Bowsprit

[Lightwave395]

I'm planning on making a bowsprit using aluminium tube 80mm - 100mm diameter. Anybody know what wall thickness would be appropriate ? The sprit will project around 1 meter forward on a 12m boat. Can't get the dimension from my existing spnnaker pole as it's carbon...

 

[B27]

I'd be interested to know the answer to this!

I think it might be a bit 'how long is a piece of string?', because unless something else breaks first you can always put your kite up in more wind until the sprit breaks.

 

[westernman]

Can't help much.
My bowsprit is about 20 feet of which 15 feet sticks out from the bow.
It is solid tree trunk and weighs a ton or two and terrifies other boats when I am coming into the marina.
You would think that they don't like it up 'em.

 

[Bristolfashion]

This site gives some wall thicknesses. I've also seen 2.5 mm for 80 mm spinnaker poles.

Spinnaker & Whisker Poles : Sailspar

 

[ashtead]

Why not go for stainless steel box section ? It’s what we have at 12.8m . Sits in the spare bow roller Mightily strong for a furler and chute pressures- won’t an ally pole bend upwards? Won’t you need a downward stay with ally pole ? Maybe you have in mind fitting a selden pole though? If you what a diagram of our box section pm me etc - appreciate we have double bow roller though.

 

[Pinnacle]

I fitted one of these to our Beneteau 411. https://support.seldenmast.com/files/595-261-E.pdf

IIRC, the wall thickness was at least 3mm.

 

[Daydream believer]

I'm planning on making a bowsprit using aluminium tube 80mm - 100mm diameter. Anybody know what wall thickness would be appropriate ? The sprit will project around 1 meter forward on a 12m boat. Can't get the dimension from my existing spnnaker pole as it's carbon...

Will it be unsupported or will there be a bobstay? Apart from supporting the end of the pole this will have a bearing on the support at the deck & how strong that needs to be

 

[flaming]

I'm planning on making a bowsprit using aluminium tube 80mm - 100mm diameter. Anybody know what wall thickness would be appropriate ? The sprit will project around 1 meter forward on a 12m boat. Can't get the dimension from my existing spnnaker pole as it's carbon...

Sounds like you don't have the tube?

Why not go for carbon... If you ask your local rigger they may know a "donor" pole that is now shorter than it once was....

And I assume you're factoring in a bobstay?

 

[B27]

Was there not a Lightwave with a conversion to a J120 style of sprit?

I have been thinking about a small sprit for my little bilge keeler.
My idea is more about getting the tack away from the pulpit and cruising with a bit asy in not much wind than what we used to get up to in Laser 4000s and the like.

If the sprit is a dumb piece of tube and it breaks long before the mast, that may be a Good Thing.

 

[oldmanofthehills]

Why not go for stainless steel box section ? It’s what we have at 12.8m . Sits in the spare bow roller Mightily strong for a furler and chute pressures- won’t an ally pole bend upwards? Won’t you need a downward stay with ally pole ? Maybe you have in mind fitting a selden pole though? If you what a diagram of our box section pm me etc - appreciate we have double bow roller though.

My first one on a Mac 26 was 2 inch admiralty steel bar, to get a 1m reach. Certainly did the job and so stiff it didnt need whisker stays but might have weighed well over 20kg.

The operation however was a success in reducing weather helm, so did the same on our LM27, also enable bigger genoa. The yard suggested stainless steel box section, I guess of about 1mm wall thickness. Yard said that provided bobstayed and whiskerstayed, then absolute stiffness not really an issue provided it does not concertina or get damaged too easiliy.

Additionally fixing plate on fore deck plus stay plates on end, are easy to weld to square section

 

[rogerthebodger]

What you really need to do is to determine what the loads would be and the a cantilever bending analysis can be done.

A bob stay will resist any upward load but there is always the sideways load.

My bowsprit is made front sched 40 stainless steel tube

 

[oldmanofthehills]

What you really need to do is to determine what the loads would be and the a cantilever bending analysis can be done.

A bob stay will resist any upward load but there is always the sideways load.

My bowsprit is made front sched 40 stainless steel tube

Fortunately such analysis can be done by any competent engineer by eye and familiarity with the materials, as was done for the past 500 milenium. My admiralty steel bar was overkill..

Whisker stays are an easy addition for side load if using hollow section though can make anchor retrieval more fidley. Once one gets to wall thickness comparable to scaffold pole (3 to 4mm), then there will be no bending in a much longer bowsprit even without side stays, and I only used solid square section on my first attempt as did not have welding gear to put plates on scaffold pole.

I intend to go for short demountable extension bowsprit so I can fly cruising chute clear of the genoa furler, and think 40 or 50mm ali pole will do for my 27ft boat

80mm ali tube with 2mm wall seems fine for longer 1m bowsprit if the OP does not want something fabricated, but 48mm ali scaffold tube with 4.5mm wall thickness is available so personally I would save the stress and go for that

 

[rogerthebodger]

Fortunately such analysis can be done by any competent engineer by eye and familiarity with the materials, as was done for the past 500 milenium. My admiralty steel bar was overkill..

Whisker stays are an easy addition for side load if using hollow section though can make anchor retrieval more fidley. Once one gets to wall thickness comparable to scaffold pole (3 to 4mm), then there will be no bending in a much longer bowsprit even without side stays, and I only used solid square section on my first attempt as did not have welding gear to put plates on scaffold pole.

I intend to go for short demountable extension bowsprit so I can fly cruising chute clear of the genoa furler, and think 40 or 50mm ali pole will do for my 27ft boat

80mm ali tube with 2mm wall seems fine for longer 1m bowsprit if the OP does not want something fabricated, but 48mm ali scaffold tube with 4.5mm wall thickness is available so personally I would save the stress and go for that

That is exactly how I did mine but as a competent Engineer I would not do it by eye for someone else.

My gut feel would be a 6 mm wall but this could be considered overkill but again I would not wish my bowsprit for fail far from shore

Mine is also overkill as my boat is a crouser not a racer so a little overweight is not an issue.

Yes whisker stays can be used but it makes it complicated when the bowsprit needs mounting or dismounting like mine

 

[blush2]

Was there not a Lightwave with a conversion to a J120 style of sprit?

I have been thinking about a small sprit for my little bilge keeler.
My idea is more about getting the tack away from the pulpit and cruising with a bit asy in not much wind than what we used to get up to in Laser 4000s and the like.

If the sprit is a dumb piece of tube and it breaks long before the mast, that may be a Good Thing.

You may be thinking of Blush. We tried a couple of ways of fitting a removable sprit for our assymetric. The first was aluminium tube sitting on the bow roller with a bobstay and lines to counter sideways movement. However these turned out to be ineffective and the tube bent where it came out over the roller one day when we were raising the snuffer, bending the roller in the process.

Version two used a length of carbonfibre mast from a 1720 with a Selden ring slightly offset to avoid the forestay and a pad eye type fixing inboard. This worked Okay until a trip through the Raz de Seine when a sudden wind change caused the assymetric to flog. The pole shattered under the shock.

After that we went back to using a block for the tack attached to the base of the bow roller.

 

[oldmanofthehills]

That is exactly how I did mine but as a competent Engineer I would not do it by eye for someone else.

My gut feel would be a 6 mm wall but this could be considered overkill but again I would not wish my bowsprit for fail far from shore

Mine is also overkill as my boat is a crouser not a racer so a little overweight is not an issue.

Yes whisker stays can be used but it makes it complicated when the bowsprit needs mounting or dismounting like mine

2 3/4"od x 1/4" wall Aluminium Tube | metals4U

6mm thickness available with 70mm tube. The weight penalty of going from 4mm/50mm to 6mm/70mm is probably a kilogram or so (cant be bothered to calculate) and compared with anchors, chains, winches etc all up by bows it will make no difference to handling except possibly for pure racing boat where most normal stuff is stripped out, and hyper light hyper materials are used.

 

[Lightwave395]

This site gives some wall thicknesses. I've also seen 2.5 mm for 80 mm spinnaker poles.

Spinnaker & Whisker Poles : Sailspar

Thanks for that, seems like 3 mm wall thickness might do it. My proposed sprit will have 1 metre projection and 1 metre on the foredeck with the inboard end fixed to a padeye when in use and supported by a ring similar to the selden product. I simply resent paying 4 figures for the selden kit when I can make my own for less the £200... A 2m length of 82mm or 100mm alloy tube is less than £60, the ring is easy enough to fabricate as is a delrin plug for the end.
I could use my carbon pole but this add-on is intended for my cruising life in relatively light breeze so won't be under huge pressure and as such the pole is over 4 metres long so unwieldy !

 

[Lightwave395]

You may be thinking of Blush. We tried a couple of ways of fitting a removable sprit for our assymetric. The first was aluminium tube sitting on the bow roller with a bobstay and lines to counter sideways movement. However these turned out to be ineffective and the tube bent where it came out over the roller one day when we were raising the snuffer, bending the roller in the process.

Version two used a length of carbonfibre mast from a 1720 with a Selden ring slightly offset to avoid the forestay and a pad eye type fixing inboard. This worked Okay until a trip through the Raz de Seine when a sudden wind change caused the assymetric to flog. The pole shattered under the shock.

After that we went back to using a block for the tack attached to the base of the bow roller.

There was also a LW called 'Haley's Dream' which had a full blown copy of a j boat sprit - she went to South Africa some years ago

 

[B27]

There was also a LW called 'Haley's Dream' which had a full blown copy of a j boat sprit - she went to South Africa some years ago

Whereas Marionette just turned into a J120 IIRC?

 

[steveeasy]

I'm planning on making a bowsprit using aluminium tube 80mm - 100mm diameter. Anybody know what wall thickness would be appropriate ? The sprit will project around 1 meter forward on a 12m boat. Can't get the dimension from my existing spnnaker pole as it's carbon...

Ive a prodder? I fitted to a previous boat, not quite a bowsprit. Too short for your needs. It was 80mm and wall thickness was 3mm I think. You could use your bow roller. Just make sure you weld a flat on the tube to spread the load or any pin used to hold it down will be too much force on a small area.
Steveeasy

 

[billyfish]

I'm using my spinnaker pole but it only sticks out about 18 inches. Get nervous at about 15kts of breeze