Yankee staysail combos and the slot effect.

[GILow]

So, I’ve heard of the slot effect from multiple sails and I believe it is relevant to cutter rigged boats in particular.

Certainly when I switched my Swanson 42 from a 100% high cut reacher to a 70% yankee and staysail combo the boat became a lot nicer to sail. Faster with less heel.

And of course I ended up with two smaller and more easily managed sails which suited a solo cruiser.

Now I’m going through the whole sail choice debate with the new boat, a Kelly Peterson 44.

The boat was designed with a great big 125% genoa and an oddly high cut staysail, if the drawings on sailboatdata are to be believed.

By now everyone has come up with their own preferred sail plan, and most owners seem to have changed things. For a start, the big genoa tends to snag on the spreaders plus the boats are known to suffer from weather helm. But very few owners seem to use their staysail, citing poor sail track placement among many things. (Easily fixed I would think).

I would like to go with the staysail/yankee combo, but I’m not sure how big to cut the yankee. At what point will I start masking the staysail too much and, more importantly, at what point do I lose the slot effect?

I feel like there must be some formulas involving the J measurement, the beam at various points and the distance the staysail is set back from the bow.

[GordMay]

Here is an excerpt from Paul Bogataj's (Aeronautical Engineer) excellent paper, "How Sails Work" [1], that speaks to this interaction of headsail and mainsail, and then relates it to an actual sailing phenomenon (rather than just theory):

Originally Posted by P.Bogataj

” ... Sails in Combination.

Each sail by itself is much simpler than the combination of a foresail and mainsail as in the sloop rig. The sails are operating so close to each other that they both have significant interaction with the other. The most interesting feature of this is that the two sails together produce more force to pull the boat than the sum of their forces if they were each alone. Earlier, upwash was identified as the increase in flow angle immediately upstream of a wing. There is also a corresponding change in angle, called downwash, just behind a wing, where the flow leaving the wing has been turned to an angle lower than the original flow.

This is the cause of the well known “bad-air” that a boat just to windward and behind another boat experiences. The mainsail of a sloop rig operates in the downwash of the forward sail, causing the flow angle approaching the mainsail to be significantly reduced from what it would be otherwise. This decreases the amount of force that the mainsail produces. The observed affect commonly referred to as “backwinding” is partially a result of downwash from the foresail, but is also due to the higher pressure on the windward side of the genoa being very close to the forward, leeward side of the mainsail, causing the flexible material of the mainsail to move away from that higher pressure.

The foresail of a sloop rig operates in the upwash of the mainsail. The wind as far upstream as the luff of a genoa is influenced by the upwash created by the mainsail. Hence, a jib or genoa in front of a mainsail has a higher flow angle than it otherwise would have by itself, causing an increase in the amount of force that the forward sail produces. So, while the mainsail is experiencing detrimental interference from the foresail, the foresail benefits from the interference of the mainsail. Notice that more air is directed around the curved leeward side of the foresail. This causes higher velocity (lower pressure) and more force. The net result is that the total force of the two-sail system is increased, with the foresail gaining more than the mainsail loses.

There is a converse affect to a windward boat receiving “bad air” (downwash) from a boat ahead and to leeward. A leeward boat gains additional upwash (“good-air”?) from a boat just to windward and slightly behind that acts like a lifting windshift until it moves ahead of the windward boat. This is the same phenomenon from which a foresail of a sloop rig benefits.

Another consequence of the difference in flow angles that the two sails experience in each others’ presence is that the mainsail must be trimmed to a much closer angle with the boat’s centerline than the foresail, which is able to be trimmed to a lead position well outboard. This angle represents the difference in upwash on the foresail and downwash on the mainsail due to each other. ...”

[1]“HOW SAILS WORK” ~ by Paul Bogataj, for North Sails
Understanding How Sails Are Designed To Create Power
➥ https://www.northsails.com/sailing/e...ils-blue-paper

➥ https://www.docslides.com/marina-yar...ails-are-wings

[GILow]

Originally Posted by GordMay

Here is an excerpt from Paul Bogataj's (Aeronautical Engineer) excellent paper, "How Sails Work" [1], that speaks to this interaction of headsail and mainsail, and then relates it to an actual sailing phenomenon (rather than just theory):



[1]“HOW SAILS WORK” ~ by Paul Bogataj, for North Sails
Understanding How Sails Are Designed To Create Power
➥ https://www.northsails.com/sailing/e...ils-blue-paper

➥ https://www.docslides.com/marina-yar...ails-are-wings

Interesting, but the extract appears to be about the interaction between main and foresail. Not sure if that can be extrapolated to yankee and staysail.

[hpeer]

I had sort if similar issues. Heavy cutter with foresail on a sprit and a staysail on a boom.

The sail which came with the boat, purchased by the PO, was too big and overlapped the center of lateral resistance.

I found the designers intended sail plan and the proper #1 which is about 70% to 80%. It is making a nice difference in weather helm. As you report the boat sails flatter and faster.

My staysail is a good size hank on, mounted to a boom, with a reef point. It is very useful, the most useful sail on the boat.

I have seen some, but not much “slot effect” advantage. If I can get something more out of the stay sail fine, but I think it useful even without the slot effect.

But I tend to that each boat, and Master, is sufficiently different that is is hard to say with confidence.

In my experience trying to suss out my weatherhelm problem I came across very little that was helpful in a quantitative way. And I received much poor advice. A fellow who is not a naval architect but close (maybe better) got me to take a bunch of measurements of the boat and worked out some basic numbers. This revealed part of the weatherhelm issue.

Also in this process I talked to a number of sailmakers who provided a vast array of contradictory and poor information. Quite frustrating.

If there is something that provides the kind of data you are seeking then please let me know, I have not found it.

My advice would he to seek out other owners and see what their opinions are.

I have over 5’ between my head stay and staysail stay and 3’ from the staysail leech to the main luff. I have no idea what those measures are on hour boat, but they would seem important.

[Don C L]

I believe the KP44 was designed as a sloop with a staysail, as opposed to a cutter, though I do know it is called a cutter in the literature. And again I can’t recall the difference exactly in mast location for each, but it is different. Just adding a staysail to a sloop doesn’t make a cutter. I had grown up with cutters having a yankee and a staysail and sloops having jibs. The first boat I looked at buying was a true cutter. In any case I’d like to check again the KP44 rig, but as I recall many years ago sailing one, going upwind with the staysail didn’t help. My thought was that any benefit of the slot was lost to drag of turbulence. Off the wind was better for having both up. I believe that flying a yankee (high cut clew) on your boat will result in greater weather helm.
What a great boat though! I loved sailing that one on rough blustery days!

[JPA Cate]

Jim and I were just talking about this, and he said that boats designed as sloops generally have the mast about the 3rd station, and cutters, about the 5th, so quite a bit further aft. This allows you to determine whether your new boat was designed to work with a big genoa.

If it was designed as a sloop, making that sail much smaller will really limit the boat's light air performance, which is some of the nicest sailing anyone ever has, if they have the sails to support it and a boat that can manage it.

The staysail would probably be used at 20 kn or more, and would be way more comfortable for going to weather...for instance if it were Qld and SE trades, and you're headed south. A 120 is not a lot of overlap. Make a sail bag for on deck stowage for it and plan to drop it into it and wait for light airs to get it out again. Not being able to have a furler forward on your boat will mean whatever the cut of the headsail, it is going to have to be dropped when it is not helpful to progress. Where you are going towards, you'll likely need the genoa power more than the staysail for heavy airs. And, I'd still recommend an assy for light airs off the wind.

Ann

[GILow]

So, just measured.

Boom is 5 meters long and the J measurement is 5.2 meters.

Does that make it a sloop or a cutter based on mast placement?

Either way, it is known as a Peterson Cutter, even to the point of having a special logo on the mainsail. (See photo attached.)

I’ve also attached a sail-plan devised for a KP44 called Beatrix (seen recently in Cygnet, Tasmania). Their boom is shorter than mine but I think that’s a modification. Many KP44 owners have shortened their boom to reduce weather helm.

[GILow]

Originally Posted by JPA Cate

Jim and I were just talking about this, and he said that boats designed as sloops generally have the mast about the 3rd station, and cutters, about the 5th, so quite a bit further aft. This allows you to determine whether your new boat was designed to work with a big genoa.

If it was designed as a sloop, making that sail much smaller will really limit the boat's light air performance, which is some of the nicest sailing anyone ever has, if they have the sails to support it and a boat that can manage it.

The staysail would probably be used at 20 kn or more, and would be way more comfortable for going to weather...for instance if it were Qld and SE trades, and you're headed south. A 120 is not a lot of overlap. Make a sail bag for on deck stowage for it and plan to drop it into it and wait for light airs to get it out again. Not being able to have a furler forward on your boat will mean whatever the cut of the headsail, it is going to have to be dropped when it is not helpful to progress. Where you are going towards, you'll likely need the genoa power more than the staysail for heavy airs. And, I'd still recommend an assy for light airs off the wind.

Ann

See post above, but not sure why you think I can’t have a furler?

[JPA Cate]

I don't think you can have a decent furler if you plan to drop the mast in its tabernacle. A foil type furler wont tolerate that much bending, so you're looking at a more "stubborn" furler. Even an assymetrical spinnaker sail that furls on its furler makes a huge snake on the deck. With dacron fabric for the headsail, that is going to be a very large diameter snake, with no UV protection, or a bigger one, with.

Looks from the drawing like that is closer to station 5 than 3, so we can assume it is planned as a cutter.

Ann

[GordMay]

Originally Posted by Don C L

I believe the KP44 was designed as a sloop with a staysail, as opposed to a cutter, though I do know it is called a cutter in the literature. And again I can’t recall the difference exactly in mast location for each, but it is different. Just adding a staysail to a sloop doesn’t make a cutter ...

Don't they call that a "Slutter"?

[GILow]

Originally Posted by JPA Cate

I don't think you can have a decent furler if you plan to drop the mast in its tabernacle. A foil type furler wont tolerate that much bending, so you're looking at a more "stubborn" furler. Even an assymetrical spinnaker sail that furls on its furler makes a huge snake on the deck. With dacron fabric for the headsail, that is going to be a very large diameter snake, with no UV protection, or a bigger one, with.

Looks from the drawing like that is closer to station 5 than 3, so we can assume it is planned as a cutter.

Ann

I'm going to try to track down the same sort of furler I have on the Swanson. It comes apart and dismantles from the stay with the stay in place.

It has been wonderfully reliable and works well.

[GILow]

Originally Posted by GordMay

Don't they call that a "Slutter"?

I think that's a solent rigged cutter.

[El Pinguino]

Originally Posted by GILow

I think that's a solent rigged cutter.

Abbreviation of sloop and cutter -- slutter

I believe my Vertue was a slutter with inner forestay from upper spreaders to about 12 inches aft of the stem.

Likewise my present boat which has inner forestay maybe two feet from the stem.

[Dockhead]

One data point:

I have a cutter rigged Moody 54 which came with a 120% yankee and self tacking staysail.

I sail in higher latitudes with a lot of strong wind and often days at a time upwind. When I went to have new sails made some years ago, I ordered a 95% blade jib besides a new yankee. I imagined using the blade just for upwind passages.

WELL, the blade worked so well that it's almost all I use when cruising The is noticeably more powerful ONLY sailing off the wind and even then, only in very light wind. I use the yankee only racing in light wind, or very rarely (less than once a year) for some long offwind passage. The blade has noticeably better lift to drag ratio so much less heeling, which means less leeway and much better performance upwind.

If I could do it over again, I would not have ordered the yankee at all. There are just too few situations where its better than the blade. I would have something like a light larger genoa, maybe 140%, or even a code 0, for those days when the blade isnt enough.

And that in fact is how my friend's Discovery 67 is set up - self tacking blade and light weight 140% genoa on a solent rig. I sailed that boat 4000 miles across the Atlantic this year. The sail combo is brilliant - just the right change of gears going from 95%: to 140%. I do miss the staysail, however.

Just one data point; YMMV.