Water jetting

Discussion in 'Metal Boat Building' started by Nick.K, Nov 30, 2011.

  1. Nick.K
    Joined: May 2011
    Posts: 328
    Likes: 25, Points: 18, Legacy Rep: 103
    Location: Ireland

    Nick.K Senior Member

    I have an empty 40ft sail boat hull in steel, I have replaced the deck and cockpit and now need to prepare the steel for paint. The hull had a fire and then sat for many years. Some areas are painted, some rusty (but sound) and some new steel with millscale. I was planning to grit blast with crushed glass but the dust is a problem as there are many boats in winter storage. I can rent locally a water jetting machine which I am told is suitable for steel prep (cost €500/day). I have no experience with this. How effective is water jetting on mill scale, rust and paint? How many hours of jetting should I expect?
    I am thinking of applying Jotamastic (by airless spray). Any comments?
     
  2. Submarine Tom

    Submarine Tom Previous Member

    How many PSI will you get at the nozzle?

    -Tom
     
  3. Nick.K
    Joined: May 2011
    Posts: 328
    Likes: 25, Points: 18, Legacy Rep: 103
    Location: Ireland

    Nick.K Senior Member

    I don't know the specs for the machine I can rent, I was told about it by a steel engineer.
    What range of pressure should I be looking for?
     
  4. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 487, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    Unless you get an industrial water blaster, which is considerably more powerful then a top of the line Lowe's/Depot pressure washer, you'll just make more rust.

    The usual choice and the economical one, is to media blast the hull shell. Yep, it's dusty, but you can tent the hull shell and contain the dust and debris. Sand is the common and inexpensive media, but coke and walnut shells also can be used, which leave a better finish.

    To specifically answer your question, you need a minimum of 5,000 PSI (350 kilo/cm2) to remove scale and surface rust. Of course heavily rusted areas will need more pressure or abrasives.

    All this said, you can do a really good job with chemicals, preceded and followed with pressure washing. Naturally, most of these chemicals are going to cause your marina to scream, unless you can completely tent the hull and ground, contain all of the goo and rust and dispose of off site. There are also some biodegradable products that you can use. The cost of all of these products will make you scream, so you're back to abrasives and tarps on a Sunday morning when no one is looking.
     
  5. Submarine Tom

    Submarine Tom Previous Member

    Ya, what he said.

    -Tom
     
  6. Nick.K
    Joined: May 2011
    Posts: 328
    Likes: 25, Points: 18, Legacy Rep: 103
    Location: Ireland

    Nick.K Senior Member

    Thanks for the replies.
    In the end I will probably blast with crushed glass as planned, I have been involved in blasting a few hulls, but I am interested to learn about the alternatives hence the thread.

    For €500/day machine rental (no labour) I would expect more than a top line pressure washer...

    Sand is cheap yes but causes silicosis, it is also illegal as a blast media in Ireland.

    What profile does water jetting create? How does this compare with the profile created by media blasting as a key for the coating?
    Has anyone experience of maintaining hulls that have been coated with jotamastic following jetting, I read that it can be applied to humid surfaces, but how tolerant is this?
     
  7. grumpygrady
    Joined: Oct 2006
    Posts: 18
    Likes: 1, Points: 0, Legacy Rep: 15
    Location: florida

    grumpygrady landlubber

    have you thought about using dry ice blasting
    works great and nothing to clean up
     
  8. Nick.K
    Joined: May 2011
    Posts: 328
    Likes: 25, Points: 18, Legacy Rep: 103
    Location: Ireland

    Nick.K Senior Member

    No haven't looked in to dry ice. I want to do the work myself, not only for the cost but I think I would do a better job than a contractor. Trawlers being blasted in the yard often have shadow areas behind frames and poor attention to detail. The contractors seem to clean gravestones and patios most of the time.
    Nick.
     
  9. grumpygrady
    Joined: Oct 2006
    Posts: 18
    Likes: 1, Points: 0, Legacy Rep: 15
    Location: florida

    grumpygrady landlubber

    1 person likes this.
  10. SamSam
    Joined: Feb 2005
    Posts: 3,900
    Likes: 197, Points: 63, Legacy Rep: 971
    Location: Coastal Georgia

    SamSam Senior Member

    Glass is made from silica (sand), it's kind of hard to see the difference. Use a good air supplied hood, no matter what. Why is sand illegal as a blast medium?
     
  11. Nick.K
    Joined: May 2011
    Posts: 328
    Likes: 25, Points: 18, Legacy Rep: 103
    Location: Ireland

    Nick.K Senior Member

    I'm not a chemist or an expert... but if you look it up you'll see that grains of sand shatter in to splinters of silica small enough to enter the cell structure of the lungs and cause cancer similar to asbestosis. Glass is fused silica which shatters in a different way in to particles that are too large to enter the cell structure. Sand is illegal for blasting in many places because of the health risk.
    Nick.
    Still curious about water jetting. Has no one information to share??
     
  12. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 487, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    It's not difficult nor unreasonable, to take precautions when blasting, regardless of media type. Anyone that isn't doing this needs to have their head examined.
     

  13. gonzo
    Joined: Aug 2002
    Posts: 15,439
    Likes: 1,011, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    You can use a slurry blaster. It is a regular sandblaster with a water input at the nozzle.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.