Sheer bolts for O.S Max .21 and K&B .21 using Dubro .21 bracket

[riggerman]

Hi all last summer while running my Dunlap W.O.F 30 with UL-1 2030 kv o/b i struck a under water log at 45 mph tore the 3/8 birch ply transom out and pulverized the R.E.K lower unit . I retro fitted the tunnel with a O.S .21 lowerunit i have from my nitro racing days looking at the install got me to thinking if the same incident where to happen again what would stop the same damage from happening once again ? On my inboard set-ups i install a bottom nylon sheer bolt on the rudder and turn fin. What do outboard racers use on the O.S Max .21 lowerunit ? What about the K&B .21 lower with a Dubro 3.5 bracket ? Pictures would also be of great help. Regards Andris Golts.

 

[Jerry Dunlap]

Very interesting question - to which I don't have an answer regarding the O.S. 21 lower unit since I've never even considered replacing the machine screws in the lower unit. When I was running K&Bs, I used the DuBro mount with the black fiber insert and it did sheer when the boat ran over something or into the beach. There's reasons I know that.

I guess that wasn't all that much help. Maybe another IW member has a suggestion.

JD

 

[Brandon Atwell]

I've personally never seen anything done to existing mounts, but I have seen a custom mount made that when the lower hits shore, log, other boat, etc. it kicks up the motor to save the transom and lessen the force of the impact.
An idea you may look at is replacing your bottom bolts of the mounts with Teflon bolts, as when it hits something it would snap the bolt and only leave you with replacing the bolt and resetting your trim? I'm not sure how viable it would be over all, we typically use them for our cowlings and when they flip it typically shears the bolts rather than rip the cowling at the screw holes.

 

[Kris Flynn]

I would be more focused on having a good, strong transom. But nylon bolts like Brandon said would be a good place to start.

 

[Grimracer]

I prefer to take my lumps.. if I wack a turtle and or what ever I accept the risk.. I try to bolt tight and dont want anything shearing or moving around on me.

Grim

 

[SayMikey]

Ive seen entire a few complete outboards go to Davy Jones locker. Fasten as secure as possible.

 

[Davidrichmond]

I ran across this photo, don't know anything about who makes it but it gives good ideas.

 

[Jerry Dunlap]

Screenshot_2018-04-16-21-46-04.png

I ran across this photo, don't know anything about who makes it but it gives good ideas.

That was actually offered by K&B to lift the prop thrust angle in a corner to keep tunnel boats from hooking.

JD