When I reread my post of yesterday I realized many of you might be confused why I introduced “pilot ladders” into Luigisoft’s initial question. The reason is that many of our more modern practices at sea are inherited from those of times long ago. So let me explain a little and hopefully clarify and probably pique some interest in the possible use of those manropes on sailing ships and how we have a carry over to more modern times.
The ship-side “ladders” on sailing ships of old are solid fixed in place treads, if fitted. They are there to assist in boarding and disembarking the ship. As has been pointed out they would be next to impossible to hold onto as a person ascends or descends the ship side. So it only makes sense that a rope or ropes would be used as grablines (we now call them manropes). Continuing on from that the rope would have been fixed to a secure point at the ship side or on deck and be available as necessary. Would it be left in position at sea? I personally doubt it but we cannot know for sure. I would expect the deck of the vessel to be cleared and secured of all non-essential gear before departing on a sea voyage.
So back to my comment about pilot ladders. In the marine industry the pilot ladder is used, as might be expected, primarily for the boarding and departing of the pilot, crew and other ship/shore personnel. It is not constructed like the sailing ships of old. They are portable and constructed in such a way (regulated) that when deployed the wooden treads tend to lay against the ship side. Much like the fixed solid treads of the sailing ships. However, the ladder is constructed of two separate ropes on each side (not manropes). These ladder side ropes pass through the treads. The treads are then secured and spaced using chocks above and below the treads (sometimes called widgets). These are then tied off with line. This means when climbing or descending the pilot ladder there is sufficient room for your hands to grip the side ropes. You do not grip the treads and it would be foolhardy to do so. This tends to reinforce the discussion of not using the fixed in place treads on a sailing ship.
The use of manropes has been around for a long, long time. Their use now is also to assist personnel when using the pilot ladder. They are useful when descending the ladder where you can use a manrope in each hand in a sliding motion. If you think about it your feet are on the treads and you use the ropes as hand holds. Ascending is a different matter. It is really awkward to climb a pilot ladder using the manropes especially in a sea and swell. It is, to me, safer to grab the side ropes of the actual ladder as you climb up.
Did I base this on conjecture? No, I have climbed and descended the damn things a great many times. If you want a scary thought think of a very heavy swell off Cape Town, a rise and fall of way, way more than 30 feet, wind howling and nervous panic trying to climb a pilot ladder as the boat you just stepped off at the peak of the wave, now drops in the trough and quickly rises fast beneath you feet as you climb faster than a sword in teeth boarding party

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A long winded post but I think if we have to guess and wonder about practices of old them maybe we should look at similar sea practices of today to give us a clue.