Duckworks - Micro Repair

To the uninitiated, the boat looks somewhat like, well..., a box. The hull cross-section is, in fact, perfectly rectangular - something which leads many observers to underate the design and write it off as a crude and simple piece of work.

Well, Micro is a very simple boat to build, but only a genius - someone like Phil Bolger or Naval Architect C. Raymond Hunt - could design such a hull and make it work well (Phil Bolger was heavily influenced by C. Raymond Hunt, among others). It takes understanding to get the best from Micro and her free-standing cat-yawl rig, but if treated properly, she is an exceptionally good performer, while at the same time being cheap and quick to build, self-righting and self-bailing, roomy, and comfortable - all in a 15-1/2' x 6' x 18" package.

Cricket - a Micro which I built back in late 2001/early 2002.

In this photo you can see some of Micro's unusual features - flat bottom, extreme rocker, and rectangular sections. Very few people could design such a boat and make her a success. Very few people understand why the hull and rig work so well.

I'm not going to go into detail about the design aspects of the boat, but I will say that on one occasion I sailed her against a well-handled Navigator and an equally well-handled Penobscott 14, and even though I had two passengers, we beat both boats to windward, and pointed just as well. Now, the conditions were ideal for Micro, in that we were on a lake with about 10 knots of breeze and almost flat water, but her performance was superb, surprising me as much as anybody. It might have been different in a steep chop...

This particular Micro has been back to my various workshops on a number of occasions in order to have cockpit modifications made, and to have repairs carried out. Most recently, she came back to me after having been in collision (head-on) with a concrete floating walkway/wharf beside a boat ramp.

The damage was fairly localised around the bow transom and forward topsides, but Micro has a wonderful self-draining well, right between the bow transom and the forward bulkhead of the cabin, with a strong set of floorboards filling the space between the bow and the cabin bulkhead. The floorboards are at approximately the level of the painted boot-top (i. e. the division between the green and cream just above the waterline in the photo below).

My youngest boy, Steven, standing in the forward well back in 2002. He is standing on the forward well floorboards.

When Cricket hit the concrete walkway, the point of impact was head-on, almost exactly at the level of the floorboards. This not only damaged the bow transom, but also forced the floorboard panel back through the forward cabin bulkhead with great force. Micro is not a particularly light boat, carrying 195kg/412lbs of cast lead in her keel, weighing-in at around 500kg/1/2 a ton - so the damage was substantial.

Winch post pad covers most of the external damage. The paint is the original Hempel Polybest two-pack polyurethane which I applied in 2002! Notice how the plywood of the topside panels has de-laminated and split away from the bow transom framing. The damage is much worse than it appears.

Photograph of the inside of the forward well, looking towards the bow transom. The two large holes in the transom are the steps of the boarding ladder! (there is a hydrodynamic reason for the transom at the bow, but that is another story). You can see how the bow transom has been driven backwards through the topside panels, and that the framing has suffered serious structural trauma. In addition the planking-to bow transom epoxy fillets have been more or less destroyed.

Looking aft at part of the damage inflicted to the forward cabin bulkhead by the floorboard assembly. The boxed opening at the far left of the photo is the cabin ventilation opening - lets air through but keeps water out...

External damage to the starboard side of the bulkhead after initial paint removal...

.... and the same on the port side. Doesn't look too bad, but represents serious structural damage on close inspection from inside and out.

Micro has a primary structure made from 6mm/1/4" marine plywood for the most part, with a substantial amount of 3/4" framing timber throughout in various widths. The boat relies on her large volume and surface area for her structural strength, and is well designed from an engineering perspective. However, like many aircraft, she is structurally strong, but vulnerable to point impacts.

Bulkhead damage.

Damage to the bow.

The point of this post is to let people know that with careful planning and execution, a wood/epoxy boat can almost always be repaired to as good a standard (or better) as when she was built. Do not skimp on the process and avoid any temptation to "plaster over the cracks" so to speak - attention to structural detail is essential.

I may write more about the repair process use on this boat in a later post (no promises), but here are a couple of photos of the structurally complete repair, with only a few remaining paint details to be finished.

Just some painting to be done over the white two-pack epoxy primer/undercoat visible at the forward end of the keel, and some black boot-topping to be painted as well.

The repair process involved a lot of debris removal, fabrication of simple jigs to regain the correct hull shape, re-lamination of damaged plywood sheeting, plenty of epoxy, epoxy fillets, and glass fabric reinforcements
- but the boat is alive and well!

The moral of the story is to build your boat properly in the first place, and repair her with care if the need arises. Have an open mind and be prepared to be innovative and to improvise. There is no reason why a home- built plywood boat should not last several lifetimes, even if damaged along the way. In fact, if you are not under too much time pressure, the process can be both challenging and rewarding.

More Detail on the Micro Repair

My recent post (above) about repairs to a Phil Bolger Micro generated quite a bit of interest, and the following comment from Dave is an example:-

Thanks for the blog post, Ross, but you left out the details on the very parts I'd be most interested in seeing!

So if you do a future post on the nitty gritty details of truing up and patching the damage, I'd be most interested in that.

Well, after taking initial photos of the damage, I didn't many more during the repair process, simply because of time pressure - but here is a brief look at some aspects of the job: -

Initial job was to do a rough paint removal around all of the damaged areas to get a clearer idea of the extent of the damage, and to remove components/timber which had been destroyed. It also allowed ventilation and thorough drying.

A small puncture wound on the forward/starboard topsides on the outside, and

... the corresponding spot on the inside.

Brutal removal of paint, damaged timber, and broken epoxy fillets.

See above

Paint removal from around the forward bulkhead on the interior of the cabin, where the floorboards had punched through.

Most of the work shown above was done using a heat-gun and a variety of sharp scrapers. The paint was all two-part epoxy primer/undercoat and two-part polyurethane topcoat (I know, because I built this particular boat myself fourteen years ago!) and removal was not ever going to be easy. However, the heat-gun and scraper combination is a good choice as long as you are very careful about never overheating the material and damaging epoxy adhesive and paint in locations which are not part of the repair. Other primary tools include chisels, 4" angle grinders, drills, sandpaper - and elbow grease!

Next stage was to carry out a more gentle sanding using (in this case) a 5" random orbit sander, going down through the grits to about 120 or 180. On the internal areas, the job is more difficult to achieve, and I made heavy use of a Fein Multi-master detail sander and plain, simple sandpaper on a sanding block, or folded triple. Hard work!

See above comments

I don't have many photos of the next stage, but it mainly involved pulling usable components back into position using a variety of improvised tools such as lengths of purpose-cut steel angle-iron with holes drilled at strategic locations, and also temporary through-bolts and backing pads. This work can be very satisfying if done properly, and with attention to detail. The key is to have an open mind, and to be prepared to be bold with your surgery.

Once I was happy that my bracing would all work, and that all interfering debris was removed from joints, I opened the whole lot up again, and even spread damaged components further apart (using wedges and chisels etc). With the components held apart, it was relatively easy to treat all surfaces with un-thickened epoxy resin and hardener in order to prime the mating and damaged joints using disposable bristle brushes. This is a very important step if you expect to achieve a good structural repair. With the work area well primed, it was then a matter of applying a rich mix of epoxy/hardener combined with the recommended structural glue/filleting powder additive.

With the structural epoxy mix worked into all joining areas, I screwed, bolted, or clamped the repaired sections together, which is why the previous work dry-fitting the bracing and jigging was such an important step. Where appropriate, I applied structural epoxy fillets at the same time.

The above two photos show steel angle braces screwed into position over the epoxied repair. In the case of Micro I had the luxury of using straight sections of steel to hold things in place, but on more conventionally shaped boats the same thing can be done using shaped and bent timber splints. I'll show an example of this in an up-coming post on a Whitehall repair.

Interior shot of the repaired bow transom, topside planking, and forward bottom planking. This was taken while the initial epoxy work was still wet and ugly. This work was followed by additional cosmetic epoxy filling.

Exterior shot of the starboard, forward topsides repair taking place. The actual puncture damage is quite a small spot underneath the centre of the plywood pad.

Matching plywood pad on the inner surface of the topside panel. As you can see from the exterior shot above this one, I placed twelve screws through the hull and into the internal plywood pad and pulled them in tightly over the epoxied repair. Note that both pads have been covered in a film of plastic to prevent them being glued to the hull permanently. All of those screw holes had to be repaired later, but the repair turned out well. The plywood pads were large enough to take up the curve of the topside planking when screwed together.

Structural work complete, glass applied where required, fill and cosmetic work done, and the two-part epoxy primer/undercoat applied (the white paint - three or four coats)

Topcoat (two-part polyurethane) applied, with just some minor black line work to be done between the green topsides paint and the off-white bottom paint (that is my little step ladder reflected in the paint by the way).

Because of a lack of photos, time and space, this has been a very brief overview of the job, but it may give you some inspiration. Unfortunately, I have no photos of the way I repaired the forward watertight bulkhead of the cabin - a job involving more steel angle bracing and numbers of temporary through-bolts, nuts and fender washers. In a post in the not too distant future, I'll show the repair of a glued-lapstrake Whitehall tender which suffered very serious damage to her hull in an accident. Most people considered her a write-off, but we were able to give her a new life.

Just a word about repairing screw and bolt holes. Many people simply fill the holes with thickened epoxy and sand the surface smooth after curing. I do not do this because the "cylinder" of hard epoxy in the screw hole intersects the surface of the repair at 90 degrees, and is sure to result in a circular crack in the paint after cycles of expansion and contraction due to temperature changes over time.

My approach is to heavily chamfer the hole on the inside and outside surfaces using a wide countersink or by dishing-out the surface using a sander. Then I fill the hole and the chamfered areas - this gives much less of a stress-riser where the epoxy fill intersects with the surface. However, if the repaired holes are going to be covered with a layer of fabric set in epoxy, this step is not necessary.

Ross' plans are in the Duckworks store.

www.baysidewoodenboats.com.au

rosslillistonewoodenboat.blogspot.com.au

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