Boom tube material and fastening

[dr gary turner]

Water pick up????

 

[John Finch]

I bought some of the tape suggested by Bob and it works great for insulating my wires on the electric boats. Looks cool too.

 

[Jim Allen]

delete

 

[Jim Allen]

Water pick up????

Gary,

Dual pickups, adjustable depth with movement in & out. I do not allow the pickups to drag in the water at speed.

JA

 

[dr gary turner]

Thanks, very neat.

 

[BobBonahoom]

Thanks to Stu Barr for sharing the info about that tape at Hobart.

 

[Aaron Lavigne]

I use telescoping tubes with solid rods for mounting sponsons. The woven graphite tubes have 95% unidirectional carbon fiber running longitudinally. The tubes are covered with a light glass scrim & have a high stiffness to weight ratio. I combine these tubes with solid carbon rods made from continuous carbon fibers in a epoxy base. The rods are extremely stiff & lightweight because they use high modulus carbon fiber. The solid carbon rods pass through machined aluminum mounts which have an "O" ring groove in their ID's. The aluminum mounts are grooved on their OD's & are epoxied in place with #406 colloidal silica thickened # 105 resin. The same epoxy mix is used to glue the plugged end carbon tubes into the sponsons. The solid carbon rods are held in place with 1" length carbon tubes epoxied to each side of the hull's aluminum mounts. I slip the tubes mounted in the sponsons over the solid rods & tape them in place with 3M nylon reinforced scotch tape. Tubes glued into the sponsons are mounted in a jig to ensure that all locations are square & identical. An aluminum reinforcement is necessary on the skid fin side. Sometimes in larger hulls the reinforcing plywood piece is replaced with linen based phenolic.

Jim Allen

I did it similarly on my first rigger I built after speaking with you Jim, but it is difficult to adjust the angles unlike other setups that use shims. Do you or anyone have any ideas on how to be able to adjust the angles slightly with this setup?

 

[Jim Allen]

Aaron,

I use precisely machined wedges similar to the ones shown in the photo when planing angle adjustments are necessary. Take a look at the Graupner SX4000. It has a simple method for making very slight angle adjustments. A similar method is used on A-3 tether hydroplanes.

JA

 

[Aaron Lavigne]

Are you taking you make the wedges to put on the bottom of the sponsons? Most riggers usually adjust at the tubes

 

[Jim Allen]

Are you taking you make the wedges to put on the bottom of the sponsons? Most riggers usually adjust at the tubes

Aaron,

Yes. They are machined in a fixture on the milling machine. I use the same method to machine, in various materials, full wedge shaped rudders & skid fins that are as thin as .0625". Sponson wedges are usually made from plastic, fiber glass or carbon fiber.

JA

 

[Aaron Lavigne]

Interesting, thanks for the info...

 

[NOVALTD]

JIM ALLEN: I like your rudder and turn fin and wanted to know if there is any flex from either one in making high speed turns? Also what grade of metal do you use for .0625 ? stainless steel that gets heat treated after machining? You do nice work? Thanks for the help.----DAN------

 

[Jim Allen]

Dan,

There is no flexing of the rudders or turn fins. This would include rudder blade mountings & rudder pivot points. The same applies to skid fins & sponson mountings. All aluminum rudders are made from 1/4" thick 7075-T651 grade aluminum. Bigger riggers use 1.500" wide blades & smaller riggers use 1.250" wide blades. Rudder lengths vary from 3.500" to 4.250". All rudder blades are full wedge type with tapered backs. All aluminum blades have a full hard steel insert where the shear pin is located. Some have rounded bottom edges & some have sharp bottom edges.

Aluminum skid fins are made from 3/32" thick 7075-T651 grade aluminum. All are supported down to the, at speed water line, with 1/8" thick carbon fiber backing plates. Some are bent & some are mounted on angled mounts. An angled mount with a straight blade is much easier to set up than a straight mount with a bent blade. Steel .0625" to .125" thick rudders or skid fins are machined from AISI-O1 tool steel. They are hardened after machining, sometimes by using a holding fixture & then tempered to a spring temper. All are a full wedge shape over their entire width & length. Some have rounded bottom edges & some have sharp bottom edges.

There may be some grades of stainless that could be used for these parts. However, machining may be more difficult. I have machined .0625" thick skid fins from blue spring steel material. This can only be done with carbide mills & a very rigid fixture. Grinding of the spring steel, even with coolant, is not possible because of the heat that is generated.

JA

 

[NOVALTD]

Jim: Thanks for the reply. Have you ever used a cutting knife that has been heat treated and had success running them. ? My feedback from modelers is that they have had success with no tracking problems. Thanks Dan

 

[Jim Allen]

Dan,

I have never used a cutting knife for a rudder blade because there are many different steels that could be used to make a quality cutting knife. Using a blade that is already hardened would make any machining operations such as drilling, milling, cutting, grinding, etc. very difficult. The benefit of a true wedge shape, the vortex that is formed from that shape & what it does as it passes through the water should not be overlooked!

Maybe this is a reason that skid fins & rudders are shaped as shown in the photo.

JA

 

[Jack ODonnell]

Dan, Cant take credit for this as I got the info from Tyler Garrard. The hammer knife from McMaster-Carr part # 3956a12. These are a full wedge. They work very well ! J.

 

[NOVALTD]

Thanks Jim and Jack for your replies---DAN-----

 

[Aaron Lavigne]

Jim- Can you explain how a turn fin is made? Can you wire edm the overall shape, including the "knife" edge and then bend on a certain press? Is the 7075 aluminum strong enough? I need to have a turn fin made and been talking to a few shops and they don't want to do it. I don't have the tools to do it myself and like the hook of a machine's work rather by hand

 

[Jim Allen]

Aaron,

All my turn fins are not made the same way. Both aluminum & steel fins can be cut with a band saw, shear, EDM or CNC mill. The width of the knife edge will determine if the leading edge is filled or milled to shape. Filling is easy & works well on the Andy Brown type aluminum skid fin shown in photos #1 & 2. Both aluminum & steel fins can be bent in a manual break or in a quality milling machine vise such as shown in photo #3. My testing shows that the 7075-T651 aluminum fin is rigid enough if it is .0938" thick & it has the .1250" thick carbon fiber reinforcing support.

.0625" thick skid fins never have their knife edges filled. They, as well as .1250" thick steel rudders & .2500" thick aluminum rudders, are milled to a true wedge shape front to back. The milling is always done in a holding fixture. The necessary angle of the fixture's side or base is calculated with trigonometry.

JA

 

[Aaron Lavigne]

Thanks for sharing, do you mean "FILED" rather than "filled" I am not sure what you mean by that. Seems like I will need to find a shop who can do the proper bending