Out Board Engines and transom height

Hi,
I am making my self a small 18 feet boat to go out fishing.
The Sea Conditions here are rough compared to most countries.
The inital design I was told to do the trandsom height at 16 inches.
But recently somebody told me to change this to 21 inches and go for a long shaft engine. I was told a long shaft engine will provide more stability.
Is there any truth in this. Or should I proceed with my inital plans.
What do you guys have to say about this.
Also let me know what other improvements I need to make this more stable.
The boat need to have speed as well as must be able to take the pounding waves.

Manoj

 

Well the lower the weight the more stable the boat but then the lower the transom the more water can get in the back .... I would go for stability you can't bail out a upsidedown boat !

 

The total Weight of the hull is estimated to be around 300Kg. Maximum engine capacity allowed is 25HP around here due to security situation. Restricted with the limitations what is your idea.

Manoj