Mid Boom Sheeting - need help with attachment points

I am converting to mid boom sheeting on my custom built 42' steel hull cutter and I need help figuring out where best to attach the 3 blocks.

The old end boom system ran directly through the middle of the cockpit and behind the binnacle/helm, making for a real knee banger. I also want a more comfortable cockpit arrangement and to be able to leave my bimini top up full time.

My boom is 19' long and the new traveller location is just over 10 feet from the mast. So - just aft of center on the boom. The Mainsail is 475 sqft which gives me just under 3500 lbs of load in 20 knots fully unreefed. I can't say I'd ever run fully unreefed in 20 knots so I like this number.

I figured on going with 3 boom attachments using 3500lb working load blocks on each. Splitting the load in half, I think these should work fine.

My question how do I figure the best boom attachment points? Do I use an angle of the sheet to the block? Should I offset the 2 aftmost blocks so that they split the center of the traveller?





I also have a garhauer solid vang that I was thinking of adding later on. Does this have an effect? I was reading about problems with mid boom systems and solid vangs.

See more of the boat here.
http://www.steelbluepearl.com/


Thanks in advance for your interest and help.

 

I want to konw too.

 

You could use a rolling yoke, like a capital C with the opening upwards and rubberised rollers at the open ends, slide along the boom to the approximate area and it will settle its place, the bottom of the C has the attachments for multiple block/s

I have seen them but don't know if designed to meet your load? - - The C part as I recall was made of cast aluminium or magnesium-alloy

 

masalai,do you mean the tralveler "c" type?

 

Possibly, I have only seen it a couple of times and thought a boom end position to be more appropriate and efficient - - - requiring less human effort to haul in etc as far better leverage option and no significant loss in wind-age or of weight of boom and attached fittings - - - AND - - - a boom that was inherently more robust because it had no holes, fittings for mid-position stresses...

Just my in-expert view and observation - ask a rigger for a definitive answer...

 

Yes ,we all think it is a reasonable way to put the blocks in the mid of the boom,but I have seen the book of " the priciple of yacht", said that a certain scale should be kept ,so I have doubt about the position of that ,except the strenth of the boom,any other reasons?

 

where the sheet blocks can be placed "on deck"? and access to main-sheet winch.

I do not see any significance in difference of strength of boom. You may also be loosing a lot in running the sheet back to the mast and then aft to the winch/cam-cleats and the additional losses in "spreading the load" - - consider the losses in 2 separate 45 degree hauls - for what advantage?

If the sail is well cut then the load is spread and best taken by the vang (a huge load) and the boom end attached sheet blocks, leading down to the track set on deck for the traveller system. - - - If the mainsail is old and a bit "baggy" then loading will be more to the ends? - - - but then I am no expert, just an interested observer, - who would like to encourage you to think it out and analyse the options and ergonomic efficiencies....

 

you would be better off using two triples
how much room is above your head in cockpit? could you do this? the designer originally had the traveller through the cockpit!!! I designed the arch, works well if you have the room over head
you can see the boom brake, essential stuff, I had never seen one til I came to Australia

 

Thanks LJ, - - I bow to your superior knowledge - I was struggling somewhat. - - - would you care to assess and correct my posts in this thread, if in error?...

 

Awsome rig Lazyjack! Looks comfy.

So are you suggesting I run one triple block attached on the boom directly centered on the traveller (traveller being the second triple)?

Would it not be stronger to split the load over at least 2 boom attachments? I hate to make swiss cheese of my boom but....

 

I understand the advantages and disadvantages of mid boom sheeting. That isn't really the question.

What I'm interested to know is if there are any set calculations I should use in determining the placement of the blocks on the boom.





I think LJ might be suggesting this rig;


There are many options to choose from as you can see from this Harken page;
http://www.harken.com/rigtips/mainsheet.php

 

Example

Pardon my poor artwork, could not find a picture that was clear. According to the Harken website and if you think about it, moving the sheet to mid boom will double the forces on the main sheet (halving the moment arm). To gain mechanical advantage most mid boom boats use a multiple pulley set up. In the picture, the dotted line is the mainsheet. The solid is the vang. The tail of the mainsheet is attached to a becket on a double that is attached to the traveller. It goes vertical for one loop, then aft around a second block, back to the double then forward up through a block then down to a turning block at the base of the mast. This last leg can be changed to suit the boats layout, but generally the companionway top and/or hatches get in the way so routing to the mast keeps the sheet clear of things. The distances between the fore/middle and middle/aft boom blocks are the same. I don't think the exact distance is that important, but don't want to make the angles at the double too big, want most of the pull to be vertical rather than fore/aft. The blocks and soft vang are attached to large bales that are attached to the sides of the boom so that that the attachment bolts are in shear rather than relying on skinny threads in aluminium. A hard vang would be bolted to the bottom of the boom.

 

I know the dodger plays a role in where the foreward block can go but having no dodger yet I am a bit flexible on that.

Yes - I will be using boom bails. Matter of fact I had trouble finding 4" wide boom bails so I wound up ordering these Boomkicker Bails.


Here's the blocks I ordered for the boom attachments;
Garhauer Blocks

My first go round with Garhauer so - we'll see.

I have an email in to Harken technical advice so I hope to hear back from them soon.

 

Garhauer is a great company to work with. The equipment is top notch, customer service second to none. Give Mark a call, fax or email diagrams/pictures they will come up with what you need. It is factory direct, you might have a slight delay if they have to make something.

If you haven't done so yet, walk around some of your local marinas and take a look at how other boats are rigged. Might give you some insight.

 

There is no set rule for spacing the blocks on the boom.

It has to do with the beam strength of the boom and the local wall thickness and attachment method.

If the boom has enough beam strength so the entire downward force of the mainsheet can be taken at the vang attachment location close to the mast, the mainsheet does not need to provide *any* downward force.

If the mainsheet is attached at the clew of the sail and lead aft, the boom is not required at all ... like a jib.

When the sheet is lead vertically, the boom is required to take the horizontal load and the boom is in compression (force toward mast)

When the sheet is moved to the mid-point on the boom, the compression load does not change, but a bending load is created and the sheet load is doubled. The bending load is equal to the increase in sheet load.

At the vang point, the sheet load is higher again and the bending load higher still. If the vang does not lead vertically to the deck, it also creates a very large compression load in the boom and can actually bend the mast forward at the gooseneck. The bending load is also increased.

If the boom is strong enough not to fail under these loads (most are these days), the question becomes how much point loading can the sheet attachment point take? If you use eye straps and screws to attach the boom block or blocks the limit is the tensile strength of the boom wall and the screws. To increase the safety margin it pays to distribute the load over several attachment points. Better yet use a loose footed sail and strops to hang the blocks and spread the load over a greater area and eliminate the stress risers at the holes you just drilled.

The argument for three blocks rather than a single triple is the internal friction of a 6:1 tackle. It is next to impossible to keep the lines from touching and adding friction when the sheet is trimmed or eased.

I agree that Garhauer is the biggest bang for the buck in traveler systems and the quality is higher than you would suspect for the price.

Here is a quick and dirty sketch to show the loads.

Cheers,

R