It has something to do with the ui bug that displays shuttles as a character's main ship.
engines effect shuttles more. the tooltip in inventory is referring as if it was on your shuttle.
when you equip it on your ship, the game changes its stats to the ship version.
+ 4 is still decent bonus. most cruisers have less than 15 overall
There is the base stats shields, engines, and weapons, and then when equipped those stats are modified by the ship class. So like a shield (where the modifier if clearest) has a rating of 5000. On a cruiser, with a 1.0 rating, it stays 5000. On an escort, with a 0.9 rating, it drops to 4500. On a science ship, with a 1.3 rating, it rises to 6500. Base x modifer = total.
Engines do the same thing. I don't know what the exact modifiers are for engines on cruisers offhand, but its a fraction of the base rating. Depending on engine type (combat or hyper impulse), the ratings are also modified by your current engine energy. Even with the best hyper-impulse engine and lots of energy devoted to it though, cruisers still turn slowly. Thats just how they are.
The majority of cruisers (but not all) have an engine modifier of 0.15.
However, as far as turn rate goes, each ship also has it's own base turn rate. You can see this modifier in the description of the ship at the shipyard listed as 'Base Turn'. http://www.stowiki.org/Federation_Cruiser lists all the Federation Cruisers base turn rate in the 3rd last column s well.
This base turn rate is in turn affected by your skills, engines, current travelling speed, and other factors.
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