bowsprit retrofit

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Member
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Kiel, Germany
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
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Bruno

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

 
L

LionIsland

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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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Kiel, Germany
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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boardhead

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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

 
Zonker

Zonker

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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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boardhead

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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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Kiel, Germany
@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

 
B

boardhead

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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

 
B

Bruno

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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

View attachment 336202
Click to expand...
What's that between 13 and 14?

 
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Member
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Kiel, Germany
Bruno said:
What's that between 13 and 14?
Click to expand...
 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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