On two occasions now I have hit underwater obstacles (ahem) and lost PB fins entirely.



Both times the fin bolt remained in the board while the fin and barrel nut disappeared into the briny depths. I am a bit mystified by this. Each time there was loud "click" , but hardly any felt impact. It seems those barrel nuts just strip out very easily. has anyone else had this experience?

Yes, on several occasions. Lost the fins and had trouble getting back (walk of shame). But decided to change my rockets to tuttle box. May damage the board with the same impact bit feel a bit more secure with two bolts instead of one.
Bit more of a range of fins in tuttle also.

The barrel nuts in fins are machined with more thread clearance than usual so they will strip out. Saves the board, but losing a $200+ fin can be hard to bear.

But - IMHO and unconfirmed, just my theory..... if anything corrodes it is the brass nut, not the bolt. As soon as the barrel nut looks a bit green or a bit "off" I reckon replace it as the excessive thread slop has just become a bit too sloppy as the thread has been eaten away like 0.1mm and that is enough....

I think that is the power box design. The nuts are made from brass so that when there is an impact the thread sacrificially strips and the fin is lost - rather than a tuttle box impact that can rip the box/ back out of your board.

I guess that things to consider are - when the power box was designed, fins weren't that expensive and you would rather lose one than have a damaged board.
-they probably thought 99% of these impacts would involve rocks/ the beach and therefore the fins would be retrievable.

Can you replace the brass nut with a harder metal?

I agree with windsurftom. I think they are designed to break this way and save the board and leave the box intact. The shape of the box has a sort of curve in it, which I think helps this.

If you have a tuttle, nothing is going to move it, without tearing the back out of the board.

If you want an intact fin and box, then US-box is the way to go.

I am surprised the barrel nut doesn't stay. With the fins I have lost, half the fin head breaks, then the rest of it drops out. I retrieved one of them once, and you can see how it cracks through the location of the barrel nut.

It's not difficult to rig a tuttle box to release a fin rather than destroy the box. You just have to round off the top front of the fin head so it will rotate out of the box without jamming. You dont have to remove much and you should be careful not to because the tuttle works by supporting the fin head at the front and back edges.

None of the boxes addresses the problem of how you retain a fin after bashing it hard. The big disappointment of the new slot box is that it continues in that tradition. The old US box comes closest at failing gracefully because it allows a fin to rotate out of the box without actually coming out. The fin might punch a hole in the board behind the box though.

It's really not too much to ask of a fin box design that it allows for quick reinsertion and retention after a solid bash on a reef or a turtle. It's not difficult to design a fin box to house a tether.

As far as stripping cross nuts is concerned that's never happened to me. Ive always been left with the cross nut and bolt but no fin. I very much doubt that they are threaded with anything other than a standard tap and any looseness is purely due to galvanic corrosion.

Power box fin heads have a frail connection between the cross bolt and the fin. There's just a skinny little bit of composite with a bolt hole through it between the cross bolt and the board. You often see them cracked above the cross bolt.

^^^ I spoke to Pio at MFC when I reckon one of his let go too early and he confirmed a non standard thread clearance. (I suspected as I was trying to figure out if it was M6 and wondered why my measurements seemed off)

Regardless - I agree corrosion is an issue as the tiny "lands" on the threads will go pretty fast.

I have only lost the fin and nut, so it was a barrel nut issue.

As per another thread here, the corrosion of the nut can break the fin head so I think they need to be replaced from time to time. If you always remove the fin then it will dry out faster and corrosion / nut swelling will perhaps be less an issue?

We already have too many box designs but yes, a tethered fin would be great! And US box is so good for fin retention ... hmm thinking cap on...

I'm not sure that replacing the barrel nut with a stronger metal wouldn't just tear the stainless bolt through the board. Given that it's only about 1/4 inch of fibreglass. I've always hoped that the barrel nut would be more sacrificial than the board. All seems a bit of a trade off, with both having their merits, and inherent problems.

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NotWal said..

It's not difficult to rig a tuttle box to release a fin rather than destroy the box. You just have to round off the top front of the fin head so it will rotate out of the box without jamming. You dont have to remove much and you should be careful not to because the tuttle works by supporting the fin head at the front and back edges.
.

I've been thinking about doing that since I smashed my last one. Replacing a fin is far less expensive than repairing major structural damage to a tuttle box.

One question, do you use a brass/nylon thread in this case to allow the fin to rotate out freely without pulling the stainless screw down through the top of the board?

What is the "new slot box"??

Clarence

As far as the screws are concerned the simplest way is to install the screws without washers so they can pull through the deck. You can use washers after that. I had some plastic cross nuts made but it was a fiddly job and the bloke who did it said he wouldn't do it again. I haven't bashed a fin since :)

The Slot box:
www.seabreeze.com.au/forums/Windsurfing/Review/Tabou-The-All-NEW-revolutionary-Slot-Box/

Hmm, i guess if it were a loose fitting thread, then repeated tightenings over time would cause excessive wear too.

Another thought, in Qld at least, powerbox fins presumably aren't so harsh on the turtles (though I'm not a fan of being finless and offshore).

I think about new design where the fin could be mounted on the hinge back and secured with release mechanism at the front.
Something like your snow skis do when stack on obstacle- release the boot.
But here fin still remain hanging on the hinge at the back.
Definitely could save the fin but looking at proportion of such accident - like loosing fins - I don't thing will be worth to modify complete line of new windsurfers.
But is could be possibly made in garage by smart handy man as retrofit and sold in limited numbers to those that need it.
Small business opportunity there....
Maybe you Mark ?

If I design such solution I would rather modify the fin itself to incorporate this hinging machanism.

Standard of the shelf fin base could be cut in half and upper layer will stay mounted to the board bottom half will release and hang on the hinge or even piece of strong rope/line.

Another point, make sure you're actually screwing the screw in at least 10mm (10mm the dia of most barrel nuts)
cause if you're only holding by a couple of threads is not going to hold.
6mm thread is 1.0mm pitch so that 10 full turns
Different brands of fins have the barrel nuts positioned at varying heights were you may need shorter or longer screws
Go to the local Bolt Barn (cheap) and get some new 6mm SS screws of the right length or cut to suit, make sure you allow for washer thickness

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Mark _australia said..

^^^ I spoke to Pio at MFC when I reckon one of his let go too early and he confirmed a non standard thread clearance. (I suspected as I was trying to figure out if it was M6 and wondered why my measurements seemed off)

Regardless - I agree corrosion is an issue as the tiny "lands" on the threads will go pretty fast.

I have only lost the fin and nut, so it was a barrel nut issue.

As per another thread here, the corrosion of the nut can break the fin head so I think they need to be replaced from time to time. If you always remove the fin then it will dry out faster and corrosion / nut swelling will perhaps be less an issue?

We already have too many box designs but yes, a tethered fin would be great! And US box is so good for fin retention ... hmm thinking cap on...

But crap to put them in & out..