Spinnaker size...How to work it out?

[Tim O]

I’m looking out for a second hand spinnaker for my Colvic Countess 33 ketch rig.

I have a pole but no kite!

how do I go about working out what size sail would be suitable? How critical are the luff and fit lengths or is there some flexibility here?

im not in the running for a new purpose made sail so it’s going to have to be used or nothing.

 

[sarabande]

No sail data on Yachtsnet, and really there is not much difference to the spinnakers of similar sized cruising boats. Westerleys will be a good start point.

The size and dimensions will vary according to the function, e.g. reaching spi, or a full downwind one.

If I had to buy one spinnaker for a cruising boat it would be a radial head which seems to stand well in a variety of conditions.


Here are some ideas on finding area from existing measurements.

Calculating your sail areas - a guide by upffront.com

 

[lw395]

There is some flexibility.
Even if the pole height is fixed at the mast end, you can angle it up or down some.
If the kite is a bit big, a longer pole will make it work.
The length along the luff tape is going to be maybe 15% more than the straight line?
So, a bit of trig, pole length, halyard height, pole height up the mast.

 

[Concerto]

You need to know the I and J measurement of the fore triangle. I is the luff length and J x 2 equals the foot length. Treat these numbers as the maximum size, but do not go below 90% or the spinnaker will be too small.

If you do not know these measurements then ask a sailmaker for a quote for a new genoa and they should provide these in the quote.

 

[Tim O]

Just going to measure I and J on the actual boat now

 

[Tim O]

Ok so....

dimensions on my boat

Pole length 11’10” (pole height is adjustable on a sliding track)
J 11’10”
I approx 35’6”

known dimensions of the spinnaker I’m pondering are
Foot (clew to clew) 17’
Approx 28’ top to foot (assume centre of foot so that would make the luff....
Luff approx 29’3”
Total area 238 square feet

Am I right in thinking this might be on the small side especially given a pole of basically 12’???

on the other hand as I’ve never flown a spinnaker before maybe smaller is better??

 

[Tim O]

Any thoughts on these measurements?

 

[lw395]

It's a bit short in the vertical.
But in the absence of a better match, if the price is right for the condition, might be OK.

I assume your boat is masthead rigged and the kite halyard emerges just below'close to the forestay?

Personally I think big spinnys are the way to go for cruising as you want progress and entertainment in light airs rather than max speed when white sails are already giving you fair speed.

 

[Tim O]

Yes kite halyard emerges pretty much right at masthead close to forestay attachment

 

[Concerto]

Definitely on the short side for the luff as the pole will be very high when set. If you lower it down on the halyard, the spinnaker will swing too much. This is due to the head of the sail being able to move and affects the centre of effort of the sail. You could get a sailmaker to let another panel in to the spinnaker to make it a better fit.