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post #4391 of Old 05-23-2013 Thread Starter
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Polar speeds: Elan 400 - Pogo 12.50

Originally Posted by opc11 View Post

I was wondering how one uses a polar chart to determine planning speed? If it's too much of a hassel, no worries. I'm just not very familiar with polar charts. it seems like a very interesting way to understand performance. In general, how accurate are these charts compared to actual performance?

The accuracy of a Polar speed chart depends on the program that is used by determine it and on the provided information. The ones made by main designers are very accurate (some few tens of a knot difference at most). They were made using very expensive and sophisticated VPP programs that were used to design and perfect the boat. They have as reference all data regarding the boat and can calculate, based on fetailed and complex CFD criteria, the performance in each wind condition and point of sail.

You can buy on the net a 50 bucks VPP, a very simple program that will accept just some reduced boat data (anyway you don't have acees to more) and has as base, not a very complex CFD core, but just some simple mathematical relations. You cannot expect to have very reliable information from there. The difference of efficiency it will be like the one between a hand calculator and a very powerful and sophisticated computer. In fact most complex CFD programs need not a powerful PC to be run but really a big industrial computer.

Regarding Polar they are not very different than the speed predictions showed on the ORC files, not in a Polar way. ORC predictions are also very reliable.

The predictions are only valid to a given boat in a given configuration, I mean keel and sails used and regards ideal conditions flat sea, high performance sails and a perfect trim with the boats in minimum sailing condition.

If sea conditions and waves has not a great importance in what regards downwind sailing (the waves can even help in what regards speed) that is not the same in what regards upwind sailing where the waves will slow the boat down and more a beamier one than a narrow one.

Regarding Polars that is just a way of showing that data in a graphic way. When it is not said otherwise all wind and angle speeds are true and as most of us just don't sail with true wind speed or wind angle that can be confusing.

When a Polar has two side (a full circle) many times it has true wind and angles on the right side and apparent winds and angles on the left side.

Around the outside edge of a Polar is normally the place were the wind angles are indicated (on top of rays), the boat speeds are marked by irregular lines inside the polar. Each of those irregular lines is correspondent to a given wind speed. The correspondent wind speed is marked on the irregular line (small numbers) or it is given by a color code. The boat speed will be given by the intersection of those irregular lines with the several circular lines inside the polar, or its relative position regarding them. Each of those circular lines correspond to a speed that is normally marked on the lateral edge of the polar in front of each semi-circle.

Now it is simple, as we have seen the downwind speed is more independent of sea conditions than the upwind speed, so the data is pretty reliable on normal conditions for those wind speeds. We are talking about 40ft boats that have a hull displacement speed of about 8.5K. Normally speeds immediately over 8.5K are not called planning speeds but semi-planing speeds. What you could call planning speed has not a uniform definition but I would say that regarding this size of boat we would be talking of speeds over 11K, not surfing waves, but a constant speed.

Now have a look at the Polar of the Elan 400 and compare it with the one of a Pogo 12.50 (that notwithstanding the name is also a 40ft):

Now, we can see that the speeds upwind are very similar, even if I believe that with waves, dead upwind the Elan will be slightly faster, downwind we will have a boat that is a semi-planing boat (Elan) against a planing boat (Pogo) and the difference in speeds after 75 starts to build up and at 135/140, the best Pogo wind angle, they are just huge.

Even considering the best downwind speed angle of Elan (150) the differences are very big:

At 100 with 16K wind the Elan will be making 8.5K, the Pogo 10.5K with 20kwind the Elan 9K and the Pogo 11.5K, with 25K the Elan 9.3, the Pogo 13K.

At 150 with 16k wind the Elan will be making 8.7K, the Pogo 10.5, with 20k wind the Elan 9K, the Pogo 11.5K, with 25K the Elan 11K, the Pogo 15.5K.

Consider that 25K is not a lot of wind to go downwind. At 25k the apparent wind on the Pogo it will be something like 13K.

These are speeds with unloaded boats and with a cruising load, even if the type of cruisers that use these boats travel light, the differences to the polar speeds will be bigger and differences of 1 or even 1,5K in what regards planning or semi planning speeds are to be expected but even so that makes the Pogo a boat capable of planning speeds while cruising (as our friend Eric had verified) while the Elan will be pretty much limited to semi planing speeds, unless on rare occasions where the conditions allow it to sail downwind with 30 or 35K.

Then the Elan speeds downwind at 150 will be 13K and 15K respectively. Even with a light cruising load the Elan will go to planing speeds with that kind of wind, but it will be just in much less occasions than on a Pogo 12.50 that needs a lot less wind to do that.

That is what I wanted to say on the post regarding the Elan 400. Note than in that the Elan is not worse and probably is better than other performance boats like the Salona 41, the Dehler 41 or the First 40, but to compare with a Pogo in what regards cruising boats and speed downwind we would need a boat like the JPK 38.

The real limitation of the Elan 400 is weight. Have that boat with less 1,5T and with a more "tendue" hull (that the difference of weight would make possible) and we will have probably a boat to match the Pogo downwind and to beat it upwind. That weight is not impossible or hard, just a lot more expensive



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Last edited by PCP; 05-23-2013 at 08:30 AM.
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