Re: Long shaft vs 20 inch shaft motor on Pontoon?
I like the idea myself. Others may chime in and disagree with me,but here's where I'm coming from. I'm building my first toon,which is a straked triple. Not a lot of experience to offer there,but I do have a lot of experience setting up mono hulls. I done a lot of research and watched videos of pontoons on plane. Plus I slept in a Holliday Inn Express.
My transom tube is patterned off of the typical factory center tube. When I hung my 20" Merc on it the 1st thing I noticed was it looked like it was mounted 2" too high,and it was bottomed out on the transom. The 2nd thing noticed was the transom angle wasn't enough to utilize much of the motor's trim. It'll blow out way before it's trimmed out. There's no way the bow will lift enough to keep the prop hooked up. I already planned on using a manual jackplate,but quickly decided on a hydr. unit because I believe I'll actually have to lower it once on plane to be able to apply much trim. So, I mounted the motor on the jackplate in a neutral position to start with,so I can raise or lower it.
This is the opposite of how you adjust on a fast v-hull. These long front heavy toons run much flatter,and will require much more leverage to raise the bow with thrust. My bet is once it's on plane I'll be able to drop it to give it more leverage for a better angle of attack to climb higher. Only need to keep it high enough to keep the cav plate out of the water. Question is where you cross the line of drag vs. HP when trying to create more lift. Things get really deep at this point,as there is almost infinite variables. The point of "is it worth it" has probably already been crossed. Kinda like trying to jerk the front wheels off the ground in a Grand Torino. Sure it can be done,but....
Something else that helps with thrust/bowlift on heavy dirty water hulls is a lot of rake angle on the prop. Looks to me like my straked center tube will trap a lot of air,which will mix (dirty water) and exit at the prop. The higher rake angle will draw water into the prop from further away,rather than directly in front. It also creates a tighter thrust cone which blasts further back into cleaner water for more "push". Not a lot of choices in high rake props in our pitch range. The best I've seen is the mercury Enertia,which is what I choose. I also bought some transom wedges (yet to be installed) to tuck the motor further under,and give it a more usable trim range.
I've also seen a lot of spray hitting the outboards like you mentioned. If you do indeed end up dropping for better trim the 25" shaft would definitely help. Was planning on making deflectors for mine once I see exactly what I need. Sorry for the long read,that's how my brain works. Again,this is just my theory. We'll know some results when I splash it soon and begin playing with the setup of this "Grand Torino".