The 80 degree back tilt of the front rounded taper, and 80 degree front tilt of the back rounded taper are plenty of structure to carry both the up and rocking forces of the foil in the finbox. Don't need any kind of contact along the inside of the top.

The genius of Tuttle's design is that a deep finbox can accept fittings of various heights without compromising strength. Just look at his drawings to see this. His finbox goes up to 5 inches deep, while most fins and foils have heights ranging from 1.5 inches to 3.5 inches.

If you absolutely must fill in the top to get a load carry inside the finbox along the top of the foil, then you are committed to using just one foil for that box.

I use three foils for one finbox. They all have different heights, they all fit in nice and tight to the tapers, and they all work perfectly.

Why limit your options when you don't need to?

segler said..
The 80 degree back tilt of the front rounded taper, and 80 degree front tilt of the back rounded taper are plenty of structure to carry both the up and rocking forces of the foil in the finbox. Don't need any kind of contact along the inside of the top.

The genius of Tuttle's design is that a deep finbox can accept fittings of various heights without compromising strength. Just look at his drawings to see this. His finbox goes up to 5 inches deep, while most fins and foils have heights ranging from 1.5 inches to 3.5 inches.

If you absolutely must fill in the top to get a load carry inside the finbox along the top of the foil, then you are committed to using just one foil for that box.

I use three foils for one finbox. They all have different heights, they all fit in nice and tight to the tapers, and they all work perfectly.

Why limit your options when you don't need to?

The shim I made is removable, if I get another foil will probably have to make a shim specific for it.

Ian85 said..
Update. Today I used my SB Alloy Race+ in the SB HWR formula board without a fin box spacer. Worked the foil mast head deep into the fin box by rocking back and forth while tightening screws to fully engage the tapers. Went maybe 200 m from shore taking things easy and the front of the foil head pushed up 15-20 mm. Heard a crunch/cracking noise then crashed due to change in mast position. So basically due to the massive forces involved via "foiling" the foil mast head (which is a plastic fitting over the aluminium mast) just deformed and twisted upwards / forward within the fin box. The leading edge of he mast has pushed into the board infront of the fin box and I also think the front of the fin box has split. This brings forward my thoughts of fitting a replacement dedicated foil box.
I guess there are lots of variables relating to type of foil and board, it's intended use, mast with or without a flange, fin box strength and depth, spacer or no spacer, weight of rider and rig, etc etc.
Ian

Sorry to hear, is that a foil ready DT box? I only see reviews from 2011 for that board, so pre foiling. But a plastic mast head just sounds wrong, I would contact SB. I thought of putting a foil on my JP Magic ride (power box) with the Chinook powerplate, but was worried I would damage the board, so sold it and got a Goya Bolt with a foil ready DT box (no issues so far after several severe impacts). Learned later that lots of guys with power box boards had lost their foils using the powerplate due to the single M6 power box screw breaking. Kings sells a carbon foil ready DT box called the King Tut for $125.

OK, that makes two instances when the DT finbox on a SB HWR formula board split apart at the front taper.

So, we have to be a bit careful about mounting foils into the DT finboxes on old formula boards that were originally designed for strong side forces of 70 cm fins, but not for the up and rocking forces of foils.

I have thee old Mike's Lab formula boards that seem to have strong DT finboxes for foiling. After three years of foiling on them, I have had zero problems.

Nowadays, the foil-ready boards have strongly reinforced finboxes. At least we all hope so.

Use a leash.

Sandman1221 said..
So following your advice his board and foil ended up getting damaged.

Huh? Who's advice?

ZeroVix said..

Sandman1221 said..
So following your advice his board and foil ended up getting damaged.

Huh? Who's advice?

Segler's, just read Ian's posts and you see see why I said that.

It's simple engineering. Point loads cause failures. Everywhere. Bike frames, fishing rods, windsurfing masts and booms, you name it.

Spread out loads, such as full top to bottom contact of BOTH front and back rounded tapers at their 80 degree rake angles, have a much lower risk of failure. Anything you do to defeat the full contact of both tapers along their ENTIRE lengths will cause problems. Shimming, filling in the top to get a top contact, all that stuff, defeats the full taper contact--along the ENTIRE lengths--of both tapers.

This is not complicated.

Even with full contact, if the finbox fails, it was weak to begin with. It was originally designed for side forces and was not strong enough for the fore and aft rocking forces. Many (or perhaps most) DT finboxes are already strong enough, but we know about these these examples that are not.

I am a retired Boeing scientist. I worked for 30 years with mechanical engineers. I will never shim or fill in a DT finbox. I'd rather avoid failure.

Use a leash.

segler said..
I am a retired Boeing scientist. I worked for 30 years with mechanical engineers. I will never shim or fill in a DT finbox. I'd rather avoid failure.

Use a leash.

You're a fellow from Boeing and you're telling us to use a leash?

What has worked for me, with a 20 year old board and no foil flange, is to put the foil head in so it is super, super tight, full contact both sides as far as it possibly can go - and then figure out a shim that will fill in the gap at the front to spread the load and prevent any rotation. And rotation does otherwise occur even when super tight and snug if there is a void above the front and no flange - it might be slight, but even slight implies some point loading.

Because its already at the super tight point, I found it impossible to make a rigid shim that was precisely the right depth. I therefore used a very hard rubber block which deforms very slightly at the super tight point (you can feel it by the increased pressure to screw it into the identical super tight depth).

So, in summary, I think it is not a good idea to make a shim that prevents the tuttle head from going in as far as it needs to go to be super tight both sides, even if that means its sunken in a bit, but once you have got to that point, if there is a void - especially above the front - then its best to fill it so the front bolt cant get pushed up by the very large up force on the front of the tuttle head when foiling. I think thats what Starboard recommend.

The other side of that coin is to reinforce and increase the size of the fin bolt washers - especially the back one which takes a lot of force and cant be too big (mine is 6cm diameter...). Its interesting that Starboard have got that a bit wrong on some of their boards - especially the iqfoil - there isnt room to put a large washer in the oval shaped sunken bolt holes. Fortunately upside down anti twist footstrap plates fit perfectly and are now officially recommended (may even come with the new gear as standard now).

ZeroVix said..

Sandman1221 said..

ZeroVix said..

Sandman1221 said..
So following your advice his board and foil ended up getting damaged.

Huh? Who's advice?

Segler's, just read Ian's posts and you see see why I said that.

Your comment is childish and disrespectful. You need to spend time talking to people that make foil boxes or that have a lot of experience with them over 30+ years. There is nothing wrong with what Segler said. He is correct. A foil approved box is different from a deep tuttle box. The disclaimer from Starboard is for foil boxes, not the SB deep tuttle box from Ian85. There is a reason why I purchased a foil box from Seatex for my Gaastra Vapor 3 board. Does it hold up with my 105kg and 70cm fin? Yes. Was it designed to hold up with my carbon racing foil? No. I understand the limitation and design. I opt not to destroy the board and replace the box. Just a piece of mind. It is only a matter of time until a board that doesn't have a foil box gets damaged. By that, I am not talking about using it to learn, rather when you put a lot of force on it with a larger mast. Most of us are mature enough to know what and who to listen to. But blaming Segler for the board getting damaged? Really?

www.seatexboards.com/deep-tuttle-vs-deep-tuttle-foil-ready-differences/

Just stated the facts, he followed Seglers advice which convinced him to stop using the shim and next time out the board and foil got damaged, I do not see anything childish or disrespectful about that.