Author Topic: New bowsprit  (Read 858 times)

Offline Charles

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New bowsprit
« on: May 26, 2017, 04:54:18 PM »
So, the bowsprit (stick that pokes out of the front) on my sailing yacht got bust in a gale last winter, so I need to make a new one - woodwork in this post, some machined parts to follow...

First, source some clean straight-grained Douglas fir, I need a finished spar of 115mm diameter, so I'm aiming to make a square blank of 120mm, so that's 4 pieces 120 x 30 x 3.7m long. These pieces were laminated together using epoxy, first, a thinned coat wait 30 minutes or so for it to soak in, then an undiluted coat, stack the piece up and use a vacuum bag to clamp the pieces together. The supporting trestles were carefully spaced while the epoxy cured, so the spar (should) be neither sagging nor hogging...

Once cured, the blank was passed through the thicknesser to bring it down to its finished size of 115mm square.

Next, a slot was cut right through at one end to accept the outhaul sheave, this was simply chain drilled, and cut with a chisel as for a mortice and tenon joint. The axel hole was also drilled at this stage.

Next, I made a spar gauge from a scrap of wood, positioning the holes with the mill to make it accurate, and the holes plum. The lines this tool makes are the edges of a perfect octagon. Next step is to make remove the corners with a power planer to make the square section octagonal. Then using a hand plane make the octagon round, then sandpaper to remove the plane marks.

I reckon this job has now generated 1 entire wheelie bin full of wood shavings...

Offline Charles

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Re: New bowsprit
« Reply #1 on: May 26, 2017, 05:04:35 PM »
Machined parts to go with the wood work...

At one end of the bowsprit there is a sheave used to pass a rope around which pulls the sail out to the end of the spar...

First the bearing, simply a piece of phosphor bronze, 25mm OD, 20mm ID, part off at 12mm.

The sheave itself started as a piece of 12mm Delrin sheet, roughed out on the bandsaw. A hole was drilled and tapped in the centre to allow mounting on a mandrel, then it was turned down round at 120mm diameter, a rope groove cut into the edge with a homemade form tool, the faces relieved with the same form tool. Next, gripping it on the outer edge the centre was bored to 25mm then the phosphor bronze bushing pressed into the centre using the tailstock. The axle was turned from 316 stainless, an M6 hole was made in each end, this will secure a small plate on each end which will then be screwed to the spar to retain the axel in place.

Offline ieezitin

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Re: New bowsprit
« Reply #2 on: May 26, 2017, 07:55:06 PM »
Nice... any pictures of your boat..

Anthony..
If you cant fix it, get another hobby.

Offline Pete W.

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Re: New bowsprit
« Reply #3 on: May 27, 2017, 05:40:12 AM »
Hi there, Charles,

Thank you for your interesting posts.

Please could you post a photograph looking at the spar end-on to show the relative orientation of the growth rings of the four pieces of timber?  How did you choose that orientation?

And I ,too, would like to see pictures of your boat. 
Best regards,

Pete W.

If you can keep your head when all about you are losing theirs, you haven't seen the latest change-note!

Offline Charles

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Re: New bowsprit
« Reply #4 on: May 27, 2017, 05:46:28 PM »
Pete / Anthnoy,

Boat pictures below...

one of these is a still from a Round the Island Race promotional video, whole thing here: https://youtu.be/ciIz48CfWE8

Pete, see also a picture of the inboard end - slightly shiny as I have coated the end grain with epoxy. The joints run horizontally when in use. The timber stack was not selected on the basis of much science; in theory, the only load the spar sees is compression, so grain orientation shouldn't matter much. I would have preferred quarter sawn timber, but I had to work with what the timber merchant had in stock, in this case, the 4 pieces came from two boards one of which was slightly lighter & softer than the other, the light board forms the middle two layers leaving the dense ones for the outer. There is a point load where the bowsprit traveller rests, so I would prefer that to bear on the harder timber.

Other than that, the section is massively over spec for the compression load, however, it will inevitably see unfair loads occasionally - dipping the sail into a big wave for instance, stuff like that is impossible to predict or model mathematically, so the only way to allow for it is to massively over-engineer...

Offline Charles

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Re: New bowsprit
« Reply #5 on: May 29, 2017, 04:49:00 PM »
The inboard end of the bowsprit needs two cheeks which locate either side of the Sampson post, then a pin passes through the cheeks and the post to secure it in place. The cheeks are made from lignum vitae - not necessarily the best possible timber for the job, but plenty good enough and I happen to have some in a suitable size - planned and bandsawed to size, then a plywood disc turned on the lathe is used as a guide for the router to round over the ends. Next, the other end is cut and chamfered on the band saw, and finally, they are clamped back to pack and the hole for the pin is bored.

Nex they are carefully aligned on the bowsprit and glued on, one at a time. Glue alone would probably be enough, but for piece of mind I want to put some bolts through, however, i don't want the bolt heads visible. So some 12mm 316 bar was parted off to give two 120mm lengths, each end of which was chamfered, then drilled tapped M8. Next, the fixing positions were marked and 25mm counterbores cut to a depth of about 12mm, then the through hole was drilled, the 12mm bars driven in and button head screws and penny washers screwed into each end.

Some offcuts from the same board where chucked up in the 4-jawand turned down to just under the counterbore size, these were bonded in with 5-minute epoxy, once cured they were trimmed with the pull saw and planed flush.

Offline seadog

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Re: New bowsprit
« Reply #6 on: May 30, 2017, 06:38:55 AM »
Nice job Charles and a nice classic boat. I remember seeing a mast being fabricated at Shepards Wharf about 20 years ago. I was amazed to see that it was fabricated from glued strip and was hollow. It was around 18" diameter at the step.

Graham

Offline Charles

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Re: New bowsprit
« Reply #7 on: May 30, 2017, 05:57:37 PM »
Graham, Thanks for looking. I did consider making this spar hollow, but the effort required relative to the minimal saving in weight, particularly at deck level didn't seem worth it here. I've also heard of people using cedar cores to reduce the weight.

The outboard end of the spar carries a fitting called a cranse iron, mine is fabricated from mild steel & galvanised - not by me, this one came with the boat. Turned another plywood disc on the lathe, then used the router with a top guided trim bit to remove most of the material, finished off with the pull saw and a chisel when the router ran out of depth. end grain at both ends was coated with epoxy, this makes the timber a bit harder in the areas that bear the load as well as keeping the water out of the end grain.

Next, I needed to make a small flat for the end plate of the axel to sit on, so I made a 20mm bushing to fit in a hole saw mandrel, then used the hole saw to cut an outline around the axel hole, then a chisel to remove the material inside the outline - about 20 minutes of tool making for 20 seconds of drilling....

That's the woodwork finished, so first coat of varnish can go on.

Almost the last bit is the end plates for the axle, just big washers essentially, cut from some scrap stainless sheet with a hole saw, except that the holes saw and the pilot drill don't seem to want to run concentrically tonight, so took the pilot bit out, cut some blank discs, then mounted each disc in the lathe to make the hole.

Offline vtsteam

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Re: New bowsprit
« Reply #8 on: May 31, 2017, 05:00:24 PM »
Nice job on your new bowsprit, and very nice boat, too. :beer:
I love it when a Plan B comes together!
Steve
www.sredmond.com

Offline Charles

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Re: New bowsprit
« Reply #9 on: June 26, 2017, 04:34:19 PM »
Remaining steps on the bowsprit - apply varnish, I use Sikkens Cetol, not quite a yacht finish, but only needs four coats, last a couple of years and is really easy to prep & reapply.

Then assemble all of the metalwork, make up a traveller from some D12 hollow braid Dyneema. A traditional traveller would be a galvanised steel ring, wrapped in leather, these are expensive to buy, or a faff to make (150mm diameter hoop in 12mm bar). So I thought I would try a modern alternative, seems to work well on a dry run, we will see how it fairs at sea next weekend...

Traveller line is just 8mm braid on braid, spliced into the shackle at both ends and threaded through the cleat at the inboard end so it can't get lost.

Onto the boat, shrouds and bobstay made up from more D12. Bobstay fitting is the final piece of new metalwork, hewn from solid 316 on the lathe. Really should have radiused the corners, but ran out of time/enthusiasm... Will take it off at the end of the season and finish it properly.