Skip to main content

Hull thickness and backing plates

As the weather is getting cold to work with sealants outside, installing the thru-hulls will have to wait until the spring. If we'll have a cold spring, I might make a tarp around the hull to be able to heat it up, but we'll see. Until then the interior can be easily heated and working inside with epoxy is possible, so I'm planning to install backing plates. The hull of the Scanmar 35 seems to be quite thick, I measured it to be 14mm (solid GRP laminate + the gelcoat), but some extra thickness might still be good to help distributing the load.


I consider this as a "once in a lifetime" job and since there is time till the spring, I want to do it properly, so hopefully I'll never have to do it again. Some people use plywood for backing plates, but I was concerned that if anything goes wrong and water gets in it will rot. So I followed the advice of this article instead and got some solid G10 GRP sheets in 3mm and 10mm thickness from eBay. The plan was to use the 10mm sheets for the larger seacocks and 2x3mm for the smaller ones.


To cut them to the right size is a challenge. These plates are so strong that only metal holesaws can cut them. The wood saws will just burn the sheet. Even with metal saws it nearly burned down my Bosch driller to cut through the 10mm, so I suggest working with thinner sheets and epoxy them together if you need more thickness. The 3mm sheet was quite easy to cut. To have a perfectly centered cut, it's a good idea to start with the big holesaw first but don't cut it through completely, then use the same center hole with the smaller holesaw, then switch back to the bigger one and finish drilling out the last few millimeters you left. Also don't start with the largest plate as I did (I know, it was stupid) because if you mess it up, you will waste more material. Now I will have to order some more sheets to properly finish this job.

Comments

Popular posts from this blog

New ventillators and smoke detector

Three old ventillators have been replaced last weekend as they were falling apart due to UV damage and the brittle plastic. We could get the exact same type of ventillators, so we went with those. No new holes to drill and hopefully these will do the job for another 10 years. I was also considering changing them to solar vents, but I haven't read too good reviews about them. They're usually much bigger (need a bigger hole) and according to many they tend to die after a year or two (either the battery or the fan motor gets loud) and doesn't worth the extra price tag.  We're going to add some regular 12V car fans inside before we sail to warmer climates. We already have a gas/propane alarm in the galley and now I also installed a battery powered smoke detector. Very easy and no drilling required, it's held in place with velcro strips.

Bilge reinforcement - Part 2

Read Part 1 After careful surface preparation the bilge reinforcement project continued with laying the new laminate. I consulted with West System on what fabric to use and they suggested biaxial cloth without chopped strand matting. The biaxial cloth is very strong and CSM would just add weight. We bought 600g biaxial cloth with 45/45 degree strands from Composite24 and West System 105 epoxy with 206 fast hardener at a local marine store. Cutting fabric The work was done in sections, we started with the bilge bottom while the keel nuts and washers were removed. It got 4 layers that come up 10cm at the bilge sides. This area gets most of the load and we wanted to reinforce it by connecting the fabric on the bottom to the fabric layed on the sides. 4 layers on the bottom In theory these types of reinforcements are suggested to be done either with the keel dropped or while the keel is hanging so it's not compressing the joint while the boat is standing on th

Scanmar bilge and swinging keel

The Scanmars, at least the SC33 and SC35 that I'm aware of, are infamous of developing a swinging/pendulum keel due to the lack of structural members in the bilge that could prevent the hull from flexing sideways at the hull to keel joint. Here is an article (in Swedish) that mentions this problem (svajköl) and here is another one from a surveyor who claims it's a weakness in the construction. The "stringers" on these boats are only floor beams, that are not going down deep enough to the bilge to act as a structural element. If the boat had harder groundings the attachments of the floor beams could delaminate from the hull, making the hull-keel joint even weaker and the swinging keel symptom worse. When we were on a boat hunt we've checked out almost all the SC33 on the market at the time and paid attention to how their bilges looked like (and in the end bought a SC35 without reinforcements 😄). As the bilge on the SC33 and SC35 looks very similar, I thoug