Aftmast rigs???

[brian eiland]

How about revisiting this early idea,..

Aft Mast Alternative on Big Trimaran

Aft Mast Alternative on Big Trimaran

Several years ago I was asked by a gentleman to come have a look at a big Kurt Hughes trimaran he was building, a 63 footer he had stretched to 65, if I remember correctly. He was VERY concerned about handling that big mainsail on that big sloop rig.

I spent a week as his guess at his home, and in the huge building shed outback he had erected to build this ultimate retirement vessel for himself, his wife and his kids. He is of Romanian decent, and has that Italian flare for home wine making (and drinking). Needless to say we had LOTS of fun drinking all sorts of home brewed wines and talking boats.

Here is what I came up with as a solution for him in the context of my mast aft theme. Please realize I was restricted by the existing structure of the vessel that was already well under way in construction...i.e., bulkhead placements, crossbeams, etc, etc.

There was one other nagging question he had, that needed to be taken into account....what if the fwd leaning mast idea would not work?? This persisted to be such a big question in his mind that I had to give considerations as to how my mast aft design could be converted back to a more standard rig configuration without a great deal of expense, and/or trouble to him.

I still chose an 'all-3 sails-furling' arrangement....my single-masted ketch concept. But I made the mast rake almost half (6 degrees) of the original design. And the mizzen sail was made a bit larger in proportion. Thus this rig could be converted to a straight standing cutter rig with the mizzen becoming a more traditional mainsail attached to the aft edge of the mast. Or a new larger mainsail could be constructed for the mast that could be extended upward (taller), but still stepped in same location. The cutter jibs would then both be fractional, but would not require modification.

Over all I sought to give him the same total sail area as the original design by Kurt Hughes. This was approx 1900 sq ft. BUT notice what happened to the mast height on these two versions !! The original sloop rig carried a 92' high rig. My rig carries that same sail area on a 73.5 foot mast !

With this significant reduction in the rig height and the 'all-furling' feature, I imagine this gentleman will be able to handle this rig without a lot of additional assistance, and he may well be able to SAFELY sail into higher wind conditions without as much fear of being overpowered....his sailing efficiency should be improved.

I had two options for the lower backstays here...1) anchored to the aft crossbeam ends (at a 13 degree angle to the mast), or 2) anchored to the ama hull ends (at a 20 degree angle). Both of these backstay angles are better than my original design, thus even less loading reqired of these backstays to offset the inner forestay loads.

I felt this shorter rig could justify a slightly shorter daggerboard, and we opted to move that daggerboard slightly forward to both be in a better balance with the new sail plan, and to better fit in with a saloon modification he had already planned.

To my knowledge at this time he is still thinking about his final rig design options, and his boat building processes have been put into a holding pattern while he attends to converting that original building shop into a multiple house dwelling. He then intends to build an even bigger shed to complete final assembly of this very big beamy trimaran. I'm not sure if he will chose my mast aft option, but I would be willing to place bets on it being a very good one for his needs.

[NOTE: The second aftmast sketch represents a more current modification of the masthead attachment of its backstay, and the ama anchoring of the lower backstay(s). These are just suggested options to the original sketch, and certainly not final ideas]

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RunningTideYachts.com
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Lets return to that big trimaran I was working on to reduce the size of that VERY BIG FULL BATTENED MAINSAIL,.....imagine trying to get that sail down with some gust pushing it up against the rigging,...impossible in sudden gusting conditions!

Then look at the reduction in overall rig height with my aftmast rig,....for same sail area.
(The original sloop rig carried a 92' high rig. My rig carries that same sail area on a 73.5 foot mast !)


(BTW,...latest thoughts would have the mizzen sail attached to the mast tube for reduced drag)

 

[brian eiland]

Okay Nick, lets return to this force diagram of yours.
It appears that the force in the single line backstay is about 2.8 times as big as the yellow line forestay (2.8 as long of a vector).

Now what if I was resisting that forestay force with 2 backstays sharing the load,...1.4 each,...about half again the forestay load in each one,.. reasonable

the force diagram by Nick,....

So a few postings ago I determined that the 2 backstays could be carrying 1.4 tons each for a forestay load of 1 ton,.....under the 10 degree mast raked condition.

How about if we reduce that rake of the mast to 6 degrees like the big trimaran? As near as I can determine at the moment it would reduce the loads in those two backstays to 1 ton each for each 1 ton of forestay load.
(PS,...I was unable to plot this out in vectors at scale at this particular time as I have a broken wrist on my right hand screwing up my drawing and typing skills )

 

[brian eiland]

Look how consistent that slot is for the aft-mast configuration.

now imagine that mizzen sail attached to the mast as i have reconfigured the rig

That wish-boned mizzen sail could even be carried to the top of the mast as the lower pair of backstays are wrapped around the front of the mast tube,...so do not interfere with any track (whatever) on mast. The wishbone boom would be made short enough to swing inside those backstays.

Cuts out a lot of that 'bare mast drag' that so many were concerned about.

 

[brian eiland]

Some reaching conditions with that original model of the aftmast rig. On the multihull's wide beam we could rig the control lines of those headsails to tracks-cars-blocks along the edges of the decks rather than having to utilize poles of any sort.

 

[brian eiland]

Another nice looking aftmast motorsailer. Looks like a basic non-sailor could get big benefits from such a vessel.

 

[brian eiland]

Semi-crabclaw

You are correct with regards to crabclaw lifting in a puff and thus spilling excess wind while maintaining forward thrust. Ours does so as well though it is a semi-crabclaw - hanging off the innerstay. At first I thought it a flaw. I have come to regard it as a self-tending sail that rocks.
Incidentally, running downwind when we set wing and wing we gain a full 2 knots over both set to same side. Still working my way though Marchaj and Gentry with regards to the interaction or perhaps non-interaction of the genoa and staysail (crabclaw-ish) but can vouch for the phenomena empirically.

deserves repeating, particularly with the photo

 

[brian eiland]

I've had my share of naysayers to the aftmast concept over the years.

Lets take a trip to Thailand and experience one of these vessels first hand,...( I need a vacation)

HK40' - Power Sailing Catamaran.​

Mast-aft rig - Wikipedia http://en.wikipedia.org/wiki/Mast_aft_rig
EDITED: I do NOT understand why I can not directly link the page I wanted to this discussion?, so I suggest you go to this linked page, scroll down to 'reference #10' and look thru those multiple photos.