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Scanmar bilge, 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 stringer frames on these boats are only floor beams, they are not going down deep enough into the bilge to act as a structural element. If the boat was grounded, 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 of the Scanmar 33s on the market at the time and paid attention to how their bilges looked like (and in the end bought a Scanmar 35 without reinforcements, haha). As the bilge on the SC33 and SC35 looks very similar, I thought it might be interesting to see how the bilge looks like on various Scanmars.

Some owners were claiming that there was no symptom of swinging keel, (we couldn't verify though without lifting the boat). Others reinforced the bilge either on their own or by professionals. The easiest way to be able to see if the keel is swinging is to lift the boat and move it a bit by the crane to develop enough momentum. If the crane is a rolling one, standing on the ground around the boat (to prevent the whole boat from moving), it's also possible to push the keel hard and spot a few centimeters of play. Note: the play may also come from loose keel bolts, so double check those too.

I also heard that the issue is affecting only the first two years of the production, later the yard claimed that they reinforced the construction and eliminated the issue. I'm not convinced about this, as many owners reinforced their boats even though they were from 1984 (4th year of production) and we saw boats from 1984 without any (factory) reinforcements too.

Examples of various Scanmar 33 bilge structures:


Scanmar 33 bilge pic 1

Scanmar 33 bilge from 1984, floor beams and keel bolts seem original, without any serious reinforcements.


Scanmar 33 bilge pic 2

Another Scanmar 33 bilge, probably original, without reinforcement. Interestingly the floor beam is even weakened by "hole drills", not sure why or if it's original (to have more storage in the bilge, or to have better access to tighten the bolts?).


Scanmar 33 bilge pic 3

This example of a Scanmar 33 is from 1983 with added reinforcements. The floor beams are glassed down to the bilge and a "new" structural member (the thinner one on the left) is added.

Scanmar 33 bilge pic 4

Another view of boat no.3's bilge, note the cut in the cabin sole that had to be lifted to gain access to work on the other port side of the reinforcement.


Scanmar 33 bilge pic 5

A Scanmar 33 from 1984. Also a good example of added reinforcements. New, somewhat thinner structural member added between the existing floor beams.

Scanmar 33 bilge pic 6

Other angle of boat no.4, with better view of the glassed down floor beam.

Scanmar 33 bilge pic 7

The smaller "beam" towards the bow is original, it's the platform where the keel steped mast is standing. (Looks the same on our Scanmar 35 too.)


Scanmar 33 bilge pic 8

This is another Scanmar 33 where reinforcement was added possibly to prevent the swinging keel. I assume this work was done by the owner. Thin, small beam glassed in between the original floor beams, the cabin sole was cut and lifted.

Scanmar 33 bilge pic 9

Same boat (no.5), I sadly forgot to note the production year, but I think it was from the first few years. Better view of the added beam and the original floor beams here.


Scanmar 33 bilge pic 10

This Scanmar 33 (year unknown) had reinforced stainless steel plates under the keel bolts. I think they didn't change the keel bolt rods to longer ones. At the plate towards the bow (on the right) it seems that the bolts are not long enough to properly secure the nuts, also it looks like there is some delamination above the plate at the corner. Could be due to a grounding?

Scanmar 33 bilge pic 11

Same boat (no.6) with the reinforced keel bolt backing plates. The beams were also extended down to the bottom of the bilge (on the left).

During the fall we're planning to reinforce the bilge on our boat by laying extra layers of fiberglass over the existing beams to have a strong bond to the hull and possibly bringing them down to the bottom of the bilge. We also have to consider keeping the access to the keel bolts, which are basically M20 stainless steel threaded rods screwed into the keel and secured with a backing plate and a nut inside. We're also thinking about adding an extra, smaller structural member.