The length of a sprit - does it count...

[tjfmmaes]

Hi

I have purchased a Selden gennaker bowsprit kit for my Jeanneau Sun Odyssey 28.1 to make it easier and better to fly my rolling furling gennaker. There is no room to fly it directly from the pullpit. The question is now how long to make the bowsprit stick out in front of the boat. Obviously, it needs to be long enough to let the halyard clear the forestay etc and to give some clear air.

I have seen several lengths out there, from extremely long, 1.5 to 2m to very short (about 30cms). I do understand that the length of the sprit will affect the freeflow of the air and most importantly the balance of the boat and the tendency to luff up / fall off, so I am interested in what the "optimal" (if there is such a thing) length would be.

I have asked the sailmaker and they said about 45cms. I have asked Selden and they said to take it to the maximum rated, which is about 60cm for the 72mm sprit I have.

Is there anyone who has some idea on how to calculate the best place/length for a neutral balance and best power (I will tend to use it in lighter airs only, up to about 13Kn) ? I don't know if there is a formula or something or just best guess and trial and error will have turn up and assist?

To help out, here are some figures which I guess you might need:
LOA 8.70, Hull length 8.50, LWL, 7.10m
Main 18m2, Genoa 20m2, Gennaker 44m2
I=10.10, J=2.88

Any help is greatly appreciated

Thanks

Tom

 

[BAtoo]

Go full length - you can always cut some off if you don't like it....

 

[Quandary]

No. resist the temptation to be greedy, the sprit comes in two diameters and with a table included in the fitting instructions giving the ratio of extended to inboard length for each size. (Selden have an excellent website well worth a look for anything to do with yacht rig).Unless your gennaker is well undersize or you are absolutely sure it will never be up when the wind gets up stick close to these figures. I have one, on a 33ft boat which when projected, extends about 750mm. past the ring and I have seen it bend when close reaching. The force on the deck fittings can be considerable and it is not worth doing serious damage for an imaginary improvement in performance. If you have a long foredeck you can put in an extra padeye which allows two positions for the inboard end allowing it to be pushed forward in light weather.

 

[exfinnsailor]

Depends how long your poll is and its diameter .. Ours is forward of the pulpit by about 600 mm which if I remember was about max .. Here .. So our Beneteau has a displacement of 4500 and we used a 72mm Pole .. The fun part is trying to fix the forward eye to something solid .. Had a bracket made from 10mm Stainless but still managed to bend a couple of bolts so think big .. Apart from that good fun .. The sheet that goes down the pole is useless and impossible to adjust tighter even with a winch .. You can let it out but thats it .. So you need it in the right place when you hoist .. Straightest run you can get .. Ours sits to one side when not in use .. The retaining eye is on the Starboard side and the pole goes out under the guard wires on the starboard side and lines up with the centre of the bow roller .. It seemed impossible to fit straight out over the bow .. Happy playing /forums/images/graemlins/grin.gif

 

[William_H]

As all the other posters indicate I think your concern about boat helm balance is unfounded you would not notice much difference. When reaching the gap between sails would ideally be about 800mm or more based on the size of boat based on the interaction of the 2 sails. However the structural strength of the pole is far more critical and more so with greater length. So short is stronger. olewill

 

[tjfmmaes]

Hi

Thanks for all the replies, they are really helpful. I had already had a look at the Selden website, and with a 3.0t displacement, RM of around 16 (according to Selden), my max length is around 80 cm, but I guess I will play it safer (hate to see my deck fittings ripped out) with around 60-65 (although the 2 padeyes solution for light winds might still be a handy one)

Another comment on here made me think as well - do you fly the genoa with the gennaker when reaching as one post implies? I am of the generation that hoisted the spi, dropped the gennie. But I have seen those crazy kids sailing around with both up, so when (what course) would that be sensible?

Thanks a lot

Tom

 

[Twister_Ken]

Two sails - the theory is that unless quite close to the wind, flying two foresails without much separation is slower because air would get very turbulent between them, and stall. Also as the wind comes aft, so the outer sail is in the wind shadow of the inner one. More efficient to have attached airflow on one sail, than unattached airflow on two.

 

[William_H]

If you are reaching and the sails have suitable separation then 2 should be better than one. IMHO leaving both up will be fine although you may find the reacher won't set well with the jib up. It may come down to whether you want to pull the jib down (roll it up) or leaving it working. You choose olewill