Turn Fin Mounting to hull bracket

[Mike Rappold]

I have built two 1/8th scales in the past where I bolted the turn fin to the mounting bracket using two screws. One screw is steel 8-32 and the other a brass 4-40 screw. The thought being that like a rudder, if the fin was to strike something in the water, the brass screw would shear and kick the fin up. This prevents damage to the hull.

I have not had to use this functionality yet - luckily. I am building a new 1/8th scale and was wondering what is everyone else doing for the fin mounting - bolt it directly to the bracket or provide some form of shear pin?

As a related reason for asking, with my current designs the bracket used to attach the hull is bolted to the rear of the sponson. This is rigid here but I believe that the fin is flexing farther back. The real Unlimiteds use a turnbuckle type device to attach the fin to the side of the hull. I like this idea to keep the fin from flexing. However if I go this route on my new hull, I then loose the shear pin feature.

Hopefully I made this clear enough to understand. Any info on what everyone is doing in this area would be apprecaited.

Mike Rappold

 

[SayMikey]

I have built two 1/8th scales in the past where I bolted the turn fin to the mounting bracket using two screws. One screw is steel 8-32 and the other a brass 4-40 screw. The thought being that like a rudder, if the fin was to strike something in the water, the brass screw would shear and kick the fin up. This prevents damage to the hull.

I have not had to use this functionality yet - luckily. I am building a new 1/8th scale and was wondering what is everyone else doing for the fin mounting - bolt it directly to the bracket or provide some form of shear pin?

As a related reason for asking, with my current designs the bracket used to attach the hull is bolted to the rear of the sponson. This is rigid here but I believe that the fin is flexing farther back. The real Unlimiteds use a turnbuckle type device to attach the fin to the side of the hull. I like this idea to keep the fin from flexing. However if I go this route on my new hull, I then loose the shear pin feature.

Hopefully I made this clear enough to understand. Any info on what everyone is doing in this area would be apprecaited.

Mike Rappold

I use a shear pin most of the time you trash the fin but the bracket stays intact as well as the hull....Mike

 

[Russell Bear]

I have built two 1/8th scales in the past where I bolted the turn fin to the mounting bracket using two screws. One screw is steel 8-32 and the other a brass 4-40 screw. The thought being that like a rudder, if the fin was to strike something in the water, the brass screw would shear and kick the fin up. This prevents damage to the hull.

I have not had to use this functionality yet - luckily. I am building a new 1/8th scale and was wondering what is everyone else doing for the fin mounting - bolt it directly to the bracket or provide some form of shear pin?

As a related reason for asking, with my current designs the bracket used to attach the hull is bolted to the rear of the sponson. This is rigid here but I believe that the fin is flexing farther back. The real Unlimiteds use a turnbuckle type device to attach the fin to the side of the hull. I like this idea to keep the fin from flexing. However if I go this route on my new hull, I then loose the shear pin feature.

Hopefully I made this clear enough to understand. Any info on what everyone is doing in this area would be apprecaited.

Mike Rappold

Yep, I use a nylon screw as the shear pin.

 

[Eric Bourlet]

Mike, you have hit on a very important issue about turn fins, flex kills you in the corners. We always run 4-40 all thread through a brass tube to a turn buckle from the farthest point back on the bracket through the side of the boat and into the engine mount area to keep the turn fin as rigid as possible. You should be able to do this directly to the angle bracket and still have your kick up feature if you use a large enough angle for your bracket.

 

[GRoth265]

Russell: The set up you are using sounds correct. You can add a turnbuckle to the rear of the fin and still keep the shear feature. Just use a du-bro style ball link mounted on a L bracket to the side of the boat. The 4-40 type is what I use with a allen head bolt and a lock nut thru the fin. If you hit something believe me the plastic end will pop off the ball and no damage to the boat. You can adjust it by screwing it in or out of the plastic end. The more ridged you can make the fin the better, usually you find that you have much more positive rudder response. Greg Roth

 

[Brett Buckmaster]

Gents,

Anyone got any photo's of this set up??

Curious scaleboater.

 

[Mike Hughes]

Link 1 Link 2

 

[Russell Bear]

https://www.intlwaters.com/gallery/displayimage...lbum=103&pos=47

https://www.intlwaters.com/gallery/displayimage...lbum=103&pos=48

Mounted to the 1/4" ply sponson transom with aluminum backer and another piece of ply all epoxied together with blind nuts installed inside. Should be tough enough.

 

[Don Ferrette]

I have to disagree somewhat on the use of a turnbuckle or similar bracing. The 2 biggest mistakes I see most people make is way too small a footprint where the turn fin bracket bolts to the heel of the sponson & not enough contact area between bracket & the turn fin itself. First, assuming your fin bracket is good thick angle stock, you need to have a solid backer behind the sponson heel & personally 1/4" hard ply or 3/16" aluminum plate is the absolute minimum. Then, and this makes a HUGE difference, mount your turn fin bracket using 3 bolts in a triangular pattern. 2 screws, one directly over another gives very little strength against pulling & flexing. Sometimes you need to get creative in shaping the part of the bracket that bolts to sponson heel due to non trip angles and/or deck overhang but believe me this is the way to go. Second, try to have as much surface area between the turn fin bracket & the mounting face of the fin. Too often this is an area that I see people skimp on. Also, if space allows, stagger your primary fin mounting hole & your shear bolt hole so they are not in a straight line. A properly made & mounted bracket will not flex before the lower part of the fin that's in the water does, and they all do to some degree. Try this simple test, take your fin & bracket built like I described & clamp the assembled pieces into a vise & try to bend or flex it. If it's done right you're going to find it really doesn't move & you are capable of exerting far more pulling force on that fin & bracket clamped in a vise than that 15 pound scale hull will ever put on it. Not meaning to start a debate here fellas, rather a suggestion from what I've seen in the field.

.... just my 1 1/4 cents after taxes. B)

 

[Mike Hughes]

A properly made & mounted bracket will not flex before the lower part of the fin that's in the water does, and they all do to some degree.

It is not the bracket that we are trying to keep from flexing. It is the whole thing from flexing on the back of the sponson. Over a few races the mount begins to dig into the glass or wood and get out of alignment no matter how big a foot it is. By adding the turnbuckle support this is eliminated. May I ask how you keep the fin line up with the center line of the boat? A small washer under the edge of the bracket makes for a large adjustment. With the turnbuckle there is no need to have special shims or worrying that they may fall out and there goes the setup. It is a very simple solution that gives you easy adjustment to the fin. Some baots don't allow for a large footprint by the way. Espesially the glass boats.

Mike

 

[Don Ferrette]

A properly made & mounted bracket will not flex before the lower part of the fin that's in the water does, and they all do to some degree.

It is not the bracket that we are trying to keep from flexing. It is the whole thing from flexing on the back of the sponson. Over a few races the mount begins to dig into the glass or wood and get out of alignment no matter how big a foot it is. By adding the turnbuckle support this is eliminated. May I ask how you keep the fin line up with the center line of the boat? A small washer under the edge of the bracket makes for a large adjustment. With the turnbuckle there is no need to have special shims or worrying that they may fall out and there goes the setup. It is a very simple solution that gives you easy adjustment to the fin. Some baots don't allow for a large footprint by the way. Espesially the glass boats.

Mike

Well that is why I prefer at least a 1/4" heavy ply or 3/16" aluminum backer. That combined with the actual hull thickness makes for a very strong mounting point. As for the digging in that seemed to go away at least for me once I went to the larger triangular footprint for the bracket. Anytime you spread the load over a bigger area & triangulate mounting points you gain strength. The brackets are digging in because they are moving & typically I've seen that from smaller footprints using in line mounting points. I will agree that a turnbuckle makes for easy adjustments though. I spend alot of time on initial fin set up & alignment & I guess I've been lucky so far on all the scale boats I've done.

BTW- looking at your pic of this fin mounting-

https://www.intlwaters.com/gallery/displayimage...cat=10058&pos=1

There is plenty of room to triangulate your mounting with your 3rd hole being in lower left quadrant. I have yet to see a scale that could not be done this way.

 

[Eric Bourlet]

While you have some good points Don to say you disagree with the turn buckle method is interesting. You have to understand that there is always different ways to accomplish things and to just dismiss an idea by starting out with the fact you disagree is a bit off base. In addition to providing a stiff mount the turn buckle also gives you a good way to make sure the fin is square to the boat. I am sure your method works for you and I know that our method has allowed a Mutt II sport 40 to win the NAMBA nationals several times not only in the Sport 40 II class but also a win at the Nationals in 7.5 Hydro class. While I find our method to work for the reasons I just stated I would not discount your method off hand either.

 

[Don Ferrette]

Eric I said I disagree somewhat which last time I checked meant not completely.

I also agreed it offers an ease of adjustment, just wanted to share how I do it & why. B)

 

[Brian Blazer]

With a hooked turnfin, I have been using the one bolt method without a shear pin. This gives me unlimited adjustment to get the pefect ride attitude. On my bigger boats, I use a 3/8" hex bolt so I can really lock it down. On my smaller boats, I use 1/4" bolts. I have never had a fin loosen up on me with only using the one bolt. You must use Hex Head bolts so you can crank it tight.

Sounds wierd, but it works awesome!

Brian

 

[Mike Hughes]

I like the ease of the adjustment. It makes it simple. It works for me. The one thing I can say that turnfin bracket should not move when in a corner. How we get there is up who ever sets up the boat.

Mike