10ft rowing boat conversion

I have a 10ft,very stable fibreglass rowing boat.
I am toying with adding a fibreglass sheet under the hull similar to the rib design.
I believe on a rib,they have a partition under the hull that fills with water when it is not moving
which empties when the boat gets on the plane.
I may be talking rubbish,but has anybody completed as similar project?
I could be using a 6hp to 18hp outboard on it.
It doesn't quite plane with the 6hp and two people in the boat.
(it's not a planing hull)

 

If it is already very stable, why are you looking to try to make it more stable at rest?
Or does it only have dynamic stability like a bicycle when underway?

I remember seeing a water ballast system on Avon Seariders - Avon has since been taken over by Zodiac.
Here is an explanation re how the water ballast system works.
Avon Searider Water Ballast system - RIBnet Forums https://www.rib.net/forum/f45/avon-searider-water-ballast-system-23565.html
Is this what you are thinking of?

How are you proposing to add this 'fibreglass sheet' under the hull?
Anything that you add below the waterline will simply equate to more resistance = less speed.

If it is not a planing hull, then there is not much point in putting a bigger engine on it - you are probably better off just pootling along with the 6 hp motor and sipping fuel.
If the boat rows well, then her hull shape was optimised for 'slow' (ie rowing) speeds.

 

That's the idea I was talking about,,the lower hull fills and empties.
I have another boat,and this one is just a project.The rowing boat is already very stable at rest,
The gunnels and the seats (which go down solid to the hull )are waterlogged.
This makes the boat very heavy,I intend removing the seats and using the single plank idea,
and removing the timber out of the gunnels and replacing it with something lighter and waterproof.
I have customised a new set of aluminium oars,and the boat is effortless to row,even in it's present state.
I was thinking that maybe when I lighten the boat in weight,I may lose some of it's stability,and the lower hull filling with water may be a possibility.
I may use the boat for a bit of inshore sea fishing,so a bit more stability would be handy.

 

VB, you could perhaps try editing your post above to remove the superfluous photos?
And have you got a photo showing the inside of the boat?

You said :
"I was thinking that maybe when I lighten the boat in weight, I may lose some of it's stability, and the lower hull filling with water may be a possibility."

But it sounds like this extra weight is in the gunwhales and the seats, so the centre of gravity of the weight is already a significant distance above the keel.
Taking this weight out can only improve things all around - you are not going to make her less stable as a result of doing so.
So go for it, and don't worry about your stability - if it is good now, it can only get better if you replace the gunwhales and seats with lighter weight materials.