if its a Crew you see yourself using at lvl 100+ then you better Max out Experts and get 100+ Vets

Experts give 1% of ability your sailor has. Veterans give 4% of your ability your sailors has. Rookies give no ability but they can be promoted to experts by playing or buying them.

You always want experts and vets, because the best your sailor can have is 60% experts 40% veterans.  That is 60% + 40% x 4 = 220% of the ability of your sailor's stats

http://www.nf-guides.com/sailors.htm#b2

the game is based on the True Ability of sailors

if your sailors are full this TA can be calculated as [4*Vets+Exps] * AbilityDisplayed.

you can see that vets are 4 times better than experts. rookies can only cut your TA.

refer to the adalbert's guide on ability for more informations, you can find it in tips and tactics, maybe in the old section.

so, try to gather the most experts you can and, on the vet conversion increaed events, try to convert them all. you can easily do experts going to GB and AAing.

you can also buy or win experts and veterans but with vets packs you can only go to 109 vets, then you 'll have to converts