Last summer at the boat ramp I watched a bloke lose his tinnie halfway down the river because the “rope” holding it to the trailer was some frayed nylon cord he’d had lying around the shed since 2015. It had gone stiff, brittle, and let go the second a wave hit. Cost him an afternoon and a fair bit of dignity in front of a full car park. A decent length of proper marine polypropylene rope would’ve cost him about the same as a counter lunch.
If you keep a boat on a mooring, tie up at a jetty, or just need something reliable for rigging and towing, the rope you use matters more than people think. Here’s how to actually rig a dock line that holds, using 10mm black polypropylene rope, step by step.
Why polypropylene rope for dock lines?
Polypropylene floats. That’s the big one. If it drops off the bow while you’re pulling into a berth, it sits on the surface instead of sinking under the hull and fouling your prop — which nylon or cotton rope will happen to do more than once if you’re not careful.
It’s also resistant to salt water, doesn’t rot like natural fibre rope, and shrugs off UV a lot better than most people expect for the price. A 10mm x 50m black polypropylene rope gives you enough length to cut multiple dock lines, spring lines and a spare for the anchor, all from one coil.
The trade-off is that poly rope has less stretch than nylon. That’s actually a plus for dock lines — you want the boat held snug, not bouncing around on a stretchy tether every time a wake rolls through. For anchoring in rougher conditions where you want shock absorption, nylon is the better call, but for tying up, poly does the job properly.
Step 1: Work out how much rope you actually need
A rough rule most marina hands use: each dock line should be about two-thirds the length of your boat, minimum. For a 5-6 metre trailer boat, that’s roughly 3.5-4 metres per line. You’ll want at least four lines — bow, stern, and two springs — so a 50 metre length gives you plenty to cut down with a bit left over for fenders or securing gear on deck.
Cut with a sharp knife against a solid surface, not scissors — polypropylene is tough stuff and blunt scissors will just fray the strands instead of cutting clean.
Sealing the cut ends
- Run the cut end briefly over a gas flame or lighter until it melts, then press it flat with something metal (not your fingers) before it cools.
- This stops the rope unravelling at the ends, which is the main reason poly rope looks “cheap” after a few months if it’s not done.
- Do this straight after cutting, while the rope is fresh — don’t leave frayed ends sitting in salt water, they’ll only get worse.
Step 2: Tie a proper cleat hitch, not a granny knot
This is the one skill that actually matters at the dock. A cleat hitch holds firm under load but comes undone in two seconds when you need to leave in a hurry.
- Take a full turn around the base of the cleat.
- Cross over the top to form a figure-eight — under one horn, over and around the other.
- Finish with a locking hitch: twist a loop and drop it over the horn so it can’t slip.
- Never wrap it around and around like a garden hose — that jams under load and you’ll be sawing at it with a knife when you need to cast off fast.
Step 3: Rig your spring lines correctly
Spring lines run diagonally from the bow to a mid-dock cleat and from the stern forward, stopping the boat surging back and forth with the tide or wake. Most weekend boaties skip these and wonder why their boat’s nose keeps knocking the pylon.
Run a spring line at roughly 45 degrees where possible, and keep it as taut as the tide will allow without pinning the boat rigid — you still need a little give for the water level to move without the line taking all the strain at one point.
What about general jobs around the boat and shed?
Not every job needs marine-grade rope. For general towing, securing tarps, tying down gear in the ute tray or general workshop use, the 10mm x 25m twisted work rope is the more practical (and cheaper) option — same diameter, shorter length, built for everyday jobs rather than saltwater exposure specifically. Keep the marine rope for anything that actually touches the water regularly, and use the work rope for the dozen other jobs a rope gets asked to do around a boat, trailer or shed.
Rope maintenance that actually extends its life
- Rinse dock lines in fresh water every few weeks if the boat’s kept in salt water — salt crystals work into the fibres and accelerate wear at pressure points.
- Coil rope loosely and hang it, rather than leaving it in a wet pile on the dock or in a locker — permanent kinks weaken the strands over time.
- Check where the rope runs through a cleat or chock every season. That’s where chafe happens first, and it’s the point most likely to fail under load.
- Replace a line once you see fuzzing along its length or a section that’s gone noticeably thinner — don’t wait until it snaps mid-manoeuvre.
While you’re sorting the boat gear
If you’re doing a general check around the house at the same time — a lot of boat owners are also the ones up the ladder each spring — it’s worth clearing gutters before storm season rolls in, since blocked gutters overflow straight past the downpipe and undo half the point of having them. A set of gutter guard brushes stops leaves and bark building up in the first place, so it’s one less job competing with boat maintenance for your Saturday.
Rope is one of those things nobody thinks about until it fails at the worst possible moment — usually when the tide’s dropping, the wind’s picked up, and you’re trying to fend off a $40,000 hull with your foot. Spend the twenty minutes getting your dock lines cut, sealed and tied properly, and it stops being something you think about at all.