Q. How to use destroyers effectively

I use them for heavily armored boarding ships.

Well if you have a good CSM, you can make two bombarder class destroyer - and one Fast Ammo Carrier built on a Destroyer Hull, One Escort Destroyer

Depends on how big your Destroyer hull At lest lvl 4 or 5 for these designs

Bombarder - .
Should have around 3 or more CSM, and max engines as this ship is not meant for ship to ship, but to sit at max range from stations, planets and pound them in to the ground. One Long Range Pd - two Medium Range Pd - two short Range PD. Ecm and Combat sensor, Fill reming space with armor Any Space left over add two ordnance supply pods, and supply pods.

- Also works good as a warp point defense ship, just put it just a little bit inside its max range to the warp point.

Strategy - Max Range

Escort Destroyer -
Max engines - One CSM - ( You want it to stay with your Bombarders ) 3 Long range Pd, 6 Mixed Medium Range Pd , 3 Short Range Pd - try and get different fire rates on your Pd Combat sensor + Ecm and max armor - Fill space left over with Shields and more PD

Strategy - Max Range Escort (custom Strategy) Sets Seekers targeting as highest on both self and friendly, then Fighters) ( in stock a Pure PD ship will lose to = cost fighters 100% of the time)

Fleet the Escort and the Bombarders

FAC - Fast Ammo Carrier
Max Engines - 1 Long Range Pd 1 Medium Range Pd 2 Short Range Pd Max Armor - Combat Sensor fill at least 50% remaining space with ordnance pods, Fill rest with shields , ordnance pods and supply pods.

Strategy - Dont get hurt

Good for glassing planets, - they stay out of range of most direct fire weapons- and can kill most Archer style frigates in small groups.

Hope this gives you an Idea

I would be wary of using CSM only ships.
Weapon mounts of weapon platforms can allow some guns to equal or exceed the range of CSMs.

I would also prefer to use Quantum Torpedoes, they have comparable range and power, are 10kt light, fire a little faster, and I think are also a little cheaper.

I try to make my ships survivable, since I am in a position where my defense is better than my offense.
My main Destroyer design (a defense ship) has crew and a master computer (at least one enemy race has the toxin injector weapons on their ships), neural combat net, ecm, combat sensor, phased shields, shield regenerator, security station, 50kt of armor, 100kt of weapons (usually 2 meson blasts and 2 anti-proton beams or the equivilent), 2 pd beams, medical bay, quantum reactor, max engines, plus a basic sensor and the required stuff.
The recon variant only has 40kt of weapons (2 meson blasters or equivilent), 45kt armor and 1 pd gun, it uses freed up space to add in a tachyon scanner, long range scanner and a mine sweeper.

They generally operate as support for my cruisers, which are my main torpedo ship, in addition to working with some frigates. They are also used to watch warp points that see little enemy activity and also to act as secondary planetary defenders (hence the medical bay).

It may sound expensive, but I lucked out and found a few huge planets with the right atmosphere, and recently got the atmospheric modification plant, so I don't have to much trouble building and maintaining more expensive ships now that I can support more resource production...

Before I got the neural combat net and started using master computers, my destroyers also had a single CSM or quantum torpedo, and an extra gun.
the upgraded version of an older one (an attack ship) has 30kt armor, replaces the pd beams with pd blasters, for weapons it has 2 large mount anti-proton beams, 2 large mount meson blasts and a quantum torpedo, plus an ordance storage. I have not actually made any yet since I have a few cruiser designs that are my main attack ships.
For planetary attack, I would make a desisgn that would remove the 2 large mount meson blasters and torpedo and toss in 3 large mount planetary napalm and an extra 10kt of armor.

One idea I never really used, removed 5kt of armor from the attack ship, and give it 10 meson blasters for weapons.
Flying shotgun anyone?

"Aiming? What's that? We just point the front of the ship at them and hit the big red flashing button that makes stuff explode!"
*lol*
(got Orks on the brain now...)

On a side note, can't fighters be equipped with small weapons useful against planets? Such as small cluster bombs, small anti-matter torpedoes and small rocket pods?

DD's are a good size for ramming ships also.

'Ramming 101' in the FAQ's has good info:
http://www.spaceempires5.com/en-US/node/3380

The DD is basically a bigger frigate. It gets less defense bonus for size, but it can still easily match the frigate's speed. Conventional wisdom is that frigates beat destroyers in simple direct-fire combat due to the defense bonus.

So one needs to look for applications where bigger is clearly better, and the d-bonus doesn't mean as much.

I have yet to do it in SEV but a tactic I occasionally used in SEIV was to equip a DD with a planet destroyer, stealth/cloaking and the bare minimum of other stuff.

Fly (cloaked) into enemy system, enter orbit of one of their larger planets, decloak, destroy planet, cloak and fly back home for repairs & resupply. Rinse and repeat. Eventually return with a planetbuilder ship to reconstitute anything that got dusted.

Handy to nerf an empire that was getting uppity when I didn't have sufficient military on that side of the quadrant to keep them in their place.

In SEV I have yet to have need of such a tactic, and Stellar Manipulation is so much more expensive to research that I'm unlikely to even try. But those fond memories of old times...

Back onto topic, I often have my destroyers equipped with supply-using weapons so I don't have to worry about ordnance. Their solar panel(s) keep both the engines and guns topped off, so no matter where they are they're still effective.

Fly (cloaked) into enemy system, enter orbit of one of their larger planets, decloak, destroy planet, cloak and fly back home for repairs & resupply. Rinse and repeat. Eventually return with a planetbuilder ship to reconstitute anything that got dusted.