Aft Mast Ketch Rig

[beiland]

BTW, over the years I have had a LOT of naysayers as to the EXTREME forces that might be seen by the backstays. I would invite you to visit this little force review,...... larger yes, but not so excessive as some have expressed.
Aftmast rigs??? - Page 23 - Boat Design Forums

BTW, note that the forward jumpers are NOT perpendicular to the mast as convention would have it, but rather they are in alignment with the aft jumper strut. I've also tried to make use of 'continuous-line' type rigging that passes over turning 'pads' attached to the mast, rather then numerous 'terminal points' with their inherent swaged terminals etc.

[44'cruisingcat]

I don't claim to be a designer, but even for a headsail only rig I'd have thought it would make more sense to have the mast base further forward, in the normal position, and have the mast raked aft to move the mast head to the same position as the drawings. This would mean the forestay tension would be much higher, backstay tension much less.

The setup in the drawings would produce far higher backstay tensions than forestay tensions.

[beiland]

Originally Posted by Bruce626

No, my builder constructs his designs and I didn't want to open discussions on such a large modification - expense, and after all, what do i know anyway? Its just a regular condomaran I want to live on - likely for just coastal cruising and living on the hook here in the central Philippines where there are almost no marinas. This thread has a couple of pictures : http://www.cruisersforum.com/forums/...s-61072-2.html and i will be gooing down to see it later this week so perhaps some more pictures - I am not sure if it is rigged yet - it is typhoon season here.

Did you get her fully launched and out cruising?
Do you have some more details on her rig?

[beiland]

Originally Posted by 44'cruisingcat

I don't claim to be a designer, but even for a headsail only rig I'd have thought it would make more sense to have the mast base further forward, in the normal position, and have the mast raked aft to move the mast head to the same position as the drawings. This would mean the forestay tension would be much higher, backstay tension much less.

The setup in the drawings would produce far higher backstay tensions than forestay tensions.

Yes, my backstay loading is higher, BUT is it really that much higher,...and is it unmanageable or dangerously higher.

Look again at my review:

A) In the best case where I have carried the backstays to the very sterns of the vessel (the 14.5 degree angle they make with the mast tube), my backstay load is 1340kg when the forestay load is 1000kg.
This would represent about a 33% higher backstay load than a traditional sloop rig with a straight standing mast and a forestay and backstay at equal angles. That's not too bad, and it is even better at the attachment point on the hull(s), as this load gets split into two legs as it transitions to its bottom attachment points.

B) In the worst case where the backstay makes a 10 degree angle with the mast tube, my backstay load is 1900kg when the forestay load is 1000kg,...almost double the load. But again this load is split into two as it transitions to the hull attachment.

C) What some folks miss is that I actually have 3 backstays in my rig design,...the lower, hounds-attached one (1) that transitions into 2 at the sterns of the vessel, and the third one (3) that goes from the masthead over the aft jumper and down to the base of the mast supporting structure in the hull/cabin bulkhead. So my individual backstay loads are not so hugely great as some folks speculate.



A lot of the 'backstay loading' questions arise when we are talking of maintaining a tight, non-sagging forestay. I would invite a lot of multihullers to take a closer look at their 'very shallow' backstay angles (the shrouds), ......and their loading must be supplemented with the mainsail/mainsheet tensioning.

My main point in bringing up the new Lagoon design was their renewed interest in power of headsails vs the mainsail

[Bruce626]

The other day a large trimaran (perhaps 50'+, large to me anyway) returned to Tambobo Bay on southern Negros in the Philippines where I am currently moored. It has its mast in back of the cockpit - much further aft than mine. I don't know much about it but took a closer look to see how it was rigged. One could see that there is a mounting for the mast at the 'normal' mast position - it apparently sits on a large ball; the mast appears to be sitting on a similar ball aft of the cockpit. The rigging is odd (to me anyway as I don't know much about rigging) - the attachments for the stays/shrouds appears to be pentagonal with the point being at the stern. The five connections to chain plates (?) seem to be done with large diameter ss cables in loops that are about four feet long and a minimum of a half inch thick, possibly larger, which are secured at the center of one side with four large cable clips on each end. At the top end of each loop is a large telephone pole ceramic insulator that one sees guying power poles. Woven through the tops of the insulators running to the top of the vertical mast is 14mm (?) Dyneema (?) woven line in a continuous series of 'V's. Apparently like some geodesic dome kind of thing - I don't really understand how it works, but it apparently does; and it is said to sail quite well to windward. The headsails are huge (to me anyway) judging from the fore triangle and the length of the spars/booms that go on the foot (the larger one looks longer than my 42' mast). It all looked very interesting but I don't see any application to my much smaller rig - probably because I don't understand how it works - but it clearly must work since the boat apparently came here from Hawaii. Judging from the size of the supporting cables, I suspect that there are some significant loads involved. My much smaller rig just broke a half inch ss attachment point for my port forestay bridle - I would not have thought it could do that, but it did.

[beiland]

Arvel Gentry's passing & a Legacy of His Articles


Originally Posted by brian eiland
The gentleman who most deserves credit for finally getting the explainations of sail aerodynamics corrected, and upon which several excellent books by Tom Whidden and C.A. Marchaj are based has recently updated his website to include many of the technical papers and magazine articles he wrote on the subject originally. I had mentioned his name, Arvel Gentry, in these postings previously, but I could not make a direct reference to his many documents as they were not posted on his site at that time.

From his site,

"I got involved in the technical aspect of sailing because I started racing. Reading the sailing books and magazines, I began to realize that most of what was written about the aerodynamics of sails was wrong, or certainly very misleading."

"The explanations for how lift is generated were based on popular myths. The description of the interactions between a jib and mainsail, the "slot effect", did not make much aerodynamic sense."

"I was soon launched on a quest to discover how our sails really worked. Over the years this resulted in a number of technical sailing papers and magazine articles. The technical sailing papers are archived in this section of my web site. My magazine articles can be reached from my Home page."

"If you are interested in sailing aerodynamics and how your sails work, you have come to the right place. All of my sailing technical papers and magazine articles are archived on the Technical Papers and Magazine Articles pages.

his original website (no longer any good)

Arvel Gentry has just recently passed away, but thank goodness someone has taken the job on to preserve his numerous papers. Check out the forum subject thread with a number of links to his papers:
Arvel Gentry's site

As is mentioned on the first posting of this subject thread that old website link is no longer valid. Rather have a look here for some fascinating articles of his on 'how sails work' :
Articles - Página web de arvelgentry

PS: His work contributed to my fascination with a primarily headsail rig.

[beiland]

Just posted a summary of some testing that verifies some of the aerodynamic claims I have been making about my aft mast rig.


http://www.cruisersforum.com/forums/...ml#post1903468
Wind tunnel and CFD investigation of unconventional aftmast rigs

.....CONCLUSIONS
In the present paper an unconventional rig has been investigated in comparison with a standard sloop rig by means of wind tunnel tests. The traditional sloop yacht rig used as a reference is a Comet 51’a Vallicelli Yacht Design & Co 51 feet IMS cruiser-racer. Several unconventional configurations have been tested, all characterised by an “A” shaped stern mast without mainsail in single-jib and double-jib configurations. Aerodynamic data available from experiments have been used to perform some performance prediction at full scale by means of a VPP code.

Both experimental tests and VPP calculation show that the double jib
configuration with overlap gives the best performance and also the same configuration without overlap gives better results in comparison with the standard sloop solution.

Numerical investigation have been carried out using RANS simulation in order to better understand the aerodynamic differences resulting from the experimental tests. Simulation results put in evidence a slat effect in the overlapping jibs configuration leading to more attached flow on the aft jib allowing for an higher pressure drop on the sailplan.

[beiland]

Originally Posted by SkiprJohn

Aloha Sandy,
Oh, sorry. I guess I do know what a ketch rig is. Had one for many years. Great for cruising but kind of unhandy for day sailing.
Kind regards,
JohnL

I recently caught up with an old friend who skippered Olaf Harken's unusual vessel Procyon for its first few years. I showed him a model of my 'single-masted ketch' concept, and then just the other day I sent him a copy of this 'sailing challenge' I had issued to a few naysayers in the past.



You know I can still imagine sailing a big 65-foot catamaran with this mastaft (single-masted ketch) rig right off the mooring, and back to the mooring, without the engine, by myself, with so little effort that I might take it out having only a few spare hours to kill, or for just a carefree daysail.

I wouldn’t have to uncover any sails, nor recover them when I returned to port. I'd likely start out with just unrolling that central 'mainstaysail' then add in more sail area as needed.

I would be less concerned with reefing by myself if the wind were to really come up. If I were short-handed at sea, I would have many of the benefits of a ketch rig, without the necessity of slab reefing the main and mizzen sails of the traditional ketch rig. I'd be in a constant state of helm balance as I could tweak the mizzen sail for that.

That about sums it up. I would like a 65 foot cat that I could take sailing by myself, and that might even be easier than a beach cat. Try hoisting a full batten mainsail on a 65 footer by yourself, or even a 40 footer. Most folks over 50 will have second thoughts, or will just unfurl the jib and forget about hoisting that traditional MAIN sail.

I'm 65 (now 73), and I could sail this 65 foot cat by myself with this aft mast rig. And with the balance and low power afforded by the smaller 'mainstaysail' I could sail this vessel right off the mooring or maybe right off a side-to-dock slip.



Here's a challenge Mr Naysayer..... Le Mans start. I'll be out sailing before you get the covers off your mainsail, and when we return for the day, I'll be at the bar sipping on a Margarita watching you put away your vessel for the day.


Randy sent me a reply this morning...

reminds me of Procyon,,,we would have her sailing before the crew could put away the fenders