Re: Motor upgrade
^^^^ All 4 responses are right on track. There may be 1-3 mph hidden in your rig as it is but the WOT specs on the motor and the gear box ratio are needed along with the prop pitch for the calculations. From there, it can be estimated how much improvement can be made before you change motors. At 4,300 rpm there's something wrong for sure and you're probably not getting 35 HP as it is. The motor is either weak on HP or it's over propped or maybe some of both. The point is you may have a few mph hidden in your rig and when you change the motor to a 50 HP then you know you're working from a base of 12-14 mph instead of 11. If you can get the specs mentioned then the calculations can be run to see where you're at.
Yes, not much happening at 8-10 mph underneath but once you start picking up speed you will hit some splash resistance under the floor especially in the turns pulling tubers. As mentioned, the under skinning is one of the biggest bangs for the buck there is on pontoon boats. There are plenty of threads on this so just do a search on this site for "under skinning".
The bottom line is, with a "strong" 50 HP motor, a premium prop selected for your rig and under skinning you may see 20 mph or so. If you can get it to turn then you can give the junior tubers a decent ride but as mentioned, it won't be a power house.
I would inspect the transom and make sure its solid so the additional HP won't cause a problem. I've seen pontoon transoms that you wouldn’t want to put a 9.9 on and then some that look like they are built for the rated HP plus. Just shake it around and look at the way it’s secured. You can buy heavy duty transoms if yours in marginal.