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how mold a bow bumper?

Posted: 23 Sep 2006, 20:40
by Hiljoball
I would appreciate any suggestions for making a bow bumper.

I have made a master out of balsa and cast a plaster mold.

I put some vasaline in the mold as a parting agent.

I filled the mold with silicone sealant. It set on the surface but did not set through after 24 hours.

Should I layer the silicone to let it cure, or just fill the mold and leave it much longer?

Any other suggestions or materials?

Thanks in advance

Posted: 23 Sep 2006, 20:50
by awallin
silicone('bathroom sealant') mixed with microballoons is what we use, I think...

Posted: 24 Sep 2006, 01:27
by Bruce Andersen
I don't have any experience with bathtub sealant, but you can cast RTV silicone (Dow Corning) into that mold to produce bumpers. Since most of the molded silicones are nearly impossible to stick to the bow, I have put a small (2-56) flat head bolt or two (CA them in place) into the flat portion of the bow (where the bumper will attach), then affix the female mold to the hull and cast the RTV directly onto the bow. It will mold itself around the bolts, encasing them and providing firm attachment to the hull.

2 caveats: the mold/hull interface needs to be perfect or you will have a bump, and you will have to trim the top of the bumper to match the contour of the deck once the mold comes off.

bow bumper

Posted: 26 Sep 2006, 16:53
by ralph kelley
Unless one is going for some limited production, any molding approach seems to be a lot of trouble.

I simply cut out the appropriate shape from the rubber sole of a cheap flip-flop. (They come in a variety of colors, so get one that matches your hull color system.)

I have, on occasion, glued and approximately shaped piece of rubber to the bow and finalized the shaping with a knife and Dremel tool.

This rubber is not silicone based and are easy to glue to the bow. I use contact cement. Waterproof, cheap, quick, easy to do.


Posted: 27 Sep 2006, 02:51
by Bruce Andersen
how do you get a smooth surface on the sandal rubber once you sand it?

Posted: 27 Sep 2006, 22:21
by Barry Chisam
On the subject of sticking RTV rubber mouldings I was having success using good quality glazing silicone. However, after my supplier changed brands this no longer worked. After lots of trials I found a silicone that worked. It is made by PACER the makers of ZAP cyano glues and is simply called silicone sealer. It comes in a 44.5ml tube and I have found it in both my local model shop and large hardware store here in the U.K.. I have added pigment to it and thinned it a little with spraying thinners to get a thinner joint, all worked well. Hope this helps.

Barry Chisam

Posted: 28 Sep 2006, 03:27
by cfwahl
There are two general formulations of silicones available nowadays: acid cure using acetic acid, and the newer neutral cure types. Acid cure silicone is not so environmentally friendly, and in fact is now restricted or banned in places; however it has the catalyst for the curing reaction within itself, so it doesn't just cure from outside-in, and may therefore cure through faster. That feature is also one of the problems with acid cure silicones: that they always have some acidic residue that can attack adjacent materials over time.

Neutral cure silicones list a 3mm cure depth in something like 8 to 14 hours. Eventually it will cure, but it may take longer than one thinks it should.

The last time I made a bumper was 20 years ago; so I was probably using acid cure silicone. Started with a balsa plug sanded in place on the hull and finished with model dope and waxed, then put in a box with plaster. It cured fine in a day or so, and was adhered to the hull using more silicone.

If I had to do it now (might get the chance), I'd drill some holes in the bow plate to "key" the silicone.

Posted: 06 Oct 2006, 17:00
by Peter_Nicholls
I have put a small (2-56) flat head bolt or two (CA them in place) into the flat portion of the bow (where the bumper will attach), then affix the female mold to the hull and cast the RTV directly onto the bow. It will mold itself around the bolts, encasing them and providing firm attachment to the hull.
Surely if you have even as little as 3mm of bolt protruding from the bow and you then mould a 10mm bumper to it you will reduce the 10mm of elastomeric material to 7mm thus contravening the IOM rules.

I for one would not be very impressed if after being "bumped" I found 2 or more small puncture wounds in my hull, unlikely at a bolt protrution of just 3mm but certainly possible with bolt lengths of over 5mm.

Once the bumper is moulded there is no way of telling if there are any hidden pointy bits

Hidden Bolts

Posted: 06 Oct 2006, 18:01
by Ralph Knowles
Peter, I heartily agree with you. Being 'T' boned by a Port tacker is bad enough without a couple of hidden 'pin punches' adding injury to insult.
If this was a mode of construction that had caused a punch damage, then the owner should be 'bombed out'!
10mm is small enough bow protection without any hidden 'weapons'.
I fitted 20mm to the bow of my Kite and still kept it in length measurement. Unfortunately, purchased hulls leave you no option but to have about 10mm, unless major surgery is carried out.
I use black neoprene, or similar, foam, which I glue on with cyano in block form and the shape it with a new 'Stanley' snap off style blade. Any irregularities are then sanded to shape with glass paper.



Posted: 07 Oct 2006, 07:51
by Bruce Andersen
good point on the "points". My bumper conveniently tore off in shipping and when I re-mold it on, I'll take the screws out and let the RTV mold into the (now empty) screw holes for the same effect - to stick the bumper more firmly onto the bow plate.

Posted: 09 Oct 2006, 01:45
by Hiljoball
Thank you to all who replied to my question.

My second molding attempt using silicone caulking seems to have worked, after leaving it for three days to cure.

If the silicone turns out to be too fragile in use, then I'll try to recast using a molding rubber.

Thanks again