Mast Rake

[Don Guillette]

Mast: There is a discussion about mast rake at a Catalina 30 site I frequent and the following article is one I just submitted. the discussion started out as mast bend but the mast on typical production boat can't be bent in the traditional meaning of the term. They can only be raked backwards. Here's the article;I sail with a lot of beginner/intermediates and my approach is simple - I tell the skipper to sail the boat as they normally would. Invariably, the skipper at some point will order backstay tension. I ask them why and get interesting answers. When you rake the mast (lean it back) you are actually moving the mast aft. Here's what is happening. There are a couple of things in play and they are called center of effort (CE) and center of lateral resistence (CLR). I hate to go into this but it is necessary. CE applies to the mainsail and CLR applies to the hull. I could spend hours on those terms but I'm going to over simplify. Look at your boat from the side and picture your mainsail. Visualize the middle of the sail and focus on a spot about 5' aft of the mast. Now draw a line straight down to the keel. That is the CE. The CLR is the center of the keel between the water line and the bottom of the keel. Man, I'm glad that's over!! So now we have two imaginary lines and what do they mean? If you could line up the CE with the CLR line the boat would easily steer straight. If you rake the mast AFT, the CE line moves behind the CLR and you have just induced WEATHER HELM. If you could rake the mast FORWARD, you have moved the CE forward of the CLR and you would have induced LEE HELM.  Windsurfers rake the mast aft all the time when they want to turn into the wind. I could have shortened this by merely saying that to induce weather helm rake the mast aft but you need to know the WHY of what you are doing. Why would anyone want to induce weather helm in the first place? The reason is that you get LIFT from your sails, keel and your rudder. Inducing 3 to 5 degress of weather helm to the rudder produces lift and makes the boat go faster. So now you know the whole story of mast rake. Mast bend is a whole different story.

 

[Alan]

Mast bend is a whole different story.

Don, I think that many skippers get mixed up about the difference between the two and many think that they are one and the same.Mast rake, IMHO is the most overlooked rig tunning error that skippers make. I wish that I could more easily adjust my mast rake from day to day dependant on wind conditions. Short of putting a hydraulic headstay adjuster on, I can think of no easy way to do this. So I set my mast rake at the beginning of the season, and it stays that all year. My mast is about 50' off the deck and I set the rake at 17".

 

[Moody Buccaneer]

Mast Rake adjustment

Lets be VERY clear here.Make rake is a static rig adjustment. I have yet to see a production boat that is set up to allow Rake adjustment while underway.Rake is adjusted by setting the mast butt (no adjustment on deck stepped rigs like Cat 30's), wedges at the partners (on keel stepped masts), and forestay length.The geometric CE of the sail plan is an average of 10% of waterline length forward of the geometric CLA of the hull and fins for most boats (every designer has their own "rule" for how much). This is called "lead" (pronouced leed). On a 25 foot LWL like a CAT 30, 10 % is 2.5 feet or 30 inches.There is NO WAY to get 30 inches of rake adjustment without changing the forestay. A typical 40-45 foot forestay would have to stretch more than 30" ... no way without the forstay breaking. A wire will deform at around 50-60% of rated strength, so range of adjustment with tension is limited to 0-40% or so.At 40% tension, 1/4" dia wire made of 316 alloy stainless steel (most common), is rated 7090 lbs. 40% of 7090# is 2836#'s. At 2836#'s tension a 40 ft wire will stretch just under 2" (1.96")There is no way that 2" stretch at the masthead will move the CE 30".The CE is below the masthead. The CE moves fore and aft less than the masthead moves. Its simple geometry.

 

[Jon W.]

Mast rake question.

I’ve heard for years the standard advice to rake the mast aft to improve windward performance. In doing this, is increasing weather helm the only factor involved? In other words, if I’m happy with the amount of weather helm my boat has, should I leave the rake where it is, or is there some other benefit to increasing the amount of rake besides adjusting weather helm?My 42-ft mast has only about two inches of rake, but I’m assuming the actual amount of rake needed varies according to a boat’s basic design.

 

[Alan]

Mast rake question.

Jon WMast rake is not used to adjust weather helm. It is true that increasing rake will increase weather helm, but there are many other factors that go in to weather helm besides rake.The main advantage of increased mast rake as Don stated, is to gain pointing ability. One of the prices that pays for this is the increase in weather helm. The key is finding a nice balance between the two. Putting a new main on your boat will also change WH and allow for more rake.What I suggest that you do is try making incramental adjustments to your forestay length and see how the boat handles it. I spent a year of trial and error to come up with the best combination for my boat. I came up with a rough formula of 2.5-3% of mast height for my rig.

 

[Jon W.]

Not sure question is answered.

Alan,I'm asking what is it about increasing the mast rake that improves pointing ability.In re-reading Don's post, I think he’s saying it's to increase weather helm. Quote: "Inducing 3 to 5 degrees of weather helm to the rudder produces lift and makes the boat go faster".That's what I've always assumed, but I've just never been sure if there was something else to it, because advice is often given to increase the rake without first asking how much weather helm there is to start with.I think my boat has a healthy amount of weather helm, so I don’t want to change anything unless there is some other factor involved in mast rake, besides helm balance, that leads to improved pointing. Sometimes it almost sounds like there is something magical about mast rake. I’m just trying to make sure I’m not missing something.

 

[Alan]

Mast Rake / weather helm

I've been wrestling with a good answer for your question. I don't know where to draw the line in the discussion to answer the question. I can refer you to a wonderful book 'Sailing Theory and Practice' by C. A. Marchaj. This book is quite technical and assumes the reader has a significant math background. It might be easier to ask that one just accept the idea that rake increases pointing and move forward from there. The net result is the same in the end.Don is right, inducing weather helm to the rudder does produces lift and makes the boat go faster. For a complete explanation, refer to pages 195-201 of the book.

 

[Moody Buccaneer]

To windward

Rake effects the balance of forces.The total force that controls leeway is created by the fin and rudder. If the rudder is more efficient that the foil, giving more of the work to the rudder (taking it from the fin) reduces the leeway angle.The efficiency is related to shape. Long skinny foils like glider wings (high aspect ratio) are more efficient than short stubby wings like fighter planes (low aspect ratio).High AR foils produce more lift per degree than low AR foils.Say the rudder has twice the efficiency of the keel. Adding rudder angle will reduce the fin angle needed for the same force. The fin angle is the leeway angle, the boat points higher.

 

[Ron10127]

Getting close.

My take on mask rake is to help the boat get closer to the wind by tighting the luft of the head sail. This creates a finer entry and you will come a few degrees closer to the wind. If your weather helm increases, now is the time to shorten the headstay. Off the wind you would loosen the headstay and the sail would shape better for that point. This is the way I understand the reason for tighting the forestay.

 

[Moody Buccaneer]

Right you are

Forestay tension has a great effect on the shape of a headsail and very little effect on mast rake.To reduce weather helm (due to heel) you want to de-power the sail and make it flatter. Forestay tension does this.Adding rake increases weather helm due to a change in balance, The last thing you need when you have too much weather helm is more rake.Increasing backstay tension reduces power and heel (thus reducing the weather helm) more than the very small change in rake increases the weather helm. The result is a net reduction in weather helm.

 

[Jon W.]

Not genetically possible.

It’s not in my makeup to accept a fact without at least some understanding of why it is. If there is something about mast rake, independent of helm balance, (or sail shape, or keel rudder theory) that aids pointing, then I must know what it is. Otherwise, it’s too much like Hoodoo-Voodoo! I will try to check the referenced book. Thanks

 

[Bil sv Makai]

Our Voyage 380

Now the discussion of rake has started try for Rake and mast prebend. Our 2003 Voyage 380 has rake and prebend. Many cruisers come up and tell us that we have a proablem. It stands out.The effect. As a catamaran it allows us to sail upwind at 35 dgrees apparrent while maintaining a slight weather helm and good speed. We can keep the knots up and the distance short, but we are still faster off the wind a bit. We go faster and further but get there even sooner.I am starting to see more boats adding rake. Our hunter 335 had a major prebend and the Bay oyster skip jacks have major rake and now we afre seeing old technology applied to improve the overall up wind sailing for boats.

 

[Alan]

Ron

You've fallen into the trap about mixing up mast rake and headstay tension. It's a very common error. They are not at all related.Headstay tension controls the sag in the luff of the sail. This is independent of the amount the mast tilts forward or back. It is possible to have a loose headstay and still have the mast raked aft. The tension applied to the headstay is supplied by the cap shrouds and backstay(or runners, in the frac rig)

 

[Moody Buccaneer]

Mast rake by design ...

There is a big difference between adding to the designed rake and a design that has large rake built in.More rake is not always better.The right rake is always better.A standard rig C30 sails best with very little rake, adding rake makes just makes the boat more hard mouthed than she needs to be.Schooners are designed with lots of rake and no one ever accused a schooner of being fast upwind.Helm balance is a balance of forces. If the boat balances better with the sail effort aft, you can move the mast in the boat with the same rake or rake the mast to move the sail effort.No one has been able to prove to me that rake alone makes a given sail area more efficient.Rake on a mast is like sweep angle on a wing. Gliders don't have swept wings.

 

[Don Guillette]

Weather helm

To the beginners /intermediates that might be following this forum - I hope you found the discussion on mast rake interesting but it is a bit beyond what you need to know about sail trim at your level. My suggestion is to master all the other sail trim controls for the main and jib and save mast bend and mast rake for the end of your learning period. If you have a conventional production boat you can't bend the mast anyway so mast bend is one less thing to worry about.When I am the mainsheet/traveler trimmer I have a set step by step proceedure I go through to trim the main and the last step is to induce about 3 to 5 degrees of weather helm so the rudder can provide lift. A neutral rudder does nothing for you and actually is a waste of the lift benefit you could get from it. On the older C30's like mine inducing the correct amount of weather helm was very easy. Here's how I did it - the rudder post has a cap on it that is used for the emergancy tiller and it sticks up through the cabin sole right behind the helm. That is the guide I use for 3 to 5 degrees. I watch it move to the 3 to 5 degrees position as I merely pull the traveler up. I would not use the backstay adjuster to induce weather helm. Sometime I do crank on the backstay adjuster just because it is there!! Personally, and this is my personal opinion, if I did not have a backstay adjuster I would not go out and buy one but I'm sure there are folks out there that swear by them.

 

[Moody Buccaneer]

Don's Right KISS

I'd pay good money for a main trimmer that watched rudder angle and knew the basic steps to good trim *then* played the traveler.Keep It Simple Sailor is good advice for those that are learning the trade.As Don says, there is a system, every good trimmer has one. They do it without thinking, but they do the same things in the same order every time.Don has the additional ability to communicate what works to others, so newcomers don't have to start from scratch.(He also has the patience to deal with a rowdy crew like us) Bravo Don, and thanks for being here.-Randy

 

[RichH]

3-5 degrees is spot-on for most boats

Not only does the rudder begin to help support with the keel against side loads but 'angles' the keel to a better angle of attack .... and besides 'pointing better' the entire boat is being 'lifted to weather' by the keel and rudder towards the wind .... the keel and rudder are now 'flying' and generating 'lift'. The keel and rudder are foils just like wings and sails and they can generate LIFT if sailed at the proper angle of attack. 3-5 degrees of rudder offset is probably about right for most production boats.Dont believe me but just watch the wake coming out the stern of the boat .... when 'everything' is perfect that wake will show that the keel/rudder are causing LIFT.

 

[Smitty]

But doesn't the weather helm

contribute to the heeling moment?Since the CG is above the CLR (center of lateral resistance), doesn't additional lift (forward and to windward force) add to the heeling force generated by the sails?? If so, is the 3 -5 degree rule the best compromise? -Smitty

 

[Moody Buccaneer]

Heel vs Weather helm

Weather helm and lift from the keel and rudder are not the same thing.The windward lift from the hull = leeward lift of the sails.Because the sail lift is above the CG (Centre of Buoyancy is the real pivot point) and the hull lift is below, both forces combine to heel the boat.When you increase weather helm, you are not increasing the total lift, just shifting some from the keel to the rudder. The total lift always equals the sail force.A sail boat cannot have greater force from the hull than the sails. All the force comes from the sails. Some of the sail force drives the hull forward (we hope) and part of the sail force drives the hull to leeward.At any steady speed, Sail Drive force = Hull Drag force, and Sail Leeward force = Hull Windward force.To generate lift, the hull (keel and rudder) have to have a trim angle, just like the sails. The hull trim angle that creates Hull force = Sail force is the leeway angle.Weather helm is the balance between rudder and keel force.As Sail force increases, the boat heels more. As the boat heels the Sail drive force is further to leeward from the centre of the boat so weather helm increases with heel.

 

[Don Guillette]

Good Trimmer

Moody B: I work for free - a "tinney" (beer)or 2 after the race is all I ask!!Some of you that are members of the C30 ass'n may know Max Munger (sec't/tres of the national organization). Anyway, I race with him at the C30 National regattas as the mainsheet/traveler trimmer. After these events I am truely exhausted plus my neck hurts from looking up. The advantage we (myself and a couple of other guys) have is that we have sailed with him enough that we almost know exactly what he wants for every point of sail and wind condition. The jib trimmers and I work together and the result is that all Max, as boat driver, will ask for is an inch here or there and that it is.Speaking of exhaustion: one year we raced in Santa Cruz with winds in excess of 40 knots. We were fighting for 1st out of 19 boats - we ended up 2nd. On Sunday, after 4 tough races over 3 days my wife and I decided to drive home to LA and make it in one shot. I was so tired that I only made it as far as Santa Barbara. I have worked with a number of middle to back of the pack race crews in So Ca and also crews preparing for the Newport-Ensenada race and the first thing I notice is none of them are working as a team. The first thing I do is separate folks into a job - it is hard for a second baseman to play the outfield. Then I teach them the job. I have an outline of the job for the jib and mainsheet trimmer. They don't have to use my proceedure after they understand the job - they can develop their own - but they have to start somewhere.One piece I always see missing with jib trimmers is the use of the fairleads. One of the reason I'm not the jib trimmer is that it is too much work!! There are 4 things you are adjusting on the jib (and main) and they are draft depth, draft position, twist and angle of attack. Do you know how many of those elements are controlled by the fairleads? ALL OF THEM!! Yet how many of you actually adjust your fairleads? I'll bet not a lot. In my proceedure, the jib trimmer adjusts on EVERY tack.