I don't care for Sentinel Frigates, either. I took it once as a starter ship, regretted it, and I'm disappointed if it shows up from one of the adv. labs.
Like others have said, it's really the 10K energy cost that's a killer - some starships are cheaper! That, especially combined with the massive cluster penalty. It has good damage output and it's tough, but when you can effectively only have one at any particular place, it feels like it has little impact, compared to a lot of the other bonus units.
To me, any ships with cluster penalties are a micromanagement headache: If I have the energy I'd like to keep a set with the fleet, and a set per planet, but then when you bring your fleet to one of your planets, you immediately get bitten by the cluster penalty.
Suggestion 1 - keep the high energy cost and cluster penalty, but make them awesome one-per-planet defenders:
- Energy cost and cluster penalties remain the same, reduce ship speed to 10 or even less, add a high-range cloak detector, add a built-in forcefield generator the size of the Mk 3 Riot-starship's that units or turrets could fire out from without penalty. Damage to the forcefield would go directly against the sentinel's hp, like a force field generator. (I think this would effectively make them fairly fragile since they'd be taking a lot more hits as the protected the covered units/turrets, so maybe their health would need to be bumped up too.) This is my favorite suggestion.
Suggestion 2 - make them energy-cheaper and more useable in quantity:
- Reduce the energy cost to 500, maybe increase the material cost a bit, double or triple the unit cap, and reduce the cluster penalty to something like .7 or .8, reduce the per-shot damage back to the old values
Suggestion 3 - reduce energy cost, make them a fortress-like unlockable instead of a bonus shipm build from command Def menu.
- Reduce the energy cost to 1000, increase material cost a bit, remove cluster penalty, no wormhole travel. This would let you build 'em all in one system, but you'd be stuck with them all there.