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Bowsprit sizing question

3K views 10 replies 8 participants last post by  braidmike 
#1 ·
Ready to start the design of a short bowsprit to rig a light air genaker on a furler. Have the load data from the sailmaker, curious if any engineering types can point me in the right direction. I really like some of the retractable ones on the market, but it's not really necessary to retract, and they are a lot of money! I would like to use 'fiberglass' tubing, would be easy to work with and mount, and its readily available. Any ideas/help would be appreciated.
 
#2 · (Edited)
I was speaking with a sailmaker here in Tampa (I'm thinking of doing something similar, just no furler) and he mentioned that I could add 10% of my "J" (which is almost 12' in my case) with minimal phrf penalty. You can make the aft section as long as practical; the longer the more stable, but probably not necessary to be too long. There are several makers of kits and they all have dimensions listed for different sized boats that you might use as a template. Selden makes one with a table listed on their website- Gennaker bowsprits : Seldén Mast AB

Edit: just read this- http://www.practical-sailor.com/blog/The-DIY-Spinnaker-Sprit-11307-1.html

Great info!

Ray
 
#4 ·
What ever you use could be 'strengthened' by a dyneema 'bobstay' down a ring in the stem. It would handle the upward pull and put less emphasis on the sprit's own strength.
 
#6 ·
Over the last few years, I have seen dozens of 'bowsprits' for code zero type sails that were bent. As a matter of fact we saw one today on a 50 foot Lagoon. They were all aluminum tube and I believe they were all too thin walled.
When we do ours, I will pay the bucks and have an engineer spec it out for me, not a rigger.
 
#7 ·
I too have seen bent aluminum bowsprit tubes. Fiberglass, however, when it gives basically explodes. I would not use fiberglass for a bowsprit. A bent tube can be repaired/replaced and even used for a while. I'm not an egineer, so there may be others who know more about this than I do, but without some real evidence, I'm going towards Capa's plan of an engineered (metal) tube.
 
#11 ·
Very true. Finally found some comparison modulus numbers, and 6061 Aluminum is definitely the better choice. The strength of the aluminum for the same cross sectional area is 4 to 5 times that of FRP. Will probably cheat and find an 'engineered, manufactured' one and just double it! I would like it to bend before pulling the deck off... so we'll see if the whole package can be installed with a ripped sail being the safety valve before anything else goes. We sail very conservatively, but want to be prepared for the unexpected.
 
#8 ·
Do Selden or others post breaking strengths or detailed dimensions on their websites? You could use this info to choose tube sizes.

Titanium tube could be fun too. In Seattle the Boeing surplus sales yard always has it for sale by the pound. May be able to find it elsewhere.
 
#9 ·
A couple of things...

1) retrofit poles are not designed for code sails generally. If you add a bob-stay they can take the load, but not unsupported. If done properly the bob-stay will result in a crushing failure (like a soda can) instead of a bent pole. So it is pretty easy to tell what happened. The problem is the huge verticle forces a code sail applies to get enough luff tension.

2) titanium... Meh. It isn't any stiffer than aluminium, much heavier, and much more expensive. A thicker wall, or a larger aluminium tube are much better options. Adding a bob-stay better still.

3) carbon fiber is really the best option. But it is obviously very expensive compared to aluminium.
 
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