trim, torque steer, and max rpm

[eli_lilly]

Hello,

I took my Johnson 90 out for its first test drive today. It's the first outboard I've had with power trim. After noting the rpms at WOT throughout the trim range, I have what feels like a trillion questions:

- When I am trimmed to the point where there is no torque steer, the WOT RPM is 5200.

- If I trim out, RPM goes up and so does speed, such that I can get 5500 RPM. However, at this trim level there is substantial torque steer to the left. Dropping off plane switches the torque steer to the right (because the tab is adjusted full left).

I can't figure out if the correct course of action is to consider the 5500 RPM trim-out position the "proper trim", or whether I should drop down one prop size and use the position of less trim? My thought is that the trim that generates the highest speed is the hull's best trim, but there is so much torque steer and the trim tab only adjusts a tiny bit left or right (unlike my old Merc).

Some advice from those more experienced will be greatly appreciated.

Thanks,
-E

 

[Chris1956]

Re: trim, torque steer, and max rpm

E- That is pretty normal for a high performance boat. They tend to pull right when under trimmed, neutral and then left when max trimmed. You might be a bit over propped since you are only getting 5500 at max trim.

On my speed boat I run at "steering neutral" for cruising at ~2/3 throttle, and save max trim for special high speed WOT ocassions. .

Can you raise the motor up on the transom? The could raise the performance somewhat. Is it possible to get a larger trim tab, or adjust it more?

 

[eli_lilly]

Re: trim, torque steer, and max rpm


Thanks for your reply, I am going to switch from a 19p to a 17p, then. The information on how to trim is particularly useful, I'll just operate in a similar way. As for my trim tab's adjustability, it's unlike what I am accustomed to on a Merc in that it has a raised line on the backside that fits into an intentation on the lower unit, which only allows for a few degrees of adjustment. I am going to see if there is a tab that doesn't have this raised area on the backside.

Thanks,
-E

 

[Dhadley]

Re: trim, torque steer, and max rpm

Going to a smaller pitch prop is only the last, very last, thing you want to do to gain rpm. In a case like this going down in pitch will almost always reduce efficiency.

If the boat pulls to the right, move the tab to the right. At mid ramge you'll find a place in the trim range that the steering torque goes neutral. If the motor is already high enough that that tab is out of the water when you're wide open you can add a torque tab to the skeg, which will obviously still be in the water.