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hello all the wise people of Tennant land.

 

I'm a long time reader but first time poster.

I have a T10 stretched to 11m Dakota cat style i am hoping that i may be able to draw on your collective knowledge.

Except for the stern extension and a small inboard blister on the cabin the boat is built to plan.

I will probably ask lots of basic questions but i come from a dinghy background not multis. 

I believe i am suffering from excessive fore and leeward stay sag as the breeze increases and was wondering weather my rig structural set up may be causing my problems. I have a non rotating rig that has a mast base arrangement that clamps around the beam and is supported by a strut and stainless flat bar strap. I'm wondering if this is loading the main beam in funny ways and what might be done to correct this.

I'll attach some pix if I can

 

sorry for the long winded description

 

Ken

20180728_134539.jpg

20180707_122128.jpg

20180727_085233.jpg

2015_02214340gregdickins_web.jpg

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Others will likely have more insight into rigs on boats the size of yours, but at least in smaller boats, leeward sag would be expected upwind, yes? What makes you feel it's excessive? Downwind, what are you doing with your mainsheet? This acts as your backstay. Again I can really only speak for smaller cats but it's extremely important to have enough tension there to avoid risking the integrity of your mast. If you're relying on dinghy experience you may be letting the main out further than you should.

 

USA233.jpg

Chris "Toothdigger" Blake and "Fast Eddie" Hansen, F16 USA233, on Lake Winnebago at Oshkosh, Wisconsin, July 13, 2014.

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Does this mast base rest on the main beam or does it sit slightly above the main beam and rely on the strap to hold it up. The latter is common practice. It could be you need to tighten the strap and use heaps more mainsheet tension. What purchase have you got? Does it go to winches? 

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My sagging stays are when going upwind. I'm thinking it is excessive because when sailing against boats of similar size but probably more weight as they are bridge deck boats as the breeze increases my pointing angle gets much worse as the breeze increases.

The mast base arrangement sits on the main beam with support from the strap, possibly this needs to be re-engineered back to the original design of the support post through the beam to sit above the beam with most loading on the strap. More tension on the strap may be part of the solution.

I think I have  about 8:1 purchase on the main not going to winches.

Any ideas on what sort of rig tension i should have?

My seagull striker sits vertical on the beam would angling this back towards the forestay improve its effectiveness or affect? rig tension 

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8:1 and no winches sounds way too little to me.

http://hem.bredband.net/b262106/pages/controls/index.html


 

I see no running backstays so the main sheet is what tensions the fore stay.

Are your sails of stout enough material? My first set of sails were underbuilt dacron things (1986) and de-formed badly as the wind picked up. Reefing the main was more about controling shape to be able to point rather than avoiding capsize.

 

These days I don't have this problem thanks to the wonders of modern materials. My lee shroud goes real floppy but as long as I haul in my main sheet the fore stay is OK.

 

Back to the mast base: Is there any play, anything to alow the mast to lean over? It doesn't look like it but... If not there will be some interesting loads on the mast base. Any signs of stress cracks?

 

/Martin

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Everybody

 

Sheet tension is an idea i hadn't considered  but will certainly give more thought, could be improved without much effort. Boom vang on a dinghy does wonderful things.

 

Interesting comments about sail material. I am confident that the main is suitable for the job as it is not that old, the jib may be a bit under so could be stretching as it is loaded up, maybe a bit full for a cat as well.

 

The mast base is is set up to allow rake adjustment but nothing sideways so i'm sure it is getting loaded up in all sorts of odd ways. I'm sure i have seen some deformation whist sailing but put it down to imagination. I have crawled all over the boat looking for cracks but can find none. Looking through the mast beam i'm sure i can see a couple of marks (like a crease ) where the mast base would be but have been unable to verify with a camera, maybe to close for clear focus.

 

Any and all thoughts are greatly appreciated

 

Ken

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Rotating mast solution:

http://hem.bredband.net/b262106/pages/launching/index.html

(scroll down to mid-page)

 

Dolphin striker for alu tube:

http://hem.bredband.net/b262106/images/old_dolphin_striker.png

 

I don't think rotating/non-rotating is important for mast base loads. Sail-mastbend interaction is another cup of tea though.

 

/Martin

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Nice looking Cat.

 

When the breeze comes up we crank the main on a small winch in our liitle GBE, as much as we dare to, vertical creases appear in our carbon main. Dont look at the sails, they may start to look terrible, only look at the speedo. 

 

Our dolphin striker is sleaved through the main beam and the vertical load of the mast is transferred on to the striker, there is no bend in the main beam just compression loads. It looks like yours is not like that and the main beam might bend a bit when you load it up.

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hello all

ive been absolutely flat out.finally got a bit of time to get back to my boat.

thanks to those who have taken the time to reply.

Martin- what a great site you have lots of info. i'm very interested in your composite beams, does this let you do away with the dolphin striker?

Freedom thanks for the compliment on boat looks

ScottiE any info will be appreciated

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would anybody have any photos (close ups) of mast bases used on Tennant style cats.

does non rotating make any difference to the loading on the mast base?

 

Ken, for what it's worth, here are pics of the mast base on my Dakota 33 (Pilkington-built, exported to the U.S.)

 

There is a chrome plated bronze hemisphere that is not shown in the photos. It is fitted flat to the bottom of the mast. The curved part fits into the delrin bearing. You can see the "boot" sitting off to the left. The entire step is welded to a dolphin striker strut which goes through the crossbeam.

2010-06-30_12-39-41__0110.jpg

2010-06-29_18-22-16_0670503725_1515.jpg

Mast rotation hardware.jpg

Dolphin striker and engines nacells.jpg

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Martin- what a great site you have lots of info. i'm very interested in your composite beams, does this let you do away with the dolphin striker?

The composite beams started out with a composite dolphin striker. We designed and built them because the alu beams were not up to the job -- wracking while sailng in waves bent the fore beam. It happened twice despite beafing up that beam after the first time. Later I came in contact with a Belgian Spyder owner who broke his fore beam and an American whose dolphin striker ripped out of the main beam. Not fun but it was this we feared would happen when we saw the drawings. Hence, the change in dolphin striker design.

 

The the job market made me move from my native Gothenburg to Stockholm and the boat ended up on a mooring rather than in a marina berth. I found the dolphin striker sometimes 'interacted' with the mooring in an unfavourable way and it had always been a bit of an occupier of much needed space on the trailer. Eventually I removed it and added a fair ammount of unidirectional carbon to the mast beam -- a net weight gain of about 1 kg if memory serves.

 

So, yes, no dolpin striker but not from the outset.

 

Added bonus: you actually feel a bit protected from the elements by the mast beam despite it only being 35 cm deep.

 

/Martin

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thanks again for the replies.

 

myosys -

i assume the dolphin striker floats up and down through the beam.

does the beam have anything welded into it to keep water from entering the boat?

are the stainless plates top and bottom of the beam to locate the dolphin striker and stop wear on the beam?

i see some "rope" from the bottom of the dolphin striker to the intermediate beam, were you having problems with the striker moving around?

 

martin-

i could be very tempted to go with bigger(read higher ) composite beams a bit further into the future. the weather protection is important.

i'll get this problem sorted first

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i assume the dolphin striker floats up and down through the beam.

does the beam have anything welded into it to keep water from entering the boat?

are the stainless plates top and bottom of the beam to locate the dolphin striker and stop wear on the beam?

i see some "rope" from the bottom of the dolphin striker to the intermediate beam, were you having problems with the striker moving around?

Hey kda,

 

Yes, the dolphin striker, which goes through the beam and is welded to the mast step, is free to move up and down.

The beam itself inserts into a socket laminated into the hull. I've attached a photo showing how far the socket goes into the boat. It is integral to the bulkhead.

 

Yeah, I think the stainless plate performs a bunch of functions; wear, centering, and strengthening.

 

The rope is actually a stainless wire with a turnbuckle. According to the importer, Pilkington built the boat with this to stop the beam from rotating when "under extreme load." That's never happened and we're really had the boat in some hairy stuff. I've since replaced it with dyneema. It would take some load I suppose, since the mast crossbeam is elliptical in profile, so if it rotated you could imagine what would be happening to the hull!

2007-01-06_15-23-40_0420209507_8037.jpg

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again thanks for the replies.

 

ScottiE and myosys the plans and pictures look to be what i will have to get organised.

if anything else comes up please share it.

 

nice looking interiors on the boats. are they finished in gloss?

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Can’t take credit for the interior of mine - superb finish by the original builder. They used POR15 and brushed it on by hand - not that you would know - he’s a panel beater by trade so that’s why! It’s realy hard stuff and doesn’t chip.

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