Catalac 8M upgrade Rudders and install Skegs

by Didier Grieten
contact Didier Grieten

When I bought my 8M, it was equipped with the older style lifting steel blade rudders.The rudder blades however were in very bad condition, with considerable corrosion and rust. Also, the wood support structure had to be replaced as it was rotting away and looking at the rudder blades showed they required some TLC as well.

In some areas thickness was reduced to 50% and rust still working to get it even thinner… This meant some serious work on the rudders was required… Here’s what it was supposed to look like:

Catalac 8M rudder replacement
Normal rudder blades

I had three options, the first being replace the wood, remove the heavily corroded areas and weld in new material or just entirely replace the steel blades with new ones, get them galvanized and throw on a few coatings of paint… The second one was to replace the whole rudder setup with the newer style Catalac skeg / rudder setup. 

Tom Lack had received complaints about windward performance of the lifting steel rudders and in 1980 he redesigned the rudders and gave the Catalac 8M and 9M new underwater profiles. This was a newly designed skeg hung rudder system. I heard this modification was a success and greatly improved the windward performance of the boats. Since this upgrade was on all Lack family designs going forward, I figure this will improve my 8M’s handling and tracking. Here’s an example of the redesigned skeg hung rudders:

Catalac 8M rudder replacement
Newer Style Skeg and Rudder

Third option, design something new. But as this is not a racing boat I figured I’d stay with a known working setup so I did not even consider going that way as it might involve a lot of trial and error… 

After some inquiring on various forums and this web site, I found out (obvious?) pros and cons of both Catalac setups 

Original Lifting steel blade Rudders
Prokeep rudders out of water when not sailing
no rudder fouling
no rudder constraints when ‘beaching’ common in some UK areas
Directional stability provides for snake tracks (more work on helmsman / autopilot)
Tendency to stall ie create turbulence when turned quickly
Heavier – a single rudder with its wood support block weighs about 20 kg
Poor windward ability
New Style Skeg Hung Rudders
Probetter tracking stability
improved windward sailing (no figures provided)
lighter – rudders and skegs I built weighted 20 kg for both port and stbd
Con:not always clear of grounding when ‘beaching’

I don’t intend keeping the boat on a mooring where she will ground twice a day, which was the original intent of the lifting steel rudders, so after some consideration, I decided I would redesign the worn out rudders to the newer skeg hung rudder Catalac design. 

I’m sorry to say that I’m unable to provide improvement figures on this redesign, as I never sailed the boat with the original rudder configuration. It may be a bit late in the game, but if anyone has done this conversion and can provide data on the Catalac before and after conversion, I would be more than happy to incorporate his findings/data into this article! 

Phase One: decide on rudder/skeg dimensions… Initially I was thinking of building big skegs. After all, the whole purpose of adding skegs is to improve boat handling…

However, a second thought slowed me down. Skegs are a good thing, but keep them a reasonable size. I didn’t want to end up with a boat which wouldn’t tack anymore due to too much stability! Over sizing the skegs might mean backing the headsail to tack, maybe even having several attempts before a successful tack or even worse, I’d have to start the engine just to tack…

As I had no intention of going through trial and error sessions I decided to stay close to the original dimensions.

No one seems to have any dimensions or data readily available but the webmaster of this site (Rick) provided me pictures of his boat on the hard. From the pictures it wasn’t difficult to determine the dimensions of his rudders and skegs… 

The total rudder/skeg surface being a bit smaller than the surface of the original steel blades, I made my rudders just a bit larger than Rick’s rudders, not exaggerating as I did not feel I needed to match up the surface area with the surface of the steel blades. (so maybe even bigger rudders would be OK)

The drawing below this sentence shows the dimensions I used, and could be used as a template or draft for others…

Catalac 8M rudder replacement
Dimensions for skegs, rudders and Rudder ‘top’

The above drawing shows dimensions for skegs, rudders and Rudder ‘top’, this last one is added onto the top of the rudders and I did this to make my life easy. In this way, the top of the rudders are the same thickness as the original wooden blocks and I did not need to adapt the rudder hinges and other connecting pieces 😉 

I then purchased a few marine grade plywood panels: 18mm plywood for skeg & rudder main part and
10mm plywood for rudder top. I then cut out all required parts, 4 of each : 

Skegs : 4 sets 
Rudders : 4 sets 
Rudder top : 4 sets

building new rudders and skegs Catalac 8M
Skegs, rudders and Rudder ‘top’

Next step is working with a wood router. Shaping the leading edge of two rudder panels. Rudder & skeg leading edge radius 1.8 so circular as width = 3.6 (2*1.8 panel glued) Skeg trailing edge radius 2.1 to approx keep distance between skeg & rudder equal, I estimated the distance between both would be about 5 to 6 mm (big guess number, but I think it is about right.

rudder profile
Rudder Profile
building new rudders and skegs Catalac 8M
Shaping the leading edge
building new rudders and skegs Catalac 8M
Next: The two skegs panels trailing edge :
building new rudders and skegs Catalac 8M
They appear to match-up — seems ok :

The skeg leading edge was treated the same with a router as the rudder leading edges …

The panels were then glued together with epoxy glue. I used woodscrews to hold the panels together… Screws were countersunk and covered with epoxy, then sanded smooth:

building new rudders and skegs Catalac 8M
Holes filled and sanded
building new rudders and skegs Catalac 8M
Holes filled and sanded

The rudders trailing edge was treated a bit differently :

building new rudders and skegs Catalac 8M
Trailing edge

The rudder top parts then were copied from the original rudder head blocks to match the boat rudder mounting detail. There was some routing involved and all fixings to the rudder changed over from woodscrews to nuts and bolts. I epoxy glued bushings in every rudder attachment point. I figured in theory, no wood will ever be in contact with water as all is covered in epoxy…

The rudder trailing edge, no router, no saw, we have abused our planing tool to do this one…

building new rudders and skegs Catalac 8M
Mounting bolts

At some point after most profiling was done, the involved areas were smoothened out with a wood file…

Next step was to attach the skegs to the hulls. I saw two options… Bolt the skegs through or glass them on. The Catalac skegs were glassed to the hulls, they were not part of the original hull mold.

My initial idea was to bolt the skegs through the hull into the rear wet locker.

In investigating this option I discovered the wet lockers had a double bottom. If I persisted in bolting the skegs onto the boat, this would actually mean removing the bottom of the wet locker just to be able to access the bolts and then creating and re-installing a new locker bottom or at a minimum install a big inspection hatch.

In considering both options again, I reasoned that if the boat stuck an underwater object, resulting in ripping the skeg off and causing a gap in the hull, that the damage would obviously be under that locker behind the main rear bulkhead. I could see no large difference between having the bolts ripped out with the reinforcement or a glassed skeg ripping out a hole when hitting something at speed. The result would be the same, and the boat would be in no danger of sinking. 

This pretty much dissuaded me from going through the difficulties of bolting and remanufacturing the locker bottom. I glassed the skegs to the hulls under the rear lockers. I made sure to have a very strong hull attachment expecting the skeg to break before anything being ripped off the boat structure… 

So to glue/glass the skegs to the hulls, the procedure I used was the following :

– Installed the rudders

– Put a thick cloth around the rudders to provide for space as both rudders and skegs will need some spacing and they both will still receive an epoxy underwater coating.

– Aligned the rudders straight, presented the skegs to the hulls and, taking into account the skegs are a few cm longer than the rudders. I then corrected the top of the skeg to the hull curvature (I had to check this on the boat as both hulls did not seem equal so I could not easily have measured this). Then we glued the skegs to the boat with epoxy glue, using the portable workmate bench in the photo.

building new rudders and skegs Catalac 8M
Gluing the skegs to the boat
building new rudders and skegs Catalac 8M
Gluing the skegs to the boat

After completing the installation, as the weather permitted, I overlapped 6 layers of fiber/epoxy. Every layer of fibermat about two to three cm over the previous one. This has taken some time, as I had to wait for good weather conditions and unfortunately I have not taken progress photos, but I suppose you understand what I did… If anyone is in doubt please contact me and I will try to explain in detail…

After this all that was left was the underwater epoxy coating….

Here is a photo of the nearly completed skeg/rudder installation. (in this picture the stainless steel skeg shoe is not installed yet, there is a plug in the bushing in order to keep it clean from excessive epoxy. The shoe has been bolted to the skeg through this bushing)

building new rudders and skegs Catalac 8M
Nearly Finished

I still need to clean and repaint the metal parts (hinges and bar connecting to the steering quadrant), but other than that, I’m done!


By Rick

Owner of a Catalac 8M and Catamaransite webmaster.

1 reply on “Catalac 8M upgrade Rudders and install Skegs”

Hi Rick.
I am the proud new owner of Duetta 1996, 900 Sail number 911.
It needs a little work (sail drives service and engines recomissioned) and the rudders. The trailing edge on one has a small amount of rot. I’m planning to plane the bad one back to good wood and match the good one then sand and epoxy them. The metal work is pretty good but there to a fair bit of slack in the hinge bolts (there’s probably a better name for them) I have access to a lathe and am intending to ream the holes back to round, bush and replace the bolts with double nuts or cotter pin. Do you have any comments or has anyone else come up with another suggestion. I’m on a tight budget otherwise I would probably get them remade is stainless so any suggestions for painting the metal prior to refitting would also be great received.
Thanks in advanve.

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