Reinforcing a boom

[rpwillia]

A friend has a 49' Beneteau (really sweet boat) and he heard about some issues with the boom (some guy mid-Pacific busted his in half). He wants to eventually do some open ocean sailing so he has been looking at what he can do about it. Lewmar who made all the hardware gave him a nice deal on higher load blocks and that looks like that will solve that problem. The other issue is the boom itself which is made by Charleston Spar and they have a reinforcing kit that slides inside and is held in place with a lot of rivets. My suggestion for something similar was to add an additional bail and block and spread the load over a longer part of the boom. The disadvantage is that it will require more line, and tend to be slower to pay out and in.

I did attach a picture of the current set up and drawings of the possibilities.Thoughts?

 

[kenn]

Do the research. Really, how frequently do booms break, and on what make/models? Without more info, I would first look at hardware, particularly the gooseneck joint.

 

[rpwillia]

Trying to be circumspect

Do the research. Really, how frequently do booms break, and on what make/models? Without more info, I would first look at hardware, particularly the gooseneck joint.

Since there aren't all that many B49's out there and there have been one, that is cause for some caution. Researching the traveler and blocks revealed their rated load was 49% of the theoretical load the sail plan could put on the boom. New blocks solve that problem, but that leaves the question of whether to reinforce the boom with the kit or if spreading the load out on another 12" with an additional block and bail will be enough. The one failure was halfway down the boom not at the gooseneck.

 

[Ted]

Bending a boom is not as rare as you may think. Here are some popular problems.

Some boom sections were not designed to withstand the high loads that a loose footed main sail exerts on the outhaul slide. I've seen the outhaul slide pull free of the bolt rope groove when the width of the groove expanded under load. This happened to my Kenyon boom. The previous owner had two aluminum reinforcing plates welded to the outboard end of the boom at each side of the bolt rope groove where the outhaul slide sits.

Booms can bend at the vang attachment point. This can happen while vang sheeting or because of an uncontrolled gybe. An internal sleeve is probably the best solution for preventing this from happening again.

Unless the existing main sheet purchase power is insufficient, I would rather install an internal sleeve rather than additional blocks. The additional line and added friction will make easing the sheet in light air difficult, not to mention all that extra main sheet in the cockpit.

 

[Ray Bowles]

Ted is right on about the loads of loose footed mains. Adding the extra block simply spreads the load on the boom more evenly down its length. It does nothing to increase the boom strength. The boom insert actually increases the boom wall thickness and thereby strengthens the boom. If the extra line required for the extra block is agreeable with the owners sailing style then he can also do that improvement.

If the new inner liner is a very tight fit then I'd question the need for a lot of rivetts. A small number of rivetts will locate and hold the liner in place. It is an expensive issue to make the wrong repair so I'd encourage a lot of dialog before making the hard choice.
Ray

 

[Bill Roosa]

Put an engineer eye on your solutions and have the following comment.
the first (top) solution is a 4:1 purchass the other two are 6:1. You have to consider the other loads to get an idea of how the boom is actually getting sheared and bent but that data is not on the diagram.
Is the sail loose footed ?
where does the tack attach to the rig? At the mast or the boom?
What is the outhaul purchas? (2:1 would be typical but it is best to ask)

let me know and I can draw up the shear and bending moment diagrams and tell you where it will break. There may be some simpler sloutions than a boom insert.

 

[Bill Roosa]

For the record there is a very good reason you want to secure a boom insert with a lot of rivets. This is best demonstrated with two yard sticks. Place two yard sticks one on top of the other and let a large portion hang out over the edge of a desk. Try pushing up and down on them. Now clamp the free end so the top and bottom yardstick cannot move with respect to each other. Now try moving the pair. Pretty big difference aint' there. all that stiffness is due to the single clamp and if you work the sticks it will "tear out." Hence the need for a lot of rivets.

 

[Ted]

Bill, I'm adding an internal reinforcing sleeve to my boom this season. Do you have any suggestions as to attachment method, fastener spacing and fastener locations?

 

[FourPoints]

I ordered a new main last fall that should be arriving in a few weeks, previous sail had bolt rope on the foot fed through a track on the boom, new main is loose footed with a single 2" slug for the track at the clew.

with a 27' boat, should I be at all concerned with the additional stress on the end of my boom moving to the loose footed main? I would think not, but since it has been raised, it is only prudent to ask.

 

[FourPoints]

hmm, Just noticed that you have a 29 footer Ted, and your PO had problems... now I am a little concerned.

I'm going to be in Huntington on Sunday (Girlfriend's Mother's Birthday), Are you going to be at your boat working on Sunday? If I have time, and alright with you, I may want to pop down to Bay Shore to look at your boom to see how solid your Kenyon boom is compared to my Z/US Spar boom...

 

[Ted]

FourPoints, Come on down. Weather permitting I should be working on the boat all weekend and would be glad to talk about this issue with you. I will send you my cell phone number and we'll work out the rest later.

Depending on how your boom extrusion is designed, you may have reason for concern. A friend of mine with a 33 foot boat experienced the same problem last season. Many boom extrusions were not designed to take the additional clew load that a loose footed main exerts. My new boom is the identical shape as the old one but the detail of the foot groove has changed to strengthen that area. You can also purchase reinforcing plates specifically for this purpose or they can be fabricated by most handy sailors.

 

[Bill Roosa]

Hey Ted
I'd have to have the boom cross section info and all the sail, sheet and gooseneck attachment info to advise.
Generally you would want to put the reinforcement where the largest bending moment is as that is typically how they fail. I've never heard of one failing is shear. the largest moment is going to be dependent on the location of the sheet attachments points. That is USUALLY toward the middle of the boom but not always.

 

[rpwillia]

I know some will ask the rest

Put an engineer eye on your solutions and have the following comment.
the first (top) solution is a 4:1 purchass the other two are 6:1. You have to consider the other loads to get an idea of how the boom is actually getting sheared and bent but that data is not on the diagram.
Is the sail loose footed ?
where does the tack attach to the rig? At the mast or the boom?
What is the outhaul purchase? (2:1 would be typical but it is best to ask)

let me know and I can draw up the shear and bending moment diagrams and tell you where it will break. There may be some simpler solutions than a boom insert.

Bill, It has in-mast furling and the sail is loose footed. The tack attached to the mast. Have no idea what the outhaul purchase is - I will find out and re-post.

Here is some more info - dang I wish my sailmaker was so diligent. http://www.neilprydesails.com/pdfs/b49 Tuning Guide.pdf

 

[RichH]

For strengthening, the 'classic' remedy is to weld (tack stitch-welding, can be solid) a bar (or small C-channel or angle or 'bridge') onto the bottom surface and 'middled' to about 11/32 to 5/8th of span length of the boom.

A boom (+ sail), if not trapped overboard buried in a wave, will probably fail via 'induced' buckling failure ... the buckling driven by the max. outhaul tension putting the (simple) beam axis in compression, and the 'inducment' will be the side forces created by the mainsheet attachments (max reaction load) that creates the 'sigma' side force. Easy calculation using simple assumptions ... and your line drawings. If you have the detailed SPAR NUMBER or SECTION PROFILE NUMBER of the extrusion, would be even better; as the spar mfg. will have these buckling mode and other failure mode calculations, etc. already on file.

If the boom section number isnt available .... (later) if this boom is currently ON the boat suggest you crank on FULL calculated load and measure any deflections that are encountered. Support the boom aft end with a 2x4 etc. from the cockpit, put on full 'normal' winch load to the mainsheet, draw a taught string on the top of the boom from the gooseneck to the aft boom end .... measure the deflection (gap) at MIDSPAN of the boom in inches. Your boat was most probably designed with a mechanical Factor of Safety of 2 to 2.5 ... so we can 'back-calculate' the 'strengthening' needed, if any.

Best is to get the booms 'section number and mfg.' if possible from your boatbuiilder.

 

[Bill Roosa]

Hey rpwillia, with loose footed and in-mast furling I can tell you that the section forward of the most forward sheet attachment is in almost pure compression and will not be subjected to bending loads so you strengthen efforts need to be aft of that. I'd still need the actual dimensions or relative dimensions of the sheet and boom vang attachments points. Just the distance from one end to the attachment point and the boom length.
Is the boom a constant section or does it taper at one end?

 

[rpwillia]

Hey rpwillia, with loose footed and in-mast furling I can tell you that the section forward of the most forward sheet attachment is in almost pure compression and will not be subjected to bending loads so you strengthen efforts need to be aft of that. I'd still need the actual dimensions or relative dimensions of the sheet and boom vang attachments points. Just the distance from one end to the attachment point and the boom length.
Is the boom a constant section or does it taper at one end?

1. The mast is a constant section with no taper.
2. The outhaul is a sliding car with one floating block on the outhaul and a turning block at the end of the boom and an internal turning block at the gooseneck end to bring it down to the deck, and a turning block, at the base of the mast.
3. He believes the outhaul is a 4:1 purchase, but I have attached pictures he sent.
4. The sheet attachment points are at 74.75", 86.75", 98.75" (this is the bail where the bitter end of the sheet is) and 110.75"

I will be down at the marina and will probably meet him and can get other measurements if you need them.

This is almost as interesting as which is the best anchor.

 

[rpwillia]

More stats

Boom is 13' long
530 sq ft of sail area