Rooster Tails

[buckeye_paul]

I own a 15 ft. 1970 Starcraft V hull Runabout with a 1970 85 hp Johnson outboard. I have a problem with rooster tails. At higher speed two rooster tails shoot up very high and also hit the underside of my motor cover. As yet I have not stalled out due to water on the motor, but I feel it will happen sooner or later. This is an old motor that runs very well, but the cover seal isn't in great shape. I heard recently that my motor may be too low. It is set in the lowest of four holes. My question is: How high should I go with the motor? Is there a specific height for this type of boat? Any help would be appreciated. Thanks

 

[Adjuster]

Re: Rooster Tails

The cavitation plate of your lower unit should be aligned up/even with the bottom of your hull. This is the optimal starting point before making fine tuned adjustments. I would think your rooster tails would be caused by your motor sitting too high, not too low??

 

[Adjuster]

Re: Rooster Tails

By the way thats a pretty big motor for that boat. I would think overpowering could increase your chance of rooster tail.

 

[H20Rat]

Re: Rooster Tails

By the way thats a pretty big motor for that boat. I would think overpowering could increase your chance of rooster tail.

85 on a 15ft trihull is VERY common. I've had a couple under 16 trihulls, the smallest engine was an 85, the largest was a 125.

 

[metriccrescentwrench]

Re: Rooster Tails

That sounds like spray, possibly from the AV plate, not really a rooster tail. (My Checkmate with a 200 HP Johnson on a jackplate throws a real rooster tail ). I'd raise the engine one notch at a time and see what it does, you'll probably pick up a few miles per hour as well.

 

[sublauxation]

Re: Rooster Tails

Are you sure it's not a speedo or a depth finder transducer causing the spray? Before I got mine placed right I had a wall of water coming up.

 

[86 century]

Re: Rooster Tails

My 18' starcraft American did the same thing till I moved the eng up so the cavitation plate was even with the keel. In my case it made a huge improvement in performance as well. Good Luck

 

[Frank Acampora]

Re: Rooster Tails

That is very common with an engine sitting too low on the transom. At speed, water impacts the lower unit and rides up it, sometimes spraying into the motor well, sometimes hitting the underside of the engine. About three-four inches above the cav plate there is another much smaller plate that sticks out in front of the lower unit. The purpose of this plate is to stop water from riding up the lower unit. However, if the engine is set too low, this plate can not do its job. As stated above, if the cav plate is below the hull bottom, raise the engine until the cav plate is even with the bottom of the VEE. This is the starting point and you may be able to go even higher for better performance.

 

[buckeye_paul]

Re: Rooster Tails

Thanks to everyone for the advice. I have "Smart Tabs" brand trim tabs that I felt could be causing the problem. I took the trim tab plates off and tomorrow, weather permitting, will see if that has any affect on the problem. If not, I will unmount the transducer and see what happens. I can't remove the speed sensor without plugging holes.
It appears that the cavitation plate is at or very near the level of the keel. If testing the other options that I described above does not help, I will try moving the motor up as that looks like the hardest option.

 

[Willyclay]

Re: Rooster Tails

If it is your transducer, you might try what one of my buds did to solve that same problem on his 15ft Whaler. He cut off a piece of four-inch PVC pipe, about 3-4inches, and glued it to his hull around the transducer with 3M 5200. He cuts slots for the wire and head. Good luck!