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

[Kurtis Houk]

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