Understay for forestay support

Option A sounds like a simpler and possibly more reliable solution to me.

It also seems to me that the typical beach cat spin pole may be a bit undersized to deal with the loadings being placed on it in the configuration for option B.

That is a concern, namely, the arrangement in option B would place large compression loads on the CF pole, something CF isn't particularly spectacular at handling.

or is it torsion CF struggles with? i always get those mixed up.

In any event i could always run an aluminum tube from the front beam to the forestay, hook all my wires to that, and slide the CF tube inside.....

i'm just really not crazy about fabricating a 112 inch long bow foil. haha

You might considering a system similar to what the supercats use. Seems much easier to make at home. You would have to modify your jib some, but not much. You may be able to see some of it here.

Basically you made your bridles wires as you would in a non-NA rig. Use a SS rod goes below your forestay to a ring around the spin pole. A pair of cables locate that ring to either side using the bridle tangs.

http://www.thebeachcats.com/classifieds/catamarans-for-sale/p13103.html

Yeah i would, but because i am setting it up to single hand, i really need a furler, which wont work with that configuration.

seems like a large job and a fair amount of cash needed (new extrusions, new tramp, etc)

why not just buy a 18square, super cat 20, mystere 6.0xl or other wider boat?

Well, i have the 5.8 and its ready to go, besides 18 square masts can't support a jib, much less a chute.

I need a new tramp anyways, the old one is tearing apart. I have a ton of sailboat parts in my inventory. everything from cabling to spreaders to you name it. I also have a metal fabrication shop. Everything from welders to mills.

take out the cost of the tramp (i have to buy one anyways) and all the parts and pieces i already have, and this will cost me 350 bucks. <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

The idea is to do it in such a way that everything (except the added boom) can be removed and the boat can be set up as a stock 5.8 again.

all sounds like great reasons to me. especially,

I also have a metal fabrication shop

take lots of picks, I would love to see them

Where do you sail?

I am accumulating the parts and pieces and finalizing the design in sketchup. should be underway in the next week or so. Maybe a build thread would be in order... hmmmm

It looks like the aluminum sprit idea will save a fair amount of weight vs. the bowfoil, and what weight it does have will be further aft. I do think it is prudent to use an aluminum extrusion rather than rely on the carbon tube, 6 pounds or so is a small price to pay for piece of mind...

The boat is in Muskegon, MI.

Then you should have plenty of time to get this sorted out. At the rate we're going, ice probably won't be off the lake until June.
It's been a long time since I've seen the big lakes ice up like this.

Quite the winter we're having!

First update:

I was able to gather all of my supplies and inventory together. I have enough blocks and cleats to hold me for the next decade i think. I also gathered together all of the rigging i had leftover from an E-scow project several summers ago. I will be using that for the spinpole support. the workshop was adequately messy, so i decided to get started.

The first thing i decided to do was to build the bracket which will attach to the end of the dolphin striker bolt. The DS bolt will go through it. The rear section of the understay will attach to the back of it, and angle upwards to the rear crossmember and the aft end of the spinnaker tube. Then the front section of the understay will attach to the front of the bracket. (or understay backbone) two cables will attach here. One will lead directly under the jib, furler and forestay, and the other will lead through a spreader extending down from under the forestay, and forwards to attach at the tip of the pole, underneath the spinnaker. My earlier post has pictures of a similar design.

Right. so i had to make the bracket. The first thing i did was to determine the target distance for the bracket to protrude underneath the front crossbeam, assuming it would be immediately above the DS strap. (putting it below would require drilling a 9/16 hole through a 1.5 inch aluminum strap. too risky) The objective was to maximize the distance down to increase the amount of vertical downward pull on the forestay, while minimizing the extent to which it would protrude under the beam, to avoid hitting many waves. Taking into account the deflection rates of the selected spinnaker pole, i plotted the curve of force vs. angle of shroud, and decided on 10 1/2 inches depth of DS strap, and roughly 10 inches down for the understay backbone.

Having arrived at that measurement, i was able to plot the triangle created by extending the rear understay to the rear beam from the DS backbone. I then determined the angle to be approximately 8.5 degrees.

I took 2 pieces of 1x1/2 inch steel bar from my inventory. I bent one of them to the 8.5 degree i needed. This way, i can use the old staymaster adjustable shroud i have in inventory. I the took the second piece, made it shorter than the first and placed it on top, so the angled section extended aft of the stacked part. Then i welded them together all the way around. I grinded the weld filets smooth, and drilled a 9/16 hole in the center (for the DS bolt) and holes in the front and back for the fore and aft sections of the understay.

Then i painted it.

Then the truck from ALRO steel showed up. I recieved my raw materials for the new crossbeams.

I recived 1 24 foot 4" x .125 wall aluminum tube, a 24 foot 2 inch tube for the spinpole, an aluminum strap for the DS, and some other odds and ends i'll get to later. Total cost was 345 dollars delivered to my garage.

Not wanting to waste time, i dug in immediately, and cut the 4 inch tube to size, which happens to be 11 1/2 feet long.

I then removed the front beam, so that i could use it as a template for drilling all the many holes i would have to drill.

After i removed it, i brought everything down into the basement shop and began measuring, and remeasuring, and then measuring again. First up was the center DS hole. I was careful to us several gradations of bit size to ensure a clean centered hole. I was able to maintain a tolerance of <1/16 in.

I then moved to the DS strap. I retained the fitting from the old tube which is basically three bolts welded to a stainless plate. It was alot easier to use this kind of an arrangement as the alternative would be to make up three compression tubes, and have 3 through bolts on either side.

After alot of drilling i was able to get the new strap mounted, and cut to size. The objective was to have the end of the strap be exactly 12 1/4 inches from the end of the tube to ensure a proper fit on the hull, my tolerance was <1/16 of and inch again. With a sigh of relief i moved on to the bolt eyes.

In order for the understay arrangemnt to work i needed side to side support for the pole to have the whole thing work sans bridle foil. I got 2 grade 9 bolts and 2 screw on eyes, and attached them at both ends of the pole, just above the DS strap. the eyes have a breaking strength of 3800 lbs, so i should be good.

Finally, i pulled everything off, and build compression tubes for the DS bolt, amd the two pole shroud support eyes. Getting the compression tube for the DS centered on a 9/16 hole, 6 feet down a 4 inch tibe was.... interesting....swearing was involved.

After that i put the whole thing back together, tensioned the DS to put prebend in the tube, and the front crossmember is done!

Note- i selected a DS bolt with 180,000 PSI tensile strength, 2.4 times stronger than stock, which should handle additional loading from the design.

That's all for now. I'm having a party on saturday, and will be enlisting the attendees to help me install the finished cross member.

lookin good!

Making progress. Got to the point of doing a full mock fitting using lines instead of cables, and a board as a spreader. Still a lot of details to finish that's for sure, but everything has fitted together exactly as planned. No surprises.....yet.

More pics

The spin pole appears to extend to the rear beam...is that temporary?

Looks like the DS rod goes through the pole, seems it might see some interesting loading.

The pole is permanently affixed to the rear crossbeam. You will notice that i mocked up the understay system with extra line i had laying around. a cable goes from the back beam to the bottom of the DS, then two cables extend from the bottom of the DS forward. one goes under the jib, and the other goes through a spreader to the pole end. Thats my downhaul for my jib, and my chute.

Because of the loading angle, pulling those cables tight will not only pull down, but also back, and that's why the pole goes to the back beam. It has been shaped and sets on the back beam flush, and will be bolted secure, allowing any y-axis loading to go through the center pole in column.

The DS rod WILL be getting loaded up. but the loads will be directed THROUGH the bolt on the Z-axis (understay pushing up, mast pushing down) not across the bolt on the y-axis. to ensure that, the pole had to go all the way back.

Also like i said earler, that DS rod has an 180,000 PSI tensile strength. Stock Stainless has 75k.

There are advantages to having the pole there too. I am setting the boat up as a singlehanded boat. the pole gives me a place to attach my mainsheet, and using thru hull blocks, i will have my spin halyard and retreival lines on the center pole via flip flop blocks. a few other controls will be routed there as well.

Huh. that's different. I'm still trying to wrap my head around the advantages of that over the traditional method of pinning the spin pole to the leading edge of the front beam and supporting it from the forestay. With the spin pole running all the way to the rear beam and through the dolphin striker rod would also be pretty tough to disassemble for trailering but it doesn't sound like that's on your list of needs.

Any side to side or fore/aft moments induced by the mast are usually carried by the front beam and how it's keyed into the hulls so the need for a pole there were eliminated in the 80s. The early versions of the Nacra 5.2 (I had one) had a bar that connected between the front and rear beams to prevent the beams from rotating. It sat in the middle of the trampoline and was commonly referred to as the

knee knocker

. It was pretty obstructive. Later versions had the front beam pinned to the hull straps (literally a rivet that was drilled through the straps and into the beam) in order to carry the moment arms induced by the mast loads and prevent rotation.

One concern I would have with that pole, and this is from some direct experience...is that in a capsize, it looks like it's possible to slide your leg(s) between the trampoline and that pole and get trapped (and/or injured) there. That would be my biggest concern.

how are you incorporating that lawn mower?
my guess is you will be pointing the blades aft?

What is going on at the DS/spin pole intersection? Do you have any sort of reinforcement? My thoughts were that the pole might want to move around the DS rod and cause a weak spot.

The pole flexes and moves on my boats, with only one traditional hard point that's not much of an issue. I think you would have to be more careful having 2 hard points.

Jake,

I hear ya. The reason i need to bring the pole all the way back is to bring the loading securely back to where it is originating, at the back beam attachment point for the understay.

This is more a safety measure than anything else. Keep in mind that because of the pretty severe angle (i didnt dare drop the DS any lower) there is going to be alot of tension on the y-axis to get enough force down on the Z-axis. when i say alot, i mean upwards of 800+ lbs. I did not want this to simply be absorbed by the front beam, the flex from the beam alone could make tuning a bear. Instead all that force goes down the central member, to the origination point of the loads. That way it keeps the

collateral

y-axis forces isolated from the rest of the boat, and rig, while retaining the z-axis loads placed under the forestay.

Check out the GC32's, or the AC45's they did the same thing, for the same reason. with the small exception of using carbon.

I had not thought of getting trapped. perhaps i can have a sleeve sew into the tramp...

as for the lawnmower, i plan to attach a 30 foot articulating sprit to the base of the mast, supported by a boom and truss system tied to the mast head, which allows 360 degree movement using an electric motor and swivel. The

chopper

will automatically engage at the 1 minute gun, clearing a 30 foot circle around my boat, and guaranteeing me a slot to leward at the start. but i'm in touch with RC to see how this will affect my D-pn.

yes it is reinforced. there is a compression tube inside the spin pole just like there is in the front cross member. Additionally, there will 1/4 inch thick by 6 inch long by 1 inch wide plating on the top and bottom of the center pole to spread loads. the pole itself will be bolted onto the DS rod quite tightly hence the compression tube. it shouldnt move. <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

2 hard points was a challenge though you're right. i had to hand shape the back end of the pole to make sure the span was perfect, with load on the back, and the DS hole centered...

Do you have a Y structure rigging going back to the corners of the rear beam? The reason the AC72 and AC45 carry the rigging under the chin of the pole is to make the entire platform more rigid. It helped carry the headsail loads and control hull wracking. Most didn't actually have a center member from front beam to rear beam. That pole might help with the rotation of the beam and carry a little of the mainsheet compression loads - but not that much. The only center member they (Oracle) had was for to form an aerodynamic end plate for the wing. The pod between the main beam and rear beam was not considerably structural.

Don't get me wrong - I'm in admiration of your work and applaud what you are sharing with us. I'm just interested in the technical aspect of it and I think you can make it simpler.

Also of note...and I experienced this from helping to redesign and rebuilding the waterstays on this boat last weekend. Synthetic rigging will stretch and kick your butt in an application like this.

We still have gigantic turnbuckles to go on the rear waterstays shown here: Loads on these stays probably approach 8,000 lbs under the main beam.

That thing (the second picture) definitely looks like an EC winner <img src="<>/smile.gif" alt="smile" title="smile" height="15" width="15" />

<img src="<>/grin.gif" alt="grin" title="grin" height="15" width="15" /> It's a beast. I'm still shocked at how well it goes and how dry the bows stay. It's feather light on the helm too - I'm in awe of the skill Alan showed in designing and building it. There are two boats that are potentially faster than us but we have a comfort advantage. It will be interesting to see which one is more important.

camp stove with a wind screen and you're all set !