bowsprit retrofit

Zonker

Super Anarchist
9,732
5,693
Canada
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. 

 

salt.ed

New member
8
3
San Diego
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.

 

boardhead

Anarchist
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.

 

boardhead

Anarchist
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.



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.

 

Zonker

Super Anarchist
9,732
5,693
Canada
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"...

 

boardhead

Anarchist
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.

 

Max Rockatansky

holy fuckfarts!
3,846
985
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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toolbar

Member
278
63
Kiel, Germany
@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

 

Rasputin22

Rasputin22
13,915
3,470
 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.png

 

toolbar

Member
278
63
Kiel, Germany
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

 

Indian Chief

Member
304
24
Sydney
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

 

boardhead

Anarchist
@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.

 

toolbar

Member
278
63
Kiel, Germany
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.  

untitled (2).png

 
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boardhead

Anarchist
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!

 

Hunky

Member
56
5
NW Alaska
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!
Hope I'm not violating nature by responding to an old thread. Apologies if so.

I'm getting close to launch of my 30' KHSD cat and have an undersized used mast at the moment, yet to be installed. I've been advised I could add a small bowsprit with a wire for hanking on a genoa or other sails, in addition to the furler jib on the forestay off the cross beam for light air.

Unfortunately I don't have much access inside the bow area. One side I could reach through the deck 8" access hole and use long sticks to reach near the waterline, which wouldn't be easy, but the other side has a repair that blocks access to anything below it. It is about the second stringer down. The hull is 6mm thick of mahogany/light glass cloth.. stem is sandwiched 4# foam by 3mm ply each side.

Boardhead's method may work to some degree - curious about all the steps. Is the G10 tube going through both sides of the hull? I think so. And the chamfer is on the hull material rather than the tube, if I understand. I suppose by roughing up the outside of the tube and applying epoxy on it and the boat surfaces through which it goes may add some strength to the pulling force. The 1/2" dia. pin is stainless? I assume horizontal through middle of tube so if it weren't sealed the best it wouldn't leak too much.

I've added the crossbeam photo to show what I have there (looks like it may be sideways.. sorry).. I suppose that welded ear for the forestay could be made to work also with the attachment of the bowsprit. If I have to drill another hole through it for mounting hardware might be possible.

PXL_20210721_185927883.jpg

PXL_20210720_001612120.jpg

 
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