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Old 01-06-2023, 18:34   #1
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Can you do a mast rake with non-swept back spreaders?

Can you do a mast rake with non-swept back spreaders? what about a pre-bend?

I guess it’s a pretty self-explanatory question

I can’t picture this. I can’t picture why it does or doesn’t work.

If I have a mast and I want to bend it backwards and make a rake, do I need to have swept back spreaders? The spreaders do not touch the shrouds I don’t think. The spreaders are only on the double diamond.
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Old 01-06-2023, 18:57   #2
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Re: Can you do a mast rake with non-swept back spreaders?

You can certainly do rake and prebend on a rig without swept spreaders. How you do it depends on so many things.
Do you have two lowers each side or one? What is the fore and aft relationship between the lowers and uppers? What is the relationship between the mast heel and the partners? Fractional or masthead? Is the mast heel squared off or angled? Forestay tension? Backstay tension? Baby stay ? Running backstays?

Some resources:

https://support.seldenmast.com/files/595-540-E.pdf

http://www.dedekam.com/sailtrim.html

These both have a ton of useful info.

Or a lesson from a good pro rigger might be worth while.
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Old 01-06-2023, 19:05   #3
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Re: Can you do a mast rake with non-swept back spreaders?

Mast rake and prebend are different things even though they are sometimes used interchangeably. Imagine mast rake as more of a static setting that one would determine while tuning the rig, the rake angle will depend on the forestay length and aft support, which could be either from swept back spreaders or lower shrouds in perpendicular spreaders. I have perpendicular spreaders and my mast is raked aft, not crazy but it is obvious when seen from the side.

Prebend can be performed at different points of sail and wind speed, usually done with an adjustable backstay; fractional rigs can have more prebend than mast head rigs where an inner forestay helps.

Catamaran is a whole different beast, we have a monohull where I can tune the rig based on helm, leeway etc.
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Old 01-06-2023, 19:21   #4
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Re: Can you do a mast rake with non-swept back spreaders?

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Originally Posted by dfelsent View Post
You can certainly do rake and prebend on a rig without swept spreaders. How you do it depends on so many things.
Do you have two lowers each side or one? What is the fore and aft relationship between the lowers and uppers? What is the relationship between the mast heel and the partners? Fractional or masthead? Is the mast heel squared off or angled? Forestay tension? Backstay tension? Baby stay ? Running backstays?

Some resources:

https://support.seldenmast.com/files/595-540-E.pdf

Sail trim: "Illustrated Sail & Rig Tuning"

These both have a ton of useful info.

Or a lesson from a good pro rigger might be worth while.

I have a fractional rig with double diamond spreader configuration.

The upper shrouds attach near where the forestay attaches. I think they are just a little bit below the forestay attachment point.

maybe about halfway down I have a pair of lowers. So there is just a cap shroud and a lower on each side.

both shrouds on each side go to the same termination point which is aft of the mast. so, the upper and lower both go to a chain plate that is aft of the mast. Well aft.

by mast heel, I think you mean the mast step? It is deck stamped. It’s a Catamaran. So no partners. If I have that stuff correct.

nothing has ever been tensioned yet on the rig. So I don’t know any of these tensions.

my Rigger was saying that it was not possible to do a pre-bend or rake with the straight spreaders. Because they are not swept back. But it didn’t seem right. he is significantly more of an expert than most people. So I am just taking him at his word but I need to understand what he means.

Checking out the resources now
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Old 01-06-2023, 19:25   #5
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Re: Can you do a mast rake with non-swept back spreaders?

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Originally Posted by Knotical View Post
Mast rake and prebend are different things even though they are sometimes used interchangeably. Imagine mast rake as more of a static setting that one would determine while tuning the rig, the rake angle will depend on the forestay length and aft support, which could be either from swept back spreaders or lower shrouds in perpendicular spreaders. I have perpendicular spreaders and my mast is raked aft, not crazy but it is obvious when seen from the side.

Prebend can be performed at different points of sail and wind speed, usually done with an adjustable backstay; fractional rigs can have more prebend than mast head rigs where an inner forestay helps.

Catamaran is a whole different beast, we have a monohull where I can tune the rig based on helm, leeway etc.
That was pretty helpful to picture.

so I have perpendicular spreaders. Thatís the correct word I guess. I didnít know the word.

I think my spreaders only deal with the double diamond part of the fractional rig. I donít think they touch my shrouds at all. Thatís why Iím so confused. I donít understand why he was saying the perpendicular spreaders canít have rake. Unless he was saying they canít have pre-bend.

Itís hard because when I try to question him on something like this he gets kind of frustrated. And I can understand why. Heís talking to an idiot. Lol I donít know what I donít know. When it comes to the stuff.
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Old 01-06-2023, 20:26   #6
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Re: Can you do a mast rake with non-swept back spreaders?

Rake is easy, and doesn't depend on spreaders at all. Just lengthen the forestay, and shorten the backstay(or whatever your rig has instead of a backstay), and you have some rake.

To get bend you might not be able to. Not because of the straight spreaders but because you only have 1 pair of lowers. If you had 2 lowers on each side, to separate chainplates forward and aft of the mast, then you can easily get bend by adding tension to the forward lowers.

However, and I'm not sure because I'm not an expert on this, you might still be able to to get bend because it is a fractional rig. Because the forestay attachment is lower than the backstay, tension on the backstay will tend to push the center of the mast forward and create bend. But without a true backstay and without forward lowers I think this is unlikely.

Expanding on what knotical said, rake means the mast is leaning aft (or possibly forward but unlikely) instead of being exactly plumb. It is set when the mast is stepped, and isn't usually adjusted. Mast bend is typically adjusted while sailing by tensioning the backstay. Adding bend will tend to flatten both the main and jib sails so it becomes another way to trim for varying wind conditions.

The determination of whether you have either and how much must be discussed with your sailmaker, as a sail needs to be cut for the correct amount of rake and bend in your mast. You are probably better off having neither than not having your sails match the way your rig is set up.
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Old 02-06-2023, 02:02   #7
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Re: Can you do a mast rake with non-swept back spreaders?

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Originally Posted by wholybee View Post
Rake is easy, and doesn't depend on spreaders at all. Just lengthen the forestay, and shorten the backstay(or whatever your rig has instead of a backstay), and you have some rake.

To get bend you might not be able to. Not because of the straight spreaders but because you only have 1 pair of lowers. If you had 2 lowers on each side, to separate chainplates forward and aft of the mast, then you can easily get bend by adding tension to the forward lowers.

However, and I'm not sure because I'm not an expert on this, you might still be able to to get bend because it is a fractional rig. Because the forestay attachment is lower than the backstay, tension on the backstay will tend to push the center of the mast forward and create bend. But without a true backstay and without forward lowers I think this is unlikely.

Expanding on what knotical said, rake means the mast is leaning aft (or possibly forward but unlikely) instead of being exactly plumb. It is set when the mast is stepped, and isn't usually adjusted. Mast bend is typically adjusted while sailing by tensioning the backstay. Adding bend will tend to flatten both the main and jib sails so it becomes another way to trim for varying wind conditions.

The determination of whether you have either and how much must be discussed with your sailmaker, as a sail needs to be cut for the correct amount of rake and bend in your mast. You are probably better off having neither than not having your sails match the way your rig is set up.


Oops. I neglected one thing in my description of the rig. And it made it into your post.

There is no backstay. There is a forestay, a cap shroud and a lower.

I think the plan is to have some kind of running backstays for adjustment purposes and to help with spinnaker loads.

The shrouds are way aft of the mast.
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Old 02-06-2023, 03:27   #8
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Re: Can you do a mast rake with non-swept back spreaders?

The rake part is easy. The pre-bend a little more complicated depending.

I'll tell you how I have done mine and then maybe you can apply that to your setup which is different.

To rake the mast back more you just add length to your forestay then tighten the rest of the stays and shrouds as needed. Opposite if you want to rake forward some.

When I raced beach cats all those years, we'd rake back more as the wind increased then back forward as it decreased.

Same with pre-bend. More pre-bend as the wind increased etc.

On the beach cats, we controlled the pre-bend with the single set of diamond wires. but unlike you our spreaders were adjustable also. I believe I had my spreaders at a 2"-3" rake depending on the boat (Nacra 6.0 or Nacra F17) then the diamonds wire cranked down to about 700 psi on the Loos Gauge in strong winds which gave me 1 1/2" to 2" of prebend in the mast (this on the Nacra 17)

I got the amount of tension number from other racers of the same boat (trial and error)

The mast on the Nacra 17 was carbon fiber so you could also but a lot of bend in it with the mainsheet. It was controlled on both boats by an 8 to 1 block.

Those beach cats had no backstay either which caused me to break one carbon fiber mast since I didn't have the main sheet tight enough and the spinnaker was up. (got it by a 25 knot gust ahead of a cloud line in Pensacola Bay)

On my present boat, a Bristol 27, the spreaders are fixed.

You control rake as above but to set pre-bend you increase the pressure with your turnbuckle on the forward lowers.

I had maybe 1 1/2" of per-bend in my mast. To get that I had all other shrouds and stays at 400 on the Loos Gauge and 500 on the forward lowers.

I recently took the pre-bend out when I replaced the chainplates, It's not going to help my boat point much better anyway. The Bristol 27 simply doesn't point well

I may put the prebend back in when I replace the rigging and rake the mast a little also then leave it since I'm not racing.

When racing we adjusted the rake and pre-bend according to the conditions at each race

See drawing of Nacra 17 mast and single diamonds. Plus one of adjustable speaders
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Old 02-06-2023, 03:33   #9
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Re: Can you do a mast rake with non-swept back spreaders?

I've explained this a hundred times, if you have a set of double diamond spreaders I imagine you have sweep back spreaders? some angle? 5,10, 15%

The tension on the diamond or diamonds is done with the mast on the ground, well supported and centered, there by tensioning and balancing tensions the mast starts to take a banana shape, pre bend.

Why is it done on the ground? because the mast rests without parasitic tensions, and it is easier to leave the mast centered on the ground, from there you can make adjustments once the mast is placed, hence not to place cotter pins in the turnbuckles until it is finished and placed in the mast step.

This is determined by your rigger when tensioning the wires.

I would not play with the mast rake in your case, it is fractional, you said it, it is determined by the uppers and the furler, once it is in place it has to bite evenly into the mast step, to much rake and the mast will rest on its heel, no good.

What I tell you, get the prebend on land with the diamonds, get it up on deck and the uppers and lowers just have to get lateral support and act as backstays and keep the furler taut, that's it.
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Old 02-06-2023, 03:42   #10
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Re: Can you do a mast rake with non-swept back spreaders?

Rake and pre-bend are two completely different components.

Rake is used to reduce the effect of drag caused by vortices at the mast head. Adjusting rake on monohulls can reduce the drag from "helm" forces under different wind conditions. Adjusting the mast rake on a beach cat, especially one without daggers, can have a profound influence on the windward ability. Wind surfers use the same technique of raking the mast to the extreme.

Pre-bend or mast curvature. In its simplest form, with a three point rig (forestay and 2 shrouds), typical on lots of cats the diamonds and spreaders determine the shape of the mast and hold it in column while the shrouds simply stop the mast from falling over.

Without swept back spreaders there is no way to impose pre-bend (fore and aft curvature) of the mast, it is impossible because of the geometry. Forces from the diamond stays over the spreaders is in only one plane.

If the spreaders are swept back is it possible to impart forces in two planes hence the ability to curve the mast - pre-bend. This feature will be present even with the mast laying on the ground and once set is fixed.

If the mast has flat (un-swept) spreaders the mast alone can only be straight (fore and aft). However, if you add additional shrouds bend can be achieved and adjusted. Six part rig, the baby stay can be tensioned such that the middle part of the mast is pulled forward. This can be further enhanced by tensioning the backstay (a la monohull) that allows adjustment while sailing and is used mainly in racing to enhance the sail shape under varying conditions.

Essentially these features are used to cope with the dynamic forces on a light weight flexible mast. The main reason for fixed pre-bend is to fix the direction of flex, prevent inversion (collapsing) due to pumping of the mainsail in heavy weather. The lower shrouds or in some cases running back stays limit this motion and prevent the opposite from happening.

As far as your boat (Chuto) goes the the pre-bend is down to the mast section and sailmaker. With in line spreaders any pre-bend must be done using the baby stay. There will however be a sweet spot for the rake that will depend on the boat and I would suggest contacting the designer to establish the best angle.
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Old 02-06-2023, 05:22   #11
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Re: Can you do a mast rake with non-swept back spreaders?

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Originally Posted by Tupaia View Post
Rake and pre-bend are two completely different components.

Rake is used to reduce the effect of drag caused by vortices at the mast head. Adjusting rake on monohulls can reduce the drag from "helm" forces under different wind conditions. Adjusting the mast rake on a beach cat, especially one without daggers, can have a profound influence on the windward ability. Wind surfers use the same technique of raking the mast to the extreme.

Pre-bend or mast curvature. In its simplest form, with a three point rig (forestay and 2 shrouds), typical on lots of cats the diamonds and spreaders determine the shape of the mast and hold it in column while the shrouds simply stop the mast from falling over.

Without swept back spreaders there is no way to impose pre-bend (fore and aft curvature) of the mast, it is impossible because of the geometry. Forces from the diamond stays over the spreaders is in only one plane.

If the spreaders are swept back is it possible to impart forces in two planes hence the ability to curve the mast - pre-bend. This feature will be present even with the mast laying on the ground and once set is fixed.

If the mast has flat (un-swept) spreaders the mast alone can only be straight (fore and aft). However, if you add additional shrouds bend can be achieved and adjusted. Six part rig, the baby stay can be tensioned such that the middle part of the mast is pulled forward. This can be further enhanced by tensioning the backstay (a la monohull) that allows adjustment while sailing and is used mainly in racing to enhance the sail shape under varying conditions.

Essentially these features are used to cope with the dynamic forces on a light weight flexible mast. The main reason for fixed pre-bend is to fix the direction of flex, prevent inversion (collapsing) due to pumping of the mainsail in heavy weather. The lower shrouds or in some cases running back stays limit this motion and prevent the opposite from happening.

As far as your boat (Chuto) goes the the pre-bend is down to the mast section and sailmaker. With in line spreaders any pre-bend must be done using the baby stay. There will however be a sweet spot for the rake that will depend on the boat and I would suggest contacting the designer to establish the best angle.
That was like a textbook. Awesome. Thank you.

I need to have a 3.5 to 4 foot rake distance at the top.

But like Neil pride said doesn’t that make the mast sit on the aft edge? Especially considering my very limited pre-bend ability with the un swept spreaders?

Is the solution to build up the area below the mast step so that it’s not horizontal and it has a slope to it?

These are questions I can’t ask my Rigger because he’s going to get annoyed at me
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Old 02-06-2023, 05:22   #12
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Re: Can you do a mast rake with non-swept back spreaders?

Check out Hunter boats.....they don't use a backstay.....rather induce mast bend via the spreaders...the so-called B&R rig.

B&R rig

https://en.wikipedia.org/wiki/B%26R_rig
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Old 02-06-2023, 05:27   #13
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Re: Can you do a mast rake with non-swept back spreaders?

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Check out Hunter boats.....they don't use a backstay.....rather induce mast bend via the spreaders...the so-called B&R rig.

B&R rig

https://en.wikipedia.org/wiki/B%26R_rig
These rigs are exactly what I donít have. They are swept back spreaders. The rig I bought has non-swept back. My spreaders are at 90į to the centerline.
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Old 02-06-2023, 05:42   #14
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Re: Can you do a mast rake with non-swept back spreaders?

If you can get the mast to rake back with rig tuning but it's not sitting on the step well once you're done, I don't see a reason why you couldn't angle the step to match.



Being that you're adding running backstays, even if you can't get much pre-bend into the rig, you'll have the ability to bend the mast dynamically with the running backs.
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Old 02-06-2023, 05:44   #15
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Re: Can you do a mast rake with non-swept back spreaders?

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Originally Posted by Chotu View Post
That was like a textbook. Awesome. Thank you.

I need to have a 3.5 to 4 foot rake distance at the top.

But like Neil pride said doesnít that make the mast sit on the aft edge? Especially considering my very limited pre-bend ability with the un swept spreaders?

Is the solution to build up the area below the mast step so that itís not horizontal and it has a slope to it?

These are questions I can ask my Rigger because heís going to get annoyed at me

That rake (3.5-4ft) is typical for a cat like yours and the angle is not extreme.

What does your mast step look like?

Typical arrangements are a flat plate with cheeks and a tang on the bottom of the mast or cheeks on the mast foot that sit either side of up-stand rail. In both cases the mast articulates on a substantial pin. Some racing monos have a curved pressure plate and slotted fixing to allow larger movements.
If yours is pin jointed and there is not enough range to prevent the heel jamming then your solution to create a wedge under foot plate would work.
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