I have a 14' sea nymph which tops out at 21 mph ,gps measured, with just me in it. The boat weighs 150lbs, the motor about 80lbs, me 215lbs, plus gas and oars. The boat is a v in the bow and flat at stern. It has a 15" transom and the motor is a 20" long shaft. Loaded with fishing gear (including 2 heavy deep cycle batteries and trolling motor)  I top out at 17.5 mph.

Any idea on how much faster my top end would be if I remove the 5" extension and convert the motor back to a short shaft ?

Also I have been looking at a grumman 14' boat that is a v bottom all the way to the stern, has a floor in it, and has a 20" transom. I figure the floor has to add at least 100 lbs of weight. How fast do you think this boat would top out with the same 15hp motor ?

Just a wild guess, but you would pick up 3-4 MPH by removing the extension on the current boat. The same 15hp on the different, heavier boat would probably slow down 3-4 MPH.  Just a guess.

That is exactly what I thought. So maybe with the longshaft 15 on a longshaft transom boat even with the added weight of the heavier boat the top speed would be about the same.

It will be close, but remember that the deeper hull will have more drag due to, well, there's more hull in the water.

I figured I would revive this thread since I have new data. The same boat in the original post will do 17.5 mph (gps measured) with an 8hp short shaft motor and just me in the boat. I have lost some weight since I started this thread, I'm 185 lbs now. With a second person in the boat, also weighing 185 lbs, the speed slows to 14.5 mph. These speeds were measured on totally flat water.

I find it interesting that the boat only lost 3.5 mph when going from a longshaft 15 to a shortshaft 8. Apparently the extra 5" sticking down in the water slowed be down quite a bit. One variable not accounted for is I do not know the prop pitch of the 15hp because I sold it.

I should have my 6hp outboard running soon and I will measure the speed with that and post it. If I recover my 25hp I will measure that as well.

My 14 foot v hull utility does just about 14 mph with my wife, me and an ice chest and two anchors, I am running a 9.8 hp mercury on it, it is a good small lake boat, if I want to go fast I use my 16 foot Starcraft super sport with the 85 hp on it, now it will do about 45 with the same gear on board, but utils are really not designed to be go fast boats.