Why a Yawl or Ketch instead of a sloop

Brian,

That thing looks like it could be VERY fast on a reach! I'm guessing it might be a real handful to reef - how would you do it? Did you notice the downhaul lines (are they vangs?) on the poles that go to the after top corner of the sail? I'm wondering if you keep those tight? Ease them as you bear off? It would be fun tuning this thing.

BTW - as the square top mains get bigger and bigger and the top batten (usually set at about a 45 degree angle) gets longer and stronger, do you suppose they'll start calling them "gaffs" eventually?? B-))

I heard from one of the crew that Speedboat had two main halyards to get the mainsail up. Effectively, they had a throat halyard and a peak halyard, it wouldn't go up with just one. Pretty funny if we're back to gaff headed mainsails.

Beau

 

Well I dont think you should extrapolate that to a modern ketch.

To windward every sail aft needs to be trimmed in a little more to get a decent sail shape, when the mizzen is close to the main it's severely compromised and needs to be closer to the centre line than you want. This adds to both heel and then to heel induced weather helm with all its attendant vices.

Getting a good slot between the leech of the main and the luff of the mizzen makes quite a difference to the sheeting angle (look back at the maxi ketches pic going to windward) . Alternatively re-try your measurement with a reef or two in the main just to illustrate whats going on.

This is another reason that ketches don't work well on smaller boats...there's just insufficient room to properly implement the rig for windward work.
Schooners are even worse since the biggest sail is the most affected.

 

Mike,

I've tried it on every ketch and yawl I've sailed on in the last few years using a Velocitek. This includes some really high performance ketches - maxi stuff. Every time, when hard on the wind, the boat goes slower with the mizzen up. It's MUCH worse with a Yawl than a ketch, but that's probably because the yawls aren't modern rigs. With the maxi ketch the best set up was to actually reef the main as you say. But, it was still slower than without the mizzen.

Having said all that the old Fischer & Paykel was still faster when hard on the wind without the mizzen set, regardless of the trim of the mizzen. You could get the boat going faster by bearing off, then the mizzen would work. But the VMG to windward dropped steadily. Some of the old Kiwi crew told me that was what they'd found with her in the round the world race, and were thankful that they almost never went up wind.

BV

 

Which I understood to be the reason for the rig choice, horse's for courses etc They wanted a good reaching boat... or is my memory failing me..... again!

 

Nope, your memory is just fine. Even on the most recent Volvo race JuanK said that if he designed a boat that could do the upwind leg to China (during which most of the boats fell apart) they would be too heavy to win on the runs and reaches. So Volvo/Whitbread boats are clearly optimized for running and reaching. Note: they are all sloops.

But, the reason that the Whitbread ketches existed was because of a giant hole in the rating rule. that hole was closed and since you've seen only sloops. The ketches did NOT exist because they were better or faster. They existed because the designers saw a wonderful hole in the handicap rule that allowed them to built a boat that was 25 to 30 percent bigger than a sloop and rated the same.

Even on a reach, when people don't force artificial rating artifacts on the design, which is why we have the lovely Concordia Bermuda Yawls of the CCA and the Whitbread ketches, you end up with sloops for performance.

Having said all that, we weren't actually discussing racing boats. We were talking about a guy who wanted a cruising boat. Thus, my earlier post about taking the mizzen down so you could get good upwind performance and then putting it up again when you wanted all that yawl stuff to happen.

 

Huh? I always thought that a ketch was a more powerful rig on a reach... there you go.

 

Zed,

Power is a funny measure. A ketch that has the same sail area as a sloop will have a lower center of effort, meaning it will cause the boat to heel less for a given sail area. But, because the mizzen is operating in the backwash of the main, even on a reach, it will be a little less effective. Sort of like having a competitor sailing right in front of you if you were a sloop, you go slower.

What makes this really complex is that a sloop has a much longer leading edge to its sails. That long leading edge is being acted upon by wind that is completely undisturbed, and therefore cares more energy or power. So the taller mast of a sloop with the same sail area as a ketch will give it a higher level of efficiency.

In situations where both boats start to carry a spinnaker, the sloops spinnaker is far more effective because it's larger than the ketch and because it is out in front and away from any other sail (unlike the mizzen staysail or spinnaker of a ketch). Modern sloops carry a spinnaker any time the apparent wind is 80 degrees or more, so that means on most reaches the sloop has the ability to set a lot more sail area into undisturbed wind.

In addition, the wind speed goes up the farther you get from the water. There is quite a dramatic increase in wind speed as you rise from the sea surface to ten or twenty feet above the surface and it continues to increase for quite a distance. This effect is even more pronounced as the waves become larger because there is almost no wind in the trough in the lee of a big sea and a lot of wind at the wave tops. The taller rig of the sloop will reach up into the stronger winds above the waves and gather more energy than the ketch, even though the sloop and ketch have the same sail area. So, in large seas the sloop will be more "powerful".

So, there isn't a really simple answer. The ketch is more "powerful" in relatively flat water where its sails aren't in the lee of the waves and where the stability of the boat is an issue. The sloop will be more "powerful" in large seas and in situations where the boat is not over-powered. This, of course, is what makes yacht design so much fun. There is not an obvious "right" answer all the time, it is entirely dependent upon the wind speed, sea state, and apparent wind angle.

We haven't even discussed one of the stronger arguments for a split rig, and that's if the masts are separately stayed, you might be able to loose one and still be able to sail home. But, that's another can of worms to open.

B

 

Beau,

Given the above why do you place the ketch as more powerful in flat water? Are you not stating the efficiency of the single rig will win the day in all cases? Is this if the additional mizzen staysail is used? Or is the single attribute of lower center of effort sufficient to win the day in flat water?

How similar is this point to the differences between a high and low aspect main on a sloop?

Thanks

 

"I always thought that a ketch was a more powerful rig on a reach.."

It is , when the wind is free enough to set the Mizzen Staysail, frequently as large as the main.

Hard on the wind , unless there is HUGE space between masts , the mizzen on a ketch or yawl is to lean against when using the sextant.

FF

 

It's a bit anecdotal without detail isn't it.
I'd really like to see the polars wind speeds sea conditions and just what angles you were sailing what your sail trim was like etc relative to each design. This would be an intersting paper but I'm not aware of any corresponding data that supports this view for a well designed ketch rig.

We are probabaly better off considering equal masted schooners for a start and then decreasing the mizzen size incrementally and looking at the lift drag relationships of the mizzen.

 

What I was trying to say, and perhaps not well, is that for a given fixed sail area (where both the sloop and ketch have the same sail area), and for the same hull shape, and when off the wind (reaching), then the ketch will be more powerful because the center of effort is lower for about the same driving force. This will cause the ketch (or schooner) to heal less and boats are generally faster when standing upright. I did not add the variable of increasing the sail area of either the sloop or ketch, in this section of my post. I was trying to simply point out that when the ketch is not disadvantaged by having its rig in the wind shadow of the waves, its lower center of effort will allow it to be faster - all other things being equal.

Now, if one adds the mizzen staysail or spinnaker to the ketch - as well as whatever large headsail or spinnaker one could set on the ketch's mainmast; then, one should add the spinnaker or A0 or whatever large sail the sloop would carry. Again, if we hold sail area constant, the ketch will have a lower center of effort. The problem when one does set mizzen staysails is that they usually end up in the backwash of, or creating a wind shadow for, the mainsail. It's almost impossible (except on ketches like NZ Endeavor or Fischer & Paykel) to set a mizzen staysail without it being too close to the main.

Regarding your question on lower aspect sails on sloops, if you remove the various rating rules that have altered sail plans dramatically, I would guess that the most effective sail plan would have a pretty high aspect ration. But, that one might want to lower the aspect ratio to avoid the polar moment of inertia becoming too high from the weight aloft and the spinnakers getting too large. (In flat water the Polar Moment wouldn't be a factor.) I honestly don't know if the better efficiency of high aspect ratio sails wins out over the lower center of effort of a lower aspect ratio sail plan. Its an interesting question.

 

Mike,

I sure wish I had a way to test this stuff. I'm just a sailor who has dragged his instruments along on boat after boat measuring things to get the boat going faster. My old skipper used to tease me about being the only bowman who brings his own Velocitek to second guess the navigator.

To give you a little more data, which is obviously still anecdotal, the numbers I have are from tuning up and racing boats hard on the wind in various sea states. Usually, these boats sail between 32 and 37 degrees apparent wind angles and with wind velocities from mid-single digits to the low 30s. Due to some of the particularly terrible characteristics of some of the boats, the mizzen had to be taken down in some of the boats simply because as the wind speed increased there was so much weather helm the boat wasn't going anywhere due to rudder drag. This is not the fault of the rig, it's the fault of the designer.

I do wish I were a naval architect and had the resources to model these trade offs as you describe. I do know that a lot of folks have been down this path and that there must be a reason that there are no serious racing boats that are split rig unless you are up against a serious flaw in the rating rule or against some maximum size limit that makes a mast simply too big to use.

 

Single-masted Ketch

At which point in these conversations I might make reference to this other subject thread where I re-addressed my original reasoning to develop a 'single-masted ketch' idea for a cruising vessel.
Aft-mast Origination & Justification

...and please, lets not re-argue the backstay forces and compression loads involved...I'll concede that critisium....but just look at the possibilities if it were workable

...lots of 'ketch' references here also

 

You can get quite a good idea of what is going on by playing around with JavaFoil.

The attached images consider thin foils in a left to right air stream.

Both foils have 10%. The single foil in the image is set to give maximum lift.

When the second foil is introduced behind and below the first it has quite an impact on the first it due to the upwash. The foresail is no longer at the maximum lift condition.

To get back to maximum lift the the boat needs to be sailed higher. The lift increases quite a lot but the drag also goes up.

Adding a third sail does the same thing. You can point higher and get more lift but the drag goes up even more. The lift to drag deteriorates.

The respective values are:
Single foil Cl = 0.973, Cd = 0.0779, L/D = 12.4
Twin foil Cl = 1.798, Cd = 0.2793, L/D = 6.4
Triple foil Cl = 2.897, Cd = 0.7167, L/D = 4

All these foils have an AR of 4 and are of identical size.

To get the full picture you would need to do the wind vectors in relation to the hull.

My feeling is that the third sail will be close to self defeating because the gain in pointing ability will be lost in extra drag. Sure the lift component goes up but it will be more than offset by the extra drag on the sails.

When freed up the extra lift will not be countered so much by the sail drag. Here the drag is acting more as a heeling force than countering the drive.

Rick W

 

Rick, this is wonderful! Thanks!! Could you point me to JavaFlow, I'll google it. It sounds like a great tool. The first thing I'm going to try is moving the "mizzen" a lot further aft so there isn't an overlap between the "main" and the "mizzen".

B