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post #1746 of Old 10-19-2011 Thread Starter
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Yes David, I am sure I had posted something about it but cannot find it so I will try to make a new post about the basic stuff.

first look at this picture:

The graphic represents the increased force needed for two 10m heavy displacement long keel boat due to drag while the speed increases. One boat has a prismatic coefficient of 0.53 and the fuller one (but with the same beam one) 0.62.

We can see that between 5 and 7.3K speed the one with thinner entries need considerably less force. We can see also that over 7.3 the fatter boat has a very slight advantage over that speed. We can see also that for taking advantage of that benefice we will need about 3 times more force than the one that needed to sail the boat at 6K, an huge force that can only be generated downwind with big winds and huge sails with enormous stress to the rig and boat.

A negligible advantage considering that the advantage of the thinner boat with less force (less wind and sail) will be much more exploitable.

I don't have a graphic with the same boat, same hull and half the weight but what you would give a much fuller curve that would show that the boat would need less force for getting the same speed and that's because with half weight the boat would have a lot less wet surface and therefore less drag. Things would improve, in what regards the needed force, if we take out the full keel and put a narrow fin keel and spade ruder: less wet surface again, so less drag.

On a light boat the end of the curve would not close so sharply meaning that the boat given adequate power could go much more easily over hull speed and on that particular point a more fuller shape can have advantage because the power needed to put the boat over hull speed is not unattainable and the needed force will be less with a fuller shape. Off course a fuller shape will gain there but will lose on the force needed at smaller speeds, or against waves were the drag would increase more than with a thinner shape.

So the factors are less wet surface, less prismatic coefficient and more power. Regarding power it is convenient to remember that a more fuller shape will also provide more form stability increasing drag but also the power needed to overcome it. That makes for instance an Open 60 or a 40class boat able to get a good performance against the wind and waves but that at the cost of more needed power (more sail area) and more pounding.

So let's have a look at the boats that you were talking about, the Salona 38 (the Salona 37 has the same hull) and the Dufour 375:

The Salona has a slightly bigger waterline: 10.15 m to 9.89, less 660kg of weight (6300 to 6960), less 23cm of beam (362 to 385) and a considerable better prismatic coefficient not only because it has less beam for the same lenght but also because it has thinner entries.

The Salona also has considerable more ballast (420kg more) for a similar keel with the same draft (2270 kg to 1850 kg - 1.95m draft) and an even bigger difference in ballast/displacement ratio : 0.360 to 0.266.

The bigger form stability of the Dufour is not able to compensate the difference in the righting moment given by the much superior D/B ratio of the Salona and so this one has the power to carry much more sail, more 26 m2 (88.4 to 67.0).

The Salona will have less drag due to a better prismatic coefficient and to a less wet surface (lower weight) it is more stiff (more righting moment) and has much more power (more sail area/righting moment).

That curve between the Salona and the Dufour will show that the Salona will need noticeably less power to a given speed and has the Salona has more power available that will make it a much faster boat even if near hull speed the differences would not be so noticeable.

The bigger difference will be with lighter winds (less than 12K) were the Salona will be considerably faster, especially in very weak winds (less than 9K). And also in high winds, downwind were the superior beam of the Dufour will not be able to compensate the much lesser weight and the ability for the Salona to carry more sail. For the recorded experience (mine and from others) with this kind of boats, downwind with a lot of wind it will be difficult for the Dufour to go over 10k while for the Salona that difficulty will happen at about 12K, and I am talking in sailing in about 30/35K wind with waves that would help the boats to surf. Boats like the Salona can go under Spinnaker with an experienced crew to speeds of 16K and that is just not possible with the Dufour.

The Dufour will also lose clearly in the superior capacity that the Salona will have to accelerate in all puffs of air (less weight, more sail) on the capacity to close downwind and also in the capacity to maintain speed upwind against waves. Here the lesser prismatic coefficient of the Salona will permit him to have less wave drag while its superior righting moment (power) will make it able to sustain speed while the Dufour will stale.

The Dufour will only have an advantage and that is that in medium winds probably the boat will sail with less heel than the Salona, due to its major component in form stability.

In what regards reserve stability and seaworthiness, the Salona will have a much better righting moment at 90, a better AVS and a smaller inverted stability.

David, I have taken so much time answering you because your's was a good question and one that will explain the difference in two different boat concepts and not necessarily between a Salona and a Dufour 375. If we compare a Benetau 37 or a Bavaria 38 with a Xp 38 or a J 125 the results are not going to be much different.

Finally as last comet about those differences, something that you have already discovered on the Dufour 34e (even in its soft version) in comparison with the Jeanneau 36i: The performance boat is much more nervous and agreeable (fun) to steer.

A final warning: That huge difference in sail (21.4m2) between the Salona and Dufour is not only due to the superior righting moment of the Salona but because the Salona has more sail for a given righting moment (like all really performance boats towards cruisers). That will make it probably a boat that will need to reef earlier (or at least at the same time) and a more nervous boat that will demand a more experienced sailor (not necessarily a more dangerous boat, quite the opposite but one that is more difficult to sail).

That's why most performance cruisers have a softer version, with a smaller mast and less sails for the ones that want to have the power and the safety without having a nervous and more demanding boat. However much of these detuned and less expensive versions also have a lesser righting moment because the hull is made out of a different material (epoxy versus inferior and heavier resins) and therefore more heavier or because to a smaller draft does not correspond the needed increment in weight to maintain the same righting curve. Pay attention to that.

Regarding to the Salona that I am trying to obtain I will want it with more 250kg on the ballast (that is the same 2270kg but on a 2.25keel), just to carry all that sail without the need to reef earlier, to have a more powerful and less nervous boat with heavier weather and also one with a better reserve stability. The better of two worlds



Last edited by PCP; 10-19-2011 at 09:19 AM.
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