Sunday, August 4, 2013

Extended Transoms: The Process

rev. 10-29-2013

I am now the proud owner of a PDQ 34 Altair, the only one, from what I understand. Rather than rehash my reasons, I will refer you to a prior post. Was it worth it? For me, yes, but this is not a small project nor for those not very comfortable with fiberglass. Well, perhaps someone with time to read, fiddle around, and learn, though I suggest learning on something less ambitious first. Perhaps it could be a good step towards convincing you that building a boat from scratch is for the birds, when there are so many good used ones out there.


The boarding ladder has to come off (the bolts are screwed into the threaded backing plates, so this is simple). The transom edging has to peal back (secured with polyurethane--not so easy). 500 pounds (as least it seems so) of tools must be delivered to the work site, including many that end up being of no use. The top side of the transom flange requires heavy grinding to produce the 12:1 taper required for bonding (the outside bevel will be ground after molding).

The Mold

A discarded 18-gauge aluminum (0.040 inches) sign provided the perfect flexible form, allowing me to match the existing line of the hull very smoothly, including the sharp turn at the chine (a little manual pre-bending was required at the sharper bends). Extending the form with furring strips allowed it to span the rudder and allowed lines tensioned by secondary winches to provide very firm but easily adjusted pressure, vital to the accurate positioning of the form. However, a ring of self-drilling sheet metal screws (1" x #8 of the sort with the drill bit in the tip--the plain pointed sort would not pull-up properly) placed along the flange was still needed to eliminate all gaps and to insure the form was tight to the hull form. Though originally I had figured on installing a false transom on the form and legs to the ground for support, with the sheet metal screws in place the form became quite rigid and no further bracing was required, even to withstand vigorous laminating roller usage. The form was waxed and coated with PVA mold release; the PVA was almost certainly over kill and I would not use it next time. Keep the PVA and wax away from the flange!


Laying up the required amount of triax on the hottest day of the year (100F) was both painfull and a lesson in exothermic reactions. At times my daughter rubbed the back of the form with wet towels and ice while I worked fast. One of my favorite hot weather tricks is to keep the epoxy and hardener in a cooler with ice, so that at least the mix pot does not heat too quickly, and it also helps to keep the pot off of hot surfaces.

Triax seems to be the best choice to build strength and thickness when working with epoxy. Mat has a binder that dissolves in polyester resin, allowing the material to go limp, but this does not happen in epoxy resin. Roving is too lumpy and bonds poorly on its own.The triax wets nicely, goes limp, and conforms very well for its weight on all axis. My source was Jamestown Distributing.

I used 8 layers of 17 oz. triax for the basic lamination, with 3-4 additional 4-inch strips of triax over the joint and edges, sandwiched in between the large triax sheets. A natural lap also created a beam down the beam centerline (I used sheets for the right and left, rather than one sheet, for handling reasons and to better fit up to the existing transom).  In retrospect, this may have been slight overkill, but I've thumped the stern into a dock before and will again, since I back into my slip every time.

Use a grooved laminating roller to squeeze excess resin from the laminate and remove air. It is also very benificial to have a helper during the main laminating step, to mix epoxy and cut glass, particularly if things begin to heat up. If you like to work more slowly, use a slow hardener for this process.

No coring. The complexities and structural compromise makes little sense, to save a few ounces in such a small area with hardware mounting and impact potential.

Drill limber holes. I did not include a leak-prone access hatches, so there needs to be a place for air pressure to go, plus any possible water leaks from the hardware. I drilled a 3/4-inch hole for each extension at the lowest point, sealing the edges (curiously starboard was cored with Corecell, port with mahogany plywood). They have not leaked a drop, and if they do, it will be into a crash tank.

Pulling the Mold

Self explanatory; with just the slightest tug on one corner it falls away. The poorly wetted fabric is in the sections that I was planning to cut away. Somewhat wasteful of materials I suppose, but time efficient.

The form literately fell away. The port form hit the ground  before we could get a picture!


At first I feared trimming a bit, since I would be cutting 1/2-inch FRP. However, I found that an 4 1/2-inch angle grinder with an abrasive metal cut-off blade goes through FRP like butter and is very easy to handle, the only down sides being that there is a lot of dust and the cuts must be straight (a series of short straight cuts can approximate a curve, and a grinder with a 25-grit disk makes it smooth). I can't imagine anything much better.

 I have ground the bevel on the outside of the join at this point and am ready to glass that in. The joint should be as strong as the rest of the boat; is is certainly far thicker.

Building the Steps

After bonding and taping, before final fairing

The steps were more like a carpentry project, but with fiberglass materials and methods. I purchased two 2' x 2' x 1/4" FRP sheets from McMaster/Carr and used these for the platform, riser and backing plates. 1" x 1/4" strips were beveled and bonded to the above hull mold in the same manner that cleats would be used to mount wooden shelves, the platform bonded and filleted in place, and then reinforced with glass tape at all joints, inside and outside. The taped areas were ground down both for better tooth and to provide space for the tape without leaving a bulge. All construction is FRP.

Do give the steps enough slope to shed water quickly. I matched the existing slope of the last step and that seems about right.

At first I sought the original PDQ vinyl trim, so that I could match it all the way back. I Googled a bit and learned that some boats reinforce with FRP instead, and that sounded better to me. Stronger. learned that the manufacturer wants a 100-foot minimum order. Perhaps if enough PDQ owners got together, but meanwhile it is simple not available. In the end trimmed the existing molding to fit and called it good. I did not want bumper trim dragging in the water.

Mounting Hardware
Glycerine makes a convinient tap lube that is easy to rinse off.

Without interior access using a simple washer and bolt would not answer. Additionally, since several of the fittings are highly loaded (cleats and bolt hangers for or securing light craft, and the boarding ladder) backing plates would be needed. For backing plates I simply bonded a 1/4-inch FRP plate in the appropriate areas, resulting in 1/2-inch total thickness. In place of a nut I simply threaded the FRP directly, which I have done on numerous occasions. So long as the FRP is greater than 1/2-inch thick (about triple the thickness of a nut) and the threads are well formed, the bolt will fail first. While I could have used nuts, I wanted removability without an access plate. If I am wrong I can always add an access plate later.

While I am a proponent of butyl tape for bedding large items that may require removal (hatches and winches), I prefer polyurethane for smaller items, as it adds to joint strength and helps keep the fasteners tight. The bond is not so great that removability is a problem. On the other hand, 3M 5200/4200 are too expensive, so I've generally switched to Locktite's PL S40 window sealant. Since I began using it 8 years ago, I'm comfortable with its durability in a marine environment. I used to patch a fishpond 6 years ago, and it is still doing fine.

Ready for paint. The trim conformed nicely. Note the inside edge is smoothed for sitting comfort. The bolt hanger (one on each side, only $3) is for clipping kayaks and tenders while at anchor; more compact and less painful to sit on; I heard stories of small cleats tearing very substantial holes in very tender places.

I had to cut 2 shallow indentations in the starboard transom rim for the ladder, to get the deployed angle I wanted. I had placed the step riser at a greater angle than prior because I liked the look better, the step is much lower, and thus the angle is different. With the indentations I got the ladder angle I wanted and the ladder gets side support. A 1 1/2-inch hole saw did a nice job, drilling from both sides.

The extension mold faired very well to the existing hull lines.
Nothing to disturb smooth flow or to increase transom suction.

Matching is the eternal challenge, made more challenging by stained gel coat. Match the lightest spot and it looks like a suborn clean spot. Match something more realistic and you better never compound the gel coat. Interlux's "off-white" turned out to be a fair match for my stained and faded gelcoat, as though a Chesapeake boat is ever going be without a few tannin stains; my marina is too close to the marsh for that to happen.

Then there is balancing solvent with temperature. Add considerable extra when hot or the paint will dry too quickly to self-level and maintaining a wet edge will be impossible. I started with a new "flagship" brush from West Marine, but went to an  old favorite varnish brush From Home Depot as the brush marks were less; a new brush seems to get finer over time if well maintained. Once you get it reasonably level, don't go back, you will only make it worse; have faith it will level if left on its own. Keep it thin on vertical surfaces and consider tipping off with vertical
strokes when working in tight places or really any time, as i feel like I get few sags that way.

Other than that, pretty straight forward. Three coats topsides, 3 coats bottom paint (normally I would only do 2 coats of bottom paint, but this is a fresh start). No barrier coat is required with epoxy construction.

Don't forget non-skid on the steps--fresh gloss paint is "slicker-n-eel-sh_t," as they say in these parts. Sprinkling coarse salt on a thick cost of wet paint (the salt dissolves in the rain leaving little holes) will take the shine off and add texture without introducing bathing suit shredding grit. The appearance and function is a very close match to the PDQ gelcoat non-skid treatment. A very traditional method, seldom discussed.

I may add some hull graphic or additional striping to further hide the transition, or perhaps simply to highlight her new tail feathers. I haven't decided.

Zoom as close as you like--no sags


A few pictures tell the tail. The Results (below) put it to numbers. One thing we noticed immidieatly was that you can now hear the wake from the helm, as it is just a few feet further back. But is is quiet.

Very close reaching at 7-8 knots. The platform goes under a wave now and then, which hurts nothing.

 Broad reaching at 8 knots. Dry enough that I'm sitting on the other side to capture this image. At this point 2 crew members had moved onto the tramp, lifting the stern. Sitting on the port platform (no ladder) underway is very cool, though someone certainly ought to keep an eye on you.

The steps in summary form. I hope you get tired just reading this.
  1. Pull boat and block.
  2. Sand the bottom as needed and prep for bottom paint. Might as well do it at the same time.
  3. Compound  (do not wax)  the last few feet of the boat or you will never get a good paint match (you won't be matching the true color).
  4. Grind off 6 inches of bottom paint in front of transom.
  5. Bevel the inside of the transom flange 12:1 with angle grinder and 25 grit disk.
  6. Remove boarding ladder.
  7. Construct metal form. Wax.
  8. Install metal form. Tighten the lines with the secondary winches. Lots of screws to insure tight fit.
  9. Coat form with PVA, just to be sure. Make CERTAIN you stay a few inches from the flange; the wax will be enough.
  10. Mask the edge molding and the SS handrail. Way up, as epoxy loves to spatter.
  11. Lay-up hull extensions with strips of 17 oz. triax. 8 layers average, with more on the edges, joint and rim. Depending on how fast you build this up the epoxy will exotherm, so be prepared to cool the form with wet towels as needed. If using West Systems, use 206 hardener in hot weather.
  12. Pull forms. Easy. They practically fell off.
  13. Remove mold release agent (water and scrub for PVA, xylene for wax).
  14. Fill bumps left by screws in forms.
  15. Grind 12:1 bevel at seam on outside.
  16. Fill bevel with triax and 6 oz. cloth as needed to fill. About 6 layers. Cloth leaves a nicer surface finish.
  17. Trim extensions to shape (grinder with cut-off wheel). Shape with course sanding disk.
  18. Add 1/4" FRP rim to last 14" of outer edge. Grind to shape.
  19. Rough-cut platform from 1/4-inch prelam FRP. Test fit and mark locations of supporting cleats. Make certain there is enough slope to shed water.
  20. Add any backing plates required for boarding ladder and cleats. Bond with thickend epoxy.
  21. Bond and tab 1"x1/4" cleats with thickened epoxy and glass tape. Short screws can be handy for holding while curing.
  22. Trim platform and place on cleats in thickened epoxy. Smooth epoxy underneath (you will be tabbing over this and so need a fair surface).
  23. Tab in with triax from underside. Not fun.
  24. Add additional thickened epoxy and form filets on all corners and joints.
  25. Repeat same steps for step riser.
  26. More thickened epoxy to smooth joints. Like finishing drywall, but harder to sand.
  27. Grind all transitions smooth. Go for your finished shape at this time, as the glass tabbing will be relatively thin and should not be ground away.
  28. Tape all seams. There are lots of them; both sides, aft edges, platform to lowest step.
  29. More fairing. Start using finer cloth on angle grinder and finish sander.
  30. Paint. 3 coats Perfection using a fine brush and great care. Sand lightly between coats. 
  31. Non-skid on the steps. Coarse salt in wet paint works very well.
  32. Mount hardware. I chose to thread the FRP directly, as 1/2-inch FRP holds threads very well. Seal with polyurethane sealant.
  33. Paint the bottom.
  34. Tape boot top. Replace the whole strip if you have the energy. Xylene and a sharpened drywall knife.
  35. Pay the yard and launch.
  36. Go sailing. Compare numbers with the last time you had a clean bottom (doesn't sound right...).
  37. Write blog post. Share the pain.

The Cost. While I didn't keep a detailed log, the rough consumption was this:
  • 12 yards 17 oz. triax
  • 3 yards 6 oz. cloth
  • 2 gallons epoxy
  • About $1200.00 in total "stuff."
  • About 45 hours labor + 12 hours of helper. I labor at a kinetic pace and I made very few false starts; if you like to relax or if this is new to you it could be double or triple. 
I figure this comes to about $600/ft vs $3,125/foot for the entire boat. If you have to pay a vendor (I got a quote of $7,800) the economics are less appealing. If I count my labor at a standard shop rate and worked  slower (I would have if paid by the hour), I can see how they got there.

Things I would do differently (advice to the reader--a very short list)
  • Use slow hardener if working in the summer.
  • Countersink and fair the screws holding the furring strips to the form. Saves faring later.
  • Mask off the hulls and trim better before starting. I was hot and my brain was cooked.
  • Dimensions: Perhaps 30 inches, but no longer than that for docking reasons. There is no sailing advantage and I'm happy with 22 inches. Perhaps a few inches higher would be drier, and some might prefer to run the outside higher for appearance.
  • Lay-up schedule; no changes.
  • Materials; no changes. 

Higher Sides?

Would higher outboard sides be better, like those on the Seawind extensions? Perhaps the higher sides give a better line and they certainly make the boat look bigger, if that matters to you. However, since one of the primary reason for the extensions was to ease boarding by my elderly parents (and the rest of us when carrying things)from low docks, the higher sides are a non-functional impediment. As for the passage of water and waves, very few strike above the first step and I can see no hydrodynamic reason for the extensions to be any higher. Some mono-hull sailors have argued that sugar scoops make no sense for offshore work at all, but cat sailors have not found this to be true and it is certainly of no concern to the coastal sailor. Making the forms higher on the outside would be a trivial matter, only a little more glass would be needed and the fairness of the sides before I trimmed them down was very good. The difference in labor and weight would be slight, the major concern being more critical paint matching. As boarding/swim platforms and for cleaning up the wake, they work just fine.

Though they are clearly well out of the water at rest, at speed the boat drops several inches into a displacement wave "hole" and further yet if everyone is in the cockpit or aft. Another foot would be good.

The Results
The dingy is lashed to the hull at 2 points.
There are good reasons to cu the inside edge low.
  • The hull form came off the mold true within 1/16-inch of anything I could have planned. Two thumbs up for the sheet metal mold.
  • The finish and detail fairing deserves only one thumb up. I will revise this in 2 years when I haul for bottom paint. But I got a good color match and it passes from 20 feet and closer, perhaps, if your glasses are scratched. 
  • Boarding is greatly improved. This is a major plus for family members with mobility issues. But even for the rest of us, loading the dingy with "stuff" is simpler and boarding kayaks is now a breeze, even in lumpy conditions.
  • The ladder now lies flat on the bottom platform rather than up the steps.I hated having to work around the ladder when it was on the steps.
  • Access to dingy and access to the water. This goes beyond simple boarding. Want to fish with your feet in the water? Wash out a pan? Work on the tender outboard or simply pull/replace the drain plug? Everything is easier.
  • Pitching. Certainly it must be reduced, but controlled measurement is impossible.
  • Weatherlyness. Any reduction in drag, resulting in better water flow over the foils, has to help. The net improvement in VMG is more than the sum of the parts.
  • Speed. Motoring at full throttle we have increased from 7.3-7.4 knots to 7.5-7.6 knots (measured by GPS, average 2 directions, both times with fresh bottom paint). The waterline length increase would suggest about 0.25 knot increase is possible, but since the hull form was not changed, 0.2 knots is more realistic and is what we saw. A small improvement, but worthwhile. The fuel savings at a constant speed to should pay for the upgrade in ... about 120 years.
  • Length vs. speed. Would even longer be faster? I don't think so. I have not changed the entry and the extended transoms are out of the water ~ 2 inches at rest with this 2-foot extension. Other boats with transoms that drag more, or cruisers that load more heavily may find additional length would help. But since the rudders are not typically relocated the affect on handling should be considered before going too long. Though sail boat extension are generally successful, there have been stories of extended power boats that lost steering control.
  • Load carrying. Though most of my sailing to this point has been with a rather light load, when we go cruising I notice the drop off in performance as the water line is pressed 2 inches lower at the stern. I believe the streamlining will help even more when overweight. If I had a very over weight boat or cruised full-time I would go 30 inches.
  • Docking. Since the extensions are 50% out of the water at very slow speeds, just sort of rounding the transom, there is perhaps a reduction in both yaw drag and backing drag. The docking length is not affected, as the dingy still hangs ~ 18 inches past the transoms.We can't feel any difference.
  • Handling. We've noticed no negative affects. More speed means faster tacks (positive), any drag aft should lead towards better balance (positive--comes out of irons faster too), and we haven't noticed any negative affect in quartering seas. It seems like it might be inclined to surf a little sooner, but controlled observation is impossible.
  • Irons. Seems less prone to getting caught and comes out more quickly, simply backing up with the helm over. I think there is clear improvement, the result of length aft.
  • Weight increase. About 35 pounds each, or about 35 pounds per foot of water line (the over-all figure for the PDQ 32 is about 230 pounds). I over built them, but I envision backing into a few docks (note as of 10/2013; I have. the pilings have dents, not the boat).
Was it worth the time and money? For me, yes. Though the process was an exhausting mid-summer exercise (we picked the 3 hottest days of the year--stupid), everything went as smoothly as could be expected with very few surprises and with the level of effort I expected; not my first FRP project.

I always find it fascinating to watch a large pile of money turn into a large pile of glass, epoxy and disposables, and then into some rock-hard shape that we have beheld only in our imagination. Like so many projects, if you do it right the work blends in with the existing and somehow it seems like little was changed, like it was always that way; both the reward of good work and its curse. By comparison, mount some new toy, like solar panels, and in a few hours you can see what you've done, though even then the finer details are subtle or hidden entirely. But do something sloppy and it will be obvious forever.


    1. Looks great! Wow! Thanks for posting all the details! I've only gotten as far as building the steps. I'll be back!

    2. Not as much work hourly that I thought you might have to deal with but you sure do have a great step aboard! Well done.

    3. This comment has been removed by a blog administrator.

    4. Thank You, nice work, very helpful for my own project

      Cajun Pat