backup plates for Scotty surface mounts?

Hmmmm......the self tapping ss screws into the piece of starboard sounds like a good idea. Where these suckers mount is extremely difficult for me to reach thru the 8" center hatch and get a washer & nut started on a bolt.

Art nice avatar! Did you and Tote get some signs made up?

The place I'm thinking of mounting is a little difficult as well, especially with my short arms. 

Will using self tapping screws without washer and nut hold well? 

is there any reason why you don't want to use a blind rivet? or better yet the 3 prong klamptite rivets? I have used well nuts on ram balls for FF bases and they work pretty well. I would venture to guess that well nuts would hold as good as starboard and self tapping screws will. another Idea is to use 3" aluminum flat bar from home depot and cut it with an angle grinder. come to think of it I haven't ever used a backing plate on the standard scotty bases, they are pretty big and spread the load out as is, on the small ram ball mounts I always do.

I don't own a rivet gun, and wasn't sure I wanted to get one just for a single time use, but they do seem like the easiest way to go.  hmmmmmm....

I will say that I picked up a rivet gun at Harbor Freight years ago for a "one time" job and used up all the rivets in short order on other projects.  I don't think you will ever regret having one on hand.  Get one that has an assortment of tip sizes. My two cents.

if your really broke like me, get a harbor freight rivet gun and a pack of rivets, then use the gun and return it the next day. I just said i dint need it found my old gun

So I'm considering rivets as an option again.  I understand stainless steel is preferred over the aluminum, right?  I've been looking on the scotty website and couldn't find any info, but what size holes do I need to drill and what size rivets would I use for the regular scotty 241 side/deck mount?

When a big halibut, wsb, striper, etc hit......whatever you attach with better be strong. Rivets and well nuts don't spread out the load far enough for my peace of mind. I've seen rod holders snap right off which is a lot better than a well nut or rivet pulling through the side of my kayak and leaving bigger holes. Took Art's advice and went and got a scrap piece of 1/4" starboard at Tap for a buck today. Unfortunately Tap is right across the street from West Marine so I guess the new rod kind of negated the cheap starboard purchase. 

So I'm considering rivets as an option again.  I understand stainless steel is preferred over the aluminum, right?  I've been looking on the scotty website and couldn't find any info, but what size holes do I need to drill and what size rivets would I use for the regular scotty 241 side/deck mount?

The size of the holes you drill should match your rivet size, I suggest going a little smaller and forcing the rivet in when you pop it for a more watertight installation.  The rule of thumb is "the tighter your rivets fit before you pull them the stronger you rivet will be."

Stainless rivets will destroy a $15 rivet puller, if you are lucky you might be able to squeeze a small one. Then something will snap, I've seen it happen twice in my shop in the Navy. Stainless rivets take a hydraulic riveter and that costs some dough, but it is so over kill and ridiculous to remove them from soft metal let alone plastic it is not even worth it. 

Got ya.  Now I just got some thinking/planning to do this weekend.  Need to take the yak out again and see where the best place to mount will be.  Thanks for all the help.

Use the Klamptite reg rivits don't hold that well   http://kayakfishinggear.com/rivets-10perpack-blackanodized-316-seepricediscounts.aspx

Regarding back up plates.   

Give the nearest TAP plastic place a call or just visit them for them boards.  They should have scraps lying around for free.