It's not likely to really be adequate to the job, your speed will be greatly slowed down by headwinds, and unless it's a long shaft you may have trouble in even minor chop keeping the prop in the water.
This scenario can become quite dangerous if you unexpectedly get caught in bad weather/sea conditions.
I have a Yamaha 4hp on my Columbia22..I can make maybe 4kts BUT its tough coming in against an outgoing tide or against headwinds over 15kts..
The 22 has an outboard well in the cockpit so the prop stays in the water in all conditions...I would get at least an 8hp if I were you..
I plan on adding a 6hp four stroke to my transom mount and use my 4hp for back up and the dingy
Which a few of the boatyard pundits said was too powerful for the C22.
I gotta say that this is a terrific engine, powerful enough to get out of the way of anything I need to get out of the way of, I could probably give a fellow sailor a tow and at low RPM it seems to sip gas.
I got it reconditioned for $600. It's a two stroke from the late '80's.
I would think a 6hp might work OK, but a 4 might leave you wanting when you need it most.
I actually have a 15Hp Honda on my 26T, it is WAY more power than I need but I am often glad to have more, not less. I would stick with a 6Hp or better for your 28. My greatest fear would be when the weather kicks up and you're you've doused the sails and motoring is the safest thing to do. You dont want to be in the situation where you cant steer the bow into the wind and current because of lack of power.