Aftmast rigs???

I'm mulling similar questions in considering how the aft mast ketch might be rigged to a center cockpit boat. Perhaps keep those sheets above a permanent bimini of small-gap trampoline material so they don't get snagged but you still have open air.

 

There's plenty of people who like to steer from leeward, and even more who may just like to relax in a seat without moving every tack. It looks like an ideal position for a non-sailor to sit, looking forward, up high and out of any spray and with a great view. Even dinghy/small cat/boardsailers like me sometimes steer from leeward when cruising, so we can check out shorelines/transits/good views/starboard tack boats/ferries etc.

Personally I find that jib sheets flog with more speed and force than mainsheets, because the boom slows the sideways motion of the mainsail's clew dramatically.

I take the point about moving the centreboard to adjust CLR.

 

Carbon Fiber Mast, amateur built

Not all carbon fiber mast need to be built in a commercial facility. Eric Sponberg developed some technics that some may find interesting. He has recently retired from designing, but has left us with new website that contains some of his histories and links to some of his extensive developments.

Here is a link to his mast discussions, and a small quote:

http://www.ericwsponberg.com/free-standing-mast-designs/

 

Phaedo³, St Maarten Heineken Regatta, 2016

Interesting comparison:
The MOD70, Phaedo³, owned and skippered by Lloyd Thornburg and co skippered by Brian Thompson won it's class and beat the course and race record around St Maarten.
(Is that an aft mounted mast trimaran with what looks to be the use of 2 headsails at times )

https://vimeo.com/158057506


Performed rather well in that regatta

Brian

 

What works for a 70' racer, for certain specific reasons, does not necessarily work for a cruiser.

 

This thread is hilarious.

Its morphed from mounting a rig near the stern and attempting to get performance out of headsails (WAFTAM), too claiming Phaedo3 is an example of
the aft-mast rig concept initially sprouted.

Surely putting more bouncy forward ,by lengthening the bows, in a vessel type prone to nose diving, has little to do with the original concept discussed in this thread all those pages ago.

No Brian , Phaedo3 gives your barrow pushing , no credibility whatsoever.

 

I have also mentioned that these sorts of racing boats are more a case of bows out the front than masts at the back. No I don't think its pedantic. There is a difference. This type has evolved from lighter faster boats carrying huge sail areas downwind which could easily pitchpole if the boat slows down in a seaway. Nothing to do with easy cruising what so ever.

But that does not mean aft mast cruising boats could not be a good option.

 

I think of MOD70's as more a matter of moving the bow forward rather than the mast aft. The idea was to get them away from being a square boat to one that was slightly more biased to length than beam. It seems to have worked out well, sure there have been some capsizes which is not really unexpected with such a high performance craft but they have been leewards rather than pitchpole type capsizes.

 

I suggest that you broaden your horizons and take into consideration the whole of the platform rather than just the rig. For instance take into account the heeling that both monos and trimarans do in comparison to a relatively flat sailing catamaran, and with that in mind what happens with those traditional athwartship heeling forces that become more hull depressants that in the case of level sailing catamarans.

Perhaps you might look thru another 'radical idea' I have about lifting (vertical) forces from headsails,...look at postings started at #99 to #103
http://www.boatdesign.net/forums/sailboats/sail-loading-rig-rig-loading-vessel-2293-7.html#post769753

Or maybe you might find this Herresoff observation interesting
http://www.boatdesign.net/forums/multihulls/herreshoffs-catamaran-reasoning-5843-2.html#post35032

Cheers from hilarious.

 

Hi Brian - To do this properly I'd start with "what is the Rm max" or its a cat so Rm5deg or at hull lift.? There is a boat around here with large foresails using furlers and the main is on a furler attached to the rear of the mast, so it furls about 300mm aft of the mast. All so its easy to use. The manual analysis goes like this Hm (heeling moment - aeroloads) = Rm (static balance) this gives you the athwartship loads, so you can start vectoring away from this moment to get the transverse loads in all the bits. You must keep the transverse loading and longitudinal loading separate until the end then you can superimpose them (add them up if you assume a linear domain solution). To estimate the longitudinal loads you need to know the forestay load or the backstay load. Since we don't know these we estimate it from the cut of the foresail at the design condition. All of these calcs are usually to size the cables so are conservative. If the the flying shape of the jib has a 2% catenary sag or allowance (at its flying speed of say 25 knots or 30kn etc ask your sail make what the sag is for each sail) we can again assume that if the sail is in its flying shape the membrane loads are producing a catenary load matching a 2% sag (high performance sails have <1% sag and produce huge catenary loads) So then you calculate the load on the cable with a 2% sag and that's your forestay load. Then the longitudinal cables must be in equilibrium with this load. Then impose the pre-load and your close. If it was a mono we'd do it at 30deg say and include the weight of the rig as its heeling. This calc is also based on the assumption of linear static equilibrium. This assumes the rig is very stiff and does not change shape at that load. If the rig had a bendy top for instance we'd need to assume its a non linear problem and do a non linear analysis to account for the change in geometry at that load. ie load history affects the final shape. In linear analysis domain load history does not affect the final geometry or shape. usually I run a linear and non linear analysis to check no NL effects are there. If they are then the work has to proceed in the NL domain and hand calcs are out the window. eg if the cables are varying diameters and therefore stiffnesses vary a lot then the assumption that the load is shared via the vectored direction is not true. The connection may share load due to direction and rigidity of each element so the load in each element may not be correct using direction alone. So I'd need more info to check what you have done. Peter

 

Reduce Aero Drag of Bare Mast

Several weeks ago i was playing around with this idea of twin, thin, flexible sheet materials that might attach to either side of the mast and have short trailing sections that would slide against one another.

While there are some interesting possibilities, I came back to the more simplistic solution of just having a single trailing 'flap' of material attached to the rear of the mast to help with that disruptive airflow off of a bare mast. According to most theories this splitter plate should reduce the drag of my bare mast by perhaps half as much.

I don't see any need to complicate the construction of the mast tube itself by incorporating a 'slot/track' at its rear side,...rather just glue on an extruded track, perhaps just a plastic one.

Then I imagine a 4-6 inch wide piece of HD (low flexibility) sailcloth (splitter plate) could be run up this track to provide the drag reduction membrane.

 

Have to come back to this subject once I find a few sketches I was playing with. And Tom Speer has expressed some concerns that this single splitter arrangement won't be enough?

 

50' cat with jib-schooner rig

Meantime I have introduced a subject thread dealing with an alternative rig proposal termed a jib-schooner.
50’ Jib-Schooner Sail Plan, Aero Analysis



What intrigues me here is the CFD analysis by Doyle Sailmakers that touts the good upwind performance of the 2 parallel jibs,...without a mainsail.

What concerns me is:
1) the drag of 2 bare mast vs my single one.
2) the non-overlap of the two jibs,
3) the somewhat lacking sail area for light air performance,
4) and the handling of a 'fisherman sail'.

 

160' Staysail Ketch, all roller furling sails

And I happened across this rig design for a 160 foot ketch

 

Doesn't that seem like very carefully chosen phrasing: "The owner was of the opinion that..."