Building an Aluminum Radar Mast

[Roger G]

I’m looking to build and install a radar mast on our boat and I was hoping for some engineering advice. The mast will be attached to the back wall of the cabin. The wall is fibreglass over ½” plywood. Here is a pic of where I plan to attach it. The white round cardboard tube is where I would like to attach it.

This is a mock up of the attachment point. The mast will be made out of ¼” aluminum tube 3” diameter. The mast will be welded to this 5” x 7” x ¼” mounting plate. The wall will be sandwiched between this and another 5” x 7” x ¼” backing plate.

The mast will have a 30 degree back rake and be 10’ long with a 20 pound radome mounted at the top. What I am wondering is whether the base mount will be enough strength to secure it, or should I add a couple of braces? I could attach a couple of braces to the railing at the flybridge shown here if necessary but I would rather not.

Any thoughts would be appreciated.

 

[viking north]

At 10feet long with a 20lb. load I would add braces to something more substantial than the rails. The whipping action of the 20lb. top load will induce some movement at the rails and this cycle loading on the solidly bolted fastening plate will cause it to crack at the weld. Just a matter of time. I would run the 3 in. dia. dome mast all the way down to the deck to a welded or mechanically fastened plate bolted to the deck. Tie the pole into the cabin wall by use of a stainless U bolt. If it's necessary to run your dome wiring into the top area of the cabin simply weld a T pipe stub to the mast and feed it thru the wall with 5200 or equivalent as weather caulking.
P.S. don't forget to drill a small drain hole say 1/4 at the base to prevent water freezing and splitting the mast.

 

[PAR]

A 10' long mast with a 20 pound weight will have 200 pounds (static) load on the mount, so nope, the bulkhead will likely buckle. Now, once the boat is underway and bouncing around and flying off waves, the cantilever loads will triple or quadruple on the welds, mount and it's fasteners. This doesn't even include a reasonably safety margin.. You'd be much better off if the mast was straight up, not canted 30 degrees, but if you insist on this angle, you'll need a much bigger plate and backing plate on the other side. Personally, I'd like to see a compression post, spaced off the bulkhead a bit, taking the loads to the deck as well as easing the mount's strain.

 

[JSL]

Looks like a TollyCraft 30. A Radarch might be a better choice

 

[Roger G]

Thank you all for the input.

Yep, it is a Tollycraft 30 Sedan. I’d rather not go with a radar arch if it can be avoided.

The cockpit in our boat is quite small so running the mast to the floor is not a great option.

A vertical mast with no rake is an option but I do like the 30 degree angle as it matches many other angles on our boat.

Are there any advantages to mounting the mast to the flybridge deck above rather than the wall? The deck is fibreglass over ¾” plywood. Would adding a short section of ¼” thick 3 ½” diameter pipe as collar to reinforce the mast at the base help? Perhaps adding three or four triangular gussets at the base? Are there proven design plans available for purchase? Thanks again for your help, as I would really like not to make a mess of things.

 

[viking north]

Keep in mind that there is a definite health hazard aspect here and the radiating element housed within the dome should be a safe distance above head height for the entire 360 deg. radiation pattern. In other words if the radar dome is only three feet or so off the flybridge deck it is not advisable that the deck be occupied with the radar operating.
The idea of mounting to the flybridge deck is an option but welding on gussets conjures up commercial fishing boat engineering. A cone shaped base support would be more yachtie. As such you are still gambling with messing up the factory fit and finish of your boat, If as mentioned above a proper radar arch has been engineered and is readily available for your craft, why mess around with home built add on's.

 

[Barry]

Without being a naval architect and determine the acceleration of the radome and mast due to rolling or pitching, it will be difficult to figure out the loading in the bolts that will hold the mast to the bulkhead.
But statically there is less of an issue and then if an appropriate factor of safety (factor of ignorance) can be applied to limit failure.


The 10 foot mast at 30 degree back slope will give you a length of 8.66 feet above the center of the mounting brackets and the top of the mast will be sloped back 5 feet of the face plate of the mounting bracket.


The loading of the mast of the weight of the radome and the weight of the mast will impart a moment (torque) to the bracket and the screws that attach the bracket will have to oppose this torque.

So you have a 20 pound radome force acting at 5 feet from the bracket face which will produce 100 foot pounds of torque and the weight of the 3 inch tube of 25 pounds times half distance of the 5 feet overhang which will add another 62 foot pounds of torque. This will give a total of 162 foot pounds of torque that the bracket and screws will have to resist

As the formulas for stress and load calculations use moment instead of torque, I will use moment from here on.

The bracket from the top hole to the bottom of the bracket appears to be 6 3/4 inches and the distance from the lower holes to the bottom of the bracket is .75 inches

Taking the bottom of the bracket at which to calculate the sum of the moments and making a big assumption that all the screws will share in the load equally which means that bracket or the bulkhead will not bend under load.

The calculations show that for static loading, each screw will have to hold 130 pounds in tension.

Additionally there will be the weight of the mast and radome what will impart a shear load to the screws/bolts as well but will be basically the weight of the mast/radome divided by the 4 screws.

Again, static load only.

The additional loading due to the acceleration of the radome/mast will add additional forces to the screws.

The further apart the screws, the less load that will be on them.

So the two things I would do, not knowing about the bulkhead strength is to increase the length of the bracket to get the screws further apart and use a thicker bracket base to minimize distortion in the bracket plate which will help keep the loading on all the bolts the same. The more distortion, the more the load on the screws will be.

Then I would add to the mast a bracket above the canvas a gusset that will bolt to the top of the roof, properly sealed etc so that the weight of the mast etc will be carried by this bracket. This upper flat bracket with a brace will then make the distance between the upper new bolts and the lower bracket lowest screws larger which means that each screw will have to take even less load.

Not sure if this helps,
Note also that there might be a chance that there will be wires in the bulkhead above the door crossing from one side to the other when drilling the holes.