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Problem is there aren't 2 identical boards with different fin boxes fitted to try back to back. The screws aren't doing anything to stiffen up the connection it's depth and surface area that gonna make it stiff and both Tuttle and powerbox have pretty much the same surface area in the Box.
I suppose there are two questions here .
1 , what is ultimately the strongest box before failure and ,
2 , which box creates the tightest fit so you dont get fin wobble .
I would think both boxes are similar in resistance of getting ripped out of the board . Possibly less chance of splitting a PB box when hitting something . Stripping one nut would need less force to let the fin come out than having to strip two nuts ???
The tuttle fin only wedges in on the ends so if there is any gap on the sides it will wobble .
The PB fin wedges on all sides and sits on the bottom so if the bottom mating area is a bit high it will also wobble .
A deep tuttle probably wins because it has a lot longer side to side mating area that holds the fin reducing wobble for the same amount of gap . A normal tuttle should be similar to a PB.
From an engineers point of view , and , someone who uses both boxes 
^^^
I think it could be more . Heavy me pushing extra hard between swell gulping for upwind with a 52cm fin . Thats a lot of leverage .
Has anyone broken any box from just sailing with sideways force ?
I've repaired a few chinnook US strong boxes that snapped the peg, then rolled. But not sure if that was just sailing load.
Nobody has mentioned it yet, unless I missed it, so I will.
I hate both Tuttle and Power boxes for one reason only, the board manufacturers who install those assume that everybody who uses that board, likes to have the fin in the exact same place. No need for forward or back adjustment, because we know best.
BS, we don't all weigh the same, and we don't all have the same sailing style, or needs.
So I've always considered it a form of arrogance on the part of the board builder, who uses those for mass consumption.
Totally matters to a good sailor. Fin shifts held with one screw and spinout can occur.
Less engineer, more windsurfer is required to answer your question. Ask any good windsurfer to use ONE bolt on his tuttle box.
That's where actual windsurf experience comes in, over useless theoritical think tanking.
Power and Meritex is centered single screw. Centered meaning no shift and balanced loading.
Trim is 1 screw and holding tab, for 2 points retension.
Did you know Tuttle is actually designed for 2 screws, 1 back and one front? Like any other part designed for TWO screws, using ONE compromises strength and holding power allowing shift/movement causing excessive point loading which stresses the system. Why do you think Tuttle systems are generally a tight fit? And TWO screws are used by all good sailors..except you?
agree that due to the placement of the screws Tuttle needs 2 to fit the fin but they dont add to stiffness once the fin is in. One bolt in the centre would work just as well.
Only advantage with 2 bolts on Tuttle is it allows you to pull the fin in more at one end than the other to get is to sit flush.
Well, keep sailing your boards with one screw tight, the other loose or missing, and good luck to you.
Now if you do decide to snug BOTH screws, well, welcome to the real world, away from the think tank world.
Something no one seems to have mentioned is the failures that happen when you hit something immovable like a coral-head.
in my sailing life I have hit coral heads, a concrete channel marker/reinforcement, and sand bars. The first two ended up in breaking the fins. They were powerbox so they broke and just dropped out. In the case of the concrete channel marker, I went back to find the fin at low tide.
They seem to break right through where the barrel nut is and then the fin drops out, leaving the box intact. I have not ever had a damaged fin box or board.
What happens with tuttle? Anyone care to share?
