Jerry,
Your boat will displace the same amount whether or not there is foam in the keel, so it would only add buoyancy if the boat is swamped and there is not enough there to save her in that case. I am of the firm opinion that there is not a expanding foam that will not eventually absorb water. There has been much discussion on this in the Ensign fleet. People have been replacing the saturated expanding foam flotation from Pearson with new closed cell expanding foam and having their boats gain hundreds of pounds five + years later and suspecting the new foam of absorbing water. Some people now pull out the foam and replace the flotation with 2 liter pop bottles. Also, I can't tell much from the picture, but I would suspect an improper bottom job of causing the blisters before I would suspect water in the keel void. These old girls generally don't have the same problems as the new production boats. $0.02
Mike
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