DIY Hobie mirage drive fins?

I found YT video where a guy makes DIY fins for his mirage drive (it's a long and rambling video, but if you look at about 1:40, the set on the right is the one that seems to work reasonably well):



I might try something similar, as I need a couple of pairs of turbo fins, and the authentic Hobie version costs $70 each, while even the fake ones are $40 each.

But, I was thinking that it might be possible to create a mold from a turbo fin, and then make fins similar to the real deal using pourable liquid rubber. I'm wondering if anybody has any idea what kind of rubber the fins are made from, or any knowledge of a pourable version of rubber that might hold up to the abuse that the fins take...

I saw someone discussing this on a Hobie AI forum on Facebook and thought it was pretty interesting. Look forward to what ever you come up with!

Very interesting, curious to see what you come up with.

I think the Hobie fins and others are probably two or three types of materials - HDPE or similar for stiffness in the shape/body, and some sort of rubber for the edges to help provide a smooth "flap" motion.  Maybe some sort of nylon or other really stiff material for the shaft sleeve.

For DIY, the best parts will probably be the trickiest. Maybe use butyl rubber, like the thick black bungee or bicycle tubes? That stuff is pretty strong when in thicker pieces, but I don't know how it will hold up to sun and saltwater. Would be easy to find and work with tho...

I've dabbled w/ pourable silicone for creating molds of small objects -- I think a 40 on the Shore A scale might get fairly close to approximating the rigidity of Hobie fins. I bet you'd be able to make the fins more resistant to abuse by incorporating fiberglass cloth. Wetting the cloth out might be a little tricky given the high viscosity of the pourable silicone but I doubt that's insurmountable.



 

I'd like a pair of jumbo turbo fin's please 

I'd like a pair of jumbo turbo fin's please 

Ha ha! Yes, that's the plan. The masts for the turbo fins are 14", and I've already made a couple of 16" masts. Sorta a "cart before the horse" thing...

I've dabbled w/ pourable silicone for creating molds of small objects -- I think a 40 on the Shore A scale might get fairly close to approximating the rigidity of Hobie fins. I bet you'd be able to make the fins more resistant to abuse by incorporating fiberglass cloth. Wetting the cloth out might be a little tricky given the high viscosity of the pourable silicone but I doubt that's insurmountable.

I saw something that is listed as 50A and thought that it might be worth a try:

https://www.alumilite.com/resins/flex-urethane-50/

Do you think that would not have enough flex? Also, the idea of including fiberglass is interesting. This person repaired a torn mirage drive fin by melting a wire mesh into it, which is kind of the same idea...

https://www.stripersonline.com/surftalk/topic/658257-repairing-a-torn-fin-hobie-mirage-drive/

I saw something that is listed as 50A and thought that it might be worth a try:

https://www.alumilite.com/resins/flex-urethane-50/

Do you think that would not have enough flex?

The flexy part of the fin is pretty thin, I'd think that 50A would still have a good bit of flex in a structure so thin. It looks like the manufacturer recommends a degassing setup... A few times I mostly got away with using a tall pour in lieu of a degassing chamber, but I'm not sure how well a tall pour would work when you're pouring the liquid into a female mould. But also a few bubbles in the medium probably wouldn't be the end of the world in this particular application.

Also, the idea of including fiberglass is interesting. This person repaired a torn mirage drive fin by melting a wire mesh into it, which is kind of the same idea...

https://www.stripersonline.com/surftalk/topic/658257-repairing-a-torn-fin-hobie-mirage-drive/

Thinking more about it, the fiberglass may be more trouble than it's worth... My understanding of fiberglass tech is pretty nascent but I think you'd have to find a cloth that plays well with whatever medium you end up using, and then you'd have to develop a technique that consistently saturates the cloth.

If you go the two piece female mould route, I'll be interested to see how you deal with the void for the mast. I can think of two general approaches:
(1) Design the female mould in a way such that the mast void is represented by the mould,
(2) Incorporate a mast in the casting process to create the mast void.

I'd probably set things up so that one half of the female mould contains almost the entire fin, and the other half of the female mould basically just caps off the top and holds whatever creates the mast void -- tried to represent this idea in the below pic... red represents half of the mould, green represents the other. Basically the red side would go as high as possible while still allowing you to pull the casting out of the mould.

One other thing -- if you do create female moulds, and you wind up using a silicone to create the moulds, be mindful of the fact that even trace amounts of sulfur can inhibit silicone curing. Absolutely something that can be mitigated, but should probably be on your radar.

This all interesting but looks to me technologically difficult. The combination of stiffness and flexibility and incorporating a rod shaft hole is a challenge along with thick to thin dimension in the fin. Perhaps, like a wooden oar for a rowboat, maybe a solid plastic fin would work but if one needs all the structural elements and material elements of a Hobie fin to make something as good as Hobie it will be a tough project. Good luck though. I'm all for a cheaper alternative.

I saw something that is listed as 50A and thought that it might be worth a try:

https://www.alumilite.com/resins/flex-urethane-50/

Do you think that would not have enough flex?

The flexy part of the fin is pretty thin, I'd think that 50A would still have a good bit of flex in a structure so thin. It looks like the manufacturer recommends a degassing setup... A few times I mostly got away with using a tall pour in lieu of a degassing chamber, but I'm not sure how well a tall pour would work when you're pouring the liquid into a female mould. But also a few bubbles in the medium probably wouldn't be the end of the world in this particular application.

Also, the idea of including fiberglass is interesting. This person repaired a torn mirage drive fin by melting a wire mesh into it, which is kind of the same idea...

https://www.stripersonline.com/surftalk/topic/658257-repairing-a-torn-fin-hobie-mirage-drive/

Thinking more about it, the fiberglass may be more trouble than it's worth... My understanding of fiberglass tech is pretty nascent but I think you'd have to find a cloth that plays well with whatever medium you end up using, and then you'd have to develop a technique that consistently saturates the cloth.

If you go the two piece female mould route, I'll be interested to see how you deal with the void for the mast. I can think of two general approaches:
(1) Design the female mould in a way such that the mast void is represented by the mould,
(2) Incorporate a mast in the casting process to create the mast void.

I'd probably set things up so that one half of the female mould contains almost the entire fin, and the other half of the female mould basically just caps off the top and holds whatever creates the mast void -- tried to represent this idea in the below pic... red represents half of the mould, green represents the other. Basically the red side would go as high as possible while still allowing you to pull the casting out of the mould.

One other thing -- if you do create female moulds, and you wind up using a silicone to create the moulds, be mindful of the fact that even trace amounts of sulfur can inhibit silicone curing. Absolutely something that can be mitigated, but should probably be on your radar.

Good info! I don't think I'm going to get to this any time soon, but it'll be a good winter project...