Fin Keel vs. Full Keel

[Dockhead]

Originally Posted by speedoo

I'd call it a "cruising fin". A "racing fin" would be deeper.

OK. Does everyone agree with that? What about the bulb thingy at the bottom? Does the deep canoe body of the boat change anything in the classification?

[Dockhead]

And how about this one? This is a Pearson 365, some places described as a "modified full keel"; in other places as a fin. I'm confused. It has a deep canoe body like the first photo. In fact, the underbodies of both boats look similar to my (amateur) eye, with just a somewhat longer and shallower keel on the Pearson. I'm confused.

[bob perry]

I'd also call it a cruising fin or to be more descriptive, a "low aspect ratio fin with a bulbed tip".

To my eye it is clearly not enough of fin for that sized boat. But I assume the geometry was biased towards making sure the shoal keel shown fit onto the same keel flange as the standard more deep draft keel thay also offerred. Without that pragmatic restriction I suspect the designer would have made the root chord longer.

I'd call that Pearson keel a very low aspect ratio fin.

[speedoo]

Originally Posted by Dockhead

And how about this one? This is a Pearson 365, some places described as a "modified full keel"; in other places as a fin. I'm confused. It has a deep canoe body like the first photo.

That's definitely a modified full keel. The difference vs. your first example is how the trailing edge of the keel is swept aft to provide additional directional stability. I don't think the hull shape has anything to do with keel nomenclature, other than the likelihood that certain hull shapes typically are found with certain keel shapes. For example a "wineglass" hull shape will almost always have a full keel, because for one thing a full keel will always be encapsulated, never bolted on like a fin.

[Amgine]

Sorry dockhead, referenced an earlier post and not your image (I really should use quotes more often.)

Deep hulls may gain some benefit lateral area, but in part that will be affected by the shape of the hull - slack bilges with deep deadrise act very different compared with a flattened canoe shape or a perfect arc.

As for the keels... fin to full keel is a continuum. A Baltic Trader's keel began at the stem waterline, so my cutaway full keel might be called a fin by somebody who compared it to that style. Few would argue a Dark Harbor 17 is a full keeler, except the wood boat builders.

One might define the differences between a cruiser's keel and a racer's keel as one of robustness, or mean vs extreme. A cruiser's keel needs to do other jobs equally well in addition to go fast, while a racer has fast as the priority. So, for the racer an extreme keel - despite its possible fragility, or anchorage limitations, or ability to dry out on, or susceptibility to snags, or ... - is not generally a drawback. But for the cruiser each of those and many more is probably a major drawback.

[Dockhead]

Originally Posted by bob perry

I'd also call it a cruising fin or to be more descriptive, a "low aspect ratio fin with a bulbed tip".

To my eye it is clearly not enough of fin for that sized boat. But I assume the geometry was biased towards making sure the shoal keel shown fit onto the same keel flange as the standard more deep draft keel thay also offerred. Without that pragmatic restriction I suspect the designer would have made the root chord longer.

I'd call that Pearson keel a very low aspect ratio fin.

Thanks -- extremely interesting.

The first photo -- the one with the slightly bulbed keel -- is a 15-meter LOA, 20-ton cruising boat with 2.3 meter draft, designed by Holman & Pye in the early '90's. It's not a shoal keel but rather the standard (and as far as I know, only) keel option, but it's not a volume production boat.

Interesting you say it's short.

[Dockhead]

Thanks, guys! I really learned a lot from your comments.

[bob perry]

Speed:
Some fins can be "encapsulated" or molded in also. I had done many like that.
I would certainly not call that Pearson keel a "modified full keel". It is a fin. It is just a very low aspect ratio fin, but it is still a fin.

I call a "full keel" a keel where the leading edge of the keel starts at the forefoot and extends aft to have a rudder on it's trailing edge. Like a Westsail, you can't see where the forefoot ends and the keel begins. That's a full keel.

Now we can begin to pull the leading edge of that long keel aft ( like my Babas and Tayanas) and at some nebulous point, where you can begin to differentiate between forefoot and keel leading edge I would call that keel a "modified full keel". There is certainly some room for argument in what defines a "modied full keel" but I'm comfortable with my defibnition.

I think these terms are important so that we know what each other is talking about.

[Hiracer]

Originally Posted by keelbolts

Whoever said that a fin keeler is better to windward than a full keeler is mistaken. .

You have just stated that Bob Perry, one of the more notable and successful sailboat designers, doesn't know what makes a boat go to windward.

Originally Posted by keelbolts

Windward ability is about resistance to lateral motion and few fin keelers have as much lateral plane, and thus resistance to lateral motion, as a full keeler.

This is where you are mistaken. This is where your thinking has led you astray. You can't talk about windward performance without talking about lift. Lift is what determines windward performance. Lift comes from the leading edge of the keel and deeper keels have more of it.

Originally Posted by keelbolts

One of the reasons why the majority of racers have fin keels is because they often race downwind..

Again, flat out wrong. Sorry, fin keels are on racing boats because they are faster upwind. There is no real argument about that.

Think of it like this. In a triangular race course you might have one downwind leg, one windward leg, and one beam reach. The distance for all three legs might be the same, but the upwind leg will take longer and therefore will be the leg most likely to determine the outcome of the race. Ergo, race boats are optimized for windward performance and 100% of them have fin keels. They might travel an equal distance on all three headings, but they will most certainly spend a disproportionate amount of time on the windward heading.

Now, having said that, it is theoritically possible for an anomalous result. One particular full keel boat may go to windward better than one particular fin keeler. Perhaps the full keeler had a clean bottom, good sails, and able captian. Sailing against a dirty bottomed fin keeler, with blown out sails, and less accomplished captain, it would not be out of the realm of possibilities for that particular keel boat to beat that particular fin keeler to windward. But it would be a mistake in attribution to say the keel was responsible for the windward performance.

[speedoo]

Originally Posted by bob perry

Speed:
Some fins can be "encapsulated" or molded in also. I had done many like that.
I would certainly not call that Pearson keel a "modified full keel". It is a fin. It is just a very low aspect ratio fin, but it is still a fin.

I call a "full keel" a keel where the leading edge of the keel starts at the forefoot and extends aft to have a rudder on it's trailing edge. Like a Westsail, you can't see where the forefoot ends and the keel begins. That's a full keel.

Now we can begin to pull the leading edge of that long keel aft ( like my Babas and Tayanas) and at some nebulous point, where you can begin to differentiate between forefoot and keel leading edge I would call that keel a "modified full keel". There is certainly some room for argument in what defines a "modied full keel" but I'm comfortable with my defibnition.

I think these terms are important so that we know what each other is talking about.

Thanks, Bob. Yes, isn't your shoal draft Islander 32MkII's keel encapsulated? I guess that would have to be called a fin keel, so I generalized too much with my "never" comment.

The reason I thought the Pearson keel was a modified full keel is that at first glance I thought it was shaped exactly like my Kelly Peterson 44, which has been consistently described as a modified full keel. But perhaps the Pearson's more sharply defined leading edge beginning point is what makes it not a modified full keel.

[Dockhead]

Originally Posted by speedoo

Thanks, Bob. Yes, isn't your shoal draft Islander 32MkII's keel encapsulated? I guess that would have to be called a fin keel, so I generalized too much with my "never" comment.

The reason I thought the Pearson keel was a modified full keel is that at first glance I thought it was shaped exactly like my Kelly Peterson 44, which has been consistently described as a modified full keel. But perhaps the Pearson's more sharply defined leading edge beginning point is what makes it not a modified full keel.

The Pearson keel is described both ways, more often as a "modified full keel". As a result, I never knew what it really was. I guess I'll have to defer to authority and experience, and think of it as a "very low aspect fin keel".

I agree that it's very useful for us to have our terminology straight, so we understand each other.

[keelbolts]

It's a fin keel. What's going to keep it from falling off when the wind blows?

[keelbolts]

When I say full keel, this is what I'm thinking of.

[bob perry]

Dock:
Your Kelly-P has a low aspect ratio fin. It's a fin.
My Islander 32 in both shoal and deep draft models has a moderate aspect ratio fin although with the extremes we see today I could argue that those fins are also
low aspect ratio". Who ever told you your keel was a modified full keel was flat, plain wrong.

I'll call him Bruce. He's a Kiwi and he is a top engineer with the world's leading structural yacht design firm. He is a college trained naval architect. He came up to the beach and spent the night last night. We talked boats into the wee hours. That conversation would have been very awkward if Bruce and I did not share a common vocabulary. That's why I think terminology is important. It allows accurate communication.

[bob perry]

Keelbolts:
In your pic I see a distinct forefoot and a distinct leading edge to that keel so I would call that a "modified full keel".