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bowsprit retrofit

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I am currently making plans for adding a bowsprit to my 48 foot catamaran and am looking for some input.

I intend to use a 1700mm long, 120mm diameter aluminium tube with a 5mm thick wall. The dimensions are derived from the published weight/size specs of commercial offerings. I will add two waterstays to the bows of the hulls. 
 
I am not sure about the best way to connect the bowsprit to the front beam and to connect the tackline/waterstays to the bowsprit. 
 
I think that making the sprit swing vertically is practical idea, so I currently intend to weld  - or rather: have someone weld - two aluminium plates (with holes for a 10mm pin) with 15mm space between them to the beam. On the bowsprit side I would have a fitting plate welded to one side (see attachment). I am not even sure that such a weld is possible.
 
I read about another option on the Catana mailing list: A boom gooseneck fitting with a pin is bolted to the front-beam and a delrin plug  with a corresponding whole in the middle goes into the bowsprit. The bowsprit is held on the gooseneck fitting by the backward pull of the waterstays. This seems like a very simple method, but i couldn't figure out, what kind of gooseneck fitting might work here. Is a dinghy-sized fitting big enough?
 
At the front of the bowsprit I would simply bolt on an deck eye plate. This would only need to take the horizontal loads, the vertical loads of the tack/stays could be  taken by wrapping around the other side of the sprit.
 
Paul

untitled (3).png

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Don't they usually add stuff to the front beam around the attachment point to stiffen it up?

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http://www.time-for-a-catamaran-adventure.com/light-wave-woods-32-37-sailing-catamaran/add-a-screecher-turbocharge-your-cat/

Smaller catamaran than yours, but lessons should be similar since he seems to have also decided to allow the sprit to swing up when not needed.  

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Currently changing bowsprit whisker setup on a 39’ 4.5t cat set up so it can swing up rather than painfully havIng  to detach the connecting pin and retract and store box pole on the catwalk to reduce overall length to reduce berthing cost. 
 

Has carbon front beam with the pole base fitting between the carbon flanges glassed on the front of the beam and pinned. 
Aluminium or ss base plate would be fine ( not carbon) of course  and remember aluminium and carbon fizz if not separated. Like my seagull striker is doing currently (pun intended). 

But anyway you’re post made me want to warn you the loads on angled whiskers  is huge. The compression loads are huge too. 
Also the whiskers etc need to line up fairly closely to the tack(s) otherwise the pole will bend and the compression will do the rest. Bang.  

Also I’ve seen the side of a cat  hull tear out where the hull where the fitting for the whisker was bolted through wasn’t beefed up properly. It had a pad On the inside to spread the load but a bread board sized section of hull ripped out. 
 

the whiskers can be string and simply tied around the pole and if necessary a strop leading forward to stop them sliding back. 

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Thanks for the ideas.

The Forespar attachment looks neat, but at 500 USD it is as  expensive, as what I hope to spend for the whole setup.  :-)

The point about carbon/aluminium is taken. Should I laminate my own attachment I will use one or two layers of glass to separate the carbon from the aluminium.

In my case the sprit will not reach much further to the front than the bows, because my front beam is a bit more than 1m aft from the bows. So the angles for the whisker stays should be better. Also I have a solid walkway across my front net with two aluminum tubes, that will take the compression load off the front-beam and transfer it to the main cabin. 

I'll take extra care to strengthen the area around the whisker attachment points with a few extra layers inside and outside (plus the pad in the inside)

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Your pole spec reads way overkill - particularly given it's shortness and efficient rigging anchor location. Sounds like it would way upwards of 15 kg and it need not. I am buzzing around laminating up a new daggerboard today but while it's sat next to me here are some shots of my cat and tri installations. If I have time I will pull the cat pole off (takes minutes) and weigh it - tbc!

15761781553511838635954.jpg

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Be aware that welding aluminum (6061-T6 is likely the extrusion alloy for the forebeam) dramatically weakens it locally.

It is wiser to create a wrap around aluminum plate you weld your ears to, and then drill and tap or rivet the mounting plate to the forebeam.

If you must weld, don't weld to top and bottom of beam which are more highly stressed than the sides.  If your dolphin striker is already welded on, the beam is already locally welded so carry on and weld!

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Ain't no welding on those assemblies Zonker!

All designed to put materials that perform well in compression in compression and materials that shine in tension in tension whilst avoiding any bending loads.

The cat pole weighs 2.7 kg (6 lbs) including both ends to handle the chute on the outhaul line and a furler on the hook. It is 2m (6' 6") long.

The (not welded) base offset to clear the anchor weighs maybe a pound. I have a bunch of blanks, that get custom finished and anodized with an acetal bearing cup to suit the application and will post some pictures of them when I get back to PA.

The daggerboard core is coming together nicely - - - 

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@Zonker Someone already had a welding party on my front-beam (see attached picture): anchor roller in front, seagull striker on top, the ears from the catwalk on the back - all welded. How localized is the material change though welding the aluminum? As aluminium is very good at transmitting heat, I guess that it spreads quite a bit. If the whole are is already weakened (but has still coped for 25+ plus years), then I guess hat it was oversized in the beginning and - if I understand you correctly - won't get worse by careful additional welding.

Regarding weight/strength: The 120mmx5mm aluminium tube would be 8,5kg for a 2 meter section. To get that down less than 3kg, it would need to be for example 60mm with a wall strength of 3mm. That seems really small - @boardhead, do you use a screacher with your bowsprit as well, or just a spinnaker? I based my sizing on the Selden bowsprits, which usually don't have whiskers to the side to take off some load, so maybe that's why they need to be much stronger.

Is there no fixed connection between the cup on the beam and the end-fitting you have in the tube? What holds it there, if the whiskers are loosened?  

I was thinking about moving the bowsprit attachment slightly to the side as well. Otherwise I would have to cut off the anchor roller and weld that on to the beam 20cm further out again. Doesn't the offset placement lead to more risk of bending for the tube?

Paul

DSC_3074.JPG

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Welding aluminium properly is involved, the design, preparation, pre-heating, actual welding (argon arc)  all have an impact on the final outcome. Long term your beam will have flex fatigue issues so just because it has lasted 25+ is no guarantee it won't fail next time you go out sailing - it's more about use than age. Because the beam is a separate assembly there is no reason for it not to have been made properly in a specialist shop but for you to have some local Joe blob on some weld in-situ is fraught with issues - it certainly needs serious localized pre heat and that in itself is a problem.

The key to lightness is ensuring the pole stays in column and with an inboard cone that transfers the wall compression to a central bearing and an outboard fitting that collects the rigging and tack loads and sends the collective thrust down the pole centerline a much smaller, lighter tube gets the job done.

I do carry a reacher/screecher as well as the chute and do so frequently and the loads are serious (but remember that these reachers are for light airs really) and the complete pole does weigh six pounds, a hair over with the non-skid on top.

The inboard cone stays in the mounting cup exclusively by  compression so the tube is able to move a little avoiding any bending loads. The cone is deep enough in the mounting block that the rigging has to be silly slack to unmount it.

We have these poles on a variety of cats, tris and even a monohull dating back to 1992 without issues. 

The tube on my St Francis sprit is 3" dia with a 3/32" wall.

1576243527592-1943110216.jpg

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End fittings blanks and some finished

1576252408224-190093020.jpg

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8 hours ago, toolbar said:

@Zonker Someone already had a welding party on my front-beam (see attached picture): anchor roller in front, seagull striker on top, the ears from the catwalk on the back - all welded. How localized is the material change though welding the aluminum? As aluminium is very good at transmitting heat, I guess that it spreads quite a bit. If the whole are is already weakened (but has still coped for 25+ plus years), then I guess hat it was oversized in the beginning and - if I understand you correctly - won't get worse by careful additional welding.

Regarding weight/strength: The 120mmx5mm aluminium tube would be 8,5kg for a 2 meter section. To get that down less than 3kg, it would need to be for example 60mm with a wall strength of 3mm. That seems really small - @boardhead, do you use a screacher with your bowsprit as well, or just a spinnaker? I based my sizing on the Selden bowsprits, which usually don't have whiskers to the side to take off some load, so maybe that's why they need to be much stronger.

Is there no fixed connection between the cup on the beam and the end-fitting you have in the tube? What holds it there, if the whiskers are loosened?  

I was thinking about moving the bowsprit attachment slightly to the side as well. Otherwise I would have to cut off the anchor roller and weld that on to the beam 20cm further out again. Doesn't the offset placement lead to more risk of bending for the tube?

Paul

DSC_3074.JPG

What's that between 13 and 14?

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1 hour ago, Bruno said:

What's that between 13 and 14?

 I guess you mean what looks like a black line and could be a crack. It's just dirt. The boat looks more than a little bit messy right now. We had a very close look at that part of the beam when I discussed it with the welder - the picture is a bit older. 

Paul 

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3 hours ago, boardhead said:

End fittings blanks and some finished

1576252408224-190093020.jpg

Looks great. I think I will go that route and not do any more welding... 

Paul 

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The HAZ (heat affected zone) when welding aluminum is quite small - just outside the weld bead. But if it's a linear weld, it's still a weak line in your beam. Sort of like "tear here" dotted line.

Welds at the end of a cat crossbeam are not that bad. Because 90% of the load is at the forestay (the rest from the tramps), the beam bending diagram looks something like this. So the beam ends are only lightly stressed in bending and the rest is vertical shear loads.

photo-34-1024x401.jpg

Therefore - at the middle of the beam, where you have a forestay and seagull striker, avoid welds ACROSS the top and bottom of the beam. That's like cutting the top and bottom flange of an I-beam.

A piece of flat bar that runs transversely to act as lug for the forestay is fine, but wide seagull strikers than run fore/aft on the top of the beam are not nearly so nice. 

 

 

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Look at Trogear.com.  They make nice but very pricey A frame carbon sprits.  They sell mounting hardware and have good pictures for ideas rigging the sprit.

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The geometry of an A frame and the expense of a carbon fabrication makes for a relatively heavy, costly option.

Aluminium alloy in compression and Spectra in tension are well performing, readily available, inexpensive and easily fashioned materials.

Thanks for the Trogear.com lead, interesting.

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20 hours ago, Zonker said:

The HAZ (heat affected zone) when welding aluminum is quite small - just outside the weld bead. But if it's a linear weld, it's still a weak line in your beam. Sort of like "tear here" dotted line.

Welds at the end of a cat crossbeam are not that bad. Because 90% of the load is at the forestay (the rest from the tramps), the beam bending diagram looks something like this. So the beam ends are only lightly stressed in bending and the rest is vertical shear loads.

photo-34-1024x401.jpg

Therefore - at the middle of the beam, where you have a forestay and seagull striker, avoid welds ACROSS the top and bottom of the beam. That's like cutting the top and bottom flange of an I-beam.

A piece of flat bar that runs transversely to act as lug for the forestay is fine, but wide seagull strikers than run fore/aft on the top of the beam are not nearly so nice. 

 

 

Properly designed the alloy extrusion is almost entirely in compression - very little shear.

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Because of the seagull striker countering the forestay loads?  There is still a significant shearing force between the vertical up of the forestay and the legs of the seagull striker pushing down (because they are not in the same horizontal position) But shear forces are rarely going to be the problem. Shearing forces are mostly carried by the sides of the extrusion and bending forces top and bottom. The bending forces will dominate how the beam is designed because they are likely to be an order of magnitude greater than shear and a hollow extrusion has much more shear material than it needs.

By the way any cat owners ever given any guidance on how tight the seagull striker wire must be? I didn't do any calculations on my boat when the beam got replaced and just did it "really tight"...

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I did say PROPERLY designed!

The cable takes all the tension but of course it stretches and the beam compresses. Under maximum loading, ideally, the beam needs to be dead straight so it's necessary to set up some pre-bend for the static set up as the cable (shitty stainless steel typically) stretches a lot.

Best method is to check the beam for straightness upwind in a blow with the runners set up.

In my experience, prior to that, renew and up-size the seagull striker cable and check it often for fatigue and stranding.

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I have prebend in my gull striker, something over an inch, but once installed with stick up it’s level. Agree with earlier post, observe the rig loaded and tension accordingly. 

My sprit is a carbon tube which telescopes out, and the martingales are Dux, spliced through perforations in the hulls (like beachcats). No welding, no drilling the fwd beam, and fairly light.

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Anchorage for Dyneema martingales

1576680586886177338630.jpg

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@boardhead What do we see in the second picture? Is the v shaped section or the right the bottom? I just see a metal pin in a pointed corner. I dont see how the shapes in the bottom picture corresponds to the dyneema going into the hull. Mabye my mind is just a bit slow right now after inhaling too much solvent today.

Paul

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On 12/13/2019 at 1:11 PM, toolbar said:

 I guess you mean what looks like a black line and could be a crack. It's just dirt. The boat looks more than a little bit messy right now. We had a very close look at that part of the beam when I discussed it with the welder - the picture is a bit older. 

Paul 

Before I did anything on that forward tube, I would wash it well and get some Scotch Brite pads and scour off any oxidation and take a closer look at that area that Bruno pointed out above. I thought the same thing that he did and to dismiss his concern with 'just some dirt' is a bit dismissive. I scaled it up and it sure looks suspect for a stress crack radiating out from where the weld bead around the end of the plate meets itself (as they tend to do...). You would be doing yourself a huge favor in buying some of that dye penetrant kit to use on the whole midsection of the tube after a good cleaning and buffing lets to see just what is happening. I had a surveyor inspect my rig who would have me stop grinding him up the mast at ever fitting.  I thought he was just being considerate and letting me catch my breath at the winch but noticed he had one of those squares of green Scotch Brite and the smallest bottle of WD-40 and was wearing those jewelers glasses with the little loupe that flipped down over one eye. He would spray and buff and then wipe clean and then rinse and repeat until the aluminum or SS was bright and shiny and then would put the old Mark 1 enhanced Eyeball to it and could find the least little flaws. He would give each swage fitting socket the same scrutiny and I'm sure his thoroughness saved me from an impending dismasting. 

    Can't hurt to take a really close look in your case.

image.thumb.png.12845c0a9b7d4a9124c846c871ac7af6.png

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I will check it more closely. I want the boat to be sound and will not close my eyes to potential problems. :-)

But for that specific line I am pretty sure that I remember touching it with my finger a few days later and shifting it downwards in its entirety, like a hair that had landed there and caught some dirt. On the other hand I see that continuation upward to the side of the weld, so maybe my memory is wrong.

Paul

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After having the whiskers out to the hulls (which my cat was never designed to have) i was concerned about the load and twisting effect on the hulls so we redesigned in consultation with an experienced multihull engineer and came up with what i believe a magic solution. The boat no longer feels loaded up. if i was parking it in a marina berth i'd have one of the side lines on a friction ring purchase set up that could be easily detached and the pole swung sideways. Also, the spreader set up allows the pole to hold its self up rather than using the seagull striker A frame. Sailing upwind we always have a halyard on it as well to help support it.  

BOWPOLESETUP.JPEG

bowpole2.jpeg

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8 hours ago, toolbar said:

@boardhead What do we see in the second picture? Is the v shaped section or the right the bottom? I just see a metal pin in a pointed corner. I dont see how the shapes in the bottom picture corresponds to the dyneema going into the hull. Mabye my mind is just a bit slow right now after inhaling too much solvent today.

Paul

A 30mm hole gets drilled from inboard the stem which breaks through inside the bow. Then a G10 tube is inserted and epoxied in place with the end shaped to match the inside of the bow. Laminate over the tube inside the bow. Drill and tap (thread) a cross hole in the tube. Taper a bolt so that it threads into the top portion of the cross hole and the end of the taper snugs into the smaller hole in the bottom. Push the Dyneema loop into the 19mm tube from (30mm OD x 19mmID tube) and insert, screw in and tighten the tapered bolt. The thread is sealed with teflon pipe tape. 

Strong, light and clean so as not to generate a soaking rooster tail as the anchor and cable cut through waves on the lee bow.

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OK. I am preparing to install the bowsprit now and and need to add the attachments points for the martingales/waterstays in the process. 

I can't do it the way I planned, because of a watertight bulkhead in the front section. The opening is too small to do any real work in the tip of the bow. So I will need to add the u-bolt/the attachments point aft of the bulkhead. 

Version a) As depicted in the attached image: Add G10 plates on both sides of the hull, replace core with thickened epoxy where the bolts go through, add a few layers of laminate on the inside to strengthen the connection of the backing plate to the bulkhead. Then add a conventional u-bolt fitting. 

Version b) The alternative would be the G10 tube as supposed by @boardhead - but in this case the way to the other side of the hull would be about 60cm and not just the 10cm in the tip of the bow as in his pictures. But maybe the tube could just be laminated to the bulkhead and wouldn't even need to go to the other side of the hull? Or is it better to go all the way, so that some force is also transferred over there?

Any comments or hints?

BTW: we checked the front cross beam thoroughly with a 3-step-crack-detection-spray after removing the paint in suspect parts and found no cracks in the aluminum. Better safe than sorry.  

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The geometry is way better at the stem waterline. I would use a G10 tube (buy bar stock and bore it out?) 1 1/8" bore x 2" outside diameter. Drill a 2" hole on a  center set  back maybe 3" from the bow (so 2" of material left ahead of the hole). Slide the tube right through and mark each side at the surface, remove and cut at that line then 45 degree chamfer 1/4" (assuming a 3/8" hull thickness). Drill a cross hole midway along the tube and cement  a 1/2" diameter pin right through  then slide that assembly into the the bow through hole, cement in position, fill the chamfer each end and apply a glass patch if you feel more comfortable. Pass your 5/16" dyneema line around the cross pin and splice it to length. Go enjoy your new sprit!

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