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Mainsheet block attachment to boom
http://www.hobie.com/au/en/forums/viewtopic.php?f=14&t=7495		 Page 1 of 2
Author:  skipper0802 [ Sat Aug 04, 2007 8:08 am ]
Post subject:  Mainsheet block attachment to boom
Good Morning All,
Last weekend while out sailing I noticed that when I've fully sheeted-in (block-to-block, traveler centered), the mast is rotated to it's stop limit. I've three attachment points on my boom for the upper blocks from the previous owner. I'm thinking that the block-to-block condition is also shoving the boom forward causing the mast to rotate. Is this correct? As the wind was light that day, it shouldn't have caused the mast to rotate to it's stop. I think I should move my upper blocks to their "most forward" mounting position. Possibly there's a measurement from the front of the boom to the attachment point(s) that should be observed?

Comments anyone?

Thanks,
Eric
Author:  hobie1616 [ Sat Aug 04, 2007 11:55 am ]
Post subject: 
You want the mast rotated to have a good leading edge for smooth air flow. Look at the profile of a airplane wing for a reference.
Author:  skipper0802 [ Sat Aug 04, 2007 12:10 pm ]
Post subject: 
Yes, I do want the mast rotated as required. But I think the boom is getting shoved forward causing un-needed additional rotation. Do you think there's any way to test this?

Thanks.
Eric
Author:  hobie1616 [ Sat Aug 04, 2007 8:01 pm ]
Post subject: 
The mast should rotate to the stop on the mast base.
Author:  skipper0802 [ Sun Aug 05, 2007 12:04 pm ]
Post subject: 
More info...I measured from the mast rail to the center of the traveler and it's ~72". From the mast rail to the 1st of three boom tangs is ~82.5". The three tangs are spread about 4" apart. Please refer to my pictures below.
Image

Image

Referencing the assembly manual, figure 54 on page 19 shows the bitter end of the main sheet fastened at the center tang. The parts location illustration on page 25 shows two tangs with the bitter end fastened to the rear block. I understand these references are meant as guides. I also noted that my block arrangement and the two previous all pull the boom at an angle from the vertical. I think the horizontal component of this force contributes to inadvertent and undesired rotation of the mast as the boom gets pushed forward. If I mounted another tang on the boom so as to bring the block group over the traveler's center (where most of the time is spent anyway) I think that this horizontal force would be minimized or canceled. This should allow the mast to fully conform to the airfoil shape directed by the apparent wind's entry angle.

Another point is that when the traveler is fully outboard from center, the distance from the mast rail would be longer putting the blocks directly over the traveler.

I suppose experimentation is in order. :D I would appreciate any comments you care to render.

Eric
Author:  hobie1616 [ Sun Aug 05, 2007 6:11 pm ]
Post subject: 
When the main is sheeted in, the boom will move forward pushing the mast into a fully rotated position. If it looks like the mast is rotated too far then the mast base is probably worn. It looks like yours is an older boat. The base is probably worn and should be replaced.

Your main blocks look like a mix of Seaways and a 18 lower block/cleat. With the height of the lower block you're unable to sheet in far enough for optimal performance. Repalce all the blocks with a low profile 6:1 Harken set.

Double grommet your tramp. The boat will be stiffer and will sail better.
Author:  JRagg [ Mon Aug 06, 2007 8:08 am ]
Post subject: 
Is that boom off of an 18? I was under the impression that booms on the older 16s (such as mine) only have two block hangers. The hanger that is the farthest forward on yours is about where the farthest aft one is on mine.

I'm still sporting the original 5:1 seaways as well. As Hobie1616 said, the front block on yours is not from a 16. The 5:1 setup that I have has two single blocks hanging from the boom (one with a becket). The bottom block stands pretty close to vertical when I'm sheeted in hard.
Author:  Sail Revolution [ Mon Aug 06, 2007 9:45 am ]
Post subject: 
Somethings wierd. Measure the boom extrusion and post it here. Could you snap a photo of the gooseneck? We'll get to the bottom of it.
Author:  MBounds [ Mon Aug 06, 2007 12:34 pm ]
Post subject: 
There are several things going on here:

The forward two hangers are the original bales. The aft one was put on to accomodate an old style (circa 1980) low profile system (the blocks were bigger than the current systems and by tilting it aft, you were able to get another inch of mast rake). It also had the advantage of preventing the mast from counter-rotating - ever.

The mainsheet blocks are an agglomeration of several systems. Did anybody notice that there are 3 beckets in that system? (There should be only one). Regardless, the way it's rigged should work reasonably well for recreational sailing, unless the rotation stops are really worn.

Post a photo of the mast base / step so we can take a look at the rotation stops.
Author:  Skipshot [ Mon Aug 06, 2007 1:26 pm ]
Post subject: 
In the posted photos I've also noticed the bitter end of the main sheet has a stopper knot in it to keep the sheet from pulling through the sheave. Tie the end to the becket on the lower block instead.

And what's with the outhaul line? I suspect the boom isn't a Hobie boom.
Author:  srm [ Mon Aug 06, 2007 1:56 pm ]
Post subject: 
A couple comments-

-It actually looks about right to me. A set of low profile mainsheet blocks would probably be worth considering.

-Remember that as the boom swings out, the location of the bottom mainsheet block will move back in relation to the boom bales. If you move the bales forward, you may end up reducing (or eliminating) the forward pressure on the boom when you ease the sheet/traveler for dowinwind sailing. This could cause the mast to counter rotate.

-From the picture, it kind of looks like the boom is bent.

sm
Author:  skipper0802 [ Mon Aug 06, 2007 10:08 pm ]
Post subject: 
I haven't been back to the forum for a short bit. Some notes:

-this is a mix of blocks. the sheet bitter end is actually in bowline to the lower becket (examine the 1st picture).
-I'll measure the boom extrusion end-to-to end and post it.
-I'll post a picture of gooseneck too.
-The "outhaul" you see outboard of the last bail is actually a "topping lift" that I'm using to simulate the sail holding up the boom.
-She is an older boat....'74 or '76 if memory serves. I'm sure one of the previous owners filed the mast base stops to a certain degree. I'm sure that some of the rake settings I've tested while sailing have ground away parts of the stops. I'll post a picture of this too.
-Yes, it works quite well for recreational sailing...I've no problem cranking the boom down block to block. This was when I noticed the mast was shoved around.
-SRM's second comment has me thinking...I pondered about earlier
-Yes, I'm drooling over the 6:1 set I see in the catalog.
-The boom appears very straight when in hand. I think the digital photo doesn't lend justice here. I'll have to gage via a straight edge.

Will be back soon. I really appreciate your comments all!
Eric
Author:  Big Jib & the Uni's [ Tue Aug 07, 2007 5:47 am ]
Post subject: 
srm does have good points

also think about how the rig "stands up" when the jib is raised and tensioned.

Jeremy, please do give us a measurement to the bail on a brand new boom. I wonder if some older boats had differening bail dimensions?
Author:  skipper0802 [ Tue Aug 07, 2007 9:51 pm ]
Post subject: 
Here's a pic of the mast base/step. As you can see, about a quarter-inch has been filed away. The remaining corners of the rotation stops are chewed-up. This likely allows more rotation than I might want. Undue rotation can't be good for the mast tang and related rigging.

Image

Will post the aforementioned pics soon.

Regards,
Eric
Author:  skipper0802 [ Wed Aug 08, 2007 8:37 am ]
Post subject: 
More info:
The boom is ~110 1/2" endcap-to-endcap not including the gooseneck pictured here:

Image

The gooseneck does extend about 1 1/4" from the boom when pulled. I suppose this is by design.

Eric
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