Osmosis in integral FG water tanks
Hi, Steve - I have a few thoughts I can share with you. And I''m sure you are posting a similar thread on the HR owners site http://www.classic-hrs.com/ altho'' you might also consider posting on the UK site http://www.hroa.co.uk/discus/ as there are a number of Nab 35''s/HR 35''s over there.
It sounds like HR added some fiberglass panels to the cabin sole since our 1975 Rasmus was built; we could pull up individual wooden floor panels and expose the entire subfloor, bracing, tank access, etc. And our experience does answer - sorta - one of your Q''s: our hull had zero blisters even tho'', when I pulled the tank''s one large access plate off (fiberglass panel, gasketed & bolted all the way around), I did find some minor blistering on the tank''s gelcoat (which only makes sense). So...at least at that point in the boat''s age (15 years), tank blistering hadn''t (yet?) caused a problem. OTOH I can tell you I just spent time with a shipyard owner who told me this is one of the issues they face when peeling and Hot-Vac''ing blistered fiberglass hulls. They look for concentrated blistering in areas where the integral water tanks exist and, if they find it, then they go after the tanks as well. (You might look closely at some old photos you have or otherwise reflect on what hull blistering you found before starting your project. If you didn''t see this localized, increased evidence of blistering, you could consider skipping tank repairs totally). It''s also important to remember a yard is motivated by two things you are not: they have to warranty their blister repair job for 5 years and so want to eliminate as much localized failure possibility as they can, and more work means...well, more work and more billing. Even a reputable yard such as I''m referring to has no disincentive to recommend add''l work. You can afford to be more discriminating in whether to commission exhaustive tank work or not.
Personally, I think your lengthy description of the difficulty of accessing all of the tank has answered your own question. It sounds like you''ve been very thorough in your hull treatment process, and I can appreciate you want the overall quality of the repair to be retained. But afterall, it is a boat...and not a terribly expensive one, either. And you own it presumably to sail it, not work on it (altho'' my wife would laugh at ME for saying that). Were I in your shoes, I would go about the business of opening the portion of the tank I can reach sufficient for a good repair, dry out the tank and execute a repair on the starboard side of the tank, thereby limiting the *potential* for moisture penetration to only one side of the hull. And as an added sanity check, consider for a moment just how little hull area is adjacent to the remaining tank area - it''s very small relative to the overall surface of the hull.
WRT your question about bladder tanks, when we left Annapolis for the Caribbean, we felt we needed more than the 50 gals of water in that one tank (no, it wasn''t 60...) and so we chose to add two bladder tanks. This is one of the best things I did to the boat for cruising purposes, altho'' that may not be in your plans. I put a 10 gal bladder under each cockpit locker''s sole (removed the ply sole sections and slipped the bladders under the transverse supports). This uses space otherwise unused and puts the weight right where you want it. I ''T''d a supply hose from each tank into the central main fill hose to the main tank, planning to use gravity to allow the bladders to drain into the main tank, installing a ball valve for each supply line. I did not put effort into installing fill hoses on each bladder as I thought I''d fall back on those bladder tanks infrequently, so I had to gain access to the bladders each time I filled them. As it turned out, this worked well for us but it''s an inelegant approach which I''m embarrassed about a bit in retrospect. The bladders could not be totally filled (due to encroachment of the transverse supports on the swelling bladder shape) but we probably got an extra 18 gals of water aboard using this approach. I realize this isn''t the purpose for which you asked your question, but mention it in case it is of use.
You no doubt know the 80/20 rule. IMO you''re now working the last 1-2% of the blistering issue and I encourage you to do what you can to wrap up the project. I don''t think water tank contamination will be likely given the thickness of your hull, you can remove water from the tank when the boat is left idle (winter, or lousy sailing weather periods), and you can seal at least some of the tank, anyway. In short, worst case: you will have a small problem that will take an extended time to appear and you can take other measures to mitigate it. Best case: there is no problem.
Good luck to you.