*WARNING: Unless your shaft is very new, like this one was at just a few months old, be VERY, VERY careful using this method.
I hate to even suggest or share this method but it "can" work. There are shaft couplings that can be removed this way, if you are very careful.
Please be aware that it is VERY, VERY easy to bend a gear box output flange, making future alignments near impossible, or actually break one see photo after this one. A better method is to have a plate made at a machine shop with the same bolt pattern as your coupling and use it to press the coupling off and NOT use the transmission flange.
If using this press off method it will be IMPERATIVE that you take both the coupling and the shaft to a machine shop and have it tested for run out and then perform a "fit & face" before re-installation.
It takes VERY, VERY little force on these couplings to throw them out of true. Throw it out of true and you'll cause shaft whip and will have a boat that is physically impossible to align. Do not cut this corner and do not over tighten the "press bolts".
This coupling came off easy compared to many. Still when I took it to the machine shop it was out by 7 thousandths. Lucky the gear box flange still spun true. I suspect this shaft came from the factory with an improper "facing" as appeared to entirely lack any evidence of ever being "faced". This would have made for an impossible alignment.
1) Insert a deep drive socket that is slightly smaller than the shaft size between the center of the shaft and center of the transmission hub. See the picture below for a close up of the socket between the coupling and the transmission hub.
2) Insert four long threaded bolts, preferably without shoulders (the part on longer bolts with no threads). This boat was only two months old and the coupling was not that tough to get off but it had already begin to rust on. Luckily the layer of rust was not enough to disturb the "fit". Other boats where I know they are rusted on I would use fine threaded bolts and a custom made "pressing plate" NOT the gear box flange.. Be sure to use washers between the coupling and transmission hub and begin tightening EVENLY.
3) After some initial tightening, and with the bolt pressure still on the shaft and coupling, you may need to go outside the boat and tap the prop shaft towards the bow, yes the bow, with a wood or lead mallet. Remember this is NOT a driving blow more of a "tap". This is not a pounding with the mallet, just a light strike. If you hit it hard you can brinell/dimple the bearings and or races in the gear box and ruin them. Then when back inside a heat gun can be used to warm and expand the coupling. Do not use a torch with the PB Blaster. A heat gun will work wonders. Heat and rotate, heat and rotate.
Use a scrap piece of maple between the mallet and the shaft to prevent potential damage to the end of the shaft from the hammer if you don't have a soft metal or wood mallet. Then re-enter the boat and continue tightening until the coupling is off the shaft. Apply heat to the coupling while it is under slight pressure will help expand it and hopefully aid in getting it off. Please DO NOT over-tighten the bolts! If it does not want to come off please STOP and DO NOT damage your gear box trying. Remember it takes very little force to throw these flanges out of true.
4) Optional: After the machine shop visit bring the coupling home to clean and paint it with a rust proof paint.
*WARNING * WARNING * WARNING: Be very careful NOT to get PB Blaster near ANY engine or transmission seals. True penetrating oils will EAT engine seals causing catastrophic failure of that seal. The most common seal DIYers destroy is the transmission output shaft seal. Be very, very careful using PB Blaster on your engines coupling bolts and DO NOT use the spray feature when working that close to seals. If you need to use a penetrating oil on coupling bolts fill the PB Blaster cap with the penetrating oil and then use a Q-Tip to dab PB Blaster on the bolts being very careful not to drip ANY on or near the transmission output shaft seal.