I'm soliciting comments before I build this 9:1 cascade (a higher resolution image is attached):



The things I like about this design are:

• It costs about US$100, since I already have a SailCraft Boom, hexa-cat base, and 3/8" mainsheet.
• All blocks are well within their rated safe working loads, assuming 1250lb total working load. (1250lbs is the SWL rating of the strongest Harken small boat ball bearing traveller car.)
• The sheet loads are perfectly balanced to not twist the boom.
• Line loads are aligned with block sheaves
• Relatively simple construction.
• I can add another cheek block and another 40mm block to convert to perfectly balanced 13:1 for about $25 more. (By cascading a 2:1 into a 6:1, the block loads are much lower than in a 3:1 into 4:1 cascade.)

My only [minor] concern about the sketch is the block at the front of the boom. When the mainsheet is eased to the limit, the load from the primary to this block will be imperfectly aligned. If this proves to be a problem, I will fix it by replacing the cheek block with a single on a very short pigtail in the same position.

Comments?

Glenn:

I count 8 to 1 with the cascading system. What am I missing?

I see a 4 to 1 outside the boom and a doubler inside the boom. I do not think that the sheet coming out of the boom to the bottom block adds any purchase as there will be no more line comimg out of the bottom block than pulls on the doubler block inside the boom.

I hope your boom is stronger than mine, small Tiger round boom, as it looks like too much of a compression load. If you are putting that much in compression it will be ready for any side force to bend or break.

Later,
Dan

Dan,

Will the boom get closer to the bottom block as the sheet is pulled? (answer = yes) Then I think there will be more line coming out of the bottom block than pulls on the doubler block inside the boom.

Jamie

This was all covered in another thread. (http://www.catsailor.com/forums/showflat.php?Cat=0&Number=85080&an=0&page=1#Post85080)

1# of tension on the main sheet gives 2# on the secondary line, for a total of 9# between the traveller and boom.


Pull 9" of line and the boom moves 1" and 8" of line comes out the end of the boom. More line does come out the bottom block than pulls on the doubler.


Yeah, the system would not even fit in the round Tiger boom (and possibly not in the earlier rectangular one).

I have a Sailcraft Tornado boom that measures 1.25"x3.5", and Tornado booms are regularly used to house cascade systems. You can even buy them from Marstrom with much pricier internal cascades.

Thanks for the feedback,
--Glenn

it's 9:1

Very Beautiful Glenn! Doesn't your boom have an internal outhaul system that may interfere?

GARY

I have tried the two singles at the bottom before and it gets pretty ugly, also the top is not actually balanced as there is more load on the right hand side, and by putting the blocks on the outside you are giving it more leverage to twist.

Basically you will always have uneven loads in this system whilst you are running the blocks on the sides of the boom. the best way we have designed (yet to build) is to have it all in line. 3:1 inline (block with becket at bottom ans singles inside boom) at the back and 3:1 inside the boom, this should be very free running and gives enough purchase.

Thanks!

I've removed it, though I will probably trim and use the Sailcraft outhaul car, which travels in a slot in the top of the boom and is a diamond-shaped piece of
stainless that looks something like this, where '*' is a delrin roller, 'o' is a hole, and '.' nothing:
. .___
. / o \
_/_____\____________ _ _
|\* . */
| \_o_/
|
|______________________ _ _

I'll just cut it down to this:
. .___
. / o \
_/_____\____________ _ _
|\*___*/
|
|
|______________________ _ _

Macca,

Thanks for the feedback!

I don't see what you mean when you say . If you look at the aft view, and note that the becket is mounted on the side of the mast, then it should be clear that there is no torque to twist the boom around its long axis, since each secondary line on the outside of the boom is matched by another on the opposite side that is equally loaded. (That's why the becket is placed where it is: on the side.)

It is true that there is more boom compression load on the right side of the boom than the left. That might bow the boom microscopically, but should not twist it.

Thanks for the note about the singles low causing trouble. I am anticipating some possible trouble there. I could just use a double (or tripple) and keep it down low to minimize the angle, but I think I want to try this out for jollies, and possibly create a silicone+line composite structure to keep those blocks loosely organized, but where they can still move.



That would certainly eliminate any boom twisting.

I'm trying to stay away from anything less then 4:1 in the secondary because I don't want to spend the $$ on high load ball bearing blocks for the secondary system. The downside is that I must use more blocks as a result, meaning more friction. The up side is that I can use much cheaper blocks. Also, since the extra blocks are in the secondary, where I'll be using small diameter vectran, I expect drag won't be an issue.

Anyway, your claim that the system will twist has got me riled, and I guess I'll have to build the system to prove you wrong. ;-) While I'm at it, I think I'll at the extra blocks to make this a 13:1 / 9:1 convertable system since you seem happy with yours, though mine will use 2:1+4:1 and 2:1+6:1 to achieve the advantage, where yours uses 3:1+3:1 and 3:1+4:1.

I'll keep you posted.

--Glenn

The system wil lfor sure load more to one side than the other, but it is not such a big problem, we have this problem on our system and it it fine but we have just tried to minimise the issue. Also it is only really adding friction right at the end of the travel when you have max sheet tension, you might not need as much as we do so you will not have the problem as much.

My concern is that when you sheet out the boom was partially move sideways and skew the pull of the lines off the vertical and have them wear on the sides of the cheek block or jam themselfs between the sheeve and sides.

But it is definately a good and simple design. Lets see if we can get it better still.

Wouter

I expect the boom to rotate around it's round gooseneck pin, so no problem there, I believe.



Thanks.



Definitely.

Thanks for the feedback,
--Glenn

Glenn,

did you ever get around to testing this system? Would be very interesting to hear about your experiences with it.

Glenn,

a few years ago I made a similar system for my old panthercraft, the only difference my system was 1:10, I think.

Nice setup!

How did you like the system? Is there anything you would have done differently?

No, It was working really great! It was one of the best system I have ever used.

Now I checked Glenn drawing again I identified a lot of differences, but that setup also could work.

Glenn, you out there?

I've fabricated the 9:1, and will post pictures soon. I'm very pleased with the result.

I haven't sailed with it because I have yet to fit an outhaul, and have been too busy to sail for 18 months (ouch!), but I have managed outfit my new mast, build a mast step with integral downhaul and mast rotation control, interface the new boom to the new mast, and build the 9:1 system. I hope to be back on the water soon. Hopefully before I have time to add a self-tacker and snuffer. ;-)

--Glenn

OooOOoOoo, you got to post pictures of it all!

Here it is.

I've not yet mounted the outhaul, or even settled on the design. Suggestions?

Oh, I've not cut the red line to length yet. I'll do that the first time I use the system.

Careful observers will note that the aft internally-mounted cheek blocks are co-axial, but aligned differently. Each is aligned inline with the expected loads. (One is vertical, and the other is tilted 45 degrees aft.)

The Ronstan cheek blocks were a perfect fit inside this Sailcraft extrusion. The aft large sheave is a Harken, and I used T6061 1/4" AL plate to make the aft fairing.

See my earlier diagram to get an idea how this system works.

All internal blocks are mounted with screws all the way through the boom, and internal Nylon spacers, to prevent line snags inside the boom.