minimum screw size

[fred1diver]

hey everyone, I picked up a swim ladder for Cool Whip this winter and I'm wondering what the minimum screw size would be to hold her on my transom?
would 4 #10 screws be sufficient or should I go bigger?

 

[rgranger]

Can you through bolt to a backing plate?

 

[Joe]

Can you through bolt to a backing plate?

rgranger's comment addresses the more important issue. You can use fender washers if fabbing up a set of backing plates is too daunting. The point is.... through bolting the hardware is highly preferable which means you can simply choose the bolt size that fits the ladder's mounting holes. Just a wild guess but I imagine 1/4 inch standard thread, flat or oval head bolts. The appropriate sized fender washers and nyloc nuts. Of course, you'll have to determine the bolt length based on ladder location. My ladder is on the transom with very easy access and I think the bolts are 1 1/2.
Don't forget the proper mounting sealant for waterproofing. I like "LifeCaulk" but any polysulfide adhesive/sealant or butyl tape will work.

 

[dlochner]

There will be a lot of force pulling away from the hull, so a proper backing plate is essential. ¼-20 screws should be adequate. I doubt the Grampian's hull is cored so that won't be an issue, however if the mount goes on deck, it is likely cored so sealing the core would be essential.

 

[shemandr]

I put one on my runabout. It had to be on the transom. At 265 lbs (Then) if I tried to climb up the side, the boat would list so much it was impossible to climb. I still needed to put a rail on the stern to grab on the way up.
I used thru bolts and backing plates made of plywood and some of Maine Sail's butyl tape. Screws have good holding power but I've seen enough failures - largely when used in tension - to not trust them. Once that screw hole is "Pooched" you've got a problem. You prolly can't use a bigger screw because of the hole in the ladder frame. Placement of the ladder can be very specific - low enough to have 2 steps in the water, high enough so the ladder doesn't drag and at the right athwartships position to be able to climb over what ever rail you have there and whatever is mounted on it. You might not be able to relocate it. Thru bolt.

 

[Tally Ho]

I just rebed the bolts on my swim ladder...4) 1/4” bolts on each (2) hinged plate...with backing plates under each.

Greg

 

[FastOlson]

A #10 machine screw or a quater inch... either will be fine. There are likely three per attachment plate on each side. And yes on a backing plate. I used some G10 leftover bits for backers on ours.