A little boat repair help please

[Timm R Oday25]

After way too long we finally got the Oday launched . Unfortunately the dinghy decided
to leave the safety of the trailer on the way home while I was doing 75 !
From what I know the material is a type
of rotomolded  polypropylene .
Any idea of how I can fill in the missing plastic ?

[Charles Brennan]

Timm, Once again, a vague question in search of a specific answer! 
I LOVE it!! 

"Rotomolded polypropylene" is at least a start; notwithstanding the fact that that most rotomolded products use polyethylene, not polypropylene, so please verify the material with the dinghy manufacturer before you start.
(I'm not in the mood to type the word "polypropylene" much less the word "polyethylene" a dozen times, so we'll just keep calling it "plastic" in this post.)
Yes, it can be repaired by anyone with reasonable skills and access to a heat gun and/or a soldering gun.
You will need good ventilation in your work area, as polyethylene terephthalates or bisphenols are NOT something you want to be inhaling, if you value your endocrine system or nervous system.

"Any idea of how I can fill in the missing plastic?"
Ah! The traditional vague data!
How MUCH missing plastic?
A 1 inch by 6 inch gash, or a 6 inch by 8 foot section missing?
Involves two very different repairs.

In general, (about the only way you CAN answer a Timm question) one reinforces with some form of metal (preferably stainless steel) melted into the edges to provide a substrate for the plastic insert piece to adhere against.

One way is to bend small wire in a continuous "Z" shape and press it against the plastic with a soldering gun tip until it embeds in the plastic.  This provides a substrate to lay a section of plastic against.  Heat the area, starting at one end with a heat gun and press the plastic material into the wire substrate, as the plastic softens.  When you reach the edges, melt together the old plastic against the new plastic and fair (smooth) the edges together.  They sell soldering iron tips with a flat surface to achieve this.
See link:
https://www.amazon.com/s?k=soldering+iron+plastic+welding+tip&gad_source=1&hvadid=693914849028&hvdev=c&hvexpln=67&hvlocphy=1015134&hvnetw=g&hvocijid=11966986612225293239--&hvqmt=e&hvrand=11966986612225293239&hvtargid=kwd-418443352928&hydadcr=17034_13426372&tag=googhydr-20&ref=pd_sl_9opqj7oo5s_e_p67

They also sell soldering gun tips for this:
https://www.zoro.com/weller-soldering-tip-pk-2-6160/i/G607352370/?utm_source=google&utm_medium=surfaces&utm_campaign=shopping%20feed&utm_content=free%20google%20shopping%20clicks&campaignid=19725397595&productid=G607352370&v=&gad_source=1&gclid=EAIaIQobChMI5LnRoe2OhwMVczgIBR2vggXgEAQYDCABEgKlE_D_BwE&gclsrc=aw.ds

Another way (if the patched hole is on the largish size; without pics, we're just guessing aren't we?) is to use a coarse (coarse: way bigger than window screen) stainless steel mesh to bridge the gap of the missing area. Melt it into the edges as per above and apply your plastic onto the mesh. and melt it into the substrate.  This way the mesh is providing the tensile strength and the plastic is providing the filler, in the same way fiberglass cloth provides a mesh and resin provides a filler when repairing/building a boat.  If a large problem area, it might even need plastic applied from both sides of the hole (if reachable).

The most common source of plastic for repairs, is Ye Olde Home Depot 5-gallon bucket.

If you have small areas to repair, or for example, splits on the seams, the kayak guys have been fixing their dumb mistakes for years with plastic welding rods, like this link shows:
https://www.amazon.com/JounJip-LDPE-Plastic-Welding-Colors/dp/B09R6MVQMG/ref=asc_df_B09R6MVQMG/?tag=hyprod-20&hvpos=&hvpone=&hvptwo=&hvqmt=&hvdvcmdl=&hvlocint=&psc=1&mcid=f370e53a72cd39a0861bf684b3f6428a&hvocijid=16728569523473306006-B09R6MVQMG-&hvexpln=73&gad_source=1

Here's another:
https://www.boatoutfitters.com/welding-rods-polypropylene?gad_source=1&gclid=EAIaIQobChMIperO_umOhwMV0kT_AR06TAVREAQYBiABEgLjlfD_BwE#216=708

And of course there is always YouTube, the app that makes Instant Experts of us all.
Here is a typical link:
https://www.youtube.com/watch?v=GWtgkLyqkZ0

Another:
https://www.youtube.com/watch?v=d5crkk0HMOU

Check those out and you should get a general idea of what is in front of you.
I helped my son fix a kayak that he got cheap, because the owner did not know about plastic welding rods, etc.

Hope this helps,
Charles Brennan

[Monroe]

There is an epoxy called GFlex that I have used for small repairs on a "plastic" kayak that works very well. I may have done something wrong, but it did not adhere to stainless steel.

Monroe