Bowsprit Replacement

Discussion in 'Boatbuilding' started by svnakia, Jun 5, 2012.

  1. svnakia
    Joined: Jun 2012
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    Location: Mazatlan Mexico

    svnakia New Member

    Hi,
    I have a Hans Christian 33 that I've owned since 1991. I currently have the bowsprit out to take care of some delamination in the foredeck and I'm starting to really dislike this piece of wood that the bowsprit is made from.

    The existing bowsprit is made of solid wood, not laminated. It is a tapered box section about 9 ft long in total. It is in great condition with no rot. It looks like Apitong to me. I've had it out a couple times and never really knew how much it weighs other then 'really heavy' so this time I borrowed a bathroom scale and risked a back injury to weigh it: 140 lbs!

    I know from long experience with this boat that getting weight off the ends, especially the bow, helps performance a great deal. We move the anchor chain from the chain locker to the keel before every passage because of the handling improvement. Hence I've been thinking a long time about replacing the sprit with something lighter.

    I've thought about Aluminum, but have discounted it because there are a lot of stainless parts that come in close contact with the sprit (cranze iron, pulpit, reinforcements in the sampson posts are all stainless) and because the bowsprit takes _a_lot_ of saltwater.

    I've fantasized about carbon fiber, but discounted it because of the cost and the fact that the bowsprit can carry a large out-of-column load (the anchor chair can pull very hard at nearly 90 degrees to the column of the sprit when pulling the anchor in big waves). It is my understanding that this kind of load is not appropriate to carbon spars.

    My latest fantasy is fiberglass.

    The questions I have are these: 1) If you were going to replace a wood bowsprit on a boat like a Hans Christian what material would be your choice? 2) If I build it out of FRP, how do I determine the weight of the end result (doing this to save 20 lbs is not worth it, saving 80 is)? 2a) what would be my target wall thickness for the layup?

    Thanks in advance for any help.

    John
    http://svnakia.blogspot.com
     
  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    You'll need to have the sprit engineered for you and it's not an especially difficult problem for a designer or NA, so reasonably inexpensive..

    FRP is not likely going to be much lighter then the solid wooden one you have, once the laminate is worked to the same strength and stiffness. You could incorporate fancy fabrics, to reduce this weight, but the cost goes up quickly. Aluminum is a good option, as would a hollow wooden sprit or a combination or the two. Steel is also a possibility, though weight savings might not be as significant.

    Frankly it's all guess work, without doing the math necessary to size up your sprit, in the various materials and build methods available. Contact a professional, if you expect it to save weight and tolerate the tasks you'll ask of it.
     
  3. Petros
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    Petros Senior Member

    I second PARs opinion. Fiber glass has almost half the stiffness to weight ratio of wood, so it is not likely to be lighter than a hollow wood one. Carbon would save weight but is very costly and rather fragile.

    Any different material would need professional guidance to optimize the weight and strength, even designing another wood one. What you have might be much heavier than you need.

    If it were me I would consider designing a hollow wood one, or if I had the budget, perhaps a fiberglass/carbon combination (using a fiberglass outer layer to protect the carbon layers).

    good luck
     
  4. souljour2000
    Joined: Aug 2009
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    souljour2000 Senior Member

    If it's in good shape..I'd take steps to ensure it stays that way...and re-mount it...forget about changing materials honestly...unless you really are looking for a big or expensive project that is...build one of out laminates if you suspect the original has a short lifespan from here on in...etc...but otherwise "leave that thing alone" would be my advice...
     
  5. Stumble
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    Stumble Senior Member

    I am with Soul on this one. though if you want to remove the weight, aluminium would be the material of choice. Galvanic corrosion can be pretty easily controlled. And aluminium and stainless aren't that galvanicaly active. If the parts were in direct contact for years it might be a problem, but otherwise it's fine.

    Carbon is defiantly an option, but the cost probably makes it prohibitive.

    The only other thing I could think of is a titanium tube. Cost wise probably between aluminium and carbon, less galvanicaly active, and very high strength to weight. But you would need a designer to work the numbers.
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    I think a hollow wooden or aluminum sprit could save weight considerably. Even a 20% wall birdsmouth would shave about 60% off the solid stick (84 lbs.), hell, you might even be able to mill what you have into the staves, saving the cost of buying sprit stock. You still need a pro to work up the scantlings and any competent designer can do this. An aluminum sprit would likely save about the same as a birdsmouth wooden build, maybe slightly better, but only a few percent or so.
     
  7. robwilk37
    Joined: Nov 2010
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    robwilk37 Senior Member

    well its been a while... what did you end up using? ive got the same decision to make on my 40' cutter project. please post a picture if you have one...
     
  8. troy2000
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    troy2000 Senior Member

    Maybe the old bowsprit fell on him, while he was trying to reinstall it....:p
     
  9. Titirangi

    Titirangi Previous Member

    I had a boat builder make me a replacement 3.7m bowsprit laminated hollow section with Sitka Spruce to replace solid laminated DF weakened by rot. The spruce unit is 30- 40% lighter and lasted 6yrs so far without sign of delaminating or rotting
     
  10. robwilk37
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    robwilk37 Senior Member

    ive been thinking of taking a length of 5" square stainless tube, knocking the corners off the last 4 feet or so to taper down to say a 3" octagon and weld in the missing chamfers, ending up with a sprit similar to a traditional wooden one only in hollow stainless. saves weight and likely much stronger. also allows weldments of similar metals to take the place of a crans iron. i know its a lot of welding and grinding X8 to fab and make pretty but ive got the time. my concern would be if all that weld on such a highly loaded sprit weakens or strengthens it? certainly interferes with its flex... or the same thing in laminated doug fir which leads me to ask which way to orient the lams...vertically or horizontally?
     
  11. Petros
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    Petros Senior Member

    quality full penetration welding, than annealing it (and possibly re-heat treating it-depending on the alloy), and grinding off the welded surfaces to a smooth shape would be very strong and almost as strong as if it had no seams at all. Though the highest loaded section would be along the center of its length, where you will not be welding it. So tapering the ends and doing quality welding on it will not affect the ultimate loading of the most highly loaded parts of it.

    which axis has the highest loading depends on the installation, the size and angles of the stays and bracing. I would guess that it would be loaded highest side to side, but it would take a proper examination of the plans and sail plan to know for sure. You need professional guidance if you are changing the design of the bow sprit, either a laminated wood or a welded steel tube can be stronger, but it really needs someone to analyze the loads to design one that is strong enough without add unnecessary excess weight.
     
  12. alan white
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    alan white Senior Member

    You only assume apitong but apitong is not that heavy. It is 44 lbs per cu ft. Probably, unless waterlogged, a nine foot spar wouldn't weigh 140 lbs. If you measure the dimensions and do some math, you will at least determine what density the wood is. Then you can decide on the best wood based on that. Unless the dimensions are huge, an apitong sprit wouldn't weigh 140 lbs. That would be over 3 cu ft of material. At 9 ft (and say 6" square), a sprit of apitong would have to be solid and then some. It would contain a bit over 2 cu ft of apitong which comes in at about 100 lbs. Therefore it probably isn't hollow.
    Drill a small hole to find out what is inside. Drill away from fasteners which may have solid blocking. You could potentially find it's solid and then you could consider hollowing the original spar and thereby lighten it by a good 30 % or better.
     

  13. Eric Sponberg
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    Eric Sponberg Senior Member

    SVNakia,
    As one who has engineered a lot of carbon fiber parts, contrary to what was said above, I can say that carbon is particularly robust and could be designed into a bowsprit. Yes, it would be pricey. Also, carbon fiber is a very noble material and if not properly isolated, the metal fittings will corrode before the carbon fiber breaks.

    I think the two best alternatives, already stated above, would be hollowed out wood construction, probably Sitka Spruce, or for something a bit harder as well as long lasting, Douglas Fir, and the other being welded aluminum. Most of the strength of any spar like this comes from the outer fibers, with the middle part doing little except for shear. In wood construction, where you need the sprit to be solid, such as where through-bolts are located, or at the butt end, you can make it solid there with internal blocking. For metal construction, you can use plates of different thicknesses to increase strength locally where needed.

    I agree that fiberglass is not all that good an option. It is heavier than wood, and about the same strength and stiffness, all things considered. Wood-epoxy construction would likely be easier, and it will last a long time.

    Recall that way back when, sailboats had solid wood masts, and then they went with hollow wood masts, and then they went with aluminum masts, all in the effort to save weight in the spars. So considering that, hollow wood or aluminum construction would likely be the best and most cost effective way to go.

    I hope that helps.

    Eric
     
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