Model Boat Mayhem

The Shipyard ( Dry Dock ): Builds & Questions => Yachts and Sail => Topic started by: Catalina on April 12, 2007, 10:57:33 am

Title: Tweeking For Performance
Post by: Catalina on April 12, 2007, 10:57:33 am
G'day all.

Novice would appreciate help. There are some aspects of model yachts upon which I would like comment.

1) What is the desired angle between mast and deck. I've seen recommendations for backward rake of between 0 and 7 degrees. Mine is about 8 degrees.

2) What is the desired angle between mast and main boom. I have mine set at 90 degrees.

3) What degree of freedom should the main boom have relative to the mast. Mine is set to sweep in one plane from port to starboard. Some of Lester Gilbert's photos show a gooseneck connection allowing conical rotation of the boom

With these settings and after adjusting back stay and fore stay my booms on both sails point down to the deck which looks strange compared to other yachts and seems to restrict performance.

Comments please.

Title: Re: Tweeking For Performance
Post by: dreadnought72 on April 12, 2007, 02:11:49 pm
Hi there,

I've sailed dinghies for years - I can't speak about RC sailing, but answering straightforward "angle" questions probably isn't what you need.

First off, you want the centre of lift for the sail layout just behind the centre of sideways resistance for the hull. How to test this? Your model should, when on a beam reach, want to sail straight or, preferably, turn into the wind just a little bit with the rudder straight. Angling the mast backwards or forwards is the easiest way to adjust the position of the centre of lift. If the boat wants to turn downwind on a beam reach, angle the mast back. If the boat wants to turn upwind badly on a beam reach, angle the mast forward. An alternative to angling the mast is moving the ballast - easy in a dinghy, harder on an RC sailboat - but angling the mast aft is the same as moving ballast forwards.

Secondly, you ask about the angle between mast and mainboom. It all depends on the cut of the sail. This angle is not as important as the tension on the sail: you want a flatter sail for upwind work, and a fuller sail for reaching and downwind sailing. In dinghy sailing you can slightly loosen the kicking strap holding down the boom when sailing off the wind. In dinghy and RC sailing I suppose the mainsheet helps haul down the boom when sailing close-hauled. It's the shape of the sail that you need to consider - not so much what angle it is, or, indeed, what planes the boom can move through. Forget the numbers, think of the forces and shapes.

Sailing is an art - hard enough when you're in the boat and able to make adjustments. It must be far harder for the RC crowd. But I would recommend three things, first off:

1/ Get the centre of lift/centre of sideways resistance right. If it's wrong, and particularly if you're trying to reach or sail upwind, you'll burn off a surprising amount of energy using your rudder to counter the weather or lee helm. That's bad: imagine spending ages making and polishing a hull for minimal resistance - and then sticking a big plough at the back of it.

2/ Don't sail with the sails too tightly drawn in nor too loose. Loose sails are obvious (they flap!), and so for novices to sailing it's often easy for them to fall into the habit of sailing with sails too tight, since there's often little visual evidence to give that away. But tight sails are inefficent, and if you have a jib, often doubly so - the slot between the main and jib is a force multiplier. You WANT high speed air over the back of the main to maximise your power. You do NOT WANT to stifle that slot by overtightening the sheets. Play with the sheet settings all the time. Not wildly in and out, of course, but subtly around the working side of the "luff" point. Learn to find and feel what your boat wants most.

3/ Think about the vectors. Modern, high performance sailing vessels can move at an appreciable percentage of the wind's speed. Some can sail faster than the wind, on certain headings. Think not only about the direction of the wind over the lake, but consider that direction when looked at from the point of view of the moving boat. ANY speed on a boat will move the apparent wind direction forwards. As the boat accelerates, the sheets will need to be gently tightened - but bear in mind warning 2/! Don't overtighten!

I said three things. There's an obvious fourth:

4/ Practice. As much as you can, in as many conditions as you're able.


Title: Re: Tweeking For Performance
Post by: Ian Robins on April 12, 2007, 09:57:16 pm
I sail a 1m Soling yacht and when the sails are sheeted in the main boom is roughly in line with the stern/hullside joint and the jib is roghly in line with the shrouds.
The fore/ aft angle of the mast is used to get the yacht to sail straight without rudder input. If the yacht sails into the wind on both tacks the mast needs moving forward and if the yacht sails away from the wind the mast is angled to the rear
The ideal setting is for the yacht to sail with only radio trim for adjustment
good luck