Alloy hull pitting
Printed From: The Fishing Website
Category: General Forums
Forum Name: The Boat Shed
Forum Description: Discuss all things boating.
URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=56671
Printed Date: 03 Jun 2026 at 6:51am
Topic: Alloy hull pitting
Posted By: Hamish_S
Subject: Alloy hull pitting
Date Posted: 23 Aug 2010 at 11:18am
|
Hey guys I have noticed when wiping out the bilge yesterday that there is some pitting in my hull, I will try take some photos to show you, they are small rough patches the biggest being slightly smaller than a 5c piece - the pit part being a small dot. Is this normal?
|
Replies:
Posted By: Tagit
Date Posted: 23 Aug 2010 at 11:23am
| Is the boat a Ramco? |
Posted By: Hamish_S
Date Posted: 23 Aug 2010 at 12:15pm
| na its an 4.5 year old marco 570 |
Posted By: YellowV198
Date Posted: 23 Aug 2010 at 12:38pm
|
http://www.powerofaluminium.com/html/corrosion_resistance.htm read that one it explains pitting .... I just hope its not electrolosis which can be cause by lead etc sitting on the ali and the dissimilar metals reacting.
|
Posted By: Titahi
Date Posted: 23 Aug 2010 at 12:49pm
|
Did you find a sinker in the bilge? ------------- "I love standing by the ocean and just knowing what its for" |
Posted By: Tagit
Date Posted: 23 Aug 2010 at 1:12pm
|
A few basic possibilities. 1) The alloy is not good quality marine grade. We have this on one half of a Ramco hull. One side perfect, one side with pitting that we are sorting out 2) There were pencil lines left on the hull. Apparently lead pencil if not cleaned off can cause pitting 3) There has been a sinker or some other foreign metal inside the hull. Are you looking under the floor, or in the open bilge? I would also send the photos to Marco and get their feedback as well.
|
Posted By: Hamish_S
Date Posted: 23 Aug 2010 at 2:45pm
|
I found no sinkers or anything down there, its on the hull plate in the under floor area on the boat, more concentrated nearer the back rather than in the front compartment.. Ill take some photos tonight |
Posted By: CEEBEE
Date Posted: 23 Aug 2010 at 5:44pm
|
Ensure that NO electrics are earthed to the hull. ------------- I STARTED THE DAY WITH NO FISH AND I STILL HAVE PLENTY LEFT |
Posted By: Catchelot
Date Posted: 23 Aug 2010 at 6:40pm
What he said...interesting about pencil lead causing a reaction... ------------- "The sea, once it casts its spell, holds one in its net of wonder forever." - Jacques Cousteau |
CEEBEE wrote:Posted By: Catchelot
Date Posted: 23 Aug 2010 at 7:07pm
|
Yeah the next things could be stainless fittings that are electrically bonded to the Alloy hull, but doubtful in that area close to the problem...meaning not insulated with rubber,nylon,plastic,etc...
So photos would be good mate.
I also wonder if sacrificial Zinc Anodes would be sensible on an alloy hull... anyone know?
Other theories are an earthing wire off the hull to a ground rod when the boat is parked/stored. ------------- "The sea, once it casts its spell, holds one in its net of wonder forever." - Jacques Cousteau |
Posted By: six guns
Date Posted: 23 Aug 2010 at 7:15pm
|
one lose wire can do funky things to alloy ..... ------------- skull draging and taging |
Posted By: Busted!
Date Posted: 23 Aug 2010 at 7:46pm
|
Doubtful if it is electrolysis, if so the entire outer hull surface in the affected area (below the waterline) will be chewed off. Typically there is NEVER electrolysis pitting INSIDE the hull as the contact with seawater that is part of the circuit occurs outside the hull.
If it is chemical corrosion of the ali (as in cases where a non-marine grade plate found it's way into the stack going thorugh the marking machine and ended up getting sent out with the rest of the 5083) then you end up with areas that look like white powdery worms have chewed into the ali. You grind them out for welding and end up with slots to weld up. Real ugly...
In your case as it appears to be inside the bilge in an area that doesn't get exposed to the full seawater effects of electrical conductivity and electrolysis I would be interested to find out what you use to clean the boat or if there has ever been any unusual cleaning products or chemicals spilt inside the boat. As the chemical corrosion of ali normally results in white powdery deposits I would put forward the opinion that something has been spilt in the boat that has reacted with the ali as it flowed down into the stern, hence the amount of corrosion increasing towards the arse.
Once the white powdery deposits have been cleaned off you end up with pitting. I would measure the pits, if any are more that 15% of the plate thickness consider puddle weld repairs. If they are shallow, clean back with a wire buff in a drill and treat with an ali corrosion preventer and monitor them.
|
Posted By: Catchelot
Date Posted: 23 Aug 2010 at 8:02pm
|
Good answer Evan, some cleaner chemical compound sounds more likely to be the culprit in the bilge area, can't see it being electrolysis issues... ------------- "The sea, once it casts its spell, holds one in its net of wonder forever." - Jacques Cousteau |
Posted By: Hamish_S
Date Posted: 23 Aug 2010 at 8:48pm
      Alright there ya go!
The last couple are of bubbling paint i have on a quite a few patches around the boat. Im not sure if this has anything to do with it or not.. I have a feeling I dont want to know whats actualy going on.......
|
Posted By: Tagit
Date Posted: 23 Aug 2010 at 8:52pm
| Bubbling paint around fittings is sadly quite normal. The other stuff looks a bit like our Ramco with one dodgy plate on the hull. Does yours affect both sides of the hull or only one side? |
Posted By: Hamish_S
Date Posted: 23 Aug 2010 at 9:03pm
|
I would be quite happy about the bubbling paint being normal, I have thought it was something bad specific to this boat for ages. So if its bad, but common/normal im in a funny way happy. Its on both sides of the hull. To be honest it may have sat with salt water in the bilge at one time or another, I always give it a good spray out but the thing is all the water doesnt drain out. even with the jockey wheel fully extended and its far to heavy to just lift up for all the water to drain out. after finding this i think i will dry it out with a towel every time from now on... |
Posted By: Kings Marine
Date Posted: 24 Aug 2010 at 10:30am
|
Those first 3 or 4 photos are quite common on alloy boats, especially in the bilge area - if you lift the entire floor sometimes it goes a fair way up the length of hull. Still dont know why, as you can have four boats of the same model that are perfect and then the next one will pin hole like above ??? sometimes its the boat that is very well looked after that corrodes (i.e. low hours, bilge always washed out and dried, stored inside etc) Most hull builders know about lead pencils and I would hope they dont use this method of marking any more. ------------- The Mighty Waikato http://www.facebook.com/pages/Kings-Marine/100476476662857?%3Fref=ts |
Posted By: Titahi
Date Posted: 24 Aug 2010 at 7:18pm
|
You could possibly clean the under floor really well and then cover every thing in some fisholene oil, after which cleaning becomes a simple matter of hosing..... with no need to towel dry ------------- "I love standing by the ocean and just knowing what its for" |
Posted By: Big H
Date Posted: 24 Aug 2010 at 8:06pm
|
Posted By: skidoggg
Date Posted: 24 Aug 2010 at 8:30pm
|
quote ...If they are shallow, clean back with a wire buff in a drill and treat with an ali corrosion preventer and monitor them.
busted, could you elaborate a bit on where to get this corosion preventer and what its called ..
|
Posted By: Busted!
Date Posted: 24 Aug 2010 at 11:05pm
|
Unfortunately the only complete corrosion prevention for aluminium is preventing chemicals from attacking in the first place. Typically it is the well maintained vessels that suffer bilge corrosion due to the amount of exposure to chemicals. Using incorrect degreasers and cleaners is worse than not cleaning ali hulls...
For protection, from new two pack specialist ali paint is the way to go. For bare hulls, clear coatings are also good but in the event of coatings over pits you need to go to an engineering supply outfit and find out what they have in stock. What you need to do is prevent any more crap from getting into the pits that will cause more corrosion. On the deeper pits best way to do this is ali puddle welding, on surface stuff wire buff back and apply an etching primer (ugly) or as another suggestion clean, degrease and fill the pit with Loctite two pack liquid metal epoxy paste or a similar aluminised epoxy. Note, it must be aluminised not the steel-filled loctites or epoxies!
|
Posted By: ancient mariner
Date Posted: 24 Aug 2010 at 11:49pm
Having done plenty of roofing in early years, one of the big no-no's was to use pencil on zincalume roofs, as it would corrode before long! The only metal that lead doesn't affect is copper, but anything else it will have an affect (different metals different rates). Aluminium is high on the list, higher than zinc, so the effect would be at a greater rate. Just a bit of utter useless info you ya. 
|
Posted By: Hamish_S
Date Posted: 25 Aug 2010 at 12:12pm
| Hrrm im not very good with the hands on side of things, Is there anyone you guys can recomend down here in TGA to get this sorted? |
Posted By: RC1
Date Posted: 25 Aug 2010 at 1:35pm
|
Hawkins Marine 027 548 1566 or Glen at Gfab 027 2800 708 both excellent guys, they repair all my alloy work. |
Posted By: wetdream
Date Posted: 25 Aug 2010 at 3:00pm
|
just as a side note to this pitting in alloy thing, I cant believe with all this knowledge that the manufactures still do to get it right. I have a brand new (well 7 hours on the clock) Frypan 500 something or other in here getting signwritten today and I'm gob-smacked to see just how much stainless and alloy is touching. This boat was purchased new from the Auckland boat show and already has signs of white chalky stuff around areas. What will this look like in a year. People have said there is no way they would still use led pencils, I would'nt be so confident if they are still putting alloy and stainless together with nothing between them.
------------- Like us on: facebook/stanleysigns.com |
Posted By: Kings Marine
Date Posted: 25 Aug 2010 at 4:32pm
|
wetdream I dont know if it is an issue of ignorance, or maybe an issue of lack of technical knowledge that causes many of the basic types of corrosion issues. Many boat builders hire staff to weld - not to posses the knowledge of marine corrosion. This leads to the inevitable question of, where are many of our marine technicians and marine engineers - not in N.Z. How many marine technician apprenticeships are filled each year - a whole heap less than when I did my apprenticeship thats for sure. This type of issue is replicated in many trades in N.Z. and contributes to what we know as the "brain drain" unfortunately then you can have a situation where there is a worker trying to do a job they are not trained for and mistakes will happen. I have trained several apprentices myself over the years and I can tell you that the training system at the moment for training technicians is laughable, the word "Circus" springs to mind !!!! ------------- The Mighty Waikato http://www.facebook.com/pages/Kings-Marine/100476476662857?%3Fref=ts |
Posted By: Rodinator
Date Posted: 25 Aug 2010 at 6:16pm
| I second RC1, Duane from Hawkins Marine is a top bloke, I would also recommend Les from Metal Works Innovation |
Posted By: riga
Date Posted: 25 Aug 2010 at 7:16pm
| We had similar issues with our ramco dominator under the floor boards, we went to the paint shop opposite greeton marist footy club (can't recall name) and bought some etching primer and a two pot tar like paint that went on two coats over the primer, we did this after lightly grinding/buffing the inside of the hull, it appeared to come up quite well, then replaced the floor boards using lanocote on the screws to help stop them from reacting with the alloy. |
Posted By: KBee
Date Posted: 26 Aug 2010 at 8:40am
| Alloy Cats/Kingfisher boats have repaired plenty of these types of issues. 577 0977, or 0274 778 693 |
Posted By: Uncle
Date Posted: 26 Aug 2010 at 9:33am
| I understand BayFisher mag is running an article on this very issue in the September edition. |
Posted By: Hamish_S
Date Posted: 26 Aug 2010 at 10:27am
| Tops! Ill look out for it |