Rake back the mast ....... a mystery !

Please help me !
I have tried in much forum and pages web but are't successful to find a convincing answer to my question:
Rake back the mast the mainsail aerodynamically improves the performance? ..... and why ?

Thanks

Depends on the boat .... Yes, Daggerboards??? No Daggerboards ....

The H16 likes to have alot of mast rake ... No daggerboards

The TheMightyHobie18 likes only a little mast rake ... Yes, daggerboards


To mathmatically prove it, takes some very involved calculations to prove why involving both hydrodynamics and aeodynamics. But it involves where the "center of effort" is on the sail and where the "center of effort is on the hull and underwater foils ... and how they combine.

Harry

Sorry , yes Daggerboards ...TheMightyHobie18 ..... but how mach "little" mast rake ? where is the limit ?

The limit is your ability to get a firm leech on the sail. Block-to-block on the main system with a firm leech is as far as you can go.

Cooper, mast rake hanges a few things and as a result a full explaination is complicated. But the main reason it helps is to do with lift distribution.

with the mast upright the centre boards are providing most of the hydrodynamic lift that stops your boat going sideways. In order to develop this lift your boat must go sideways a little (leeway) so that the angle of attack of the symetrical foils is sufficient to generate lift. If you rake your mast back then the cntre of effort moves aft so that now the rudders are also being asked to supply lift.

by splitting the load between the boards and rudders you have reduced the amount of lift required from the centre boards and therefor the amount of leeway you'll need to generate said lift. As a result you'll point higher.

As JW says the limit is defined by when you go vlock to block on your main, but before that happens you may find you've developed to much weather helm, making your boat hard to sail.

Hard to sail = slower, just to point that out

There are also some zany mathematics on how pressure distribution changes as the leading edge of a foil is raked aft, how it becomes more stall tolerant and loads of other stuff that is relatively "far out".
My thumb of rule on mast rake: Rake the mast aft until you can let go of the helm when going upwind in your dominant conditions and with crew/sails trimmed for best VMG.

We experimented with rake a lot while sailing the Tornado. For us the class standard ment that the rudder(s) at times was overloaded and lost grip. Then we had to sheet out to regain control. When we corrected this we became faster. We sailed at 200kgs crew weight which was not the norm. Find the setting that suits you and that is your setting

My really simple rule (no mathematics.. ) :

rake the mast forward! until you can manage the boat in downwind without pichpole or without to much risk at the given conditions (wind and waves).

Aside from the H16 trim application where you can vary the CLR, a heavily raked mast can help create a rollover vortex to delay separation on the low pressure side.

Cheshirecatman

Finally a technical explanation!

Can give a detailed explanation of this vortex?

With the limited rake of a sailboat mast it is more likely a series of inconsistent vortices which will help delay separation. The result is greater tolerance to sail trim and less tendancy to stall.
I believe there is benefit to be had from deliberately generating more controlled vortices to do this. This has been the subject of previous threads.

Cheshirecatman

That all makes pretty good sense, but would also mean that moving the whole rig backwards would have the same effect. [yes that would mean moving the front beam]

there does seem to be some benefit from having the mast angled back from the vertical. Could it be that some lift is generated? A bit like the long spinny pole on a H16 rakes the spinnaker luff and generates lift?

Some light reading on the aerodymanic component of mast rake.

Marchaj

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$999.99 ???????????? unless the book comes with a new main sail..... WhatTheHECK ???

Try this one then:

http://www.amazon.co.uk/Sail-Perfor...mp;s=books&qid=1254781855&sr=1-1

Cheshirecatman

A-class cats with their highly efficient and very vertical masts would disagree with the vortex theory.

Scarecrow's explanation is as technical as needed for improving upwind performance. The trick is finding the right balance for fastest times around the course. Balance of lift to drag as well as upwind and downwind performance.

+1 - 2 Day Acat regatta a couple weekends ago, first day I was going upwind as fast or faster than anyone, only to get my clock completely cleaned downwind (3 or 4 boats per leg). Moved the mast forward 1 pin for day two and was able to mostly hold my position downwind (always somewhat of a challenge since I'm 215#) in spite of usually deadly (to me) light air. I think optimum rake is a very situation and boat specific thing... more is not always better.

..... as imagined, there isn't a definitive solution.

There is no 1 asnwer to a question with so many variables....

Even with all the variables... the great sailors almost always finish in the top spots... so there is non-definitive solution !

Also, don't forget a/b Uni's and the location of your daggers. The further aft your daggers are, the more rake you will HAVE to run to balance all the CE's.

is that like the 'shared lift' concept of the ARC/supercat designer?

so if you moved the boards well forward, and kept the rake more vertical, does sailplan efficiency increase? Kind of like swept wings vs. straight wings in aircraft?

Depends what altitude u sail

That's the idea...but the problem then becomes maneuverability.

which might explain the experiments with gybing daggarboards?

Nah - different benefit there. The problem with forward mounted daggerboards is that they are so far away from the rudders that they, in effect, make the turning radius of the boat very large...makes it difficult to tack quickly and efficiently. Over the last several years, we've seen (in F18 and A-class) the Daggerboards get located further aft...closer to the rudder. It makes the boat very twitchy but it tacks very quickly.

I'm lost on the whole gybing daggerboard thing - I thought they did one thing (point slightly to windward to help the boat climb to weather better) but after looking at the infusion boards, I realized they did the opposite of that...so I don't know what they do...I don't have a boat with them so I haven't been overly concerned about it!

Sort of...with daggerboards further forward of the rig, the rudder needs to load up more to prevent rounding up, this is weather helm. If you go too far forward it is hard for the boat to bear away from head to wind during a tack because the boat will want to weater-vane around the centreboards (approximately the centre of pressure of hull and appendages).

It is also better to think of the centre of pressure of the sails and boards (underwater part of hull and appendages) relative to each other, rather than their position relative to the hull. The centre of pressure is fairly close to the centre of area of the sails and boards.

A little bit of weather helm is good for VMG, safety and the general feeling of the boat.

some basic explanations.

http://en.wikipedia.org/wiki/Weather_helm
http://www.onemetre.net/Design/Balance/Tune.htm
http://www.sailingusa.info/sailboat_balance.htm

Gybing daggerboards just require less leeway angle (relative to the hull) to produce side force, because they are setup with an angle of attack relative to the hull.

I'm dubious about gybing boards on cats because the leeward hull provides a significant side force when at a leeway angle. If you try to reduce the leeway angle, the boards have to produce relatively more of the required side force, probably requiring more board area, which I'm not sure is efficient.

Lee or weather helm has to do with the center of effort of the rudders as they related to their pivot axis. The degree to which the rudder helm is sensitive to various rudder rake positions depends on the center of the lateral resistance and the drive for the sail plan.

Your links are mostly related to monohulls and there is a different thing happening there. As their hulls lean, the shape of the hull presented in the water changes dramatically with high angles of heal. The boats tend to lift their sterns and drop the nose (since the bow is slab sided and the back wide and rounded). This changes the presentation of the sails to the wind and it angles the keel downward. They develop severe weather helm with too much lean and tend to lose traction on both the keel and the rudder.

Because the angle of most rudders are fixed on monohulls, it's become common practice to think of lee or weather helm as a sole function of the relationship between the center of the sail effort and the lateral resistance center of the boards, keel, or hull. However, this relationship is actually simpler than that. If your rudder is perfectly balanced between it's own center of lateral resistance (or center of lift) with it's center of rotation (pivot axis), there will be no helm whatsoever - it will be completely neutral regardless of the relationship between the sailplan lift and the lateral resistance in the water or any changes you make in that relationship. If the rudder is off-center with it's center of rotation, you will experience helm and depending on which way it is off center, it will be weather or lee helm. That helm loading experienced will be increased or decreased with mast rake or daggerboard position, but it's not the reason you have helm.

Your points regarding monohulls are correct (except the rig moves forward relative to the keel creating a moment that will bear you away with heel induced trim) but they are secondary effects. You need to also look at the boat from aft to see a moment created between the keel and rig which tends to turn the boat to windward.

Most (dare I say all) rudders have their centre of pressure aft of the pivot axis. If everything else is equal and you move the rudder pivot axis closer to its centre of pressure the force on the tiller reduces (ie. the force you feel on the tiller), but the force on the rudder blade does not change. The pivot point is chosen to maintain a managable helming force for a given weather (or lee) helm that is best for the conditions.

I think we're arguing the same thing but our terminology is different. Because most boats want to round-up into the wind and the force on the tiller is the feedback we as skippers feel for this effect, they can basically both be called weather helm.

There is a slight difference between cats and monos but the principles are the same, the various forces on the boat are separated by distances creating moments. Those forces change with heel, trim, sail settings and daggerboard position.