Hi all, have pulled apart the old style dolphin striker on my nacra 5.2 to clean up some corrosion issues and now need to put it back together, (busted the little stainless bolt that holds the strap onto the post as it was firmed corroded to the strap but that's been replaced now)
what is the theory here? should i be looking to torque up the nuts above and below the main beam to the same value to keep the main beam flat, or should there be some prebend???
Nacra put a manual (well, two page copy of a manual) with the new style dolphin striker kit they send me. It had detailed explanations on how to install and prebend the whole thing. Here is the relevant extract:
Quote
B. NEW ROD INSTALLATION
Using a rat tail file, increase the diameter of the mast support rod holes in the beam by 1/16". The original rod was 1/2" and the new one is 9/16". Test the holes with the new rod. A new compression tube is supplied however the original may work. Do not damage the threads by forcing the rod into the holes. Thread one of the nut down the rod about 1" from the non-thread end. Slide a washer down on top of the nut.
Silicon the thread above the washer generously. While moving the support strap out of the way slide the rod up from the bottom making sure the compression tube is properly in place. Fill the recess hole in the bottom black anodized mast support casting with silicone. Silicone the other side of the mast support casting. Push the rod up to allow placement of the mast support casting on the strap. Once the casting is properly located, slide the rod back down into the casting recess squeezing out any excess silicone. While holding the rod in the fully down position (into the recess in the mast support casting) thread the lower nut by hand up against the bottom of the beam. When tight, hold the nut in place and turn the rod by hand until tight. Wrap some tape around the rod in the middle of the non-threaded part. This tape will protect the rod from being scarred when holding it with the wise grips. Secure the wise grips to the rod tightly. Using a 7/8" wrench tighten the lower nut 7 half turns (3 1/2 full turns) making sure the rod does not turn. Note **8 to 9 half turns on a 11 ft wide boats. NOTE: The age, ware and strength of beam hardware will vary. The structure intent of pretentioning the Dolphin Striker Rod is to create a slight bow in the beam. (the center is up to 1/2" higher than the ends). Slide the upper washer onto the rod and then thread the nut down. Holding the lower nut secure with one wrench tighten the upper nut very firmly with the second wrench. Clean away any excess silicone with a rag and acetone. Thread the new mast ball onto the rod.
I assume you have the old system: the main difference is at the bottom of the post, how it attaches to the strap. You don't have a black anodized mast support casting, but you have a small screw going through the strap, then through an aluminum V spacer then into the rod to keep it in place.
thanks for that pepin, i noticed a slight bend forward in my rod so i can see why they beefed it up from 1/2inch to 9/16th, will swim it all in a $3 tube of clear bathroom silicon
i guess ideally i would replace it with the new assembly but as it is a lot of cash and i only sail in a small lake with no chop i reckon i can get away with the old striker system, as long as i keep an eye on it
This is helpful, but I have a followup focused on the compression tube:
How important is the compression tube? Any helpful hints on placing it inside the crossbar during installation?
I just bought up a used Nacra 5.0 with a damaged mast support rod. The previous owner had forced an incompatible mast ball onto the old (too large? custom?)rod causing the ball to explode while we were trying to step the mast. The threads on the top 3/4" of the old rod were ground off compared to about 1/2" on the new smaller-diameter replacement Murray Marine mailed to me. I've removed the old rod (it was so corroded/battered I had to hacksaw through it), and am preparing to install the replacement, but realized the previous owner installed the old compression tube around the support rod between the dolphin striker and the crossbar (!?) instead of inside the crossbar. The old compression tube is sun damaged and can't be removed from old rod. I didn't order a new compression tube (didn't know I should have till now). How big of a deal would it be to continue to sail this boat without a compression tube (obviously it has been done for a while now).
[...]How big of a deal would it be to continue to sail this boat without a compression tube
Big deal. The compression tube prevents the beam from collapsing. You *need* one, the beam is not designed to take all the pressure from the mast.
I've got a 5.2, but I've never taken the dolphin striker assembly apart, so I may be out of line, but I don't see how the crossbar is taking that much downward force from the mast. I'd have assumed that the compression tube was so that you could crank the two nuts towards eachother without crushing the crossbar, not to provide support for the weight of the mast.
Hopefully I won't have to find out any time soon...at least not the hard way.
Re: dolphin striker question
[Re: yurdle]
#192330 09/30/0905:20 AM09/30/0905:20 AM
As far as I can see it the beam takes all the pressure from the mast. The dolphin striker keeps the beam bowing up helping it resist the pressure.
In other words mast press ball, ball press metal rod, metal rod press upper bolt, which push the topside of the beam. The compression tube transfers some of that pressure to the bottom of the beam, without it only the topside of the beam hold all the mast pressure.
It also works the other way. When you install the rod and put some prebend by turning the 3.5 turns if you don't have a compression tube all the pressure is on the bottom of the beam. With a compression tube, the pressure is equalized between the top and the bottom.
I would not install a dolphin stricker rod without a compression tube.
Re: dolphin striker question
[Re: pepin]
#192337 09/30/0907:06 AM09/30/0907:06 AM
This is a truss system - when looking at the static load from the mast the front beam doesn't resist much of a downward force at all - The vertical mast load is carried straight through to the dolphin striker. The dolphin striker is then in tension to reduce the load. That tension on the dolphin striker then turns that load into a compression force on the main beam.
Pretensioning / prebending the main beam helps the entire system resist the load by returning to a neutral position when the system flexes.
Jake Kohl
Re: dolphin striker question
[Re: Jake]
#192361 09/30/0910:05 AM09/30/0910:05 AM
Your diagram implies that the striker pole slides up and down inside the beam like an arrow on a bow. It's not, it's fixed by a bolt on top and a bolt at the bottom of the beam. So when the mast presses down it pushes the beam down. The tension on the dolphin striker band comes from to places: the beam flattening and the pole pulling the band down because the distance between the beam and the band is fixed. hence everything is locked in place with a minimum of movement.
Expressing it differently the mast pushes down and something is pushing up with exactly the same force to compensate. This thing pushing up is divided between the top of the beam and the bottom of the dolphin striker.
With a compression tube the pressure on the top of the beam is divided by two as it presses both at the top and at the bottom of the tube. Without it the risk to collapse the beam is not negligible. And that's why the compression tube is a big hunk of stainless instead of a flimsy alu tube which would be sufficient to just support the two nuts.
Re: dolphin striker question
[Re: pepin]
#192366 09/30/0910:33 AM09/30/0910:33 AM
I agree with Jake and Yurdle. The dolphin striker post takes the full downward force of the mast. This force is transfered to a tensile force within the striker rod which is counterd by an axial compressive force along the main beam. With the dolphin striker securely fastened to the main beam and properly tensioned, the main beam should see no bending load due to the downforce of the mast (only compression along it's length due to the force of the striker rod pulling inward). The compression tube is there simply to prevent the user from over-tightening the dolphin striker post bolts and accidentally crushing the main beam. Case-in-point, boats like Hobie 16's & 18's (which don't use a double-nut sriker post) don't require a compression tube.
The compression tube in your boat is most likely stainless steel in order to prevent it from corroding and seizing to the dolphin striker post.
sm
Re: dolphin striker question
[Re: srm]
#192371 09/30/0911:09 AM09/30/0911:09 AM
I agree with Jake and Yurdle. The dolphin striker post takes the full downward force of the mast. This force is transfered to a tensile force within the striker rod which is counterd by an axial compressive force along the main beam. With the dolphin striker securely fastened to the main beam and properly tensioned, the main beam should see no bending load due to the downforce of the mast (only compression along it's length due to the force of the striker rod pulling inward). The compression tube is there simply to prevent the user from over-tightening the dolphin striker post bolts and accidentally crushing the main beam. Case-in-point, boats like Hobie 16's & 18's (which don't use a double-nut sriker post) don't require a compression tube.
The compression tube in your boat is most likely stainless steel in order to prevent it from corroding and seizing to the dolphin striker post.
sm
Nearly....
MOST work thus:
Mast sits on a ball; this ball is attached to the a threaded rod that goes thru the main beam and attaches to the dolphin striker. However; the rod is also fastened via 2 or maube 3 bolts to the main beam.
The beam AND the DS take the load. THis is why it us usefull to apply some positive (up) prebend in the front beam.
For a while, this was the only setting I would change on the Hurricane I owened. More wind; more prebend so more rig tension. It does NOT work on my F16 in the same way!
the prebend applied to the main beam means that the downward mast loads only deflect the main beam back to a normal flat condition...i.e. where it is neutrally loaded vertically. Because the system is then shorter than this condition, the dolphin striker is placed under a great deal of tension which translates to compression on the main beam.
Even without the pretension of the main beam and the fixed (bolted) system, any deflection the main beam experiences from the vertical mast loading is immediately translated and magnified (10 fold or better) into tension on the dolphin striker...i.e. 10mm of downward deflection of the main beam would mean the dolphin striker would have to stretch 50mm each side!...Even though it's bolted together, for all practical purposes, the upward force or downward deflection force experienced by the main beam is minuscule (and negligible) compared to the compression it's experiencing from the striker. It's truss engineering.
Engineering principles aside, HOW would I install a new compression tube inside the main beam?
Does it simply slide in from the top along with the mast rod, or does it need to be inserted from the side through the end of the main beam?
From the side.
Drop a small nut attached to a small but long line through the top hole so it goes out at the bottom. Tie the end together so the line cannot get out.
Then slide a long pole (your tiller extension for example) to push the line in the beam sideways all the way to one side. So you end up with one continuous line with one loop around the beam and one loop exiting the beam on one side.
Rotate the line until the knot is on the loop on the side of the beam, untie the knot, slide the compression tube on the line, tie the knot to make it whole again. Then you just have to pull on the loop around the beam until the compression tube is in place.
One other thing, before reassembling, check for corrosion on the beam around the holes for the mast post. Make sure the area around where the washers and nuts touch the main beam is strong and the mast post does not have much play.
And did you upgrade the 1.5" mast post to accommodate the new mast balls?
Yeah, I upgraded the whole assembly including a new post/ball to replace a hardware store bolt somebody had in there before. There is just a tiny bit of visible corrosion, but it doesn't look more than superficial.
When you install the rod and put some prebend by turning the 3.5 turns if you don't have a compression tube all the pressure is on the bottom of the beam. With a compression tube, the pressure is equalized between the top and the bottom.
I would not install a dolphin stricker rod without a compression tube.
