Beam vs Keel Depth - Which is more Efficient?

[alansmith]

Tito, also many boats beam and keel arrangements are targeted for what type of use the boat is primarily to be used for. For example a race boat might have a long shank with bulb shaped swing keel, versus maybe a short sheel type of wing keel for skinny waters. I am always fascinated looking at naked pictures ( bottoms of boats) and different configurations of keels, fins, & daggerboards... it is amazing isn't it?

[MAJICDAN]

As you go beamier with a shallower keel, you end up more like a motor sailer. The boat does not lean as much, but it does not sail as well against the wind.

[ALAIN97133]

Originally Posted by alansmith

Alain, thanks for posting pics and story. Enjoyed it.

I forgot to point out that the tandem centerboards layout came from East coast 19th century American sailboats & that when you go downwind, you raise the main centerboard which is then useless & you lower the aft one that gives you what is called great "arrow stability". Fin keel sailboats are hard to steer downwind & most windvanes (Except my Hydrovane) do a so so job but with the raised main centerboard & the lowered aft board, any windvane, electric or mechanical will do the job! Included pictures are typical European cruisers meant for offshore living from the North pole to the Tropics, except the last one which is an American 1950/1960 sailboat (S&S design maybe?).

[knockabout]

i haven't seen a twin hull design in a submarine yet and to make multi hulls sail quickly people fly them on foils out of the water / usually it's far more cost effective to research the type of boat you want and buy that design complete than cut up a perfectly good boat and try to make it into the boat you want / the grass is always greener syndrome /enjoy sailing within your vessels capability / this old girls theoretical hull speed is 5.5kts / it has centre line foils and surfs both on the foils and on the beam / we enjoy sailing slowly but then again boats are like pushbikes noone has had one flat out

[zstine]

Originally Posted by knockabout

i haven't seen a twin hull design in a submarine yet . . .

well, the Russian Akula class (aka typhoon class) has 2 parallel pressure hulls, like a catamaran, connected by a superstructure. makes for a 75ft beam, 40 ft draft... just sayin

[knockabout]

zstine, thank you / the lady nelson had a winch up centreboard for survey work and still sailed back to england for repairs when the board was broken off in a storm / dutch use lee boards to solve side slip under sail / very interesting subject /self righting sea going efficiency most likely ballasted keel boat / for reliably handling a big sea a surf board seems to survive well / recent sydney- hobart competitors appear to be a cross between the two / the fastest time between sydney - hobart used to be Joint Venture aluminium twin hull ship USMC /

[markpierce]

Keels help one from sliding to leeward.

[ramblinrod]

Originally Posted by TitoSoto

Can you fully replace a keel with a beamier design (or vice versa), or is there always a balance to be struck between beam and keel depth?

In the most simple terms, anyone can do anything; how well it works is another matter.

Ballast (as often found in keels) provides righting moment.

A wide beam provides form stability.

Both help to keep the boat upright, but there are significant differences.

Without ballast, a design providing high form stability (think catamaran) tends to sail quite flat, but is just as happy upside down as it is right side up.

Whereas, without form stability (think narrow beam monohull), a heavy ballast design will initially heel very easily and then stiffen up rapidly.

If the design parameters specify a self-righting vessel, yes, there is a balance between form stability and ballast.

If the design parameters do not specify a self-righting vessel, then ballast can be foregone for form stability.