Rotating mast with tabernacle

[Garry]

The recent discussion on free-standing masts raised the topic of rotating masts and their advantages with sprits. At first, I was not open to the idea, especially after reading the high-tech solution for the high-tech boats designed by Eric Sponberg. However, the examples of low-tech solutions got me thinking and I drew up my ideas, see below. It may still add some complication that I would not want, but it does look doable for the P28 with a tabernacle.

What do you think?

[Gordy Hill]

Garry, Garry, Garry....(sigh)

It seems you make things sooo complicated!

Of course, your's may be the best and most efficient when all is said and done....

However

I was thinking of an aluminium welded-up tabernacle (which I can get done free) It would have a heavy- walled tube rotating in a sleeve below decks and the three-sided framework and pivot point above the deck.

Powder coat it white.

This way you wouldn't have to build the tabernacle 3 degrees off center so the masts will miss each other when folded. The whole mess would pivot as you moved the mast to where you wanted it.

If you didn't use a tabernacle on the mizzen you could lift it out and lay it with the main and swing the whole lot off to the edge of the gallows and give better access to the cabin when on the trailer. This would make camping on the trailer easier, like when stopping for a nap at a rest area.

[Tom Lathrop]

Gordy,

Gary may be a wee bit overboard but his idea seems to have merit to me.  The basic swing-up base tube looks like it could be workable.  It clearly need something to lock it in the raised position and maybe Gary just left that out.  To me, it is a bit too complicated and can be simplified.  The rotation controls are unnecessary since the snotter and sprit will do that job automatically.  Fancy bearings are also not necessary.  A thick piece of heavy poly or such is adequate at the base.

The idea looks OK to me.

[Gordy Hill]

I think you're right Tom, and I'm sure Garry's idea would work, but I didn't want to agree so soon. I was hoping for a more lively discussion!

I had a 17' Silhouette MkII that has a steel tabernacle on top of the cabin. Two "ears" on the sides had holes that accepted a bolt that ran through the centr of the mast. Another hole near the base of the mast had a second bolt to hold the mast upright. Since this was a sloop the mast extended well over the stern when lowered. However. I could remove the bolt and reposition the lowered mast well forward for trailering. However, this put the mast right over the companionway and made access to the cabin restricted. I still think I'd vote for the rotating tabernacle, but I wouldn't be surprised if Garry notices a flaw  or improvement.

[Garry]

Here is my interpretation of Gordy's idea. There is a lot that I like about it. It would definitely need a "splash collar" to try to keep water from filling the fixed tube but that could be easily built into the rotating tabernacle. One advantage with the rotating tabernacle is it provides a good base for cleats. I suppose they could be mounted on the rotating mast in my original idea.

Tom's ideas for simplification may be right on:

The rotation controls are unnecessary since the snotter and sprit will do that job automatically.  Fancy bearings are also not necessary.  A thick piece of heavy poly or such is adequate at the base.

But I'm am a little wary since I'm not sure about the loads and friction for the P28 masts. Also, my reading indicates that it may be beneficial to "over-rotate" the mast at times for better efficiency. However, I DO favor simplification!

I also don't favor "fancy" bearings. My idea is to form graphite filled epoxy into the bearing shapes directly on the mast (or tabernacle post).

I also enjoy a lively discussion! *shh Now I better get back to work.~~~

[Gordy Hill]

I LIKE the graphite-filled epoxy bushing!!

[Garry]

Eric Sponberg gives the following advantages for rotating free-standing masts:

The advantages for rotating masts are

1) The leading edges of the sails are always at optimum, fair to the wind regardless of the point of sail;

2) Sails can be raised, reefed, or stowed without changing course.  This is particularly important when sailing downwind, because you do not have to turn the boat first broadside to the weather, and then into the wind in order to change sail.  You hold course and just let the rig weathervane while you do what needs to be done;

3) When sailing downwind, you can set the booms well forward of the beam with the masts pointing aft to get really great lift downwind, which is much more powerful than drag

[Ken_Potts]

  I like the rotating tabernacle as long as the sails aren't of the sleeve luff variety.  Sail tracks can be installed to below the level of the tabernacle partner but a sleeve luff would have to stop short of the tabernacle.  If I didn't have sleeve luff sails I might volunteer to try this idea   If you're building new it might be worth combining this idea with the tabernacles that are being built on some of the CS20s (the foredeck cut out before the mast).  It would allow the top of the tabernacle to be a bit lower for the same strength.  You don't really need the tabernacle to rotate much before the heel of the mast clears the deck, do you?  (of course that would lower the trailer-camping tent).

  I also think the control plate adds lines to the deck needlessly.  I'm prepared to be proven wrong, though (at this point I'm pretty used to being proven wrong:) ).  The good news here is the control plate can either be added originally and removed if deemed unnecessary or it can be left off and added if the masts don't rotate readily enough.

  An alternative to putting the control plate at the base of the mast and adjusting it manually would be placing it up by the snotter and rigging it to the forward end of the sprit boom.  My masts already rotate fairly readily when the wind is blowing nicely and I think they would rotate quite readily if the sprit boom was given a little more leverage through the use of Garry's control plate.

  I'm pretty sure if somebody asks me nicely I'll bang together a test plate, try it out on my boat and report back.  I might go ahead even without being asked...  What do you folks think, should I fire up a prototype Garry-plate?

  Let's get lively - Shoot me down, challenge me to show you the boom-actuated-Garry-plate, whatever

  Garry, If I have sea room and I want to work on something on the sails I can raise the centerboard and let the sheets fly.  The CLR moves way back without the board so the boat runs downwind with the sails streaming out dead ahead.  The few times I've tried it I've been pretty comfortable.  You made a good point about relative wind.  When I do this I have full access to the mizzen from the cockpit.  I can't reach the clew of the main but once I've got whatever I'm worried about sorted out with the mizzen I can lower the board, sheet the mizzen in hard and the clew of the main swings into the cockpit as the boat weathercock into the wind.

  I'm finding this rig to be a joy even though I routinely try to trip myself up with those clown shoes...

[Howard]

Garry:

Have studied both sketches and I'm not sure I understand the break point for the bottom of the mast. Is it going to slide through the deck to the keel, or drop down to some point on the deck to rotate on a fixed base?

Here is a thought. How often are you going to want to raise and lower the masts on a 28 footer? Even if tabernacled, how difficult would it be? The mizzen, I think you could "walk up". The main.....maybe use the same masthead block used for the mizzen staysail to get it about 45 degrees, then walk it up or use the sprit boom as a gin pole with a block and tackle. All depends on the weight of the mast, but is this something you could do on the water? Desirable feature, yes, but is it realistically doable on a 28 footer?

Perhaps an alternative is to plan on stepping and unstepping the masts at a marina or boat yard. If you do that, rotating masts just becomes a whole lot simpler. Just a collar on the deck and a mast step on the keel.

Or how about the sleeve yoke you show, the mast slides through that. Once it pivots up, it slides down through the deck all the way to the keel. At that point, your mast sleeve could be a piece of schedule 40 PVC....about 4 or 5 feet of it. Cleats for halyards, dead eyes for snotters, etc. are all on the mast.

One thing I had not thought of is that for the sail to spin the mast around (to rotate it), all the side leverage, etc. is on the snotter. The mast is the fulcrum. The sprit boom the lever and all the force is on the short piece of sprit boom that projects forward of the mast, on the snotter fitting on the SB and the dead eye on the mast.

And related to this, it has also occurred to me that a sail rigged with sprit booms generates the same forces as a sail with booms. All the tension that is generated on vangs and mainsheets with a boom are still there. If there isn't much load on the mainsheet as is the case with booms, then it must all be on the snotter end of the sprit boom as tension...pulling forward. Combined with rotational force above, the dead eye on the mast had better be built to take some abuse.

[Garry]

Just a quick response - more later:

Howard - In both sketches, the mast does not drop through the deck; it is supported only by the tabernacle. Our design goal is to be able to raise both masts manually using the advantages of the tabernacle design. That is why we are considering Graham's lightweight carbon fiber construction at least for the upper parts of the masts. If necessary, a rope/pulley system could be added to pull the base of the mast down and into the base of the tabernacle. Graham feels confident that with the carbon fiber mast and the tabernacle, raising the masts by hand is a reasonable goal.

Ken - I appreciate your offer but I wouldn't ask you to do that. If you choose to do it, though, I would be interested in your results!

[Gordy Hill]

Ken,

Is there any reason you can't just trim off the sleeve for the bottom foot or so. The downhaul rigged to a point on the tabernacle would keep the tack of the sail tight.

[Garry]

Howard - I have little experience in this area (none with a rotating mast), but I would think the sail would apply most of the force to rotate the mast, not the snotter. In Sponberg's examples with the oval section masts, the axis of the long part of the oval is aligned with the leading edge of the sail. That provides the best entry angle for the wind. The rotated angle of the leading edge of the sail is well beyond the angle of the sprit. It seems the rotation would probably settle somewhere between the angle of the sail's leading edge and the angle of the sprit (or wishbone).

Ken - I'm sure you could remove part of the luff sleeve. I have one reef point in my Spindrift sail; the sleeve stops at the reef point and about three feet of the luff is not supported so the sail can be easily dropped for reefing. I thought I might want to add a webbing loop but it doesn't seem to need it.

Tom - I am not sure the rotation control is needed. I remember seeing it on the Sea Pearl boats but now I realize that it was primarily because they wrap the sails around the masts for reefing and storage. This limits the sails to a hollow leech with no battens or with the roach supported by vertical battens. As Ken pointed out, the rotation control could be added later if desired.

[Howard]

Howard - I have little experience in this area (none with a rotating mast), but I would think the sail would apply most of the force to rotate the mast, not the snotter.

I have no experience either, just imagination and a curious mind. I think the sail will provide the force....and lots of it. But it will all act on the mast end of the sprit boom and dead eye through the snotter. As I think about it, the most strain might be on the end of the sprit boom, which is the only connection point the sprit boom has to the mast to apply rotational torque. Again, I think this is one reason why I think you would want your mast to rotate. If your mast is fixed and that sprit boom rotates a lot, there will be extra tension on the snotter and the short protrusion of sprit boom forward of the mast. Unless you let off some tension, I can imagine enough leverage being created to snap your sprit boom.

[Ken_Potts]

I've never noticed the boom being strained by the forces you mention (if I'm picturing it correctly).  The sprit boom doesn't really get pushed hard against the mast.  My original main sprit boom was an ugly knotty piece of wood and it didn't break...

[Tom Lathrop]

Howard - I have little experience in this area (none with a rotating mast), but I would think the sail would apply most of the force to rotate the mast, not the snotter.

The snotter, being attached to the front edge of the mast and the end of the nearby sprit does exert a lot of force on the mast.  On Southern Skimmer with the tabernacle, we would have needed to let out the snotter when going off wind to protect the sprit.  That is why we attached the snotter to both sides of the mast instead at only the front.  The double attachment points act like a traveler to reduce the tension as the sprit swings around. 

Letting out the sail adds tension to the snotter line.  If it is tight to start with and the main sheet is eased, it gets tighter and could eventually break the sprit.  And yes, the sail, if it is on a sail track, also exerts turning force on the mast.  Tightening of the snotter line off wind is the main reason for going to a rotating mast.

[Ricardo Ramalho]

hi everybody! nice meeting you all in this great boats forum!

I have some experience with rotating mast: normaly it is supported by stays, that makes the pivoting system a lot easier and simpler at the base of the mast (but it does make lowering and raising the mast a little more time consuming).

The control lines idea are not going to work as proposed, they will be unecessary: the wind force is to great, and the mast will turn with the sail. (if the mast needs help turning it is obviously a not very good system).

A self standing rotanting mast, as suggested, is a lot more complicated and expensive, the collar system in the drawing will scratch the mast and cause too much friction. It would require a good bearings system.

Also the wing mast format (oval) would be more indicated. I belive a rotating mast with a regular tube shape wont make great difference in performance (considering the complications involved).

[Garry]

Differing opinions on bearings for rotating masts; both from persons with some experience:

A self standing rotanting mast, as suggested, is a lot more complicated and expensive, the collar system in the drawing will scratch the mast and cause too much friction. It would require a good bearings system.

The rotation controls are unnecessary since the snotter and sprit will do that job automatically.  Fancy bearings are also not necessary.  A thick piece of heavy poly or such is adequate at the base.

I'll have to look at some loads and dimensions once we get that far in the design. Ricardo's opinion is based on a stayed mast which would generate a lot of downward force on the mast thus increasing the load on the bearings. Tom's is based on free-standing masts which is probably more applicable. The downward forces on a free-standing mast would be weight of the mast, sail and boom; the vertical component of the main sheet; and possibly a portion of the wind ofrce if there is a vertical component. The halyard force would only apply if it is brought down to the deck which probably would not be the case for a rotating mast.

And related to this, it has also occurred to me that a sail rigged with sprit booms generates the same forces as a sail with booms. All the tension that is generated on vangs and mainsheets with a boom are still there.

Howard, I partly disagree with this. Yes, the primary lift and drag forces are the same, but the the typical boom and vang are under a lot of load because they are resisting the load in a cantilever while the sprit is in direct compression. Try holding 20 lbs with your arm horizontal - it takes a lot of strength; now try with your arm straight up - its easy. That's why a typical boom is much larger in cross-section than a sprit for the same size sail.

[Ricardo Ramalho]

hi Garry! what I tried to say is that a rotating mast with stays does not require fancy bearings, only the self standing masts do, I belive. In beach cats, for example, (and some big catamarans), with rotating masts and stays, there is a simple solid plastic ball (a sphere made of nylon, or some hard plastic) at the mast base, so the mast rests on the top of the ball. The ball system makes rotation easy, and it helps removing the mast. As mentioned by another colleague, lowering and raising the heavy mast of a 28 feet boat is not going to be very frequent, nor easy. 

[Tom Lathrop]

Garry,

My first free standing rig just had a wood plug on the end of the mast that rotated on the epoxy base.  Still going strong after much use over 13 years.  I am adding a 1/8"  thick piece of poly to the bases on Lapwing just out of cussedness but it's not critical.

[Garry]

Thanks to everyone for all the input. There is lots to think about. We will see how this is developed for the P28 design.