Magic Numbers
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When I bought my boat I was very pleased it had a C-Tech mast. I had never sailed an OK with a Carbon rig. I stopped sailing the OK at the time of the cross over.
I was led to believe the C-Tech was the mast to have. Pleased with the boat and looking forward to getting back into sailing I started to find out about the things I would need know to make this boat work for me.
I was given a set of numbers which my mast was built to, F/A 64 82 65 Deflection of 600, the Side numbers, 42 63 55 Deflection 229.
I contacted C-Tech and they confirmed the numbers and they informed me the mast was built with an 80 to 85 kilo weight range.
That was good news, I am now an ageing 85kg wet through and short in stature so not much leverage over the side. Believing I had the right boat and the right mast with a sail matched to the rig I could concentrate on sailing and start to enjoy the boat.
It soon became very clear that I was struggling upwind to drive the boat forward. I had massive heeling force and lacked forward drive. Whatever I did to reduce power I could not get things moving in the right direction. I had great off wind speed which I was pleased with so at least this was a positive.
Some 3 weeks into the series at Alton Water, Robert Deaves made the comment ‘maybe your rig is too stiff?’
I half discounted this based on my numbers which C-Tech confirmed was the correct rig for me. Basically I also wanted to discount it based on the cost of replacement if it was incorrect for me. I got stuck in and persevered, not making much headway. I then broke my ankle which put things on hold.
Whilst I was laid up David, my son, (previous OK Nat Champion) sailed the boat in a bit of a blow and sailed it very well giving Robert a run for his money and declared on his return to shore ‘boy you have a quick boat there Dad’.
Interesting I thought, at least I know the boat is good. David is 6foot 1inch and his weight was 95 kilo.
I got on with the ankle thing and managed to sail on and off doing my other things putting boat speed on the back burner.
It was not until the Nationals that I really got the chance to sail against other boats in numbers and in a consistent breeze that it all started to become clear to me that things had to change. I really was struggling to sail to windward, I needed balls bigger than an elephant.
Terry Curtis took away my numbers and very soon he informed me that my mast was very stiff for my weight.
It seemed C-Tech’s idea of these numbers being for a low weight was not so. It has since become clear the numbers I have been sailing with are used by some of our front runners who are much taller and heavier than me.
Oh dear, it looks like a new mast is going to be needed. I started my enquiries about replacement and it became clear that Aardspar was to be my preferred route based on delivery dates, after sales service, cost and the ability to hit correct build numbers.
Now then, what might those magic new numbers be that would provide the right mast for me? Dave Rose and myself set about early talks over a few beers (we are getting good at this).
I decided that the Fore and Aft numbers should stay the same but reduced the deflection to 500, this ensured my sails would fit the new rig. I would only need to move the foot of my mast forward 5mm to compensate for the reduced deflection.
I now needed some side numbers that would still drive the boat and also work depowering the rig when it started to become breezy. Terry Curtis was very helpful here with some numbers he had used plus he also had a good understanding of bend numbers Aardspar have been developing/working with. I also spoke to C-Tech who produced a set of numbers they would recommend to replace my mast based on the information I supplied. Interestingly these numbers turned out to almost exactly what we have arrived at.
After talks with Terry Cooke at Aardspar we decided to go with, Fore and Aft Numbers of 64 82 65 deflection 500 Side numbers of 45 71 65 with a deflection of 265.
This should still offer plenty of power off wind low down and mid-section. The new numbers at the top of the mast should allow the rig to work better up wind for me and dump unwanted power. I think it should still work fine in lighter winds.
The new mast has been built and the numbers have been achieved. I will fit it to the boat and look forward to the remainder of the Alton Water winter series hoping for some breeze to prove my new rig is working for me.
So, if you are struggling with driving your OK up wind and overpowered off wind and not getting the satisfaction out of sailing because it is all just too hard work, have a think “is my mast too stiff and can I make my rig work better for me?”.
The first thing you need to do is know your mast bend figures and how these numbers compare to other people of a similar weight and build and who are sailing the boat with some success. Our Secretary Andy Turner has a list of numbers that he has collected over the years. Si Cox also has quite a lot of other peoples numbers, it could be worth giving either of them a call. Aardspar also have base numbers for masts ranging from a 70 kilo person right up the range. Jim Hunt our current National Champion and sail maker has a wide base knowledge of mast and sails and is always happy to offer advice. So don’t forget with a little bit of homework and help from others you can tailor make a mast to suit you.
I will suggest to the powers that be that we have a list of mast bend numbers and corresponding people’s weights in the Technical forum. This could be continually updated and become a useful library. I will also suggest there is a thread on the method of measuring mast bend.
The only down side is if you make big changes you could also find yourself having to buy new a sail, but in the long run you will have a boat much better suited to you and certainly more fun to sail. I have managed to keep my existing sails based on retaining the fore and aft bend to the existing measurements.
The other upside is, you can now sell on your old rig making the whole cost much less for greater reward, plus you might be helping others to upgrade and bring the fleet to a better standard.
I hope this is of use to some of you, at least it has given you something to read and think about this winter.
If you need to have a chat please feel free to get in touch. I don’t profess to have all the answers but I hate to struggle and if we can help each other to improve our boats then we should try.
Regards Ken.Have I got this right?
Existing C-tech = F/A 64 82 65 Tip 600. S/S 42 63 55 Tip 229.
New Aardspar = F/A 64 82 65 Tip 500. S/S 45 71 65 Tip 229.
So, the difference is S/S is in the top 2/3 bend while the tip is the same?
@Finn wrote:
Have I got this right?
Existing C-tech = F/A 64 82 65 Tip 600. S/S 42 63 55 Tip 229.
New Aardspar = F/A 64 82 65 Tip 500. S/S 45 71 65 Tip 229.
So, the difference is S/S is in the top 2/3 bend while the tip is the same?
Well done Finn the final draft got slightly distorted. The side Tip Deflection for the new mast Aardspar should read. 265 not 229.
Sorry about that well done for reading it correctly. You get a ten from kenWell done Ken,
If you can get this sorted, it removes another reason not to buy an OK.
I’m happy to share my numbers – just need to check with Aardspar first.Well first indication are that the new mast is working fine. The boat is certainly much easier to sail. Dave Rose comments that looking from his boat you could clearly see the leech opening in the gusts. The most noticeable feeling was that of not having to bust a gut to sail/drive the thing upwind. The boat needs a little bit of tweaking due to the difference in the fore and aft deflection. 600 reduced to 500. The centre of effort has certainly changed and the helm is slightly wrong. The boom is also far to high off the deck. I need to move the mast foot forward some more. I only moved it forward 5mm as a starter but this clearly not enough. I will now move the foot forward,and I will also rake the rig back so that the boom is in the region of 50/70 mm off the deck when max sheeted in.
So all in all the new mast seems to be having the right effects and first indications are the boat is faster for me.
Ken.
One Life Live It.I think a few of us might be sailing with too stiff masts sideways. Its all very well on flat water where you can stuff it and just gain height but with a chop, that ‘flick’ with the softer masts seems to make a big difference to speed. WillT went like a rocket with his older soft mast at the Nationals. Will can hike and is 95kg so if he has a soft mast that gives a message (come on will what are those bend numbers !!!) I hoped not to have to buy 2 masts which also means 2 sails as luck says you’ll only put the wrong one up for the conditions when they change anyway. I’ve decided to be less technical and just put on weight to grow into my mast instead but the food and drink bill to achieve that is close to the cost of a new mast….
Your right Lee, ref the softer mast. I can confirm the numbers for Will and Terry, both were very quick at the Nationals. These numbers were also a governing factor along with the knowledge Terry Curtis has accumulated over the years that convinced me to change.
Will Turner, C Tech fore and aft 70 90 70 def 542 side 53 75 70 def 250.
Terry, C Tech Fore and aft 85 120 98 def 656 Side 54 86 73 def 350.
The fore and aft numbers can easily be kept the same in different masts if you don’t want new sails all you need to do is have different side numbers. This is what I have done. My new Aardspar fore and aft numbers are no different to my Stiff C Tech other than the deflection which has changed from 600 to 500.
But the side is softer,45 instead of 42, 71 instead of 63, 65 instead of 55, and Deflection of 265 as against 229. These numbers for me should allow the rig to work upwind and drive the boat forward rather than creating massive heeling force. That the theory anyway.
Ken.When I returned to OK with initially a Selden Carbon mast and then an old one of Terrys I felt much more overpowered than with my previous OK with blue top metal. Put it down to age and lack of fitness.
Perhaps I am suffering from an over stiff mast as well. After reading Jims tuning tips in the newsletter today maybe i am cranking things in too much and should ease off a couple of inches in strong winds.
Unfortunately I am not in a position to try a more bendy mast so I better start eating.Hi all.
I also have a pretty large database of mast bend numbers. One thing I have felt for a while is that people were going too stiff with the sideways tip deflections and thus their rigs are becoming more static rather than dynamic. If you think logically about it try to work this out. If you suspend 10kg from the mast tip while supporting it only at the heel and deck ring, how much more carbon must there be in the mast to make it bend 10mm less?
It has to be quite a bit. So if Ken has gone from 229 to 265 his new mast has quite a bit less stiffness and probably is a lot more dynamic. This is why he feels it easier to sail with in a breeze. And it probably is!
I really think the f/a bend makes less difference than the s/w. I am sure Jim would agree that a sail can be made for pretty much any f/a bend and the f/a tip affects rake and how hard it is to pull the mainsheet in. The sideways is what lets you either sail upwind in a breeze or makes you struggle. Of course too soft and you have not enough power in the moderate and are slower downwind.
The good thing now is that unlike in the alloy days we can easily get a mast that suits us. We also have a choice of where to get them from which is a real bonus and we don’t have to just hope it is a good one.cheers
GregGood reply Greg, I really do think that the Class should have a tips and advice section based on things just like this. Prospective new members just thinking about the boat should have something to draw reference from. Maybe an OKDIA based thread plus each country should have their own section on their website offering advice of this nature. Together with a not too large a library of numbers, it should also contain a list of names to contact for advice.
Just recently a now new member asked a question on the forum about the weight range of the OK. The link to this is below. I think we were lucky that Rich could see his way forward but some not as confident could walk away finding it all to hard.(maybe another thing for the road maps to work on)
viewtopic.php?f=3&t=1228Regards Ken.
@leech wrote:
…… 2 masts which also means 2 sails …..)
Not really. Both my masts are the same F/A bend between the bands and the sails are interchangeable.
@Finn wrote:
@leech wrote:
…… 2 masts which also means 2 sails …..)
Not really. Both my masts are the same F/A bend between the bands and the sails are interchangeable.
Jim / Greg you agree? I’d have thought if you were using a softer mast as a second mast you’d also want a flatter sail as you’d be using it only when it’s in overpowered conditions
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