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Old 19-05-2013, 15:41   #1
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Bowsprit Design

any old salts out there? Am adjusting the rig on an 1980 Swanson 28 (aussie design) heavy displ.ocean going masthead sloop. The swansons have a bit of notoriety for weather helm and this particular little fat number is no exception. The swanson did produce some with short bowsprits (1 - 1.5 m, about 4 - 41/2 feet) and the jerry built i have rigged with a thick flat timber to try out is about 900mm long (outboard) and has certainly eased the problem. I want to run a cutter rig also so I need the space between heady and mast to use a staysail.
my question is about bowsprit design. I have seen plans for a few, westsail, tayana, baba etc and while these seem fine (yet a bit involved in construction), i wondered why a section of an old/ second hand/ or even a new section aluminium mast head wouldn't suffice? The top 3 metres or so.
It would seem to have all the rigging and perform very similar functions as a regular mast and be stayed similarly, just in the horizontal plane. It would seem appropriate for the age of the boat rather than a wooden spar.
wadya rekon?
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Old 19-05-2013, 15:59   #2
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Re: bowsprit design

I see no reason why not. It needs to take compression well, as long as you have the proper guys/stays to support it. You need a good solid "heel" of some sort for it to bear against that will support the compression. A sampson post comes to mind.....
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Old 19-05-2013, 16:00   #3
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Re: bowsprit design

I ain't no old salt but go for it. It's all about compression loads for a bow sprit and where are you going to put your anchor. A old chunk of 4x4 Doug Fir would be easy to attach to the boat not sure about a piece of AL mast. Best deal would be to get your anchor in front of the head stay and bob stay but then you need a stronger/ heaver bow sprit..
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Old 19-05-2013, 17:30   #4
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Re: bowsprit design

You should do a little kitchen table yacht design before you go further than musing. Find a drawing of your boat's body plan that shows the below-the-water-line hull profile. Do not include the profile of your rudder. Trace this profil on to card board. Now balance the card board on the edge of your kitchen table pushing it out until it just balances near the midships line. Mark the balance line. You have now found your hull's the center of lateral resistance (CLR). Compare this to the total center of effort (CE) as shown on your sail plan. If weather helm was designed into the rig the CLR will be forward of the CE. To reduce weather helm you will need to move the CE forward and closer to the CLR. As you say, you can do that by adding a bow sprit to cary a jib of the correct size. The size must be computed carefully. If too big it will give you lee helm. If to small it will not correct the weather helm to your satisfaction. As for the bowsprit, you will need to add a bobstay and heel at a minimum. The deck will need to be strengthened to take the compression as the bowsprit wants to slide aft under load. The stem will also need to be reinforced to take a bobstay U bolt. Some sort of arrangement at your mast head will need to be made for the jib stay. If your new bowsprit is of any length at all some sort of safety arrangement (netting) sould be provided for your own safety.

Note, when you drop your new (flying) jib the weather helm will return. You may correct that by reefing your main.

As I recall, you can find a more detailed discussion of this subject in Yacht Design and Planning by Howard Chapelle
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Old 19-05-2013, 18:16   #5
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Re: bowsprit design

"...Compare this to the total center of effort (CE) as shown on your sail plan. If weatherhelm was designed into the rig the CLR will be forward of the CE. To reduce weather helm you will need to move the CE forward and closer to the CLR... ..."
If I remember right, the combined CE of the working sail plan will never be aft of the CLR. It should be well forward, probably in the order of 5-12% of waterline length. Obviously boat design aint perfect or your boat and many others wouldnt have pretty severe weather helm. Regardless, You know you have it, and moving the CE forward with the bowsprit should help some.
I've had to do it on 2 "designed", boats of mine, one of which eventually built them with the bowsprit.
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Old 19-05-2013, 19:03   #6
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Re: bowsprit design

I would shy away from using a piece of a standard aluminum mast. On my aluminum schooner, I used an 10' piece of aluminum schedule 40 irrigation pipe, 8" diameter. You need side stays, plus a bobstay to nearly the waterline, turnbuckles at the bowsprite tip for all three wires/chains. You will need chainplates to take the side stays on the hull, and a very herky through stem fitting for the bobstay. If you are doing it anyway, make the bobstay fitting with two eyes, rather than one, to attach an anchor rode snubber. You need a samson post to take the heel of the sprit, and that should be 1/4 of the length of the sprit inboard. I assume you have a GRP boat hull, so you will have to beef up the decking where the samson post goes thru and the post should be attached to the stem or keelson area, firmly, like a mast step. Do some research. Look at several boat building books, preferably dealing with traditional boats, gaff rigged, etc. The suggestion of a doug fir 4X4 is a good one, if you can find a clear one, or nearly clear. With a metal boat, the samson post can be simply welded to the reinforced deck, but with wood or grp, you should take it to the keel and tenon it in. What all of these suggestions should tell you is that it is not just a matter of putting a stick in front and going sailing with it. The system must be well thought out and strong. Read some yacht design books and go for it.
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Old 19-05-2013, 19:54   #7
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Re: bowsprit design

thanks for the replies. I already have installed a bobstay just above waterline as per westsail design and styed up to the "cranse iron" stinless firttng with chain. This arrangement on a 300mm wide timber has negated the need for side stays. As I said, this was just a jerry set up to see how the moving of the sail plan out further helped the boat behave.
In a nut shell (help help let me out), pointed up 5degs higher, ran full headsail and main up to 20kts with moderate weather helm.
I want to put in a "tidier system" tht's all, just trying to sort out the material. I think the moving of the sail plan via bowsprit works well.
Masts are aluminium and take all the same loads if not more and are a lot taller. all the rigging is there
(or should be) for side stays, head stay and bob stay attachments.
Don't get me wrong, I truely appreciate the input, just seems it should't be rocket science and I don't see (as yet) why it wouldn't work.
As for samson posts; i've seen quite a few set ups that have a boot type of attachment at the end rather than a tenoned post type arrangement. This type of arrangement should work? reinforcing below deck I agree is necessary but why should it be connected down to the hull?.
Why the fascination with wooden spars on modern material boats anyway?
They rot, break and for size and weight per strength, I can't imagine they compare very well in consistency of material performance c.f. aluminium.
please comment, hungry for feedback
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Old 19-05-2013, 20:08   #8
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Re: bowsprit design

While not addressing the helm issue here is my experience that may give you s few other considerations.

The Papes sprit was a simple 3 by 3 box section. Tough the original builder went to great lengths to prevent it the section rusted through and I had to replace it. One of the bad things about the old design was how the bob stay would foul the anchor making anchoring a mess. Also the anchor shank had to come through a small aperture in the sprit platform. And the sprint platform was wobbly and poorly supported.

This fall I built a new sprit. I retained the basic design but added 2x2 box sections as whiskers. This allowed me to make a very secure base for the sprit platform and to add some additional strength. I moved the anchor rollers to the end of the sprit so that they fall and raise without interference.

Having a steel boat it is all welded construction, and the new sprit is in 316ss. It will surely out live me. The but is welded to the Samson post and it is also welded where it passes through the stem. The whisker sections were capped with 1/2" plate welded to the hull. All sections are 1/4" wall thickness.

Despite my best efforts the forestry attachment ended up about an inch too far forward. I ended up putting stay extender links.

We are just now finishing up topside painting and prepping.

Here is a pic in the basement before we installed it.
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Old 19-05-2013, 20:21   #9
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Re: bowsprit design

I made my bowsprit out of fiberglass, wrapping layers around a piece of 4" ABS pipe until I had about a half-inch all 'round. Polyester resin, gelcoat finish; could have faired it to perfection, but didn't bother. It is immensely strong, requires no maintenance, and will never rot nor rust. My 15-foot boom is built the same way, and serves wonderfully.
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Old 19-05-2013, 20:38   #10
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Re: bowsprit design

From this to this on my Rob Roy 23...
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Old 19-05-2013, 20:40   #11
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Re: bowsprit design

I think the mast section with masthead is a good idea if it is cheaper than your other alternatives. Just remember that your bobstay should be larger diameter wire than whisker stays and forestay because there is more load.
You can look up the relative loads in Skeene's Elements of Yacht Design.
kind regards,
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Old 19-05-2013, 20:41   #12
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Re: bowsprit design

Yes wood doesn't last forever but it is straightforward to work with. Here's a bowsprit I built for my old boat, a Cape Dory 30. Nothing fancy, it sounds similar to what you have now, but it is laminated and through-bolted with stainless rod. It was a good anchoring platform too, which was just as important to me.

No reason you cant use other materials, but wood is simple. This is Iroko, not teak.

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Old 19-05-2013, 21:21   #13
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Re: bowsprit design

yeah, nice pretty set up on the rob roy; very pretty. Bit exposed for my liking, probably just needs some rails, bit lumpy in Bass Strait for no safetys.
Plank or pole? the plank systems seem simple and seem to get around the side stay issue and an access deck is already there; the bow roller for the anchor, i guess keeps it clear of fouling the bobstay but i worry about the potential heaving load of a boat at anchor or moored in a blow with swell. i think back closer to the stem would be safer and stronger. is the fouling such an issue?
I like the way the rob roy has shaped its side planks; very pretty!
what's the consensus on plank systems v's a pole with overlaid deck of sorts. i tend to think the pole type would be structurally more appropritae if appropriately stayed.
any comments?
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Old 19-05-2013, 21:45   #14
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Re: bowsprit design

Here's a photo of my new bowsprit. It's made from a piece of old growth clear fir. The old one had been made from 2x6s glued together.

The Sampson Posts had rotted below decks taking the forward bulkhead with it. I replaced the first two bulkheads and the tonnage deck with richlite, waterproofing two compartments with Freeman hatches on each one (pics to follow).

I removed the Sampson posts all together, closed the holes, and fiberglassed a piece of purpleheart mahogany 2" thick under the fore peak.

Lets see about posting some pics. The gammoning iron was poured last Wednesday at Port Townsend Foundry.

Some pics.
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Old 19-05-2013, 21:48   #15
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Re: bowsprit design

Quote:
Originally Posted by seano View Post
yeah, nice pretty set up on the rob roy; very pretty. Bit exposed for my liking, probably just needs some rails, bit lumpy in Bass Strait for no safetys.
Plank or pole? the plank systems seem simple and seem to get around the side stay issue and an access deck is already there; the bow roller for the anchor, i guess keeps it clear of fouling the bobstay but i worry about the potential heaving load of a boat at anchor or moored in a blow with swell. i think back closer to the stem would be safer and stronger. is the fouling such an issue?
I like the way the rob roy has shaped its side planks; very pretty!
what's the consensus on plank systems v's a pole with overlaid deck of sorts. i tend to think the pole type would be structurally more appropritae if appropriately stayed.
any comments?
I always lead my rode through the chocks. I like the bowsprit on the Rob Roy. Mine was designed as a replacement to the stock bowsprit.
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