I have found that the most important thing about heat bending wood is that the grain is straight with no run out, or at the most, very shallow run out.

My current coaming form is an egg shape, and by the time we (having a friend help to place clamps is helpful) get to the sharper front of the coaming, or, if we make it past that, to the last curve coming around the coaming aft, the wood is starting to cool significantly and we have to go slower to let the lignin slip.

We found it helpful to have the shop iron standing by, billowing steam, and have the assistant apply that to these second and third radiuses as pressure is held, until the wood fibers slip. You can feel it relax -- don't be in a rush and try to go too fast -- gently pressure until it feels tight, then hold and you will feel it relax. A shop rag folded and laid against the local area that needs to relax seems to help hold steam/heat there from the iron. These are just our solutions at hand to help with the heating. With a recalcitrant piece of wood, I've also stopped the bending at the coaming apex (having clamped the tail scarf and along the port side, we wrap clockwise due to the setup of the coaming form on our table) and wrapped the wood with rags and just steamed that part with the iron. You can work it in stages like this if needed.

I've by no means become a wood bending master, but this was a very frustrating and mysterious process for me, initially, so I'm pleased with success however I can achieve it. The more you bang away at it, the more you learn what matters, and figure out how to make it happen.

You can also bend your coaming in two (or more?) pieces and scarf them together.