Here's the specifics. I've got a new to me trophy 21', with a 29"transom and a Mercury optimax 150, 25".
Prop is: Mercury Vengeance (13 1/8 X 16")
I think I'm about 3.5" too high, see pic
https://photos.app.goo.gl/ybbAwMfM3VdBv7Wg9
I can drop the motor 1 hole as it is right now. I think that gains about 1". I also bought a jack plate. Will moving the motor away from the transom help? Should I look into getting another prop? Thanks in advance.
2 problems: As you realized you are too high and second, you have a prop for a 4 ¼" gearbox...the size used on 90 thru 125 3 and 4 cylinder engines. you need to be up in the 14-15" diameter range.....that might get you a whole inch.
You said you could drop a hole, that's 2". Pushing a trophy 21 which is a lot of boat even for a 150, I would expect that the engine would have to have the AV plate pretty much even with the bottom of the hull, surely not more than 1" up.....so at 1" up, that's 3.
The big guns on here, and some still frequent the site, used 1" up for 6" setback. Install your jack plate which should get you the 6" setback, and you have consumed your additional 4".
Haven't looked lately, but if your bottom is stepped, that is the end of the "keel area"...if it had one, vs the actual transom area are not on the same vertical plane,like the former is recessed under the boat somewhat, you already have 6" of setback. Pad hulls, used on Bass Boats, use this kind of stern setup. Give them extra setback for high transom settings and more speed, and provides a convenient out of the way place to put all the plugs and access ports for the drain plug, and the live well fill and drain hoses.
Having that setback built in will allow you to drop your engine, get a prop (diameter) suited for your engine and you should be good to go. Nothing else to do.
I am assuming that you can't get up because you are ventilating terribly...rpms way up, thrust way down, if you get up it's a long slow process.
The Vengeance seems to have all Mercury SS prop attributes except for porting and maybe a bit shy on "rake"...terms defined up top in the iboats prop sales section....definitions.
Porting in the prop (port/vent holes under the leading edge of each blade), will help you to get out of the hole, as long as the prop blades can grasp the water....fresh new water, not what's behind the transom like your current situation. The ports decrease the mass of the water allowing the rpms to increase faster for a given load. Since idle to about mid rated rpms is where torque and rpm both rise and torquexrpms/5252 is HP and HP is what spins the prop which develops thrust to push your boat, you want them to get up reasonably fast...ports can help that happen, yet leave you with a goodly amount of pitch for your top end running at a good clip.....you don't penalize your top end speed for your hole shot...best of both worlds.
