988 19:5317
    A few summers ago, we were first to arrive next to a 42' power boat
    near Fisher's Is that was sinking.  With 4 electric bilge pumps
    and several "frightened men with buckets" it still was a losing
    battle.  Fortunately a local harbormaster showed up with a gasoline
    pump and after 15 minutes the transom freeboard was increasing from
    6" -a very close call.
    
    I heard the next day that they lost a rudder - probably a 1.5 or
    2 inch shaft!  
    
    So a serious motor driven pump is a great idea, the problem is how
    to handle and store a gasoline engine and fuel, or how to power
    one off the main engine.
    
    Any good solutions out there?
    

738.2The `Heck' with the pump, get a sponge!!TOLKIN::DEMOSSMon Feb 01 1988 13:0217
    I'm not too sure that the largest pump is the best way to go??
    TRUE with a 2" - 4" hole in the side of the boat you don't have 
    much of a chance to keep up with it.  More than likely the boat 
    will sink before you get the thing set up!!
      
    Now being of NAVY extract(?) the hole should be plugged with ....
    anything !!! (Your foot will reduce the hole more than half)  before
    looking for the pump!!  
    If you plug it to make it a 1/2 the size, you have reduced the water 
    intake by (approx) 1/2.  Then you can stuff the spaces to make a
    smaller spaces until all you need to keep the boat dry is a sponge!!! 
    
    Now where did I leave that bucket and sponge??
                  
    `Charlie'

738.3PLUGITGRANMA::HAJOHNSONTue Feb 02 1988 11:4110
    Could not agree more with .2.  Keep several size wooden plugs aboard
    to stop whatever flow you can.  Sizes to fit your thru hulls should
    be a must; and keep them close to the thru hulls.  Books, pillows
    and sails can also be used to stop the flow.
    
    If you can keep you cool and find the leak, you can probably slow
    it enough to keep up with it.
    
    

738.4You can't always find itSPCTRM::BURRTue Feb 02 1988 12:2437
    It's all well and good to say 'plugit', but I have to agree with
    Alan and Grant that you really need to have a good high capacity
    pump.  As someone who has had the unhapply experience of looking
    down at my cabin sole 4" under water after noticing that the pump
    was running, I can tell you that trying to find a serious leak under
    18" or 2' of dirty water when the boat is bouncing around like a
    cork is not my idea of fun.  Its even less so when the water is
    gaining on you rapidly.  That was in a glass boat-an engine cooling
    water line let go.
    
    Last Fall, on our way up to Maine to put the boat away for the winter,
    we popped a seam in a gale.  This was not a bad leak as it was only
    about 4-5" of caulking seam that let go...not more than 1.25 sq."
    in total area.  It was, however, underneath about 1200 pounds of
    (removable) inside ballast just forward of the mast step (under a 
    built-in bureau of course) and it kept 2 2000 GPH Rule bilge pumps 
    going full time to keep up with it.  BTW, we were about 75 miles out 
    when this happened.
    
    Had we not had the pumps, we would have lost the boat or at the
    least, we would have had to get the CG to bring us a gas powered
    pump and by the time that could have arrived, we would certainly
    have ruined much of the cabin equipment and probably gotten the
    engine under water.
    
    In Steve Dashew's "Circumnavigator's Handbook" he describes the
    damage control pump system he has rigged on his new boat as well
    a a somewhat less sophisticated (and less expensive) system on his 
    previous boat.  The new boat has a separate 2.5 HP diesel powered
    damage control pump housed in the engine room of his 60' custom
    cutter.  In his old boat, a modified Columbia 50 (I think) he rigged
    a power take off from his main shaft to a high capacity pump.  He
    agrees that anyone who is going to sail off shore MUST have a big
    pump for damage control and that this pump should not be electric.
    I'm looking into how I can rig something similar for Caprice.    
                                                                 

738.5jury rigged pumpRDF::RDFRick FricchioneTue Feb 02 1988 13:0316
    A pump can be jury-rigged in some cases by taking the raw water
    intake hose off of your engine and using it in addition to a bilge
    pump.   A length of hose, sufficient to reach probable points of 
    damage, that can be connected to the raw water intake hose is a good
    thing to keep around.
    
    I'm not sure what the gpm rate is on this setup, but it couldn't
    hurt.
    
    Rick
    
    * one of course shuts the seacock before disconnecting the raw
      water hose, lest they create leak #2  :-)
    
    

738.6The engine can move a lota water...TILLER::SEARSPaul Sears, SHR1-4/D27, 237-3783Tue Feb 02 1988 13:1245
   There are a number of sources for plugs. For instance i never go out w/o a
   bottle of worcestershire sauce.

   When i took delivery of Cachalot, i was poking all around down below as an
   excited new owner will do, when i noticed the hose from the galley sink to
   the throughull. Thinking to myself it looked a little rusty, i squeezed it to
   check my hypothesis. I was right. It was more that just rusty, it was totaly
   rotten. In just a few seconds, it disolved, and a 1.5 " stream of green water
   was comming in. The closest thing to grab was the Worcestershire sauce
   bottle, which it turned out was just the right size to plug the fitting and
   slow the torrent to a trickle. I learned a few things: the survey was right:
   all T/H fittings should have seacocks (they do now), think before poking, and
   always keep DC plugs nearby any T/H fitting. Also never go out w/o a good
   bottle of worcestershire sauce. 

   I believe many offshore cruisers have the ability to switch engine cooling
   water from the sea to a well-screened bilge intake source. An engine water
   pump can get rid of a lot of water. The screening is important, however, so
   as to not clog the engine coolant circulation passages (either FW cooled or
   SW cooled).

   A complete water-removal system might have many parts:

   -	Two cockpit-accessable hand pumps, each .5-1.0 gal per stroke

   -	two electric pumps, one automatic w/ float switch, the second manualy
	switched.

   -	a Y valve and sufficient strainers, pickups, hoses and filters to
	channel the engine water intake from the bilge.

   -	an aux pump fighting for fanbelt space on the engine, along with its
	hoses, strainers, etc.

   -	a seperate deisel pump (gasoline is for cars, not boats)

   -	assorted buckets

   -	rebuild kits for the above hardware (especialy the electric and hand
        pumps.)

   -	a frieghtened crew; when the wind's blowing 40 kts and you'r in the gulf
	stream, it's amazing how fast the crew can pump


738.7don't use the engine pump?PDPSRV::BERENSAlan BerensTue Feb 02 1988 14:2719
The engine sea water pump can indeed be used as a bilge pump in an 
emergency. However, the usual small marine diesel has a very small 
sea water pump -- ours pumps on the order of 200 gallons an hour at 
cruising speed which is not exactly high volume. Morever, if the pump 
or line strainer clogs the engine will eventually overheat and stop. 
Then your troubles are much worse. It would be better, I think, to 
install other more adequate pumps. 

Be aware that most bilge pump gallons per hour ratings are the maximum 
that the pump will pump with no hoses connected to the outlet. Add the 
normal hoses and through hull fittings and pump capacity is reduced 25% 
or more. According to a Practical Sailor test, the best manual pump is 
the big Edson -- a true 30 gallons per minute and with valves large 
enough that the chances of clogging are minimized. Installing the pump 
is not at all easy due to its size and its 2" diameter hose. Still, it 
is nice to have. I'm not looking forward to ever using ours.

Alan

738.8Pump outputBOMBE::ALLATue Jan 21 1992 09:2813
    I tested my Rule 1500 pump for output. (19 year old 1500 gph)
    
    The bilge on the triton is 3 1/2' + below the water line and the
    outlet on the transom is just under 2' above the W/L, so assume
    a head of 5 1/2'
    
    I got 600 GPH on the pump which uses 1 1/4" hose.
    
    The cockpit has a Whale Gusher 10 pumping at the same head.
    
    I carry wood plugs and pieces of wood and sealers to stop leaks
    if need be.  Hope I never need them.