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  #1  
Old 10-20-2009
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Estimating Boat Performance

Hey,

I'm dreaming about my next boat and I have a few questions about estimating performance. What is the best way to estimate how a particular model boat will perform, is it PHRF numbers or numbers from Sailcalc like SA/D, LWL, Hull Speed, Motion Comfort, etc.?

The reason I ask is that for some similar boats, the numbers are kind of crazy. For example, consider the Beneteau 36.7 and the Beneteau 361. The numbers from Sailcalc are very similar

Performance Comparison
LOA Beneteau 361
36.5
Beneteau 36.7
36.08
LWL Beneteau 361
31.1
Beneteau 36.7
30.25
Beam Beneteau 361
12.6
Beneteau 36.7
11.33
Displacement Beneteau 361
12790
Beneteau 36.7
12495
Sail Area Beneteau 361
685
Beneteau 36.7
666
Capsize Ratio Beneteau 361
2.16
Beneteau 36.7
1.95
Hull Speed Beneteau 361
7.47
Beneteau 36.7
7.37
Sail Area to Displacement Beneteau 361
20.04
Beneteau 36.7
19.79
Displacement to LWL Beneteau 361
190
Beneteau 36.7
202
LWL to Beam Beneteau 361
2.47
Beneteau 36.7
2.67
Motion Comfort Beneteau 361
20.51
Beneteau 36.7
23.61
Pounds/Inch Beneteau 361
1400
Beneteau 36.7
1225

It seems like the 361 should be faster. It has a 'better' numbers (LWL, SA-D, etc.) yet the 36.7 rates MUCH faster in PHRF (80 or so to 140 or so). Is it because the PHRF numbers assume spinnaker and sailcalc does not? Is it because the sprit on the 36.7 makes an assym must easier to fly? Do you need 5 beefy guys to make the 36.7 perform? Is it something about the hull / keel of the 36.7 that doesn't show up in sailcalc? Or something else?

Thanks!

Barry
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Old 10-20-2009
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Most 361 have in mast furling for the loose footed main. Not the most performance enhancing feature. Also they have a traditional drive train shaft and prop and that prop is a non folding 3 blade it does cause drag.

The 36.7 Traditional Main and a sail-drive with a 2 blade folding prop. More power from the main and less drag under water.

I hope this helps.
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Old 10-20-2009
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The first 36.7 is a one design race boat with a LOT less stability and a razor thin bulbed keel

There was a big mess within the 36.7 org about one owner who had to have a 1" thick shoe added between the keel and hull to get the stability rating to do the Bermuda race


When we race the J24 we beat the snot out of a lot of much faster boats on the first upwind leg of course they ALWAYS get it back if the next leg is a reaching leg and as always 1,2,3 had and 18 second spread after 9 miles
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Old 10-20-2009
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The extra foot and change of beam on the 361 doesn't help it any either. The bottom line is they're different boats designed with different goals in mind, the 36.7 being more focused on racing/performance.
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Old 10-20-2009
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The PHRF numbers are based on the same standard configuration (#1 genoa, standard size spinnaker, folding prop, etc.) and assume a well-sailed boat with a smooth bottom and good sails. It's probably the best single number to compare relative performance. But remember it's an average of different conditions and different points of sail. Some boats will sail faster than their rating in ligher air, some in heavier air. As tommays noted, some will do better upwind, others better on a reach or downwind.

I think some of the numbers in Sailcalc are suspect. I note that the sail area for the 361 is not the rated which uses a 100% headsail. The SA/D ratio they calculate as 20 is much too high for a boat like the 361. Bottom line is the 36.7 will sail rings around the 361.
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Old 10-22-2009
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Thanks, one more question

Hi Guys,

Thanks for the comments above.

I do have one more question. My boat is an O'day 35 with shoal draft. I believe the base PHRF rating for that boat is around 156. Mine should be a little slower because I have a fixed 3 blade prop, and a few other things. So if i want me next boat to have significantly better sailing performance, I should look for a boat with a better PHRF number. But, in order for a boat to sail to it's rating (or close to it) do you need a full crew? Would a boat like a Bene 36.7, when singlehanded, be significantly slower because you don't get the benefits of crew on the rail, ability to carry a large chute, etc.?

Please note that I am only using the 36.7 as an example, because the boat is well known, the ratings are easy to find, etc.

Thanks,
Barry
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Old 10-22-2009
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Barry,

I like the way you're going about your search -- gathering data, comparing values etc. I also agree that the PHRF numbers are useful for getting an idea of the speed potential of various boat designs.

But I also feel it's possible to put too much emphasis on those PHRF figures. You already know that they assume certain factors that may or may not be present on any given boat -- folding/feathering prop, max allowable-sized genoa (plus a full complement of other headsails), very rarely equipped with roller furling, max allowable spin pole, etc. Other factors that affect ratings are how competitive the fleet is. In competitive fleets racers are more likely to renew sails annually, purchasing racing cut sails from more expensive materials, fair the boat's bottom with racing paint, keep it scrubbed, race with seasoned crew, etc.

Another factor is that the boats that are being used for racing are generally never loaded like a boat used for family cruising. In fact, they are frequently stripped of every non-essential piece of gear that adds weight.

So while some of the figures you see for certain boats may appear attractive, those figures in no way reflect the real world effect on performance of sailing those boats with a short-handed crew, using a single roller-reefing headsail, an asymmetric cruising chute, a ten year old full batten dacron mainsail, loaded up with provisions and family cruising gear, sitting two inches lower on the waterline, towing a dinghy, etc.

How will you be using your new boat?
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Old 10-22-2009
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PHRF numbers assume a full crew in the top 5% of sailors (or something like that) along with top equipment. Very few boats are raced to the their rating. If they were, the races would be all photo finishes. The absolute number is probably not that important to you if you are just comparing boats on performance for non-racing situations. The relative numbers will be pretty representative.

Paul L
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Old 10-22-2009
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Barry, you are on the right track. Really work those numbers. Out here in SF/NorCal, the base (without any adjustments) PHRF calculations pretty closely follows the regression analysis that was discussed in another thread. So these numbers might level the playing field when comparing boats. Base PHRF for the 361 is 138 and Base for the 36.7 is 120/123 depending on regular or shoal keel. The 36.7 hands down is a better performing boat. If I was looking for a number or ratio that would indicate how closely a boat sails to their rating, I’d look at total displacement and the ballast to displacement ratio for a given LOA. Beautiful, wooden interiors are not conducive to sailing fast.

I have a fair amount of experience in sailing the 361 and was able to closely observe the 36.7’s as they started immediately after us in the Rolex Big Boat Series this year. The 361 is a pretty boat and a comfortable cruiser. The forward stateroom is a little cramped for my 6’1” frame but is very doable for two couples on long weekends. The main salon is very comfortable and nice. Sailing characteristics are a bit pedestrian for my tastes. I don’t like the Lewmar traveler and the boat’s freeboard is a little high (but man, what an interior!). My lowly C34 is faster on all points of sail (yes, I’ve raced both), but you could improve the 361 by getting decent sails, upgrading lines and junking the Lewmar blocks. The 36.7 impressed me in the BBS. They were pretty fast and lively and looked like a lot of fun to sail. When the wind piped up to 20+ kts, they did get blown around a bit and I saw them get over powered and even a round up or two. This might be a function of the fractional rig and as these guys were racing, they were loathe to reef. If your average winds are in the mid teen, I’d say this would be a fun boat to have either single, double handed or with a full crew.

Sorry, I could not find any NorCal PHRF data on your Oday 35. Our local PHRF gives only 6 seconds for a 3 blade fixed prop. When I was Chief Measurer for the C34 Class Association, I did a lot of analysis on props and prop drag and our big 3 bladed props were more like 20 seconds a mile slower. The biggest performance improvement you can make is going to a 2 blade folder, followed by new sails, new (high tech) lines followed by a faired bottom. Remember, speed costs money. How fast can you afford to go?

Last edited by GeorgeB; 10-22-2009 at 06:25 PM.
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Old 10-22-2009
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I am not interested in racing but this is all interesting information never-the-less.

gus
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