Conference unifix::sailing

    Brian,
         A lump hammer will shatter it all quite nicely as a last
    resort..........and then you can replace the lot.!!!!!!!!!
    
    However if you have access to a power source try gently heating
    the area with something like a Black & Decker stripper. Gently heat
    and keep trying to free it up. It worked rather well on a neighbours
    boat to free a stainless stanchion out of an Ali base.
    
    Good luck Pete

re .0:

If brute force doesn't work, you're not using enough of it! If the 
screws heads sheer off, so be it. With a bit of care, you can drill out 
the broken off screw. The threads in the aluminum casting are most 
likely corroded beyond use anyway. There is available in the US (and I 
would assume in the UK) something called a Heli-Coil. It is a stainless 
steel threaded insert. Drill the old screw holes out, drill oversize 
with the Heli-Coil tap drill, tap the hole, coat the Heli-Coil with the 
proper anti-corrosion stuff (liquid zinc chromate or Never-Seize), and 
screw in the Heli-Coil. Any machine shop can do this. The Heli-Coil will 
be stronger than the original tapped aluminum hole and the corrosion 
problem of stainless steel screws in aluminum goes away. (Heli-Coils 
meet all sorts of military specifications and are really neat things. 
I've used them several times.)

 
 Galvanic corrosion inhibitors work in four basic ways:

    1. Set up a voltage potential to stop electron flow from 
       the anode to the cathode (not very practical).

    2. Eliminate the liquid eloctrolyte (salt water). Good luck with this
       one on a sailboat.

    3. Insulate the anode from the cathode.
     
    4. Introduce a sacrificial metal into the electrical circuit such
       as zinc. Would you believe that approx. 25% of the weight of an
       oil drilling rig is sacrificial metal!


  Re .2 
  
       What is the life of the zinc based corrosion inhibitor you 
mentioned for the helicoil. Considering that aluminum and stainless 
are such a strong galvanic pair, I wondering how long it would take for
the zinc to be consumed. Have you used it in a very wet environment?

-Paul                 

re .3: 

Actually, stainless and aluminum aren't that bad (just don't use 
passivated stainless steel). Corrosion happens when salt water gets into 
the screw threads. The liquid zinc chromate (which has a Mil Spec 
number) is apparently effective (I actually haven't tried it). 
Never-Seize is effective -- I just easily removed some stainless screws 
from an aluminum boom fitting (no Heli-Coil) that hadn't been removed in
maybe three years. No corrosion. Also effective is low technology 
anhydrous lanolin (available from better sheep everywhere) -- I use this 
frequently for lubricating stainless screws in aluminum and for turnbuckle 
threads. Just coating the screw threads with something like BoatLife is 
also fairly effective. Nothing absolutely prevents corrosion.

    Well, I got them out without shearing - or brute force.  As it happens,
    the helicoils wouldn't have worked as an idea because of the design
    I'll try to draw it :  
    
    
    			|                   |
    			|		    | Mast
    			| _		  _ |
    			|| | <-  Tabs  ->| ||
    		       |----   		 ----|  Screw into tab welded
                        || |  		 | ||   onto foot
    			---------------------
    			|		    | Foot
    			---------------------
    			      |  |  |  |
                              ----  ---- 
    
    The tabs aren't think enough to take a helicoil so I guess it was
    the soaking in WD40 and penetrating oil for a few days and then
    slight heating and a BIG screwdriver into enlarged screw slots that
    did it - fortunately.  Now all I need is to persuade the manufacturer
    to repair it quickly for launching in 2 weeks!
    
    Brian

Oh crikey, why not drill out the holes slightly and use a through bolt? 
This would avoid the problem of threading aluminum completely.

By the way, Heli-Coils come in various lengths (usually 1, 1.5, and 2 
times the screw diameter). If the tab is thick enough to thread, it is 
thick enough for a Heli-Coil. A 1.5 times diameter length Heli-Coil in 
aluminum is (more or less) strong enough that the screw or bolt will 
shear when overtightened -- that is, the Heli-Coil won't pull out of the 
aluminum.

    Check your outboard motor tilt brackets, especially if they are the
    aluminum and stainless "fresh water types" rather than all stainless.
    
    I have the "fresh water type, and my (sail)boat has spent 1-2 weeks in
    salt water each of the last 4 years. The rest of the time (8 years) it
    has been in Lake Winnipesaukee. I believe the manufacturer is Eez-In.
    Although I thought that I washed my boat off well at the end of the
    salt water period each year, apparently I did not - or salt water is
    even more destructive to dissimilar metals than I realized.
    
    In any case, there are aluminum spacers on the stainless steel bolts that 
    pass thru the "L-brackets" mounted to the hull, where the pivoting arms
    attach. These spacers also act as bushings for the coil springs that
    help counter balance the weight of the motor.
    
    I decided it was time to disassemble the outboard bracket and attempt
    to stop any salt water damage that had occurred.
    I found that 3 of the 4 spacers/bushings were split / corroded into 
    3 or 4 pieces. As a result, I am replacing them with pieces of 3/4" pvc
    pipe which have been filled with silica filled epoxy and bored out to
    the required diameter. I will also keep the bracket well coated with
    Boeshield while in and around salt water (2 weeks on Penobscot Bay)
    this summer. 
    
    Those of you who have this type of outbaord bracket should check these
    bushings carefully - you should be able to see them under the coil
    springs without disassembly. It does take about 6 hands to reassamble.
    
    Bill