Aluminium boat tracking problems

Hi Anyone/Everyone
This first-timer(on web) has problems with a used alum. boat constantly pulling to the right. It is a 5.5m Stessl Mako(popular QLD made fishing boat). This open boat with small cuddy cab has 85hp Yamaha longshaft outboard propulsion. All items good Condition, !997 models.No hydrofoils or trim tabs. I have adj .cav.plate in increments further to right now showing 8mm adj. off centre. Motor is attached 10 mm off centre of transom. Boat tracks to right gradually at low speeds and strongly at high speeds. Hull design incorporates famous ? Stessl trak rails- one each side approx 2.5m long, 100mm high- running from mid to transom on edge of hull. Arms ache by daysend and following/quartering seas a bit dicey!
Any comment much appreciated. :!:

 

boat tracking

Motor torque will cause this. If the motor is mounted high so the prop partly surfaces, this will make the problem worse. If the motor is trimmed in relative to water surface, it will be worse again - very noticeable with power trim. The trim tab under the cav plate is the adjustment to decrease the problem and rear end should be pointed to right (looking forwards) - it should be nearly balanced at your normal trim & speed combination as it will be unbalanced at other combinations. You have indicated this tab is deflected 8mm and I have now checked my 50 HP and the deflection from front of the tab to rear at the top of the tab is 22mm.

Suspect you have trimmed the motor in too far to stop the porpoising I understand this boat is prone to and the trim tab needs further adjustment to the right.

These comments also assume there is no problem with the boat construction or damage which could add to the problem. Hope these comments are of some assistance.

Regards,

nevd