Cannon Barrage

I love this.

This is mostly used as a large AoE slow, since the damage in most cases is negligable. However, this can work to scare people of your towers, or to kill off large, pushing minionwaves from the opposing team.

This ability is very good but only onward mid game, because after they get items the dmg is lowered and the only way to get it high is AP which i dont think anyone will buy.Still it has global range and it is mainly used vs wave of minions(defending).Still i think they should make 20% of AD instate of AP or more!

This is an awesome way to ks Baron from the other team :)

Global ult, you can direct it anywhere regardless of position

Uses:

Steal Dragon / Baron
Take out really low health champs running away or hiding by turrets
Defending a turret from minion waves while away
Assisting other champs in a gank while away
Slow enemy champs from escaping or chasing teammates
Chip away damage faster when being ganked.
Reveal an area [this use is not recommended, unless you're predicting the path that a low health champ is going to take]

This is gangplanks hardest skill to master since perfect placement for maximum damage is very hard to do. Especially while in a gank since enemy champs can move around. Have exhaust at the ready for better results.

I find this skill most effective when I get ganked since the combo Cannon Barrage->Exhaust Spell->Raise Morale->Parrrley + Grog Soaked Blade Passive means gg for at least one of them. in 2v1 you end on top thanks to remove scurvy, On 3v1 you at least leave them all pretty wounded.

Another good use, not as great as the one above (IMO), is for helping your teammates run away from ganks gone sour. This ult slows so that plus parrrley (which also slows) the chaser closest to your low health teammate and you'll save their life most of the time and on top of that deny the other guys 300g (or more if the one saved is on a killstreak). Similar use is to slow the enemy down, it won't be be by a whole lot. But if you have ghost/flash it can very well be the amount you need to pick them off.

If you think that the enemy team is at dragon or baron. Throw this ult at that location. You might steal the kill. I've done it a few times, but it doesn't always work. Its an awesome trololololo though if you pull it off =)

Last piece of advice for this ult, and its a very situation specific one: defending a turret. Now, depending on teammate position, who is attacking the turret (minions/champs/both), health of the enemy champs if they're attacking, who you are laning against, etc. It might or might not be worth it to use this ult. This is one of the tougher choices I've had to make and I leave it up to you to decide.