This is an age old quandary that took me a while, some old salty sailors, and quite a few pints of grog to finally wrap my head around. Why a 100' ocean going sailboat only needs a small outboard to make the same speed as under all the force of her sails. Or also, why 100' sailboats go faster than 50' sailboats. AKA why a 50' sailboat will never beat a 100' sailboat in a race.
Displacement hulls, one's that sit in and displace water, like sailboats or rafts, need very little power to move through the water up to a limit. That limit is a function of their hull length. Any more speed than that will require logarithmically more power as the engine will need to start lifting the hulls weight and begin rising out of the water, AKA planing like a speedboat. AKA Why it takes 250 HP to push a boat not much heavier than a raft at 50 MPH when that's only 10X the speed you can row it. You're not supplying 250 divided by 10 = 25 horsepower rowing energy, right ? Maybe 1, tops.
There is a formula that takes in hull length and will produce the maximum speed. Surprisingly, there is only 1 variable. Maximum Hull Speed in knots equals 1.34 times the square root of the waterline length in feet (HS = 1.34 x √LWL). Hull length is the amount of the boat actually under water. Knots converts into MPH with : 1 Knot = 1.15077945 mph. So, for a 14' raft, the maximum speed to efficiently move would be 5.76 MPH assuming all 14' is in the water. Notice there is no weight parameter in that equation. More or less weight will require more or less power and fuel to get to or reach that maximum efficient speed.
Any more speed than that is at the expense of quickly increasingly HP needs and fuel consumed as the hull starts lift out of the water and plane. A 10,000 HP motor will not move a sailboat or raft any faster than that hull speed if enough weight is added to prevent it from rising out of the water and becoming a planing hull vs. a displacement one. Lifting out of the water.
With a planing hull, the vessel starts becoming more and more inefficient, requiring more and more power and fuel on a quickly escalating curve as the speed increases.
The rough rule of thumb for determining Horsepower Required for a Sailboat is one horsepower per 500 pounds ( ± 50lbs ) loaded. No hull speed required for this calculation. Strictly weight. Why ? Because any more power will begin moving the hull up and out of the water, planing, and becoming inefficient. Weight of the boat vs. weight of water. That simple.
I would wager a 2 HP motor would have moved you very nearly as fast as the 5HP on perhaps half the fuel and a fraction of the weight if you could have found a smaller form factor motor.
There are some smaller form factor motors already around that scale to maybe 15 # and fitting in a 8" X 8" X 36 " box. They may move a raft at about 2 - 3 MPH or more, but to my knowledge haven't been tested yet.
I have one I'd be willing to test on a raft if anyone has time and is in the Montrose area. Maybe at Ridgeway or Sweitzer if you have a registration number on your raft.
PM me if interested.