Let me start by saying I have been setting up boats for over 30 years. From lower HP boats to Go-Fast boats capable of triple digits. I will admit I have only setup a couple fish and ski boats but the concept is the same. In your post you fail to mention several very important factors required to attempt to fix your problem. What prop and pitch prop you have? Engine RPM at WOT? Type of lower unit (Prolly a TorqueMaster) and gear ratio? Engine height above/below the pad? (Bottom of the hull level on a level surface, engine lowered to where the top edge of the cav plate is level: Measure the distance from the floor to the bottom of the back of the pad (Lowests running surface) & distance from the floor to centerline of the lower unit (Bullet)=difference above or below the pad. (Yours will be below)
Lets first address the trim gauge issue you describe. An analog gauge is simply a representation of position. It should only be used as a reference so you know where it is. How much trim is needed "will vary with speed and load"!! It WILL NOT be the same for every boat, every time for every condition. You have to get a feel for how the boat is running and trim accordingly. You cannot use what someone else uses for many reasons. A digital trim gauge gives a more accurate reading. When I'm running I "Feel" how the boat is running and once it's trimmed to where I "Feel" its best, most times can look at the gauge and see it's at the same reading near every time. Years ago most boats didn't even have a trim gauge!! Also, very few boats need to use the "FULL" trim! Second, NEVER go off what some tech working at a shop says. They setup a boat which is essentially "Unloaded". All they can do is get it in the ballpark. Their customers tell them "It runs great" then over time load the boat up with their "Stuff" and return telling them this thing ran great when I firsts got it, now it runs like crap!! They have no idea of how you or anyone else will actually load your boat. ALL boat manufacturers sell a boat with the cheapest "Minimum" pitched prop they can get away with and NOT the "BEST" prop for a particular boat because they know that no 2 boats will run the same because of the issues mentioned above. I recently purchased my first Nitro and have used/tested 4 props already. A fifth is on order and I'm searching for a 6th. The prop that came with the boat is nowhere near the correct prop for my boat as setup! Also, boats will run differently from season to season (Cold/Hot) because an outboard will make more power in cooler weather as opposed to hot weather.
Porpoising as you describe can be caused by one or more things, usually too much trim for the speed. Motor too high on the transom, bow area overloaded for the set up, prop not maintaining a good "bite" etc.. or of course a combination of these. Will a jackplate work?? Maybe, maybe not. One reason a jackplate is used is when additional bow lift is required. A jackplate also allows for quicker/easier ability to change engine height. Boat/Motor, set-back, trim, lift are basically a fulcrum (longer lever=more lift capability). 2 inches in setback can also make a big difference (Good or bad). The boat you have does NOT have natural bow lift as does most bass boats and other Go-Fast boats and cannot be rigged the same, as the bow area on your boat is very heavy in comparison. Same concept but achieved differently! To get the trim needed the motor must be down far enough on the transom so when you trim up it is still in clean water allowing the prop to bite while maintaining good water pressure. Too high on the transom and you lose this ability because you actually get less trim and the prop is losing bite, gaining bite, losing bite, gaining bite--porpoising! In addition, trim too high and you actually lose speed because the angle of the lower unit/prop is pushing the transom at a downward angle instead of forward. As speed increases, bow lift increases which "LOWERS" the prop deeper in the water. This is where proper engine height comes into play. Most boats will get up on plane at many different engine heights but may or may not run best or even at all at the same given height (Hydraulic Jackplates fix this problem) A prop will also lift the stern to a degree (Some props more than others). As compared to a 3 blade, a 4 blade prop will plane better, handle better, will have more bow and stern lift capabilities and will allow you to run it higher in most cases. The steering torque you describe is a classic case of a prop trimmed too high. A 3 blade prop isn't designed to run partially submerged, or at high angles of attack as is a 4 or 5 blade prop. Steering torque will increase with them all, but the fewer blades means higher steering torque when partially surfaced. Boats such as yours is not a high performance hull which requires or even allows for a prop to be run at high engine heights.
In my opinion, considering your porpoising with lots of steering torque, your motor is too high. You need to start by lowering the motor by 1 hole and see what thats does for you. If you don't see things get better, drop another hole. If this isn't working try a 4 blade prop of similar pitch and work with engine height again!
