Thursday, March 7, 2013

Whimbrel Developments

My plans for Whimbrel have been sitting 90% finished for a long time. The drawings which have been finished are nicely detailed, and I really like the boat. But for some reason the final step of completing the plans for publication has been very difficult to complete.

Outboard Profile of Whimbrel, showing her with the original balance lug rig and free-standing mast. Note how the heel of the mast has to swing down and up through the foredack. This arrangement is catered for by the installation of a slot-type box - similar to a centreboard case, but upside-down.
Whimbrel was designed with a balance lug mainsail, and to capitalise on some of the advantages of this rig (no headsails, no sail-track or lacing, self-vanging boom, simple and effective reefing) the design was drawn with free-standing masts. An important component of the rig was to be a main-mast set in a tabernacle, so as to allow quick and easy rigging and un-rigging for trailer transport, and for reducing windage when on a mooring in bad weather. You can see the lowered mast position in dotted lines on the drawing above.

Tabernacles can be bulky affairs, and I dislike having a tabernacle which extends a long way above the deck line. However, a free-standing mast benefits from substantial "bury" of the mast within an arrangement of mast step and mast partners, or, as in this case, within a tabernacle. My approach was to have the base of the mast swing up and down through a slot in the foredeck, with sides to the box which extend all the way to the level of the mast step. That way, water can drain down the mast, or come on deck as spray, and it simply runs down to the bottom on the mast case and is then drained over the sides through scuppers - making the mast case self-draining.

The design of the slot and case was fairly straight forward, and I was very pleased with how the structural members support each other and go together in a logical fashion. The trouble is that although the mast case (i.e. a structure similar to an upside-down centreboard case) is simple to construct, it was and is very difficult for me to explain on paper. The drawings of all the components are complete in a 2D form, but I do not have the skills to operate the 3D CAD program I have on hand (TurboCAD 17). I do all of my CAD work in an AutoCAD 2D program (AutoSketch 9).

Several times I've tried to do a simple diagram as an "exploded" isometric drawing, but nothing satisfying has eventuated. I thought that the simplest thing would be to build a prototype, and photograph the case construction in detail, as I was/am sure that as soon as people see it, they will appreciate the simplicity and structural elegance.

In the meantime, one of my sons has been experimenting with a tiny jib (set flying) on a small clinker/lapstrake dinghy he built years ago. The small jib has boosted the windward performance of the boat enormously, even though the sail area of the jib is only 11sq. ft (from memory). This boat sailed quite badly with the free-standing rig as originally built, using a carbon windsurfer mast. The mast was just too soft, even with the addition of substantial alloy sleeve at the lower end.

Here you can see Dave's boat sailing with the original free-standing mast. She had initially sailed with the sail made according to the sail-plan on the plans, but that sail had been awful. Perhaps it would have been ok on a stiffer mast instead of the windsurfer stick. We then changed to an old Laser sail as shown in the pictures, but the mast was still far too soft, and the boat was a very poor performer to windward.

I've told this story before on the blog, but I suggested to Dave that we put some stays on the rig, with the point of attachment (hounds) being just a fraction over halfway up from the deck to the masthead. He was reluctant, but I eventually convinced him to give it a go and we made up a forestay and two shrouds using Dyneema/Spectra. This also allowed us to set an old jib, as a "flying jib" i.e. a jib with is not attached to the forestay with hanks.

The performance of the boat was dramatically transformed for the better, and the boat is giving great service.

My point in re-telling parts of the rig story is to illustrate that sometimes a very modest increase in complication can result in huge gains in performance. Now, back to Whimbrel....

The revelations about performance brought about by the addition of the jib to Dave's boat got me thinking a little outside the box I normally occupy. I tend to be a bit obsessive when it come to rig simplicity, because I hate clutter in a boat when rigging and un-rigging at the boat ramp. Our experiments with the dinghy rig above have brought me to the point where I'm considering some different rigging arrangements for Whimbrel, and the design evolution continues - perhaps it was fortunate that I hung off from completing the original drawings.

In the above drawing you can see the proposed new rig for Whimbrel. The hull is unchanged, as is the mizzen, but the main part of the rig is now a lightly stayed, gaff-headed mainsail with a staysail set on the forestay. The staysail can either be hanked onto the forestay, or could be permanently attached to the forestay with a simple roller-furling tackle set at the tack.

Because the mainmast is now supported by stays in the form of a forestay and two shrouds, the mast no longer requires the support of a tabernacle which runs deeply into the hull. In this case the tabernacle is short and extends only to the level of the deck, and the foredeck is no longer cut up by a mast case and slot. So the mast is significantly shorter at the foot, and the diameter of the mast is reduced from 92mm (3-5/8") to 65mm (2-9/16").

Whimbrel set up for the night with a boom tent rigged and the mizzen sheeted flat to hold he head -to-wind.  I can't see the anchor rode - it must be some of that invisible rope I sometimes use!

I'm continuing to consider some options for the cockpit, and will up-date soon.


  1. My enthusiasm for the design only grows as your ideas and developments proceed, Ross. Those knife-blade jibs really do make a difference. That's a whole lot of boat packed into a short LOA.

    Rick H.

  2. Although performance might be improved with the newly proposed rig, I would urge you not to drop the original lug rig (despite the difficulties in providing plan detail). The new rig instantly adds cost and complexity, daunting to a new sailor. It also eliminates it as a prospective plan for those needing an unstayed rig that can be dropped rather quickly...

    Offering both sailplans would be an excellent addition to a fine hull design...

  3. Hi Ross - I agree with your idea of offering both rigs for the flexibility of the plan, depending on the needs of the builder. As I have said, I do prefer the simplicity of the unstayed rig, but opinions vary...

  4. My case is one of sailing all of the time out of the same harbor without expecting ever to trail the boat. A trailer sailor will likely find the unstayed lug rig much handier in setting up to sail and in getting the boat back on the road afterward. There may be wider appeal in that.

    As for cost and complexity, it seems there is a tradeoff between greater complexity and cost in the structure of the boat, versus greater complexity and cost in rigging the boat. For some, a further consideration is that the jib adds a string to pull every time one changes tack, but as for basic cost and complexity, I'm not sure one beats the other.

    The main thing would be whether trailer sailors are happy with the rig. As for me, I'd be happier with the more complex rig for reasons explained later, and for not having all that penetration of the below-decks space by structures open to the weather, as in that foredeck slot for the foot of the mast to swing through.

    As I would be sailing in a rugged rampart coastal region with a steady onshore thermal that runs all year long, the spread out rig with more sail combinations and the promise of being closer-winded is very attractive, as is the shorter mainmast. I especially like the option of being able to reduce sail quickly by dropping the main to go on under jib and mizzen, either while tying in reefs or for maneuvering low-powered in tight spaces or approaching a mooring or anchorage.

    Rick H.

  5. Hi Ross - just read the article again, and thought about how Phil Bolger might have solved the tabernacle/slot problem. He used an open well forward in several of his designs, a self- bailing set-up, which also gives you a place to stow the anchor, and any other muddy wet stuff- fish, shellfish, etc. I don't think the lost space in the cabin would be too much, there's not much room up there anyway. Just a thought- I really do like that unstayed lug rig.

  6. I like Wimbrel a lot! It seems to be in the same class as the Great Pelican and Selway Fisher's Goshawk. Or is she smaller than those boats? I'm not sure about the rig though. I'll take a boat with two sails over three sails any day. I know there is a boat (Navi-something?) that is very successful with three sails but I think its a few too many strings, wires and sticks to deal with. Especially if you also have leeboards to watch.

  7. I had a Great Pelican a single summer not too long ago. I could try this. I figured tough about dropping the rig and installation a 30 HP outboard about the stern. It could have been a very great, distinctive little cabin garvey, and i believe will have planed and handled quite well. Thanks for this!

    3D CAD Program

  8. I'm very interested in this design. I will vote for two sails and unstayed mast. Im planning to downsize from a 22 ft magregor for this very reason. Love the thought given to the tent

  9. Any idea when plans will be ready for Whimbrel and Fleet?

  10. I am very interested in plans for Fleet as well.