I am trying to determine the minimum HP for a longtail that I would need to power my 18' modified V, Seaark. I have a 50 HP outboard, but want to get in shallow marshes 8-20" of water.

Would I have to remove the outboard to lighten it up?

I don't care about speed, just want to get across the water with 2-4 hunters and gear.

Any thoughts?

Either or. The 50, or a big longtail (27-35, 35 suggested). You'll hate trying to run that Longtail with anything else back there. It's not an add-on type of motor. An sd would be the best balance between the two, but a big Longtail would do the job well. Neither are gonna run as fast as that outboard, though, so decide what your overall expectations and priorities are, and go from there.

Yes, my plan was to perhaps get a small longtail and add it to the boat and outboard, much like a smaller outboard and a large main outboard. I had not thought about the longtail being blocked by the outboard.

The boat is 72" wide, I don't know the weight, but it's pretty heavy. The outboard is not a tiller, console steer.

Ideally, trying to avoid spending the big money for a large long tail or surface drive, and not want to repeatedly remove and replace outboard.

From a power standpoint, if I removed the outboard do you guys think a 15hp would push a boat this big as long as it was in enough water to float and had little to no vegetation to push through?

Yes it will push it forward at about 4.79357 mph with a 4 man hunting load. It will bog down in thick stuff. I hope your not looking at SPS/Thai longtails either.

Seems like you want the convenience of a mud motor, but dont want to commit fully financially to one. If you go the route of buying a 15hp, you will just be throwing your money away on an expensive test drive. You will be hooked once you run one, might as well get something that wont need to be replaced.

If you are not a wimp, get the 35 HP (preferably a Swomp) and put the OB in the garage till it's time to fish. Longtail mode is a bit slower but reliable...kind of moving from an auto to a pump shotgun.