The dolphinstriker: http://www.gust.ax/gallery/2008-02-10/08-04-14/

I thought about chain plates of carbon on beachcats, but I worry about wether the weight savings are worth it and if they can stand the shock loads?

Great work on the mastfoot and striker! I dont have any welding equipment and have never welded stainless either so I'll think about a way to do something similar without welding. Thanks for sharing!

I decided to go for a ss tube because that is no problem to weld with “normal equipment, alu is an other story. Guess you got a drawing from Phill, the problem with that one is that you have to drill a big hole trough the bottom of the beam and it’s there that you would need the material..
Guess it would work if you make the mast foot out of a M12 threaded rod going down all the way trough the strap, putting a spacer inside the beam and making the possibility to adjust the tension by one nut on each side of the strap. It’s not the most beautiful setup and a little bit heavy.
Had been expecting a lot more hints coming in on this thread. But guess it will come when people will see what the art school of Turku can do if they take on the painting job, then everybody can get emotional...
Can give no guarantees for the carbon chain plates, but if they resisted on Idec when Francis Joyon made his record run around the world why should they not resist on a beach cat?

Very nice metal work there Gato !

With respect to the hole on the bottom of the beam. Because of the dolphinstriker and upward pretension of the beam you don't really need the material there. The beam is mostly under direct compression and with the right upward prebend the beam bends down under sailing loads to almost hardly any bending stresses. This is why the "tube through bottom of mainbeam" as the dolphinstriker pin is such a common way of fitting. It is simple, does the job very well as it transmits the loads directly from the pin to the mast voet and it doesn't crack up or break even after years of usage. There is not much point in doing it any other way.

Wouter

Maybe this is a stupid question with a straightforward answer, but - why not use ss wire for tensioning the beam ? Or thin rod ? Round section material is best for tension loads, I assume the flat strip setup is used because it "cuts" the waves better than steel rod ?Or maybe adds rigidity in the horizontal plane ? But how large diam. should be enough for the tension there ? 5-7mm ?

I'm asking this because the flat ss strip looks too much extra material to me, for the tensioning purpose.

I think it is a good question, but also expect a good answer <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />

On The Pixie 14 the setup is with a wire. I'm maybe stupid like h.. but I hade tought that the dolphinstriker was there to take the downward forces (compression) that will occure in the mast, and then to my simple thinking the underside of the beam is taking more and you need less cross section in the strap if you don't drill a 25mm hole in it. It's also stupid in my opinion to drill a hole in the strap, because you have to use a bigger cross section, to compensate the loss of material or can that also be canceled?
I'm going with Florin, there is no need to add more weight than needed.

The answer is stiffness.

You want the mainbeam setup to be very stiff in the vertical plane. The more material in the dolphinstriker the stiffer the mainbeam setup. You can get alot more material (stiffness) in say 30x3 SS strip then a 7 mm ss rod. About 2.34 times as much. 7 mm SS wire is less stiff then a 7 mm rod, so you loose even more here.

Additionally there is hardly any difference in tension resistance between a round second and a flat rectangular one. Also it is alot easier to fit a flat strip to the underside of the mainbeam then a round rod; same for fitting the dolphinstriker pin to the strap. It is easier to bend a flat strip into a V-ed shape then a thick round rod and last but not least a frontal area of 3mm by 1500 mm is significantly less then 7 mm by 1500 mm when impacting with 1000 kg/m3 water. And and I forget that a flat strip keeps the pin in place in the horizontal plane were the rod and wires will do so muhc less. In fact the pin on my boat is not even secured in the beam. It was just slid in and there is no hole in the strap either, just a piece of metal welded horizontally to the end of the pin adjustement threading. This T-ending actually secures the pin to the strap and when under tension it can not move at all. Best part is that this setup can be taken apart in mere seconds and I have done so. Just turn the screw to detension the pin, move the T-ending up and turn it through 90 degress, angle the pin and slide the whole thing out of the beam.

In fact there is no good scientific reason to use round rods; only guts feelings to that extent.

Wouter

No, you are not stupid, you've just made a modelling error. In fact you overlooked part of the system.

When the rig is loaded the mast presses down on the mast foot which in turn pressed down on the beam AND the dolphinstriker pin. In turn the pin presses down on the dolphinstriker strap. The strap is ALOT more resistant to being pressed downward then the beam is. In fact the strap takes care of the vast majority of the mast foot loading; in the order of 75-90 % if the beam is not prebended upward initially. The strap comes under tension to resist the downward loading of the mast (compression in pin) and this results in almost equal compression forces in the mainbeam. The latter are transmitted from the strap to the mainbeam via the bolts securing the strap to the mainbeam near the hull.

This is commonly accepted stuff so far.

Now if the beam was perfectly flat before the mast being stepped then the beam will bend downwards when the rig is loaded up just like the D-striker and thus experience bending stresses. These bending stresses are highest on the top and bottom of the beam and typically an engineer want material in these places to limit these stresses.

That is your reasoning.

My reasoning and one that is commonly applied on cats is as follows.

If the d-striker strap is pretensioned upward before the mast is stepped then the mainbeam will straighten out first before being bend downward under a loaded up rig. Of course a straight beam can not experience bending stresses, only a curved beam can. So in effect if we give the beam the right amount of upward prebend we can lower the bending stress significantly under a fully loaded up rig. If the prebend upward is the same as the downward displacement of the V-tip on the D-striker under max sailing load then the bending stresses in the mainbeam are perfectly zero and the beam will ONLY be under compression loading. This latter situation is not always used in reality as it is advantagious to leave some bending of the beam but that is another topic. But I can give you some insight into the reasoning. The D-striker puts the beam under compression stresses and a slightly downward bending beam will see tension stresses on the underside of the beam. By careful selection of the upward mainbeam prebend you can have these two cancel one another out completely. In that situation it won't matter one bit how large the whole on the bottom as that part of the beam won't experience hardly any stresses at all ! The trick is to find the optimal average setting for a range of sailing conditions (rig loading)

And that answers the question

Wouter

Thanks for the answer Wouter,
Actually your answer makes me think more into it (trying not to..)
If you say flat strip is used for rigidity in the VERTICAL plane- well I don't understand why flat strip is more rigid than rod, I understand why flat strip is more rigid than rod in the HORIZONTAL plane, but why VERTICAL ?

To my understanding mainbeam rigidity is given by the stiffness of the Alu cross section and the aditional compression put into it by the straps @ tensioning pin. The longer the dolphin striker pin=more rigidity (considering the apropriate tensioning strap)

Considering that , the ultimate role of the straps is to handle tension loads, whatever it's cross section. Further more , a 1m long 7mm diam ss rod is much more rigid than a 30x3mm , same lenght ss strap - in the plane perpendicular to the 30mm side. Of course the opposite occurs in the other plane.

I understand it is easier to fix the strap to the Alu profile, but you can use an end fitting for the rod.

A 30x3mm ss strap has 90sq mm of cross section, while a 6mm diam rod has 28sq mm of cross section, hence lighter.

More to it: when the boat nosedives and the Dstriker enters the water it reacts pretty much as a wing, opposing resistance for the boat to resurface, while a rod has much less resistance for resurfacing.

Is this like a MythBuster case ?
6mm diam rod can take a lot of tension, we designed stair ramps hangin' from ceiling, supported by thin cables...
don't like the math of it but I assume even 5mm rod should do it. The tension in the rod should be higher or least equal to the compression and dynamic forces that the mast exerts on the beam.

Wouter can you clear this out for us, or at least explain where am I wrong, may be missing something.

by the time I wrote this you already wrote more to it.

Typed up a reply and then it turns out the time-out had passed and I lost everything.

I don't feel like retyping it so ...

Sorry,

Wouter

Wouter,

it is not the first time I hear about this problem from you (and others). I use Firefox at home, and when I go "back" after recieveing the timeout notice Firefox remembers what I wrote and fills out the text field. If it is possible for you, I think you should give Firefox a try.

Shall I have a go....

The strap is working mostly in tension not bending so its orientation (vertical or horizontal) is irrelevant, its stiffness is proportional to its cross sectional area only.

So as you say "A 30x3mm ss strap has 90sq mm of cross section, while a 6mm diam rod has 28sq mm of cross section" so the strap is stiffer.

Does that make sense?

All the best


Gareth

Not really...

Considering the straps alone, not fixed to the beam :
stiffness is different for the 90 and 0 degree plane on the rectangular cross section strap, while for the rod stiffness is equal in both planes.


1. If the cross section area is enough for the tension loads (and a safety factor) what relevance has the cross section shape (other than ease of mounting to the beam) ?
2. The flat strap adds rigidity in the horizontal plane but I can't see how it adds higher rigidity in the VERTICAL plane compared to rod (again -as long as the cross section area is enough for the loads).

SS has a tensile breaking strength of 5300kg/sq cm
that means a 6mm diam rod can withstand aprox 1500kg of tension or 147Mpa. Is it enough ?

Come on guys, need answers for this.

Gareth - stiffness is related to material distribution in a given cross section, not necessarily higher than rod for a certain cross section area. For example for a given cross section area you can draw several shapes with different stiffness manifested in different planes. The idea is that the whole thing (beam+d striker and straps) should be stiff.

And another thing, am I on topic ? Should we put this on the Homebuild forum ?

isvflorin,

You are getting confused between bending stiffness and stiffness in tension.

What you are doing wth this arrangement is creating a truss, The vertical member is in compression, and the straps are in tension not in bending. Stiffness in tension is related purely to its cross sectional area.

All the best

Gareth

A dolphin striker is a trussed arch as shown in figure E



from the page http://mysite.du.edu/~jcalvert/tech/truss.htm

Gareth

The design of the striker does not revolved around strength but rather stiffness.

A 4 mm wire can take the loading but won't add enough stiffness to the beam in the vertical plane. This is turn MAY allow the beam to bend to much (even with prebend) and raise the bending stresses in the beam to such a level that the beam may indeed crack or rupture. Without the beam taking the compression of the striker the whole setup fails and folds up downwards leaving the mast unsupported and destroying the whole craft in the proces.

So strength calculations alone are not sufficient one needs to include the stiffness portions as well. especially when looking at sail behaviour. A flexing mainbeam playes havoc with mainsail sheeting and trim.

That is the short version of my earlier posting

Wouter

Ok, my last q ?
So basically you need to over dimension the cross section of the straps to get more stiffness in, like being much above the breaking limit where the strap does not oscillate/vibrate from dynamic loads ? Well in that case, considering the same cross sectional area what is the advantage of the rectangle cross section. Don't get me wrong, I understood all the other stuff (I guess), but this Q wasn't answered - the actual advantage of the rectangle cross section over the round one. The similar round section for the 90 sq mm area is an 11mm diam. rod.

And sorry about being off topic, and about bothering with this Q.

A rectangle will generate less aerodynamic and hidrodynamic drag for the same section area.
An eliptic section would be better in order to minimize drag, but it is hard to find, hard to attach to the beam and hard to attach to the dolphin striker. It is also more expensive.

Other reasons to use a rectangle are:
- Price
- Ease to conform to the required shape
- Firmer attachment to dolphin striker extremity.

Anticipating the next question:
A firmer attachment to the dolphin striker increases the degree of hyperestaticity when compared to a a rod or wire passing through a hole or recess in the dolphin striker's extremity, which is the usual setup. This means "stiffness".

Rgrds,

Well, Scarecrow has a very nice setup for his striker, the thing that made me do it another way was the need for welding aluminium, I don't have the equippment for that.
Still when I made the Blade it felt quite strange to drill a BIG hole trough the mainbeam and just put a tube in it.
don't think that that is improving the strenght in any direction...