SC20TR - Bill Roberts

Hi Thom,
That is a nice looking Gennaker. Gennakers have been roller furling for years and they are made out of strong, heavy sailcloth/laminate. What does this have to do with spinnakers made out of 3/4oz rip stop material? I'm lost.
Bill

According to North Sails thats a Code 1 furling Asm spin.

thom

Hi Thom,
I think the sailmakers are playing games with sail name/type designations. What weight cloth/laminate is that sail? That is the first time I have seen a triradial sail with crosscut seams also.
Bill

Bill,

Relative to the thread we had several posts ago about superior rudder systems, what happened and why did the Super Cat break a rudder after running aground in the Steeplechase?

Hi Jake,
At the time we ran aground, and we knew we were about to run aground, we had just been hit by a 20+ knot puff and the windward hull was flying high. Eric had the tiller cranked over hard making the boat bare off. The impact with the bottom loaded the rudder blade sideways rather than fore and aft and it simply folded sideways like there was a hinge in the blade along the bottom of the rudder head. The boards and rudders on this boat are all original, 21 years old and built with 1981 construction materials and methods. Todays boards and rudders are much stronger and lighter.
Bill

Bill-

Their info shows both.

thom

<img src="http://www.northsails.co.uk/sails/racer/downwind.html" alt="" /> <img src="http://na.northsails.com/Racing_Sails/asym__spinnakers.html" alt="" />

Just to clarify for those of us who do not use this stuff daily:

The "Phi" in Mr. Roberts Euler thin column formula is actually the constant "Pi". This solution applies when the boundary conditions are Moment = 0 on both ends (so make sure the ends really are pin supported in the actual structure).

That was a nice explanation Mr. Roberts.

It is funny to hear people comment that the SC-20 is old. Old like the A-11 Oxcart apparently.

Best regards
-colin pitts

Ho Colin,
Thanks for your suggestions and support. I appreciate your help.
Bill

Bill-

How much larger is an RC30 rudder than a ARC22 in general terms like width, length, chord.

thom

The following comment by Bill is at the same the reason why the strut is the heavy weight solution.

>>This is a stability equation and has to do with 'buckling failures'.

A trick to get around the buckling problem is to induce the buckling failure mode yourself. = PREBEND spinnaker poles. Due to the prebend it can not buckle downwards anymore.

By bending the pole upward you force the failure mode to be directed upward which can be stabilized again by the little wires from the midpole to the bridles. See F18 pole setups. This is lighter (spectra dyneema HT line), cheaper and easier to repair. This has the advantage that also sideways buckling is stabilized which may not be the case with just the strut.

This means that the reason to opt for a strut is only one. Lengthening the luff of the jib. Supportwires to the sides are then still needed to stabilize the pole to the sides or the pole needs to be made stiffer to have sufficient "I" when you want to do it without support wires.

Another trick is to make the angle of the tip support wires relative to the pole as big as possible. The I-20 does this I believe. Here the tip support wires are not lead back to the bridlewires but have their own attachement point forward of the bridle wires.

For cats up and including 20 foot a tube of 40 mm by 2 mm is stiff and strong enough for the all used spinnaker sizes (not hooters). For 18ft and all 16 footers 40 mm by 1.5 mm is by far enough.

I've used 3.55 mtr. by 35 mm by 2 mm for several years and this was on the borderline of buckling WITHOUT midpole support wires. With them I had no problem what so ever not even in windforce 6 (25 knots) although I wished I hadn't pulled the kite then.

40 mm = about 6/10 inch 2 mm = about 1/12 inch

Weight of a 3.75 mtr. by 40 mm by 2 mm pole incl support lines (dyneema) = 2.5 kg = 5/5 lbs

Weight savings on a 3.75 mtr 35 mm by 2 mm pole EXCL strut = 2.5 - 2.2 kg = 0.3 kg = 3/4 lbs. Any guesses what a strut of say 2 ft weights ? Bills 3/4 inch SS strut weights 400 grams = 1 lbs. And than it is not stabilized in horizontal plane.

Two lines of dyneema and prebend in the pole add only grams to whole setup.

The tornado guys are only using the strut to keep the jib area the same to what they had before. It is not a stability device as they still have the lines - wires for that.

Wouter

Hello Thom,
The 30 rudder is 10 inches longer than the 22 rudder. It is also about 1 inch wider on the chord and it is about 1/16th inch thicker at max thickness. For exact measurements call Tom H.
Bill

Hi Wouter,
Prebending the spinnaker pole uses up some of the compression strength of the pole on the compression side. Straight columns can carry more compression load than curved, prebent, columns. If it were not so then the Romans built all those columns at the Parthenon incorrectly. Some spin poles are set up with what appears to be prebend but under load they are straight.
Good Sailing,
Bill

>>>If it were not so then the Romans built all those columns at the Parthenon incorrectly.

Isn't the parthenon build by Greeks ?

Besides I don now of the Parthenon that the construction is crooked on purpose. This is actually the brlliance of the piece. If they used straight lines than the building would not appear straight visually. There is actually no staright line in the contructtion. By incorporations curves and crookedness the architect compensated for the imperfections of the human eye and brain when the object it looks at isn't relatively small and in one plane.

Netherless you are very much right that a prebend pole theoretically has a lowered buckling load. However the by forcing the buckling direction to be upward (by weakening it a little in this direction) one can use lines or wires to stabilize the setup. Of course lines and wires can only provide a pulling force and not a pushing force. Hence the buckling mode of downwar needs to be prevented as that can't be stabilized with support wires to the side.

Of course when a line between the pole and bridle strop is used (Three support wires to the middle of the pole) than the whole system is closed and not prebent is necessary. Even though it is handy to lower the tip of the spi pole.

Anyways the Prebend lowers the buckling load but adding the support lines increases the buckling loads again. So much so that the new setup how as much higher buckling load. You loose a little but that is corrected out over by winning alot. End result is a big net gain.

>>Some spin poles are set up with what appears to be prebend but under load they are straight.

Yes this could be the case. I haven't looked at it like this. On the boats I sailed this wasn't noticeable.

Wouter

Mike-
Thom,

I think there's some confusion here...What I call a strut, I thin some call a pelican striker. I've always thought a dolphin striker points down toward the water while a pelican striker points upwards...is this wrong?
Anyhow, the Tornado runs stays from bridle tangs to the mid pole at bottom end of strut. In the picture I showed, these stays are the light brown vectran line below the bridle stays (stainless). There is only one other pair of pole stays, from the bow tips to the end of the pole (again using vectran). There is no stay from the main beam.

Parthenon--built by Greeks, a temple with surrounding columns, non-straight for aesthetic rather than engineering reasons
Pantheon--a dome built by Romans in Rome

Both are remarkable feats and facinating to study and see; neither have anything to do with sailing or spinnaker poles

Wouter,

This is what I was wondering...why is the strut necessary on the Tornado even with the jib luff below the bridle & down to the pole? The mid point of the pole cannot move upwards and side to side appreciably because of the vectran stays attached there and down to the bridle tangs on the inner gunwales. What does this strut get you other than redundancy? Is the concern the midpole will bend upwards when loaded by the jib luff tension...because of the very slight stretch of the vectran stays? Or, is the angle from the pole to the bridle tangs not sufficient to handle the loading?

Mike.

Mike-

As I said above when Bill gave his view " I stand corrected" but Wouter says the opposite. All I know is mi boat is in the pic. It works and I really don't know which is which...

thom

Mike and Thom,
A strut is a beam which carries a load along its axis. It has nothing to do with up, down, front,rear or sideways. In the catamaran industry a Pelican stricker is a strut that extends upward like on the front beam of a big ocean going cat and is associated with the forestay. A dolphin stricker is a strut that extends downward under the main beam of many beach cats or downward under a spinnaker pole. If some class wants to call a strut a jack rabbit, so what. Let it go. It is time to move on.
Bill

Mike,

What I found was that a spinnaker pole cna easily be made strong enough to resist the spinnaker loads on normal sized beach cats. No strut is required. However there can be advantages using such a strut.

-1- If the strut is loaded up then the bridles can be taken higher which reduced the toe in bending moments in the hulls.

-2- It keeps the luff of the jib straight without alot of jib tension. Remember that tension in the forestay keeps the luff straight and no such tension is present between the bridle strop and pole. You don't want to load up the jib luff to much as that curves the leeach and may pull the draft to far forward.

-3- When the vectran / Wire support line run very horizontal then these line do not stabilize the pole in upward or downward direction. Today many try to get the pole as low as possible. Prebend can take care of this though.

-4- If the luff of the jib is tensioned than it will put a significant load on the pole which is far greater than the forces required to stabilize it in the vertical plane. Maybe the support wires run vertical enough to stabilize the pole but they will hardly ever run vertical enough to take the luff tension of the jib.

-5- A spi pole needs to kept up by at least one element. As all the support wires run downward it means the one element is required to pull it up. Often this is a ring or wires going down from the bridle strop to the pole. With an elongated jib luff this could be a wire also but why not have the advantage of at least point 2 and 4 as well ? In same cases it may pay to have point 1 too. If you are going to have an element there then why not use it to the max.

That is the reason why I use it. I made my boat wider and didn't want to load the boat up more so I raise my bridle strop and keep the angle to the hulls constant. In order to get the jib area in and keep the pole up I opted for a strut. I think I will load it up to medium level. I don't want to end up with a heavy thick road because of buckling considerations.

I don't know why the Tornado guys did it. I hear they changed nothing to the bridle height so I guess the answer must be found somewhere in points 2 to 5

Is it necessary ? I don't think it is. It can be done in other ways at well. May it be the msot attractive option ? That is likely.

Wouter

Marstrom Tornado's are built with some millimeters toe-out, so when you crank the main sheet, the pressure on the forestay straightens the hulls. I think it was 7 mm, but I'm not quite sure.

Marstrom used to have som PDF's on their web site from Multihulls Magazine about their building process, but it looks like they are removed on the new site.

I have never seen a Tornado with any kind of dolphin striker or compression strut on the bowsprit. When the 'sport' rig was introduced, one of the big issues was that no rebuilding was neccesary. You could just add the neccesary equipment and go. (Nevermind all the old non-Marstrøm platforms that could not take the strain of the bigger sail area).

I was the one claiming the Tornado DOESN'T have a dolphinstriker.

It has a strut between the pole and bridle strop as seen on all Tornado's with the new rig.

Can anybody understand now how much confusion it causes by calling this element a Dolphinstriker or Pelicanstriker ?

Wouter

I am wondering -- has anybody EVER hit a dolphin or a pelican with any part of their boat?

I don't know about hitting a dolphin, but during the H-20 Continentals in September, one of the boats straddled a massive sodden log, maybe two feet in diameter, floating just above the surface like a tropical ice burg and caught it with their dolphin striker. It bent the stainless rod parallel with the tramp. Ouch

Hi Rolf,
If the spinnaker pole is stalilized at a mid length point, the jib tack for example, then the effective length of the pole is half the actual pole length and a smaller and lighter weight tube can be used.
If a strut tube is put in place between the jib tack at the spin pole and the intersection of the forestay bridle wires, two important options open up. One is that the forestay bridle wires can be made longer, a taller bridle, which reduces the toe-in loads on the hulls. Second the jib tack can be set at whatever height above the hull deck is desired. Longer luff sails produce more forward thrust and make the boat go faster.
If a strut, vertically downward, is installed on the underside of the pole immediately under the vertical strut connecting the top of the pole to the forestay bridle, and this strut has a tension wire connecting the lower end of the strut to the ends of the pole, then it is possible to lower the outer/forward end of the spin pole which allows a longer luff spinnaker. Again longer luff sails produce more forward thrust and make the boat go faster. The arrangement I have tried to describe here is a lower spinnaker pole and the pole can be straight under load and longer luff jibs and spinnakers can be used.
I have no idea why the Tornado class did what they did. That is their business.
Bill

Yes Mary!

I was swarmed by a pod of good sized dolphins a few years back on an old hobie 18. They are right along side us, zooming past and darting directly across the bows with only inches to spare. We were moving at about 8 kts. At one point, one of them seemed to enjoy matching our speed while swimming between the hulls just in front of the main beam. What a site! At some point, another one thump against us somewhere along the port hull. An amazing day!

I've also hit a seal on that same boat. We were in a good breeze, boat has moving 15 kts easy. My rookie crew was mumbling something while facing forwards...I could not make out the words over the sound of the wind. Moments later...THUMP! "What was that?!" I asked..."The SEAL!" came the reply...."Didn't you hear my warning you??"

Ahh, that piece of rigging is not about any kind of compression on the Tornado (I'm refering to the Tornado, altough the discussion is a bit broader..).

The small cylinder going from the bridle intersection and down to the pole is to restrict the maximum luff length of the jib with downhaul applied in a measureable way. As per the Tornado class rules.

Lowering the tip of the pole could be handy trough a dolphin striker, but hitting waves would be a consern. Also, most classes has a max luff length of spi's as well (The Tornado also has a rule about minimum height of tip above deck).

I think the names dolphin striker and pelican striker are much more memorable than 'compression strut'

Mary,

Dolphins get hit by dolphin strikers all the time. You might have seen some of the ones that have been hit; they have a hole on top of the highest portion of their body.
GARY

Be careful, Gary, you might start a new mariners' myth.

When I sailed the Hobie 18 I always tried to find the pod of dolphin which had the highest number of the ones that had already been punched by a dolphin striker. Then my chances of hooking-up with one was the greatest.

Once the protruding end of that dolphin stiker would snag in the animal's scar-hole, the boat can really fly. The dolphin's tremendous power allows him to actually lift the boat some and this gets you right into planing mode where he can then scoot you along at tremendous speeds.

I miss the striker on that boat so much! The Mystere that I sail on now, has a flat bottomed striker so I have to actually hook onto their fold between their beak and skull with the trailing edge of the striker's bar. Much more difficult to engage this hook-up and also more difficult to maintain it for any good distance.

Those days of riding the "seahorses" are sorely missed!

GARY

Gary you have been to sea too long...