In Part 2 we defined our goals for rake and prebend. Now let’s do it.
Rig Tuning, Part 3—6 Steps to a Great Tune
19 CommentsReading Time: 9 minutes
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Hi John,
this is really great stuff, however one thing keeps me pondering: in case you have no rodkicker and have your boom supported by a topping lift only, the weight of the boom will produce a pull from the mast top and a pressure point at the gooseneck, thus generating a pre-prebend that cannot be eliminated. In such a situation, would you advise to (a) remove the boom for setting initial rake and prebend, or (b) simply ignore this as the amount of inaccuracy would be so minimal it wasn’t worth the hassle?
Hi Ernest,
Wow, you are really thinking about this, just as I hoped would happen. And, I had never thought of that!
That said, I don’t think it would be enough force to make a difference, unless you had really hawked-down on the mainsheet, but, now that you have pointed it out, this is certainly something I would check for if I was getting a funky bend that made no sense.
And one more: your summary lists setting the rake before moving the mast base. But if I understand correctly moving the base will itself again alter the rake, so shouldn’t one redo step 2 if the mast base has to be moved?
Hi Ernest,
In theory yes, but practically with the small amount of rake on most cruising boats, and particularly keeping in mind that we are measuring at the goose neck, probably not enough to matter.
But once again, the key point is that you are really thinking this stuff through, so you are going to be able to come up with a good tune, pretty much no matter what you are faced with.
The bottom line is that once we understand the basic geometry at work, and what the desired end result is, which you clearly do, we can handle the details, regardless of whether or not I have expressly written about each of them.
Sorry, one more – when thinking this through I believe when adjusting prebend the rake might change (grow) as well – if I get you correctly the prebend should NOT change the rake, thus the prebend is reached by “bulging” the mast forward (a bit), more or less keeping the mast top at the same location. Am I correct?
I tried to make a sketch for this question here – https://ibb.co/n43Ded – if I am correct the distance marked in green should be the same before and after setting prebend?
Hi Ernest,
Yes, you have that right: bend does not change rake. Or to put it another way, the mast head is held one place by the fore and backstays. That said, measured rake at the goose neck, will change a little. I touch on this in Part 2.
A good read. A few thoughts that may have been in there, but I missed them.
My experience has been mostly with mutihulls, which like B&R rigs, have no backstay and generally no vang. This leads to several differences:
1. Mast bend comes primarily from raked diamond wires. Mainsheet tension may have some effect on fractional rigs, but it is minor.
2. If the mast rotates, this can have major impact on draft for two reasons. First the cord of the mast section is added to the draft. Second, rotating the mast pushes cloth into the center of the sail (prebend pulls it out, but a rotated mast is straight), increasing draft. Thus, it is common to over rotate off the wind and under rotate beating in heavy air.
3. Forestay tension comes almost entirely from mainsheet tension. Ease the main in a gust and the jib instantly becomes more full due to forestay sag, defeating part of what you were trying to accomplish. Thus, playing the traveller or reefing are better alternatives than a loose, twisted main. A small jib upwind is also a common solution, since it is hard to maintain a tight forestay. This is why it is common to see performance multihulls beating in heavy air with a small jib hauled out slightly to maintain slot, eased traveller, and a tight mainsheet.
Hi Drew,
Yes, all good points. As I said in part 1, I decided to just focus on masthead rigs that do not rely on swept backed spreaders so as to keep the series manageable. And even with that it’s going 5 parts and over 7000 words!
Hi John,
I unstepped my mast for the first time and had new standing rigging made last year. upon setup it appears the starboard aft side of the mast is touching at the partner. I used the original wood wedges for the first thousand miles to let the new rigging stretch before installing spartite. I got the boat leveled and installed a rieker 1 degree inclinometer, which is a great addition btw. using the plumb bob method my masthead is still off 3″ to port at the gooseneck but is just barely touching the afth starboard side at the partners….i can move the butt about1/16th” fore and aft but there is no play at the butt for athwartship adjustment. have you come across this? would it be better to have a bit of bend at the partners to allow for the spartite or let the starboard side of mast rest on the partners? thank you for the great article!
Tyler
Hi Tyler,
Yikes, it seems that either the step or partners are badly out of true. The only other thing I can think of is that there is a big wow in the mast low down, which is throwing things off, but I assume you have checked for that?
The right way to fix this is to determine whether it’s the step or the partners and then fix the core problem. I really don’t like the idea of inducing a bend at the partners to compensate. This is a highly loaded part of the mast and I think there is a risk of stress cracking if you do that. Also, I don’t like the idea of having the mast hard up against the partners, although I guess it’s the lesser of too evils, as long as it is not carbon. (Carbon masts require at least 1/2″ of spartite cushioning at the partners)
I guess, if it were me, and I did not want to take on fixing the boat, I would lean the mast a bit more to starboard to produce at least 1/4″ of clearance to port for the spartite with the mast straight athwartships and rake and prebend where you want them. Far from ideal, but I think the best of the three options.
That said, before doing anything, I would recheck everything you have done so far, starting with the first chapter of this guide, to make sure there was not an error in levelling the boat or setting up the mast. Also make absolutely sure you have rake and prebend right since you say the aft port side of the partners is the problem, therefore less rake, or more prebend, might also have a positive effect by moving the mast forward at the partners.
Hi John.
Seeing this whole process written down in an article does make it seem very laborious indeed. That’s what it felt like the first time I did it too.
However, for anyone contemplating giving up and just going with poor tune, I would like to share some encouragement:
Our boat has a longish keel with an attached rudder. It was always known for being hard to steer with an amazing amount of weather helm when beating in a breeze. So much so that I had to support both legs on a coaming, sitting sideways on the cockpit seat, and pull with both hands on the tiller like I was deadlifting weights any time a puff came along.
I started learning about sail trim, which did help some. But still, steering with one hand only was not possible in a stiff breeze.
Enter rig tuning. After spending two long days learning a process very similar to the one You describe, the result was simply astonishing:
the boat sails with one hand on the tiller, heels far less as tuning the rig helps flattening the sails much further, and weather helm is modest.
After learning with some trial and error, getting a good tune is easy and the process seems logical once learned – takes a few hours every spring nowadays.
As a bonus, proper rake & bend makes the boat look a lot better.
Time very well spent!!
Hi Vesa,
Thanks for providing the encouragement. As you say, a good tune is vital, but sadly I’m guessing that less than 5% of cruisers bother. We have a lot more on weather helm here: https://www.morganscloud.com/2016/06/10/ten-tips-to-fix-weather-helm/
Hi John,
This is about the pre-bend. My boat (also designed by McCurdy & Rhodes) doesn’t have a permanent inner stay; I added myself a removable one, which I used to induce the pre-bend. If I compare the mast with a long bow, the compression applied by the forestay and backstay should hold the amount of pre-bend even after the removal of the inner stay. But that didn’t happen. The mast went back to straight, the rake was OK and the backstay was at 20% of breaking load.
The inner stay is attached to the mast at 75% and I thought that it would work better if I set something closer to the middle of the mast. I made a temporary stay out of Dyneema and set it around the mast right above the spreaders. Everything worked out well, the mast was well aligned in both planes, the rake and pre-bend looked great but when I released the temporary stay the mast straighten up again.
I followed your “order of operations” and I believe I did everything right. Is it possible to maintain pre-bend without a permanent inner stay? Any thoughts/suggestion? Thanks.
Hi Horia,
Yes, prebend won’t stay in just from compression since that would take a fantastically stiff boat, and even if it did it would not be good because a wave impact could cause the mast to rapidly invert, which is a good way to have a gravity storm.
A few questions. Is the mast deck or keel stepped? Do you have fore and aft lower shrouds or just one lower each side. When in the sequence of tuning did you put the mast blocks (or spartite) in?
Also, it would be worth reading through this chapter again, where I did deeply into prebend: https://www.morganscloud.com/2018/06/03/rig-tuning-part-2-understanding-rake-and-bend/
That said, I did miss one thing: we can also prebend the mast with blocking at the partners, but let’s get into that after you answer my questions above.
Hi Again Horia,
Oops I did cover inducing prebend with blocking.
Hi John, thanks.
The boat is a Seafarer 29, deck stepped, one lower on each side. I already read everything multiple times. Also the standing rigging is new, so there could be a initial stretch issue until things settle. The lowers are 1/4″ and everything else is 7/32″, 1/19 wire.
There is a furler on the forstay and I’m not able to check the tension. So after I had the rake right, I tighten both turnbuckles but the final fine tuning was done just at the backstay where I kept the tension between 17% and 20%, the forestay feels right at hand and the rake stays in place, around 3/4 of the mast fore and aft diameter, to alow for a litthe more in heavy weather. As a side note I went around the marina and check the forestay tension on a dozen boats, by hand, and almost all feel too lose. To me.
Also my thinking is that at high winds, when I decide to set up the inner stay and the running backs, I can induce pre-bend.
Hi Horia,
Thanks for the answers. In that case there is no way to induce prebend without adding a staysail stay or baby stay. Something must pull the mast forward out of column to induce prebend, see the red rigging in the graphic above. As you say, when you set up the inner stay and runners you will be able to induce some prebend.
Greetings from EU.
Rig: In question there is 24 ft sailboat with 4 lower shrouds angled 2 forwards, 2 aft, attached below spreaders, 2 cap shrouds, backstay that can be tensioned with blocks, and forestay. So simple rig. Mast is 8 meters approx.
Problem: mast was stored in middle-to-aft bend(mainsail track up) and did attain that. I did store it turned other way around and in few months it mostly cured problem – it slowly bent toward middle-fore bend and being straight. However there is some middle-to-aft bend in mast still, how one would compensate that? Cant set rake first since mast is not straight middle-to-fore-and-aft wise. I can guess that mast needs to be straightened first with lower shrouds, both stays, thus setting rake and bend at same time. Messy. Any ideas?
Hi Martin,
Most mast tubes have a bit of a wow in them, but unless it’s extreme, it’s not a problem. You can still set the rake, just as described. Rake is measured with a plum bob (spanner) from top of mast to goose neck, so a wow in the middle will not make any difference. After rake is set you can pull in prebend with the forward lowers and in the process get rid of the wow. All is described in the last two chapters before this one.