Magic Numbers (mast bend figures)

[Ken]

I have posted this here along with the Gossip Column at the request of the Secretary. Ken.

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.

One Life Live It.

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