Jay wrote:
If a person strapped TWO of those Suzuki 2.5hp outboards (only 29 lbs.
each) to the back of the boat, one on each side, what would be the
effect? 5hp? Double the speed? Double the noise? Nothing?

In your case, not much.
At 2.5 - 5.0 hp, you're dealing with a displacement hull. So the
difference in those two hp's doesn't do much for speed (noise yes,
fuel yes, speed no).
But if you started with 10hp, which would almost plane (but not quite)
your boat, doubling the hp to 20hp would make a huge difference in
speed. That would be enought to switch from displacement speeds to
planing speeds. Big difference.
So everything matters. Weight, drag, hull shape, like that. But the
biggest difference in hp/speed ratio is when the hp is enough to get
the mass over the displacement/planning hump.

Rick