Elastosil M4503 is a condensation-curing soft white silicone I am now using for the PIKANTO bow bumpers. The RTV-615 I used earlier was an addition-curing silicone and cured in 60 minutes at +100C. This one will not cure faster at an elevated temperature. The datasheet suggests 24 h at room temperature, but I found 48 h was needed.
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!
Based on two slightly different MDF prototypes we made an aluminium mould for a bumper today.
In other news, hull nr 2 has been de-moulded and looks good. I'm designing the jib-support, the mainsheet-post, and the radio-mounting next. Then 2-part moulds will be milled for these components.
