Don't want to tag on to DeepVs post. This is just for my own curiosity more than anything else. I'm wondering if this is correct or not.


If you plan to add a whale tail, you may need to consider that the prop you have will make an impact on the performance.

When I was researching options to improve time-to-plane, I learned that the prop thrust makes a difference.

In this first image, the top prop has a wide thrust which puts more down pressure, and loses up pressure to the surface, and therefore pushes the bow down. A wider thrust will also lose forward thrust.
The bottom prop has a narrow thrust which gives more forward thrust, which pushes the drive forward and therefore lifts the bow.



This second image assumes a whale tail instalation.
The top prop may have the upward thrust deflected, lifting the drive unit, and pushing the bow down.
While the bottom prop may not have as much of an effect from the whale tail due to less upward thrust.



Again, this is just my thoughts on the physics of this apperatus. I'm wondering if this is correct or not. I've never seen any explination of how these things are supposed to work.
I also had to type this quickly because I gotta run to a meeting.

Feel free to laugh if this is just SO wrong. I've got thick skin.

I think thats part of it HF. The rake of the prop is what determines how wide or tight the thrust spreads.
The main thing about the whale tail is the shape of them, they are shaped like an airplane wing, the top is curved and the bottom is flat. As the water travels across the longer surface of the top it's velocity increases and it's pressure drops. the resultant pressure differential between the top and bottom is the lift created by the "wing". I wish I could post a drawing of this, but I am not an artist.

Good point MM. I wasn't thinking of the "hydrofoil" aspect of the wing.

I guess my statement above may not be so much of HOW they work, but more of WHY they may work better for some and not so much for others??

I don't know that the hydrofoil effect makes much of a difference, but it could.

In my mind, it is the angle of the tale that creates the movement.

In reality, it is likely a combination of all three.

I put a whale tail on my 20' bowrider. I didn't notice much difference in ride or performance except it didn't seem to porpoise as much. That being said, a slight adjustment to the trim of the lower unit could fix any porposing issue.

HF, in your research and your drawings, could you 'splain to a dummy like me what makes one prop have a wide thrust and another a narrow thrust? Is it the pitch of the prop?

Later I'll try to find the web site that explained it. But for now, I'll be happy to quote Memory Maker just a few posts above.

I should put one of those below my inboard props...