Dinghy Mast Design

Hi

I've been reading Frank Bethwaits High performance sailing and realise that his square backed wing mast is now a fairly old concept. I am into low RE aerodynamics so the effects of boundary layer turbulation are very familiar to me. I'm wondering if there has been any further aero study and development that has lead to a demonstratably more efficient mast concept.

If not, Franks mast may still be the best proven technology around despite current fashion. Any info would be appreciated.

Also I have an old MG14, the rig is now 20 yr old with a round mast. I'm still comming to terms with consistantly trimming the boat but it does at times show surprising promise for an old boat. Depending on where my investigation leads and commercially available options, I'm thinking of trying to make a mast. Yes I realise that's pretty tricky to match the main luff curve to the mast, get the right camber and mast flex etc right over a range of conditions. However it should be a pretty challenging experiment. Can anyone point me towards helpful info on mast design and construction?


Regards,
Ken

 

Any book on marine design like Skeens will do ,

but the really fast modern stuff seems to be on windsurfers.

If you could find a rig big enough it would be fast as anything .

FAST FRED

 

Ta Fred.

The MG14 is restricted to 1 rig, sail area and rig height. The area is very small for a 14 footer but boats are efficient and fast. Efficency is the name of this game. This is why I am asking for quantitative research in the area and info on constructing a mast with the right flex characteristics.

Can you be specific about resources?


Regards,
Ken

 

I think the main reason the whole concept faded was because it was so impractical in aluminium. Reading between the lines the Tasar mast is well short of optimum. Also they're ******** to tune. I have one of the original Bethwaite wooden ones and its stll a pretty impressive bit of kit when you get it right but desperately heavy.

There are no good resources on the subject of dinghy wing masts. All the work since then has been done with poles. I'm quite sure that there's a lot of unfinished business in the concept, especially with modern materials and in particular combining materials so that like the original you have much of the mast non-structural - maybe moulding on a PVC flexible fairing on top of a semi square section carbon tube. No need for an internal mast track like the originals - a glued on plastic track will be fine and still leave you the trip on the corner.

But, but but. You need to be prepared to make a lot of prototypes and lose a couple of seasons learning how to both fabricate the rigs and then learn to tune them.

 

I'm doing the same thing, but I have lots of freedom. The boat I'm working on is un-stayed and gaff rigged.

(you can stop laughing now)

FW's mast were stiff with bendy tips, the tips were tuned for gust response. I'm going to build the masts (it's a schooner) stiff for the length of the luff. Above the throat the masts taper. The gaffs will be tuned to react to gusts and if need be I can build several to evaluate performance. Each gaff can be tuned for fore and aft bend with peak halyard tension and attachment point(s) to vary the length and prebend of the unsupported tip.

Twist is positively controlled with gaff vangs. The foremast gaff vang leads to the mainmast. The main gaff vangs will lead to the transom.

Only the main has to be automatic to keep the boat on her feet. Athwartships bend can be tuned by moving the attachment point of the vang.

Once I get the rig built, I'll work with Quantum to make sails.

Unless someone comes up with a better plan, I'm going to use the front 20-40% of a 63x Naca foil for the mast and gaff sections. I figure the flow at the spars will have a laminar boundary layer as long as the pressure gradient is positive.

I'm not sure that anyone has tried to optimise a gaff rig or schooner in the last 50 years. I'm hoping that I can approach the upwind performance of a stayed Ketch or Yawl and keep the Schooner's superior performance on other points of sail.

(commence laughter)

 

Interesting about the gaff rig schooner. Will the masts rotate?
I'd use more than 20% to 40% of the section. A good laminar section will maintain laminar flow over more than 80% chord at small angles of attack.
Small angles are the aim of a rotating mast but the real point is taking the flow past the thick point of the section so the high speed flow is converging onto the sail very early. To attach turb flow on the sail early and so minimise the size and effect of the separation bubble. At least that's my understanding of it.

If the masts don't rotate or only rotate with the boom, I don't think aero masts can be much if any advantage because such masts open a wider separation bubble on the lee side and that is the most important side by a factor of 3 to 5 times.

Regards,
Ken

 

Gaff Rigs

Mr. Hough, I'm not laughing at your gaff rigs-I think they have a lot of potential.
I've built a series of production rc sailboats using exclusively a version of a gaff rig . On one version seen on the F3 below the gaff is carbon and sticks(rotatably) into the top of the mast.On America One it's just a plain, though horizontal, gaff. Both systems use peak and throat halyards but I discovered accidently that an upper outhaul on the gaff-horizontal on both types really adds to sail shaping.Both mains have excellent gust response....
These ideas were tested against triangular sails of the same area on equal boats and proved to my satisfaction that they were far superior.
Adapting the upper outhaul maybe too much hassle but it works well.
F3 rc foiler: http://www.microsail.com/pictures/mfoilerf3_14a.jpg
America One:
http://www.microsail.com/pictures/a1c.jpg

 

Thanks,

The masts will be simple affairs, with the finest back yard engineering.

I know what the maximum static load will be. The hull is very fine (24x5), the stability curve is positive to 128 degrees with no steep rise to induce athwartships shock loads. With the fine entry I don't anticipate high shock loads for and aft. The rig has to be short to keep the power up so if I hose the mast design I won't be into a second mortgage to build new ones.

The rig will probably be unstayed if I can get enough jib luff tension from the mast rake and foresail sheet tension. If not I'll add back stays to the foremast. PBO with UHMW deadeyes for adjustment ... it is a schooner after all ...

Since you've built RC boats, you'd find the jib club setup quite familiar. The club pivots aft of the jib tack so part of the halyard tension is transferred to the leech. This should act to control twist and make the sail effective at broad reaching angles. The jib sheet will be on centre if I can get enough twist control, I'll add a jib club traveler if I need to get more leech tension upwind.

At this point I'm not planning to rotate the spars, but I could be talked into it. Rotating the main mast would be easy and not add much weight. Rotating the foremast is a bigger problem due to the forestay and jib.

On a gaff rig the effective luff is broken into two parts, the gaff section stays aligned with the sail automatically, I'm not sure that much would be gained by rotating the masts also.

I've given gaff outhauls some thought. I plan to include a static adjustment at the end of the gaff. Allowing adjustment from the deck would be tricky. If I use gaff bend for gust response, it would change the gaff outhaul tension and make the head of the sail fuller just when I want it to be flat. If I can figure out a way to hold the adjustment from the deck while the gaff bends I'll do it.

-Randy
(Mr. Hough is my father)

 

Gaff Rigs

Randy, for what it's worth: on the model rigs what I've found is that the size of the head/gaff determines how easily the sail can be depowered automatically. It's controlled by vang tension but will always twist off in response to a gust. My gaffs are quite stiff and the sails are adjustable in the direction of or by a peak and throat halyard in addition to the two outhauls. The gaff is molded (horizontal area)so that the outhaul adjustment doesn't change peak tension...
I don't use the "normal" model jib boom: I use either the Hoyt type boom or the "normal" type with the jib pivot on a traveller(JPT= jib pivot traveller) so that the leading edge of the jib is on the CL of the boat upwind...If the jib forestay isn't on or intersecting the boat centerline it doesn't point as well.This same system was used on a full size boat called Cayuse which can be seen in the book "Maximum Sail Power". They liked the performance but they didn't like the foredeck being essentially off limits when the jib luffed.

 

Doug,
I'm a firm believer in model testing. Once I have a design that I think is right, I'm planing a 2"=1' scale RC model to test sail area distribution and verify construction method.

Are you referring to the size of the head and gaff as length or gaff diameter?

I'm planning to limit/adjust foresail twist with a gaff vang to the main mast. The foresail gaff will bend in the plane of the sail only, to provide draft control of the foresail above the throat.

The main gaff vangs will attach to the main gaff part way out the gaff so only the tip of the gaff past the vang attachment point can twist off in response to gusts.

Off the wind the gaff vangs will provide twist control independent of leech tension. I'm hoping the rig will not need boom vangs so gooseneck loads will be low.

I hadn't heard that the balanced jib club points lower than a Hoyt system. I'll have to give that some thought. When I built a RC 1 metre I thought that the system was very clever and solved the jib twist when off the wind problem very nicely. I don't think I can make a Hoyt system work since the jib tacks on a bowsprit. As the boat bears off the leech load would be a torque on the sprit and making it stiff enough to support Hoyt jib boom would put extra weight on an already too fine bow.

I could use a traveler car to make the jib boom attachment point adjustable ... it would allow the jib boom to be aft and the jib luff on the boat's Centre Line upwind and provide jib vang tension off the wind ... hmmm

Thanks for sharing that tidbit I hadn't considered the effect of moving the jib luff to weather.

-Randy

 

square top jib / gaff

Randy, you might find this worth experimenting with. I've used it on a model as well as on my 16' aeroSKIFF foiler. The head of the jib is supported by a "gaff" that is just a short length of carbon tube. Then the halyard is attached to the carbon tube at about 33% aft.
What happens is that the upper third of the jib rotates slightly to weather but in this case because(apparently) it is a "masthead" jib there is no (noticeable as yet)detrimental effect on pointing and the whole rig viewed from above appears twisted instead of having just two separate sails individually twisted. Don't have enough info yet to say this works any better than normal.
aeroskiff_trial009.jpg
http://www.monofoiler.com/images/aeroskiff_trial009.jpg
Model:
http://www.microsail.com/pictures/aeroskiff013f.jpg
---------------------------
On the subject of this thread: the mast on my 16' foiler is a round carbon tube and stepped into the deck; it supports itself initially like an unstayed mast. The main has a zipper up the center of the luff that allows the shrouds out thru an opening; it is hoisted normally but you have to pull the zipper down as the sail is hoisted. Then you connect the shrouds. The mast has a rather large masthead crane that supports the jib with it's integral forestay/gaff combo.The crane+ backstay is necessarry to get proper jib luff tension.
Future versions would use sliding versions of camber inducers inserted as the sail is hoisted and would probably give extraordinary sail shape with virtually no mast interference.

 

Very nice! ... but I can't use it ...

One of the design goals of this boat is to maintain a traditional schooner profile, while getting better than traditional schooner performance. The main targets are Don Martin's Rocket 22 and the Martin 25 fleet.

I don't think I can get the same windward leeward speed average as the Rocket 22, but I think I can get pretty close on a random leg course.

In Bethwaite terms the R22 has a power/wt ratio of about 7.5% upwind and the schooner is coming in at 8.2% The design windspeed for the schooner is getting down to about 10 knots true. Vancouver sailing is mostly light air, I want the schooner to plane before the R22. In VTW below 10 I have a fighting chance.

Donny's boat looks high tech and performs very well. I just want to see the look on his face when a low tech looking schooner is too close for comfort and hopefully ahead on corrected time.

It would very very satisfying to have a plywood boat built from Home Depot lumber approach the performance of a vacuum bagged composite boat on the race course.

My other target is a vintage 6 Metre that cleans up in the Wooden Boat Festival races.

The boat is to be a sleeper.