Conference unifix::sailing

    In my opinion, the captain has the obligation and responsibility to
    ensure that the boat is properly prepared, most especially when making
    a long offshore passage. It appears to me that the captain of this boat
    was negligent. It is accepted practice that all electrical wiring be
    carefully secured at short intervals. Besides, I see no excuse for
    routing heavy cables so that they could come into contact with moving
    machinery (eg, propeller shafts) should the cables come loose. This
    disaster looks to have been totally avoidable. It is fortunate that
    nobody died or was injured. I would guess that the captain would most
    likely lose a lawsuit for damages. As it is, the loss of the boat will
    tend to increase everyone's insurance rates and make getting insurance
    for ocean passages just that much harder. 
    
    West Marine, and others I assume, sell high-current fuses and circuit
    breakers to cope with shorted wiring. We have a 250A fuse (mounted at
    the batteries) in the cables to the starter motor and a 50A circuit
    breaker at the battery end of the cable from the batteries to the
    electrical panel. 
    

I confess that I initially agreed with Alan's assessment. However, not having
seen the installation, it's premature to jump to this conclusion.

For example, it's possilble that the cable WAS secured to a bulkhead with
staples of inadequate size or holding power, and that the staples vibrated
loose during operation. If the captain installed the cable this way, it
would simply be a case of bad judgement, not negligence. It's also possible
that the builder built this time bomb into the boat.

Most boats that I've seen do not have fuses in the primary power circuit.
Personally, it's my opinion that the + cables for boat batteries should have
a fusible link in the cable immediately at the battery post. (Many automotive
battery cables are designed this way.) Sadly, this apparently isn't required
by any boating industry standards, and the cables typically sold for battery
hookup in boating stores do not have this feature.

Interestingly, the description of this fiasco in -.? implies that the
short to the rotating prop shaft was discovered before the fire, and the
fire didn't happen until the motor was put in neutral. I.e. if the crew member
who discovered the short had pulled the cable away from the prop shaft
immediately, the disaster would have been averted. (Of course this is asking
for considerable bravery, pulling a wildly sparking and potentially HOT cable
away from a rotating prop shaft!)

While I'm speculating... It's also possible that the cable would have simply
melted away at its contact point without further ill effects, if the motor had
not been put into neutral! This is because the rotating shaft would continuously
expel any molten metal at the contact point thus keeping the contact resistance
fairly high and preventing the high currents that caused the cable to overheat.

--RS

    The fuseable links on autos are in the  line to the alternator and
    accessories. The main battery cable to the starter solenoid is not
    fused. Since starter motors draw 200-300 amp while cranking, which can
    last many seconds, it would be a very tough fuse problem. A short on a
    battery cable would likely limit to a similar current based on the
    batteries own internal resistance.
    
    The only real answer in this case was proper straping down of the
    cable. I am generally appalled at the sloppy wires dangling around
    in most boats.

    re .13:
    
    The high-current fuses that West Marine sell have nominal ratings of
    up to 250A or more (I don't have my catalog handy). These fuses will 
    carry the rated current for some time and open fairly quickly at
    substantial overloads (again, I don't have the specifications here). 
    
    The cold cranking capacity of a battery is the number of amperes that
    the battery can supply for 30 seconds at 0 deg F without the battery
    voltage dropping below 7.2V. The cranking capacity is much higher at
    the temperatures most of us sail in. The cold cranking capacity of a
    100 A-hr deep cycle battery is likely in the 700 to 800 A range. The
    battery short-circuit current is (I assume) higher still. Clearly, the
    risk of fire or other unpleasantness is very high if a starter motor or
    other heavy cable short-circuits. It seems to me that a 250A fuse in
    the starter motor cables provides significant protection.
    
    Inadequate restraint of wiring is only part of the problem. The
    batteries themselves need to be very strongly mounted so that they
    don't move or come loose under any circumstances. I haven't seen very
    many battery installations that I would consider safe. 
    
    Yes, the wiring standards do allow fairly long cables without
    over-current protection, which is no reason not to add such protection.
    For example, the cables from our batteries to the battery switches are 
    only about 12" long. The 250A fuse is at the battery end of the cable
    from the battery negative terminals to the common ground point (engine 
    block) since that was the easiest place to mount it.
    
    Alan
    
    
    
    about 12" long and even if 
    
    Alan
    
    

��              <<< Note 2177.14 by UNIFIX::BERENS "Alan Berens" >>>
��                           -< fuses are a good idea >-

��    re .13:
��    
��    The high-current fuses that West Marine sell have nominal ratings of
��    up to 250A or more (I don't have my catalog handy). These fuses will 
��    carry the rated current for some time and open fairly quickly at
��    substantial overloads (again, I don't have the specifications here). 

    I have an older west catalog here, and they have a 300 amp
    fuse listed.  It's intended to be used in the supply line
    to an inverter.  $89.95

    I suppose it could be used in a cable to the starter, but...

    If the starter were to draw more than 300 amps in some
    situation, the fuse could blow, leaving you unable to
    start the engine.   Possibly bad, depending on the situation.

    299 amps of 12 volt current could still start quite a fire.

    The 1994 West catalog has high-current fuses for about $10 each,
    mounting block about $30. The time required to open at over-current
    varies with the current and ambient temperature. We've been using a
    250A fuse with our 25hp diesel for a year now without problem. These
    things are inexpensive enough that carrying a spare is financially
    practical. I prefer the risk of having a fuse blow to the risks of no
    fuse at all. Besides, if you blow a 250A fuse trying to start the
    engine, something may be very seriously wrong with engine anyway.
    

Specifications for the Blue Sea high-current fuses available from West 
Marine:

 current (percent   time to open
of rated capacity)    (seconds)

     130                600 
     140                 60 
     160                 14 
     180                  7 
     200                  3.5


Fuses are available with capacities from 100A to 250A in 25A increments.

Note: Most (if not all) engine starter motors will overheat if used for  
more than a short time (probably less than a minute). Thus a 250A fuse 
should be adequate for a starter motor load of up to 350A or so.

How much current does a starter motor draw? Most engine service manuals 
don't say, but it appears that (from a Ford heavy-duty truck manual and 
a VW Golf manual) the typical starter motor running current is 200 to 
300 amperes and that stall current is perhaps 500 amperes. These numbers
are for engines ranging from a 1600 cc diesel to a 9200 cc heavy-duty
truck gasoline engine. If the starter motor stalls (ie, can't turn), 
something is seriously wrong and blowing a fuse is probably a good idea.

Interestingly enough, by the way, the intermittant maximum current
rating (ie, for engine starting) of battery selector switches is in the
range of 325 (Cole Hersee) to 360 (Perko) amperes. 

Alan