Cracking problem

[Hubzol]

Hello Guys,
I would like to ask your help.
I am building a mahogny motorboat. The seams of the mahogny boards crack a litle bit after few days of varnishing, it started in the workshop but on the sun it was more powerful.
I attached a picture.
- I used 3 layers of wood (2 pine, outside mahogny 3/8 each).
- I used polyurethane glue (waterproof, Technobond D4)
- I used acrylate dispersion base wood filler
- I used yacht varnish (AkzoNobel brand)

Which was the bad choice and what can I do?
I really wouldnt like to install a new layer of mahogny with a different type of glue...
Thank you and have a nice day

 

[gonzo]

It is doing what is normal for wood: moving with changes in temperature and humidity. You would have to encapsulate the wood in epoxy and then use a UV blocking clear finish to minimize it.

 

[Hubzol]

Thank you Gonzo, I already started to sand off the varnish, i will apply stain again and the epoxy after that.
I hope it will help.
Thanks

 

[fallguy]

It won’t help to redo it all. Wood expands and contracts.

And it expands for two different reasons.

It will shrink when it dries and expand a bit when taking on moisture.

The other reason is heat. When the sun hits the boats; it heats different parts of each board and they shift and move, The movement can even be different within a single board as dark spots heat more than light spots.

About the only way to mitigate this is a light layer of fiberglass, like 4-6 oz woven. The fiberglass creates a more uniform surface and the expansion and contraction is more even.

If your boat was built inside a climate controlled area, when it goes outside, it will take on moisture and the wood grows. If finished in winter or dry season, it’ll grow in a rainy season. Built in cool weather; it’ll grow when warm.

The boat looks great.

@gonzo is correct .. this is all natural for wood

 

[Hubzol]

Thank you Gonzo, just epoxy could help or I need fiberglass cloth, too? I really dont like it but if I have to...

 

[fallguy]

Epoxy alone will not stop the movement of the wood. It will create a harder seam that is more pronounced when it cracks and it will.

Epoxy and glass doesn’t stop the movement completely either, but it makes it more uniform movement.

 

[Hubzol]

Thank you

 

[gonzo]

The glue is advertised as good for indoor use or outdoor with surface protection and not suitable for static loads. That indicates that it should not be submerged or in high humidity environments. Also, it will creep under continuous loads, which may be part of the problem. What kind of varnish are you using? If it is too brittle it will crack.

 

[Hubzol]

Thank you for everybody the help and advices!

 

[Dave G 9N]

As has been said, moisture expansion is the most likely problem. From the limited information available, I can only speculate. If the layers of strips were laid perpendicular, The thickness of the strips may have been the cause. The seams opened as the boat was warmed up and was worse when heated by the sun. That says that the mahogany was shrinking more than the pine as it was dried by the increase in temperature. I think that the thickness of the strips might have made the problem worse, based on the assumption that the layers of strips were laid up perpendicular to each other.

Moisture can only be controlled by completely encapsulating the wood to reduce changes in moisture content. There are no coatings available that are completely impermeable, but most will slow the diffusion of water vapor. A layer of fiberglass is often applied to encapsulated wood. The strength, stiffness and low expansion of the glass fibers perpendicular to the grain prevents checking and cracking.

Any significant change in dimension will be caused by changes in moisture content, not thermal expansion. Heat will dry wood rapidly, so thermal expansion is not a significant part of the overall change. Thermal expansion in wood is so much smaller than moisture expansion that it is rarely a problem or even measurable. This is explained in much more detail in the Wood Handbook than can be done here. There is a link below.

If laid parallel, the differences in radial and tangential expansion between species will stress the wood. As moisture content increases pine will swell more than mahogany and the mahogany will be stretched and tend to crack. As the moisture content decreases, the mahogany will be compressed and the pine will tend to crack. This indicates that the strips were not laid parallel because the sun would dry the wood and the larger shrinkage of the pine should put the mahogany under compression. The varnish should slow drying, so it is not likely that the mahogany dried much faster than the pine.

If the strips are perpendicular, the longitudinal shrinkage of any species of wood is very low, so some shrinkage cracking can happen regardless of the different species used. If they are laid perpendicular, then the differences in moisture expansion are less important, and the thickness of the strips will have more of an effect. If the layers were thin as in plywood, the much greater stiffness and low expansion in the grain direction stabilizes the wood. Thicker layers are less easily constrained, so as the thick layer dries the outer surface can crack.

I do not think that the adhesive was the cause of the cracking, but a stronger adhesive might have prevented it from happening so soon. Technobond D4 is a moisture curing polyurethane glue, and for the Americans here, may be similar to Gorilla glue. Both Technobond and Gorilla glue expand when they cure. Most PU glues expand by foaming because the cure process releases carbon dioxide. Thechnobond may foam like gorilla glue, but hopefully not as much. Structurally, this tendency to foam requires heavy clamping and tight joints. Foaming results in very weak joints if not clamped securely and moderately weak joints if the joints are very tight and well clamped. Technobond may not foam as much, but I have no way to know that. In the States, Gorilla glue has a poor reputation among boat builders because it foams so much. Gorilla glue can be used to fill small cracks, up to 6 mm or more wide with a low density foam.

There is a considerable difference in shrinkage between mahogany and pine. I may not have chosen the same species of mahogany and pine as were used in this boat.
Shrinkage:
Austrian pine Radial: 4.1%, Tangential: 7.3%
Honduran mahogany: Radial: 2.9%, Tangential: 4.3%


Index Wood handbook: Wood as an engineering material | US Forest Service Research and Development https://research.fs.usda.gov/treesearch/62200 ---> Chapter 4: Moisture relations and physical properties of wood Chapter 4: Moisture relations and physical properties of wood | US Forest Service Research and Development https://research.fs.usda.gov/treesearch/62243
Austrian Pine | The Wood Database (Softwood) https://www.wood-database.com/austrian-pine/
Honduran Mahogany | The Wood Database (Hardwood) https://www.wood-database.com/honduran-mahogany/

 

[Hubzol]

Thank you for your thoughts!