it depends on how much you shoot your target down before starting boarding and if you suceed in fulfilling all boarding Support Missions - and with rookie Marines you can forget to board big targets right from the start, you have to train them on smaller Targets like weak Freighters first

OK, thanks for that.

To capture a ship you need to damage the main hull down to atleast 20% with fresh troops, select the ship to be boarded and open up the info window and look at the "repel boarding" info the lower the value the better chance to capture, anything over 100 forget it and take down the hull value, anything under 75 should be ok to capture but you may loose troops.

Thanks. I reduced a titurel's hull to 7%, captured it without losing marines but lost it to reivers soon after.

I us the ship Boarding recistance and usually bring it between 15 and 19 and that usually works fine but there is a mod needed to stop other ships shooting the ship you are trying to board like allies or other's as I was boarding a ship with my marine officer and 50 marines then an HOA heart of albion ship jumped and blew it up