Learned something new about trim tabs...

[jeffk]

In a similar vein, the trim tabs can also be used to reduce or eliminate porpoising. When the wave spacing and current running characteristics of the boat cause the bow to bounce up and down continually, using the trim tabs to force down the bow can be very effective. As noted, there is more drag from the tabs and probably pushing through the waves makes the engine work harder but I just push the RPMs up to compensate and enjoy the smoother ride. Yes, you will burn more gas. C'est la vie.

However, just thinking this through further, when the pounding caused by porpoising would cause me to drop speed to just about planning (18 MPH), it's fuel inefficient. Using the tabs allows me to get closer to the "sweet spot" of fuel efficiency around 25 - 35 MPH as the boat lifts further out of the water. The fuel cost of using the tabs may not be all that much?

[Dave R]

Originally Posted by jeffk

However, just thinking this through further, when the pounding caused by porpoising would cause me to drop speed to just about planning (18 MPH), it's fuel inefficient. Using the tabs allows me to get closer to the "sweet spot" of fuel efficiency around 25 - 35 MPH as the boat lifts further out of the water. The fuel cost of using the tabs may not be all that much?

I installed a gauge in my boat the measures speed and fuel flow and compares them to display near real-time MPG to two decimal places and it updates once per second. Running with the tabs down at a full plane is substantially more efficient than running at barely planing speed (regardless of tab position). I lose about 10% economy when I run the tabs down at speed. I lose at least 30% when I am barely on plane.

[jeffk]

Originally Posted by Dave R

I installed a gauge in my boat the measures speed and fuel flow and compares them to display near real-time MPG to two decimal places and it updates once per second. Running with the tabs down at a full plane is substantially more efficient than running at barely planing speed (regardless of tab position). I lose about 10% economy when I run the tabs down at speed. I lose at least 30% when I am barely on plane.

Really neat. I was envisioning that someone could monitor this to find out but not without installing special equipment. Nice to know.

[Woody38]

The only boat I had with tabs was the 37 foot Egg Harbor and they worked wonders. On my boats without tabs when crossing a wake I would come along parallel to the wake then steer into and over while steering the opposite direction when on top of the wake. Like a lazy S curve. Seemed to work fine for me.

I am a retired workaholic and continuing aquaholic.

[Descant]

Originally Posted by jeffk

In a similar vein, the trim tabs can also be used to reduce or eliminate porpoising. When the wave spacing and current running characteristics of the boat cause the bow to bounce up and down continually, using the trim tabs to force down the bow can be very effective. As noted, there is more drag from the tabs and probably pushing through the waves makes the engine work harder but I just push the RPMs up to compensate and enjoy the smoother ride.

In my experience, porpoising is often caused by improper outboard/outdrive setting i.e. the drive is too far out from vertical. You may be able to stop spanking by bringing the engine in a little, then you can use the trim tabs for all the other things described in this thread.

[jeffk]

Originally Posted by Descant

In my experience, porpoising is often caused by improper outboard/outdrive setting i.e. the drive is too far out from vertical. You may be able to stop spanking by bringing the engine in a little, then you can use the trim tabs for all the other things described in this thread.

I might agree except that I can be traveling along for quite a while without a problem, i.e. the engine trim is fine, and then, without a change of speed, the porpoising starts. You can correct by changing the engine trim or you can use the trim tabs. Either one can force the bow down, which is the effect you want.