Flight Risk needs a new booom, and I'm contemplating a T
shaped design similar to ones seen on ORMA 60s (see attached sketch). The original boom (see next pic) had to be lengthened when it's gooseneck was moved down 22" from its original position on the wingmast, to the main crossbeam, just below the maststep. This changed the angle of the underside mainsheet traveller (ala 18 meter square) that serves as vang.
The original boom was also strengthened (w/ a carbon tube bogged and slipped inside) at the same time it was lengthened, and still there was noticible flexing. So the compression forces seem big. (The main is an EP squaretop with a 34' luff and 11' foot). So now the boom's too heavy as well.
So the proposed plan is to vacuum bag carbon uni to the flat surfaces of a couple of Sitka spruce planks, cut to profiles similar to the sketch, screw 'em together in the T cross section, hi density fillet either side of the joint, layer of 2" tape followed by a 4" tape, tape all three edges.
So it's advice on the details I'm asking for.

Thickness of the boards (.75", 1", 1.25")?
Width of boards @ widest point (8", 10", 12")?
Weight of carbon uni (6 oz)?
Does carbon come in a woven tape (what weights)? S glass?
Are the tape widths proposed for joint reinforcements appropriate/sufficient?

I think the design offers a lot of strength and pretty light weight, and I think there's less than $500 in materials. But I'm open to other affordable ideas. I can get a 2.25" x 10' or 12' carbon spin pole blank for around $450 retail, but I still need to modify it for the underside traveller. And is a pole like that stiff enough for this application?
Other souces for carbon poles? Other concepts entirely? Baseless crticism and ridicule?
Thanks!

Dave

Here's a pic of the current boom.

Well, I do not have any advice on the boom but, wow, that boat looks nice. First of all what type of boat is that and is that a custom paint job?

Sorry to get off topic I was just impressed!

Wouldn't a triangle tube be stronger and stiffer?

Don't get hit in the head by that thing <img src="http://www.catsailor.com/forums/images/graemlins/frown.gif" alt="" />

If you attach the mainsheet to the sail via a loop similar to the 'A' class setup, there's no need to make a truss of the boom member.

Just an observation Dave, the tack of your sail (and therefore the set of your sail) would surely be happier if it was held in line with the rest of the luff, closer to the mast, by a slide in a track?

I'd go round carbon - www.forterts.com. Tony can engineer you the right size tube, and its going to be a lot less work than building a T boom.

If you need a custom attachment at the travelor end of the boom, I'd modify the Round Forte section by adding carbon and epoxy to suit your needs.

Or you can use a LOOP around the tube, and shackle it to the sail.

It seems to me the reasons the ORMA booms are shaped the way they are don't mean anything to the boat you're applying the design to.

If I understand it correctly, the flat section of the boom on such such boats serves two functions -

One is that it forms an end plate of sorts for the main. Unless the foot of the main is close to the flat section, there will be no end plate effect. The picture of your boat shows the foot of the main to be well separated from the boom. Thus no end plate effect, and your class rules may specifically not allow end plates or may calculate your sail area differently if they are used.

Two - the flat area makes a nice platform for controlling the bottom part of the main when reefing. Again, I don't think this applies to your boat.

I'd go for a carbon tube with the appropriate main traveler built onto it. As there are no benefits of the t-section for your boat, I'd think it would only get in the way, add windage, and hurt like hell when you hit your head on it.

But I could be wrong...

Can you tell us in what direction the boom flexes ?

Of what material is the current boom ?

My current analysis is two fold.

First where does the flexing come from ? With the boom fitted to the mainbeam there is absolutely no reason to have the boom under compression. On beach cats the boom is under compression to actively rotate the mast but clearly this is not the case with your setup. In effect you can just release the car on the underside of the boom as fixing it is absolutely pointless. The car on top holding the sail still needs to be fixed to set the draft in the bottom of the sail, however this loading should be nowhere near the level that is required to bend the boom under compression. And if it is then your boom has a serious design flaw; one that is not to difficult to solve. But more about this if this is truly the case, which I really don't believe.

I think the boom is flexing downwards because your forces don't line up well. If your leech car is much to the rear of your mainsheet car (on boom) then these loads create a bending moment that acts all along your boom. A first quick fix is to fit a slide to your tack corner of your sail and hold that corner close to the mast. It appears a couple of centrimeters can be won that way on the total misalignment of maybe 15 centrimeters. This is already a 15 % improvement.

Is it possible that you load up the boom and sail and give us a picture of the flexing boom. For I suspect the solution is really simple and easily homemade for 100 bucks.

If the misalignment of the boom cars are the reason for the flexing then what you want in the way of weight is a non-prismatic boom. Meaning one that is not constant in its crosssection. This means additional weight can be won with respect to a straight carbon section. Not too mention alot of money.

Pretty much the advantage of carbon is that it is twice as stiff for a given weight as aluminium. Aluminium and other metals are pretty much equal in their stiffness/weight ratio's. Forget about glass laminate as the has a worse ratio between stiffness and/weight then aluminium. This carbon advantage is ONLY attractive when a designer is seriously limited in the volume AND weight he can accept for a given element. If either one of these is NOT limited then their is no reason to spend more money on carbon as a simple adjustment of the design will result in exactly the same performance.

Homebuilding a boom for your boat can be as simple as getting cutting out some ply sides (non-prismatic) and just gluing two strips of aluminium to the top and bottom, closing of the box. For weight reduction you could cut large holes in the sides as was done at the rear of your current boom. At the point of the cars you'll just glue in a piece of solid wood (between the ply sides).

The advantage of this setup is that flat alu strips can easily be bend to follow the non-prismatic shapes you have cut in the sides. The ply sides are only there to stabilize the alu strips that take all of the load. Of course the alu strips can be replaced by carbon if you really want but alu strips is a simple way of quickly getting results by an amateur. The advantage of ply is that it is MUCH lighter then carbon laminate. Think 2.3 times lighter and don't underestimate the load baring properties of ply. In effect ply is nothing more then a naturally produced carbon based fibre matrix. Glues or screws are easily available to fit the box together. This much lighter ply can possibly allow you to homebuild an equally performing boom for less weight then a constant section carbon boom and for less cost. Professionally made custom non-prismatic booms are far too expensive to even consider.

Like I always say. Carbon is for lazy engineers.

Wouter

I like your composite boom design. can you develope it a little?
Carbon is for lazy engineers. Think I like that one even more...

I can but I'm doing this between two "really-must-do" design projects and another "really-want-to-do" design project. So I'm a little pressed for time. Meaning I will only persue it further if it actually is seriously considered to be build.

Ake, if you are going to use it on your mini650 then we surely can continue, just let me know.

I may have something you may like. You are going to use a heavy boom vang on the mini boat right ?

What do you think about a robust but lightweight hollow timber tube of varying crossection reinforced with two alu strips on top and bottom. It is made from curved timber planks. And then rounded on the sides by routing and a little bit of sanding. This leaves a flat surface (but curved lengthwise) to secure the alu strips to, you can also replace the alu with carbon cloth if that is what you want.

This trick (but excluding the alu or carbon strips) is used on traditional Dutch sailboats to get lightweight tapered timber masts without upsetting the traditional feel of the boats.

I love what you are doing with the mini, I regulary check your webpage.

Wouter

No hurry the winter is long, but I am quite serious about it. Are considering a wooden mast and boom for my PE 21.
I kind of like those little odd ideas.
Then I have also the plans for a Blade F16 coming from Down Under.
By the way, coming down to Belgium for Christmas.

Will the mast be rotating ?

Profiled or round.

I will type something up over the coming time and send it too you.

Belgium ? Well everything I got in the way of boats is tucked away in some commercial winterstorage. I live in the city. So I can't show you any stuff, but we can can drink a beer over discussing boat design.

Wouter

If you go carbon pole, I would use a 4" dia.

The compression strength is increased by the cube
of the increased area. 2.5 to 4 is huge.

I don't believe Wouters analysis of being able
to rid yourself of compression loading is correct.
Unless you can get the mainsheet to pull on the boom
at 90 degrees and you can get the clew to align
at that point (you can't) you are doomed to having boom
compression. No big deal. If your existing deflection
is minor, consider living with it.

Nice boat.

"Carbon is for lazy engineers"? Wouter, I would like to introduce you to Mr. Tensile Strength and Mr. Density .

It will be a rotating Wing profile. Like the idea of a beer.

Wow! That's a wonderful response! Thanks guys!
Here's more in hopes of answering questions and or clarifying things.
The current booom started life as a 2" hand laid fiberglass tube. Malcom immediately (after sailing it the 1st time), wrapped most of it with layer of Divinycell and another layer(s) of glass. The day the wingmast snapped (another story) I noticed the boom flexing to leward enough to cause concern, it was a 15 kt day, max. Part of trying to analyze why the mast broke where it did, Malcom and I figured it couldn't hurt to take the compression force off the mast and transfer them to the main crossbeam, which I'm pleased we did.
By dropping the boom down to the crossbeam, it needed 14" of additional length to have the outhaul work properly. Bogged up a very stiff, thick walled, 8' carbon pole and slid it inside. This did actually stop the flexing really, but at the end of the season I could see stress cracks in the paint in the area just beyond where the 8' tube stopped. So some signs of weakness, the wrong angle for the mainsheet traveller, and excess weight brought me to thoughts of a new boom.
Seems not everyone is catching the concept of the underside traveller for the mainsheet, which I like the theory of, and will try to relate as clearly as I can manage. Here's how Malcom, the boat's designer and builder, described what he was after.
His main goal was to be able to use the main traveller to spill power in the gusts, which allows the main to retain it's shape, w/tight leach, not twist off and spill air. He claims that with skilled crew running the traveller (4 to 1 purchase, continuous sheet), he could keep the boat on it's feet, accelerating in the puffs, not losing energy by heeling excessively or allowing the power to spill out of the main with too much leech twist (associated with the rise of the boom) when you dump the mainsheet. By dumping the main traveller instead of the mainsheet, with the underside traveller on the boom, the aft end of the boom swings to leward without rising (leech twist, spilling). The pull downward on the boom (and clew) is always perfectly vertical and constant regardless of how far sheeted out, neither adding nor subtracting from the boom compression.
So the theory sounds nice, but I can't say that I've been able to test it much, as I mostly sail the boat solo or with unskilled crew. And I don't yet trust the traveller as much as I do the mainsheet in a panic dump. It's still a new and somewhat intimidating boat when fully powered up, and I'm trying real hard to keep it upright , at least until I have a more comprehensive plan to right it than I do at this time.
John and others, good eye! Sorry for the sloppy rigging! I just threw the main up the day I pulled the boat, so I could take the picture to try to have a chance at determining the angle between the underside traveller and the top edge of the boom. The lowest sail slide isn't even in the track! I normally lash the tack directly to the mast when sailing.
Thanks for suggestions on pole diameters and sources, I will pursue those options too, though cost is, unfortunately, and issue.
Anyone else have thoughts on triangular sections?
Wouter, you once again are very generous with your thoughts, though I don't have a real clear picture of the design you're offering. Simple sketch maybe? Seems like the T concept I offered shares the intent of maximizing material where it's most needed.
Keith, your right in that I don't need any of the properties you mentioned, the endplate effect, sail storage platform, and ? . But it does seem to offer substantial resistance to flexing out of column, in both directions, with a minimum of weight, and within my modest skills. I don't like the windage either though. The design
does allow easy accomodation of the underside traveller.
Please keep the comments coming, I'm far from decided on the best course of action here.

The boat was built 5 or 6 years ago by Malcom Davy, Downunder Boatworks, Kelseyville, CA, a boat restorer who built it for himself, as a challenge and a project I think. It's 24' x 14', maybe 750 to 800 lbs now, between Malcoms early modifications and additions, an my repairs and reinforcements (ongoing this winter), cylinder moulded, canted hulls, assymetrical daggerboards (very effective!), sweet rudder system, 36' Barlowe design, Gougeon Bros technique, wood/epoxy (and now carbon uni) wingmast, and waycool central pod system for storage.
I drug the boat home to WA a year ago Oct, spent the winter sanding it, and had my car guy paint it. Corvette Yellow, Viper Red, $900 for the paint alone! Have proceeded to trash the paint job in one short season, shoulda found all the week spots and practiced docking FIRST, then painted it!
Overall, despite a lot of early breakage, I'm absolutely enamored with it! It has a powerful, silky feel to it in light air, strong creation of apparent wind, and produces plently of adrenaline starting around 14 kts windspeed. It's the challenge I was looking for , I expect to be thoroughly entertained for years to come.

.Dave 509 276 6355

Dave
You are my hero for singlehanding that beast. Wow.

I think most of us get the constant leech pressure idea. My Acat has a curved travelor that accomplishes the same thing, but with much more simplicity. The clew of the sail is attached to the mainsheet via a loop and none of the leech tension is transfered to the boom. The outhaul is attached to the clew, so the compression forces on the boom are minimalized.

N6.0 does this will a free floating car under the sail on a loose footed sail.

I think the issue you need to solve first is lining up your clew with the travelor so that the force is 100% downward. This will relieve pressure on your boom. From the pics you posted it doesnt look like the clew and the travelor are lined up. Recutting the sail might work. Or moving the travlor, or both.

Bill

Wouter

Thats the stupidest thing I've ever heard from you.

Bill

Dave, just a quick question. Your boat have assymetric boards, have you got much experience with other cats? If so, what would you say the performance and handling difference between assymetric boards and symmetric boards are?


If a boom can be buildt as stiff and light with a ply/alu composite, why use expensive carbon?
You guys should begin to get used to Wouters pointed and exaggerated style so you dont take stuff like this too seriously <img src="http://www.catsailor.com/forums/images/graemlins/laugh.gif" alt="" />