It doesn't matter.

When I got my first free boost, I ask everyone I know this question "when to boost".

The answer is not matter too.

Both the growth and the current ability will be boosted by 20%. Just use it ASAP to enjoy the boost. lol

Highly recommend boost ur Elite Engine first if you are a BB player. I am kinda of mean, so I save my boosts till my brought some Elite Eng/Rep as I want to max the advantage.

Enjoy you boost! :D

As others have said, it doesn't matter when you boost since the ability will be increased by 20% anyway.

The perfect time to boost any sailor, is when you decide that you want to definitely use that sailor in the future. If at lvl1 you decide that you will take that sailor to 120, then boost it then and don't wait to 120 or you will be spending a lot of time with 20% less ability. At lower levels, that 20% sometimes is a great help.

A friend and I were also curious of this, so we conducted a test for this. We bought 2 Elite Accuracy gunners at level 1. We also bought 2 Boosts. We immediately boosted one of the gunners at level 1. After that, we leveled both sailors on the same ship up to level 12. At level 12, we boosted the other Accuracy sailor, and the results of each sailor were the same. As the others said above, it doesn't matter when you boost the sailor.