Bayliner 175 BR/MerCruiser 135 pulling to left after 30 mph

[Captain Coromandel]

We hit a rock with our Bayliner 175 BR and the local repair shop replaced the lower half of the drive. They had a shorter ratio unit in stock so it now has about 1.8 instead of the original 2.0. To compensate this they used a Mercury Enertia propeller 48-898994 which to my understanding is 14 1/2 x 17. I read somewhere that the original black max is 14 1/4 x 21.

The problem is now that after 30 mph the boat starts to pull to the left. Top speed is 47 mph @ 4250 rpm (previously 43 @ 4800) where you really have to hang on the steering wheel to keep it straight. The adjusting fin is turned to the max on left side.

Is this normal behaviour or is there something else wrong with the boat?
Repair guys say they'll fix it with smaller prop but I'm not fully convinced with them anymore. Will the boat now always be pulling to left if I want to keep the lower rpms (which i like since we are purely cruising) due to the lower gear ratio which requires even bigger prop than original would have?

If the repair guys are right and this can be sorted with prop selection, which parameters should be adjusted to get it right? Or do I need the original gears in the drive to get this right?

Or what, am I missing something totally?

 

[hwsiii]

Re: Bayliner 175 BR/MerCruiser 135 pulling to left after 30 mph

Do you have the power trim on the outdrive up or down when you are running, some boats will dig the bow in and tend to turn when the bow is not high enough and they need to have the power trim in a slightly raised position to pick the bow up.

H

 

[Captain Coromandel]

Re: Bayliner 175 BR/MerCruiser 135 pulling to left after 30 mph

Trim position doesn't seem to have any impact on the problem.

I notice I mixed some things in my original post. The current 1.8 is of course longer than original and therefore would require a smaller prop to run equal speeds and equal rpms.

I guess the bottom line is can this ratio/propeller lottery really cause such amounts of pulling or is there something bigger like the whole drive being twisted or something. The bigger the prop, the more it potentially pulls sideways, right? Now due to the longer ratio the new propeller is much smaller than original and still causing a lot more pull. Can the difference in behaviour between aluminium and steel be that big?

Before the accident the boat was running straight, 0-43 mph and on all trim heights.

 

[hwsiii]

Re: Bayliner 175 BR/MerCruiser 135 pulling to left after 30 mph

Captain, if what you are describing as the trim fin is turned to the left all the way then that is the problem and the faster you go the worse it will get. Here is a picture of what I am talking about. Just think of it as a rudder on a boat.

H