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Author Topic:   RADAR Mast or Lightweight Gin Pole

anthonylisske posted 02-17-2012 11:12 AM ET (US)
For two years [I have] been toying with the idea [of installing a RADAR mast or a lightweight pole]. I am gonna make a go at it, and I need advice on what schedule and pipe to use. I have 3-inch schedule-80 pipe, 3.5-inch O.D with a wall thickness of about 3/8 inch, and liked it at first; but I think it is overkill. [I] would like to go 2-inch schedule 80, 2.35-inch O.D with a wall thickness of about 1/4-inch. This way I can run it through the gunwale on my 1972 smoothside Outrage without really messing up glass. [The] hole would be similar in size to a rod holder. The height of mast would be about 8-feet to bottom of radar base. The main points of contact would be at base plate on floor and aluminum plate on top of gunwale. This would be install through gunwale next to fiberglass back seat, so I might box-in the gunwale with plate aluminum for strength.
I know the 3-inch schedule-80 pipe would work, but do you think a 2-incher would do it? I may be missing data needed for advice, so please ask for any tech info you would need. Thanks in advance. Anthony
anthonylisske posted 02-17-2012 12:59 PM ET (US)
Oh yeah, I am talking about Aluminum pipe
dfmcintyre posted 02-18-2012 07:26 PM ET (US)
Without any tech info, I think for a pole that long with a 14 - 18lb object on the end, I'd personally go with the schedule 80, 3" pipe.
And I'd also place a backing plate under the gunwale. Is the "trough" (classic 21 owners know of what I describe....) wide enough for an schedule 80 though?
Any thoughts on having the ability for quick removal? I've got some ideas.
Regards - Don
jimh posted 02-18-2012 08:15 PM ET (US)
This is a very simple bending moment calculation. You need to find the bending moment on the mast. Then you must know the tensile strength of the material. With that information you can easily do a force analysis to see if the mast will fail. The point of failure will always be at the lowest point of the mast where it is no longer supported.
To calculate the bending moment you need to know the projected area of the mast and anything attached to it. Then you need to know the wind pressure on the mast. If you will be towing the boat on the highway, use 86-MPH for the wind. That will be the wind when you drive 55-MPH into a 30-MPH headwind. At 86-MPH the wind pressure is 30-lbs/1-foot^2.
Sum the moments above the mast from its point of support. For tubular structures you can derate the projected surface by 0.66.
Once you have the bending moment you can see if the material has enough tensile strength. Aluminum that is used for masts is usually 6061-T6. You can look up the tensile strength. There are some stronger grades, but for marine use the 6061-T6 will probably be better for corrosion resistance. The tensile strength will be about 35,000.
You cannot really approach this with a by-the-seat-of-the-pants analysis. You will be very surprised how much bending moment can be created and how much strength is needed. There is a reason that most masts are made of steel. Steel can have much higher strength. Steel used in masts is often 120,000-lbs tensile strength.
jimh posted 02-18-2012 10:38 PM ET (US)
Also, most masts are made from tubing. Pipe is typically designed to carry liquid and to resist the bursting pressure of the liquid. Pipe is not designed to carry large bending moments.
anthonylisske posted 02-19-2012 08:52 PM ET (US)
Thanks for the replies Don and Jim.
It will be close on the 3 inch. I will remeasure tomorrow, but I think it can take the 3.5 inches. It is just a matter of ugly. Lol.
I might put it on the inside of the gunwale rather than through it.
Don, I would love to hear any suggestions on the quick disconnect. I was gonna make it permanent, (since the radar wire would make it a bitch) but, I was thinking of putting a mitered plate about 12 inches above gunwale. So I could rotate the tube and rebolt at a 45 degree angle.
I will re work the ideas tomorrow.
Thanks
jimh posted 02-20-2012 09:09 PM ET (US)
For calculating the stress on a mast, I found this interesting calculator on-line at
http://www.math.niu.edu/KARC/mast/
I entered the following data as input for the calculator:
Safety factor = 2
Wind loading factor = 0.004 (default)
Outside diameter = 2
Inside diameter = 1.75
Total length of mast = 6 feet
Height above support = 5 feet
Antenna 1 height = 5 feet
Antenna 1 area = 2 square-feet
(Leave antenna 2 and 3 at zero values)
The calculator predicts the mast will survive at 99-MPH wind and fail at 140-MPH wind.
jimh posted 02-20-2012 09:19 PM ET (US)
To see more examples of calculating the bending moment on a mast, see this good paper at
http://www.va3cco.com/towersheight.pdf
These calculations are for masts fixed to the earth or a solid structure. On a boat you will have some shock loading of the mast as the boat slams into waves, so it would be prudent to allow a large safety factor in the design.
conch posted 02-21-2012 10:21 AM ET (US)
Great calculator Jim,it looks like a radar mounted to a pole mast is very doable even using the water pipe rating of 30000.
Chuck
jamesmylesmcp posted 02-21-2012 02:51 PM ET (US)
Anthony, here's an interesting read.
http://www.soundingsonline.com/features/technical/ 235432-installing-radar-on-a-small-boat

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Author	Topic: RADAR Mast or Lightweight Gin Pole
anthonylisske	posted 02-17-2012 11:12 AM ET (US) For two years [I have] been toying with the idea [of installing a RADAR mast or a lightweight pole]. I am gonna make a go at it, and I need advice on what schedule and pipe to use. I have 3-inch schedule-80 pipe, 3.5-inch O.D with a wall thickness of about 3/8 inch, and liked it at first; but I think it is overkill. [I] would like to go 2-inch schedule 80, 2.35-inch O.D with a wall thickness of about 1/4-inch. This way I can run it through the gunwale on my 1972 smoothside Outrage without really messing up glass. [The] hole would be similar in size to a rod holder. The height of mast would be about 8-feet to bottom of radar base. The main points of contact would be at base plate on floor and aluminum plate on top of gunwale. This would be install through gunwale next to fiberglass back seat, so I might box-in the gunwale with plate aluminum for strength. I know the 3-inch schedule-80 pipe would work, but do you think a 2-incher would do it? I may be missing data needed for advice, so please ask for any tech info you would need. Thanks in advance. Anthony
anthonylisske	posted 02-17-2012 12:59 PM ET (US) Oh yeah, I am talking about Aluminum pipe
dfmcintyre	posted 02-18-2012 07:26 PM ET (US) Without any tech info, I think for a pole that long with a 14 - 18lb object on the end, I'd personally go with the schedule 80, 3" pipe. And I'd also place a backing plate under the gunwale. Is the "trough" (classic 21 owners know of what I describe....) wide enough for an schedule 80 though? Any thoughts on having the ability for quick removal? I've got some ideas. Regards - Don
jimh	posted 02-18-2012 08:15 PM ET (US) This is a very simple bending moment calculation. You need to find the bending moment on the mast. Then you must know the tensile strength of the material. With that information you can easily do a force analysis to see if the mast will fail. The point of failure will always be at the lowest point of the mast where it is no longer supported. To calculate the bending moment you need to know the projected area of the mast and anything attached to it. Then you need to know the wind pressure on the mast. If you will be towing the boat on the highway, use 86-MPH for the wind. That will be the wind when you drive 55-MPH into a 30-MPH headwind. At 86-MPH the wind pressure is 30-lbs/1-foot^2. Sum the moments above the mast from its point of support. For tubular structures you can derate the projected surface by 0.66. Once you have the bending moment you can see if the material has enough tensile strength. Aluminum that is used for masts is usually 6061-T6. You can look up the tensile strength. There are some stronger grades, but for marine use the 6061-T6 will probably be better for corrosion resistance. The tensile strength will be about 35,000. You cannot really approach this with a by-the-seat-of-the-pants analysis. You will be very surprised how much bending moment can be created and how much strength is needed. There is a reason that most masts are made of steel. Steel can have much higher strength. Steel used in masts is often 120,000-lbs tensile strength.
jimh	posted 02-18-2012 10:38 PM ET (US) Also, most masts are made from tubing. Pipe is typically designed to carry liquid and to resist the bursting pressure of the liquid. Pipe is not designed to carry large bending moments.
anthonylisske	posted 02-19-2012 08:52 PM ET (US) Thanks for the replies Don and Jim. It will be close on the 3 inch. I will remeasure tomorrow, but I think it can take the 3.5 inches. It is just a matter of ugly. Lol. I might put it on the inside of the gunwale rather than through it. Don, I would love to hear any suggestions on the quick disconnect. I was gonna make it permanent, (since the radar wire would make it a bitch) but, I was thinking of putting a mitered plate about 12 inches above gunwale. So I could rotate the tube and rebolt at a 45 degree angle. I will re work the ideas tomorrow. Thanks
jimh	posted 02-20-2012 09:09 PM ET (US) For calculating the stress on a mast, I found this interesting calculator on-line at http://www.math.niu.edu/KARC/mast/ I entered the following data as input for the calculator: Safety factor = 2 Wind loading factor = 0.004 (default) Outside diameter = 2 Inside diameter = 1.75 Total length of mast = 6 feet Height above support = 5 feet Antenna 1 height = 5 feet Antenna 1 area = 2 square-feet (Leave antenna 2 and 3 at zero values) The calculator predicts the mast will survive at 99-MPH wind and fail at 140-MPH wind.
jimh	posted 02-20-2012 09:19 PM ET (US) To see more examples of calculating the bending moment on a mast, see this good paper at http://www.va3cco.com/towersheight.pdf These calculations are for masts fixed to the earth or a solid structure. On a boat you will have some shock loading of the mast as the boat slams into waves, so it would be prudent to allow a large safety factor in the design.
conch	posted 02-21-2012 10:21 AM ET (US) Great calculator Jim,it looks like a radar mounted to a pole mast is very doable even using the water pipe rating of 30000. Chuck
jamesmylesmcp	posted 02-21-2012 02:51 PM ET (US) Anthony, here's an interesting read. http://www.soundingsonline.com/features/technical/ 235432-installing-radar-on-a-small-boat