Jack Plates and Setback ???????????

[RonS]

Leo's jack plate thread got me to thinking.

There was a time when I bought big boats with big motors.  I never understood why setback was a good thing for the outboard. I always bought them because the rigger I trusted told me I needed them.

I understand a jack plate. Especially a hydraulic powered one you can raise and lower on the run.

 I see in new boat designs they are kind of designing setback into many hulls, especially bass boats.

I guess my question isn't really about jack plates but just setback. What exactly, does setback do for the performance of the boat?

My thoughts were-If the propeller were the fulcrum point so to speak, the farther away from the stern you could get the prop, the quicker and farther it would pick the bow up out of the water. To my simple thinking, this would help you get on plane quicker.

Is that the point of setback?

[TimM]

It's something like that in my opinion Ron.

 From an engineering perspective, yes, moving the fulcrum point (which I think is the point where the motor is first rigidly attached to the hull) away from the transom, laterally, does create a moment arm and increase the torque applied to transom and that does generally help get a boat on plane faster.

The wake behind a boat in motion rises after passing under the aft most edge of the hull bottom thus forming a standing wave.  An additional benefit of a jack plate is that by moving the prop laterally away from the hull it gets closer to the peak of that standing wave. As a result you can generally trim the motor higher than if you didn't have the jack plate.

Tim

[Glenn Hawkins]

Hi guys I'm back from 4 days of BMW Sales Manager training classes that were held in South Carolina.  5.5 hrs and 360 miles.  I'm glad to be home!

Jack Plates allow you to raise the motor vertically so that less of the lowerr unit has to remain in the water.  Of course, you can only raise it to the point that the water pick up is still flowing a solid stream through the water pump.

Jack plates will also allow  you to change the set back from the transom.  The reason for set back is to reduce cavatiation.  The futher back the motor is mounted from the transom the less disturbed the water will be at the lower unit.
Too much set back on a smaller boat will cause it to float stern low and bow high .
With a jet you really want the foot/intake as close to the transom as possible.  This allows the intake the best flow of undisturbed water.  This keeps the jet from cavatiating as well.
Hopes this helps!
Thumper

[RonS]

Thanks guys. Makes more sense about the setback now.

[dleo6446]

BandDad has hit most all of the points about setback that need to be touched.  The bottom line about set back is that it increases the motors performance because it gets  the propeller (or jet) into "cleaner" water.  Cleaner in ths case being defined as more pure water. 

As your boat is operated, the water directly under the hull is disturbed and it creates some bubbling as well as turbulence and that takes away from the performance of your propeller (or jet).  Some of the bass boats can be ordered with 13" of setback on the motor...now you are talking the big bucks.

What I hope to achieve is improved performance running upriver, and with the shallow water lever engaged, maybe some improvement in the depth of skinny water I can run in.

The down side, I have to give up my back rest when running the trolling motor!!