Most model-yacht classes require a soft bow-bumper to be installed on the bow of the boat, to prevent serious damage when the inevitable collisions happen.
We milled a bow-bumper mould for the PIKANTO a while back. A first test was made using the normal Frekote+wax release agent (which we use when moulding boats with epoxy+glassfiber) in this mould and a standard bathroom silicone. Not very successful. The silicone stuck to the mould, cured slowly, etc.
Googling for this a bit it seems there are separate release agents made for silicone-casting. The simplest solution I found was to use Vaseline (a.k.a. petroleum jelly). This can be diluted in some hydrocarbon solvent to make a thinner Vaseline release-agent.
We've used RTV-615, a two-component silicone, previously in the lab for various projects, so I decided to try it for bumpers also. It looks like this, and is mixed in a 10:1 (A:B) ratio.
The closest thing to Vaseline I found lying around was this High-vacuum grease, which I applied to the moulds undiluted. In the future it's probably better to dilute it a bit to get a thinner mix and a thinner coat of release-agent on the mould surface.
I mixed 22 ml of the RTV-615 (a transparent liquid), added about three tea-spoons of white microballoons to produce a thick white mix, and poured this into the mould. At room temperature the RTV-615 cures in about 6-7 days (!), which is clearly too slow for me. However, at +100 C the curing time is reduced to just one hour. After an hour in the oven the bumper felt fully cured, and released from the 2-part mould by applying light pressure. Voila!
The bumper looks good without any major air-bubbles visible, and weighs about 16 grams (never mind the decimals 🙂 ).
I would be interested in hearing about how other builders make bow bumpers. Anyone know where to order some RTV-615? Use the Add Comment link above!
3 thoughts on “Building Bow Bumpers”
We have made molds of the "finished" bow of a boat, using a two part polyurathane molding compound from http://smooth-on.com . Actually, we found an art supply store locally, that carries the smooth-on products, used for mold making. Brush onto the hull and bumper "plug". It is pretty thick, but can be thickened even more with most powered fillers. Use polyurathane for the mold because silicone won't stick to it. Be sure to carry the mold back a few inches onto the hull. When cured, mark where the hull ends on each side of the mold, then peel the mold off of the hull, and remove the plug from inside it. I left the very top of the mold open.
Rough up the bow flat with course sandpaper, slip the mold back onto a hull to your marks, then use a 2 part silicone and slowly pour into the top of the mold. The mold peels off after cure. If the bumper wants to peel off too, it can be re-attached with some high temp rtv.
We also tint the silicone with a few drops of water based, acrylic paint, to match the hull color. usually 3-6 drops of paint is enough. Works really nice.
Photo at http://www.flickr.com/photos/hew_hamilton/4307503921/in/set-72157604340965096/
With your mold, much of this is not needed, just the silicone and some paint!
I also mould bow bumpers using 2 part silicone rubber. Craig Smith advised me of this. It is manufactured here in Australia. Buying a 1 litre pot, with 50 ml hardener, will mould over 50 bumpers. We normally use 15-20 ml of part A and just 1 or 2 drops of hardener. Cure time ranges from 15 to 30 minutes, dependent on ambient temperature.
Of 2 different hardness types, I chose the firmer compound. This is very white in colour, and doesn't require extra pigmentation. It does not require any release agent, it simply peels out.
check Ralphs online shop at: http://www.rt-sails.de/contents/de/d73.html
He offers a two part silicone which cures in about 15h and gives you a pot life of 90 Min. The base silicon is white and it can be colored with pigment.
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