hot-mop tar over plywood for budget houseboat hull construction?

[Squidly-Diddly]

Any reason this wouldn't work? IIRC there are a few grades of hot-mop tar and some are heat resistant for sitting in blazing sun all day and still remaining semi-walkable and others are more flexible to maintain seal in cold weather and where movement (from expansion and contraction) may occur when still at near freezing temps. I'd go with the low-temp more flexible for a houseboat hull.

Main benefit is cost, as well as at least initial reliability and some added strength as tar will glue everything together, and fast cooling time for quick builds, and subsequent coats bond perfectly to prior coats, as will any repairs using either hot-mop to melt in or any of several cold bond products.

Of course hot-mop tar is extremely dangerous to work with, and more so if doing something weird for the first time.

Can't get a fix on cost per 100# keg, but I've heard the cost of materials is negligible.

I'm thinking of open top 4' deep pontoons out of 4x8' plywood or OSB with 2x4" frames/bulkheads every 4', everything mopped inside and out. Maybe mop the frame and bulkhead pieces prior to installing, then mop again once installed, so if a leak and flood occurs the inside wood should still stay dry and a leak wouldn't start a chain reaction of swells and leaks.

I wonder if a hot-mopped hull would be major environmental violation in most areas. Never heard of anyone complaining about run-off from roofs and roads and pretty sure I've seen tar coated pilings everywhere.

 

[gonzo]

You will find out why sailors used to be called "tars".

 

[bajansailor]

Would you still use (eg) epoxy to glue everything together, perhaps with screws as well?
If you precoat everything first with tar, then you would have to sand off the tar in way of the epoxy joints, which could get a bit tedious?

Is it possible to apply paint over this tar coating? I am thinking not - even coal tar epoxies leach through paints that are applied over them.

Maybe the best reason for not doing it, is that nobody else appears to have done it so far....... and if they had, and they were successful, I am sure that we would have heard about it

 

[gonzo]

No, nothing goes over tar. However, tar does go over everything. It also gets the surfaces to over 200 F (93C) in a sunny day.

 

[Squidly-Diddly]

Would you still use (eg) epoxy to glue everything together, perhaps with screws as well?

No, I'm thinking not even screws, just common sinker nails and low quality plywood for a low cost, low tech build, and hot tar over everything.
One reason I'm suggesting this, besides the other thread about houseboat building on a budget, is I remember being really impressed at how, decades prior, a hot tar job had really seeped into, glued together and preserved different pieces of wood of an old roof structure I was tearing down, even though that was just an unintended consequence.
It seemed as good as epoxy for the purpose but at I'm guessing less than 1% of the cost. Unlike epoxy or other glues or paints, hot tar is useful and water tight almost instantly. It will be ready by the time you put the mop and bucket away and walk back. In any corner of the world there should be a Hot Tar contractor or two, and even in Silicon Valley, CA is pretty cheap to have them come to your location and do the dirty work.

I'm also thinking, semi-long term, it would be workable to haul out, knock a few of the bigger barnacles off, and re-tar top and bottom, inside and out. Much unlike a fussy fiberglass, expoxy or re-paint, it would be fine to just slop the hot mop over everything a few times and walk away and put it back in the water 2 minutes later, providing you can flip the hulls upside down during the hot mopping. You could do this on any sand and if sand sticks to still warm tar its OK and if tar slashes onto sand you can toss the sandy tar into the pot and its all good and forgiving. Might even be considered Eco-Friendly in overall lifecycle. No sanding, masks, solvents, masking tape, spray cans, tyvek suits, power tools, etc required. Just old school propane fired hot-pot (and operator) already in use for shower pans and roofs.

 

[Blueknarr]

As Gonzo says

You'll find out the root of "tars"
Everything on the boat will get some tar on it
Tarring was standard treatment a few centuries ago. It barely worked then. Why ignore centuries of technological advance?

 

[Mr Efficiency]

I would say adhesion would be questionable. As for painting over, two problems, stains bleeding through can be fairly easily fixed, with specialty sealers, even aluminium-pigmented paint is supposed to stop it, but a bigger problem might be your finish paint creeping on the shifty substrate that softens in a hot sun.

 

[kapnD]

Most roofs are horizontal surfaces, to which tar is applied to the upper surfaces, sometimes on mild slopes, but never on steep surfaces, as gravity plays a big role in keeping the stuff in its intended location.
Good luck applying hot tar to anything steep or upside down, it’s quite runny in its spreadable state. (And HOT!)
You’ll likely get better adhesion on rough surfaces, maybe CDX, but check the specs on the glue, it might not hold up under the extreme heat involved.
Try some test patches on plywood, maybe using some fiberglass fabric to add strength to joints.
The newer “hot melt” roll roofing products seem to be taking over the mop down market , you might want to investigate that as well.

 

[Squidly-Diddly]

I'm thinking the hot tar would be applied horizontally and the square cross section 4x4' pontoons would be rolled during the application to present a new flat surface. Yes, I was thinking of rough CDX, good tip on plywood glue and extreme heat. Might use Pyro-Guard fireproof (and rot proof) plywood which doesn't cost much more if heat is issue with standard CDX or OSB. Pretty sure tar will stick real good to dry plywood and stay stuck under water, but the part that is above water might get soft in the sun and want to droop or otherwise de-lam, so either have enough overhang to prevent direct mid-day sun or drape some sort of covering over the gunnel and down to the waterline as sun-shield. Direct sun will attack roofing tar which why they put gravel over tar roofs.
This would be for pontoons under a (mostly) stationary houseboat, with some "house" overhang, so shouldn't be any tenant/tar contact, so no "tars" aboard. They might be open topped, for storage or to house various system and holding tanks, so just line with cheap blue vinyl tarp to keep tar at bay.
"It barely worked then. Why ignore centuries of technological advance?" Barely for ocean going ships, this just has to sit in calm water. Aluminum pontoons cost big money. This is for an extreme low budget build using low quality scrap wood and basically free hot mop tar that exists everywhere.

 

[gonzo]

basically free hot mop tar that exists everywhere.

Only in the land of your imagination tar is given away for free. On planet Earth it has to be paid for. Further, you need to have a hot tar kettle and the fuel to heat it up. Then there is the cost of mops and other ancillary supplies. Finally, tar is not used on its own. Roofs have fiberglass laminated with it so it won't alligator, crack and delaminate. Finally, the hot tar is covered with gravel for protection.

 

[Mr Efficiency]

I think there are primers specified to be used under some tars, and timber is not the most forgiving of substrates for any coating, in fact one of the least, varying according to wood species, so the right primer would be essential. The extent to which re-inforcing fibres can be mixed in before application, while retaining the flow of it, might be a matter for experiment.

 

[KeithO]

I think there could be an argument for an aluminum unpainted barge type hull being the most cost effective over time since it never needs to be sand blasted or painted and just anti foul on the bottom to keep the growth off. Yes, its a bit more tricky to weld aluminum, but I think that is more than offset by the long term advantages. A steel barge would probably be the cheapest in terms of materials, but blasting and painting would not be cheap unless you happen to own a spare screw compressor like Doug from SV Seeker... The painting is never going to be cheap and most of us do not have such good vendors as Doug has in Tulsa. Today any decent supplier could supply you water jet cut plates that are very accurate with notches and tabs to control the position of all bulkheads and allow a significant amount of welding to be done on the outside when assembling it. Then it would be a matter of doing sealing welds for watertight compartments on the inside.

Somewhere you are planning to put a superstructure on top of the barge and there has to be a balance between the cost of the "house" and the cost of the barge and the risk of finding it all on the bottom...

 

[Squidly-Diddly]

I think there are primers specified to be used under some tars, and timber is not the most forgiving of substrates for any coating, in fact one of the least, varying according to wood species, so the right primer would be essential. The extent to which re-inforcing fibres can be mixed in before application, while retaining the flow of it, might be a matter for experiment.

Some painter once told me paint or similar will bond to lower quality porous wood like cheap plywood very well because it soaks into fissures and large grained surface so it resists cracking and peeling. I've done a few demo jobs where tar had been splashed up on plywood and IIRC it was always still very stuck even decades old in varying mild CA weather.
Yeah, some fiber not a bad idea but I'm thinking they could be just about anything that will stand the heat for a short time such as old cotton or linen (besides purpose made fiberglass). And that seems to be mostly an issue with roofs that do lots of contracting and expanding. Also thinking 1/2" plywood at 90deg stitched to 2x4 frame with nails will have less of a movement issue than a caravel planked over frame ship. Shower pans normally last for 50yrs or more and I've seen Hillbillies just bust out the tile and repair with hot mop and throw down a piece of plywood to keep tar off bare feet and it works for years and doesn't leak (2nd story bathroom).
Last I checked a Hot Mop guy will come out for $150 or two pans for $200 and they don't ask "how big?", and that barely covers driving over and getting out of the truck around here.
I'm thinking about eight 24' x 4' x 4' pontoons in 4 rows space 4' apart to make a 48'x 28' platform, and additional 4' overhang all around for 52'x32' building deck. The pontoons could be individually flooded down and detached for maintenance, one or two at a time, leaving the main structure floating in place.

 

[Squidly-Diddly]

Somewhere you are planning to put a superstructure on top of the barge and there has to be a balance between the cost of the "house" and the cost of the barge and the risk of finding it all on the bottom...

yes, this would be a Shanty for sure where low2no cost would be its main benefit, but as stated in other post the pontoons would be removable and replaceable so upgrades or additional floats between the TarWood pontoons a possibility.
Quite often there will be several hundred sheets of perfectly good 1/2" CDX on offer for FREE that were used for temp protection during construction, so I'm trying to come up with good use.

 

[KeithO]

Free plywood in this day and age where its over $30/sheet ? That must be one of the reasons homes cost so much in CA ?

I have previously come to the conclusion that marine plywood is more expensive than PVC foam as a core material for a composite hull. And it would be considerably heavier too. Yes, one would have to add more fiberglass to the facings which means more laminate and more epoxy but the end product would still be better.

Now you mention pontoons, so now I'm a bit confused. Perhaps all you are trying to build is a deck to provide structure and attach pontoons on the bottom ? Pontoon logs have gotten bigger in diameter over time and have pretty nice robust attachment flanges on the top so if you use enough of them, I'm sure that can work. Leave some inspection covers so that you can drop bilge pumps into them if needed and a way to detect water intrusion.

yes, this would be a Shanty for sure where low2no cost would be its main benefit, but as stated in other post the pontoons would be removable and replaceable so upgrades or additional floats between the TarWood pontoons a possibility.
Quite often there will be several hundred sheets of perfectly good 1/2" CDX on offer for FREE that were used for temp protection during construction, so I'm trying to come up with good use.