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Old 19-12-2020, 00:18   #1
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Bowsprit rigging geometry

I am seriously considering adding a bowsprit to my Pearson 35 and would appreciate tips, pictures, experiences from knowledgeable members. I currently have a lovely 4ft long teak double bow platform that extends out about 2ft but I came across a 7ft, 5" square teak square heartwood post and thought "hey add a cranse iron and three hull fittings and three stays gives me a bowsprit!" There is of course the usual question on what to do with bow roller and how best to avoid anchor fouling bobstay. Should I trice the bobstsy or put a lower 2nd shackle fitting down under the bobstay fitting to reeve a snubber through, or will using a bridle snubber suffice to avoid fouling the anchor rode on the bobstay...etc, Usual stuff.

But this is my greater conundrum: as I understand it bowsprits are under compression load much like spreaders on a mast. But spreaders are at least supposed to bisect the shroud angle to create compression rather than pull on the spreader tip. This requires the spreaders to have a few degree tilt up (though in real life most boats have droopy spreader syndrome.)

I would assume therefore that for the same reason a bowsprit is also supposed to bisect the angle formed between the bobstay and the jibstay (or forestay) but that doesn't seem to be the case IRL. Is this just neglectfulness like droopy spreaders or because bowsprits are meant to be more like levers than spreaders, with the distal tip lifting, the proximal heel pushing down as well as aft, and the gammon iron in the middle is a (immobile?) fulcrum?

I see in some boats they use dolphin strikers to expand the bobstay angle to make it closer to being bisected but I don't think it is literally possible to have a bowsprit bisect the bobstay-forestay angle because then the bobstay chainplate would have to go well below the waterline. I guess the length of the bowsprit makes a difference here

Sigh maybe I should I just stick with the bow roller I have and give up on the bowsprit? I hear good things about bowsprits for long distance trading cruising though...
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Old 19-12-2020, 03:27   #2
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Re: Bowsprit rigging geometry

If you look at most boats with bobstays, you'll see that the bobstay is significantly beefier than the jibstay which it opposes. That's to make up for the disparity in loads that comes from an angle not bisected. The bowsprit shrouds on either side need not be so heavy.
If you put the anchor roller a foot or so out from the stem along the bowsprit, that's a pretty common place--mine rides mostly on a 3/4" pin going through the sprit.
Chafing between bobstay and anchor rode is common, but thousands of boats have surmounted that. I have a length of PVC pipe over my Dyneema bobstay that just rolls as the chain moves past. I can also reeve a snubber though a fitting on my bobstay chainplate, so that the rode effectively starts there when I'm anchored, giving a better rode angle for less scope, and no bobstay interference. It's definitely worth bothering with in swingy, rolly anchorages. If you're making a bobstay chainplate, I'd strongly suggest incorporating that feature. It need be nothing more complex at first than a hole for a shackle pin. The rest can be designed later.
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Old 19-12-2020, 08:26   #3
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Re: Bowsprit rigging geometry

thanks I already scored a cranze iron on the auction site, the description says it is 4 15/16th round (not sure if that's interior or exterior)
Sheez replacing anchor platform with a bowsprit is a bit of a detour in a refit! Thankfully I haven't positioned my windlass yet. I'm seriously considering getting side-mounted anchor rollers to be attached to the sides of the sprit say around the middle of the sprit. I can perhaps adapt the top mounted bow rollers to be mounted sideways, with an axle for 2 rollers (one each side) going through the center of of bowsprit

A PVC sleeve is great idea thanks I get that the bobstay is beefier but my gut just doesn't see how that can overcome tight angle geometry to present a totally counterbalancing load,
Looking at some older boats or the dutch sailing barges, their bowsprit seems to have a really high upwards tilt! I see some people use chain as bobstay too

I suppose in an ideal world the forces would be totally equal but in the real world it is an approximation and so in addition to the bobstay the bowsprit itself is made beefier and secured very well onto the deck to deal with not just a compression force but also a pull up. So I suppose 5" thick post of teak sticking out about 3' should ok ... They sell 6" sq cedar posts in lumber yards too
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Old 19-12-2020, 11:28   #4
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Re: Bowsprit rigging geometry

The extra (lower) hole in the bobstay chainplate works great for mounting a shackle and running your anchor rode snubber through the shackle. I run the snubber up to the deck from the shackle, so I can release the whole assembly (rode and snubber) in an emergency. Don’t terminate the snubber at the near waterline bobstay chainplate shackle. Don’t ask me how I know this.
I wouldn’t sweat the forestay and bobstay angles not symmetrical about the cranse iron. Your proposed bowsprit should be sized appropriately to have enough bending strength to deal with it not being in pure compression. Spreaders are a totally different animal due to windage, weight aloft and connection issues at the mast itself. I’d stay away from cedar posts - cedar is not known for its strength.
The bowsprit should follow the shear line rather than being pitched well up as in old barges. Try climbing a steep bowsprit in a bit of a sea - why would you complicate your sea life needlessly.
I would add significant budget for a good pulpit to make your proposed bowsprit not serve its traditional role as widow maker.
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Old 19-12-2020, 11:43   #5
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Re: Bowsprit rigging geometry

Quote:
Originally Posted by Cyrus Safdari View Post
I am seriously considering adding a bowsprit to my Pearson 35 and would appreciate tips, pictures, experiences from knowledgeable members. I currently have a lovely 4ft long teak double bow platform that extends out about 2ft but I came across a 7ft, 5" square teak square heartwood post and thought "hey add a cranse iron and three hull fittings and three stays gives me a bowsprit!" There is of course the usual question on what to do with bow roller and how best to avoid anchor fouling bobstay. Should I trice the bobstsy or put a lower 2nd shackle fitting down under the bobstay fitting to reeve a snubber through, or will using a bridle snubber suffice to avoid fouling the anchor rode on the bobstay...etc, Usual stuff.

But this is my greater conundrum: as I understand it bowsprits are under compression load much like spreaders on a mast. But spreaders are at least supposed to bisect the shroud angle to create compression rather than pull on the spreader tip. This requires the spreaders to have a few degree tilt up (though in real life most boats have droopy spreader syndrome.)

I would assume therefore that for the same reason a bowsprit is also supposed to bisect the angle formed between the bobstay and the jibstay (or forestay) but that doesn't seem to be the case IRL. Is this just neglectfulness like droopy spreaders or because bowsprits are meant to be more like levers than spreaders, with the distal tip lifting, the proximal heel pushing down as well as aft, and the gammon iron in the middle is a (immobile?) fulcrum?

I see in some boats they use dolphin strikers to expand the bobstay angle to make it closer to being bisected but I don't think it is literally possible to have a bowsprit bisect the bobstay-forestay angle because then the bobstay chainplate would have to go well below the waterline. I guess the length of the bowsprit makes a difference here

Sigh maybe I should I just stick with the bow roller I have and give up on the bowsprit? I hear good things about bowsprits for long distance trading cruising though...
You may find this helpful. A friend spent several years gutting and rebuilding his Cape Dory 36 and documented every step.

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Old 19-12-2020, 11:48   #6
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Re: Bowsprit rigging geometry

As long as both stays are adequate it really doesn't matter. The bob stay takes more abuse usually anyway so make it big. OTOH it is much shorter so stretches much less than the headstay under load.
Are you trying to correct weather helm? or make more room to a quasi cutter rig?
I'd like to see a post aft of the bowsprit to take the compressive load and they are handy for tying off anyway.
A bowsprit complicates the windlass though.

I put a bowsprit on my Rawson 30 and one on my Seawind 31. Both worked out well.

The Rawson was a "propeller blank" of sitka spruce which comes I think 8" x 8" x 8 ft. Light and strong but very prone to dings as the wood is fairly soft. It was very troublesome to keep varnish on.

The Seawind had a teak plank I made up and through bolted to the fore deck with a backing plate and 1/2" SS bolts. My logic was that was adequate just like bolted on chainplates are adequate. I made the teak plank maybe 2+ inches thick and probably 10" wide. I carved it concave on the bottom to fit the deck shape. I used Cape Dory Silicon bronze fittings for the stays etc.
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Old 19-12-2020, 12:09   #7
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Re: Bowsprit rigging geometry

The idea is to get a light asymmetrical or gennaker flying without hassle of spinnaker pole etc..for singlehander. I'm totally redoing standing rigging,, already adding a removable inner stay and I'm installing windlass now. So it is time to make these decisions.

Indeed the bowsprit like any other spar should be calculated to do its job but being a non engineer im going to have to go with oversizing. I figure I can't reasonably fit anything larger anyway. The retractable spirits are 70mm aluminum extended 36 inches off the bow so that can provide a basis for comparison if someone could do the math.

Cedar - White Atlantic cedar if I can get it - has been used in spars along with fir and spruce for a very long time. Teak would be nicer but Heavier and I'm not sure I want more weight on the nose. There that Oregon cedar cypress too...

Honestly costing this out along with considerations of windlass location and anchoring issues... it may be more economical to go with one of the retractable pole options, rather than a permanent fixture. But they are OH SO UGLY and my foredeck is already crowded
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Old 19-12-2020, 12:59   #8
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Re: Bowsprit rigging geometry

If you secure a copy the late Brion Toss' "Rigger's Apprentice," it has load calculation formulas for bobstays and other shrouds, and good discourse on all matters of standing rigging. My edition was dated in that Dyneema wasn't around at time of writing, but load formulas and geometry don't change anyway.
There is a school of thought that believes the bowsprit heel should take the compression load without it being pinned at the gammon iron--essentially that the gammon iron serve only as a partner of sorts. I pin mine on both places to spread the load, but I'm not an engineer.
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Old 19-12-2020, 13:06   #9
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Re: Bowsprit rigging geometry

Quote:
Originally Posted by Cyrus Safdari View Post
The idea is to get a light asymmetrical or gennaker flying without hassle of spinnaker pole etc..for singlehander. I'm totally redoing standing rigging,, already adding a removable inner stay and I'm installing windlass now. So it is time to make these decisions.

Indeed the bowsprit like any other spar should be calculated to do its job but being a non engineer im going to have to go with oversizing. I figure I can't reasonably fit anything larger anyway. The retractable spirits are 70mm aluminum extended 36 inches off the bow so that can provide a basis for comparison if someone could do the math.

Cedar - White Atlantic cedar if I can get it - has been used in spars along with fir and spruce for a very long time. Teak would be nicer but Heavier and I'm not sure I want more weight on the nose. There that Oregon cedar cypress too...

Honestly costing this out along with considerations of windlass location and anchoring issues... it may be more economical to go with one of the retractable pole options, rather than a permanent fixture. But they are OH SO UGLY and my foredeck is already crowded
Cant you just fly the asymmetrical with a pendent on the tack? Do you need it further forward?
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Old 19-12-2020, 14:46   #10
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Re: Bowsprit rigging geometry

Another vote for Brian Toss...great guy who left us great books.
You might take a look at classic marine.co.uk but I don’t agree with all of it.
I did a 5&1/2 foot total length on a custom Bruce King 31. Four foot over stem.
About 5”x5” clear laminated vertical grain Doug fir. Lyle Hess sold me the gammon iron , bronze. The crance was custom cast. Bolted by two bronze cross plates with four 1/2” bronze bolts to plates under the deck so no holes drilled into the spar. Custom welded bronze anchor rollers fixed under, again so the 3/4” axle was not drilled into the spar. Fitting at the waterline was two 3/8 x2&1/2 plates bolted with 1/2” button heads to backers in the hull. The bobstay and a pulley could be attached so the snubber worked as described by a post above.
The bobstay to spar angle was kinda fine but I never saw the need for a dolphin striker. Whisker stays were 1/4”. On the whole, it worked perfectly.
You need a bronze star on the end.
A star to steer her by.
In a hurricane, the boat nosedived under the pendant to the mooring ball and broke the spar at the stem and took the rig down. I’d never use a mooring ball
with a bowsprit again. The bowsprit extended about 4” past the crance and at the tip there was another bronze ring to prevent splits. This was just enough to catch the pendant.
Happy trails to you and merry Christmas
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Old 19-12-2020, 17:45   #11
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Re: Bowsprit rigging geometry

Quote:
Originally Posted by Manateeman View Post
Another vote for Brian Toss...great guy who left us great books.
You might take a look at classic marine.co.uk but I don’t agree with all of it.
I did a 5&1/2 foot total length on a custom Bruce King 31. Four foot over stem.
About 5”x5” clear laminated vertical grain Doug fir. Lyle Hess sold me the gammon iron , bronze. The crance was custom cast. Bolted by two bronze cross plates with four 1/2” bronze bolts to plates under the deck so no holes drilled into the spar. Custom welded bronze anchor rollers fixed under, again so the 3/4” axle was not drilled into the spar. Fitting at the waterline was two 3/8 x2&1/2 plates bolted with 1/2” button heads to backers in the hull. The bobstay and a pulley could be attached so the snubber worked as described by a post above.
The bobstay to spar angle was kinda fine but I never saw the need for a dolphin striker. Whisker stays were 1/4”. On the whole, it worked perfectly.
You need a bronze star on the end.
A star to steer her by.
In a hurricane, the boat nosedived under the pendant to the mooring ball and broke the spar at the stem and took the rig down. I’d never use a mooring ball
with a bowsprit again. The bowsprit extended about 4” past the crance and at the tip there was another bronze ring to prevent splits. This was just enough to catch the pendant.
Happy trails to you and merry Christmas
Captain Mark and his never enough eggnog manatee crew
Cool
Got the bronze star already
How about some pictures if you have any. Would love to see roller arrangement
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Old 19-12-2020, 22:58   #12
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Re: Bowsprit rigging geometry

Cyrus,

Post #4 has the calculation for bobstay, based on headstay breaking strength. This calculation takes into account the unfair load caused non-bisected angle.

https://www.cruisersforum.com/forums...es-220163.html
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Old 21-12-2020, 21:33   #13
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Re: Bowsprit rigging geometry

Thanks

Rather than a new bowsprit I've decided to use my existing anchor roller platform that juts out about 2' as a bowsprit by removing one of the two bow rollers to fit a cranse iron. I plan to put the remaining anchor ahead of the cranse iron to min issues with bobstay. I had hoped to go out at least 3' but 2' is a good compromise
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Old 04-03-2021, 08:35   #14
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Re: Bowsprit rigging geometry

Sorry for hijacking this post but couldn't find another similar post. Can anyone tell me what this is for? Thanks in advance.
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Old 04-03-2021, 08:45   #15
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Re: Bowsprit rigging geometry

It’s a crossbow, for firing arrows at pirate boats. Can also be used for whale fishing.
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