What is the Advantage of a Keel Stepped Mast?

The idea that leak potential should be a determining factor here is just plain silly. If properly installed a mast partner will not leak. If you have leaking mast partners something is wrong and needs to be fixed.

Some of the arguments presented here are rather half-baked. If you are realy trying to decide, get a copy of The Riggers Apprentice, or if you are feeling more technical you can Get Skene's Elements of Yacht Design.

There are two good reasons to have a deck stepped mast: It is easy to put up and down if you have to deal with bridges, or if you do a lot of canal travel. It can be cheaper to construct. The mast will have to be heavier to have the same stiffness as a keel steped mast, it is simple enginering statics and is reflected in any scantlings table for boat design.

You can certainly build a deck stepped mast suitable for ocean voyaging. You can not build a keel stepped mast that can be self-raised to deal with low clearance situations, or at least not that I have ever seen.

[jzk]

What about the problem of lightning strikes on deck stepped masts?

[delmarrey]

Lighting strikes are unpredictable no matter what mast you have. It will take the EZest route to the water. A keel stepped mast would have to be sitting right on the lead/iron to make any difference. And I dought that would be the case.

Mine has a SS plate bolted to the keelwood, then a plastic shim, the mast base and then the mast. Then there is a battery cable running from the mast to the center keelbolt (no ground plates). Weather that helps or not, I hope I'll never find out.

A deck stepped would have to have a cable running thru the deck, down the compression post and to the ground plates. Or any other route desired........................_/)

[SkiprJohn]

Aloha All,
Charlie has a voice of reason. I've had both and each are good. Just need to maintain the rigging and mast steps properly for them to last a boat's lifetime.
JohnL

[BobB]

All in all this has been an interesting thread and I agree Charlie has summarized this rather succinctly. But a week or so ago, I mentioned Hallberg Rassey as a bluewater boat that had deck stepped masts. So why everyone was discussing the pros and cons, I went ahead and wrote Hallberg Rassey, asking what their reasons were. FYI Here is their reply:

Thank you for your email.

Both keel stepped and deck stepped masts have their advantages and disadvantages. I am convinced that for a cruising sailor, the deck stepped mast has by far more advantages than drawbacks. The main advantage is that a deck stepped mast cannot leak. With a keel stepp mast the risk of leakage is high. It also looks better with a nice wooden support rather than the mast coming into the interior. A keel stepped mast brings more noise into the cabin if some rope or halyard is smashing against the mast. In theory, it is possible to make a keel stepped mast slightly thinner than a deck stepped mast, but in reality this almost never makes a difference, since there is not an unlimited supply of mast thicknesses anyhow. So that means you usually end up with the same mast thickness anyhow. In theory it is also a little easier to trim a keel stepped mast, but the difference is so small, that a cruising sailor cannot measure the difference. So if you are not out there to win the America's Cup or the Volvo Ocean Race, but having fun and enjoy nice sailing as a cruising sailor, a deck stepped mast is the obvious choice.

As with all installations, you can make them in a better or not so good way. On a Hallberg-Rassy, we today for example install a metal beam that is cast into the hull grid just under the mast support. That helps carrying the loads further. We also use stable forward and aft lower shrouds on all boats from 36 ft and up, to get a really sturdy and easy to trim mast.

Regards
Hallberg-Rassy Varvs AB
Magnus Rassy
CEO

I don't think this really changes anything (Charlie's summary still seems valid), but it would be interesting to hear directly from manufacturers and designers their thoughts.

All boats are compromises; that's why there are so many choices and so many diverse opinions, especially on this website. It's a great way to learn.

[swagman]

I'd agree 90% with Charlies summary of the pros / cons - but have to continue to argue on what I see as a key issue in the hard / soft spot created in a rig by a deck stepping arrangement.

1. Charlie and others have assumed any failure in a keel stepped rig will damage the deck - yet suggests the same would not happen if it were deck stepped. If the assumption is that with a deck step the mast will break at the stepping point, then one must surely agree that is obviously a weak point also? After all if not the weakest point - why assume it would break there.......

2. Practically I've only been on one yacht where the rig has come down - but been on several where we've lost a shroud and yet not lost the rig. All were keel stepped. I suspect if any of those rigs had been deck stepped, we would have lost the rig.

3. I've also sailed out of a region where lots of river based boats needed to drop rigs when underway to facilitate low bridges. Apart from the very smallest (say 22 footers) few would risk dropping rig with an unsecured base-point like a deck step. Almost 100% fit hinges in the lower mast section and handle the dropping safely.

And finally IMHO I suspect whether deck or keel stepped, if a mst wishes to break it probably go above the spreaders. In that case the top section will come down like a spear and probably take out some deck before going overboard - either way.

Again, all in my humble opinion (IMHO) of course.

JOHN

[Charlie]

I've been on one boat that has lost its mast and was sailing right next to another that lost its mast I have been on a few that have lost shrouds. The two boats that lost there masts were race boats on one the owner missed getting the running back on in a gybe and as I stood at the shrouds admiring the gybe I had just completed and the mast kept right on going. Fell slowly to the side of the boat (the sails acted like parachutes) and then we took the pins out of the shrouds and the mast sank to the bottom. There was a stub maybe two feet high sticking out of the deck. On the one I was next to we were sailing upwind trying to eek (sp?) out some clear air. Iwas looking at the trim on their main when the mast shattered in two places3' and 8' off the deck.

the times I lost shrouds they were intermediates. and we dropped sail and rigged a spare halyard to take the shrouds place and motored back. I lost a headstay going up wind when the skipper plowed into another boat and we broke the headstay. the jib held the mast up till we turned downwind and put a halyard up to the bow. All ot these cases were on deck stepped masts. Never lost a cap shroud.

In terms of a roll I don't know what would happen.

Swagman IMHO I would not say that the thru the deck fitting at the mast is not a weak point rather a strong point. In many cases it is strong enough to hold the mast in place while the top end shears off. On racing boats with small masts the deck does not suffer as much damage. on cruisers with tree trunk masts it does matter b/c the mast is strong enough to tear up the deck when it is unstayed. A deck stepped mast would have to bounce around on the deck to puncture the deck. In either case a swift break won't cause as much damage as when the rig bounces and slapps from side to side causing punctures.

[Stardate2010]

Bolger had a design years ago that appeared in the boatbuilder mag.
It was a 48' steel sharpie, double ender, swing keel, with a mast that was keel/deck stepped in that there was a 3' tabernacle on deck which had the unstayed mast pivot through an opening slot in the deck all the way to the keel. He designed it with a dipping lug sail. It was a teaching platform for skinny waters, and the mast could be lowered with a jin pole in 5 min or so for going under l0' bridges. Seems like the name of the boat was Dawn Treader.

Alan

[tdw]

One of the interesting points to come out of this discussion is the issue of dropping a deck stepped mast to go under low bridges.

Years ago, over in Western Australia the Swan river yachtsmen often used deck stepped masts in a tabernacle to allow for this. I tried it on a 28' sloop and it was doable but it was not something you would want to do every day or when short handed. This was in pre furler days. Today , with a furler on the forestay (Furlex) it would be nigh on impossible to drop the mast in that fashion due to the intricacies of dismantling the furler this would be impossible with this type of furler. It would also be (ahem) interesting, to say the least, to be attempting to drop 50 plus feet of aluminium extrusion to the deck so you'd figure that this advantage of a deck step is really only applicable to small boats with moderate rigs unless you have a large crew.

From an experience point of view I've owned two yachts with deck stepped masts.

The first, glass but not cored, had a real problem with rig tension and deck movement. The mast was stepped over a metal base that straddled two bulkheads. Once you tightened up the rig there was a serious problem with the bulkhead doors jamming. Admittedly it never got any worse than that but I never satisfactorily cured the problem.
I realise this thread ran out of steam a couple of weks ago but I've only just caught up with it.

Current boat is also deck stepped but she is steel, with a compression post of admirable size. No problems at all with the set up except that the hole through the deck to carry the wiring (situated directly under the mast step) leaks. Grrrrr..... It's on the to do list and may not be a huge problem to remedy but it does make something of a mockery of the no leak argument.

Other than that no problems, baby stay keeps everything in place no matter how short and steep the seas but I've yet to do any long passages offfshore with this boat. Down below is definitely quieter than it would be with a keel step.

[swagman]

HI tdw,
I sailed for close on 20 years out of Perth river clubs to go offshore most weekends.
All yachts doing the same dropped their rigs to go under the three bridges at Freo - and few larger yachts (say over 22/24 foot) had deck stepped rigs.
Most had (and still have) keel stepped masts with hinges built in just above the deck.
And trust me, it may look worrying to an outsider, but it does get easy with practice.
Sue and I often went out for a cruise two up on our 40' Adams with a hefty cruiser 55 foot mast section - and we handled it two handed with no eletrical help. Kept you fit!
The biggest yacht in my time to do this regularly was Pacemaker - a 50 foot Ben Lexan sloop with a 70+ foot rig - and whilst we did need a jack under the mast to loose and regain tension quickly - mob handed when racing even this rig was not an issue to drop and rehoist.
Cheers
JOHN

[CSY Man]

Thing's like water pissing in through deck collars that never seem to seal?

Nah, had my 1979 vintage boat for 8 years with no leaks and no repairs or adjustments to the mast/deck collar.
No leaks, not even a drop in big hurricanes with big rain.

Feel much better about my keel stepped stick than some of them "modern" deck stepped jobs.

Guess I am old-fashioned and like sturdy stuff, espacially off-shore.

Also like the ease of maintenance: have replaced standing rigging and chainplates without pulling the mast..And no leaks, ever...

[Boracay]

I think the question should have been worded "What is the advantage in a deck steped mast?"

[tdw]

Originally Posted by swagman

HI tdw,
I sailed for close on 20 years out of Perth river clubs to go offshore most weekends.
All yachts doing the same dropped their rigs to go under the three bridges at Freo - and few larger yachts (say over 22/24 foot) had deck stepped rigs.
Most had (and still have) keel stepped masts with hinges built in just above the deck.
And trust me, it may look worrying to an outsider, but it does get easy with practice.
Sue and I often went out for a cruise two up on our 40' Adams with a hefty cruiser 55 foot mast section - and we handled it two handed with no eletrical help. Kept you fit!
The biggest yacht in my time to do this regularly was Pacemaker - a 50 foot Ben Lexan sloop with a 70+ foot rig - and whilst we did need a jack under the mast to loose and regain tension quickly - mob handed when racing even this rig was not an issue to drop and rehoist.
Cheers
JOHN

G'day John,
Thanks for putting me right on the deck step v hinged mast bit. I confess I didn't realise that. I bet it did keep you fit. Isn't it wonderful what people will go through to have a sail ?
Cheers
Andrew

[Jeff H]

I have posted this before but here tis again. I personally strongly prefer a properly designed deck-stepped mast over a keel stepped mast but once again this is an area where opinions can differ widely. There is no right answer here. There is a contingent that thinks that the only proper way to step a mast is on the keel. There is a logic to that but it is a logic that can be engineered around and which comes out of a historical context that is less relevant with modern materials.

To start with the basics, the base of a mast has a vertical and horizontal thrust to it that tries to push it down through the bottom of the boat and also sideward off of the mast step. In normal conditions the down load is several times greater than the side load. Beyond the loads imparted to the boat, there is also the issue of the loads that happen internally in a mast. When you look at the structure of a mast it is really a truss standing on end but it does not completely act as truss because the components of a truss are not supposed to have bending loads on them. Ideally the loads in the mast are primarily axial (acting along the length of the mast) rather than in bending (acting perpendicular to the long axis of the mast). Of course masts do have fairly large bending loads imparted into them. The two most often cited reasons for keel stepped masts being considered stronger is the way that the bending loads (moments) are distributed within the mast itself and the way that the mast imparts its loads into the boat.

If the goal of designing a mast is to reduce bending moments within a mast, the greater the number of panels (segments between shrouds and other supports) the smaller the moments tend to be. In the days when single spreader rigs were most common a keel-stepped mast added one extra panel, the segment between the mast partners at the deck and the keel. This has become less significant as bigger boats have routinely gone to multiple spreader rigs and moment connections at the deck mounted mast steps.


In terms of the way that the mast imparts its loads into the boat, masts are generally located in the area of the cabin trunk and because of the shape of the cabin (i.e. the deck folds up at the cabin side and horizontal again at the coach roof) this area, if not engineered for side loads is more prone to lateral flexing than would be the keel. One idea behind a keel-stepped mast being stronger is that with a keel stepped the mast is not supposed loads are put loads into the deck.

In reality, this ideal is rarely accomplished for a number of reasons. First of all, if the mast is not tied to the deck or the deck tied to the keel near the mast, either with a tie rod or a tie from the mast to the deck and a connection from the mast to the keel, the downward force of the mast working in opposition to the upward loads of the shrouds can pull the hull together like a bow and arrow lifting the deck and separating the joint between bulkheads and the deck. You sometimes see this type of separated bulkheads on inexpensive or early fiberglass boats with keel stepped masts.

Not only do keel stepped masts impart vertical loads into the deck (through the ties mentioned above) but they also typically end up imparting side loads as well (if they are going to reduce the moments in the mast as mentioned above). This somewhat reduces the structural advantages of a keel-stepped mast to next to zero assuming that a deck-stepped mast is properly engineered, and that is a big if!

There are several things that I consider critical to engineering a deck stepped mast properly. Primary is having a jack post below the mast. A jack post is a vertical member that carries the vertical loads of the mast to the keel. My preference is to have an aluminum jack post rather than a wooden one but a wooden post can work as well. The other issue is the distribution of the side loads. Ideally there should be a bulkhead or ring frame adjacent to the mast that can take the side loads and distribute them into the hull. These are obviously more complex to do than simply having a fat spot on the keel for the mast step to land on.


My objections are to the purely practical. Keel stepped masts mean that there is always water in the bilge. This water comes in at halyard boxes and other openings in the mast and nothing you can do will stop that. Second, it is way harder to step and unstep a keel-stepped mast making the boat more subject to damage in the process. Beyond that if you loose a mast (I have lost two in my life) it is better in my opinion to loose a deck stepped mast because a keel-stepped mast is more likely to damage the deck when it fails and a deck-stepped mast is easier to clear away. The keel stepped mast advocates point out that you are more likely to end up with a bigger stump after the mast fails. I am not sure that that is the case if you are able to tow the rig as a drougue until things quiet down enough to rig a jurry rig. I am not sure what you do when the boat is being beaten to death by the upper portion of a mast that has buckled 20 feet off the deck at the spreaders. .

My preferred set up is a deck stepped mast that has a welded flange on its bottom that is through bolted through the deck into the top flange of a structural aluminum jack post. My new boat has a keel stepped mast. It is my intent to pull this mast and have it modified to that arrangement if I ever go offshore with her.

Jeff

[Amgine]

I beg to differ, Jeff H, regarding "and nothing you can do will stop that..." I've owned a keel-stepped boat which had no way for water to get down through it - it was a solid wood mast. And there are two boats in the marina with aluminum extrusions - but no through-mast rigging; both use blocks attacted to cap. One appears to have wiring up the mast, the other has nothing on the sticks (junk rigged.)

Anecdotally, it seems I hear of more incidents of deck-stepped masts falling over than keel-stepped masts. Probably this is due to the cheaper design also being implmented in a cheap manner by some builders.

I'm agnostic in this particular argument. I've owned both; current boat has a deck stepped mast.