The harpoon is pretty powerful, yes, but it's just as risky.
If you use it in close range, you will face a ship ramming you. A galleon can handle the damage and maybe a junkers armor too, but if your armor is low on health or gun in total you will have to face a bit of permanent damage and you cannot escape it yourself. If you hit the enemy wrong then they can still shoot while ramming you, not so good.
If you miss in long range, you will be faced with a rather long reload time. However punishing that is depends on the scenario (you wanna pull the enemy away from your ally or pull them back into the fight to kill them in a 2v1?).
I mean, fine, go 5 harpoon mobula so the enemy can't move, but you can't move either and you might kill yourself in the process.
The harpoon works only so good, man, don't make it worse. If we were to (e.g.) half the force the harpoon currently applies will not be a solution, it would still overpower most ships engines.
The harpoon is an utility gun in not that it disables ships, but in that it changes engagement (at least that's what I think the harpoon is supposed to do). And it fails miserably at that, with the exception of certain scenarios (galleon back seems to be reliable enough).
I have a harpoon that is good for ramming ships or forcing ships to ram me in close range (even though there are better and more secure ways to kill ships in close range), but is really bad at everything else.
I will be honest here, I want a harpoon that allows me to pull ships back into battle incase the enemy wants to escape, which is not possible because physics.
I want a harpoon that allows me to force long range ships to go into medium/close range, which is not possible since the duration last for 5 seconds and that doesn't work for most of the time, I have not tested that reliably though.
I don't want a gun that makes me play bumper karts in mid-air.
What do you think the harpoon is supposed to do? If the force of that gun were to be reduced so much that half of the ship can simply move away from it, then I don't see the point of having the harpoon in the game. Or we are going back to the old harpoon, which negates every effort that has been put into this gun to make this gun better.
And if it takes too long for the harpoon effect to kick in, it might just be too risky to use and people would think if another ship or gun combination choice wouldn't be better instead.
What do you want the harpoon to do? Fish some birds?
Btw, that new heavy gun sounds nice.