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So I'm building a boom. I have an idea how much stuff to put in it so it won't break, but it'd be nice to get some confirmation.

Any folks that have access to their boom (especially if it's carbon), could you lay it on two supports at the ends, put a known weight in the middle, and tell me the deflection?

To be useful, I'd need to know the distance between supports, the weight used, and the amount of deflection. This is with the oval boom in the vertical orientation.


I've got the tooling made, the foam cut, a test panel curing with +/-45 degree carbon uni on 4mm foam, and I'm ready to start laying up the four sides of this thing. It'll be fun to compare my deflection figures against the C-Tech boom (or any other successfully used section).

So if you can help, thanks!

I'll try to come up with an arrangement to check the deflection of my boom this week end. I have a Forte boom and 100 lbs of lead. To be accurate, I think I'll need more weight then that though. I'll see what I can come up with and report early next week.

Hi Chad, What shape? Oval? Have you thought about a triangle shape?

Tim, that'd be swell. Not looking for breaking load mind you, just the deflection against a known weight
-of course, it has to deflect enough to be measurable...

Rocky, it's a rounded rectangle shape, tapering in height at the ends. There's a drawing on my blog a couple of posts ago. A triangle would need more elaborate tooling to use the positive pressure / caul and clamp method that I'm wanting to use, to no significant structural advantage that I can see. I don't think twisting or racking will be a problem, which a triangle shape would address. Unlike big boat booms, by the time you get a sidewall that is robust enough to take some abuse, it is more than strong enough to handle its primary structural loads. I think.

I will get you some numbers off of my C-Tech boom tonight or Saturday. I believe I have a hundred pounds of leftover lead somewhere. It would be interesting to see the difference between manufactures.

So, for this bear of little brain, you are talking about measuring like this (below) at max. distance?

Yes, just like that! Thank you for finding the pic that shows what my many words were trying to esplain.

Just took a spare 15-20 minutes in Photoshop this morning to do the drawing.

But how to measure this? (e.g., create the orange line). Stretch a string real tight across the boom and tape it down, then measure?

8' framing lumber is going to be too short to lay on the boom, end to end.....Laser level?

Well then, extra thanks for going to the trouble!

I figured:
1) two saw horses, about 107" apart
2) place boom, wedge it in upright position
3) place tape measure on ground underneath with tape extended and locked, so it just touches the bottom of the boom. Record dimension where tape disappears into spool
4) add weight, in sufficient amount to make a small but measurable deflection, say at least 6mm or 1/4"
5) adjust tape so it just kisses the bottom of the boom again, and record new dimension.
6) subtract the two measurements.

I use Stanley tapes in the silver case, and they will allow me to clamp a setting, then they will hold the setting as the tape is pushed down by the deflection. Depending on your own setup, some variation of this ought to work. Worst case, just put something on the ground under the midspan, and take two measurements from the bottom in the deflected/non-deflected conditions.

No need for expensive new lasers, unless you were just itching to add one to your arsenal!

I finally got some deflection measurements on my boom. Distance 107", Weight 100#, Deflection .120 in. or 3.04 mm.

Thank you, Jon!

No progress on my boom today- just another sanding and bog session on the deck. Thursday's boom sidewall test panel turned out well- good toughness (it'll easily survive a smack to my noggin, or similar abuse), and the weight came in just below expectation. So I have my sidewall base laminate schedule decided, I think I'm comfortable with the top and bottom schedule, and I'll keep thinking about any additional local reinforcements. I hope to do the first of the four laminations tomorrow.

Last lamination yesterday, pulled the wraps off this morning. Looks the part, just not glossy, nor perfectly smooth- my top and bottom surfaces overlap the sides a little, so the sides won't be flat unless I add a crapload of filler:



And the weight at this point:

So I'm .4 pounds over the C-Tech boom, although I still need to cut about 2" off each end. That will put me about .2 pound over C-Tech, although I'll have a lower tip weight due to the tapered shape and laminations.

The real test will come when I test load it, but that needs to wait a couple days- the temps have gone chillier here for a few days and I need to let it cure more thoroughly. It's too big to fit under my heating blanket, so I've stuck it up by the ceiling where things are always warmer...

It's been lots of fun making, now to wait and see if all for nought!

Well, as my bicycling magazine pointed out, you can almost always fix a carbon part if you are wiling to grind out the bad and replace with new. And I think that's a skill we all have in spades around here.

Looks great. k.

.25" deflection with 180 pounds resting in the middle. Pretty close to the same ratio Jon reported.

My belly is sore from resting 180 pounds on it while measuring the deflection...

Hopefully optimistic that this will work. It feels a little soft side to side, so I'll need to decide if it's worth the weight to add a strip of uni down each side. Since I'm not end boom/bridle sheeting, there will be some side bend induced. It's hard to know how much...

Your right in there on defection. Go with it! If side deflection is noticeable when sailing then address it, otherwise have fun. Easy for me to say as I tended to over build some myself. Well done!

Edit: I used 100# weight for my numbers. Not 180# you might be closer to the same deflection then you think.

I can get you side deflection numbers if it would help.

jray wrote:Your right in there on defection. Go with it! If side deflection is noticeable when sailing then address it, otherwise have fun. Easy for me to say as I tended to over build some myself. Well done!

Edit: I used 100# weight for my numbers. Not 180# you might be closer to the same deflection then you think.

Yeah, that's what I meant- your .16/100 is real close to my .25/180. If you can get some side deflection numbers, that'd be swell too. Don't go to any trouble, though.

I'll start with some light air days I'm sure- can't help it here in the summer- so I can keep an eye on it. Yeah, for now I'll go with it.

Kinda wanted to sand and clear coat it now though. Feels better to rock up to the fishing ramp with a big shiny stick, than a dull industrial tube, ya know.

No problem, I will get you some side numbers tomorrow afternoon. Hate the thought of you showing up for the first launch with a dull boom.

My numbers for the 100# down deflection were .12 if that helps some.

Side deflection at 107" and 100# is .30

Thanks. I know already that's lots stiffer than mine, sideways. Hmmm. I'll get a measurement and decide form there.

As a resource; Jim Lee of the left coast Dart project is starting to make noise about making 2" x 4" booms. He's going to use infusion and high pressure air bag to compress the laminate.
He rang me the other day to ask how long to make his mandrel so he could cover the i550 crowd.

I'd like to see his results! It's been fun to follow his boat (and other stuff) build for the last few years. Haven't seen anybody else infusing carbon though, although I don't get out much...

Finished boom. I added a layer of uni to each side, and it now supports my weight sideways easily enough. After cutting the ends to length, cutting in the outhaul exit sheaves, and gluing in the gooseneck tube I gave it a coat of clear epoxy to jazz it up a little. No need to fear condescending looks at the fishing ramp now!


Outta batteries on my scale, will report the final weight soon.

Edit: 6.7 pounds.