Keel-stepped vs deck-stepped masts - pros & cons

[Neil]

In my continuing quest for knowledge, I considered the two main methods for attaching the mast to the boat. At first glance, a keel-stepped mast has the advantage of strength and security and only losing out on cost and ease of lowering the mast. Are there other factors that should be considered?

 

[Ex-SolentBoy]

In my continuing quest for knowledge, I considered the two main methods for attaching the mast to the boat. At first glance, a keel-stepped mast has the advantage of strength and security and only losing out on cost and ease of lowering the mast. Are there other factors that should be considered?

Leaks

 

[Deleted member 36384]

Keel stepped needs to have adequate drainage at the bottom of the mast. The pools of water that gather there can cause significant issues over the years: corrosion of the mast foot alloy or rot where wood shims are used.

On older keel stepped masts the partners can be awkward to replace if they are damaged or missing. Ill fitting wedges are usually a poor substitute and its worth having new ones manufactured to fit correctly. Mast boots to seal the deck penetration can be an issue on older yachts as well as the material starts failing and does not seal correctly. It may be difficult to replace because of the shape and taper.

I don't think a keel stepped mast has any additional strength or security over a deck stepped mast, assuming both are supported adequately. On occasion deck stepped masts have issues with sagging decks where the mast foot support has failed for various reasons: poor design, over tensioning, wear and tear GRP damage.

 

[snowleopard]

A keel stepped mast is normally a lighter section as the support given by the deck makes it stiffer. The other differences are generally pretty negative. Leaks at the deck, leaks down the inside, difficulty of stepping (crane needed, no possibility of pivoting), potential damage to mast and boat in a dismasting, extra cost up front.

 

[jonjo]

Are there other factors that should be considered?

Noise. A keel stepped mast is a sound conduit whether caused by an internal wire slap or loose running rigging or general wind through the rigging.

Just as sound is conducted downward, a keel stepped mast sucks heat out of the main cabin on a cold day.

A mast is more intrusive in the accommodation than a bulkhead or mast compression post.

Pro's? Your yacht would have something in common with HMS Victory.

 

[jonjo]

On occasion deck stepped masts have issues with sagging decks where the mast foot support has failed for various reasons: poor design, over tensioning, wear and tear GRP damage.

I think any seasoned surveyor would assume this to be the case until there is conclusive evidence there is no sagging of the coachroof.

 

[jonjo]

A keel stepped mast is normally a lighter section as the support given by the deck makes it stiffer.

I am struggling to picture this. On a deck stepped mast the length to be supported in 7ft less.

 

[sailorman]

Deck stepped, cap shroud breaks = mast over the side
Keel stepped ditto = mast breaks at lower spreaders
DS needs the deck supporting to transfer rig loads KS doesnt the loads are taken directly by the hull

 

[V1701]

If no mast support post with deck stepped, can get some compression of the deck. Can often be rectified by strengthening where appropriate, e.g. the mast support beam on Albin Vega...

 

[snowleopard]

I am struggling to picture this. On a deck stepped mast the length to be supported in 7ft less.

Let me try to explain. The key factor is that the mast is held rigidly at keel and deck which is sufficient to support the first few feet above the deck in the same way as my unstayed mast is held up.

To see the effect, take a length of bamboo about 3 ft long, support it horizontally at one end and 6" from the other end. see how much force is needed to bend it by say 3" by pressing down in the middle. You will notice that the unsupported 6" will rise at the same time. Now clamp that end down and try the bend again. It is much more difficult.

 

[Tanqueray]

Let me try to explain. The key factor is that the mast is held rigidly at keel and deck which is sufficient to support the first few feet above the deck in the same way as my unstayed mast is held up.

To see the effect, take a length of bamboo about 3 ft long, support it horizontally at one end and 6" from the other end. see how much force is needed to bend it by say 3" by pressing down in the middle. You will notice that the unsupported 6" will rise at the same time. Now clamp that end down and try the bend again. It is much more difficult.

Exactly! (Sometimes referred to as 'Pin Effect').

Whatver your decision it is essential that the bottom of the mast is properly secured in both Fwd/Aft and athwartship directions (think about the stresses at the secure points if both the very top and the very bottom of the mast can both 'orbit'.

IMHO deck stepped masts are strictly for local based motor sailor types of vessels (which can increase X-X / Y - Y andf wall thickness to compensate). Not for serious offshore work where sail is the primary means of propulsion (Multihulls excepted for a number of reasons).

 

[V1701]

...IMHO deck stepped masts are strictly for local based motor sailor types of vessels (which can increase X-X / Y - Y andf wall thickness to compensate). Not for serious offshore work where sail is the primary means of propulsion (Multihulls excepted for a number of reasons).

I think you need to look at the rig as a whole...

 

[Tanqueray]

I think you need to look at the rig as a whole...

Agreed. (though it's sometimes difficult not to generalise when there are so few specifics as to the exact requirements).

Steve

 

[Marakei]

keel stepped mast

+1 for the effect of a keel stepped mast on the accommodation, particularly in a little boat.

See the Newbridge Coromandel for illustration.

 

[Tranona]

IMHO deck stepped masts are strictly for local based motor sailor types of vessels (which can increase X-X / Y - Y andf wall thickness to compensate). Not for serious offshore work where sail is the primary means of propulsion (Multihulls excepted for a number of reasons).

Is this just your opinion or do you have some evidence to support your statement?

 

[Seajet]

I had a boat with a keel stepped mast, it was nothing but a pain with the leaks, noise and unwanted cooling effects mentioned.

Also when the boatyard lowered it to restep, they obviously didn't play 'The Golden Shot' much, as it went down like a piledriver precisely 3" off centre, disimproving the woodwork below.

My boat with a deck stepped mast is certainly not a motorsailor and I have taken her across the Channel at least 18 times, other have crossed the Atlantic without the rigs toppling.

Nothing wrong with a deck stepped mast if the design is right re compression loads etc.

 

[snooks]

IMHO deck stepped masts are strictly for local based motor sailor types of vessels (which can increase X-X / Y - Y andf wall thickness to compensate). Not for serious offshore work where sail is the primary means of propulsion (Multihulls excepted for a number of reasons).

Thankfully there are thousands (if not millions) of sailors out there who haven't listened to your opinion and have set off around the world with their deck stepped masts.

 

[Twister_Ken]

Generally, size also comes into it. Bigger boats, more rig tension, more beef needed in deck section if deck-stepped = more weight where it is unhelpful.

From totally unscientific observation, looks like about 32-34 feet is the upper limit for deck-stepped, maybe unless it's not a very powerful rig.

Re heat suck, you can put a jacket around the below-decks portion of the mast, which does a good job of insulating it.

Another advantage of keel-stepped is that all the wiring emerges below decks and you don't need possibly dodgy cable glands or connectors to get the electrickery below.

 

[Salty John]

Most points have been well covered above. Here's my thoughts:

http://saltyjohn.blogspot.co.uk/2012/06/deck-or-keel-stepped-mast.html

 

[Tanqueray]

Is this just your opinion or do you have some evidence to support your statement?

As the first four letters of my post say; It Is My Humble Opinion!

(after 40 years as professional marine engineer with YM Ocean for 25 of them)

It is self evident if you look at the masts in any marina. Increase the mast height (hence why I said "where the sail is the primary means of propulsion"!) and you will need to take action.
That action can include (but is not limited to) increasing wall thickness, increasing x-x / y-y, or increasing rigging angle (hence the increase in number of spreaders with increasing mast height).

Think of the deck as the first set of spreaders and the benefits are obvious.

Are there boats that circumnavigete with deck stepped masts? - of course there are - but without exception they use a combination of the above to compensate for the lack of deck support.

Is that a bad thing? Not necessarily (I have always favoured boats more towards the motor sailer end of the scale myself). But you cannot avoid the basic engineering priciples that ultimately limit mast proportions and 'pin effect' or deck support is a basic principle.

IM MY HUMBLE OPINION!