practical proa sailplans
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nice , that looks like Sydney harbor - 18’ skiffs?
that parasol for the jib is whacky, but laminar flow is king, obviously, no matter how you get it.
Yes, 18 ft skiffs. The “parasol” is intended to be an end plate to minimise the vortex off the square jib head. I reckon it also provides additional mast support. ” Lumix” has won a few races, but hasn’t set the world on fire to my knowledge…..
You mentioned tear drop mast sections and fixed booms…..If the teardrop is intended to act as an aerofoil, then to get the most benefit, it should over rotate relative to the boom. Over rotation is generally controlled with a mast spanner either at the mast pivot or just below the boom.
Rob
.....As soon as you are off the really close hauled courses though and get into a close reach, then more foresail area (relative to the main), gives you more driving force per unit of heeling moment, so if you’re not sailing triangle courses and not trying to get the absolute best velocity made good (VMG) directly upwind, then larger foresails are good for you.
Marco ,
I think you have it the wrong way round…... as a foresail is eased off from closehauled it is more difficult to keep a good shape due to excessive camber, lost leech control and increasingly being shadowed by the mainsail the further you go off the wind. The bigger the foresail, the bigger the problem. Mainsails are better off the wind than a foresail and fully battened square top mainsails definitely have to be the way to go. Not just because of tip vortex issues, but also because you get more sail area up higher, on a shorter mast. You also get some automatic gust response in that the top battens flex off in a gust, feathering the top, where you want to dump power the most….
I think I didn’t express what I meant very clearly there. 😊 I went and looked up the exact values in ‘Sail Performance’ (Chapter 15, in the subsection ‘Aspect Ratio’, page 255 in the version I have); what I meant is that when you are close-hauled, say at a heading angle of 20°, then a smaller foresail and higher aspect ratio sails are better, but when you bear off to a close reach, say a heading angle of 27°, then putting more of your sail area into the foresail apparently results in better performance, in particular it results in more driving force per heeling moment. On a triangular course the smaller jib and higher aspect ratios (his example: fractional genoa AR 3.2, mainsail AR 4.6) will thus serve you well, but if you want better all around performance then a somewhat larger foresail and slightly lower aspect ratios are better (e.g. much larger (masthead) genoa AR 2.5, smaller mainsail with AR 3.2).
As I recall similar conclusions were referenced elsewhere in the book as well (I didn’t want to spend ages looking for it though), where Marchaj talked about the distribution of sail area for the Soling class (SA is limited by class rules, distribution between mainsail and foresail is left to the designer). The boats with small jibs are the fastest on triangle courses, and the the boats with genoas the slowest, but triangle courses put a real premium on rigs with good windward performance, particularily if the class rules allow spinnakers, so that’s not a good indicator of good all around performance, especially reaching performance is sold short. Most dinghies are designed with triangle courses in mind, so that’s worth noting when thinking about what sort of a rig to put on a boat. Alternatively, if you can allow for both smaller and larger jibs (and perhaps reef the main accordingly), then you mgith be able to have both, so long as you can get the boat to balance.
And I agree with you fully where the trim of the jib for reaching or off-wind courses is concerned, on most dinghes I’ve sailed the shape of the jib is just a nightmare when on a beam reach. While not allowed by most class rules, a pole (sort of like a spinnaker pole but with a downhaul) to hold the clew of the jib (or simply having a wide enough deck) can do wonders for reaching performance though, because then you actually can trim the jib properly. 😊
Cheers,
Marco
On a side note, I really like the Wharram ‘wingsail’ too. IMO it really isn’t a wingsail at all, just a high aspect ratio gaff rig with a very large mast pocket, but no matter what you want to call it, a round mast and a big mast pocket is a really simple way to reduce flow sepation behind the mast on all points of sail.
The tradeoff of course is that getting the sail on or off the mast becomes a bit of a pain (though in most cases, why would you really need to?), and performance-wise you more or less give up the option of using full-length battens, have a bit of extra drag when reefed, and you probably don’t have quite as good a section as you could get with a rotating mast. In exchange you save the complexity of having to mount the mast on a bearing, you can just swivel the sail around the mast however you want, and still have a reasonable profile, and you don’t even have to worry about having a mast track. I think that makes it really appealing for proas!
Marco
Rob
“You mentioned tear drop mast sections and fixed booms…..If the teardrop is intended to act as an aerofoil, then to get the most benefit, it should over rotate relative to the boom. Over rotation is generally controlled with a mast spanner either at the mast pivot or just below the boom.”
interesting. So what do you lose if the teardrop simply heads to wind and then you sheet in via the boom? Should the mast point higher or lower than the sail?
Marco
I really like the Wharram ‘wingsail’ too. IMO it really isn’t a wingsail at all, just a high aspect ratio gaff rig with a very large mast pocket, but no matter what you want to call it, a round mast and a big mast pocket is a really simple way to reduce flow sepation behind the mast on all points of sail. The tradeoff of course is that getting the sail on or off the mast becomes a bit of a pain
would a zipper up one side of the aft and of the pocket be ridiculous?
and performance-wise you more or less give up the option of using full-length battens,
yes that is a drawback
In exchange you save the complexity of having to mount the mast on a bearing, you can just swivel the sail around the mast however you want, and still have a reasonable profile, and you don’t even have to worry about having a mast track. I think that makes it really appealing for proas!
right, the mast pivot just seems over complex, though I’m sure I could make one.
I’d like to think about stayed, sans gaff, with some sort of a cutoff peak idea with battens, maybe a triangle of batten that rides up the mast on semicircular cars (half rounds of 4” PVC pipe…) to hold the peak semi rigid. Maybe 3-4 battens fanning out like ladies collapsible fan, making a roughly quarter round peak?
...
A three masted schooner rig on a 34 ft boat is definitely one mast too many, because it gives a rig with a CE even further off centre (aft) than a 2 masted schooner rig
...
Rob
Catching up on some older discussion here…
Rob,
Why would a 3 masted schooner have a further haft CE than a 2 masted?
As long as masts are evenly distributed along the hull and each rig has same sail area; it’s not true.
Let’s consider as an approximation / rule of thumb that for a “regular” marconi rig (mast and main sail, I am not talking about crab claw rigs, and exclude jibs at this point), the CE is about 25% of the sail foot length behind the leading edge (i.e. the mast).
If you have the same sail shape and size on all masts, with the same booms, the OVERALL CE will be 25% of the boom length aft of the midship point. It does not matter if it is a one mast, 2 masts, 3 masts, etc configuration. So if we consider that the total sail area stays the same between the different configurations, the boom length decreases as the number of masts increases, so actually, the CE location marginally get closer to the center of the boat, excluding jibs, once again.
Cheers,
Laurent
Laurent,
The answer to this question is aerodynamic, not geometric per se. In any rig going to windward with multiple inline sails, the following sail will be operating in the backwash (which is downwash in aeronautical terms, turned sideways) of the leading sail. Because the leading sail is “bending the wind,” the backwash will cause the apparent wind angle on the following sail to be closer to the bow of the boat; therefore the sail will need to be sheeted harder to centerline to produce drive.
The sail force resultant angle of the following sail will be more directly to the lee rather than forwards, and the sail will produce comparatively less forwards drive out of a given total force than it would if it was running solo. And, of course, with the resultant more directly to leeward, the sail will be producing relatively more side force.
At the same time, the following sail will be altering the local apparent wind as it approaches the leading edge of the leading sail, due to the build up of a higher pressure region on the following sail’s windward side: the local wind speed is slowed down, but the angle of attack of the leading sail is increased; the sail can be sheeted farther out, the resultant moves forwards, and relative to the centerline of the hull, the leading sail is now producing more forwards power but less side force.
The combination of the two effects is that relative to the hull, the center of forwards power is moved forwards, and the center of sideways pressure is moved aft, because the leading sail is producing more of the power and less of the side force, while the following sail is now producing more side force but less forwards power.
If you add a third sail to the train, this sail must be trimmed even more in parallel with the centerline of the boat, as the angle of the relative wind, being bent by both the leading sails, will be almost parallel with the centerline of the hull. In fact it will probably need to be over-trimed to windward in order to produce any power at all. In this situation the third sail will be producing nearly all side force, or even producing some drive force towards the stern, slowing the boat down.
You can see this in play on any ordinary monohull ketch more weatherly than a Tahiti Ketch; with jib and mainsail flying, it is completely useless to fly the mizzen (the third sail) on windward courses, save perhaps for helm balance. The proa Cheers suffered from this effect; skipper Tom Follett was disappointed in the boat’s ability to windward.
So, the more masts you stick up, the farther aft the center of side force moves. The size of the effect depends to some degree on the distance between the sails. If you can get more than about 2 1/2 to 3 chord lengths between each of the sails the effect is reduced- but it will still be there.
Marchaj has a lengthy discussion of this in Aerohydrodynamics.
So what do you lose if the teardrop simply heads to wind and then you sheet in via the boom? Should the mast point higher or lower than the sail?
The mast should point a little higher, ie the lee side of the teardrop should effectively become an extension of the lee side of the sail. That way you are not only reducing mast drag and turbulence to the lee side of the sail, but increasing effective sail area by roughly half the mast area with a slightly greater forward component to the lift it produces.
And the mast doesn’t necessarily have to be teardrop (which is pretty bad for aft edge compression). Frank Bethwaite used to make Cherub and NS14 wing masts which were a thin elongated D section and I see that that the current favourite NS14 section is a flat or pointed ellipse. My NS14 has the aluminium equivalent of the carbon fibre one below.
Rob
Re 3 masted schooner rigs:
Geometrically, I agree with Laurent….....Aerodynamically I agree with Diazo.
Rob
I see what you mean Diazo.
Cheers,
Laurent
Rob
The mast should point a little higher, ie the lee side of the teardrop should effectively become an extension of the lee side of the sail. That way you are not only reducing mast drag and turbulence to the lee side of the sail, but increasing effective sail area by roughly half the mast area with a slightly greater forward component to the lift it produces.
got it
And the mast doesn’t necessarily have to be teardrop (which is pretty bad for aft edge compression). Frank Bethwaite used to make Cherub and NS14 wing masts which were a thin elongated D section and I see that that the current favourite NS14 section is a flat or pointed ellipse. My NS14 has the aluminium equivalent of the carbon fibre one below.
can you explain edge compression? not sure which edge.
But with any asymmetrical mast on a proa, the rotation must be a full 180 shunt to shunt. Extra mechanics, extra forces, extra controls. This is making the pocket sail round mast look pretty good. I was thinking about the two ‘problems’ removability and battens.
The batten thing I think you can fix with batters fixed to rings which ride up the mast. The joint of batten to ring is a bit tricky but I think a simple hinge would do it. Or a Wharran style lacing, or a bonded flexible gusset - leather or some silicon rubber…And you could accommodate a square top this way without gaff.
For removability - don’t know, nor sure how strong the forces could be. Velcro,or zipper, zipper with a velcro cover, or with occasional tabs with snaps should do it, without messing up the flow too much. ?
Rob, Laurent, Marco, Johannes, thanks for your sophisticated aerodynamic knowledge - its helped me ramp up my understanding of the issues.
Simon
Diazo
fascinating. complex.
Regarding the ‘following sail’. This discussion has been largely about cat schooner rigs, and people have ‘ignored the jib for the moment’. But isn’t a jib in a sloop doing the same thing as the leading mainsail in a schooner? And what of the ‘slot effect’ is that an outmoded concept?
And how does the relationship between the two sails change as the distance apart increases? Is there sense in putting masts as far fore and aft as possible, rather than a boom-length apart?
And what about the biplane rig,which is getting some attention lately. The arguments I’ve heard here and elsewhere are that on a fast craft with high apparent wind, blanketing of the lee sail will not be a problem. How does this jive with the ‘downwash’ argument?
Simon
As usual, combining performance, seaworthiness, ease of handling/reefing while reducing complexity and cost is an ‘insoluble pancake’ (And if you don’t know the origin of that term, I highly recommend ‘the third policeman by flann obrien - one of the funniest books in the english language, imho)
So far, two masts, round but stayed, square topped battened sails with pockets seems to be a good compromise. In my case, deck-sweeping booms on semi circular tracks, yoked together, with some adjustment of yoke length for sheeting in aft sail.
Add small jibs to taste, if weather helm is problem, though I’m hoping that windward mast placement will negate some of this.
and for downwind fun and games, Im going for trade-winds style. Two jibs (drifters) flown from the aft mast, one to port, one to stb. The advantage with the three mast plan was that you wouldn’t have to drop and re-haul these when shunting, just drag the tacks to the new fwd. Oh well, maybe I can fly them from the mid point between the masts from the mid point of a wire yoke from the mastheads…, or some kind of a dual halyard setup - one up each mast. yep, that should work 😊
Simon
can you explain edge compression? not sure which edge.
If you look at most modern mast sections today, they are usually some form of oval or ellipse with the aft edge flattened to get more material around the mast track, which is the area of maximum compression fore and aft because masts are usually less supported fore and aft than they are sideways.
Rob
This discussion has been largely about cat schooner rigs, and people have ‘ignored the jib for the moment’. But isn’t a jib in a sloop doing the same thing as the leading mainsail in a schooner? And what of the ‘slot effect’ is that an outmoded concept?
And how does the relationship between the two sails change as the distance apart increases? Is there sense in putting masts as far fore and aft as possible, rather than a boom-length apart?
And what about the biplane rig,which is getting some attention lately. The arguments I’ve heard here and elsewhere are that on a fast craft with high apparent wind, blanketing of the lee sail will not be a problem. How does this jive with the ‘downwash’ argument?
Simon,
Basically with a sloop rig, the sails are sufficiently close together to behave as one aerofoil, Although there is some loss of overall efficiency, it is not as great as with a schooner set up where the sails are further apart and upwind the front sail modifies and disturbs the air flow to the aft one. Maximising the distance between the masts doesn’t seem to help, because you need more distance than you can get even by putting masts on the bow(s). and there is a price to pay in terms of pitching in a seaway with the extra weight in the ends of a boat.
If it wasn’t for the risk of being caught aback, you could go the other way, ie, bring the masts closer together so that the two sails overlap sufficiently so that the two sails behave as one aerofoil again.
Often what also seems to get forgotten with a sloop rig when compared to a mainsail(s) only rig is that you can have nearly double the sail area for pretty much the same amount of mast drag and weight aloft and nearly half of that sail area doesn’t have mast turbulence in front of it.
I am a fan of biplane rigs particularly on proas for all sorts of reasons, but my theory regarding blanketing is that it only happens around 80-100 degrees apparent wind, when any boat is going pretty quick anyhow. But hard on the wind the sails are completely separate but you have a windward / leeward (call it a lee-bow) situation and especially if the windward sail is smaller than the leeward one, there may well be an improvement in efficiency of the leeward one with less detriment to the windward one. And deep downwind, there is no blanketing at all, so an overall gain in efficiency….....Only one way to find out.
Rob