Polyurethane sprayed-in foam insulation inside steel hull

Hi! I'm new here. I'm hoping to get some thoughts on the insulation design choice for a 35-years old steel hulled boat I'm purchasing pending the survey.

The inside and out hull is protected with the Devoe system: Catha-coat inorganic zinc primer (a 2-part cold galvanizing compound) followed by Bar-Rust epoxy barrier coat. The whole interior was sprayed with 1 1/2" of polyurethane foam with the exception was the bilge area by the original owner who built her from a bare hull. So it was insulated below the water line, which seems uncommon from what I've gathered from reading through forums. Most areas were sheathed over the foam with 1/4" plywood.

So I realize having insulation that prevents condensation by stopping air infiltration and having hull access for easy inspection is pretty much mutually exclusive. I do understand his design choice. But after speaking to a surveyor I'm concerned over the fact that these areas can't be visual inspected. The surveyor gave me the buyer beware speech, which was appreciated, stating that even after testing the thickness of the hull plating there's no way to guarantee there's not corrosion issues within inside the hull.

The vessel appears to be very well taken care of. I suppose I'm just weighing if I can live with the fact that the condition of the interior hull is unknown becoming an expensive ordeal to deal with in a few years. The only way I can see to know for certain is to deconstruction the cabin then remove the foam, which isn't something I'm looking to do for a vessel in this price range.

 

I'm lining up with the surveyor and his buyer beware speech. He might be just covering his own *** but he also may be telling you something that you need to consider seriously. The surveyor will have used an electronic device that is capable of determining the thickness of steel plate, from only one side, just by applying some conductive gel to the place that is to be measured, then pressing the wand into the gel. No that is not magic and yes it is reasonably accurate. But then how many places would the surveyor have been able to measure within reason? 35 year old steel boat....foam "insulation"...plywood covering the insulating foam........Hmmmm ?????

 

Yeah, what messabout said - but what I see missing is the "sampling" method. Rather than facing a complete refit, do a few selected tests, and see if there is a reason to be afraid. Pick an area where the stringer meets the frame in about 3 places, 2/3rd of the way down the hull side, but also under the water line.

At least then you can make a more educated guess on whether it's worth progressing. The owner might be happy to have the results available for other buyers as well, so it may not be a big issue to make a few divots.

On the face of it, it seems the original build knew his stuff, so there is hope..

 

What is the condition of that part of the hull interior, that isn't hidden behind the insulation foam ? Of course it will always be a gamble what is hidden out of sight, but the exposed part offers some clues, and also what $ are you risking is also a major factor in any decision.

 

The entire interior except the bilge and around the bilge were foamed over. No signs of rust though around the bilge. You have a good point about how much $ is at risk. This is mainly what I'm weighing. The boat is a great value for a blue water boat, but it's still a lot of money for me. It's very well equipped for my needs. Nevertheless it would be a huge hit if I ran into an expensive refit right away. 10-years down the road I wouldn't mind doing a refit but right I'm looking to cruise.

This is a great suggestion. Thanks. I'll run this by the owner and surveyor. A sampling is surely better than nothing at all. Not sure there's actually stringers but I think doing it along where the transverse frame meets the longitudinals would suffice. I'll post a pic of the interior before the foam was sprayed in after it's approved by admin.

 

Here' the interior prior to the foam. Where might be the best spots to take samples from?

 

As you are looking for evidence od corrosion, look where moisture is most likely to collect. Top of longitudinals just forward of transverse. Deck hull juncture. Penetrations, such as port lights or anchor hawshole. Ventilation, electrical or plumbing thru hole (edit) pay attention to internal sources oh humidity such as the galley or heads.

Also check the removed foam for signs of moisture or corrosion

All indications point to excellent original construction. I be surprised if there are issues

 

Blueknarr said it better than I could. I'll second penetration points ,especially where they are near water usage, like heads and sinks.

 

I've participated in a few insulations of steel boats with spray polyurethane foam. PU is an excellent material used right as it – if applied properly – will "get in everywhere" thus avoiding cold spots where moisture can accumulate. It will also provide some sound proofing and render a boat unsinkable (for some time at least) if you have enough of it. On the downside it will burn (and the fumes are very toxic) and contrary to some belief it will get soaked if submerged for any extended period of time. And once soaked it will take a very long time to dry, so long that it most likely will be cheaper and certainly faster to remove it and apply new foam (or something else).

Normally about 90 per cent of the cells are sealed so humidity will be able to travel through the PU foam. As a result two criteria have to be met in order to secure a proper function. If they're not met the insulation may actually prove to be a large disadvantage to the boat.

First of all the areas to be insulated needs to be properly coated with something that provides corrosion proofing and good grip for the foam. If the foam delaminates from the hull you'll have a pocket where moisture can accumulate and feast on the hull.
Second, the PU thickness needs to be sufficient given where the boat is to be sailed. Moisture will move towards the cold areas so the insulation needs to be so thick that the moisture doesn't pick up the scent of the cold hull (figure of speech). I'm at 60 degrees north and IIRC we applied no less than 50 mm of foam in boats that were to be in water all year and go everywhere.

As for inspection I'd remove some foam as low on the boat as possible (ie below the water line). If it sticks to the hull down there you should be ok, if not you need to investigate further.

Also, if done a long time ago the PU was better as they used freon which supposedly lead to better cell structure.

And PU is quite sensitive to temperature and mixture when applied. Press with your finger, of it's spongy there was an excess of polyol, if it's brittle there was an excess of isocyanate. It should give away to some extent but applying "maximum finger pressure" shouldn't leave a permanent mark other than on the surface. You could also to a density measurement if you can get a large enough piece to do it accurately. About 45-50 kg per m3 is to be considered good for spray PU.

 

2" (50 mm) sounds right for cold waters, in tropical conditions you need 2" of foam to have an effective icebox.

 

Properly done I have seen many boats with years of problem free sprayed in place foam. But, always good to check it out.
This may sound kind of 'out there' but I have seen it work and done it myself.
You can do a 'scratch test' on the foam surface to find possible voids or separation: you scratch (or tap) and listen/hear a difference in the 'hollow' tone from one part to another and you may have a void that can fill with water & corrode.