Pontoon Boat & Deck Boat Forum

I purchased a new 90 Mercury 4 stroke "2017" for a 2009 Suntracker 21 FSB. The logs have lifting strakes.
Was told that this was the same as my old 2009 90 hp Mercury efi.

The shaft is shorter than my old motor. When they first mounted the motor I took it out for the break in and ran just fine.
As I got near the end of the break in period and was able to go full throttle the boat ran about the same as my old motor with one problem. An alarm went off. I took the boat back to the shop and they determined since the motor was shorter they needed to lower the motor all the way down and put "water scoops" on the water intake as the alarm was caused from overheating due to loss of water.
With the motor mounted all the way down, you cant trim the motor all the way up or it will hit the steering arms and possible break them. So they mounted a trim limit so now it wont go all the way up. When parked in water the motor trimmed all the way up bottom of motor is still in the water.
Motor is fully broke in now and I took 4 men out on boat probably 900 pounds and boat is rated for max 1900. The boat would not go about 16 mph. I took the boat back and they tell me its fine. I am not buying it. The motor does run fine but I don't think this is the right motor or it needs a jack plate or something.
With just me on the boat it runs 28-29 mph. My old motor would get to 31. Load it up with folks and now it goes 16. My old motor did struggle loaded down but it would still get to low 20's.
I have a 4 blade 13 pitch prop.
Would love some input in what I should do. Should they have put the Bigfoot on the boat? They tell me it will go a lot slower.
I think the motor is now mounted too low. Only with 1 person on it does it come close to my old performance.

Thanks

You could probably get a few more MPH when under load, but I wouldn't expect more than 20 MPH under that kind of load. Can you answer these questions?

Did you get the non-Command Thrust 90 4S EFI?

Were you measuring speed with GPS?

Do you have an accurate Tachometer to measure your RPMs at WOT when running solo vs loaded?

How were the passengers dispersed on the boat? At the bow, even or stern-heavy?

Were you able to give the engine a little trim up while underway - to get the toons to plane a bit on the lifting strakes?

The Command Thrust or BF gear ratio would give you better performance under load, but you'll give up some topend overall. I'd recommend first trying a little trim up while on-plane and making sure your weight is evenly distributed. Next, verify you are in the correct RPM range at wide open throttle for that engine ~5000 - 6000 RPM (don't trust your gauge - have the dealer hook up a SmartCraft handheld). You should have the correct prop, but testing the RPM range will help determine that. I would avoid using a jack plate. With my experience, it's more trouble than it's worth. Just upgrade to the 25" shaft, if you don't already have it, and raise the powerhead up a couple holes.

Yes they should have sold you the command thrust (new name for big foot) for a pontoon. Was there a reason they went with a 20" when your last one was 25" If I read it correctly.

Dealers have a responsibility to get ultimate performance out of any motor sold, and selling you ashort shaft motor was a royal screw up. He should allow you to return that motor and swap it for a long shaft version for just paying the small difference in price.

If I was going to repower any boat with 90 hp, I'd at least want to better my horsepower and go 115 hp. I think the Merc motors of those sizes are relatively new motors--not the same as your old motor.

The motor is the non-command thrust.
I was measuring speed with my fish finder that has gps and my phone that has an app to measure speed "speed box"

The boat was set up with ski bar, lifting strakes and the old motor would pull skiers and tube as long as you didn't have a lot of people on the boat. I don't think this new motor set up will do it. It struggles under a load and the rpms drop. Its "bogged down"

With just me and one person I can get up to 6000 rpms under the heavy load its more like 4000 rpms.
Seems way under powered or too deep in the water and too much drag.