An inflatable dinghy is not a displacement hull vessel..... if that were the case you wouldn't be able to get it up on plane with a properly matched outboard. That's just not the way theoretical hull speed works..... Besides.. if you set your tow distance properly... so the dinghy rides the front of the stern wave... the resistance will be reduced tremendously. There's a big difference between towing and dragging....speeds....
By switching to a high thrust prop... the boat will still reach its maximum speed.... admittedly at a higher rpm...
Edited to clarify.
Joe, to some extent I will agree with you, but there is going to be a trade off that will actually cost more energy than it produces.
its for the OP to decide if what little gain is worth the end cost. my calculated guess from experience would be about a quarter knot gain for the cost of the prop and a decent rise in the hourly fuel consumption..
assuming he is running at his maximum rpm, changing the prop is either going to cause the engine to labor more as it pulls a higher pitch, or go slower thru the water with less laboring...
Walt gives a good explanation using an automobile in place oo the boat, where the gearing can be changed with the movement of a lever, rather than the swapping of components.
its kind of the same thing, just a different vehicle... the only thing lost in the equation between the two is what we would call "prop slip"....
if he was over or under propped towing the
small dinghy, it means that he is also over or under propped when he isnt towing it.
and I will agree that changing out a severely mis-propped boat, can greatly improve the efficiency, but with the numbers he is showing, this isnt the case.
my entire life has been spent working and/or playing with boats.... mostly smaller ones.
some have been very high powered speed/racing boats and others just row boats, with or without homemade propulsion.
and even though there are a countless number of smarter people than me on any given subject, and some can spew forth calculations that would make most of us wonder, and some who can articulate their thoughts much better than I can mine, I will still argue what I DO know
... (but always in a friendly, if outspoken manner)
and I do know that no matter how big or small the object is that is moving thru the water, there is a speed at which it will move with very little measurable effort, but when it needs to move faster, the effort needed becomes greater.
it makes no difference if it is submerged or floating on top of the water, or what shape it is, or how its powered. it may be called a light or heavy displacement, semi-displacement, planing or something in between. but it will still take more power to move it faster thru or over the water,
if it is in contact with the water.... (and when there is too much power supplied, the object will either come apart or leave the surface of the water)
as for the OP's problem, at the top of his rpm range where the production of more torque/ horsepower/speed is least efficient, he is already squeezing the very most out of what horsepower has to work with. by adding some more drag to his boat, its taking away the speed that is already barely being produced... its not an unreasonable fact.
by re-propping, he may gain a slight bit of the speed lost back again, but it is going to come at a much higher cost in fuel.
you can change the prop or the gearing below, but you can not change the output horsepower of the engine itself.
Nothing is free, but I will agree there can be trade offs...
