Mainsheet Blocks & Routing

[David Sanner]

I've been thinking about venturing into the world of boom bail placement, and mainsheet routing.
I think the original routing on the MK I has a lot to be desired as well as the placement of the boom bails.

My Mark I setup (see photo) is a 6 to 1 purchase (or 6 1/2 to 1 if you count the sheet going forward).
The sheet starts at a beckett on the boom and winds up at block at the base of the mast. 
From there it is run directly back to a fairlead under  the traveller track and then through a rope clutch. 
(The original ran the sheet to the gooseneck then to the base of the mast and then trough the
deck organizer... loads of friction.)

The Mark II booms (which I assume are of similar strength) only have two bails for the mainsheet
each with a double block on them.  It is also a 6 to 1 purchase (though maybe a 7 to 1 if you count
the two lines that run forward).  I like the routing much better because the sheet runs straight
back from the block on the cabin top.

I've been looking at my boom and the sheet angles and I keep thinking the MKII  looks better.
It's a trade off between how far aft the bail is and the angle the sheet makes with the boom (pulling
straight down is better than at an angle).   Seems to me the Mark II has a slightly better purchase since
each end of the mainsheet goes through a block on the deck forward and I think it might be
slightly more effective because their are four lines on the aft bail, the bail that has the most
effective pulling angle.  (The aft most bail on the MKI, even though it's further out
is less effective because some of the force is wasted pulling the boom forward instead of down)

Though Catalina may have made the change from three to two bails to save money I
think they might have decided that the booms are strong enough to work with only two
bails and that this setup might also be better, more efficient.

There are a lot of factors from the strength of the boom to the force on the gooseneck so
I don't think I'm going to make any changes without taking a few more measurements
and asking a lot more questions... & probably calling Catalina... but I'm wondering if anyone
had any other thoughts on the subject?   Like, the 8% gain is not worth the
trouble ... or I've routed my mainsheet years ago like the MKII and just use the
forward two bails. 

[rirvine]

David:

First the standard MKII arrangement is 1 block aft on the boom, 2 blocks on the traveler, and 2 blocks forward on the boom - 6 :1.  2 blocks, 3 blocks, 2 blocks would be 8:1  

The MK I arrangement is more efficient as the center blocks pulls almost at right angles to the boom with the other two to distribution the load along the boom.

The two blocks on the MK II boom are approximately in the same position as your forward and aft blocks.  Assuming the booms are the same, there should be no trouble is changing your arrangement to use the forward and aft blocks on the boom only.  I would not recommend using the forward and center blocks on the boom as that leaves considerably longer length of the boom unsupported.

There is an easy way forward – try it.  It the boom does no break all is well. If the boom breaks replace it with a MKII boom and now all will be well.

Good luck, Ray

[David Sanner]

Ray:

I was hoping someone else had already been the "guinea pig." 

Perhaps someone at least replaced their Mk1 boom with a MkII boom and heard
something about the different design.

I believe you have the 2 blocks, 3 blocks, 2 blocks or 8:1  purchase.  However the two lines
that go forward to the cabin top pull at less than a 45 degree angle to the boom so I'm thinking
they don't contribute nearly as much as the ones on the aft block so effectively it's less.

The mechanics are tricky, especially as shallower angles (tension)... and they
change as traveller car moves outboard or the sheet is eased.  The boom is being
compressed by the forces from the sail and being bent (compressed on top, stretched
on the bottom) by the mainsheet blocks pulling straight down... and compressed and bent
by the aft blocks.  I'm sure rigging designers have a not so simple simplified diagram
of the forces on a boom... and like everything else, there are trade offs.
(Anyone know a good book on modern rigging design?)

I've been looking at a few pictures of the MkII and it looks like the aft most block is attached
somewhere between the aft two on the MkI but closer to the middle.  The forward bail on  the
MkII looks slightly forward of the MkI forward bail.  Should we get our measuring tapes out?

I guess I'd like to make it easier to sheet in by reducing the friction in the blocks, fair leads
(better routing) and make the purchase that I have more efficient by attaching the bails
and deck blocks in the best place.   I assume Catalina refined their rigging with the MkII
but maybe it was just to save money or aesthetic reasons.

I think I'm going to start with a protractor and a tape measure but maybe I should just
buy a new 3/8" hi-tech mainsheet and some teflon spray start thinking about something else?

[rirvine]

David:

You are correct, Crew's Nest does have 2, 3, 2 blocks for the mainsheet but that is not the factory standard.  Yes, boom end sheeting is the most mechanically efficient but it requires the traveler to be in the middle of the cockpit - taking up too much valuable room.  It also increases the bending load on the boom because the boom is then only supported at two points - both end.   In your case the bending load is reduced because the boom is supported at 4 points leaving just the aft 3-4 feet unsupported - hence in bending.

Ray

[Ron Hill]

David : I don't believe that you'll gain that much more by doing a change from what you already have.  Beside the holes are already drilled and the three bails should distribute the load thru the boom better than 2 bails.  I'd leave it alone.

One thing that I'd recommend is that the blocks on the boom be NON swiveling.  Keeps the lines from twisting the blocks.   

[David Sanner]

Thanks Ron... I think I am going to keep things as they are though it would be slightly more
efficient to pull down from a spot about 6" behind the middle bail. (the bails are about 20" appart)
If I moved the two aft bails to the ideal spot on the boom it would only increase the sheets
efficiency by 5%-8%...    not worth it for me.

Here's the work:

If you like this kind of stuff here a bit of analysis for effective downwards force on the clew.
(overly simplified, treating the boom as a lever arm, downward clew forces only)

I looked at it like this, the closer to the clew (further out on the boom) that you pull the more
force will be applied to the clew but the further aft you pull thanthe spot directly over the
traveler the less downwards force you are applying (because you're pulling at an angle)

Turns out most efficient/effective spot to pull down from a traveler that's 70" from the mast is
on the boom 80" from the mast.   This spot applies the most downwards on the clew for a given
force on the line... the effective force turns out to be a bit over half the force on the line.

Doing a bit of basic geometry I came up with this:
Effective Downwards Force @ clew = (EndOfTraveller + x)/LengthOfMainsailFoot * cos(atan(x/heightOfBoom))
where x is the distance away from the traveller (x=0 is point on boom directly above traveller)
Effective height of the boom (from where the bail connects to the bottom of the block on the traveler is about 27".

Using gnuplot I plotted the above equation with the following values.
plot [-15:25] (70 + x)/144 * cos(atan(x/27))   (see attachment)

The bails are currently at 55, 75 & 95" from the mast or -15, 5, 25 in relation to the traveler position.

So from the graph you can see the most effective spot is about 10" further out than the
traveler or 80" from the mast.

Ok... enough of that.

[kurt]

My Mk1 boom snapped into two pieces at the 3rd bail this past weekend and will be doing the upgrade to the Mk II boom which will have two bales.

When referring to two blocks in this thread do I need to get two blocks for each bale connection or 1 double block per bale?

Also, the Mk II boom will have internal reefing that has the lines coming out of the top of the boom at the gooseneck side to run to the reef cringle and back aft for single line reefing.  Personally, I like my current MkI reefing setup which uses two lines and keeps the lines out of the cockpit.  I suppose I can take reef 1 aft to the cockpit and possibly leave reef 2 up front and tie off on the cleat on the mast that the Mk1's use for the reefing cringle line.  However is the winch helpfull when using a single line reefing when led aft or not necessary? 

Thanks

Kurt