Osmosis signs after time out of water

Ben998

New member
Joined
3 Nov 2004
Messages
153
Location
St.Lucia today, Martinique asap!
www.manateeofhamble.blogspot.com
Hi All,
Do osmotic blisters disappear after a boat has been out of the water for an amount of time, say 18 months? Im assuming that as the hull dries out the blisters (if any) will disappear, but should still leave tell tale signs. However, these signs may be difficult to spot as they would be under the antifoul. Presumably if I get the antifoul removed by blasting it would show up any old blisters and would leave holes in the gel coat?

The reason Im asking is that Im looking at a boat thats been out of the water for about 18 months and the surveyor has noted moisture readings and signs of osmosis on the rudder but not on hull. That is all I know from an initial phone call as I haven't received the detailed survey yet.
Whats the probability of the rudder having osmosis, and the hull being osmosis free? Its a 10 year old Jeanneau 37. Could the rudder and hull have been built in different places using different materials? I may be able to negotiate for the vendor to pay for the treatment but until I get the survey back I dont know what else it may need.

cheers,
Ben
 
G

Guest

Guest
Generally blisters only reduce due to the pressure inside reducing with being out of water ..... but the moisture inside then will mostly remain. The reason for the blister is the density of the liquid that gets in to the laminate increases when it disolves various inside ... to a point that its density is too great to pass back out again .... VERY simple explanation !

It is common for rudders to have osmosis but is a problem that is not so serious as on a hull .... grinding back, filling and painting is fine here.

A 10yr old Jeanneau without osmosis guard or treatment previously ?? It's most likely to have osmosis or moisture to some degree- whether its to blistering stage ... I'll leave to your survey ....

So answer is - Blisters do not disapear - they only reduce in size, the problem does not go away ....
Yes it is possible for rudder to have and hull be free - but unlikely unless hull has been previously treated.
 

maxi

Member
Joined
8 Nov 2002
Messages
973
Location
Hampshire
Visit site
Can't tell from here what the osmosis position may be on the rudder, but, very high moisture content readings are the norm on the majority of rudders. High moisture content is not an automatic forecast of osmosis.
 

boatmike

Well-known member
Joined
30 Jun 2002
Messages
7,044
Location
Solent
Visit site
Very common to have rudder problems due to water getting inside the rudder itself. The boat may be entirely unaffected. Best cure would be a complete new rudder as it may be degraded internally. If the owner will let you a hole drilled in the bottom of the rudder may well confirm it to be full of water. If it is, surface treatment will not solve the problem. it should be rebuilt or replaced.
 

DickB

Member
Joined
20 Dec 2002
Messages
785
Location
Surrey, UK
Visit site
Rudders are often subcontracted out as I found out when I replaced my Bav42cc. After delay upon delay I rang Bavaria in Germany and they said that the Italian manufacturer had not delivered the new rudder. Possibly that is why there is a difference in the standard of biuld...
 

Birdseye

Well-known member
Joined
9 Mar 2003
Messages
28,487
Location
s e wales
Visit site
The rudder moisture might be due to difference in build quality, but it likely could be due to water ingress past where the rudder stock is laminated in. As a consequence, most osmosis companies wont guarantee treatment on a rudder or a skeg

As it was explained to me, if the boat has osmosis the hull wont dry out just because the boat has been left out of the water. Ever. The whole point about osmosis is that the thick soupy solution behind the gelcoat has molecules too big to get through the porosity in the gel. Thats why you get blisters - volume behind the gel building up with no where to go eventually causes bulges visible on the surface.

So conversely, if the hull really is dry, then there are grounds for hope. The query at the back of my mind is twofold. Firstly one well known surveyor reckons that most ortho poly boats at 10 years have the beginnings of osmosis. Its inherent in the resins used or so one resin supplier says . The second worry is how difficult it is to get accurate moisture indications. As an illustration of this, I had a situation where the surveyor of an epoxy company came to check that my boat was ready for coating and pronounced it perfectly dry. When he had gone, I borrowed a Sov meter and found damp patches in a number of places. The reason was that I had a lot more time, and was way more systematic than he had been measuring moisture on a 12 inch grid all over the hulls. Ironically, the rudders were dry.

I dont know what the answer is. People will say that osmosis isnt the killer it was once thought to be. But having gone through 2 osmosis treatments, the sheer frustration and time wasting involved would make me very reluctant to accept a boat where there was any risk of it at all. But the seller might not like scraped patches at 12 inch spacings.

The least I would do is to buy or borrow a meter and really carefully check myself. Or maybe get a surveyor to do a really systematic check. Or buy a new boat.

But then I am a witterer


.
/forums/images/graemlins/frown.gif
 

oldsaltoz

New member
Joined
4 Jul 2001
Messages
6,005
Location
Australia, East coast.
Visit site
G'day Ben,

You asked << Do osmotic blisters disappear after a boat has been out of water for 18 months>>

The very small blisters can shrink to the point they are very hard to locate, larger blisters will also shrink to some degree but will be seen.

<< these signs may be difficult to spot as they would be under the antifoul. Presumably if I get the antifoul removed by blasting it would show up any old blisters and would leave holes in the gel coat>>

The blisters are formed in the laminate, blasting off the antifoul will make them less visible and you will not see holes in the gelcoat, the water has passed through the gelcoat, gelcoat is Not Waterproof in the long term.

<< the surveyor has noted moisture readings and signs of osmosis on the rudder but not on hull>>

This would indicate you do not have a problem in the hull, repairing a rudder is not a huge task and there are a few things we can all do improve the situation. Mass produced rudders often contain an open cell filler or are made from closed cell foam but have air voids between the them, this void will fill with water and cause damage.

A well made rudder (for a 25+ footer) will have its tangs shaped to fit the stock and lay horizontal, and shaped to fit rudder foil profile; welded to the stock and cut so that the weld runs half way around the stock, rather than bits of flat bar butt welded vertically on the back. Top tangs should be at least 100 mm down from the interface, so that if/when an object/heavy line gets trapped between the rudder and hull, the top of the rudder will break, leaving the hull intact. The bottom tang should be at least 250 mm from the finished bottom, again to allow it to break rather than rip the stock tube from the hull. The internal areas should be filled with closed cell materials only, and all air gaps should be filled with closed cell spheres and resin.

As to water ingress via the rudder stock to rudder interface, a couple of '0' rings with a little Vaseline on them glassed in will all but stop this problem.

<< Whats the probability of the rudder having osmosis, and the hull being osmosis free>>

Boat manufacturers and rudder manufacturers use much the same practices, a bit like a farmer and a gardener, they both know how to do each others task, and they could in theory do each others jobs, not sure the outcomes would be the same though.
Many boat manufacturers use others to build their rudders but most boatbuilders build their own.
So in short, if your boat has been mass produced, its a very good chance your rudder was, and little chance it was the same crowd.

I'll finish with a note on moisture readings. first point use them as a guide only, point 2, the readings are only as good as the operators experience, point 3, you will get variations in readings depending on what is on the other side of the section you reading, water tanks are an obvious one, but timber will give a higher reading be it your engine mount blocks or ribs. like I said it depends on the operator, if you get a reading done and the surveyor does not get on board to what is located where, your readings will be rubbish.

Avagoodweekend......
 
Top