Any Engineers who can do the calculations?

[rogerthebodger]

I build a bow sprit for my cruising chute a couple of months ago and with a total length of 1.5 meters and I used 1.25 sched 40 stainless tube thats about 35mm i/d and that is certainly strong enough.

I think 3 mm thickness may be a little light due to the potential of the tube deforming at the point of high bending moment caused by shock loading

 

[srp]

Thank you for that. Whilst the figures look a bit lower than I hoped....

I would have worked it out the same way as MisterG. To give it a practical twist, imagine a bit of projecting scaffold tube. You (or I) could easily swing on it without it bending permanently, (12 stone is about 76kg).
I would be happy hanging 4 bags of cement off it (100kg), which is the safe limit with a factor of safety of 2.5.
MisterG's calcs suggest it would bend permenantly with around 10 bags (250kg or 1/4 tonne) hanging off the end, which seems about right to me.

I know I've added little or nothing to the discussion, but I'm just thinking aloud.

 

[Norman_E]

I build a bow sprit for my cruising chute a couple of months ago and with a total length of 1.5 meters and I used 1.25 sched 40 stainless tube thats about 35mm i/d and that is certainly strong enough.

I think 3 mm thickness may be a little light due to the potential of the tube deforming at the point of high bending moment caused by shock loading

Have you had a chance to test it yet? How much unsupported length is there when you use it? I think your boat is bigger and much heavier than mine, so if 1.25 shedule 40 tube is strong enough for you, 2 inch schedule 40 should be plenty strong enough for me.

 

[rogerthebodger]

Only tested it in light breeze in the marina but 2 of us (180 kgs) swung from it and no noticeable deflection. Overhang is about a meter and is attached to the centre web of my bow fitting with 2 of 16 mm dia pins one being my anchor rode retaining pin.

My boat is 49 ft over deck about 52 with bow sprit rigged and displaces 22 tons all up but my mast is only 15 meters so about 3 meters shorter than the designer specified.

Just one thing to consider is the weight that you will have to handle to rig it unless you have some good way to lift it in front of the bow

 

[Norman_E]

Based on the calculations given on the previous page your overhang seems to be rather long for the size of the tube.

As for handling the weight, I will only have to do it once, as the bowsprit will slide through a plastic block, held in a stainless fitting bolted to the port side of the bow fitting. The aft end will sit on a fitting bolted to the lip of the anchor locker, with a cross pin through it, which can be withdrawn to allow it to slide back. All the fore and aft forces will be taken by the tack line rather than the bowsprit.

 

[rogerthebodger]

The calculations on the previous page is working out the maximum permitted load for various sizes of tube as per your OP but what is the actual load on the end of the bow sprit under various wind speed / sail configurations.

Once the loads that are imposed are specified can you determine if the arrangement is up to the job in hand.

So can any one work out the actual loads on the tack line for various sail sizes and wind speeds.

 

[Blueboatman]

Just to add.
A nice snug generous length of sleeve at the bow will minimise fears of the pole deforming..feeding it through a couple of u bolts ( seen that ) is of course not best practise..
Empirically one might be tempted to be a conservative engineer and do what others do at that size of prodder and rig. The real unknown is the variable loading in a squall just before you get the sail in..And theres stainless and stainless.
A mock up and test sail with a scaffold tube would give an idea of the bend, too.
All very 'undigital'

 

[misterg]

Am I right in thinking that the yield strength of 170Mpa is the minimum for 316L with ordinary 316 being higher, and typical values for both being about 300Mpa?

Yes, 170 MPa is the minimum for 316L in the annealed state, and 316L has a lower yield strength than 316.

Google shows a 'typical' yield strength quoted as 290Mpa in the annealed state. Yield stress could be a lot higher in practice for cold drawn seamless pipe due to work hardening - I believe 600MPa+ is theoretically possible, but what you can get hold of in real life is another matter.

The loads I worked out above just scale with yield stress - take your pick!

If you don't want to sleeve the tube where is meets the front support, make the support very close fitting around the tube, or put a short 'slug' inside the tube at the bearing point to guard against it crushing.

In the end, whatever the numbers, it should look right.

Andy

 

[misterg]

To add:

There are photos of the Southerly 32 offering on the web. (The bad news is that they're on the R0cn@ web site )

http://www.rocna.com/wiki/images/1/14/Southerly-32_09.jpg

http://www.rocna.com/wiki/images/a/a0/Southerly-32_01.jpg

http://www.rocna.com/wiki/images/7/77/Southerly-32_06.jpg

The overhang doesn't look that big to me - maybe 400mm?? If you knew your Rocnas, you could scale the dimensions of the bowsprit from the photos. (If you're not sure of the dimensions, I'm sure a post to the forum on the subject of Rocna anchors will get some responses

)

(It also looks like you could blow the tack of the cruising chute and launch the anchor with one smooth and swift movement...)

Andy