Moisture on inside of baseplate

[Froggy]

Well we've had our boat less than two months and already, if i was buying again, there's a whole host of different questions i'd ask and things i'd look at. The more I examine the boat in greater detail the more faults I'm finding. Only today i found a section of one of the front doors below the window rotten through, with just the paint hiding the issue - until you poke your finger at it. But that's relatively minor and shouldn't cost too much to repair with filler, or even to replace the door. Of more concern to me is moisture on the bottom of the boat (on the inside of course!):

 

After pulling out the panel housing the shower/bath drain pump switch from the side of the bath a few days ago and noticing a couple of the floorboards directly under the pipes are rotten through (fortunately the rest of the boards supporting the bath seem sound but i need to take a closer look in the not too distant future), I've started looking a bit more closely 'behind the scenes'. I'm glad I did this because upon lifting the cushions and base panel away from the convertible sofa/bed in the aft room yesterday, beneath which most of the pumps are housed along with the toilet flush storage tank and holding tank, i discovered a lot of damp on the floorboards. It turns out our flush pump is leaking so we've temporarily disconnected it and are using bottles of water to flush the loo. I know this has only been leaking for a few weeks because I previously investigated this area to inspect the pump when we thought it had failed a few weeks ago (it turned out that we had a separate flush tank we hadn't been aware of and it had simply ran dry). At that time the floorboards were pretty dry and i can verify this from a photograph i took at the time. Incidentally, we discovered we had a separate flush tank after pumping the toilet out and putting toilet blue down a hole in the roof that we'd assumed fed straight into the holding tank, only to find our toilet flushing blue! It is of course possible that this chemical concoction has caused the seals in the pump to fail. But this isn't my main concern. There was a small gap in the boarding where you could see the bottom of the boat, which was understandably damp due to the leak, but which additionally looked rather rusty, and i was rather concerned that when you prodded this gently with a screwdriver or the like it seemed to 'give' and you could see moisture being disturbed an inch or two away. A couple of people i've spoken to suggest that this may be either the surface paint lifting or what one of them called 'mill bloom' or something similar, i.e. that it was probably just a bit of surface rust only a millimetre or less deep. This is reassuring but i won't be convinced until we've had the boat back out of the water for blacking in a few months time, at which juncture I'll have the area thoroughly examined. But there's more!

 

Now as a result of this discovery I then had a closer look underneath the little removable board at the back of the aft room, which i'm assuming from stuff I've picked up over recent weeks is the cabin bilge. This is nicely painted and bone dry, as it has been on previous occasions, but with my new-found cynicism this time i pushed my hands underneath the boards to an area not visible to the eye, and sure enough there is damp there. This has me concerned. It may of course simply be the moisture from the leaking pump finding its way to the back, or it could be the sign of something worse, and the fact that the previous owner or the broker painted the visible floor area with what looks like a coat or two of boat primer just has me feeling even more cynical that they may have been disguising an issue. We had the boat surveyed before purchase and the baseplate was deemed sound, but what if it is rusting from the inside? I've been further confused by reading a thread on here suggesting that water in the bilge area is fairly normal and can affect the way a boat is ballasted. Is this really the case? Surely water inside the boat beneath the floorboards will eventually rust the baseplate through from the inside? I would appreciate any advice on this.

[Tony Brooks]

The odds are when considering the whole post that it is of little consequence. The bilge area can get damp/wet on most boats. In fact some with through or all in one bilges drained the front well deck via the cabin bilge area into the engine bilge for pumping out and many of them are still around. They tended to have 6mm or less base plates. Some boats drained their showers into the cabin bilge so that the water could be pumped out by the engine bilge pump.

 

Even if the base plate looks really rusty 1mm of steel makes about 8mm of rust so it is rarely as bad as it looks.

 

You will find my boat has(I am almost sure) a somewhat rusty bilge but the area under the trap is painted. This is not to try to cover up problems but its the only area I can get at AND a rust free painted surface is nicer to sponge water from than rusty steel.

 

Water in the accommodation bilge could be domestic water leaks (as yours seems to be), condensation or or window/vent frame to hull leaks. All to be expected over time. The important thing is to try to do your best to ventilate the bilge area as well as you can - which will be not very well unless external bilge vents were built into the boat from new, very few have them.

[jake_crew]

Its condensation. Insulation will help reduce it, but the temperature gradient bewteen warm cabin and cold hull will create condensation. Air gaps allow ventilation (good) abut allow allow damp air in (bad).

 

Following the cold weather of a couple of weeks ago (when the basin froze over), I checked the tug a few days later and found inside of the hull wringing wet. The hull was still cold, but the weather had warmed, and the moist air had condesed on the cold metal.

 

There is no insulation as its a working boat.

 

A week later, with the weather staying warmish, the condensation had all gone.

Edited December 31, 2016 by jake_crew

[Froggy]

Thanks to both of you for your help. Tony, your detailed and knowledgeable reply has put my mind at ease somewhat and I will enjoy celebrating the New Year on the boat a little more tonight. With regard to the comments from both of you regarding condensation, would I be right to assume that this is likely to be a bigger problem in the winter than the summer, or could it be vice versa given that summer would produce more extremes of heat during a 24 hour cycle? What would be the best way to vent the bilge area given that the only access i am aware of is the small inspection panel in the floor at the rear of the aft room? We are using an electric dehumidifier in the cabin areas to combat the moisture and it's proving pretty effective. Is there some device on the market that could do a similar job for the bilge, e.g. by blowing dry air down it? I'm assuming that it would be many years before the rust produced by such condensation would be a serious issue, but can't help wondering how relevant a surveyor's ultrasonic measurements from outside the hull can be, given this factor.

 

A Happy New Year to both of you and to anybody else reading this .

[Bee]

Tony is right. Don't panic. It is always good to be able to get the floor up easily but I doubt whether 1% of boats are this well thought out. When fitting a boat out most people just get a heap of 3/4 ply and bung it down then build everything on top of it. If you have fixed cabinets or beds or anything like that it is a good idea to cut out nice big areas of floor and then replace all the trim etc but with lots of vents. This is a horrible, difficult and grim job and not many do it. I don't think I would bother but I did put in a lot of panels in the floor with brass ring pulls to lift them, it looks boaty and is useful but its a lot of work.

[nicknorman]

Although bilge ventilation is a "good thing" I'd just cite a word of caution about it. The base plate and lower sides will be at canal temperature which will be very low at this time of year, let's say between 0 and 5C. Let's say the boat is being lived on and heated to 22c with a seemingly dry relative humidity of 50%. When that air at 50% relative humidity is cooled to 5C by contact with the base plate, condensation will occur. The more air you pump through the bilge, the more condensation (ie water) there will be since the base plate will be kept cold by the canal water.

 

Of course if you are running a dehumidifier and not heating the boat much so that, let's say, the internal temperature is 10C with a RH of 50%, then when the air hits the 5C base plate there is no condensation and there will be the expected drying effect.

 

In summer the canal and hence baseplate temperature is much warmer and so condensation doesn't occur, a good airflow in the bilge will help to dry out any residual dampness.

[Froggy]

Tony is right. Don't panic. It is always good to be able to get the floor up easily but I doubt whether 1% of boats are this well thought out. When fitting a boat out most people just get a heap of 3/4 ply and bung it down then build everything on top of it. If you have fixed cabinets or beds or anything like that it is a good idea to cut out nice big areas of floor and then replace all the trim etc but with lots of vents. This is a horrible, difficult and grim job and not many do it. I don't think I would bother but I did put in a lot of panels in the floor with brass ring pulls to lift them, it looks boaty and is useful but its a lot of work.

 

That's a lot of work for us, Bee, especially given that this is our first boat and funds have already been stretched to breaking point (in my case at least!). It's time now for me to break out of my extended hibernation from work after taking a voluntary redundancy package and restore some cash to my account, and I'm not looking forward to it! We have our Safety Certificate expiring in early March and will also be getting the boat back out of the water for blacking and new anodes sometime in the spring so, along with other repairs needed such as repairing/replacing a rotten door, it's a case of economising as much as we can.

[Tony Brooks]

Although bilge ventilation is a "good thing" I'd just cite a word of caution about it. The base plate and lower sides will be at canal temperature which will be very low at this time of year, let's say between 0 and 5C. Let's say the boat is being lived on and heated to 22c with a seemingly dry relative humidity of 50%. When that air at 50% relative humidity is cooled to 5C by contact with the base plate, condensation will occur. The more air you pump through the bilge, the more condensation (ie water) there will be since the base plate will be kept cold by the canal water.

 

Of course if you are running a dehumidifier and not heating the boat much so that, let's say, the internal temperature is 10C with a RH of 50%, then when the air hits the 5C base plate there is no condensation and there will be the expected drying effect.

 

In summer the canal and hence baseplate temperature is much warmer and so condensation doesn't occur, a good airflow in the bilge will help to dry out any residual dampness.

 

 

This is why I specifically said "external vents". Internal ones are likely to make matters worse, especially if the boat is being lived on.

[Froggy]

Although bilge ventilation is a "good thing" I'd just cite a word of caution about it. The base plate and lower sides will be at canal temperature which will be very low at this time of year, let's say between 0 and 5C. Let's say the boat is being lived on and heated to 22c with a seemingly dry relative humidity of 50%. When that air at 50% relative humidity is cooled to 5C by contact with the base plate, condensation will occur. The more air you pump through the bilge, the more condensation (ie water) there will be since the base plate will be kept cold by the canal water.

 

Of course if you are running a dehumidifier and not heating the boat much so that, let's say, the internal temperature is 10C with a RH of 50%, then when the air hits the 5C base plate there is no condensation and there will be the expected drying effect.

 

In summer the canal and hence baseplate temperature is much warmer and so condensation doesn't occur, a good airflow in the bilge will help to dry out any residual dampness.

 

Great advice, thanks. We are heating the boat to around 20C at the moment (not sure of the RH, but we did inherit a barometer with a humidity dial in the aft room so could probably use that as a rough guide). I guess it's just a case of holding tight until the early spring then and seeing what can be done at that point.

[Tony Brooks]

All I do is accept that it will happen in winter so when we are back home I leave the rear trap open and sponge any water out when I visit the boat. You soon get to know how much to expect so if its more you can investigate.

[Bee]

I think this proves that sometimes you just can't win with boats (!)