Building a new Boom

[Noah]

Ok guys, have at it.

I want to build a new boom for my Nordic Folkboat. The current one is bascially a 2 X 4, and is wobbly as hell. When sailing hard it almost seems that you get a standing wave in the boom. Scary stuff when that thing is flexing 8"-10" and going back and forth.

Also, I will be using the sail from a Melges 24 so I need to extend my boom 1 ft. It is big and roached out, and should give my under canvased boat more drive. My total sail area will now be around 325, up from 250. The melges main is 250 alone, and my jib is only 75 sq ft. Also, the melges sail is very flat, and has a great shape to it. Plus it looks cool.

So, how should I build a very strong lightweight boom? I'm open to anything here. I sourced aluminum T section, but it was going to run around $425 for a 12'6" peice!

It looks like I could get the Kenyon D-section boom for around $350 to 400. ( http://www.rigrite.com/Spars/Kenyon_Spars/D-boom.html )

Or I could make my own out of wood. What is the best way to do this, and what would it cost. I'm guessing that you would make some kind of a "T" section using sitka spruce, but are there otherways?

Go wild on this one.

Noah

[ 06-06-2002, 02:34 PM: Message edited by: Noah ]

[Ed Harrow]

This kind of reminds me of picking up a tired old (original) VW bug and stuffing it full of Porsche 911 motor. Two questions:

What is going to break?

How will the boat's balance be impacted, I think you'd be in for a very strong weather helm, perhaps not best for an older boat that needs a bit of work?

You could certainly build the boom out in a T section. I think Bruce Bingham's (?) book Practical Yacht Joinery (or similar) discusses building such.

[Noah]

Well, right off I don't think that anything is going to break! In heavy winds I will still use the Folkboat sail. Unfortunately often times the wind is right around 8-10 knots, which is great for sailing but slow in my heavy under canvassed boat.

Also, I don't think that it will screw up balance or add weather helm at all. The boat is very well balanced as it is, and generally speaking the jib doesn't do much, as it is only 75 sq feet. The Melges sail has similar luff and foot dimensions, the only thing is that it has a very large roach to it. I have already dealt with the backstay issue, so I don't think that there should be any problems. Plus I have the Melges sail and it was free. It is in much better shape than my Folkboat Main.

Noah

[Noah]

I should add that I have all new standing and running rigging, including a 5-1 mainsheet, etc. The standing rigging is slightly oversized as well. Also both the chainplates are remounted with longer backing plates to distribute the load better. Plus the mast is in great shape, and fairly new.

Noah

[John B]

I reckon you should make a box section hollow boom. A nice radius on the corners. A bit of taper end to end always looks nice. Fast .

[Bruce Hooke]

Well, going back to good ol' Skene's Elements of Yacht Design; he recommends that for a mainsail of 325 square feet the boom be a hollow box spar of spruce, 3 1/2" wide by 4 7/8" high at the mid-point, with walls 11/16" thick. The boom can taper to 0.7 (2 7/16" x 3 7/16") of these dimensions at the forward end and 0.8 of these dimensions at the aft end. Actually the maximium section can be a little aft of the mid-point (say at 60% of the overall length of the boom) so that the taper towards each end has the same slope. This is probably a conservative design overall. He says that in "competitive classes" the wall thickness is often reduced to as little as 10% of the maximium boom width, but I don't see any real good reason for you to cut things close and risk a busted boom.

This strikes me as too big a boom to go to a T-section. Skene's says to use a T-section or a solid boom for booms that are 1 1/2" wide or smaller, which definitely is not your boom...

Note that this all assumes that we are NOT talking about a roller-reefing main. They are a special case because the boom may end up on it's side at times and because the sheet must attach right at the aft end of the boom.

[Noah]

Thanks for the dimensions from Skene's. My main will only be 255, not 325, so I'm guessing that I could make those dimensions quite a bit smaller.

I would be into making a box spar, and I think that it would be fun. So Sitka spruce is the best, but if I can't find any what other wood should I use?

Noah

[Bruce Hooke]

Ah..that would explain why it seemed to be coming out SO much larger than your current boom. I glanced back up at your original post to get the sail area once I had Skene's open and I clearly didn't look carefully enough...

For a 255 square foot main it works out to a 3 1/8" x 4 3/8" boom with 5/8" walls. If you need more meat to fasten the sail track to you could go to a 3/4" board on the top side (which should, of course, be straight so all the taper is on the bottom side of the boom). The taper would still be to 0.7 of these dimensions at the forward end and 0.8 at the aft end. This is still significantly larger than your existing boom, which would explain why you are seeing so much flex...

[Bruce Hooke]

On the wood to use, the weight of the boom is less critical than the weight of the mast so unless you feel like spending lots of dough on Sitka I would go for either local spruce, if you can find any reasonably clear stock, or douglas fir. Some old time lumberyards still sell rough spruce planks (they used to be called scaffolding planks but I to avoid liability they don't call them that anymore :rolleyes: ).

[Jeff Kelety]

Noah - You just want the dimensions of my boom?

jgk

[Peter Jacobs]

Noah, if you're interested in the stock dimensions I have the original Folkboat plans. I could e-mail you the part showing the boom, plus the section of the class "rules" describing it.

-Peter-

[Carlsboats]

I would consider making the boom out of a birds's mouth, oval section -- light, strong, and simple to put together. Making an oval rather than round spar is no big challenge with the bird's mouth technique. To add a little more rigidity in the vertical plane, put a substantial spline under the sail track --say 5/8" high. And I agree with the posting above about not spending extra for sitka. A little weight in the boom can be a good thing,and you can get that by using slightly more than average wall thickness. Carlsboats