First, you can't skip grits. If you start with 40 you need to go to 60, 80, then either 100 or 120, then 150 or 180, then 220. Hitting them each wouldn't be overkill. If you do this each grit will be able to work the way it's supposed to, and it will actually take less time with each successive grit. 120 grit isn't intended to take out scratches left behind by 40 grit paper. As Rob said, sand paper works best with light pressure. Using the right grit lets you use light pressure. Skipping grits makes you more likely to end up with unsanded or rough spots when you get to it. Also- sandpaper is an edge tool that needs to be sharp to work, just like a plane or a knife. When it gets dull it doesn't work right. Keep it fresh, and throw out marginal paper. Don't wait for it to get so dull it feels smooth. If it makes you queasy because it seems wasteful move to a hand plane and card scrapers. They're durable and can be resharpened over and over.

As others have said, a random orbit sander is the right tool, but it is my opinion that before you hit the ROS, you need to use a long board to fair your hull. Any sander can only fair as wide as its disc, otherwise it will follow the contours of the boat. For a ROS you're talking 6". I generally use an 22" long board. I start with 60, but don't use a ROS at 60 because it's too aggressive. It's easier to make mistakes than using a long board.

Lastly- there's an article on sanding in the latest Fine Woodworking: http://www.finewoodworking.com/SkillsAndTechniques/SkillsAndTechniquesPDF.aspx?id=34628 You may also want to look into how auto body shops do their sanding. It's the same process as fairing a boat.

Time and patience are the most important steps in good sanding.

Lastly- sometimes wood is blotchy because wood is blotchy. Maple and Cherry are notorious for being blotchy, but softwoods can look blotchy or be blotchy around grain reversals, so it may not be your sanding.

m