Everything you ever wanted to know about pre-bend

[GreenLake]

The difference in sailing characteristics between a full, keel stepped mast and a deck-level tabernacle (with compression post/mast segment below) is not well appreciated. I'm sure Tom is up to the challenge or explaining the difference and how it might translate into not only boat speed but increased safety.

In a word: Pre-Bend. If anyone wants to discuss the difference any further, let's chew on this subject in a separate thread.

Great idea. Here's the opener. Now let's pull together everything we've discussed before. Here's a loose list of questions:

1. What is it?
2. What does it do?
3. How to you set it up?
4. How does a deck-stepped mast differ from a keel-stepped one?
5. How do you sail with i?
6. Is it for racing only?
7. Why would a day sailor or cruiser care?

[tomodda]

Me and my big mouth. But, OK, I'll play. Let's start with questions #1 and #2:

1. What is it?: Mast prebend is the degree of fore-and-aft bend placed in a sailboat mast when the shrouds are adjusted. (Thank you, Google!). Now, the important part is that we are only interested in the bend between the gooseneck and the top of the mast slot... in other words, the luff of the mainsail. In theory, what the rest of the mast does isn't important. In practice, you certainly don't want some crazy bend right at the gooseneck, for instance. Also stating the obvious, but pre-bend is usually AFT, in other words the "belly of the bend" goes forwards and tips aft. Yes, there are cases with reverse pre-bend (mostly exotic masthead rigs), but that's not our case in the Daysailers, so let's not speak of them. There's also rake (angle of the mast in relation to the waterline) and tip sag (mast bends to leeward), but also outside the scope of discussion, except to note that rake affects your Center of Effort and therefore lee/weather helm.

Digression (I love digressions): They may not look it, but our aluminum masts are as bendy as a spaghetti noodle (uncooked anyhow). Pick one up at the middle and you'll see what I mean, you can shake it up and down and get a nice "boing-boing-boing" effect going. As I always tell new sailors, the stays hold the sail up and the mast holds the stays up.

#2 What does it do?: Bend that luff, baby! What does that do for us? Thought experiment: Take a flat sheet of paper, cut a curve on one long side of it. Now pull on both corners of the curved side, up and down, straightening out the curve, what happens? The sheet of paper will "belly out," the curve will go into the sheet, giving you a "sail" form. Opposite is also true, of course, if you could curve the straight edge of your paper while holding the other edges straight, you'll also cause a "belly" in the sheet of paper. With pre-bend, we're playing with those same effects on our mainsail, "bellying it out" more or less. In sailing terms, we are changing the "Draft" of the sail, making it "Full" or "Flat". The fuller the sail the more "power" it has, the flatter the sail, the more it can point upwind without stalling (think of an airplane wing and landing flaps, same thing). What does "more power" do for you? Gives you more Lift, which is not the same as raw speed, but more like low gear in a car.. more "omph." In light winds or accelerating out of a tack, you're going to need it. Downside is more air resistance and tendency to stall out when pointing close to the wind, not to mention heeling you over more. Which is why you want a flatter sail (less draft) as you get higher winds, develop more speed, or want to point higher, again just like higher gears in a car. Quick note - your mainsail is actually cut with a draft curve already in it, not flat (like our thought-experiment sheet of paper), so straightening out the luff curve flattens the "belly," not the other way around (the flat paper). Anyway, with all these interactions, the "start point" depends on how your sail is cut - curved or straight luff, and the built-in draft shape - and the exact shape of your prebend curve - where's the deepest point? In our boats, should be where the forestay attaches, but could also be at the hounds, more on that in part #3. Basically you set up your prebend to have the draft the way you want it for your "normal" or "neutral" conditions and then adjust it using your running rigging / secondary controls. We can discuss it more in part #5.

Three explanatory notes (digression again!): 1) When you get custom-made sails from a good sailmaker, you'll work with them to set your luff curve to exactly match your mast pre-bend. My sailmaker had me send him a bunch of photos + prop my mast up between two sawhorses and hang a 20-lb weight off the forestay attachment point. Worked a treat! 2) By normal/neutral conditions, I mean whatever works for you. A San Francisco Bay sailor (high winds!) will probably want a flatter cut/less prebend than me on my inland lakes. A "round the buoys" racer will probably want a fuller cut/more prebend than a distance racer. It all depends. ("REAL" raceboats also have more than one main and way more inputs to control pre-bend than our little daysailers!). 3) About draft: There are other tricks to change the location - forward or aft - of the draft, but that's controlled by your Vang, Outhaul, and Cunningham (sometimes called VOC) and to some extent, halyard. Don't want to get into all that here, but imagine pulling on the CORNERS of your sheet of paper and that's how VOC affects your sail shape. Also can discuss more in part #5.

OK, that's all I can ramble about on two cups of coffee. @GL: What do you think? I'm always open for editorial comment/corrections.

[GreenLake]

So far, so good. Here's my summary, extracted from your reply, just shortened. Perhaps I'll do that again if you answer the other questions. Just think of these as the "Cliff Notes" version, so keep giving us the long format.

Q1: What is it?

A bend in the mast. For the DS, we would bend the tip back, so the mast between hounds and gooseneck is bent forward.

(Note1: it's called pre-bend, because it's present before raising the sail)

(Note 2: above the hounds, the pre-bend is zero, because the DS does not have a back stay to the mast top)

Q2: What does it do?

It puts a curve in the luff. For any sail, increasing the luff curve flattens the sail (moves the draft forward), reducing it makes it fuller (moves the draft aft). Flatter sail points better, fuller sail has more power (good in low wind or when accelerating after a tack).

If a sail is designed for a standard pre-bend, reducing that would make the sail fuller than designed and vice versa. Having it designed for some pre-bend, allows you to adjust the pre-bend up or down.

(Note 1: starting from zero, you'd only have "up", which is one of the reason you don't want your sail designed for zero pre-bend.)

(Note 2: sailmakers that know the DaySailer will know the different mast types. They may not need a picture but still the sail is cut assuming non-zero pre-bend)

(Note 3: having some bend in the mast helps prevent the mast from oscillating in rough conditions, something perhaps more worrisome on a bigger boat)


Looking forward to your answers to Q#3 and beyond.

[tomodda]

Good summary. I do like to ramble, don't I? But I think it's good to give our future readers the why's as well as wherefores.

Speaking of which - yes, technically there is no pre-bend above the hounds. However, once you raise your main, the weight of the boom also is transferred to the mast, setting in more curve, and the leach itself acts to bend the tip of the mast. Not a huge amount, but enough. Is why my sailmaker told me to put a 20-lb weight on my mast as a trial. Deflected the mast about 2.5 inches at the hounds. Anyway, tiny details, but they add up!

[GreenLake]

All of that would become clearer once you add the remaining answers.

[tomodda]

Back when I was studying for my Maths degree, one of my professors said, "a mathematician is a machine for turning coffee into theorems." He cribbed that from a famous Hungarian, but that's beside the point. I can aspire to no such greatness, but I will myself try and transform some more coffee into sailing theory as soon as...

[GreenLake]

Standing by for your next coffee infusion

[tomodda]

Hello, today have some free time and lucidity, hope we can continue on the topic of mast prebend. Especially because I'm really unsure as to why keel-stepped is better than tabernacle (deck-stepped) for bending the mast. "Everyone says so" is not a good answer. But let's explore. So, continuing on:

3. How to you set it up? - With your stays, of course! Very useless answer because it ALL depends: do you have turnbuckles or is stay length fixed? do you have a mast jack? when/ how much should you change your prebend setting? etc.,etc. So let's unpack this a bit, and talk about stays, what do they do?:

-Hold up the mast! (NOTE*)
-Also, paradoxically, they hold down the mast, in compression, into the keel of the boat. In cross-section, you pretty much have a bow-and-arrow.
-The forestay pulls the hounds of the mast forwards. The hounds are 2/3 of the way up, where the stays meet the mast.
- Since the chainplates are abaft of the mast, the side stays pull the hounds of the mast aftwards
- Which brings us to the spreaders. What are they for and what else do they do?
oThe spreader's basic function is to hold the shrouds away from the mast and increase the angle at which they meet the mast. The greater the angle
between shroud and mast, the lower the shroud tension required to provide lateral support and, therefore, the less the compression on the mast. This means the shrouds’ diameter and the mast section can be smaller, thereby reducing windage and weight aloft.
oThe spreaders also push the mast forward, same deal - the spreaders point aft from the mast, at an angle, so the load from the side stays push spreaders and mast forwards. This force goes into the mast about 1/3 of the way up. I dunno if the attachment point is still called the "crosstrees," it was in wooden ships, but I digress. Also note that the spreaders can swing fore and aft on our daysailers, we need to accommodate the varying mast bend.

*NOTE: My own personal "take" is that the stays hold up the SAILS, and the mast holds up the stays. Considering how "noodly" the mast is, I think it's a pretty good frame of reference. Also helps me explain to my novice crew why they shouldn't freak out when the leeward stay or the forestay are slack. No, the mast won't fall down, concentrate on the damn sails!

Allright, that was my brain dump re: stays. So now you can see the basics of prebend, shorten the side stays you increase prebend, lengthen them and you decrease. Shorten or lengthen the forestay appropriately to keep the a nice curve, gooseneck to hounds. The top 1/3 of the mast (above hounds) and bottom of the mast (gooseneck) to deck, pretty much goes along for the ride, you can't adjust those (..ISH, we can talk about bending with the vang in part 5 of this discussion). Whatever you do, setting your prebend is a pre-sail activity, preferably onshore, on your trailer. Once you are on the water, you're not changing stay lengths! You want to set your "base prebend" to a repeatable standard curve (which you can adjust a bit with the vang while sailing, question #5).

One big thing to note (actually quite small) - the total prebend, as in the maximum distance between the aft face of the mast and the chord line across the bend is on the order of 1.5-3 inches, it's really not much. For instance, my boat is set up for 2 inches bend as my "base bend." This to explain my next paragraph....

So, if you have turnbuckles (DS2 and later), you can adjust your stay length by turning the screws. Make sure you do it evenly on both sides, slack the forestay after you tighten the side stays, tape up your turnbuckles once you are done. If you have a DS1, then life is simpler (and more complicated), you don't adjust the stay length at all, you adjust the mast length! Using your mast jack, a big ol' bronze "nut" at the bottom of the mast which pushed the mast up off the keelson. The more you turn it, the more the mast bends. As you can imagine, this method is pretty limited, there's only so far you can push the mast up, total play is maybe 1.5-2 inches. But that's really all you need (read above paragraph again)! Same for the DS2-DS3 turnbuckle method, really. The mast has a pretty good prebend as it is, "straight from the factory." Sidestays pull backward, spreaders push forward, forestay keeps the hounds in place, it just works out. All comes down to tension on the stays.. you don't want them slack, but you dont want them iron-hard either. We've discussed it many times on this forum. There are actual numbers you can tune to using a Loos Gauge, but I'm happy if I can strum a note on the sidestays, preferably a Low E or a G on a guitar. That's it, you have your base prebend! Either write down how many turns you made on your turnbuckles or memorize the "tune" when you strum the jacked stays. If you can come up with a method to measure where you are on the mast jack (DS1), then let me know, I find it a total pain. Maybe if I made a mark on the bottom of the mast that I can relate to the cuddy roof? Anyway, make sure you can reproduce your "base bend." Once you have it set, it won't change "on it's own", between sailing sessions, but it's have it written down/reproducible.

But, what if I'm not happy with the prebend "from the factory", I want a different base. Well, as you can imagine, change the number of turns on your turnbuckles or your mast jack, experiment. However, if you don't like the overall range of bends that you can achieve with turns, then there is one other tuning method - the length of your spreaders! Longer spreaders will push the mast further forward (more bend), shorter will have less bend. Of course, cutting the spreaders is kind of permanent, so go slowly (or have your old spreaders around for comparison).

But, but, how do I know if I have the bend "right"? It all depends! Let's discuss in section #5 How do you sail with it? To be written when I have time and lucidity again

4. How does a deck-stepped mast differ from a keel-stepped one? - As I wrote above, I don't know! I used to think that the keel-stepped mast is BETTER, tabernacle masts (not strictly speaking same as deck-stepped!) don't bend right. But the only bend we care about is between the gooseneck and the masthead.... How can anything below the gooseneck really matter? Um.. unless GL has any better ideas, we'll get back to this question later, I want to explore question #5 first. Which will involve more thinking and writing, please bear with me.

P.S. A personal note, I managed to catch COVID last week after dodging it for 2.5 years. "Only" the Omnicron variety, thank Neptune - still quite nasty, although the worst is over. Nevertheless, if what I wrote up above was mostly a fever-dream, please forgive me and feel free to edit/comment/correct!

[GreenLake]

Just saw this come in. Will need a bit of time to read it. Then I'll come back for a more in-depth reply. Your PS is scary.

[tomodda]

Thanks GL! Yeah, that was a brain dump, as usual it's easier to just write than to edit. As for my PS, I'll live, I'm fortunate that it was "just" a very bad cold for me. For others, not so fortunate and my heart goes out to them. Get vaccinated! It's not preventative (like a flu shot), it's TRAINING your body to deal with COVID (like a gym workout). T-cells, B-cells, immune system... fill in the details, you get the point. Anyway, I'm off my soapbox, here's wishing health, safety, and happiness for all our fellow DS'ers!

[GreenLake]

Tom, I thought a picture might save another kWord or so. Here is one that attempts to show the contributions from the various sources. Note that the drawing isn't to scale in the sense that the total forces shown in green must equal those in orange (plus the contribution from whatever the restoring force is from springiness of the mast). Also not shown is the net downward force from the stays, and upward force from the mast step/deck.

If the mast was bent much more, then the compression force would contribute to further bending the mast (by wanting to bring the ends together).

Showing forces contributing to mast bend

Mast-Pre-Bend.gif (10.31 KiB) Viewed 20509 times

On the left you have your keel stepped mast. On the right is the deck stepped (tabernacle) version. The difference is that the area over which the mast bends is much shorter. The dashed red line is simply taken from an oval, but I made the oval shorter on the right. It's supposed to be indicative of how each mast bends, with the straight section on top.

The point in space where forestay and shrouds are balanced depends on their relative lengths and tension. In principle, it can be freely chosen as can the position of the mast step. That means the bent mast can be aligned with the mast partners so as to pass through with zero force, or, as shown here, with some net force.

The vang is off in this picture, but the sheet is on. What's the difference? The vang is usually attached to either the mast at deck level, or the deck. If the mast can't move, because of the tabernacle or the mast partners at deck level, then the entire force of the vang on the mast is taken up by the vang, leaving the contribution from the vang pulling on the boom.

Assuming a keel-stepped mast is set up so the mast partners push forward (mast rests against the back) the net difference for pre-bend is that the mast bends in a more shallow curve (and that an equally deep "belly" at the middle of that curve should take less tension to set up).

[GreenLake]

Note that if you search the web, you'll find claims for both the keel-stepped and deck-stepped to be "easier" for generating pre-bend. Only one answer can be right. Can it?