Roach - What Size ?

Hi - I've learn't a lot from these pages and run many searches - thanks to all the people who share their knowledge.

But I still have a question I can't find an answer to : how do I decide what area of the main sail should be in the roach ?

I came across a small model rc trimaran at a car boot sale and I am planning to make a smaller set of sails to hopefully help reduce pitch poling in stronger winds

Details of the trimaran http://www.rchobby.co.uk/trimaran_r...boat_fidji.html :

Length = 65 cm, Beam = 58 cm, Mast = 92 cm, AUW = 1.1 Kg
Total sail area jib + main = 387 sq in, Jib : Main sail area = 1: 2
Estimated main sail roach area (ie additional to triangle) is 24% of main sail area

The only "facts" that I have managed to glean so far are :

- the roach is about 2.5 inch on a Marblehead
- about 15% max on a 1.3 metre mast
- with a longer boom and less roach you can get better sail control and less twist as the wind picks up.

I hope one of you can help as I am keen to learn by doing - thanks in anticipation.

 

I describe the roach as the % extra area in addition to the given by a simple triangle.

10 and 15% are typical values when defined this way. Some racing yachts have much larger roaches.

Tim B.

 

trimaran

You picked a difficult boat to get started with but congratulations on giving it a shot! Small multihulls are extremely difficult to sail in winds over 7-9 knots because of the fact that, in being scaled down, they lose a great deal of pitch stability-but you already know that. You might consider that the two "major" classes of rc multihull are the Formula 48/mini40(LOA 1.2m) and the 2 Meter-popular in France-and a real giant. A Formula 48 is a VERY difficult boat to learn to sail in winds above 7-9 because of it's proclivity to pitchpole in the blink of an eye. Smaller rc multies only amplify this problem.
You're on the right track with smaller sail area but one thing: you said "reduced twist". Actually you want a main that will twist off automatically in gusts thereby depowering the boat. This can be accomplished with a square top main : look at the photo's of the F3 foiler here:
www.microsail.com The top of the sail is supported by a very lightweight gaff that is molded to fit rotatably in the top of the mast. It is designed to allow the aft ,upper, corner("peak" in gaff terms) to be able to be manually adjusted creating ,in effect, an upper outhaul. The advantage of a modern "gaff" is that it will support the sail without requiring full battens yet give you all the advantages of a square top main and then some with the upper outhaul. Gust response on such a rig is controlled by vang tension; shape by the two outhauls and downhaul tension. With a square top you have a more or less rectangular planform that does not require much roach. Keep in mind that your battens will have to be approx. 2.5 times the amount of roach to support the sail properly.
Since your new rig sounds as if it will be used
mostly in stronger wind there is the possibility of just using an upper diagonal batten to support the "peak"-I'll post a picture below. You might be able to get away with just the upper batten as the problem on models has been to make a full batten system work effectively; it's very,very difficult on sails that will be used in light air as well but you may be able to pull it off on a heavy air set. The "peaked up gaff" I use on my boats allows the sail to be configured for any wind but even so I make mine reefable by havng a removable section of the mast and rolling up the bottom of the sail(retaining it with velcro). Hope this helps you ...
=============================
Both sites are up; check this pix for a diagonal
batten to support a square topped main. On models I think the "gaff" on the other site offers more.
aeroskiff_trial009.jpg
Address:http://www.monofoiler.com/images/aeroskiff_trial009.jpg Changed:11:06 PM on Friday, October 31, 2003

 

Tim / Doug - thanks for the super quick responses !!

Tim - what does the roach affect as it goes from 10% to 15%, does it provide more twist / less twist, or is it just a "nice way" to finish the edge of the sail off and pick up some extra sail area ? (maybe just a dumb question !!)

Doug - thanks for the detail response and encouraging support - will wait to see the pics.

Gee

 

Mostly provides a bit of extra area. Twisting the top part of the sail is controlled by the vang tension. Larger roaches will require battens to retain thier shape and a lot more tension in the vang.

The change in twist with wind speed will also change with the structural setup of the rig and the various rigging tensions. Hence it is a black art to tune a rig.

Tim B.

 

Wacth our fingers with a a short roach.

 

The main reason for a large roach and thus shorter boom is that the smaller the chord (width) of the sail then the lower the induced drag generated by the rig. However there are 9 dozen other factors to consider as above.

 

Guys - if I've understood you all correctly then predominantly the roach gives extra sail area and so power to drive the yacht / trimaran.

But as I am trying to de-power the sail for (relatively) heavier winds for my small trimaran - I could get away with just having a basic triangular sail.

Wheras for ligter winds I should keep the roach as part of the overall main sail design ?

Is my thinking correct ?

thanks
Gee

 

yep, sounds good. If you want to reduce the pitching moment, then you need to reduce the height of the sail as well as the area.

Tim B.

 

The area in the roach is high , so the CE of any sail with large roach is higher than with a hollow cut.

For full sized boats , the roach can easily go a 1ft or 2 behind the backstay as on tacking the GRP battens easily take the bend. Covering the backstay with slip on plastic takes care of any wear.

Only unusual operation is main has to be eased , just a bit, to pass battens tacking close hauled.

Hard to do on a model , but worth the effort for a cruising boat.

FAST FRED

 

square top

For the same boom length and area a square top is likely to have a lower CE(center of effort). Couple that with the superior planform and particularly superior gust response, the square top is probably the best choice for your heavy air sail on a model.

 

I can't imagine any circumstances in which I would want a triangular sail on the 12 - 16 foot size boats I sail. It would always have a reasonably generous roach. For strong winds I'd reduce mast and boom length in proportion. At your size range scale effects might make a difference I suppose, but I suspect not. I'd also be keen on having a flex top mast and some roach which yields in the big gusts.

 

In making some new sales for "heavir winds" (relative to the size of my trimaran) I was planning to lower sail height to lower the combined sail plan CE.

Looks as if Doug's "square top / gaff sail" idea will yield a benefit by helping to lower the CE - so I've started reading up on gaff sails now !!

This is a great learning process and exactly what I wanted to use this trimaran for - thanks guys - I'm soaking it all up !!

 

square top

Another view of a square top by Bill Hansen Aerosports:
WylieCat17c.jpg
Address:http://www.hansenaerosports.com/images/WylieCat17c.jpg Changed:1:33 PM on Tuesday, October 10, 2006

 

Hi Doug,

You probably know this already, but the original reason the Skiff Moths moved to an inverted T rudder was to control just this pitch problem.

The horizontal foil on the bottom of the rudder was there to pull the stern back down should the nose of the boat ever be pushed down.

Doug, have you found your foiled model multis much better in pitch than unfoiled equivalents?

So if Doug thinks it works, you can have both the roach and the pitch stability.

Also you have to look at the overall centre of the sail. Maybe a shorter rig with a bigger roach will have less heeling/pitching moment overall or allow you to carry more sail for the same pitching moment.

You can't look at roach independent of other factors.

Michael Storer