How big is too big...

[Willallison]

A catchy headline always gets the readers!
But what I really want to know, is at what point (what size boat) does it become impractical to have a deck-stepped mast?

[RHough]

When you can't add enough ballast to compensate for the heavier section?

Is this a trick question?

Properly designed, there is no limit to boat size with a deck stepped mast that I can think of.

[FAST FRED]

The usual reasin for wanting a deck stepped mast is that it can be in a tabernakle and easily lowered.

The easy way to go bigger is with a LUTCHET , where the deck has a hatch in front of the mast that can be opened .

The mast then has pivots above the deck , but goes right down to the bottom of the boat for support.
The mast is carefully weighted below the pivot point to compensate for the mast and sail weight.

AM told the Thames Barges used this setup and a boy could dip the hugely heavy rig, to pass , under sail , under frequent bridges.

FAST FRED

[Willallison]

Fred - I'm doing a 15 metre aluminium, twin keel boat. I don't want to be confined in my interior layout by the masy protruding through the cabin.

RHough - I realise that in theory there is no upper limit, but at some point - as you suggest - it must become impractical to run with the deck-stepped mast...

[RHough]

Not having a mast or a compression post in the interior will require that the deck structure handles all the compression loads. I'm guessing the RM on the boat will be something on the order of 100,000 lbs. If the beam is 14 ft or so the mast will have about 40,000 lbs of compression load.

Building the deck strong enough to handle the expected load puts weight high in the hull.

Deck stepped masts have to be stiffer than keel stepped masts, in general this means heavier.

Now you have a heavy mast and a heavy deck.

By twin keeled I assume you mean two bilge keels? The twin keeled boats I've seen also have shoal draft.

The combination of shoal draft, heavy deck and heavy rig might add up to a very tender boat.

Using a bulkhead to support the mast cuts the interior in half, a compression post or a keel stepped mast would not intrude as much as a bulkhead. The boat would have a lower centre of gravity and not need as much ballast to sail well.

If interior space is a higher priority than the boat's sailing ability, you could design a deck stepped rig at the 15m size with no problem.

[JPC]

There might be some middle ground available in a post and lintle -type construction (i.e. an I-beam at the cabin-top and a pair of compression posts, port and starboard of centerline) - this gives you a clean run through the center of your cabin. Still, you're building a structure 2 or 2-1/2 times instead of once (cabin-top plus floors), and I'd go with RHough and keel step as the starting point that I'd have to come up with a compelling reason to leave.

JPC

[D'ARTOIS]

Do what is common practice, boats over 40 'hardly have deckstepped masts when sloop/cutter rigged.

Although I realise that deckstepped masts have certain advantages I prefer to have keel stepped ones. That's of course only my opnion.

[MikeJohns]

Will
Depends on many factors but is just another part of the design spiral. There are modern sailing ships with deck stepped masts , but they achieve a good moment connecion by using a husky tabernackle that is in turn well supported and connected through to the CVK with a compression post(s).

[Willallison]

Yes, alas the design spiral has claimed its 1st victim - my deck stepped mast has spun out!
The need for a compression post, or other supporting structure, did away with my reason for selecting the deck step in the 1st place, which was to free up internal space....
Thanks for all the advice guys...