material to construct a radar/instrument mast

Discussion in 'OnBoard Electronics & Controls' started by seadago, Nov 25, 2016.

  1. seadago
    Joined: Jan 2012
    Posts: 5
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    Location: uk

    seadago Junior Member

    Hello all
    I need the assistance of an engineer to help me choose a material to construct a transom-mounted radar/instrument mast. Setup constraints as below:

    Mast must be mounted between 1 and 2' off the transom, and secured with ss brackets. This is to avoid inference with the swing arc of the boom, which in my case (I own an 30 Freedom cat ketch with wishbone booms) overlaps the vertical of the transom by about a foot.

    Mast length is 4 mts from the lowest bracket; i.e. about 4.5 mts above water line.

    On top of this mast will go a radar radome, several other transducers (GPS, VHF aereal, etc) plus -hopefully- a wind generator. Total weight estimated about 10 kg

    With this much weight aloft, design constraint is to make the mast itself as stiff and light as possible. Concern is to minimise loads on the GRP transom. Brackets will support lateral loads as well as vertical; i.e. the weight of the whole assembly. Available OTS materials are 316 SS, aluminium, pultruded GRP tubing, and carbon fiber tubing, in order of weight per unit length.

    A carbon/kevlar composite would be ideal, but cost exceed my budget.

    I don't know how to calculate the loads that will be applied to the tube with the rolling of the boat, hence unable to decide on the material and/or tube diameter and wall thickness.

    Currently thinking of using roll wrapped CF tube (specs below). The material has weaknesses, particularly to cross-sectional force applied across the diameter of the tube.

    Internal Diameter (ID, mm) 46.0
    Wall Thickness (mm) 1.4
    Outer Diameter (OD, mm) 48.8
    Stiffness I x E (KNm�) 5.57
    Weight Kg/m0.35

    Any insight much appreciated!
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    You may reconsider the height of the radar antenna. At that position, it will irradiate you with microwaves. That is a sure way to get cancer and tumors.
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    The highest load is at the top bracket. I would use 2 or 3 diameters, like 80 mm for the lower section, 60 mm for the top. Stainless is the preferred material because it bends under high load, wrapped tubes break without warning.
    The wind generator probably is not a good idea, it creates unpredictable lateral forces.

  4. seadago
    Joined: Jan 2012
    Posts: 5
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    Location: uk

    seadago Junior Member

    Thanks Gonzo, CDK.

    Re radar, unfortunately, I can't mount anything on either mast as the sails are hoisted with parrels. Height of the pole can be adjusted. I'll have a 2KW broadband dome. Manufacturer stipulates min 14' above water line, but could be higher, so the beam would potentially go over my head, even when standing up on the cabin roof. Safe at the wheel standing on the cockpit.

    Re materials good points both. Would 316 SS schedule 40 2.5-to-3.0 inch OD do? That will weight about 8 kg by itself.

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