Ballast questions please

Hi could anyone give me some guidance on using ballast please.

Here's the scenario:

I am building an aluminium hull tug boat.

I am at the stage where the hull is water tight .

It is sitting very high in the water at present.

I have loaded it with temporary ballast of steel blocks and water containers.
They weigh 33.7lbs/15.3kgs

I am quite worried that the hull is displacing so much water.

Could you any share thoughts on this amount of ballast for such a small vessel and can you suggest suitable ballast for the 
finished model please.

Here is the model tug boat. Thanks for any feedback and guidance.