Fin keel vs bulb keel - any difference in rolling motion side to side?

[NaClyDog]

Only considering the side to side rolling motion (at anchor or underway), is one better at minimizing movement if you only consider a fin keel and a bulb keel?

[wingssail]

Originally Posted by NaClyDog

Only considering the side to side rolling motion (at anchor or underway), is one better at minimizing movement if you only consider a fin keel and a bulb keel?

The two factors which will affect rolling (other than the waves) are the effective area of the keel and its shape.

A keel with more area will resist rolling more than a smaller (in area) keel. As far as shape goes, a deeper keel will resist rolling more due to its better leverage.

A high aspect ratio fin keel will be the most effective in reducing roll, in my opinion.

For bulb keels, they lose in two ways. One they are usually shorter, so they have less leverage to resist the roll (look up moment of inertia, too) and they generally have less area since the other objective of a keel is to weight down the boat to keel it upright which they accomplish with the concentrated weight of the bulb closer to the hull.

But the bulb, if it is far from the roll center of the boat, will resist rolling (moment of inertia again). So the best will be a high aspect ratio, deep draft, fin with a bulb at the very bottom.

But, when I am in a rolly anchorage and look around, all the boats, without regard to their shape or type (so presumably they would have varying types of keels) are rolling. The worst seem to be full keel boats with short masts (moment of inertia).

My view is, if you are really worried about a rolly night at anchor, pick a better anchorage. As far as the boat goes, choose a boat which sails well, it will have a good keel, and sail somewhere which is not rolly.

[Breaking Waves]

In theory, there will be a difference in the motion. A bulb, in theory, will make the motion a bit faster and more snappy but peak heel flatter; while the fin will be slower motion but with higher peak angles of heel.

In practice, with typical configurations on otherwise similar boats, I doubt you will notice much difference.

[NaClyDog]

Thank you wingssail.


I've not been a student of hydrodynamics and wasn't sure if having more surface area would make rolling easier or not. Your explanation makes a great deal of sense.


We briefly owned a cat as covid hit but sold before really getting to enjoy it. Without getting into the merits of one vs the other, we (well, I and I need ammunition to convince the admiral) would like to get back into sailing and I feel better about a monohull at this stage.

[Rumrace]

I think it’s limited to the build to get more weight deeper in yet a shallow keel and work the hydrodynamics as best as they can. The “T” on some boats is apparently for balance. Rather than heavy hulls full of bronze or lead cheaper iron on threaded rods under a lightweight hull. Evolution

[Giro]

Interesting input here. While on the subject, I’ve always been curious about just how “safe” my light weight fractional rig, 25’ sloop really is in a moderately lively west coast bay. Specs are:
Hull length is 25’
Length at water line is 23’
Displacement at 2,100 lbs
Ballast at 650 lbs
Draft is 5’ 9”
Swing keel (iron)

Does anyone, in the know, have an opinion? Please be aware that even though I have been sailing for over three decades, I have never considered myself to be a great sailor. I just want to maintain a safe sail for all on board.
Thanks in advance.

[NaClyDog]

Originally Posted by Giro

Interesting input here. While on the subject, I’ve always been curious about just how “safe” my light weight fractional rig, 25’ sloop really is in a moderately lively west coast bay. Specs are:
Hull length is 25’
Length at water line is 23’
Displacement at 2,100 lbs
Ballast at 650 lbs
Draft is 5’ 9”
Swing keel (iron)

Does anyone, in the know, have an opinion? Please be aware that even though I have been sailing for over three decades, I have never considered myself to be a great sailor. I just want to maintain a safe sail for all on board.
Thanks in advance.

How about you start a thread about rigs instead of hijacking one about keels?


Just a thought...

[Rumrace]

Sounds like you’re doing fine. I’m a power boater in sailing school.
I watched the Annapolis to Bermuda race with great interest. I noticed the Navy44 has a pretty small water line standing still but the crazy old machine get up and go lying down a wee bit. I get power boat hulls. Deep Vs are king in big water. The dynamics in a sailing hull are interesting to learn. Why call my boat almost 39’ when it has 35’ wet spot. Docking fees 😂 2’ of that’s pulpit.
My boat came in 3 keels 2 rudders or single rudder. I had no idea until someone asked which version it was. So I ended up with a shallow draft 4500 pound winged bullet and two stubby rudders. Works out cause back of the slip is shallow. Apparently it’s normally the worst performance version but in this model it’s a performance package.
Who knew. We are loaded on safety gear above requirement due to some lake racing. .

I think all boats are as safe as the day the captain , it’s looking out for the other guy the biggest concern.

[nortonscove]

The old guys here referred to what you seek as catch. The deeper and larger in surface area the greater the catch. In other words the more water it would catch or push against. Of course there are other tradeoffs for more surface area.

[NaClyDog]

Originally Posted by nortonscove

The old guys here referred to what you seek as catch. The deeper and larger in surface area the greater the catch. In other words the more water it would catch or push against. Of course there are other tradeoffs for more surface area.

Catch or ketch?


What would you consider other trade-offs? Overall sailing speed?

[Giro]

Aside from one needlessly uncalled for and completely inappropriate comment from one poster, I found this discussion quite interesting and informative.
For me, the main “take-away” is that deep is definitely better.

[SV Tom Crean]

Originally Posted by Giro

Aside from one needlessly uncalled for and completely inappropriate comment from one poster, I found this discussion quite interesting and informative.
For me, the main “take-away” is that deep is definitely better.

Tall rig and deep bulb keel are the best (but what do I know). Short,shallow keels coupled with short rigs are probably the worst.

[Brioche]

The OP refers to "minimizing movement". Movement is not a closely defined term however. Cruise ships, for example, are built with a greater degree of instability because that gives a slow roll, which passengers find more comfortable than a quick, sharp, but closely confined roll. A deeper center of gravity on the keel will slow the roll and probably make it more comfortable. A lot of surface area will not necessarily affect the roll rate (depending on the shape of the keel) but it will serve as a damper, limiting the range of movement, in the same way that a sail steadies a boat, for example, when motoring with a little bit of sail up. Both damping and slowing the roll add do comfort, although a slow roll may not be "minimized movement".

[zstine]

Originally Posted by NaClyDog

Only considering the side to side rolling motion (at anchor or underway), is one better at minimizing movement if you only consider a fin keel and a bulb keel?

yes, the more area your keel has the more damping to roll. So a traditional long keel will damp roll more than a fin. A hard chine boat will also dampen roll more than a round hull.
I see several responses discussing inertial factors. This has nothing to do with 'damping' but rather with natural frequency. A tall heavy rig and deep bulb keel moves the weight out from the center and reduces the frequency of rolling, which may be more comfortable than higher frequency roll you feel on a catamaran for example. HOWEVER, swell in an anchorage is typically at low frequency and a yacht with low natural frequency is more likely to have a much higher amplitude of roll, because the swell can act like a little shove on a swing that grows and grows..
If I was only on the boat at anchor, I'd certainly get the long, full, traditional keel. I would never actually buy that because I enjoy a boat that sails well to weather.


Also, consider the fact that higher amplitude roll means more wave making resistance and a slower, longer passage. For reduced resistance, higher frequency is always better.


I found that reducing roll in anchorages is typically simple, just kedge out a stern anchor or bridle your anchor line to a stern cleat and rotate the boat to point into the swell. Works like a charm, but I rarely saw other boats doing this!

[nortonscove]

Originally Posted by NaClyDog

Catch or ketch?


What would you consider other trade-offs? Overall sailing speed?

Ketch would be the way the vessel is rigged. When they used catch it was referring to the keel. The obvious tradeoffs are wetted surface reducing speed and the worse for tight docks, the maneuverability. They track better in open ocean and inside the seawall. Unfortunately tracking stubbornly in one direction can be costly and annoying.