Motor trim help... small rooster tail from boat.

I have a 1468 Polar Kraft mod-v jon boat and I've been running my 15hp mercury 4 stroke on it. Last trip out on the lake I noticed that the motor was throwing a small rooster tail at full speed. You can see what I'm talking about in the video below The manual trim bar is missing so the motor is tilted as far forward as possible. I have a new pin coming in the mail and should be here in time for my next fishing trip. My understanding is that motors tend to throw rooster tails when it is angled too high. So if the motor is in the lowest position, actually angled forward and the prop is pushing at a downward angle in relation to the hull, why am I getting a rooster tail still? Also, when I get the trim bar, how will I determine the proper trim angle for my boat?

Thanks for the help!

https://youtu.be/vvNZgk1C9fY

Here is my opinion, others may feel differently.

Because you are missing the trim bar you may be getting a rooster tail not from the prop but from the anti-cavitation plate being angled almost like a plate stuck semi-vertically in to the water and the rooster tail is coming off the following edge of the plate being dragged through the water at this angle.

When I set up a motor on a boat first of all I make sure your motors anti-cavitation plate (The plate just above the prop) is deep enough in the water to cover it at rest and to ride a bit below the surface while on plane. Too high and you will waste energy, get cavitation and get a rooster tail, too low and you are wasting energy dragging your gear box through the water. Use shims between the transom and the motor mount to raise the motor if necessary.

To find optimum location for the trim rod load the boat as you would normally use it. Put the pin in the first hole closest to the boat. Make a run straight out and back while using a handheld GPS to record your speed. Write down the results. Move the pin up one hole. Make another run and write down the results. Move the pin up one hole, make another run and write down the results. Do this until your motor cavitates on plane or it becomes obvious from a speed drop that you are too far out. You will find the sweet spot.

Note with the motor tucked all the way in you are wasting energy trying to force the bow down and the stern up. When the motor is all the way out you are forcing the stern down and the bow up. With a power trim model you usually start with the motor tucked in and then as you get up on plane adjust the motor out until you start to cavitate at which point tuck it in a bit. If you have a tach you can adjust or fine tune your trim using that too. You would notice that as you go past full trim the RPM's start increasing while the speed is decreasing that is the start of cavitation so you tuck it in a bit to reach max speed. But most small motors do not have tachometers so speed and your keen hearing is your best indicator in my opinion.

Good luck,

Tim C.

Are you getting any 'porpoising' or is the bow going up and down or bouncing when running fast?

If you bow is riding high you can put more weigh up front.

Let us know how it going when you get that pin installed.

Thanks for the help Tim and Bill. I'll do as you said when I get the pin on Friday and hopefully will find the sweet spot. I also have one of those big hydrofoil things on my outboard. Maybe this is exacerbating the effect that you described from the steep forward angle of the anti-cavitation plate as well?

Bill, I have not noticed any porpoising yet. With a passenger sitting on the front casting deck, when the boat gets up to max speed, she seems to ride smooth and the bow doesn't seem too high or low...