other mast sections

[Hirilonde]

There have been numerous discussion here about mast sections for free standing masts for use on Graham's boats. For the most part all have been round and most use birdsmouth construction. Is round the best structural shape? Could other shapes be as strong, or at least strong enough? I found this following section on a web page for the International 8 Meter:

Could something similar work for a Lapwing or Core Sound?

It is probably over kill to think that the foil shape would have much impact, but it does also look pretty.

[PAR]

Round shapes are employed because you want the center of gyration to be equal around the perimeter of the column.

The tear drop section you've shown above, will have a higher resistance to bending fore and aft, compared to side to side. This in itself isn't a bad thing until you introduce side loads, as well as fore and aft loads simultaneously. In a nut shell, a stick of that shape would bend off to leeward excessively and unpredictably, in regard to luff round. It would only work on a single point of sail, as all other points the stick wouldn't offer the bend the luff needs to get a reliably shaped camber. In short on some points of sail, there'd be too much belly and others not enough in the same wind strengths.

[Gordy Hill]

Round shapes are employed because you want the center of gyration to be equal around the perimeter of the column.

The tear drop section you've shown above, will have a higher resistance to bending fore and aft, compared to side to side. This in itself isn't a bad thing until you introduce side loads, as well as fore and aft loads simultaneously. In a nut shell, a stick of that shape would bend off to leeward excessively and unpredictably, in regard to luff round. It would only work on a single point of sail, as all other points the stick wouldn't offer the bend the luff needs to get a reliably shaped camber. In short on some points of sail, there'd be too much belly and others not enough in the same wind strengths.

What if the mast were free to rotate? Wouldn't the mast then always be at the optimum angle?

It might be an advantage to bend to leeward when overpowered while still maintaining a relatively straight luff.

Now I'll go hide behind something!

[Hirilonde]

What if the mast were free to rotate?

That would require a very interesting mechanism.

[Gordy Hill]

If the cross section of the bottom of the mast is round it's no problem. The masts on my CS17 rotated freely. If they didn't the snotter would constantly be changing tension.

Actually, I really like the idea. It would give the rig a much better airfoil, in fact, if you went a bit farther with the dotted line it would flow right into the sail slot. Use an internal halyard and you'd have very smooth airflow right where it counts the most.

[hokeyhydro]

I think an unstayed sprit rig should rotate. In fact I believe one would have to pin the mast to keep it from rotating, assuming round mast.

[tom151]

Here's my planned rotating wing mast - for a CS20 - 2

This would have a plug in the lower section that would span bending and compressive loads at the thwart and be rounded below the wing profile to fit into a fixed sleeve for rotation,

Was planning a half wishbone sprit, now considering a full wishbone, we'll see...

[hokeyhydro]

Here's my planned rotating wing mast - for a CS20 - 2

This would have a plug in the lower section that would span bending and compressive loads at the thwart and be rounded below the wing profile to fit into a fixed sleeve for rotation,

Was planning a half wishbone sprit, now considering a full wishbone, we'll see...

Now we're talking . . . cool rig..

[Gordy Hill]

Tom,

I think it's far far more important for the trailing edge of the mast to flow as smoothly as possible to the sail.

As soon as there's a break in the smooth airflow on the leeward side of the leading edge of your sail (wing) there's an enormous loss of lift.

[PAR]

If interested in keeping things simple, the sock luff is the best bet. To make foil sections work well you have to be able to over rotate the foil, which can add a considerable amount of complexity to the arrangement. If you use that bullet section, you'll surely want to over rotate the foil a bit. The sharp leading edge will offer some improved "penetration" but it'll also stall much sooner and a good size eddie on the windward side will appear other wise, particularly with that section. Flaps will help this to a degree and cats have done this for years, first with narrow lengths of sail cloth, which "buzzed" or flapped, though thicker gauges of Mylar have solved this for the most part. What really happens with these types of setups is you discover how much twist you really need to keep the length of the luff "engaged" and drawing. Having recently gone from a semi rigid, balanced and over rotatable foil luff, to a two segment foil, it's real easy to get caught up in improvements that account for only a fraction of a knot improvement. My new double segment setup, improves wind range usability, but speed isn't improved by a distinctive amount. My point is once you delve into these sort of modifications it's a whole new can of worms, none of which is easy to study, implement easily or justify with speed improvements.

[Howard]

Not that I would recommend it, for reasons PAR has outlined, but if a person wanted to build an oval birdsmouth mast, all that is required is to leave two staves a bit wider than the other six and glue them up opposite each other. The remaining geometry still works. Might work for something like a boom or sprit that works in compression, but it does seem logical that with an unstayed mast, you would get inconsistent bending characteristics with an oval or elliptical shape. Round or square seem to be the best choices.

I still find it interesting the length people will go to with stayed masts and such to shape their sails. All manner of rake, backstay adjusters to induce mast bend, loos gauges to tension the stays, etc. Outhauls, downhauls, cunninghams, barber haulers, travelers, vangs, etc.

Then there are these simple unstayed cat ketch rigs that somehow still manage to claw their way to weather and reasonably well to boot. A lot to be said for simplicity.

[tom151]

Tom,

I think it's far far more important for the trailing edge of the mast to flow as smoothly as possible to the sail.

As soon as there's a break in the smooth airflow on the leeward side of the leading edge of your sail (wing) there's an enormous loss of lift.

Gordy,

I used to have the precisely the same belief as you - and thought it was a good one - as it seems to be valid in so many of the situations I came in touch with when sailing wing masted boats.

Then I re-read Bethwaite, whose work comes from theory and testing, and found his work that led to his design of the successful Taser mast. I am going in this direction partly because for the unstayed masts of CS20-sized boats the section will be a relatively small one,

Additionally (my opinion, never stated by Bethwaite) the success of what might be called a 'flat back' wing may have something to do with the size/scale of the mast compared to the chord of the sail.

In any event, I'll trust Bethwaite on this one. He's definitely worth a second read if you're into this stuff (as I am an 'old' aero/hydro-dynamics guy, I get sucked in every time)

Cheers,

TomH

[tom151]

If interested in keeping things simple, the sock luff is the best bet. To make foil sections work well you have to be able to over rotate the foil, which can add a considerable amount of complexity to the arrangement. If you use that bullet section, you'll surely want to over rotate the foil a bit. The sharp leading edge will offer some improved "penetration" but it'll also stall much sooner and a good size eddie on the windward side will appear other wise, particularly with that section. Flaps will help this to a degree and cats have done this for years, first with narrow lengths of sail cloth, which "buzzed" or flapped, though thicker gauges of Mylar have solved this for the most part. What really happens with these types of setups is you discover how much twist you really need to keep the length of the luff "engaged" and drawing. Having recently gone from a semi rigid, balanced and over rotatable foil luff, to a two segment foil, it's real easy to get caught up in improvements that account for only a fraction of a knot improvement. My new double segment setup, improves wind range usability, but speed isn't improved by a distinctive amount. My point is once you delve into these sort of modifications it's a whole new can of worms, none of which is easy to study, implement easily or justify with speed improvements.

PAR,

Yes, I am anticipating the need to induce some needed over-rotation for this scheme. My experience with Gougeon type wings made me initially leery - looking at Bethwaite's work it seems (my untested inference here) that the amount of over-rotation that will be needed is substantially less for the Taser shape and scale. True I don't know why - but hey.

Couldn't agree more with your "whole new can of worms" thinking. My primary interest is to get the biggest bite of the sail-horsepower-improvement apple by getting to a rotating aero-shaped mast. For me and my understanding (subject to correction) the luff sleeve does not give enough in increased power benefits and adds severe (to me) complications to efficient reefing - but that's me.

Cheers,

TomH

[Phil Garland]

I have a Presto 30 with free standing carbon rigs...they are round sections and with the wishbone booms the masts rotate fairly easily. There is a delrin collar on the mast at the deck and at the heel into G-10 tubes so there is not much friction. The section goes from about 4" at the base to 2" at the top. They are pretty clean and if I over-rotate them a bit it's even better.

A wing shaped spar sounds good and would be cleaner to the wind but the amount of rotation becomes critical to spar bend so you would need a rotation control arm which is starting to go outside the box for boats like the Core Sounds. And you might find that fine tuning sail shape becomes more complicated.

For monohull boats that average 4-8 kts it's less of an issue than multi-hulls that go 10-20 kts with the apparent wind well forward.

For me I would spend the extra effort to have curved or wishbone booms to improve the overall sail shape over the straight sprit...but again, for most owners of a CS17 a sprit is light and simple solution.

Phil Garland

[Alan Stewart]

Howdy All, since we're talking about making our boats go faster I though I would chime in since I'm a fan of that.

Rotating wing masts are sweet but personally I think the simplicity of the CS boats is what makes them agile and fun to sail. On a 17 or 20 I think I would look elsewhere to increase speed. It's going to be really really hard to beat the round aluminum section in strength, weight and cost. If anything, I would love a set of carbon round spars for my 17. Keep the simplicity and bending characteristics but drop the weight. If a wing section masts (say a wooden one for example) gives you more power but ends up 20 or 30 lbs heavier than the aluminum stand-by then your going over sooner than the other guy in a gust and you may not come back up as quick.

You definitely want battens IMO and so too tracks on your masts. Curved spirits sound good to me. Anyone thought about running the halyard inside the mast or going to a thin wire halyard? Lines along the mast make dirty air and some serious drag. Are you sheeting and trimming constantly, watching for wind shifts, always looking up to see what the sails are doing, making sure the helm is balanced yada yada. Then there is always bigger sails. Graham will be the first to tell you he is a fan of horsepower. Most of the latest track luff CS sails are semi full batten'd with a generous roach.

I would also point out that there are sure to be gains waiting to be made below the water as well. How fair and smooth is your bottom? Even road grime from a long trailer ride can make a huge difference. Are the bottoms of your CB and rudder all chewed up or nice and smooth? Wetted surface area is a big source of drag at "slow speeds" like under 5 knots. That means the lighter the boat the less of it is in the water and thus less drag. At slower speeds boat trim is also HUGE. scooching a few feet forward in light air, using that tiller extension to get the butt out of the water and sitting on the low side to help heel the boat to leeward and get some of that bottom out of the water will pay off big time. This is dingy sailing 101. The CS rudders and centerboards are well thought out tradeoffs between shoal draft sailing ability which is what the boats are designed for and good performance. I would expect anyone who is looking to increase CS performance and entertaining the idea of wing masts might also be thinking about trading some draft for a higher aspect rudder profile. What about some rubber flaps for the centerboard to reduce drag from the slot. 420's and many other small CB dingys use those with success.

Todays C-class catamarans with solid rotating wing sails and ultra skinny hulls do their darndest to save every ounce. One teams designers estimated they stood to gain 1 boats length over the other guy for each pound of weight they shaved off the boat. One thing you could do to immediately increase performance is to reduce weight. get rid of that stuff that goes sailing with you every time but that you never use. Maybe you don't need so much anchor chain?

Just some other ideas

-Alan

[tom151]

I have a Presto 30 with free standing carbon rigs...they are round sections and with the wishbone booms the masts rotate fairly easily. There is a delrin collar on the mast at the deck and at the heel into G-10 tubes so there is not much friction. The section goes from about 4" at the base to 2" at the top. They are pretty clean and if I over-rotate them a bit it's even better.

A wing shaped spar sounds good and would be cleaner to the wind but the amount of rotation becomes critical to spar bend so you would need a rotation control arm which is starting to go outside the box for boats like the Core Sounds. And you might find that fine tuning sail shape becomes more complicated.

For monohull boats that average 4-8 kts it's less of an issue than multi-hulls that go 10-20 kts with the apparent wind well forward.

For me I would spend the extra effort to have curved or wishbone booms to improve the overall sail shape over the straight sprit...but again, for most owners of a CS17 a sprit is light and simple solution.

Phil Garland

Phil,

Thanks for your comments. Presto 30, nice; Roger Martin, terrific!

Is your Presto a stock rig? Am curious, if no mast rotation is being induced manually - is the natural tendency of the wishbone to over- or under- rotate the mast?

I do hope that the end result for the new boat will be quite a bit closer to 8 - 12 knots ;} and maybe a bit more off the breeze in a good blow.

TomH

[tom151]

Howdy All, since we're talking about making our boats go faster I though I would chime in since I'm a fan of that. Rotating wing masts are sweet but personally I think the simplicity of the CS boats is what makes them agile and fun to sail. On a 17 or 20 I think I would look elsewhere to increase speed. It's going to be really really hard to beat the round aluminum section in strength, weight and cost. If anything, I would love a set of carbon round spars for my 17. Keep the simplicity and bending characteristics but drop the weight. If a wing section masts (say a wooden one for example) gives you more power but ends up 20 or 30 lbs heavier than the aluminum stand-by then your going over sooner than the other guy in a gust and you may not come back up as quick. You definitely want battens IMO and so too tracks on your masts. Curved spirits sound good to me. Anyone thought about running the halyard inside the mast or going to a thin wire halyard? Lines along the mast make dirty air and some serious drag. Are you sheeting and trimming constantly, watching for wind shifts, always looking up to see what the sails are doing, making sure the helm is balanced yada yada. Then there is always bigger sails. Graham will be the first to tell you he is a fan of horsepower. Most of the latest track luff CS sails are semi full batten'd with a generous roach. I would also point out that there are sure to be gains waiting to be made below the water as well. How fair and smooth is your bottom? Even road grime from a long trailer ride can make a huge difference. Are the bottoms of your CB and rudder all chewed up or nice and smooth? Wetted surface area is a big source of drag at "slow speeds" like under 5 knots. That means the lighter the boat the less of it is in the water and thus less drag. At slower speeds boat trim is also HUGE. scooching a few feet forward in light air, using that tiller extension to get the butt out of the water and sitting on the low side to help heel the boat to leeward and get some of that bottom out of the water will pay off big time. This is dingy sailing 101. The CS rudders and centerboards are well thought out tradeoffs between shoal draft sailing ability which is what the boats are designed for and good performance. I would expect anyone who is looking to increase CS performance and entertaining the idea of wing masts might also be thinking about trading some draft for a higher aspect rudder profile. What about some rubber flaps for the centerboard to reduce drag from the slot. 420's and many other small CB dingys use those with success. Todays C-class catamarans with solid rotating wing sails and ultra skinny hulls do their darndest to save every ounce. One teams designers estimated they stood to gain 1 boats length over the other guy for each pound of weight they shaved off the boat. One thing you could do to immediately increase performance is to reduce weight. get rid of that stuff that goes sailing with you every time but that you never use. Maybe you don't need so much anchor chain? Just some other ideas -Alan

You're spot on Alan.

My thinking is similar: that if you want to have high average speeds you first better maximize the horsepower from the sail area you've got. Then you go on to killing the drag, and the foils are the place to start because of the huge effects bad foils have and that hurts you on all points of sail. Hull shapes for a boat that's going to be planing a lot (Bruce No over 1.6 range) pretty much has to primarily be concerned with keeping low-speed surface area at a minimum and paying attention to hull shape combined with trim will get that done mostly. I'm looking for ways to keep the chine out of the water at low speeds yet retain the flat aft sections with sharp edged chines at planing speeds (aren't we all After all - it's only plywood!

The punch list for this project is: full-battened, fathead/squaretop sails (think Presto 30 as mentioned earlier) with lots of reefs on rotating wingmasts, working sail area up around 280 sqft (main plus a nearly as large mizzen), prodder with code 0, CS20 style hull stretched to 22.5 feet, maybe a little work on the bottom shape to unstuck it easier in the light stuff, a profiled daggerboard in a close-fitted case, profiled dagger-rudder similar to MikStorer's approach on his GIS (with a twist , and water ballast in interconnected side tanks.

I want a "Florida to the Bahamas in 5.5 hours" boat - I think this one might do it! Though we all know that a multihull would make it a bit easier to accomplish

Cheers,

TomH

[Tom Lathrop]

I think Alan has the best slant on this issue. The CS is a great boat but Graham never intended them to be high end racers. They perform well but most people rarely become proficent enough at sailing the stock boat for such minor issues as the mast cross sections being considered here to become important. The most efficient of the simple sail rigs is the sleeve luff over a round mast. We go to a sailtrack mostly for easy reefing. No amount of go-fast will allow recovery from being on the wrong tack or not adjusting sails properly or not balancing the boat well or a dozen other things that are more important than mast cross section shape.

[PAR]

I'm inclined to agree. There are many areas I'd address first on this boat, but mostly windage and weight aloft reductions will yield the most results on this rig, while appendage modifications could offer some rewards too. These results will not make substantial improvements in the performance envelope, though against others of her type, certainly significant enough to be noticeable.

In regard to foil luffs, you have to have the potential to exploit or it's just an engineering excessive. The CS series is a nicely balanced, forgiving, stable platform, with good capacity and is light enough to offer some plane mode fun. I don't think there's enough potential to really consider going crazy, though again, some weight aloft and windage reduction, as well as appendage sections wouldn't hurt. Now if a CS-17 was 100 pounds lighter, maybe several inches less in beam, toss in a trapeze and a more powerful rig, well then your envelop may have something to exploit, but this isn't what the CS series is.

[Phil Garland]

Tom H...to answer your questions about the Presto 30: It is a stock boat...it is hull #1 and I am partners with Rodger Martin. We designed and built the boat with fathead sails and most of the boats have had that rig. The wishbone booms are rigged like a Wyliecat 30...Tom Wylie was very helpful and his system uses a double snotter arrangement so the masts rotate very well; aligning with the sail.

Rodger and his wife Patty have just spent 2 months on Presto in the Keys and the Bahamas and the boat has performed really well. We have set up the reefing systems so everything can be done from the cockpit and it usually takes less than a minute to put a reef in either sail. The wishbooms also have 3 lines forming a basket so when the sail is reefed or lowered it's all cradled like with lazyjacks.

I am planning to build a CS17 and do carbon rigs...not sure about the booms. We did the Presto booms in aluminum and they are pretty heavy. Keeping the weight down on a centerboard sharpie is important.