Here is the deal. I am looking for some advise and possibly a boat builder.

Recently I purchased a new 70hp 4 stroke for my 16' boat. The problem I am having its the boat is way to heavy in the back. If 2 people get in the very back it can go under water. I moved the battery and gas tank to the front for ballast and it still is alot back heavy. I have taken approx 300 lbs of sand bags and put them in the front of the boat and it seems to equal out the boat. My question is :
Is there a way to add extra floatation to the exterior of the boat to account for 300 lbs?
Any thought or suggestions would be appreciated.

What horsepower rating do you have on the manufacturers plate? Did you have a smaller lighter motor on before? It sounds like you're putting far too much weight on the transom for the balance of the boat.

You really need to provide some info on the boat and its rating as found on the capacity plate. In a perfect world, you could do some things to improve the trim on your boat. However, in the real world, unless your boat is a wooden stitch-and-glue craft, there isn't much you can do that will have an effect. Even then, you would have to go through some serious rebuilding to get what you want-- a wider stern.

Another down side is that by adding weight, such as the 300 pounds of sand bags in the bow, you are affecting the handling and freeboard of the boat. My hunch is that you'll find you've exceeded the weight capacity of your boat. That is a dangerous situation.

You may want to put the 100 Qt. cooler full of beer in the bow.

But seriously, how wide is your transom? Your boat may not be designed to have that much weight hanging off the back. Check the Coast Gaurd tag on the transom.

Even if your boat functions OK you will surely attract attention from the water constable with your bow sticking up in the air.

Fishmasterdan

First, tell us about your boat! Is it powered by a "jet pump" or is it powered by a prop!

The four strokes are heavy. If moving the fuel and batteries to the bough is not enough, I doubt that adding ballast with sandbags will do anything to improve the safety or performance of the boat. If two guys in the stern sinks the boat, you should probably replace either the hull or the motor. I bet this is not what you want to hear but you will be better off with a well balanced boat than some kind of added flotation or other bandaid type solution. I would think long and hard before modifying the hull.

The beam is 64" and the boat is rate at 1350 pounds.The motor is a 70 hp jet so its actually around 56 hp and the boat is rated for 60 hp.
The model of the boat is a valco bayrunner.

The four strokes are heavy. If moving the fuel and batteries to the bough is not enough, I doubt that adding ballast with sandbags will do anything to improve the safety or performance of the boat. If two guys in the stern sinks the boat, you should probably replace either the hull or the motor. I bet this is not what you want to hear but you will be better off with a well balanced boat than some kind of added flotation or other bandaid type solution. I would think long and hard before modifying the hull.

There is no way to add flotation to the back of the boat inside the hull. That is a function of the width, depth, length and shape of the hull itself. The hull is going to sink into the water until it displaces an amount of water that is equal to the weight of the boat. If the weight is at the back of the boat, the boat tips backward and the displacement happens all at the back, as you observe.

You can play around with the balance of the boat, but it sounds to me like you may have a bit too much motor weight for the hull. The only fix I can see is either fewer people or less motor weight. Sorry.

It looks like this could be an expensive mistake.