Bowsprit calculations.

[Norman_E]

I have an asymmetric cruising chute for my Jeanneau 45.2, which is about 11 tons in sailing trim. The sail has the following dimensions and is of fully radial construction: Luff 14.74m. Leach 12.97m and foot 8.33m. From those figures I think the area must be about 70 sq. metres. With a bowsprit I can do away with the snuffer and fit a top down furler.

What I want to do is make a stainless steel retractable bowsprit which will stick out 600 to 650mm beyond its sliding fixture attached to the bow roller and a greater distance behind the fixture to a deck mounting. The critical strength issue is the bending force applied at the bow roller mounting. Can anyone here guide me as to the size of code 40 or code 10 pipe needed? For various reasons I would ideally like the pipe to be 2.5 inch code 40 which measures 2.875 inches O.D. or smaller. I notice that the Selden aluminium bowsprit requires to be 87mm for an 11.3 ton boat, though I don't know what its wall thickness is and how much stronger stainless steel pipe is compared to aluminium alloy.

I know Southerly supplied some of their boats with a stainless bowsprit. If anyone has one perhaps they could measure it?

 

[Tranona]

Can't help on the bowsprit calculations except to say that if your sail is indeed only 70sqm then your proposal is more than adequate!

However not sure your are is correct. My cruising chute for a 33' Bavaria is a modest 65sqm, but it is small because I singlehand and the boat could easily take 75sqm. That keeps it within the smaller GX10 furler which will go up to 80sqm and 36' boat. You will need the next size up so would guess a sail are in the 90-100sqm range.

 

[Norman_E]

Thank you. The problem is that the area of the sail is not specified in the order documentation, only the dimensions. I worked on the average of leach and luff x 60% of foot which gives 60.25 sq.metres. Its the 60% that is probably a wrong guess. 80% would result in 92sq metres! I think I will phone the sailmaker tomorrow and ask if they can tell me the area, but I agree that a GX15 furler with a 16metre luff rope looks to be the right size. As for the bowsprit it would be even better if 2 inch nominal code 40 pipe would do the job.

 

[TLouth7]

Max Bending Moment M = Ze (Section Modulus) * Max Stress - find these from the web

Max (perpendicular) Tackline Tension T = M / Length L

For 2.5" section 40 this gives M = 5050 Nm, which means it could carry T = 7780 N

You could compare this to the breaking strength of your tackline (T will clearly not go above this), or you could find some other estimate for the maximum tension applied by the sail - perhaps work backwards from the Selden Ali sprit if you can find its specs. Don't forget to include a safety factor!

Source:
http://www.crp.co.uk/technical.aspx?page=141
https://www.engineersedge.com/calculators/section_square_case_12.htm
https://www.google.com/search?q=s31...e7&rls=com.microsoft:en-GB:IE-Address&ie=&oe=

 

[lw395]

The unknown is the force from the sail.
Knowing the area does not help much, the loads on a sail depend as much on the weight of the boat hanging on it.
I have a dinghy with a light sprit about 2m long and about 60mm of thin grp. It holds a 28sqm kite in 25 knots.
I reckon it would break in 15 knots on a small cruising yacht.

You could try working out the force to rip the tack off the sail, compare that to the strength of the tack line?

Of course you could take the view that if you get hit by a squall with the kite up, a bent bit of pipe is not the end of the world.
A sprit which is the weakest link might give many years of normal cruising use.

 

[Norman_E]

Thank you both. I have always used a 10mm tackline made of Marlowbraid with the sail, because 10mm is the largest line that fits though two organiser blocks on the foredeck. The breaking strain is quoted at 2850KG so that is not going to break before the bowsprit bends, but I very much doubt that the tack of the sail is that strong. I wont be using the kite in winds over 20 knots because the Genoa on its own gets the boat going fast enough downwind provided I keep it a little bit away from a dead run. The asymmetric is more for when I need to go downwind and there is not enough of it to make reasonable progress with the white sails. I think I will try to get a sprit made up in the Saniye in Marmaris whilst I am out there in September, and try it using the sail with the snuffer before I buy the expensive top down furler.

I need to have it made so that it sticks both forward and upwards from the bow so that it clears the hoop on the anchor which sits on the other side of the bow roller and retracts downwards into the port side of the chain locker, the lid of which will need a cut-out, so a wooden mock up will be the first thing to make.

 

[lw395]

The other thing with sprits is 'what does the inboard end push down on?'.

 

[Norman_E]

The other thing with sprits is 'what does the inboard end push down on?'.

I need to measure but I think that the solid deck ahead of the chain locker extends far enough back and can be reinforced if required with a stainless bar.

 

[rogerthebodger]

I need to measure but I think that the solid deck ahead of the chain locker extends far enough back and can be reinforced if required with a stainless bar.

This may help to determine the loads in a sail and thus the tack line thus the bowsprit

https://www.riggingdoctor.com/life-aboard/2015/11/18/calculating-loads

 

[Quandary]

The Selden sprit on our Finngulf 33 had limited projection and support well within Seldens table of constraints but even in quite moderate breezes it was subject to a worrying amount of upward bend so do not be tempted to cut any corners.

 

[lw395]

The Selden sprit on our Finngulf 33 had limited projection and support well within Seldens table of constraints but even in quite moderate breezes it was subject to a worrying amount of upward bend so do not be tempted to cut any corners.

Yes, I sail a dinghy with a sprit, that bends quite a lot, it's not just the loads you can put on while reaching, the bend you can achieve bearing away in gusts is quite something.
But, if the force gets excessive, a springy tube taking the impact out of the gusts might be good, and in the limit, a crumpled sprit is less worrying than a crumpled deck.

 

[thinwater]

A more logical basis, than strength of the tackline (which could be anything) is the halyard load required to knock the boat down. You can work backwards from there, adding a 3x safety factor.

The other alternative is to hoist the sail at the bow, sail it in X wind, attach a load cell, and then scale up to the max wind (square). Then you are not guessing. Load cells are pretty cheap in the scale of things.

 

[LadyInBed]

Instead of a circular X section how about square or rectangular? My gut feeling is that it would be stronger, maybe a 'proper' engineer could say.

 

[lw395]

A more logical basis, than strength of the tackline (which could be anything) is the halyard load required to knock the boat down. You can work backwards from there, adding a 3x safety factor.

The other alternative is to hoist the sail at the bow, sail it in X wind, attach a load cell, and then scale up to the max wind (square). Then you are not guessing. Load cells are pretty cheap in the scale of things.

You can do these exercises, but the biggest loads are short-term, as a gust hits or as the bow hits a wave.
The head of the kite might be pulling mostly forwards rather than generating too much heeling in a breeze.

One assumes that the cruising yachtsman will exercise due restraint, but it is possible to get caught out by a sudden squall etc.

 

[TLouth7]

Instead of a circular X section how about square or rectangular? My gut feeling is that it would be stronger, maybe a 'proper' engineer could say.

A square section is better (stiffer and higher yield strength) when the load is vertical or horizontal, but weaker when the load is diagonal (which it is in this case). One advantage of a square section is that the bracket at the bow might be easier to make (parallel cheek plates and a pin across the top.

An option with any section (but easiest with round tube) is to insert a tight fitting tube inside the main section at the region of highest moment (at the bow fitting). This makes the sprit stronger without excess weight along its whole length, and is easy to achieve.

 

[lw395]

....

An option with any section (but easiest with round tube) is to insert a tight fitting tube inside the main section at the region of highest moment (at the bow fitting). This makes the sprit stronger without excess weight along its whole length, and is easy to achieve.

You could proably make it just as strong using half the wall thickness except for the middle third, by using a sleeve inside the middle third.
It's a weigh-off of weight against complexity.
An ali sprit I was admiring also had a short sleeve outside to spread the point loads of tack, bow and deck.

 

[bbg]

I have an asymmetric cruising chute for my Jeanneau 45.2, which is about 11 tons in sailing trim. The sail has the following dimensions and is of fully radial construction: Luff 14.74m. Leach 12.97m and foot 8.33m. From those figures I think the area must be about 70 sq. metres. With a bowsprit I can do away with the snuffer and fit a top down furler.

What I want to do is make a stainless steel retractable bowsprit which will stick out 600 to 650mm beyond its sliding fixture attached to the bow roller and a greater distance behind the fixture to a deck mounting. The critical strength issue is the bending force applied at the bow roller mounting. Can anyone here guide me as to the size of code 40 or code 10 pipe needed? For various reasons I would ideally like the pipe to be 2.5 inch code 40 which measures 2.875 inches O.D. or smaller. I notice that the Selden aluminium bowsprit requires to be 87mm for an 11.3 ton boat, though I don't know what its wall thickness is and how much stronger stainless steel pipe is compared to aluminium alloy.

I know Southerly supplied some of their boats with a stainless bowsprit. If anyone has one perhaps they could measure it?

See here for sail area.

https://www.sailboat-cruising.com/sail-dimensions.html

0.8 x luff x foot = 98 m2 approximately