Conference unifix::sailing

    
    	Hi Jean- I'm putting exactly this job off till next season. Think
    I've got 3 big, 3 little. I'm also sorta-serious, but time is short and
    there's lots else to do. I am, however, going to fair them in, West and
    microlite and elbow grease- better than nothing.
    
    	A few years back, I installed a knotmeter which was flush. The
    company (SR Mariner) sent me their tool to use which was a point drill,
    holesaw and countersink bevel, all in one. So the tool exists. Now
    where to borrow or rent one. I would start snoopin around boatyards,
    when the windchill rises a little, ask folks, especially racers, who
    does that kinda thing. If your boat's already there, maybe it'll be
    reasonable to have it drilled, then do your own sealing, installing and
    fairing. If it's home, maybe leave your firstborn or MasterCard with
    the owner of the tool, and grab a monster drill and cut away.
    	Just noticed- the tool from SR Mariner WAS a loaner of course-
    think they took/returned some hefty deposit- they're not THAT nice!
    
    	Let us know how it goes- next year is my turn- and if the hull is
    cored, don't forget to gouge out a little of the balsa and refill with
    some gutsy stuff (may I suggest West and Cabosil) as to not add any
    waterletterouters as demonstrated by Howard/Fine/Howard Marine
    Architechts. Coitainly!  OOhhhhhhh, a winch guy!
    
    (apologies to the above)
    
    Scott.

re .0:

Flush thruhulls are definitely better, but, indeed, replacing a mushroom 
head fitting is substantial aggravation and effort. See note 292.4 for a 
method to make the beveled recess in the hull. You'll need to add a backing 
plate (1/2 plywood) inside the hull.

    Jean, what are your racing?
    
    FL
    

    I finished off my own boat and put all flush thruhulls in. One
    method which Tom Morris suggested was just put in a piece of pipe
    and cut it off flush. But you have to bolt in your seacocks.
    (It is a good idea to bolt them anyways).
    
    My method was to make two jigs for my laminate trimmer router,
    one to cut the outer bevel, and one to cut the flat surface.
    (I needed different size jigs for the different size thruhulls).
    I had my thruhull flanges reduced in thickness to reduce the amount
    of hull I cut away. I also had the bevel changed to match the bit
    which I used for the bevel.
    
    The jigs were held in place by the boltholes used for the seacocks.
    
    I finished the hole with a hole saw, using a jig to also find the
    center of the flat surface.
    
    Inside I put in a pad, bonded with Epoxy paste, and glass over the top.
    
    I installed the thruhull and seacock using 3M 5200, and faired the
    hull with Epoxy and micro as necessary.
    
    It was more work than just drilling a hole and not making them flush
    but I feel it was worth the effort.

    re .4
    
    How'd you reduce the through hull flange thickness?
    
    Did you also reduce the flange diameter and add a bevel?
    
    How'd you fair in the seacock bolts?
    
    - gene

re .5:                        
    
>>>    How'd you reduce the through hull flange thickness?
    
Easily done on a metal cutting lathe. I would not do it for reasons of 
safety and thruhull longevity. Better, in my view, to add a backing 
plate or a number of layers of matt and roving to increase the thickness 
of the hull. 

>>>    How'd you fair in the seacock bolts?
    
Not necessary if you use flathead bronze screws. 

    I guess I don't get it. If you want flush-mounted through-hulls, why
    not simply buy that kind (there are two, mushroom head and flush
    mount). Mushroom are available in more sizes and styles than flush
    mount.
    
    Ross Faneuf
    

    re.5
    I had them turned down on a lathe, to about 3/16th of an inch. I
    think they started at about 3/8ths which reduced a lot of the hull
    material. (I.E. had to cut out more than I wanted to get them in
    flush). I think you could probably work out a way to do the same
    thing on a drill press, using a lathe cutter clamped in the vice
    and finishing it up with a file. Probably make a wooden plug which
    could be tightened into the thru-hull with a cut-off bolt that could
    be put in the drill chuck. 
    
    Since the diameter was wider than I wanted they were reduced and
    beveled at the same time on the lathe.
    
    Fairing in the bolts was done on some with 3M5200, on others with
    epoxy and micro ballons.
    
    I think if I were to do it again, I would have the diameter reduced
    to maybe a 3/8ths flange beyond the thread and also reduce the
    thickness. The thru hulls I think are really patterned after those for
    wooden boats which could use the wide flange. 
    
    I used Spartan seacocks and through hulls. I am not sure how other
    MFG flange and thicknesses compare.
    
                       Bill

re .8:

>>> I think you could probably work out a way to do the same
>>> thing on a drill press, using a lathe cutter clamped in the vice
>>> and finishing it up with a file. Probably make a wooden plug which
>>> could be tightened into the thru-hull with a cut-off bolt that could
>>> be put in the drill chuck. 
    
I think this suggestion is exceedingly dangerous. Lathes hold the
workpiece and cutting tool quite tightly and allow taking small,
controlled cuts. The arrangement described here would not be at all
rigid and there is a near certainty that the cutting tool would jam,
probably breaking something and possibly throwing heavy pieces at you.
If you insist on reducing the thruhull flange thickness (which as I said
before I don't think is wise), have a competent machinist do it on a
lathe or do it yourself with only a hand file. I do have a lathe, I am
not a particularly skilled machinist, and I have had a couple of almost
nasty accidents as a result of inexperience and ignorance (I've gotten
very cautious and careful). No way would I attempt what is described
above. 

Alan

    I agee with Allan that one must be very careful when working with
    power tools. I do not think the drill press approach is the best
    if one had a number to do. However, for one or two it is possible.
    
    Reducing the flange with a file is a long process. Secondly, bronze
    is very soft and is machined quite easily. A process where the two ends
    of the thruhull are controlled, and holding the cutter in a machinest
    vice will work adequately. I have done similiar work often.
    
    Regarding flange thickness. I am not sure what value there is to a
    thick flange. It is not a load bearing surface. The seacock is held
    in, in may case, by flat head machine screws 5/16th I think. The
    thruhull is screwed up into the seacock, and there only needs to be
    enough pressure to hold it in place. (Remember, I am talking about
    thruhulls that are specifically designed to be mounted flush. There is
    a big difference between those for outside mounting and those for
    flush mounting).
    
    My opinion is to keep as much glass as possible when putting them into
    the boat. A wooden backing block is nice, but give me rot free, termite
    proof, no maintenance glass.

    If you don't want to use a lathe, you could also use a sinding disk, or
    even a belt sander, with 80 0r 100 grit resin-bonded disks. These will
    tear through bronze surprisingly fast.
    
    If you are having trouble mating your through-hulls to your hull
    thickness, then I would also look at other alternatives than removing
    material. Maybe you need a more substantial, well mounted backing
    block. After all, if you reduce material to the point where a failure
    is possible, you could lose the boat.
    
    On a related subject, what do people recommend for bedding compound for
    through hulls?
    
    Ross Faneuf
    

re .10:

A marine plywood backing plate with a couple of coats of epoxy resin 
and perhaps a layer or two of fiberglass cloth isn't going to delaminate
or rot. Leaving the head of the thruhull full thickness provides some 
additional security -- after all the seacock mounting bolts could fail 
also. 

re .11:

I've used Sikaflex with good results. The three types (231, 240, 241) 
have somewhat different characteristics with regard to adhesion, 
flexibility, and curing time. I used to use Boatlife, but it doesn't
seem to adhere well long-term, at least under deck fittings. (Boatlife
also turns brownish with age, which isn't so nice for deck fittings.
Sikaflex stays white.) 

    It has been a while since anyone has written in this note.
    
    I need help, I am replacing my instruments with an Autohelm Tridata and
    have to replace the old thru hulls with the new ones supplied by
    AUTOHELM. My boat is a 1986 Pearson ( fiberglass ). The thru hulls are
    the blach plastic type.
    
    How do I go about removing these?
    
    I especialy would like to remove the Depth sensor thru hull intack
    since I have sold the instruments.
    
    Any help will be greatly appreciated.

These through hulls generally have a threaded barrel with a large 
nut, typically also plastic. If you have room, you can probably loosen the
nut with a pipe wrench. There is frequently some bedding compound between
the thruhull flange and the hull. If you run a knife under the flange, you
can break the adhesion and make it easier to remove.

If it's important to save the transducer, get a piece of plastic (or metal)
pipe that's large enough to fit over the transducer barrel, but small
enough to push on the large nut. (You need to thread the entire transducer
cable though the pipe first.) You can then pound on the pipe to try to loosen
the transducer. The transducer should come out fairly easily. You may be
able to get the transducer out without resorting to the pipe trick,
especially if you clean out some the bedding compund on the inside of the hull
before you try.

The last time I removed a thruhull, it let go without a big fuss.

--RS