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Strengthening Aluminium tube - Yachtie question


Kurt Barlow
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Kurt Barlow

I'm making a short bow sprit for my Yacht (about a metre long  although the section from where its secured off will be about 70cm. From this I will secure a 25m2 assymmetric spinnaker

Not wishing to get bum banged for a Carbon fibre one I have an Alumnium tube which is 48mm OD and has a wall thickness of 4.5mm. 

The question is - is it worth filling with expanding foam to increase its stiffness. The foam has quite high compressive strength so in theory should increase the tubes resistance to bending. Will also make it float if it goes over the side 

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11 hours ago, Kurt Barlow said:

The question is - is it worth filling with expanding foam to increase its stiffness. The foam has quite high compressive strength so in theory should increase the tubes resistance to bending. Will also make it float if it goes over the side 


Sounds like a “project farm” type question.   Unless you know others who’ve done it I suspect you’d have to set up an experiment.

My guess is that it will not add that much total strength,  but may stop it suddenly buckling if the loads get too high,  instead bending more gradually. And as you say,  it would help it float.

@Chewing Grass and @onlyme are fairly practical types..  they may have some ideas.

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Chewing Grass
13 hours ago, Kurt Barlow said:

I'm making a short bow sprit for my Yacht (about a metre long  although the section from where its secured off will be about 70cm. From this I will secure a 25m2 assymmetric spinnaker

Not wishing to get bum banged for a Carbon fibre one I have an Alumnium tube which is 48mm OD and has a wall thickness of 4.5mm. 

The question is - is it worth filling with expanding foam to increase its stiffness. The foam has quite high compressive strength so in theory should increase the tubes resistance to bending. Will also make it float if it goes over the side 

Answer, not really for strength, you will find at 4.5mm section thickness it is quite strong anyway, sounds like a scaffolding pole. Just fill it with foam to make it float (just) if by any chance it breaks then review the design.

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belfastchild
13 hours ago, Kurt Barlow said:

The question is - is it worth filling with expanding foam to increase its stiffness. The foam has quite high compressive strength so in theory should increase the tubes resistance to bending. Will also make it float if it goes over the side 

Depends on the type of expanding foam. It will have to be sealed, IIRC the stuff I used before was affected by UV and was water resistant, not waterproof. Submerging in sale water could make it soak in or dissolve! You need the PVC stuff not the normal diy polyurethane stuff.
If using in salt water, might be a pita locking in salt water so would have to be sealed.

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Kurt Barlow
1 hour ago, Libspero said:


Sounds like a “project farm” type question.   Unless you know others who’ve done it I suspect you’d have to set up an experiment.

My guess is that it will not add that much total strength,  but may stop it suddenly buckling if the loads get too high,  instead bending more gradually. And as you say,  it would help it float.

@Chewing Grass and @onlyme are fairly practical types..  they may have some ideas.

Cheers. Aren't we all project farm here!

Most of the tension will be if using it on a beam reach. More down wind and the main exertion is pulling on the pole. 

The length of the pole from the bow roller will be 650-700mm. If it does bend the other option is using a steel pole but then have corrosion issues to deal with. I don't intend to launch the CC in more than 12 mph of apparent wind and will take down if it goes above 16mph. I have a snuffer for quick take downs. 

I found a research paper on foam filling of tubes but it was behind a pay wall. 

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1 minute ago, Kurt Barlow said:

Cheers

The article is about metal foams. I'm talking squirt out of a can! 

well gis a fukin clue then! why not post a link to the article? O.o

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Kurt Barlow
12 minutes ago, Kurt Barlow said:

I have to put a request in which goes no where. 

Its more interest really to see if the foam filling has any significant effect. I will do it anyway as it will make the tube float if it goes over the side

I now recall sections of dinghy mast are filled with polystyrene as a stiffening agent so there must be some benefit. 

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Chewing Grass
3 minutes ago, Kurt Barlow said:

I have to put a request in which goes no where. 

Its more interest really to see if the foam filling has any significant effect. I will do it anyway as it will make the tube float if it goes over the side

I now recall sections of dinghy mast are filled with polystyrene as a stiffening agent so there must be some benefit. 

4.5mm is not thin wall tube, thin wall tube at your size would be 1 or 2mm.

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Kurt Barlow
1 minute ago, Chewing Grass said:

Answer, not really for strength, you will find at 4.5mm section thickness it is quite strong anyway, sounds like a scaffolding pole. Just fill it with foam to make it float (just) if by any chance it breaks then review the design.

Its an Aluminium Scaffold pole. The length under tension will be 650mm long. The remaining 350mm will be ratchet strapped to the Bow roller and forward cleat with 800kg rated straps. 

The other main control is limiting use to light winds - dont launch if apparent wind speed is above 12mph. Take down if it rises to 16/17mph. 

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Kurt Barlow
59 minutes ago, belfastchild said:

Depends on the type of expanding foam. It will have to be sealed, IIRC the stuff I used before was affected by UV and was water resistant, not waterproof. Submerging in sale water could make it soak in or dissolve! You need the PVC stuff not the normal diy polyurethane stuff.
If using in salt water, might be a pita locking in salt water so would have to be sealed.

Good point.mThe foam will be inside the tube so not going to be effected by UV. 

I wonder if you can get Scaffold tube end caps?

The pole won't be submerged but  will get spray over it. 

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Kurt Barlow
1 hour ago, belfastchild said:

Depends on the type of expanding foam. It will have to be sealed, IIRC the stuff I used before was affected by UV and was water resistant, not waterproof. Submerging in sale water could make it soak in or dissolve! You need the PVC stuff not the normal diy polyurethane stuff.
If using in salt water, might be a pita locking in salt water so would have to be sealed.

Can't find any sources of PVC foam its all PU. 

Just bought 2 of these. I shall effectively seal them with some Jet Blue paste. Paint the Aluminium with white Hammerite and it will look pucka - like a real professional job. Perhaps I can brand mark it - DOSBODS marine xD

Scaffold Tube End Caps (Yellow) Scaffolding Pole Pipe Feet 48.3mm BRITISH MADE | eBay

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Kurt Barlow

Just had the aluminium pole delivered. It feels much stronger than I anticipated. The walls are 4.5mm. I will fill with foam anyway just to ensure it floats and seal off the ends. 

I'm confident its strong enough for the job in hand. 

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Chewing Grass
3 minutes ago, Kurt Barlow said:

nearly finished. Just need to clean the scraps of foam off. Not bad for about 30 Quid. 

If you are storing wood in a garage, make sure you burn the stock over winter to keep wood-worm at bay.

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Initially thought that it would possibly bend regardless of foam which would not add much stiffness, looking at the photo I'd say unlikely to bend, aluminium scaffold section in general is in tension in a tower, apply any lateral load and it does bend much easier than steel, but that looks small enough o not have bending forces to cause any issue. Your fixing points may be more of a concern - thought holes on the sidewall may be prone to wallowing out if those lashing eyes become at all loose - thread locker or stainless unlock and a padding washer might be a good idea. 

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Kurt Barlow
On 25/09/2021 at 10:30, onlyme said:

Initially thought that it would possibly bend regardless of foam which would not add much stiffness, looking at the photo I'd say unlikely to bend, aluminium scaffold section in general is in tension in a tower, apply any lateral load and it does bend much easier than steel, but that looks small enough o not have bending forces to cause any issue. Your fixing points may be more of a concern - thought holes on the sidewall may be prone to wallowing out if those lashing eyes become at all loose - thread locker or stainless unlock and a padding washer might be a good idea. 

About midway the sprit will be lashed to the bow roller with a 800kg ratchet strap. Another ratchet strap further back will secure it to the cleat. The rear eye bolts will be used to tie off the sprit particularly if the strain is pulling the pole forward. 

The fwd eye bolt has 2 nuts and 3 washers. The thread is effectively jammed withe the expanding foam. The rear eye bolts have one nut and 2 washers. 

At the end of the day the total cost of that is about £35 and a few hours work. If it lasts 4 seasons I will be more than happy. Its only for occasional use as I don't intend to fly the chute unless I have crew and conditions are right. 

BTW - do you know where you can get good quality cheapo chain - ideally in long lengths. I need 11mm for my scope and 13mm for the bridal. 

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Bricks & Mortar

That looks quality.  Sorry i didn't see this earlier.

Not sure about your fittings and fixings, but I've got alloy poles that we've had since the 80's.  Not quite in marine use, but we do find the roofs on the seafront wear out faster, so they've been regularly built where they get at least salt water spray.

I hope the foam works out fine for you.  Having used it on roofs, even where I thought it was protected, I've gone back to find it fully waterlogged.  Maybe check on it after you've been to sea a few times.
Just in case there's any need to redo the foam, I'd consider '40mm closed cell polyethylene backer rod' - I reckon it'd squeeze down the tube if you managed to clean it out nicely.  It's basically a thin pool noodle, and is used in construction to fill expansion joints, perhaps between concrete panels. 

You can get closed cell expanding foam from boat-building suppliers...  2 part liquid stuff, as I recall.  Pricey, when I considered adding buoyancy tanks to a rowing boat, but then didn't bother.

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10 hours ago, Kurt Barlow said:

About midway the sprit will be lashed to the bow roller with a 800kg ratchet strap. Another ratchet strap further back will secure it to the cleat. The rear eye bolts will be used to tie off the sprit particularly if the strain is pulling the pole forward. 

The fwd eye bolt has 2 nuts and 3 washers. The thread is effectively jammed withe the expanding foam. The rear eye bolts have one nut and 2 washers. 

At the end of the day the total cost of that is about £35 and a few hours work. If it lasts 4 seasons I will be more than happy. Its only for occasional use as I don't intend to fly the chute unless I have crew and conditions are right. 

BTW - do you know where you can get good quality cheapo chain - ideally in long lengths. I need 11mm for my scope and 13mm for the bridal. 

Have bought spares from this supplier before - good for plant/equipment and thought they might do chain, which they do.

https://www.lsengineers.co.uk/chains-plastic-metal.html

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Kurt Barlow
On 27/09/2021 at 20:09, Bricks & Mortar said:

That looks quality.  Sorry i didn't see this earlier.

Not sure about your fittings and fixings, but I've got alloy poles that we've had since the 80's.  Not quite in marine use, but we do find the roofs on the seafront wear out faster, so they've been regularly built where they get at least salt water spray.

I hope the foam works out fine for you.  Having used it on roofs, even where I thought it was protected, I've gone back to find it fully waterlogged.  Maybe check on it after you've been to sea a few times.
Just in case there's any need to redo the foam, I'd consider '40mm closed cell polyethylene backer rod' - I reckon it'd squeeze down the tube if you managed to clean it out nicely.  It's basically a thin pool noodle, and is used in construction to fill expansion joints, perhaps between concrete panels. 

You can get closed cell expanding foam from boat-building suppliers...  2 part liquid stuff, as I recall.  Pricey, when I considered adding buoyancy tanks to a rowing boat, but then didn't bother.

Went sailing today, light winds  and couldn't resist so tried it out. Worked really well

 

bow sprit2.jpg

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Kurt Barlow
On 27/09/2021 at 20:09, Bricks & Mortar said:

That looks quality.  Sorry i didn't see this earlier.

Not sure about your fittings and fixings, but I've got alloy poles that we've had since the 80's.  Not quite in marine use, but we do find the roofs on the seafront wear out faster, so they've been regularly built where they get at least salt water spray.

I hope the foam works out fine for you.  Having used it on roofs, even where I thought it was protected, I've gone back to find it fully waterlogged.  Maybe check on it after you've been to sea a few times.
Just in case there's any need to redo the foam, I'd consider '40mm closed cell polyethylene backer rod' - I reckon it'd squeeze down the tube if you managed to clean it out nicely.  It's basically a thin pool noodle, and is used in construction to fill expansion joints, perhaps between concrete panels. 

You can get closed cell expanding foam from boat-building suppliers...  2 part liquid stuff, as I recall.  Pricey, when I considered adding buoyancy tanks to a rowing boat, but then didn't bother.

Cruising chute in action this afternoon

 

 

cruising chute.jpg

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