Mast Rake

[bartley]

I just noticed a rather old discussion about mast rake and the reasons for it. 

 

In another life I taught kids to sail and to set up their boats for best performance.  So mast rake was an important topic for discussion.  There are two reasons for it.  The first is due to the effect on the "helm" or the steerage of the boat.  The sail obviously drives the boat forward but it also creates a turning moment which which tends to rotate the boat about its keel.  The sum of all of these the force all over the sail can be thought of as acting at a single point on the sail called the "center of effort" and the position of this is affected by mast rake.  It is vital to arrange things so that if you let the tiller go (by accident perhaps) the boat rotates "into the wind". The boat will then stop.  This is called "weather helm" and is really a safety feature.  The other direction of rotation  would be dangerous as speed would increase and a small boat would probably capsize.

 

However, there is another reason which is  often overlooked.  Because you have set your boat up with weather helm it will be necessary to pull the tiller slightly to windward in order to keep the boat on course.  This slight inclination of the rudder creates "lift' or "drive" under the water.  This adds to the drive from the sails but also reduces the amount of heal caused by the wind pressure.

 

I know this sounds complicated but anybody who has sailed will have felt the effect of these factors.

 

John

 

Edited Sunday at 07:07 PM by bartley

[paul ron]

im just taking a guess here but....

 

all that makes alot of sense. what i noticed most from building models, is if the rake isnt just right, alot of rigging just doesnt sit right. i also figured the rake had something to do with the wind load on the sails n masts n how stays n other rigging distributes the loads as to not stress the masts beyond their snapping point. also it seems logical raking contributes to better drive power transfer the ship, gives some forward lift to raise the ship for better speed.

 

 

 

 

 

 

 

 

Edited Sunday at 02:38 PM by paul ron

[BANYAN]

John, this may be very true for small boats such as hobby sailing boats.  It may even have some effect on larger vessels - I simply have no experience in the subject matter.

 

The following from a prominent naval architect writing in the mid-19th century may also be pertinent.

"When a sharp bowed vessel or ship has her masts to rake, it frequently eases her pitching, but never adds to her sailing, the wind having less power on her sails. However, it is necessary that a ship’s main and mizzen masts should rake more than the fore mast as separating them in this way, the wind acts with more power on all the sails when close-hauled which otherwise would not be affected and be of little or no advantage."

 

Hopefully, this adds some value to the discussion.

 

chjeers

 

Pat

 

[mnl]

Mast rake is just another knob that is useful in rig tuning. Generally as the breeze comes up a little more mast rake improves things. This is especially true for a Marconi rig where it allows you to twist off the top of the main to depower the rig and reduce heel. It generally isn’t the first control you reach for as it changes other things like shroud tension. 
 

There are a lot of other variables that factor in such as the amount and location of draft in the sails. If the sail is cut with the draft forward then a little more rake may be required to bring the center of effort back to where it belongs. 
 

Another variable is the trim of the hull. If the trim is a little more down in the stern then the rake may need to be adjusted to add or reduce weather helm. A little weather helm is good, but too much is slow. You need more rudder and hence induce more drag. 

[bartley]

Yes mini that is very true you only want a touch of weather helm.  And I did not want to complicate things with other factors.  It is also much more complex when there is more masts.  I have mostly only sailed  with Bermudu rigs where  things are fairly simple.  In the earlier discussion I read people had no idea why there should be any rake at all.  The point of sailing is another issue.  One should really change the rake as you come of the wind and I have seen people do this.  But I was trining kids up to about 18 and this is wway beyond the level required.

 

John

Edited Monday at 04:44 AM by bartley

[mnl]

At least some people do change rake going up or down wind. A Star will have the mast raked several feet forward of vertical going downwind. You do need to be careful as shrouds and backstays start to lose geometric efficiency as the mast goes forward of vertical.

 

I expect this has a lot to do with rake for a square rig. It makes it easier to set up shrouds. The yards require clearance to swing so the shrouds have to be somewhat behind the masts which reduces their effectiveness for athwartship support. If the mast is raked, the shrouds will be closer to ninety degrees to the mast at the attach point and provide better support.

 

 

 

[wefalck]

I think one has to distinguish between the rake at the design stage and adjusting the rake on an existing boat/ship.

 

As a matter of fact rake seems to be largely a question of fashion, for instance up to the 1840s ships were designed largely vertical mast and then with considerable rake (perhaps to give the impression of speed at the dawning age of clipper ships), while at the turn to the 20th century masts were essentially vertical again.

 

On the other hand, the centre of effort for a given sail will move differently,  for a nearly vertical mast compared with a raked mast. I wonder, how important that still is when the ship works in the sea.

 

As observed above the rake is primarily a knob for tuning an existing sail-plan/hull combination or correcting a ship's behaviour.

 

On large sailing ships with their proportionally small rudder surface compared to modern dinghies and yachts, the 'wheather helm' may have been less important and most of the balancing would be done with the distribution of sails set.

[popeye2sea]

Most of the discussion above applies to smaller fore and aft rigs.  Big square riggers act very differently.

 

I have read that one of the reasons for mast rake in large square rigs is to get the yards pivot points more aligned in the vertical plane. Each additional upward section of mast being stepped slightly in front of the lower section.

 

Regards,

Henry

[mnl]

I looked through a few references and most of what I found was that rake was because it looked good.

 

As far as square rigged ships go, you can look here:

 

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/783FD78DE365F79DC6C5DA209CF6A971/S0373463323000103a.pdf/selected-issues-of-optimising-parameters-on-square-riggers-to-maximise-speed.pdf

 

Interestingly the word rake does not appear in the document above.