Question about PHRF Calculation

[Tedd]

I found a formula for PHRF online, which I applied to my boat (Macgregor 26S). The PHRF number I've seen thrown around for the boat, anecdotally, is from 216 to 222.

The boat has a swing keel. If I put in the minimum draft (1.25 ft) I get a PHRF of 153, which seems unrealistically low. If I put in the maximum draft of 6.33 ft I get a PHRF of 251, which seems suspiciously high. But if I put in the average of the two (3.79 ft) I get 219, smack in the middle of the range I've seen quoted.

So, my question is, is that how swing keels are handled for calculating PHRF--the average of maximum draft and minimum draft? Or is my calculation just wrong and this is a coincidence?

 

[Jackdaw]

Unlike IRC or ORC, there is no such thing as a 'formula' for PHRF. The ratings numbers are based on educated guesses about how a (new/unrated) boat will perform, and adjusted over time based on observed results. The thing you are looking at (if its the one I've seen) was someone's misguided attempt to backfit numbers into a formula. It has been widely discredited.

Any variances in the actual numbers used for any given boat are usually a results of difference weather in the fleet's area that will affect performance, or slight variations in the relative ratings that will slide a fleet's boats numbers up or down by 3 or 6 seconds.

So to answer your question, the numbers for different keels were set based on an initial estimate of how the would do, and then adjusted. Generally, deeper is better than shoal, and for any given boat the lighter the keel the better.

 

[PaulK]

Like Jackdaw says. Unlike IRC or ORC, PHRF takes typical local conditions into account. A heavy, short-masted boat in an area that has strong winds like SFO Bay will be rated lower (faster) than the same boat in a place famous for light air - like the PNW. A light boat with a tall mast will rate lower in the light-air venue, since its weight and sail area are an advantage there. It may not rate so fast in a heavy-air location because they will need to reef to stay on their feet and the bigger waves are likely to slow them down as well. It's all relative.

 

[Jackdaw]

Like Jackdaw says. Unlike IRC or ORC, PHRF takes typical local conditions into account. A heavy, short-masted boat in an area that has strong winds like SFO Bay will be rated lower (faster) than the same boat in a place famous for light air - like the PNW. A light boat with a tall mast will rate lower in the light-air venue, since its weight and sail area are an advantage there. It may not rate so fast in a heavy-air location because they will need to reef to stay on their feet and the bigger waves are likely to slow them down as well. It's all relative.

Exactly right on local conditions.

ORC actually uses a triple (rating) numbers for low / medium / high winds, and for distance races can actually create temporary ratings for boats based on expected wind.This is almost always done on big international races.

 

[Tedd]

I didn't realize I was wading into a sectarian dispute!

So, to return to my original question, how does having a swing (or retractable) keel factor into these numbers?

 

[Jackdaw]

I didn't realize I was wading into a sectarian dispute!

So, to return to my original question, how does having a swing (or retractable) keel factor into these numbers?

not sure what you mean by that; there really isn’t a dispute of any kind.

because your boat ONLY came with a swing keel (no other option) it does not factor at all. The original rating officials looked at the boat, and gave it a rating based on the sum of all it’s relevant design issues. The keel being one.

had it had a been offered with different keels, they probably would had different ratings. Like I said in my first post.

 

[Davidasailor26]

So, to return to my original question, how does having a swing (or retractable) keel factor into these numbers?

I can’t find any examples of boats that have a swing keel and fixed keel option in PHRF New England’s list of base handicaps. The best I can find is an O’Day 30 centerboard vs fin keel. The centerboard version rates 3 seconds slower, but a swing keel where you’re actually lowering ballast weight would probably be faster than that.

If you know of a boat that was offered with both options you can look it up on PHRF-NE’s base handicap list at PHRF New England - Handicapping - Base Handicaps. Or see how it rates in different regions on US Sailing’s list at Boat Class Lookup - US Sailing

 

[Tedd]

@Davidasailor26 :

Thanks, those links will be helpful.

 

[Jackdaw]

So, to return to my original question, how does having a swing (or retractable) keel factor into these numbers?

I'll take one more swing at this.

To your original question, the smaller number has no practical use when racing or determining a rating. Keel up, it is often only the hull draft, or the draft of the keel retracted along the bottom of the boat. The boat is never sailed this way or rated this way. As I noted before, that formula you are looking is has no basis in the actual determination of ratings in PHRF, ever or anywhere.

Now maybe on another related question: How do swing keel get 'rated' by a PHRF rater?

Well, they are going to look at it, in its fully down position. They will concentrate on:
Draft (more is better)
Foil shape
Position/distribution of ballast (lower is better)

On an entry level swing keel, positioning ballast low and on a deep swing keel can be problematic due to the forces involved, which can create engineering/built problems that are costly to solve. And, the mechanism needed to lift the keel has to well engineered. Most often, the builder will compromise to keep costs low.

So on some boats, a version with a fin (old design as well) rates much faster than the swing keel version
Cat22 SK 255
Cat22 fin 222

On a boat like the Beneteau First 235 with a better designed swing (lifting) keel, the swing does better then its fin keeled sister boat.

 

[FDL S2]

I'll take one more swing at this.

To your original question, the smaller number has no practical use when racing or determining a rating. Keel up, it is often only the hull draft, or the draft of the keel retracted along the bottom of the boat. The boat is never sailed this way or rated this way. As I noted before, that formula you are looking is has no basis in the actual determination of ratings in PHRF, ever or anywhere.

Now maybe on another related question: How do swing keel get 'rated' by a PHRF rater?

Well, they are going to look at it, in its fully down position. They will concentrate on:
Draft (more is better)
Foil shape
Position/distribution of ballast (lower is better)

On an entry level swing keel, positioning ballast low and on a deep swing keel can be problematic due to the forces involved, which can create engineering/built problems that are costly to solve. And, the mechanism needed to lift the keel has to well engineered. Most often, the builder will compromise to keep costs low.

So on some boats, a version with a fin (old design as well) rates much faster than the swing keel version
Cat22 SK 255
Cat22 fin 222

On a boat like the Beneteau First 235 with a better designed swing (lifting) keel, the swing does better then its fin keeled sister boat.

This makes sense, thank you for going into detail.

Usually the ratings work, in our club my S2 7.3 with a fin keel and old sails rates faster than an Irwin 27 with a centerboard and new sails. Unfortunately for me the couple with the Irwin are very good sailors who ALWAYS sail to their rating. I often can stay with them until I run out of talent and they pull away, but that's not the boat-that's the crew.

 

[Sandy Stone]

I'd put at least some of the blame on the old sails. ; )

 

[FDL S2]

I'd put at least some of the blame on the old sails. ; )

Nah, I'm the guy using the sails who hasn't replaced them yet.....