Spinnaker Loads and Used Sails

[Ross]

Hi,

I am in the US working on a modification to an International 110 and I thought the Cherub guys may be a good resource.     I am building a masthead asymmetric, which will be quite similar in size to a masthead Cherub chute.     Has anyone estimated the halyard loads?  

I would also be interested in a used sail if there is one available.     It could be an old throw-away, does not matter - the goal is to fly it for testing before building the new sail.     Thanks!  

My sailplan is on the link here:    www.  facebook.  com/#!/photo.  php?fbid=1687246414843&set=o.  135090796850

Cheers,
Ross
ross.  weene@gmail.  com

[phil_kirk]

Spinnaker loads are a function of the boats sailing weight,  resistance of the boat moving through the water and the apparent wind angle sailied.

Loads on a Cherub are therefore quite managable. 

Most boats use a 4-5mm dynema cored rope such as Marlow's Excel racing for the halyard and either a 20mm or 30mm ball bearing block at the mast head.  This is not disimilar form most other spinaker clases that I know of. 

I believe there is a report on this website of how the spinnaker lifts the bow which calculates the loads at each corner.

I think for a dinghy of a similar size the halyard loads will not be significantly different. 

Just be aware that as you increase the weight of the boat and the righting moment from the crew (crew weight or width of racks) you can potentially increase the load on the spinnaker and develope more power from it.

Small keel boats would go up a size of rope and blocks.

The cherub spinnaker is very similar in size to the RS800's wich has been used as a substitute in the past.

[Ross]

Thanks, makes sense.   That's a great paper on spinnaker loads.   Magnitudes are in the range I'd expect for the 110 as well given wind and boatspeeds.     

Regarding spreaders, do the upper spreaders typically have the same sweep angle as the lowers? 

And regarding spreader construction, how thick of carbon skins are you guys usually laying up? 

Cheers
Ross

[JimC]

That's a great paper on spinnaker loads.   Magnitudes are in the range I'd expect for the 110 as well given wind and boatspeeds.     

Hmm, be a bit wary... the paper was based on a significantly smaller kite than the current masthead ones and its a spectacularly different weight boat. I agree that loads are liable to be around the same order of magnitude, but I would not expect them to be within the same close order - I would *guess* they'd be at least double.

[Phil Alderson]

There is an article on Building Spreaders
on the site, which gives a good idea.

I would go for a couple of layers of 300 gsm uni, and then cover with some some woven cloth at 45 degrees.

early on the upper spreaders were often quite short with a lot of sweep, but as it has been realised that they also perform a useful purpose in supporting the mast sideways while going up wind, they have tended to get longer, and often have about the same sweep as the main spreaders (although this depends a lot on your shroudbase setup.

[Ross]

So at the end of the day there's about 1 mm carbon skins over the core?  That would sound about right to me.   An aluminum Proctor spreader has 1. 5 mm wall thickness.    Are you using 400 gsm biaxial for the 45-45 top layer? 

Have you guys come up with any guidelines on "minimum" snuffer tube diameters? 

Cheers,
Ross

[Will_Lee]

http://en.wikipedia.org/wiki/International_110

300lb keel! Minimum disp 910lb!

(Those kite loads might be a bit more than you will get on a cherub. (All up sailing weight ~200 kg (=420lb) if you are heavy)

[Phil Alderson]

So at the end of the day there's about 1 mm carbon skins over the core?  That would sound about right to me.   An aluminum Proctor spreader has 1. 5 mm wall thickness.    Are you using 400 gsm biaxial for the 45-45 top layer? 

Have you guys come up with any guidelines on "minimum" snuffer tube diameters? 

Cheers,
Ross

On the ones that I have builtI have used either 200gsm woven cloth which was OK but difficult to consolidate, and 300gsm woven sock which woseasy to get a good finish, and it was easy to ensure and use.

Will makes a good point about the loads, this is particularly important when it comes to attaching to the mast

[phil_kirk]

Ross,

I would assume that a 110 sailing weight (including crew) is about 1200-1300 lbs which is about 3 times the sailing weight of a cherub.

The cherubs also have planning hull form and can reach speeds in excess of 20 knots.  In a gust or when over powered we bear away sharply carrying the same speed at a deeper angle so reducing the apparent wind speed on the sail. if caught out in a gust we capsize.

With a ballasted keel i asume that you don't capsize and if over powered in a gust the spinnaker would be eased so that it flogged.  This would put higher shock loads into the rig and halyard that we see on cherubs.

I would therefore err on the strong side when building anything.  We usually use 200 g per m^2 carbon biaxial or plain weave cloth because it is esier to work with and we do not need the additional strength of heavier cloths.
I would reccomend using an additional layer of 200gsm cloth and perhaps an additional layer of uni in your spreader layup.

I would have to measure the chute diameter to give you an acurate figure.  We often use 3 patch kites to reduce the length of the sock so bigger chutes make it easier.

[Ross]

Hi All,

Thank you for your help so far!  I have found an old I14 chute that I can cut down.   For luff length, sailmakers take 1. 05 - 1. 15 X the distance sprit tip to masthead, to size the luff length of the sail.   What works well in the Cherub? 

Cheers
Ross

[phil_kirk]

I think 1.1 x is used as a general rule of thumb.

[Ross]

Thank you Phil,
How about leech length as percentage of luff length? 
Ross

[JimC]

How about leech length as percentage of luff length? 

I think "short enough so as not to drag in the water when not filling" is probably as good a guide as any. The actual ratio to the luff will vary with pole length, mast rake etc etc

[Ross]

Good one!  That does make sense.   So a rule of thumb could be make sure the clew is equal height (up from waterline) in relation to the tack when the sail is set? 

[Phil Alderson]

If you look at 5 different kites you will see 5 different clew positions.
The RMW kites all had very high clews, and were sheeted towards the back of the boat.

The Fyfes and Hydes seem to be much lower cut and sheeted further forward.