Please help me assess the damage and formulate a repair plan

This is a continuation of the thread that can be found here:
http://www.boatdesign.net/forums/showthread.php?t=20189
The topic has shifted so I have moved it into this forum.

Upon further examination of this 21 ft wooden powerboat, there is a significant hole in the hull that will need to be repaired before the boat can be put back in the water. The current owner/builder is aware of this and plans to fix it before selling it if I choose to buy it. He would also have any bare wood epoxied first, then the whole boat repainted. My main concern is the repair of the hull and I would like to hear from those experienced with such things, what they would do.

The owner has suggested putting backing behind the hole from the inside, then filling it from the outside with a thickened epoxy mix. It seems to me the proper repair is to cut away the boards beyond the damage and apply new plywood, scarfing it in in some manner, using liberal epoxy, then painting. The hull is constructed from 1/2" marine ply.


Broad View

Starboard side of hole

Port side of hole

Hole from outside
I couldn't get any better shots from the outside because we just tilted the boat up enough for light to shine through.

Thank you for any suggestions you may have.

 

You have the right idea. The wood around the hole looks soft; possibly the reason for the hole. The purpose of a repair is to restore the full function of the hull (not necessarily the appearance) to the way it was . Anything less than cutting back to sound wood as a start will leave a serious weak area.

 

Yep, Charlie has it right. The garboard looks to have "started", letting moisture in next to the keel and under the sheathing. Remove all the suspect wood, well back into good wood and scarf in a repair (8:1 or better ratio on the scarf for plywood). You'd be well advised to check the keel and keel batten to insure they haven't been affected, which unfortunately is probable.

A backing block, filling from the outside will repair the area, but it's a poor type of repair, likely to pop out once the bottom "loads" up underway, eventually causing similar damage down the road.

It wouldn't be unreasonable to replace the garboards at this point. Garboards are "consumable" pieces, intended to be replaced from time to time, usually more often then the rest of the planking.

 

Additional Info

I may have left out some helpful information. This hole was the result of the boat getting tossed up on the rocks several months ago in one of the tropical storms that came through here. It doesn't feel like there is any softwood around the impact zone, but that is hard to tell for sure. There is a long series of scrapes (into the plywood, but only superficially along the hull then this hole, then some more scrapes. I'm glad I was on the right track as far as repair. Could someone explain what is meant by going 8 to 1 on the scarf? I am a little unclear how the scarf is to be accomplished on 1/2" plywood.
Jeremy

 

8 times the thickness of the piece your scarfing for plywood planking is the general rule. So for your planking (1/2") the scarf would be 4" long. This is a tapered scarf that goes from a feather edge around the inside of the hole to full thickness 4" back from this, all around. Search the word "scarf" and you'll get hundreds of hits on this site.

Since you have impact damage you should check for pulled fasteners, which will appear as dimples or bumps under the sheathing. Impacts loads can tear loose fasteners pretty far away from the damaged area, so don't be shy.