Slight S-Bend in Aluminium Mast

[Babylon]

When re-rigging at the start of last season, the riggers pointed out to me that our 30ft single-spreader Kemp/Selden mast has a slight S-bend in it, just below the spreaders. They predicted that I'd probably notice a slight difference in speed on one tack or another.

(The probable cause of the S-bend goes back to the end of the previous season when, beating into a F6, my crew set up the running-backstays but I didn't at first notice that the lead wasn't correct, and we sailed, short tacking, for ten minutes or so with the running backstays twisted around the lower aft shrouds before correcting this!)

During the season I did in fact notice that we seemed to sail slightly faster on Stbd tack than on Port, but only very marginally - until the last sail of the season when, for some reason, beating into a F3 against a foul tide in sheltered but open water the log-reading for speed showed 3.5 kts on Port but 4.25 kts on Stbd, a difference of 0.75 kt!

Could all this be due to the S-bend (which is stable and hasn't got any worse over the season), or could some of this have been due to other factors (ie particular conditions, log-impeller offset just to Stbd of long-keel, weed fouling the impeller more on one tack than the other, etc)?

And what can I do about the mast itself? Its structural integrity isn't compromised (in fact its a pretty bullet-proof cruising rig) and new one would be extremely expensive. Is there any way, with the mast unstepped, that the S-bend could be worked out - or would this fatigue the aluminium?

 

[dt4134]

If it is just a slight bend (i.e. unlikely to be any damage to the mast) I'd be inclined to think a decent rigger could tune it out. Was the mast stored properly to ensure it couldn't distort?

Lots of boats sail slightly faster on one tack than another. I must've spent tens of hours discussing it on various boats over the years. The rig is one potential cause as is the keel & hull - including surface prep. I assume you checked to ensure the trim was equivalent as well as taking account of changes in wind speed. Differences in the car position (even quite small ones) is quite a common cause. I've even seen it where the helmsman seemed the most likely cause just because of differences in the way the tell-tales were used on different tacks.

Differences between the wave pattern and the wind can also cause different speeds on each tack. I've even occasionally seen significant wind shear that required completely different trim on each tack.

 

[vyv_cox]

When re-rigging at the start of last season, the riggers pointed out to me that our 30ft single-spreader Kemp/Selden mast has a slight S-bend in it, just below the spreaders. They predicted that I'd probably notice a slight difference in speed on one tack or another.

(The probable cause of the S-bend goes back to the end of the previous season when, beating into a F6, my crew set up the running-backstays but I didn't at first notice that the lead wasn't correct, and we sailed, short tacking, for ten minutes or so with the running backstays twisted around the lower aft shrouds before correcting this!)

During the season I did in fact notice that we seemed to sail slightly faster on Stbd tack than on Port, but only very marginally - until the last sail of the season when, for some reason, beating into a F3 against a foul tide in sheltered but open water the log-reading for speed showed 3.5 kts on Port but 4.25 kts on Stbd, a difference of 0.75 kt!

Could all this be due to the S-bend (which is stable and hasn't got any worse over the season), or could some of this have been due to other factors (ie particular conditions, log-impeller offset just to Stbd of long-keel, weed fouling the impeller more on one tack than the other, etc)?

And what can I do about the mast itself? Its structural integrity isn't compromised (in fact its a pretty bullet-proof cruising rig) and new one would be extremely expensive. Is there any way, with the mast unstepped, that the S-bend could be worked out - or would this fatigue the aluminium?

Next time my log reads the same on each tack will be the first!

However, I have seen many masts straightened and have helped with a few myself. All were small racing catamarans, Hobie, Prindle, Dart, etc., but the same principle applies and the size is not enormously different. Strip spreaders off and set the mast up so that the bend, or one of them, is facing upwards with a padded support at each extremity of the bent length. Then stand on the peak of the bend and bounce, gently at first until you see how much is needed for progress. Take it easy and take your time. It is possible to straighten badly bent masts this way.

 

[Quandary]

Bent mast

This is the first time I have had to disagree with Vyv whose experience I respect, but having tried it in the past I would not be at all confident about removing a subtle bend in an aluminium spar his way. And if you kink it at all, it is scrap. I much prefer the idea of tuning it out when erected as in the first post, this is usually possible as many aluminium masts are far from rigid anyway. Any performance rig has to be adjusted to shape and the bottle screws are there to do it.
However if your rig depends on runners for stability, rather than simply to tension the forestay, as it was for instance on my old Sigma 38, then tuning it out may be a bit more difficult.
Worth reading the Selden rig tuning handbook which can be downloaded from their website.
I agree that log variations of 1/2 kt. on opposite tacks are not uncommon, no one can fit a transducer to a degree of accuracy.

 

[Babylon]

Re Quandary, Viv Cox, JCP

JCP - reassuring to hear about the effects of subtlties of sail trim, wind-tide, etc on different tacks.

Quandary, Viv - I think I'll stick to trying to compensate for the slight S-bend through rig-tuning rather than risk trying to bend the mast straight. Of course if it were so bad that I'd need a new mast anyway (which it isn't) then it would be worth mounting the mast on trestles and bouncing on it.

It would be interesting next time the mast is down to actually measure the maximum deflection each side - easy enough with a piece of small stuff taught alongside the mast.

BTW, the running backstays are there to counteract the forward pull of the the stays'l (proper cutter-rig) in F4 or higher. As regards the rest of the rig, she's a Vancouver 27 so an out-and-out cruising design rather than any sort of racer. Interestingly I've had two riggers tune the standing rigging, the first guy was the one who identified the problem and gave the rig a fair amount of tension in an attempt to mitigate it, the second guy retuned the rig before I'd been out sailing (after fitting a new forestay and furlex) and set it up with less tension. I think I'll get the original rigger to tweak it again this season and see what difference it might make.

Thanks for the views.

 

[oldfatgit]

Before you go to a rigger, you might try tuning it yourself. It really is not that difficult and, given the description of your mast and boat, is worth a try, particularly if the riggers have set the mast up with the head in the right position. What I'd advise you did is sketch your rig in both side and rear elevation and work out which shrouds and stays need to be adjusted to have the effect you are after. Then have a go, after all, if it all goes wrong you can get your man in to put it right. The result is that you may sort out the problem and learn about your standing rigging and how it effects the mast as well as sort out the problem for free. Finally, I found that I was then able to tune my mast for ever increasing effects season on season.

 

[Ariadne]

When we bought CAPE we had a big S bend in the mast, it looked like a monkey had set it up! It was part of the purchase deal to have mast removed and the chain plates drawn for inspection. So when the mast was off the boat we checked it after a week and all was fine. It would seem the rigging was badly done originally, it was easy to sort out our rigger tuned the mast well with no bends and we sail well now.

He also gave some good advice on how tune up/down & check the tensions without a gauge as well. A good rigger is worth his fee.

 

[William_H]

Masts

Lots of comments some I disagree with.
I did try to straighten a mast for my 21fter. I placed it under the trailer fore and aft members and put a jack under the mast at the peak of the bend. So jacking against the weight of the boat. I got a little bend out but ended up with a new mast. I can't see jumping on the mast (keel boat type) doing anything.
Yes boats do have a better sailing tack. Mine has had a better side for 26 years through 3 different masts and many different rig adjustments. It must be the hull. It seems to sail faster point higher on one tack. It is most noticebale when sailing hard close to another boat.

The only bend of a mast with single spreader intermediate stays that can be affected by the stay tension is the single curve ie middle forward back or either side. Any bend between the stays can only be adjusted (pulled out)by additional stays or jumper strut type(diamond) spreader stay arrangement.

I can't imagine how the runner if fouling the side stay could bend the mast permanently. It takes a huge bend to give a small permanent bend.
Incidentally jumper struts could help you get more tension on the cutter inner forestay without the use of runners. Small spreaders are attached near the top of the inner forestay. ie about 45cms long and set at about 80 degree angle from one another facing forward. Stays are fitted to these spreaders which run from the lower stays to the cap shrouds. Fit bottle screws so that as you tighten the added wires the mast is bent backwards at the inner forestay attachment. The tension of the inner forestay counteracts this bending force. You occasionally see these jumper struts on fractional rig boats. olewill

 

[oldsaltoz]

I'm with Vyv on this.

.
The yard haul out operator managed to put a bend in the 10m mast of a clients yacht, The bend was in the upper section.

All we did was lay it on trestles, eye ball along it's length from the base and mark the high spot, then measured the distance to the top end of the mast, placed another trestle the same distance down the mast to ensue we had the middle of the bend.

It two of us bouncing to get it back in line, I was just not heavy enough on my own, but was still the only one bouncing.

Re speed difference on opposite tacks, I was invited aboard a sea trial prior to purchase by a friend. It was pretty obvious that the speed on port was a lot better than starboard.

Checked all the obvious items but found nothing. When she was lifted for a hull inspection I noticed the keel was 3 to 4 inches out of alingment with the hull. We walked away.

I have set to sail any vessel that will indicate the same speed on both tacks, but still look to reduce it.

Avagoodweekend......

.

 

[Mrnotming]

differing speeds on tacks

All great advice. All I can add by way of the above is that the track cars are usually used as a reference when positioning the cars for a genoa or jib.Fine, but on one production cruiser the tracks were planted in different positions and the holes choosen each side would thus not be in the same place relative to the clew of the sail.Once this became apparent, after about two years, markings were added to reveal the true positions.Live and learn!
You might scoff, AWB and production progress obviates this.It does'nt. The pulley cars on my tracks were delivered with the sheaves at the aft end of the tracks. As most people will have noticed, the car will make it's own way aft(with bungee), but needs pulling forward.

Good sailing to all in 2010.