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

[Blueboatman]

I have a boat that was ( and may still be for all I know) offered with either a keel ( 'cantilevered') or deck stepped ( 'pin ended') mast arrangement.

It is considered to be an offshore capable cruiser and afaik the standard build offered by the builder is now a deck stepped, twin spreader rig.


Seajet, love the 'Golden Shot' quip re mast stepping

 

[Tanqueray]

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.

It was an opinion not advice - I have never asked anybody to abide by it

 

[Tanqueray]

Not entirely. That is how the unstayed masts on a junk rigged Coromandel were made.

However, what is missing from this "debate" is any recognition that a mast is only one component of a system and its safety and security is not just a function of whether it is keel stepped or deckstepped, it is how the loads from the sails are transmitted to the hull. Keel stepped masts date from the time when the keel was the major structural member of the boat, so loads were transmitted direct to the keel as the rest of the structure wa not capable of sharing any of that load. You only have to look at the elaborate mast steps of performance wooden boats to see how difficult it was to deal with those loads.

Once it became possible to build structures that could spread the loads more accurately the need for basing the mast directly on the keel became less important - helped by all sorts of other changes such as lighter dispalcement, better mast sections, more efficient rigging etc that have come from developments in designs and materials.

Like many aspects of yacht design and construction, things have moved on in the last 40 or 50 years, and many of the old "truths" are no longer relevant to today's designs, even though they were (and still are) perfectly valid for the designs of the time.

BTW this is not knocking design thinking of the past. The very people who were expressing an opinion about the merits of keel stepped masts were also challenging the orthodoxy of the day in other areas, and you can be sure that if they had the materials and technology available to them that are available now they would have been exploiting them.

You earlier asked me to provide evidence of my claims. I now ask you to do the same. Can you tell us why "lighter dispalcement (sic), better mast sections, more efficient rigging" change the engineering / design principles that govern mast support calculations?

'Skene', 'Lars Larsson', 'Elements of Yacht Design', 'Yacht Design Principles' and 'Nordic Boat Standards' would seem not agree with you.

Which reference authority do you wish to use to support your case?

 

[snooks]

You earlier asked me to provide evidence of my claims. I now ask you to do the same. Can you tell us why "lighter dispalcement (sic), better mast sections, more efficient rigging" change the engineering / design principles that govern mast support calculations?

You could look at the Open 60 design. Carbon spars, PBO rigging, deck stepped masts, some of which can rotate.

Admittedly not a 100% success rate, but most make it around the planet -unless their keels drop off that is

 

[RichardS]

You could look at the Open 60 design. Carbon spars, PBO rigging, deck stepped masts, some of which can rotate.

Admittedly not a 100% success rate, but most make it around the planet -unless their keels drop off that is

or unless their masts fall over!

and Mike G has suffered both!

Richard

 

[jonjo]

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.

Oh yes I can picture the mechanics now, thanks. So one positive for my keel stepped mast.

 

[jonjo]

Just one damn great gland (which nobody manufactures as a single part so you have to build it yourself[1]) to get the mast below!

Pete

[1] Admittedly there are now some good products for making less-leaky mast boots.

Could you name this product, I need one.

 

[Tranona]

You earlier asked me to provide evidence of my claims. I now ask you to do the same. Can you tell us why "lighter dispalcement (sic), better mast sections, more efficient rigging" change the engineering / design principles that govern mast support calculations?

'Skene', 'Lars Larsson', 'Elements of Yacht Design', 'Yacht Design Principles' and 'Nordic Boat Standards' would seem not agree with you.

Which reference authority do you wish to use to support your case?

I guess I should just quote Christopher Wren's memorial - look around you.

Masts do not usually fail because they are not strong enough. They fail because of some other component in the system fails. This is the case whether it is deck stepped or keel stepped. The outcome might be different, although when you read the first hand accounts of rigging failures you find some who are pleased the mast was deck stepped because it can be cut away and the integrity of the boat is not compromised. Others have valued having a stump left for a jury rig.

Not sure why you have difficulty in understanding the impact of the factors I mentioned as having an effect on rig design (sorry about the typo). If design was set in stone there would be no progress, and once again I suggest you look around you and you will see that the solutions of 30, 40 or 50 years ago, while perfectly valid in their time are not the solutions that designers use today.

Interesting that Blueboatman, who owns an archetypical "old style" design makes the point that the boat is now built with a deck stepped mast.

If keel stepped masts were inherently superior, every boat would have them. As we have seen there are significant negative aspects which can be eliminated and still produce a safe strong rig - but deck stepped. This does not, of course mean that in some designs a keel stepped mast is not appropriate. It is up to the designer to make the choice.

BTW, you did not produce any "evidence" to support your opinion, only a further opinion in relation to the relative strength of different types of mast on one design of boat - hardly relevant to today's world. I was rather expecting some analysis of mast failures that showed that deck stepped masts were more prone to failure than keel stepped masts. That of course would be almost impossible to do as not only are failures (at least in cruising boats) relatively rare, but there is little reliable data from which to draw any conclusions. Certainly insurers don't see any difference and they are more concerned (quite correctly) about the integrity of the rigging rather than the method of stepping.

 

[penfold]

Could you name this product, I need one.

The market leader is Spartite, but others are available; it's a bit of a pain to apply, there are numerous accounts online describing successful and not-so-successful attempts, also youtube videos if you like that kind of thing.

As with all engineering systems on a boat the keel/deck stepped decision is a compromise made by the designer; he must decide how the various requirements are ordered in rank of importance, interior space, weight, ease of tuning, durability, cost of manufacture. There is no right answer, either can do the job provided the rig is designed properly.

 

[Lucky Duck]

If keel stepped masts were inherently superior, every boat would have them. As we have seen there are significant negative aspects which can be eliminated and still produce a safe strong rig - but deck stepped. This does not, of course mean that in some designs a keel stepped mast is not appropriate. It is up to the designer to make the choice.

It may be worth noting that keel stepped masts seem to be more prevailant in boats designed towards the performance end of the market (e.g. my old First 31.7 had a keel stepped mast while equiverlent Oceanis 311 was deck stepped) so the designers/manufacturers seem to think there is some advantage in a keel stepped arrangement.

 

[Tanqueray]

I guess I should just quote Christopher Wren's memorial - look around you.

Masts do not usually fail because they are not strong enough. They fail because of some other component in the system fails. This is the case whether it is deck stepped or keel stepped. The outcome might be different, although when you read the first hand accounts of rigging failures you find some who are pleased the mast was deck stepped because it can be cut away and the integrity of the boat is not compromised. Others have valued having a stump left for a jury rig.

Not sure why you have difficulty in understanding the impact of the factors I mentioned as having an effect on rig design (sorry about the typo). If design was set in stone there would be no progress, and once again I suggest you look around you and you will see that the solutions of 30, 40 or 50 years ago, while perfectly valid in their time are not the solutions that designers use today.

Interesting that Blueboatman, who owns an archetypical "old style" design makes the point that the boat is now built with a deck stepped mast.

If keel stepped masts were inherently superior, every boat would have them. As we have seen there are significant negative aspects which can be eliminated and still produce a safe strong rig - but deck stepped. This does not, of course mean that in some designs a keel stepped mast is not appropriate. It is up to the designer to make the choice.

BTW, you did not produce any "evidence" to support your opinion, only a further opinion in relation to the relative strength of different types of mast on one design of boat - hardly relevant to today's world. I was rather expecting some analysis of mast failures that showed that deck stepped masts were more prone to failure than keel stepped masts. That of course would be almost impossible to do as not only are failures (at least in cruising boats) relatively rare, but there is little reliable data from which to draw any conclusions. Certainly insurers don't see any difference and they are more concerned (quite correctly) about the integrity of the rigging rather than the method of stepping.

Tranona, as I stated in my first post (11) - you can of course increase other factors (x-x / y-y or wall thickness etc) to compensate for the fact that lack of deck support gives an initially weaker structure to the mast.

You can also compensate in many other ways as you so rightly state in your post 39. ("Once it became possible to build structures that could spread the loads more accurately the need for basing the mast directly on the keel became less important - helped by all sorts of other changes such as lighter dispalcement, better mast sections, more efficient rigging etc that have come from developments in designs and materials").

If I was wrong, and what you say is correct - you would not need to compensate in any way!

The sea hasn't changed in 40 years, neither has sail pressure, neither have basic beam structure calculations etc. etc.

As materials improve, of course it will be possible to reduce scantlings, try new methods etc. But it won't change the laws of physics and mechanics!

As to your last point, I have said Sparkman & Stephens, 'Skene', 'Lars Larsson', 'Elements of Yacht Design', 'Yacht Design Principles' and 'Nordic Boat Standards' would seem not agree with you - you have dismissed them as out of date without stating why they are all suddenly wrong - I really cannot add anything more if that is the case.

Thank you for a most enjoyable knock about!

 

[doug748]

It may be worth noting that keel stepped masts seem to be more prevailant in boats designed towards the performance end of the market (e.g. my old First 31.7 had a keel stepped mast while equiverlent Oceanis 311 was deck stepped) so the designers/manufacturers seem to think there is some advantage in a keel stepped arrangement.

Yes, the usual blusterers on here trying to face all ways at once.

If you want absolute efficiency a keel stepped effort will appeal, for day to day use, a deck step wins hands down.

 

[Lucky Duck]

Yes, the usual blusterers on here trying to face all ways at once.

If you want absolute efficiency a keel stepped effort will appeal, for day to day use, a deck step wins hands down.

Leaving aside that raising and lowering the mast is probably more complicated, I'd be hard pushed to notice any difference during daily use. The mast section isn't much bigger than the compression post used on the deck stepped predecessor to my boat and I have not yet noticed any water leaking down the mast despite the very heavy rain we have had of late (unlike the First!).

 

[doug748]

Leaving aside that raising and lowering the mast is probably more complicated, I'd be hard pushed to notice any difference during daily use. The mast section isn't much bigger than the compression post used on the deck stepped predecessor to my boat and I have not yet noticed any water leaking down the mast despite the very heavy rain we have had of late (unlike the First!).

I think I should have said convenience rather than day to day use.

My mast boot is a leaky old thing and I should really improve matters.
A lot of rain came in through empty mast sheaves in the early days, running spare halyards through them solved that problem.

Keel stepped is handy if you are replacing the standing rigging, you can just remove one side at a time to take to the riggers to be copied. The rig will stand quite happily unsupported or with a halyard to the sidedeck.

 

[sighmoon]

I've not read the whole thread, but here's my experience with a keel stepped mast.

1) If you have a guest on board who you have the hump with, you can position the mast between you at dinner time, so they don't put you off your food.

2) We recently suffered a failed forestay. With the mainsheet pulling the rig back, it may well have fallen on our heads if it were deckstepped. As it was, the stick stayed up.

 

[snooks]

2) We recently suffered a failed forestay. With the mainsheet pulling the rig back, it may well have fallen on our heads if it were deckstepped. As it was, the stick stayed up.

So it may well have not fallen on your head if it was deck stepped....What was your point again??

Many deck stepped masts have a baby stay, and that would have helped keep the mast off your head

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The way I see it....

Would anybody really notice that the cruising boat they are sailing has either a deck stepped or keel stepped mast? No

Does either effect the performance of the average cruising boat, being a cruising forum and all that? No

Is the wall thickness of the mast really that much of an issue once you've added all your cruising gear down below, and 10kg radar, it's cables mount etc and radar reflector to the mast? No

So it comes down to maintenance and what happens in a dismasting.

The latter is rare enough that, unless you're racing, it's an unlikely event if you maintain your rig well, so lets almost negate that one. However if you're crossing an ocean and the worst happens and you want a stump left (which you may or may not have to climb to remove/retrieve sails) go for a keel stepped mast. If you want to ditch the whole rig, then deck stepped is the way forward. You should still have enough of both deck stepped and keel stepped masts to rig a jury rig to get you back to a bit of mud. But it's an unlikely event.

So it (very basically) comes down to maintenance and the design of boat.

It is possible to personally step a deck stepped mast but in reality you'd need a crane for both, unless your boat is under or around 30ft (yes there will be exception of the 80ft Easterly Spanglefart which can be done by one man and a small child blah blah blah) so that's another issue out of the way

Keel stepped masts require more maintenance, gaiter, mast foot, and ingress of rain, so a wet bilge from either a leak at the gaiter or coming down inside the mast.

Deck stepped masts are more vulnerable to deck compression.

Me? I have around a 6 inch square hard wood compression post from deck to keel right under the mast (and a bone dry bilge)

 

[Seajet]

Having once been on a boat when the forestay failed, I always keep spare & spin halliards clipped taut to the pulpit as Plan B.

 

[prv]

Having once been on a boat when the forestay failed, I always keep spare & spin halliards clipped taut to the pulpit as Plan B.

Hmm, I often see them clipped there - it's convenient for rigging the spinnaker in any case - but hadn't considered that as a reason. On KS I have no fewer than three wires coming down to the bow (plus a shroud per side that's slightly ahead of the mast foot), so I have no worries in that department. Perhaps when I eventually return to metal masts and pointy sails, I'll do the same as you.

Pete

 

[snooks]

Having once been on a boat when the forestay failed, I always keep spare & spin halliards clipped taut to the pulpit as Plan B.

Not always a good with furling the genoa. Alright if you can keep the halyard well forward out of the way, but a bit of slack, a roll of the wave when you don't, and it get sucked into the genoa.

 

[Seajet]

Not always a good with furling the genoa. Alright if you can keep the halyard well forward out of the way, but a bit of slack, a roll of the wave when you don't, and it get sucked into the genoa.

Well, these lines are clipped forward on the pulpit ( which incidentally has good backing pads if the forestay load ever did come onto it ! ) and easily kept taut as they're led aft, also I'm very happy to say I have hanked headsails, no roller reefing to cocoon things or crew in a Peyton-esque manner...