Mercruiser 165 - setting proper trim angle underway

[region4]

How does one determine optimum trim angle, while underway?

At higher speeds for my boat (above 25 mph), as I trim up, the steering becomes light and neutral, and the boat feels lively (all good here). As trim angle is further increased, eventually, an abrupt steering change occurs ... boat turns to port aggressively, requiring immediate correction on the steering by the helmsman, who must turn back to the right to correct. RPM remains constant.

Does this occur because the vertical fin, on the sacrificial anode, is losing its lift? And, if so, does this imply that moving to a more modern propellor, with more rake (and thus requiring an anode without the vertical fin), is a bad idea? Alternatively, could the existing prop's imperfections introduce such behavior? Any suggestions on a replacment propellor would be welcome.

Mercruiser 165 (1972 inline six) - 1.65 drive ratio (I believe)
WOT - 4400 rpm - 47mph (Service manual specs 3900-4300 WOT)
Medium/Light load - 2 passengers, 18 gallons fuel in bow tank
48-88440A-4 19 stainless prop with some cup (needs some TLC)
17' jolly roger - deepV 19 degrees deadrise - low freeboard and fairly light hull (~1000 pounds, excluding engine/drive)
Ride Guide manual steering

Thank you, in advance.

 

[Silvertip]

Re: Mercruiser 165 - setting proper trim angle underway

Trim angle is determined by the seat of your pants. Varying the load in the boat can also impact what the optimum trim angle is. There is no magic setting that's good for all conditions. The best way to set trim on any boat with any load is to get on plane. "Play" with the trim switch while watching speed and the tach. RPM and speed will increase slightly as the optimum trim position is reached. Trimming too high will likely get the bow way too high (provided there is enough power on tap) and you may run into a porpoising (bow bounce) effect. You either power through that point or adjust trim down a bit. Trim setting should not cause abrupt changes in direction (right or left) unless there is something wrong with the boats hull -- such as rotten stringers that can cause the hull to twist.

 

[region4]

Re: Mercruiser 165 - setting proper trim angle underway

I will inspect further; the hull,stringers, and transom .. but I think they are dry and sound. I certainly hope so.

One thing that I have not yet retested, at speed, is an adjustment I made to the hand-nut, securing the RideGuide cable to the steering tube, on the transom plate. There was a full 3/8" play, which is now eliminated. Perhaps that play, combined with the trim adjustment, allowed the sterndrive to shift a couple of degrees.

Thank you for your help. I find this 'boating thing' brings me a tremendous amount of pleasure.

Trim angle is determined by the seat of your pants. Varying the load in the boat can also impact what the optimum trim angle is. There is no magic setting that's good for all conditions. The best way to set trim on any boat with any load is to get on plane. "Play" with the trim switch while watching speed and the tach. RPM and speed will increase slightly as the optimum trim position is reached. Trimming too high will likely get the bow way too high (provided there is enough power on tap) and you may run into a porpoising (bow bounce) effect. You either power through that point or adjust trim down a bit. Trim setting should not cause abrupt changes in direction (right or left) unless there is something wrong with the boats hull -- such as rotten stringers that can cause the hull to twist.

 

[achris]

Re: Mercruiser 165 - setting proper trim angle underway

As this isn't really a 'repair' question, I'm going to move it over to somewhere a little more appropriate....