# Aftmast rigs???

Discussion in 'Sailboats' started by jdardozzi, May 28, 2002.

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### sharpii2Senior Member

Post 345

I gave your drawing another look.

I did this for two reasons:
1.) to see if I was right, and
2.) to see how I got it so wrong, if I wasn't.

I studied your vectors as best I could and found that you were mostly right.

If both fore stays ended at the same height as the back stay, the ratio would indeed be about 2:1.

But this isn't exactly how it is.

The outer fore stay top is about 25% higher than that of the back stay.

This gives it a 1.25:1.00 mechanical advantage.

To estimate the effect of this, I found the proportionate size of the two jibs (I assume they will both be set when sailing up wind).

I found the outer jib had about 66% of the total area of the two jibs.

Keeping this in mind, I concluded my analysis with the following formula:

((Outer jib)(0.66*1.25)+(inner jib)(0.34*1.00))*2.00 = 2.31.

So, according to my own analysis, I was way off.

I attribute my mistake to the following:

1.) I assumed that the relationship of increasing angles and their sins was linear. it is not.
2.) I failed to realize that there was going to be only one back stay, and that its lower end was moved aft to create a bigger angle between it and the mast.

I hope you don't mind if I re post your drawing, so others on this thread may look at it.

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### pbmaiseSenior Member

Update on aft-mast for Hot Buoys Trimaran in Philippines

I will break this post update into several posts. Firstly a general thank you to all on this site and the supporters that help keep the lights on. The data collected here and thoughts of others are ones that I research and consult carefully.

It seems there is almost no possibility to have an original thought regarding rigging a boat. I ran into this again this week. I have been putting on the new 62ft mast very slowly.

I.E. It is still sitting on the ground of the Cebu Yacht Club as I ponder, think, research, and rethink.

This week I thought I had the I GOT IT! Moment and thought I was all so original, only to research this on the web and discover the subject was once discussed years ago on this site.

This is a quote form Paul B. on 08-11-2008, 01:13 PM
We get too caught up in the real world issues like weight location and how it influences pitch, abnormally high loading due to reduced backstay angle (even requiring a backstay spreader arrangement), the alarming problems associated with lack of headstay tension (causing the sails to become fuller at just the time you want them to be flatter), headstay angle, aspect ratios, etc.

Okay. So the subject today is the backstay spreader pole. Or rather backstay spreader bar.

I find no pictures of one and little association with sailing. Two reference I found on the net refer to a soccer net and goal. The backstay spreader stablizes the whole net so it doesn't collapse on the goalie. I also found it in association with cranes. The spreader pole increases the crane mast stability and there is a patent on this.

So what is it on a sailboat?

Functionally it is a bar or pole that separates two backstays so they are spread apart farther off the deck. Specifically in the case of my trimaran I am running two backstays from the main hull. One from starboard and one from port side.

These both travel up and meet at the mast.

Not let us get some points very clear. Issues with placing an aft-mast on a trimaran versus a catamaran are very different. On a catamaran you are basically forced to place the back stays on the two ama. On a trimaran you cannot place the backstays on the ama UNLESS the ama have specifically been designed to handle the forces involved.

I have been inside my two ama again and again. I measure, think and rethink, and every time conclude that it is not feasible for me to run a backstay over to the ama. My boat is West System with balsa sandwich. There is too great a length between the end of the ama and the structural box wall.

I don't have this problem in the main hull (wa'a). I had full access in the wa'a to reinforce the wall all the way down to the Douglas fir that runs along bottom of the boat.

Agree, agree, agree to all those that keep harping on backstay tension to keep foresail shape. HOWEVER, if you design around this problem by using a crabclaw (a.k.a semi-crab claw, lateen, or whatever you want to call it) then foresail shape doesn't matter.

Remember the goal here in my case, and in almost everyone with aft-mast is not racing and high performance. Instead it is simplicity and safety.

Thus, logically, and also based upon my first sea trial, the real issue I have to contend with is lateral load. Physically, I was on the deck looking up at my mast while under sail and saying to myself again and again "What I need is a stay that runs out about 45 degrees behind my mast.

I essentially was trying already what I am thinking now. I was using what I call the baby sidestays to pull the backstays towards the two ama. It was working to a degree. However, as the sidestay pulled outward, it also drew the backstay closer to the mast. In other words pulling the two backstays apart was helping to wreck my backstay angle.

A backstay spreader pole, is exactly the same concept I was trying on those sea trials. Only with a spreader pole, a pole mounted horizontal high above deck pushes the two back stays apart. Since they are being pushed apart, there is no change in backstay angle.

However, the attachment point at the end of the spreader pole is now farther away from the mast in a lateral direction.

Further, by running the starboard backstay to the port side of the spreader pole, I will gain even more lateral control. When arranged in this manner, the two backstays form an X.

And this my friends introduces tensegrity!

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### pbmaiseSenior Member

Tensegrity and the aft-mast

I am really starting to get excited about this project now. Long ago I was hinting I wanted my new mast to look more like a work of art. What I was referring to was tensegrity.

Here is the Wikipedia definition.
Tensegrity, tensional integrity or floating compression, is a structural principle based on the use of isolated components in compression inside a net of continuous tension, in such a way that the compressed members (usually bars or struts) do not touch each other and the prestressed tensioned members (usually cables or tendons) delineate the system spatially.

The backstay spreader pole is a floating member in compression held in place by tendons in tension. Further, where the center of the X meets there is the opportunity to place one nice strong shackle.

I bought just such a 1" 316 polished stainless steel shackle in Thailand and have pegged it to go in this location.

What the shackle does is one amazing trick. It converts both backstays into active stays. There is no longer such a thing as a lee or windward stay. This has been done before. On monohulls two backstays are commonly joined in a triangle and then one wire runs to the top of the mast.

In the aft-mast design, there is no need to worry about the backstays being in the way of the mainsail. Therefore, the junction for the backstays does not have to be combined into a single line.

I know there are many more experienced people here reviewing these posts. So,
Before I do this, any comments please, or any other examples you may know.

Again only other examples I have located are for cranes and soccer goals.

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### pbmaiseSenior Member

Using the spreader bar as a fulcrum and vertical levers

I am documenting this here in case anyone wants to look into this further. I stress I am not going this route.

In coming up with the best way to rig a tall mast I came up with what I thought was a rather simple idea. Leverage! If I could somehow get a lever up there, I could attach the stays to the bottom of the lever.

The lever would pivot on a fulcrum so that for every pound force pulled at the bottom of the lever, the top of the lever could pull on the top of the mast with 2 or 3.

The logical point for the fulcrum is the tip of the spreader bar.

It would introduce a very high compression into the spreader bar. Much higher than it already experiences.

After doing some math on this possibility I decided it wasn't practical in my case. The lever would need to be something like 20 feet tall.

Just a free idea to anyone that wants to consider it again.

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### pbmaiseSenior Member

Its up! 5 months work in the Philippines

It isn't a work of art yet. I only have the temporary backstay on at this point. That temporary stay doesn't use tensegrity. Over the next few weeks I will be installing the new rigging I designed for low compression forces on the mast. Again, this is based upon the scale model testing I did on shore.

It will be at least 2 months before I attempt to raise a sail. I am going slow and careful. The installation went perfectly. Well almost perfect. I did not anticipate that the crane operator would forget that after the raising the mast his lifting strap would be stuck 50 feet off the deck. Fortunately I had already bought a new lifting harness so it was up the mast I went when it was only attached just a few minutes.

Philip

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### mijJunior Member

Mast aft wing sail

I've just come across this thread and I'm pleased to find a wealth of knowledge on mast aft sails. I've been experimenting with mast aft wing sails on rc model monohulls and multihulls for the last few months. I've been posting on rc sailboat forums, but not found much interest or expert advice.

It seems to me that the mast aft sail in combination with solid wing sails have some potential. One of the advantages of using a solid sail is that it holds its shape at all points to the wind. From a practical perspective, it should be possible to raise a wing on an aft mast by winch, with out the use of a crane. I'd be very interested to hear peoples thoughts, and to hear of any efforts along these lines in the past. To illustrate what I am suggesting I have provided some links to videos of the rc boats I have tested below:

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### Jim CaldwellSenior Member

Hopefully you will get some good advice here. Looking forward to your results.

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### Jim CaldwellSenior Member

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### mijJunior Member

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### mijJunior Member

A question

I have a question I was hoping someone might be able to help me with. When I position the mast aft wing sails I don't seem to be able to use the conventional methods for assessing the CE. If I position the CE in the position that I calculate that it should be relative to the CLR, I get significant weather helm. This seems to be the same whether I have a single or double wing configuration. Is this a function of the mast aft sail, or the wing, or both, and how can I calculate the correct position?

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### SukiSoloSenior Member

Forgive me if this sounds naive, but have you accounted for all the extra drag from the spar itself?. It is sufficiently far aft to give a lot of aft drag to the balance. Rigging too may affect it, as it is all effectively aft of the CLR.

Nominally a wing is more efficient and I understand the CLR is much closer to the CofE with wing rigs or wingmast rigs. It could be the order of several percent which is significant.

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### brian eilandSenior Member

Clarification? You are claiming you get weather-helm with the rigs pictured above....not lee-helm, is that correct? (the vessel has a strong tendency to turn into the wind?)

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### mijJunior Member

Brian: The images show the sail positions at which the boat seems to be reasonably well balanced (slight weather helm). With the sails even slightly further aft the boat does indeed have a strong tendency to turn into the wind.

SukiSolo: No, I didn't include the extra drag from the mast and rigging. I agree that this could be a significant factor.

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### sharpii2Senior Member

Another thing to consider is how much the boat heels.

I noticed it has a very deep fin keel.

As the boat heels, the Center of Lateral Area (CLA) shifts to windward, as the Center of Area (CA) of the sails shifts to leeward (as the CA is about halfway up the wing sail(s) and the CLA is about halfway down the fin keel).

Moving the draggy mast, with its spreaders and rigging further aft only adds to this.

Calculating a sailboat's balance, based on its rig and hull profile, is always a dicey matter, as it is really a three dimensional problem, not a two dimensional one.

I once had an instructor avoid me for over a week, as I tried to pin him down on just how far forward I should put the CA on a steel gaff cutter design I was working on.

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### mijJunior Member

Thanks sharpii2, I'm beginning to appreciate the 3D nature of the problem. I started with this rig on a cat, which simplified the situation somewhat.

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