Repair of polypropylene dinghy hull

[Bill Spence]

I have an Escape sailing dinghy, which has a 12 ft approx polypropylene hull. This material cracks easily if subjected to long term loading, and will not bond well to adhesives. It can apparently be repaired by welding, but I do not know how effective this is, and no-one seems interested in doing the work.

The hull has a substantial crack in the deck and transom at the rudder mount. I have routed out the crack, flexed the hull to open the crack, filled the crack with epoxy, and removed the flexing. The epoxy appears to have bonded to the polypropylene, but I expect it will crack again when loaded. I therefore intend to reinforce the transom with an aluminum flatbar, 2" x 1/4", about 2 ft long, which I intend to attach to the transom with stainless steel sheet metal screws (#10) into the polypropylene of the transom. I am concerned about corrosion of the aluminum or stainless screws in contact. The boat lives out of the water, so the area only get wet when it rains or the boat is in use (salt water). I could use nylon shoulder washers between the aluminum and stainless screws (but this would reduce the effectiveness (stiffness) of the connection), or I could ignore the possible problem.

Any suggestions?

[gonzo]

I have seen repairs on canoes and kayaks done with welding. They use a propane torch and a hot glue gun with special sticks. The torch is to soften the area and blend in the patch. I believe they sell the sticks at kayak stores.

[Lew Morris]

hey Bill,

You mention that the boat lives out of the water... does it also live out of sunlight?

U/V (ultra-violet) radiation REALLY plays hell on plastics, especially polypropylenes (PP). It's a useful material, but weakens substantially when left exposed for even a single summer. My little Walker Bay is another example.

You are already aware of the fact that virtually nothing will bond onto PP. At best, epoxy can be used to fill a small hole, but even then it isn't a hundred persent positive fix.

That said, I don't think you're going to have too much luck screwing through the bar into the PP hull. The hull is probably no more than 1/4" thick and that isn't much more than two or three threads of engagement. Use more screw (and holes) and you'll just be providing a "perforated" line for the transom to break along.

If there is access into the interior transom area I would though-bolt the bar onto the transom using large diamter "fender washers". An even better solution would be to use a backing plate(s) inside the hull (on the transom) to provide more surface area to spread the load onto.

Use the aluminum (it's easy to fabricate) and all stainless fasteners. Don't sweat the corrosion issue, it doesn't sound like the boat will last long enough worry about it...

By the way, how old is this boat, and what caused the crack in the first place... or should I ask... :-]

It should be easy to find a capable polyethelyene welder...new waterlines are high density poly e...and contractors doing waterline work are well equipped with materials and equipment. You might want to obtain poly welding rod from the mfgr..so it will match. It's a yellow pages thing..I'd look before the cdonstruction season begins...

best,
Bill

Bill: I have a polypropylene windsurfing board with a small gash in the underside of the board. I have been searching for epoxy remedies to permanently fill the gash. What type epoxy did you use? -Kurtis

[quote=Bill Spence]I have an Escape sailing dinghy, which has a 12 ft approx polypropylene hull. This material cracks easily if subjected to long term loading, and will not bond well to adhesives. It can apparently be repaired by welding, but I do not know how effective this is, and no-one seems interested in doing the work.

The hull has a substantial crack in the deck and transom at the rudder mount. I have routed out the crack, flexed the hull to open the crack, filled the crack with epoxy, and removed the flexing. The epoxy appears to have bonded to the polypropylene, but I expect it will crack again when loaded.

[Derrold Holcomb]

Bill,
I have a similar problem; a small hole in a polypropylene hull. Did you ever find a solution to this problem?

Derrold Holcomb
holcomb@erdas.com

[Stephen Gray]

Bill,
There is no fix for your problem. I have worked with polyprolylene,from tanks for platting,to roto casting expanded pellets. Get another dinghy,because thats the thing you don't want to fail,when the patch snaps.
Steve Gray

[wwstewartiii]

to Bill Spence:
I know this is old, but I have the same hull. I fixed the cracks in the stern with two-part epoxy. But now the well that holds the plate that holds the mast & boom has cracked. How did your repair go and what did you use?
I'm thinking of building a glass & two-part epoxy fix, any thoughts?
I didn't know the Escapes were out of business until I googled this problem. My kids liked this boat.
thanks,

[daiquiri]

Polypropilene doesn't bond well to other dissimilar plastics and resins. So epoxy might be ok for filling small holes, for example, but I wouldn't trust it would bear any significant structural load across the crack. Besides, it will soon deteriorate and become brittle when left unprotected under direct sun.

PP can be repaired by welding with a hot gun and PP sticks, as Gonzo said. You need to bevel the two sides to be welded, in order to create a V-groove. See an illustrative example here: http://www.plasticweldingschool.org/...ing-groove.php

The gun should be set to a temperature between 220-300 °C and a constant pressure should be applied to the welding stick during the process. Polymer materials' strength depends on the length of polymer chains, so it is necessary to weld slowly in order to allow the molten material to mix and re-create sufficiently long chains.

The final result and the strength of the weld will depend strongly on type of PP, on the eventual presence of fillers, on welding procedure etc. It goes without saying that the welded zone will not have the same mechanical characteristics it had before the crack happened - it will be much lower. So if it is a mechanically stressed area of the hull I would be very, very cautious and would check the welded zone for cracks after each and every boat use.

Take a look at these short but informative thermoplastic materials' welding primers:
http://www.wegenerwelding.com/pdf/we...ionWelding.pdf
http://www.wegenerwelding.com/pdf/09_Guidelines.pdf

[Fanie]

There is no fix for your problem

Unfortunately my experience also. If you want to fix it it is going to have to be a mechanical one ie pop riviting something over the hole. Not very elegant.

I haven't seen these materials welded, but if it is a simple glue gun type weld I will question the integrity of such a weld. An ultrasonic weld may be better - if you can find someone willing to give it a try.

[wwstewartiii]

Thanks for the quick response & help. This hull might be finished. The crack is at the bottom of a sump about 50 cm deep x 20 cm wide and 40 cm length. It is hard to get two hands and a wrench in there, but at least now I know what tool I'm looking for.

[Fanie]

Good grief ! cm or mm ?

500mm x 200mm x 400mm would require serious filling...

[wwstewartiii]

I was trying to be global with the metric, maybe that wasn't such a good idea. Picture a waste basket about 18 inches deep. The whole thing is PP and at the bottom of this "basket" [tabernacle in nautical terms] is a steel plate upon which the mast and boom rest. The bottom of the tabernacle has cracked. I couldn't get under the steel plate to patch the cracks and had to remove it, somewhat enlarging the crack.

Not sure where this is going. I could perhaps build a fiberglass reenforced tabernacle that I bolt/epoxy to the upper "deck."

If you are familiar to the Escape this probably makes more sense. I've not seen another boat with exactly this rigging so it is hard to describe a portion without boring you with the whole thing.

[Fanie]

Put some pictures up.

[tinhorn]

Originally Posted by wwstewartiii

I could perhaps build a fiberglass reenforced tabernacle that I bolt/epoxy to the upper "deck."

Bingo. The only PP boat I own is my wife's kayak. No holes yet, but I've already decided that if it does need a repair, a fiberglass patch, mechanically fastened to the kayak (likely with a thin gasket sealed with Permatex #2) will be my approach.

Your project sounds like it's load-bearing--I'd think a backing plate (which could also be fiberglass) is essential.