Originally Posted by stillbuilding
Can you elaborate on this cos I do not understand. Do you mean using a tie to prevent deck distortion?
Yes, most cabin
top are designed to resist downward direction forces but looks what happens when you have running rigging that is led back to the cockpit from the mast.
Take a halyard
for instance. The line is attached to the headboard of the sail and the bottom of the sail is attached to the boom which is attached to the mast tube. The halyard
is then run up to the mast head
where it goes around a sheave (pulley) and back down the mast tube. Then the halyard is attached to another sheave attached to the cabin top usually at a mast ring which is bolted to the cabin top then the halyard is redirected aft to the cockpit.
Since the keel
stepped mast is sitting on top of the keel, tightening the halyard "pulls" up on the sheave attached to the mast ring or cabin top. This exerts a force pulling the cabin top upwards distorting it. Anti-pumping rods are commonly used to redirect this force to the keel. This is a thin stainless steel
rod that runs inside the cabin from the cabin top down to the keel just behind the mast tube itself.
With deck/cabin top stepped masts the sheave is attached to the mast shoe sitting on top of the cabin top so the forces are contained within the mast tube system and the only possible force is aftwards - the mast base being pulled towards the cockpit. But this is a direction that the cabin top can easily resist.
So with a keel-stepped mast a clamping system needs to be attached to the mast tube at the cabin top level to transfer the vertical forces off the cabin top and back onto the mast tube system or an anti-pumping rod added. Basically this is one or more parts
that need to be manufactured and properly installed and maintained which can be avoided with deck-stepped mast systems.
When I converted my running rigging from being situated at/on the mast tube to being run back inside my pilothouse, I designed and had the clamping collar built as part of the deck ring that covers/seals the hole through the cabin top where the mast passes on its way to the keel.
Additionally, these days, shipping
mast tubes which are 60+ feet (18m) long from manufacturing plant to boat assembly plant is rather expensive. Deck stepped masts are about 7 ft +/- (2.2m) shorter and subsequently easier to ship. And if you look now at the really new boats you will see a joint about halfway up the mast that allows the boat manufacturer to get masts that are - obviously - only half as long when shipped to them and then assemble the mast system to full length at the boat.