Why?  The drones have sufficient range, speed and accuracy to engage the target. A lightweight torpedo would wind up reducing the payload. 

Jeff (Jefffar) said:

A lightweight torpedo would wind up reducing the payload

Also would substantially increase cost; 'real' torpedos seem to cost as much or more then missiles. 

The appeal of these USVs (as well as the danger of them) is they are cheap, use mostly commercial off the shelf components, but pack a big punch.

Just about the right amount  of punch too. An almost guaranteed mission kill on even a large landing ship, but not so much as to sink it.  

Enemy has to dedicate resources to towing the victim back to port while all the time wondering if there's another USV homing in on them.

I wonder what they did to boost the effectiveness of the warhead?

Jeff (Jefffar) said:

Just about the right amount  of punch too. An almost guaranteed mission kill on even a large landing ship, but not so much as to sink it.  

Enemy has to dedicate resources to towing the victim back to port while all the time wondering if there's another USV homing in on them.

I strongly disagree with all of that.  The resources -- one or two tugboats -- to tow a disabled warship back to port are negligible.

Plus, leaving the enemy ship mostly undamaged so it can be repaired and returned to service against your guys is shortsighted.

If anything, the Ukrainian naval drones pack far too little punch.  A mission kill is not the preferred option.  Sink that mother!!!  

YouTube

*NEW Camera Views* QUICKSINK Destroys Ship

The Air Force Research Laboratory and Eglin Air Force Base's Integrated Test Team demonstrated a new low-cost, air-delivered capability for defeating maritim...

.

gatnerd said:

The appeal of these USVs (as well as the danger of them) is they are cheap, use mostly commercial off the shelf components, but pack a big punch.

Yes, but like other improvised weapons fielded by Ukraine, they are only being used out desperation and lack of state-of-the-art weapons.

Successful employment seems limited to targets which are either stationary or moving slowly, and usually only during hours of darkness.

Daytime attacks with these cheap USVs against naval vessels which have adequate armament and alert crews are relatively easy to repulse.

YouTube

Russian warship blows up 'Ukrainian' marine drone

Russian warship blows up 'Ukrainian' marine drone: The intense clip, released by the Russian defence ministry on state television, showed how one high-speed ...

But that assumes the enemy can repair the ship in a timely fashion.

The landing ship damaged this week was made in Poland. Most of the Soviet-era ships and some of the post-Soviet ships used engines made in Ukraine. 

Spare parts are going to be hard to come by. 

Jeff (Jefffar) said:

But that assumes the enemy can repair the ship in a timely fashion.

Not exactly.  Your position depends on assuming that the enemy will be unable to repair the ship quickly enough to put it back into use against you during the war.

My position is that -- unless you know how long the war will last, and that the enemy definitely cannot return the ship to action before then -- it is better to sink it.

Jeff (Jefffar) said:

I wonder what they did to boost the effectiveness of the warhead?

The most important step seems to have been improving the fusing so that they actually go off.

There was one earlier in the war where a similar USV hit a Russian warship, but from an angle, and so it failed to whack its nose fuse hard enough? and it didn't go off. 

stancrist said:

A mission kill is not the preferred option.  Sink that mother

I agree, especially from a propaganda / morale standpoint. Sinking has a much bigger impact.

Had the Ukranians just badly damaged the Moskva, instead of sinking it, it would not have been nearly as epic and morale boost / celebrated event, even if the military effect would have been comparable. 

https://en.wikipedia.org/wiki/Sinking_of_the_Moskva

But sunk, this became an event that was literally put on a stamp. 

gatnerd said:

The most important step seems to have been improving the fusing so that they actually go off. There was one earlier in the war where a similar USV hit a Russian warship, but from an angle, and so it failed to whack its nose fuse hard enough?

Either that, or perhaps the angle of impact was so great that the fuzes did not even touch the ship's hull?

On the earlier drones, the two fuzes look to me like they need to impact at somewhat near to 90 degrees.