Stopper Knots

Stopper Knots are imperative to keeping a line from slipping through all of its blocks and pulling out. The idea is to have the line pass through the blocks as it makes its way to the cockpit, and then stay in the cockpit. 

Lets take a jib sheet as an example, the sheet comes off the winch and slips out of your hand. If there is no stopper knot, the sheet will pull through the fairlead and flap wildly in the wind as the jib flogs. You can't use the engine to bring the bow into the wind since the sheets can foul the prop. This whole dilemma could have been thwarted by using a stop knot, which would have kept the bitter end of the sheet from passing through the fairlead, allowing a much easier recovery. 

While it may seem logical to place the stop knot at the very end of the sheet, this is not advisable. Think about it, the sheet pulls away because it was under tension. Now the stop knot is jammed into a block and you have nothing to grab onto to pull it out! You needed a winch before because the line was under tension, now you have no way of getting it to the winch and it's still under load.

To make matters easier, locate the stop knot ahead of the bitter end. If the sheet were to pull away, you want to have enough tail to reach the winch and allow you to place a few wraps around the winch drum.

This does equate to a rather long sheet, you factor in the length that you would have needed, and then add the distance from a stop knot to the winch. This should be the length of your sheets.

Tree Surgeon Back Splice vs Regular Backsplice

When it comes to finishing a line, there are two good options to secure the end:

Whipping
Backsplicing

Whipping is the act of wrapping the end of the line in a much smaller line to bind the fibers and hold the end together. This works wonderfully and will last a very long time, but in situations where the ends of the line will be chafed or abused, the whipping can break and fall off; leaving the end of the line to fray.

Backsplicing is wonderful because it uses the rope to hold itself together. In situations where whipping is not the best idea due to abuse, backsplicing can shine through.

The two types of backsplices I will talk about now are the Tree Surgeons Backsplice and the Bulk Backsplice.

The bulk backsplice is quick and easy to do, but as the name mentions, it adds significant bulk to the line. This can make it hard to pass through hawse pipes and cleats. To help keep the end together, a crown knot is also tied, which adds considerably to the amount of bulk.

The alternative to the bulk backsplice is the tree surgeons backsplice. It barely increases the bulk of the line at the end and still provides the same ability to hold the line from fraying when under abuse. The only downside to the tree surgeons backsplice as compared to the bulk backsplice is the splice takes a few minutes longer to do. 

I used it to secure the end of my dinghies painter, which towed the dinghy behind us for 800 miles on our summer trip, as well as on the ends of our anchor snubber.

It is a very versatile backsplice that is worth knowing how to do.

Calculating Halyard Loads

Last time, we talked about calculating Sheet Loads. Now we will look at the loads placed on the Halyards and why they are so difficult to calculate! If you look online, it won't be easy to find a resource on calculating halyard loads because of the legal ramifications. If a halyard breaks because it wasn't strong enough when it was calculated to be strong enough, legal consequences can follow. Because of this, everyone seems to keep a tight lip with regards to halyard loads.

In a similar fashion, I will begin with a disclaimer:
The loads calculated here are calculated with formulas that are discussed in college physics and math classes. They are the formulas for calculating the tension on a line. They are theoretical values, not real world values. In the real world, shocks and freak weather systems can greatly stress a system in ways far more extensive than originally calculated. In other words, always oversize by a significant margin of safety. To be on the safest side, I would follow the recommended size given by New England Ropes in their calculator, and then choose a line with at least that breaking strength. 

http://www.neropes.com/InteractiveLineSelector/Sailing_Type.html

If you still want to know other ways of estimating the loads placed on the halyards for academic reasons, read on!

The halyard pulls against the head to keep the luff under tension. While there are many formulas and online calculators available to figure out the clew loads, halyard calculators are not as plentiful.

This is because halyard tension can be extremely variable. If you simply raise your sail all the way and let it hang without applying any tension, the halyard load will be equivalent to the weight of the sail hanging from it.

Then you begin to tighten the halyard by cranking a winch or tightening the Cunningham. These actions along with the force of the wind pressure on the sail add up to an immense load on the halyard. 

The most reliable way to calculate the halyard tension would be to sail with a tension meter mounted between the halyard and winch. This would be the only way to know exactly how much force is being exerted on that line.

Installing a tension meter inline is not practical, so other methods must be utilized to estimate the loads. One of the least involved methods is to calculate halyard tension based off of the force exerted on a 10" winch handle. The winch is simply a device that acts as a force multiplier. The number on the top of the winch corresponds to the number of times your effort is multiplied (not taking friction into account). For example, a 24 winch simply multiplies the force you put in by 24. If you apply 1 pound of force, it will apply 24 pounds of force to the halyard; if you apply 20 pounds of force, it will apply 480 pounds of force. By estimating the amount of force you are applying to the handle, you can get a ballpark figure of the halyard tension.

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The safest way to decide what size halyard to use is to follow the advice given by New England Ropes on their interactive line selector. 

http://www.neropes.com/InteractiveLineSelector/Sailing_Type.html

The line selector will tell you a specific line for the job, you can then look at the breaking strength of the line and use that as your minimum breaking strength requirement. Say it gives a size for Sta-Set X, you want to use a smaller or lighter line. Simply take the minimum breaking strength for the size of line they list and select an equally strong alternative. 

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(If you get bored with the math, skip down to the next divider line, the article resumes there)

Lastly, if you really want to do the math to calculate the loads exerted on the halyard because you are curious, please read on. I did well in Physics, so I believe I have correctly calculated all the values. I personally use the line selector from New England Ropes or the factory recommendations as my starting point. I have no problem installing a "stronger than necessary" halyard on a boat, but do veer away from the thought of installing a line with less breaking strength.

For our example, we will use a 500 square foot mainsail. The measurements of this sail are as follows:

Luff height 50 feet
Foot length 20 feet
Leech Length 53.85 feet

Tack angle 90*
Head angle 21.8*
Clew angle 68.2*

If this is starting to look like Trigonometry class, it's because it is Trig. 

From here, we can calculate the wind pressure that will be exerted on the sail, which in our example would be

Wind Pressure per SqFt = 0.00256 x Wind Speed^2 in mph

0.00256 x 25^2 = 1.6 lbs per SqFt
1.6 x 500 square feet = 800 pounds in 25mph of wind.

So in winds of 25mph, we can estimate that 800 pounds of pressure will be exerted on the mainsail.

To convert our wind pressure into halyard load, we need to calculate the forces involved using a vector diagram.

The force on the clew is:

0.00431 x 25^2 x 500 = 1,346.875 pounds on the clew

The force on the head is roughly the sum of the wind pressure on the sail pulling the sail down, and the load on the clew of the sail pulling back and down. In our example, with 800 pounds of wind pressure on the sail and 1347 pounds of clew load, we arrive at around 2147 lbs of load on the head. 

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Big picture points to take away when sizing your halyard:

• Online calculators exist that will tell you roughly what size line you need. I strongly recommend using them and following their guidance.

http://www.neropes.com/InteractiveLineSelector/Sailing_Type.html

• If you wan't to know a rough estimate of the loads involved, look at the force multiplier number on your winch and guestimate how many pounds you are pushing on a 10" handle. Multiply your guestimated work with the number on the winch and that is a very rough estimate of luff tension while the sail is being raised.
• You can do all the math involved to figure out a rough estimate for the halyard load. This information is only useful for those who would sit at the helm and wonder "how much tension is the halyard under?" I would not choose a halyard that has a breaking strength anywhere near the calculated load. The safety margin on the halyard should be tremendous. If this line should snap, the sail comes down and it will be a lot of work to replace this line (this is the same reason you should replace your halyard when it begins to show signs of wear).
• Make sure the halyard is comfortable in your hands. 
• There is a nifty graph that will tell you the halyard loads based on boat size on page 372 of "The Complete Riggers Apprentice" by Brion Toss. It is the only place I have ever seen any information on how much load is on a halyard. To give an idea of how elusive this information is, the graph is only a small portion of the page with no caption or explanation in the back of the book, past where most people would have stopped reading. I do not have permission to reproduce the image, so the best I can do is reference the specific page in the book.

Calculating Sheet Loads

When you size your running rigging, you need to make sure it can withstand the loads you will place on it. These loads will come from the force of the wind exerted on the sails. It is best to size your running rigging for the worst imaginable situation: The sails sheeted in tight to a gust of wind on the beam. This will place all of the sail area under load, which will transmit the highest amount of force to your running and standing rigging. 

First we need to understand a few points: 

What is the force exerted on the sails?
How is that force transmitted to the running rigging?
How strong does my running rigging need to be?

The force of wind on the sails is equal to:

Wind Pressure per Square Foot = (0.00256) x (Wind Speed in mph)^2

It is important to note that the wind speed is squared. This means that as the speed of the wind increases, the force it exerts on the sail also increases; but at a much higher rate! 

At 5 mph, the pressure would only be 0.064 pounds per square foot.
At 10 mph, the pressure would rise to 0.256 pounds per square foot.

This may sound insignificant, but on a large mainsail of say 500 square feet, these numbers would translate into:

5 mph = 32 pounds of wind pressure
10 mph = 128 pounds of wind pressure

Just doubling the wind speed quadrupled the wind pressure. This force gets transferred to the sheets and halyards that have to hold the sail in place.

While the actual formula for calculating the force of the wind is actually much more complicated taking in many more variables. For our purposes of properly sizing the rigging, we can trim the formula down to just these variables.

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The simple way

Harken provides online calculators on their website that can help provide an easy idea for sizing the sheet loads on their website.

For the Mainsheet: http://www.harken.com/MainsheetLoading/
For the Headsail: http://www.harken.com/content.aspx?id=9096

Samson also provides a formula to calculate the amount of force on the clew:

(windspeed in mph)^2 x 0.004 x (sail area in square feet) = Sheet Load at Clew in pounds

New England Ropes also provides a nifty tool on their website where you answer a few questions and it will tell you what line and what size to use

http://www.neropes.com/InteractiveLineSelector/Sailing_Type.html

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The not so simple way

The sheets serve to hold the sail against the wind and keep it from turning into a giant flag flapping in the breeze. 

Headsail sheets need to withstand the full load of the sails clew alone, as the single sheet is run to the winch. The mainsheet has the advantage of being in a purchase system, spreading the load over various lines, greatly reducing the loads on the mainsheet line itself.

When wind exerts force on the sail, the sail in turn wants to move. The job of the sheet is to resist that movement and keep the sail in its position. This force is shared between the tack, head and clew. If the sail were not attached, it would simply blow away; but by being secured, it exerts a force which serves to pull the vessel along. 

The luff is maintained tight by the tack and head pulling to resist deformation of the sail. The tension for the luff is provided by the halyard pulling on the head. The leech and foot are both held in tension by the clew. 

Adjusting the jib sheet angle shifts where its force is applied. When the jib car is forward, the jib sheet acts more on the leech. When the jib car is aft, the jib sheet acts more on the foot.

For our example, we will look at a 400 square foot jib under winds of 25mph. 

Our wind pressure would be:

Wind Pressure per Square Foot = (0.00256) x (Wind Speed in mph)^2
0.00256 x 25^2 = 1.6 pounds per square foot x 400 square feet = 640 pounds of force on the sail

While the force on the sail is 640 pounds, the force on the sheets and halyards will be much higher because they need to hold that force at various angles. 

The formula for calculating the jib sheet loads is:

Sail Area in square feet x Wind Speed^2 in mph x 0.00431 = Sheet Load in Pounds

Our Jib sheet load would be = 400 x 25^2 x 0.00431 = 1077.5 pounds

To select the proper jib sheet size, we would want to take into account two points:

Tensile strength
Comfort in your hands

For tensile strength, you would need a line that can withstand 1077.5 pounds as a minimum.

For safety reasons, factor in an additional 20% for safety margin and that gives us a minimum value of 1,293 pounds
For a higher safety margin, use 50%, which gives a minimum value of 1,616.25 pounds

When looking at lines, this translates into actual products that we can use on our boat safely.

Most stretch

Less stretch

Minimal stretch

With a 50% safety margin, you could use:

1/4" New England Ropes Three Strand
1/4" New England Ropes Sta-Set
1/4" (6mm) New England Ropes VPC

With a 20% safety margin, you could use:

1/4" New England Ropes Three Strand
3/16" New England Ropes Sta-Set
1/4" (6mm) New England Ropes VPC

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In my hands, 1/4" is tiny and is hard for me to hold onto for an extended period of time. For comfort reasons, I like to keep my lines bigger than 3/8"; 1/2" being very comfortable. 

A line of this size will be incredibly strong, 9,500 pounds of strength in VPC! That lines breaking strength would "in theory" be able to hold the sail in winds of 74mph

400 x 0.00431 x 74.23^2 = 9499.4 lbs
I would have reefed that sail long before then!

This is where sizing the sheets based on comfort comes into play. First calculate the minimum size of line for your sheets, then choose a comfortable size in your hands making sure that the comfortable line size is strong enough to support your running rigging's requirements. This will lead you to selecting a capable and comfortable line.

Sculling Oar: Part 8

The oar notch was cut into the block of wood using a circular saw. I made many fine cuts and then obliterated the thin pieces of wood in between the cuts to smooth out the area. 

I first made the notch 1/2" deep, but this was way too shallow. 

I then went back and made the cut a full 2" deep. This looks deep enough to hold the sculling oar in place. The sides were also beveled and flared, to allow the oar to pivot more easily. 

The next step will be to attach the base and give the oar a test run!