Tuning the mast on a DS II

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Tuning the mast on a DS II

Postby rkennedy » Thu Mar 16, 2023 9:42 am

My sails were pretty beat so I bought new north sails.
I went to the tuning guide at the north site and by the second paragraph I was already confused.
MAST RAKE AND SHROUD TENSION
To measure the aft rake of your mast, hoist a tape measure on your main halyard and hold it tight at the intersection of the transom and rear deck. This measurement, without your jib up and your rig set “snug” (no play in any shroud or forestay), should be 24 11” to 25’ 1”. Then hoist your jib and pull the rig forward through either a magic box, a block and tackle, or using the forestay (as a bow and arrow effect) until you achieve a 25’1” to 25’2” measurement at the transom. With this rig tension (approximately 150 lbs on the shrouds) the leeward shroud should not go slack until the wind velocity is approximately 10 mph. This may require more rig tension than you are used to carrying, but this tension is important in keeping the jib luff sag down to a minimum for maximum speed and pointing capability.


My 1st question is when measuring from the top of the mast to the transom rear deck do they mean the bottom of the transom where the floor meets the transom?
Or the top of the transom. I measured my mast from the top of the mast to the top of the transom and it was 25'1",
If the measurement is supposed to be to the bottom of the transom then wouldn't the DSII have less distance because of the elevated floor? Or does the bailer equal things out?
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Re: Tuning the mast on a DS II

Postby GreenLake » Thu Mar 16, 2023 6:30 pm

The top of the transom, and it looks like your measurement matches the range they give, so you are fine.

Another way to confirm whether your rake is set correctly is to sail the boat and notice whether you have to pull hard on the tiller going upwind (too much weather helm) or whether you don't feel any pull even as wind pipes up (too little weather helm). You should not need more than 5° rudder deflection to keep your boat from rounding up.

Given your measurement, I would expect your boat to be pretty balanced.

Many jibs nowadays contain a luff wire or equivalent bit of Dyneema that can take over the tension provided by the forestay. That allows you to fine tune the luff tension (and sag) with the jib halyard. You may want to rig a simple halyard tensioner.

Here's a pretty crude 2:1 tensioner (blue line) that pulls on a loop fixed to the jib halyard.

2679
(Click on image to enlarge)

I use a single block with a simple hook which makes the tensioner easy to connect after raising the mast.
~ green ~ lake ~ ~
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Re: Tuning the mast on a DS II

Postby rkennedy » Fri Mar 17, 2023 9:33 am

Thanks Greenlake.
To use that halyard tensioner do you have to have the wire in the jib ?
I assume that if you are going up wind and pulling on the tiller more that 5 degrees then you would use the tensioner to move the mast forward?

Thanks
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Re: Tuning the mast on a DS II

Postby GreenLake » Fri Mar 17, 2023 6:07 pm

The jib is the second I got from the same sailmaker (now retired). The first one had an actual wire, but for the second he switched out the wire against Dyneema rope which is just as strong as wire of the same diameter, but much lighter. So, in that sense, yes it has a luff "wire".

There are two independent things to tension on a jib: one is the tension in the "wire" from which the luff is suspended, which determines the "sag" and with it the shape of the luff. The other the tension in the cloth itself, this determines things like folds in the cloth.

On jibs without integral luff wire, the cloth tension is set by the halyard and the "wire" is the forestay which on our boats cannot be adjusted in tension underway. On jibs with a luff wire, the sag of the jib is controlled by the halyard. There's a separate "tack line" that you could rig to pull on the bottom forward corner of the sail cloth (just like a Cunningham for the main). Pulling on that would straighten the cloth independent of the tension in the "wire".

Now, many people don't actually bother with two completely independent controls. A common technique is to use a lashing (several wraps of thin stuff) to tie the cringle in the bottom of the cloth to the eye on the wire that gets connected to the stem fitting. In effect you fix the relative length of the (stretched) cloth to the length of the luff wire. As you tension the latter, you also tension the cloth. As your sail ages, you can adjust the length of the lashing as needed to give better results.

That's the setup my sailmaker suggested and what I have rigged on my boat.

As the wind increases, the pressure on the jib increases and it has a tendency to sag more. That's the moment when I add tension to the jib. And in principle the reverse if the wind slackens again. For the initial tension I look to see whether there are any "scallops" in the luff of the jib, that is when the luff isn't straight between adjacent connections to the forestay.

Tensioning the jib halyard will probably affect mast rake to some degree, but that's not the reason you do it. It's best to think of mast rake as essentially fixed: even if the jib halyard has some influence over it, you don't actually use that to control mast rake.

(Btw, cranking a vang or a really tight main sheet would also affect mast rake a bit, by pulling backwards. Again you would not use either to affect rake, but to primarily cause other changes).

With the boat properly set up, if I really have to yank on the tiller, I know I'm overpowered and drop the mainsheet a bit or put in a reef.
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Re: Tuning the mast on a DS II

Postby rkennedy » Mon Mar 20, 2023 8:58 am

Thank you for the detailed explanation.

On a Hank-on Jib without a luff wire you would probably not want to pull on the jib halyard so hard that it moves the mast forward because,
you will end up with a vertical crease running up the luff, and you may take some years off the jibs life if you stretch the luff out?
I am always curious about halyard tension, I know I have over done it when I see those vertical creases.
I would like to know what indicators people use to tension their halyards, main and Jib?

I know you said your sail maker is retired but I was wondering who it was?
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Re: Tuning the mast on a DS II

Postby GreenLake » Mon Mar 20, 2023 3:42 pm

Jib halyard tension shouldn't be used to "move the mast", but to make sure the jib is set properly.

If you get vertical creases, you've overdone it. However, if your jib has a luff wire (or Dyneema rope equivalent) you might get creases if your lashing at the tack is too tight.

If you have no integral "wire", then forestay sag will have to be controlled by forestay tension; however, that is not readily adjustable under way, hence the jibs with integral "wire". If yours isn't done that way, time to upgrade.

For some of your other questions, you might want to take an idle hour or two and read the older posts here on the forum. It's amazing what you can find out.
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Re: Tuning the mast on a DS II

Postby pbobbitt » Tue Apr 11, 2023 12:34 pm

I just bought my first daysailer II used. The guy I bought it from said he had recently replaced the stays, and I thought I'd tune the rig before I sail it...

I'm getting some really strange measurements: Distance from top of mast to transom: 25'4".

I look at the forestay, and the turnbuckle is all the way out. When I raise the tape up the jib, to get an estimate of the forestay length, I'm measuring 15'8.5" (I've read that the forestay length is about 15 feet, so you'd think the distance from the top of the mast to the transom would be short).

When I measure the length of the mast, I'm getting 22'1"...

I haven't sailed it yet to see if there is super light (or non-existent) weather helm...

Thoughts?
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Re: Tuning the mast on a DS II

Postby GreenLake » Tue Apr 11, 2023 1:09 pm

The first principle should be to sail the boat first, before you tune it. For that you need to make sure the rig tension is in the ballpark. When the mast is stepped, you should begin to just be able to pluck a low note on the shrouds. With too little tension, you get more of a rattle, and there's no need to go too far past where that sound transitions.

Then on the water you look for three things. At about 10kt of wind, the leeward shroud goes slack. Going upwind, with sails trimmed in, you should feel the need for a bit of pull on the tiller, but you shouldn't have to fight it. That's your weather helm and it should be present even when the wind slackens, just less of it. Third is to look at your jib: can you adjust the forestay sag with added halyard tension, or, if your jib doesn't have a luff "wire", is the forestay tight enough.

The mast top to transom measurement you give would indicate that you have too little mast rake. So you'd expect very little weather helm (assuming the mast height is correct, which in a used boat isn't always a given). If the shroud tension was in the ballpark and your test sail confirms that you have too little weather helm, then you can adjust and observe.
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Re: Tuning the mast on a DS II

Postby rkennedy » Tue Apr 11, 2023 2:44 pm

THe DSII mast is 24'1". So the measurement is to the top of the deck?
Do you have a tabernacle on that mast?

Last year someone at the club told me to pull the pin to let the mast tilt back. I did this one race and it was a mistake. Tilting the mast aft is not the same as bending the mast so the top points aft.

I think GL is right get it close at the dock and then go sail it.
The real problem is finding that perfect 8 to 10 knot day, whenever I have time to do some tuning it is either too much wind or not enough wind.

This is from D&R marines website.
Daysailer Mast Complete w/all Hrdw less Rigging. This product is too large to ship UPS. Please call for freight pricing.Dimensions 2.12in. wide 2.84in Deep (Lengths are 24' 1" on D/S II and 24'7" on D/S I)
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Re: Tuning the mast on a DS II

Postby GreenLake » Tue Apr 11, 2023 3:11 pm

There are two things: location of center of effort, and mast bend.

Here's an image that shows the theoretical center of effort which needs to be balanced against the location of the center of lateral plane.

Image
(See discussion here)

Any method of bringing the mast top back will change the location of effort.

However, if you can induce a mast bend, you will pull the middle of the luff forward (between head and tack). That will flatten the sail. This also changes the location of center of effort, because it will bring the actual location of maximum force on sail forward. (Like many things in sailing, you don't always get a clean single response to any adjustment, but some mix).

MastBend.JPG
Mast bend for keel stepped and desk stepped mast
MastBend.JPG (21.18 KiB) Viewed 3207 times


In a DS the spreaders are swept back and they push the mast forward a bit, so does the boom, especially with pull from the vang (the latter also causes a downward/rear pull on the main top via the leech tension, the two work together). If you have a vang, sight up the mast as you engage it.

If the mast is keel stepped the partners are just a point against which the mast is braced, they don't interrupt any mast bend. Tabernacles are not as efficient in making the mast act as if it was continuous. The picture looks more like the one to the right. That's the reason why racers don't cut their mast: they want to have the best control of mast bend.

The other fixed point is the hounds (where the stays meet and hold the mast in a relatively fixed position).
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Re: Tuning the mast on a DS II

Postby pbobbitt » Tue Apr 11, 2023 6:08 pm

I measured the height of the mast from the base of the tabernacle.

Yeah, I've been sailing Flying Scots for a while, I have a good idea of what a sailboat is supposed to feel like if it's reasonably balanced. I've also been in a sailboat that has negative weather helm, and it can get a little nasty.

In all respects, the boat appears to a be pretty well cared for stock Daysailer, never been raced (the controls are very basic as described in the manuals, no extra holes drilled anywhere). It's honestly pretty clean.

I'm going to drop the mast an make some measurements on the actual lengths of the shrouds... it seems weird that if I haul the tape up the jib halyard, I measure 15'8" to the where the forestay mounts to the deck, as this source (https://sailboatdata.com/sailboat/day-sailer-ii/) says the forestay is 15'.

I'm definately going to be taking it out soon, I've got a couple of other issues: The previous owner apparently didn't believe in battens, and when his centerboard got wonky, he replaced the cable first with just a steel cable. When it rusted, he replaced it with stainless, but I appears that he cut the new cable too long as when the blocks on the uphaul in the cuddy are 'even' the CB still has a few inches to go.

I'm sure I'll sort it out. Water temp in the bay is still in the low 50s.
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Re: Tuning the mast on a DS II

Postby pbobbitt » Tue Apr 11, 2023 6:21 pm

rkennedy wrote:THe DSII mast is 24'1". So the measurement is to the top of the deck?
This is from D&R marines website.
Daysailer Mast Complete w/all Hrdw less Rigging. This product is too large to ship UPS. Please call for freight pricing.Dimensions 2.12in. wide 2.84in Deep (Lengths are 24' 1" on D/S II and 24'7" on D/S I)


Yeah, I saw that, but then I looked at the DS bylays (https://www.daysailer.org/resources/Doc ... bylaw3.pdf), if I'm reading it correctly the length of the mast above the tabernacle should be no more than 22'6" (I'm measuring 22' even so if I'm understanding the rules, it's a plausible height).
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Re: Tuning the mast on a DS II

Postby pbobbitt » Tue Apr 11, 2023 6:22 pm

GreenLake wrote:There are two things: location of center of effort, and mast bend.

Here's an image that shows the theoretical center of effort which needs to be balanced against the location of the center of lateral plane.

Image
(See discussion here)

Any method of bringing the mast top back will change the location of effort.

However, if you can induce a mast bend, you will pull the middle of the luff forward (between head and tack). That will flatten the sail. This also changes the location of center of effort, because it will bring the actual location of maximum force on sail forward. (Like many things in sailing, you don't always get a clean single response to any adjustment, but some mix).

MastBend.JPG


In a DS the spreaders are swept back and they push the mast forward a bit, so does the boom, especially with pull from the vang (the latter also causes a downward/rear pull on the main top via the leech tension, the two work together). If you have a vang, sight up the mast as you engage it.

If the mast is keel stepped the partners are just a point against which the mast is braced, they don't interrupt any mast bend. Tabernacles are not as efficient in making the mast act as if it was continuous. The picture looks more like the one to the right. That's the reason why racers don't cut their mast: they want to have the best control of mast bend.

The other fixed point is the hounds (where the stays meet and hold the mast in a relatively fixed position).


Nice.
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