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A-class block on boom or mast?


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Building a small trimaran foiler with a cut down FP mast and 2sqm cut down Slingsby A-class main. Have seen solutions with the last block before sheeting, either on boom or mast. Which one is to be preferred and why? 

SKOT4.png

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lower one will have marginally less effect on mast rotation.

In the first image the purchase down to the traveler is pushing the boom forward encouraging the mast to rotate while the sheet at the front is pulling it aft doing the opposite.

 In the second the line running from the boom to the mast will pull the boom fwd and become a tight hypotenuse locking the mast in its rotated position. 

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8 hours ago, SCARECROW said:

lower one will have marginally less effect on mast rotation.

In the first image the purchase down to the traveler is pushing the boom forward encouraging the mast to rotate while the sheet at the front is pulling it aft doing the opposite.

 In the second the line running from the boom to the mast will pull the boom fwd and become a tight hypotenuse locking the mast in its rotated position. 

Except the pull on the mast is only 1/6 the tension on the traveler, so the result depends on the angle of the aft sheets !

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Thanks guys,

The ones I have seen, have the block in a sling around the mast. It probaly works fine,  but for me it would be better with the only off the mast, being the ratchet. Also noted all of them having the ratchet block on the sling around tha mast.

For practical reasons I might use the one with the block on the boom. This means I will have to compensate for the 1/9 of rearward force, by changing the angle of the aftsheets, maybe by only moving the rear sheave further back, to make room for the clew sling.

 

SKOT7.png

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  • 3 weeks later...
On 12/18/2021 at 4:48 PM, revintage said:

Thanks guys,

The ones I have seen, have the block in a sling around the mast. It probaly works fine,  but for me it would be better with the only off the mast, being the ratchet. Also noted all of them having the ratchet block on the sling around tha mast.

For practical reasons I might use the one with the block on the boom. This means I will have to compensate for the 1/9 of rearward force, by changing the angle of the aftsheets, maybe by only moving the rear sheave further back, to make room for the clew sling.

 

SKOT7.png

Not the mast, the mast base or beam.  The rotator is then either attached to boom vs. tramp or on the front.  Then under the tramp to the block with cleats fore and aft of the block or no cleat at all.

 

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3 hours ago, Loose Cannon said:

Not the mast, the mast base or beam.  The rotator is then either attached to boom vs. tramp or on the front.  Then under the tramp to the block with cleats fore and aft of the block or no cleat at all.

 

Not sure I understand your recommendation. Anyway, this is a small tri, the rotator is no problem, an will be postioned as in the simple sketch. The hull is from an A-class cat.

 

  

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I wouldn't change anything about how an A Cat boom and mainsheet works, if staying straight boom.  Clipping the mainsheet block directly to the clew is easier on the boom, with the pretty high mainsheet loads required, with better articulation than the  tornado type system you've drawn, and at the mast end, getting the mast rotation to automatically 'lock in' in light winds is a problem, the sling around the mast really helps this.  Because the A Cat straight boom was only loaded in compression, it and it's gooseneck were very lightly built (not much heavier than a carbon tiller extension) and surprisingly reliable, your setup looks like a 30 year old A Cat setup.

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Max….

Thanks for the input. Note we are talking about a 12sqm main so the forces will not be enormous. I only asked about the attachment of the block before the ratchet, the rest is not a problem. As I see it, when using the the block on the boom the rearwards force on it, have to be compensated for by moving the clew end blocks more rearwards. I don’t know what is meant by ’lock in’ in light wind means, so please enligthen me. As the boat will not be sailed in less 12-15 knots of wind, it might not make a big difference here? 

 

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Did a simplified load case to check the load on the tiny boom tube with 200mm between the axles  of the two rear sheaves with approximately 1000lbs(!) leech tension. Max stress on the 6060-T6 used is 215MPa. Also checked the compression forces, as the boom section is only 20mm wide and can not see any reasons to worry,

 

image.png.8ba6375cf737084d6c66673cf5f096df.png

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Not trying to influence your thinking but with the advent of boomless deck sweeper sails on the A Class ( well almost boomless with a simple 1/2 windsurfer type booms for optimum sail control ) have you not thought about using the hull and travellor as the mounting points for the pulleys to achieve your 9:1 advantage and you get quite an advantage from the deck sweeper at low wind speeds. The small bit of sail you loose from converting the standard sail is easily made up by better efficiency.

I set my small tri up as such and it worked well using exactly the same as you had planned, A Class hull, sail and mast. I simply used a 3:1 on the back and then used a 3:1 from the front beam to pull that 3:1. The tricky bit is getting the final pulley at the mid point ( best position as it pulls you onto the boat rather than foward or back ) without getting tangled and have minimal change in the line length as you let out the travellor , but if you think of it as 2 x 2:1's laying alongside of the hull with one of those leading back to the centre block, rather than centre hull. You can see the said final pulley just foward of the diagonal batten. 

The plus side of this setup is that there is minimal friction and you can probably drop to 8:1 or even 7:1 such is the straight pulls on the blocks.

image.thumb.png.16c9d821715c1285d82cfe7f1ec8f4ce.png

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Hi Waynemarlow,

Thanks for the input. The boom final design will be trial and error and it might even be a windsurfer wishbone on the existing sail. Have tried it  one of my single hand dinghies for two years.  The platform will  be ready this month and the other main parts including rig&sails are finished. Have also built a foil prototype, the final ones is being built when the platform is OK.  You can follow the build and I would really appreciate if you wanted to become a member in my FB-group: https://www.facebook.com/groups/2160774603967026 .  

Foiler.png

Mockup of the diagonal beams:

beammockup.jpg

Mockup of the rudder gantry with its linear actuator for electronic pitch control:

gantrymockup.png

hela.jpg

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