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  #481  
Old 09-19-2012
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Re: Full or fin keel?

Quote:
Originally Posted by Jeff_H View Post
Sea Hunter,

I am glad that you clarified what you are thinking. At least as you are describing it, (i.e. "Reserve buoyancy is a hulls ability to carry a load over and above displacement".) that is not in keeping with the way yacht and ship designers use the term, "Reserve buoyancy". What you are describing rarely if ever is considered as a part of yacht design.
"Reserve of Buoyancy" is a term that come from time begin of yacht design. Sometimes shortened to reserve buoyancy. There are a number of chapters in this subject and the importance of reserve of buoyancy in yacht design. Here is a quote from page 12 of "Naval Architecture" printed 1877.

".. commonly used to express the volume and corresponding buoyancy of the part of the ship not immersed, but which may be made water tight.."

"The under water, or immersed, part of a ship contributes to the buoyancy; the out-of-water part the reserve of buoyancy, and the ratio between the two has a most important influence upon the safety of the ship against foundering at sea."

It sounds like Sea Hunter described it correctly.
Bryce
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  #482  
Old 09-19-2012
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Re: Full or fin keel?

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Originally Posted by Daily Alice View Post
Bryce,

I got as far as finding VMG, metacenter and CB for a boat, but how do you arrive at " the ratio of mass righting moment to hull righting moment"? I mean what do you need first, for data, and what formulas are you using? I ask because your approach seems useful. Isn't hull righting moment graphed over various heel angles (like from 0-30 deg.)? Anyway, I'm curious, thanks.
Sorry it took so long to answer. The boat is now out of the water for winter storeage. I hope to create a new thread discussing this subject this winter. I have simulations to support my assertions.

For me, it is the defining distinction between a rough water boat and a non-rough water boat. Light weight racers are the worst, heavy weight cruisers like the big Island Packets are the best. For me, it is part of the answer to the age old question of "blue water" capable and it is consistent with what the experienced sailors know already.

The reasons I developed my simulations and mathematics was to understand the significance of different weight boats of similar size.

The problem with righting moment diagrams is that they become useless once you put a boat on waves (in rough water). So you cannot predict from righting diagrams what boat is best in rough water. You cannot even predict what a boat will do in rough water from its righting diagram.

Two boats of similar size can have very close righting diagrams. However, one will be very comfortable in heavy seas and the other can be quite dangerous.
Bryce

Last edited by BryceGTX; 09-19-2012 at 10:22 PM.
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  #483  
Old 09-27-2012
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Re: Full or fin keel?

Based on what I've read of this thread,
Full keel boats are the best general use boats,
since they are great in shallow water, deep water, long journeys and in heavy weather.

Fin keel boats are only for special cases, such as racers, powersailers and marinophiles.

Last edited by elspru; 09-27-2012 at 07:40 PM.
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  #484  
Old 09-27-2012
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Re: Full or fin keel?

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Originally Posted by elspru View Post
Based on what I've read of this thread,

Fin keel boats are only for special cases, such as racing, powersailers and marinophiles.
You might want to tell that to what is very likely the majority of successful ocean crossers, who have fin keel boats.
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  #485  
Old 09-29-2012
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Wink Re: Full or fin keel?

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Originally Posted by mitiempo View Post
You might want to tell that to what is very likely the majority of successful ocean crossers, who have fin keel boats.
crossing an ocean can be done with even a raft made of styrofoam, doesn't mean it's comfortable.
a full keel is better in heavy weather, and large waves, as it's less likely to heel with the waves, and more likely to simply go straight.

Sure you can plot your courses to avoid storms, and only go in mild weather, but then you miss out on all that rollercoaster fun .
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  #486  
Old 09-29-2012
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Re: Full or fin keel?

If you're a weekend sailor with a couple weeks vacation on the boat in the summer, (90% of us) then a well designed (S&S for example) fin keeled boat is far better. Faster, more maneuverable, less wetted surface, etc. Have had the boat 30 years, been off shore, as well--handles great.
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  #487  
Old 09-29-2012
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Re: Full or fin keel?

I'm really glad you admited to being a weekend sailor, as that fits in my view of fin keelers :-). We intend to liveaboard, on the hook as much as possible, traveling to different areas harvesting resources, with a shoreside plot for backup, though indeed we could be a small minority, that's okay :-), oh ya ferrocement full keeler mmm, sooo good :-D.

Last edited by elspru; 09-29-2012 at 10:56 PM.
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  #488  
Old 09-30-2012
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Re: Full or fin keel?

After scanning this blog, it is apparent that there are a quite a few folks out there who are able to rationalize their positions on fin vs. full keel based on what they own. "Love me, love my boat."

My current boat is is a 35' sloop with a weighted swing keel. It has other parts than the keel that impact its performance, like substantial form stability (less heeling) and inboard shrouds (allows better performance to upwind). It--like every boat--is a compromise and has its negatives, like less interior space to accommodate the keel trunk and more moving parts (swing keel and also a swing rudder). All of this allows the boat to float in less than 2' of water, but with drastically reduced maneuverability. I've sailed her from the Gulf coast to Maine over a period of 16 years and have found myself in a wide range of conditions and have to admit that I'm rather comfortable with the compromise. I've had her offshore and I've been in various conditions from light winds to sustained winds of 40+ kts.

On the other hand, I've sailed on many full- and modified full-keel boats ranging from a 26' Dolphin to a 42' Island Packet over the past 40 years. Most of my experience on full keel boats was as a bareboat skipper for a minimum of a week at a time and a lot of that was in the Caribbean, from Puerto Rico to Antigua. These keel boats were noticeably less responsive than the fin keel boats I've sailed. This could make for a challenge when tacking, for example, especially with the cutter rigged Island Packets. They also didn't do as well going to weather or in light air. On the other hand, they were more comfortable when the wind picked up to 20+kts, but some of that was due to their higher displacement. For some reason, all of the Island Packets I've been on (350, 370,380, 40, 420) required a heavy hand to steer. This might have been due to the steering design, rather than the keel, but the net result was fatigue and sore shoulders, so whatever tracking advantage is claimed for full keel boats was nullified. When going to weather, I would just use the motor. The fin keel boats--mine included--took a much lighter touch on the wheel and were more weatherly. This is a big deal on longer trips or in higher winds.

The one serious reservation I have about fin keels is their deeper draft, which makes anchorages smaller and some "shortcuts" unavailable.

There is no boat that does everything, so you pick your parameters and live with a compromise in either case. In my case, the swing keel and swing rudder needed an expensive upgrade (to make sure things move when necessary). These expenses would not be incurred with a traditional full keel (non-centerboard) boat. On the other hand, I have never needed help to get ungrounded.

My bottom line on fin vs. full keel is that to each his own.
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  #489  
Old 09-30-2012
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Re: Full or fin keel?

I agree,

I have a cutaway forfefoot keel and while they turn on a dime when the wind is just right, they are sluggish at low speed and I have had a few problem tacking when the wind was really strong just like I had on cape cod mercurys so it's not just the shape.

I approach my boat rationaly, it's a machine with nice features and flaws too. It's a little disconcerting to see people swear by the design they happen to have (sometime by luck) and get into pissing contest about the shape of their keel. All this talk is nonsense, the only persons I would trust in this matter are boat designers and naval architects. Let them discuss the merits of such and such configuration. The rest is legend and fable.
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  #490  
Old 09-30-2012
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Re: Full or fin keel?

Quote:
Originally Posted by flo617 View Post
I agree,

I have a cutaway forfefoot keel and while they turn on a dime when the wind is just right, they are sluggish at low speed and I have had a few problem tacking when the wind was really strong just like I had on cape cod mercurys so it's not just the shape.

I approach my boat rationaly, it's a machine with nice features and flaws too. It's a little disconcerting to see people swear by the design they happen to have (sometime by luck) and get into pissing contest about the shape of their keel. All this talk is nonsense, the only persons I would trust in this matter are boat designers and naval architects. Let them discuss the merits of such and such configuration. The rest is legend and fable.
Well, if you take it that way it is rather simple: Naval Architects, I mean reputable ones, have long abandoned full keel configuration even for blue water cruising purposes, even the ones that once, far in the past, designed full keel boats.

Regards

Paulo
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