# Bowsprit - Load on solid glass

Discussion in 'Boat Design' started by ghoti, Jun 7, 2022.

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### ghotiNew Member

Hi,

I am sure this is a simple question but my google-fu is failing me. I am designing a simple bow-sprit that that bolts into the existing bow roller channel. Using the calculator provided by Selden provides an estimate of 3.5kN on the tack, assuming unsupported section = support section 3.5kN pushing down on the deck.

Glass of the deck in that area is fairly well supported ~10mm solid glass and ~800mm from toe-rail to toe-rail. Spreading this across 100mmx50mm feels about right but this this project has sparked some interest, how can I determine what load can be supported by the deck. I'm sure this is 101 type stuff but I'm trying to learn. Any tips?

Thanks,
Alex

Bowsprit calculator : SeldÃ©n Mast AB https://support.seldenmast.com/en/services/calculators/bowsprit_calculator.html?id=4605&cmd=submit&pp_refocus=&pp_offset=&lang=en&RM=40&WLH=1200&HGR=14000&USL=500&DBS=500

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### gonzoSenior Member

It is a rather complicated question. You would need the design and scantlings of the boat to calculate all the loads. For example, the failure mode may be the deck to hull joint.

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### ghotiNew Member

Gonzo, it's a good point but the sprit will be loading down onto the deck, not trying to rip it up.

From the diagram in the link I am interested in the interactions at R1 and understanding what kinds of loads a 10mm sheet of solid glass can take. At this point I would not even have a guess of the order of magnitude so really just trying to get in the ballpark.

Ultimately I would like to specify a sprit that fails before the deck.

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### TANSLSenior Member

To give a serious answer it would be necessary to know the structure of the deck in that area.
(Frankly, I wouldn't know what to do with failure mode. I'm not saying that it shouldn't be considered, I don't know, that's why I would like Gonzo to be so kind as to explain how to take it into account)

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### gonzoSenior Member

The link is not opening for me.

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### rangebowdrieSenior Member

If for sail carrying there should be a bobstay and the bowsprit is, (should be,) in compression.
Perhaps I've misunderstood?

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### ghotiNew Member

Thanks All,

The purpose is to extend a tack point for an Asymmetric spinnaker beyond the pulpit. This won't be used for code sails so not planning on using a bob-stay, this will be a cantilever design.

I have attached a screenshot of the Selden calculator which estimates the loading on the tack for those where the link is unavailable.

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### gonzoSenior Member

That only shows the vertical force. What are the horizontal forces?

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### rangebowdrieSenior Member

I see now.
Not being an engineer, how does the upward force at the forward end translate?
In other words, which connection, (the stem head attachment or back on the deck,) becomes the fulcrum point?
It "seems" as though the stem point would be the strongest in tension and would be the fulcrum, thereby making the after end putting the deck in compression?

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### gonzoSenior Member

They don't translate. They are independent force vectors. To study and calculate forces, a "Free Body Diagram" is an easy simplification. You can look at examples online. However, for a proper analysis, you need to consider shock loads which can be much larger than the static loads.

rangebowdrie likes this.

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### rangebowdrieSenior Member

Ok, I'll have some studying to do.

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