I work on a/c for a living just not on boats. Similar but I am sure there are some special considerations taken for boats. I will try to help as much as possible.

-Typically an a/c unit will blow between 15-20* cooler air than it's return air temperature. If you got that then I guess it's ok.

-Again , check the return air ducts for any leaks which may be letting outside air into your air stream. A/c works on removing the heat from the air not adding cold air to the space.


-next question would be how big is the cabin? Generally 5 to 6k btu can cool a space approximately 300 sq. feet. (12k btu =1 ton of cooling = 600 square feet of effective cooling residentially) I know this is not the same with a boat but I would guess you still don't have 200 square feet to cool so I would think it is sized ok. Now check for any places that are not sealing out the outside air, That essentially adds load and could be the cause. If it's none of that than maybe you do need a bigger size a/c. But I find it very hard to believe unless your boat is black.

I hope this helps, Good luck.

Thanks for the info.....