25hp 4 stroke motor height

[targetpanic]

I posted this in the Merc section too but I think it belongs here more than there

I just purchased a Merc 25hp 4 stroke long shaft powere tilt and trim. I went through the break in procedure yesterday according to the owners manual.
The boat is a heavy 16' sylvan Expedition. At WOT the motor only gets to 4900 rpm and top speed is 11.6 gps. It currently has a 10X8P prop. The anti-ventilation plate is 2-3" below the lowest point of the keel and the water line is about 8" up from the bottom of the keel. The motor was mounted in the top holes. If I raise it up 1 or 2 holes what will this do to performance???? where should the ventilation plate be in relation to the keel?? Max rpm for the motor is 6K. I know it's not a performance boat but the reservoir has hp restrictions and I'm trying to get whatever I can out of it.

 

[steelespike]

Re: 25hp 4 stroke motor height

Looks like that boat is really heavy is your speed with a light load?
You should raise the motor to a point just before ventilation becomes a problem. Most end up with plate 1 to 1.5 inches above the bottom.
Most tollerate occasional venting to maintain best performance.
I think you will have a noticeable improvement.Weight distribution is critical and of course try to eliminate as much weight as possible.plastic instead of wood oars,light danforth style anchor.light styrofoam cooler well shaded so you need less ice,3 gallon plastic gas tank.If its a tiller get rid of and remote steering setup.If its tiller get a tiller extension so you can sit in the middle seat.weight and weight placement can mean the difference in plowing and planing.Let us know how you make out.

 

[targetpanic]

Re: 25hp 4 stroke motor height

Thanks for the reply steel. I will raise it up a couple holes and see what it does.
As for the boat itself it is a big heavy side console hull weight alone is around 1000# it has an 87" beam and is deep. The hp regulations require no more than a 25 4 stroke...this boat needs a 60-75hp to get any performance out of it...but rules are rules.

There are 4 mounting height holes. It is in the top holes. I'll raise it up 2 and see what happens. I played around with the weight the best I could while on the water and did find that moving things to certain spots did help.
When I raise the motor do I need to put shims in the gap???? What material???

 

[steelespike]

Re: 25hp 4 stroke motor height

If your raising the whole mounting bracket just be sure the bolts are quality
and have a good purchase on the transom. The lip on the bracket is just for installation.The bolts are doing the work.