comparison polarity charts twin keel vs monokeel

[georgeo]

fellow forumites, this old chestnut again! As you will be aware there has been considerable discussion as to the various merits of these two keel forms and the differnces on and off the wind.
Well, 'les Francais' have done their bit to try and put some science into this.
The new RM1060 looks a seriously 'sexy' (IMHO) bit of kit and is available in both monoquille and biquille versions.
A serious comparison between the two has now been published on the AVRM website, and attached below is the chart of the comparative advantage of the mono versuon over the twin keel version
Figures down side are wind angle, figures along top are wind speed (as I understand it). (apologies in advance if chart does not come out quite as clear as on the original, reference here http://rm-asso.org/fichiers/PolairesRM1060comparatifBiquilleMonoquille.xls)

4 6 8 10 12 14 16 20 25
36 9% 8% 7% 6% 4% 4% 3% 3% 4%
40 8% 6% 5% 4% 3% 2% 2% 2% 3%
45 6% 5% 3% 2% 2% 2% 2% 2% 2%
52 5% 4% 2% 2% 1% 1% 1% 1% 1%
60 5% 3% 2% 1% 1% 1% 1% 1% 1%
70 4% 3% 1% 1% 1% 1% 1% 1% 1%
80 4% 2% 1% 1% 1% 1% 1% 1% 1%
90 3% 2% 1% 1% 1% 1% 1% 1% 1%
100 3% 2% 1% 1% 1% 1% 1% 1% 2%
110 3% 2% 1% 1% 1% 1% 1% 1% 2%
120 3% 2% 1% 1% 1% 1% 1% 1% 1%
135 3% 2% 2% 1% 1% 1% 1% 1% 2%
150 3% 2% 1% 1% 1% 1% 1% 1% 2%
160 2% 2% 1% 1% 1% 1% 1% 1% 1%
170 2% 2% 2% 1% 1% 1% 0% 1% 1%
180 2% 2% 1% 1% 1% 1% 1% 1% 1%

VMG près 7% 6% 6% 5% 5% 4% 4% 4% 4%

VMG portant 3% 2% 2% 1% 1% 1% 0% 1% 1%
Not a lot in it is my interpretation. And superimposing the polar charts they look just about identical

 

[AliM]

Fascinating, thanks.

In May, we did a straight comparison beween our new boat (twin-keeled Sirius 310) and a single-keeled version. Both boats were almost new. The designer of the boats spent the first 3 hours on ours and a few more hours on the single-keeled one. We had identical rigs and sails, although I suspect that ours had a bit more weight in general cruising kit, aboard. At first, the wind was about 5kt, slowly increasing, until we ended up with a super ~15kt beat, with a flat sea (in the Baltic Sea, near Travemunde/Neustadt).

In all points of sail, our twin keeled boat equaled or exceeded the single keel version. She pointed as well, and was at least as fast, usually faster. The designer, by his own admission initially sceptical about twin keels, agreed with that analysis.

 

[bbg]

I don't know what the two of you think you are seeing, but that table indicates how much faster the fin keel version is compared with the twin keel. Up to 9% faster although that quickly drops off to 1%-3% faster when off the wind in stronger breeze.

2% faster = 15 minutes on a 12 hour passage. Not huge, you might say, but 5% faster is 35 minutes, and that is starting to make a difference. I would be delighted if my boat would go 1% faster.

Anyone who is primarily interested in performance will invariably go for a single keel.

 

[johnalison]

If either boat can sail at 36 degrees to the true wind, it is doing a lot better than I can.

I suspect that the differences quoted apply only to this design and can't be generalised. For example, I doubt that the similarity in performance close-hauled at higher wind speeds would be typical of many more traditional designs. This boat I saw recently might be an exception. It has twin steerable props.

 

[flaming]

I don't know what the two of you think you are seeing, but that table indicates how much faster the fin keel version is compared with the twin keel. Up to 9% faster although that quickly drops off to 1%-3% faster when off the wind in stronger breeze.

2% faster = 15 minutes on a 12 hour passage. Not huge, you might say, but 5% faster is 35 minutes, and that is starting to make a difference. I would be delighted if my boat would go 1% faster.

Anyone who is primarily interested in performance will invariably go for a single keel.

9% is massive. And the close hauled figures reveal that it's 3% faster at worst. But mostly at least 4% faster.

To put that in perspective, a 4% difference is the equivalent of an IRC rating difference of 0.040.
Which is roughly the difference between a Dehler 36 and a First 40.7.

The 6% difference to windward in a good sailing breeze (12kts) would be an IRC difference of 0.060 Which would be the difference between an X332 and a brand new X37.

Even if you averaged the upwind and downwind differences (which of course an IRC rating would try to) and said that in 12kts the single keel version is on average 4% faster, you've still just turned an X332 into an Arcona 370.

And the RMs are supposed to be the better twin keelers...

 

[bbg]

Thanks for putting it into perspective better than I did.

I am actually pretty surprised at how much faster the polars indicate the fin keel is upwind. As you say, 9% is massive.

However, I am also impressed by how little difference there is off the wind. I would put this down to the RM's very nice (narrow chord) keels which will mean there is not a huge increase in wetted surface area (compared with other twin keelers).

 

[jwilson]

fellow forumites, this old chestnut again! As you will be aware there has been considerable discussion as to the various merits of these two keel forms and the differnces on and off the wind.
Well, 'les Francais' have done their bit to try and put some science into this.
The new RM1060 looks a seriously 'sexy' (IMHO) bit of kit and is available in both monoquille and biquille versions.
A serious comparison between the two has now been published on the AVRM website, and attached below is the chart of the comparative advantage of the mono versuon over the twin keel version
Figures down side are wind angle, figures along top are wind speed (as I understand it). (apologies in advance if chart does not come out quite as clear as on the original, reference here http://rm-asso.org/fichiers/PolairesRM1060comparatifBiquilleMonoquille.xls)

4 6 8 10 12 14 16 20 25
36 9% 8% 7% 6% 4% 4% 3% 3% 4%
40 8% 6% 5% 4% 3% 2% 2% 2% 3%
45 6% 5% 3% 2% 2% 2% 2% 2% 2%
52 5% 4% 2% 2% 1% 1% 1% 1% 1%
60 5% 3% 2% 1% 1% 1% 1% 1% 1%
70 4% 3% 1% 1% 1% 1% 1% 1% 1%
80 4% 2% 1% 1% 1% 1% 1% 1% 1%
90 3% 2% 1% 1% 1% 1% 1% 1% 1%
100 3% 2% 1% 1% 1% 1% 1% 1% 2%
110 3% 2% 1% 1% 1% 1% 1% 1% 2%
120 3% 2% 1% 1% 1% 1% 1% 1% 1%
135 3% 2% 2% 1% 1% 1% 1% 1% 2%
150 3% 2% 1% 1% 1% 1% 1% 1% 2%
160 2% 2% 1% 1% 1% 1% 1% 1% 1%
170 2% 2% 2% 1% 1% 1% 0% 1% 1%
180 2% 2% 1% 1% 1% 1% 1% 1% 1%

VMG près 7% 6% 6% 5% 5% 4% 4% 4% 4%

VMG portant 3% 2% 2% 1% 1% 1% 0% 1% 1%
Not a lot in it is my interpretation. And superimposing the polar charts they look just about identical

That is on an RM - a twin-keeler with very deep and efficient bulbed keels. 98% of twin-keelers have neither. Nothing wrong with bilge-keeled boats ( I have owned one from choice ), but for most of them performance is substantially less than the fin versions of the same hull.

The person quoting the Sirius TK beating a fin version was in flat water. The rougher it gets the worse most bilge-keelers perform.

 

[rwoofer]

And the RMs are supposed to be the better twin keelers...

And that's exactly what they are - one of the best twin keelers you can buy. I struggle to think of other 35ft cruising designs (and especially ones that can dry out), which is what RMs are, doing the same kind of performance. Nearly 12 knots downwind in 25 knots of breeze. 6 knots upwind in 10 knots of breeze.

I know I've never said that twin keelers are faster than their equivalent fin variants, but what I would maintain is that well designed twin keelers can be faster than fin keelers, because they are better designed. Much of the bad rep of twin keelers has come from their overall bad design, not the twin keels themselves.

 

[rwoofer]

To put that in perspective, a 4% difference is the equivalent of an IRC rating difference of 0.040.
Which is roughly the difference between a Dehler 36 and a First 40.7.

Faulty logic as it will only occur if the race was upwind only. If you assumed round the cans then that difference would probably half.

 

[flaming]

And that's exactly what they are - one of the best twin keelers you can buy. I struggle to think of other 35ft cruising designs (and especially ones that can dry out), which is what RMs are, doing the same kind of performance. Nearly 12 knots downwind in 25 knots of breeze. 6 knots upwind in 10 knots of breeze.

I know I've never said that twin keelers are faster than their equivalent fin variants, but what I would maintain is that well designed twin keelers can be faster than fin keelers, because they are better designed. Much of the bad rep of twin keelers has come from their overall bad design, not the twin keels themselves.

Absolutely. But a well designed boat will be faster with a single keel. And even faster if that single keel is a well designed fin and bulb. And even faster if that fin and bulb can be canted to weather....

There's nothing wrong with twin keels, not everyone has speed as their number one priority. I just saw those numbers as big differences, esp upwind, not small, and was surprised because I actually had higher expectations of RM's keel design than that.

 

[georgeo]

Absolutely. But a well designed boat will be faster with a single keel. And even faster if that single keel is a well designed fin and bulb. And even faster if that fin and bulb can be canted to weather....

There's nothing wrong with twin keels, not everyone has speed as their number one priority. I just saw those numbers as big differences, esp upwind, not small, and was surprised because I actually had higher expectations of RM's keel design than that.

Thankyou Flaming. I find your posts, as ever, informative and instructional. I have looked again at the figures. As you point out, in light airs, and very close hauled, the differences quoted do add up. But suspect, unless racing, if forced to sail on that point, the donkey would be on. For cruisy type stuff though, the differences are minor, and then one has the advantage of being able to dry out etc etc. And the fin keel version of the 1060 is deep, I believe 1.9m, restricting further ones access, particularly in the very tidal waters round here.
Mind you, the new 1060 does look the bees knees and as others have commented, when looking at the polar charts, off the wind both versions go very well. If my lottery boat came in I might find myself making my way back to la Rochelle!

 

[earlybird]

The polar diagrams plot boat speed against wind angle, presumably taking no account of leeway. If the twin keel boat is worse in that respect, which seems likely, then won't its performance on the wind will be further degraded compared with the single keel?

 

[Seajet]

Thanks for putting it into perspective better than I did.

I am actually pretty surprised at how much faster the polars indicate the fin keel is upwind. As you say, 9% is massive.

However, I am also impressed by how little difference there is off the wind. I would put this down to the RM's very nice (narrow chord) keels which will mean there is not a huge increase in wetted surface area (compared with other twin keelers).

One thing; surely narrow chord on a keel primarily reduces frontal area resistance, the reduction in wetted area over a thicker chord would be relatively tiny ?

 

[rwoofer]

The polar diagrams plot boat speed against wind angle, presumably taking no account of leeway. If the twin keel boat is worse in that respect, which seems likely, then won't its performance on the wind will be further degraded compared with the single keel?

Surprisingly the RM twin keel may have an advantage there because the keels have toe-in and the leeward one will be vertical. The downside is extra drag, which obviously would effect the speed. Marc Lombard the designer explains some of the theory somewhere on the net, just can't remember where...

 

[Signed Out]

I wonder how twin shaped lifters (lee boards) á la Red Fox 200/Hunter 20 would fare in comparison. As I can see it, they should be the best, but lose out on potential hindrances inside and less capability of drying. Always one foil (providing windward raised) fairly vertical and faired to provide lift.

 

[Bosun Higgs]

There is nothing new about this. The PY racing handicaps give a fair idea of the difference since they are performance based. For the Fulmar the difference fin to twin is 4.3% average and for the Sadlers 32 its 3%, And both those boats are oldish designs so I would expect the RM to be better.

 

[Seajet]

I wonder how twin shaped lifters (lee boards) á la Red Fox 200/Hunter 20 would fare in comparison. As I can see it, they should be the best, but lose out on potential hindrances inside and less capability of drying. Always one foil (providing windward raised) fairly vertical and faired to provide lift.

Yes but where's the righting moment ? The foils - a PIA to accomodate & operate - may be spiffing for grip on the water & lift but the boat still needs to stand up to the sails; notably racing boats with such foils often have canting keels to complete the recipe.

 

[bbg]

One thing; surely narrow chord on a keel primarily reduces frontal area resistance, the reduction in wetted area over a thicker chord would be relatively tiny ?

Chord is the fore-aft measure

 

[grumpy_o_g]

One thing; surely narrow chord on a keel primarily reduces frontal area resistance, the reduction in wetted area over a thicker chord would be relatively tiny ?

You're sort of right but possibly not for the reason you're thinking. As per bbg the chord is the fore and measurement - narrower chord = high aspect ratio (or a smaller foil of course, if you keep the same length).

The narrow chord will usually result in a less thick foil as too much camber will cause the foil to stall very early as the flow separates behind the thickest point, just like an aircraft stalling in many ways (camber is the shape of the curve, so effectively the thickness vs the chord length). The effect is a big increase in drag and the loss of much of any "lifting" moment into wind. On aircraft the wing can be made incredibly slim and the airflow becomes laminar which effectively reduce the frontal area by far more than any increase due to the increased length. The effect is less in water but it still reduces the frontal on a high-aspect keel of the same "size"

I spent a couple hours chatting with some students from Kiel Uni way back in the 70's or 80's - they'd found out the cheapest beer in Kiel was in the BKYC - I'd often wondered why keels were so basic compared to aerofoil design and they happily explained why it was actually quite complex in return for free beer. They were experimenting with varying the angle of attack on the keel by rotating it as the boat sailed using a test tank model. They reckoned it made quite good gains but you'd have to find a way to vary it constantly as the boat reacted to gusts and luffs, etc.

So yes, a narrow chord will result in a smaller frontal area on a high aspect ratio thin keel with roughly the same wetted area. Basically long thin glider wings are more efficient as lifting surfaces than short stubby Cessna 152 ones.

A secondary element would be that the deeper keel could carry less ballast for the same righting moment so I guess that would reduce the frontal area as well.

 

[AliM]

Seajet "Yes but where's the righting moment ? The foils - a PIA to accomodate & operate - may be spiffing for grip on the water & lift but the boat still needs to stand up to the sails; notably racing boats with such foils often have canting keels to complete the recipe."

For RMs, the keels have heavy (lead?) bulbs so the centre of mass of the whole boat is right down below the hull, between the keels. For a single keel version, with a similar draft and also a bulb keel, the centre of mass is in about the same position. Therefore the righting moment is almost the same. For boats without bulb keels, the ballast is distributed throughout the height of the keel (either one or two) and so the centre of mass is higher, and righting moment is less.

Now cant the keel, and the heavy bulb keel in a long stick is angled to maximise the righting moment - but I don't think many ordinary cruising sailors would go for the complexity of that!