Discussion in 'Boat Design' started by nemo, Sep 8, 2011.

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### nemoNaval Architect

Hello,
I need to design a carbon bowsprit for a gennaker. I didn't find on any book any formula for calculating the tack load.
The only thing I've found is the Marshall formula but it is for sheets and I don't feel it is really applicable to this case.
Could you please advise wether there is something useful in the literature?
Thanks

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### Richard WoodsWoods Designs

The simple way would be to say "what is the breaking strength of the tack line?" and work from there

You don't say what boat you have, but most boats will use 8-12mm rope

Richard Woods of Woods Designs

www.sailingcatamarans.com

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### GreybarnJunior Member

The other way to look at is how strong is the halyard and the winch the halyard is getting loaded on to. I think the bow sprit wants to be stronger than the halyard, and any deflection limit may create that strength anyway.

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### nemoNaval Architect

Thank you for your replies. However, I think that using the tack line breaking strength would lead to an oversized sprit. A 10 mm rope nowadays has a breaking strength of almost 2 tons, I believe it's a bit unrealistic to put 2 tons on the head of a bowsprit of a boat in the range of 34'-40'.
I made some reverse engineering on the selden bowsprit and it looks like they consider, for a boat of about 7 tons of displacement, a service load on the tack of about 200kg. I attach for reference the selden datasheet: they give the max service load on the inner end of the bowsprit, it is then easy to calculate the load on the tack by multiplying the MSL at inner end X DBS/USL (see table).
This is a service load, so I should apply a factor of safety, but even with a factor 3 we are far from the 2 tons of the tack line.
It is also interesting the reply by dougfrolich to my other post in the sailboats section: http://www.boatdesign.net/forums/sailboats/gennaker-bowsprit-load-39627.html

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### michael pierzgaSenior Member

Perhaps a service load of 200kg and your reverse engineering approach would work. Some time ago I was googling hull keel attachments and came across a pdf file which contained the work done by structural engineering students analysing the structural loading on a swing keel, bow sprit ocean racer . Try google...It was in Italian.

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### Richard WoodsWoods Designs

I'm not sure that 200kgs is enough

The snatch loads when the sail collapses will at least double the loads.

Again thinking simply. What is the breaking strain of the stopper you use?

Can three crewmen hold the end of the line when the sail refills? (they will be able to hold 200kgs easily)

Richard Woods of Woods Designs (someone who has broken two gennaker bowsprits)

www.sailingcatamarans.com

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### michael pierzgaSenior Member

I agree... the bow sprit takes alot of mistreatment from colapsing sails. Halyard tension on a tight reach excedes the 200kg figure.

But you have to start your investigation somewhere...200kg might be a good start.

the breaking strength of the tack fitting will be your ultimate guide. Make the fabric tack fitting the weak point in the systen to protect the structure of the boat.

.

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### nemoNaval Architect

The breaking strength of the stopper makes more sense to me than the halyard.
A normal Spinlock for a 10m boat has a breaking load of around 575kg. I normally go racing on a J109 which has one of those stoppers.
Although I would not recommend to ease the tack line while simply holding it with the hands, it is relatively easy if passed around a winch.
Michael, I cannot find the

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