Hull shape and comfort in swell.

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Oscarpop

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Last night over drinks I was told that I had the wrong boat for our current sailing .

I was complaining that the swell at 2m on the beam was bloody uncomfortable especially combined with upwind sailing, for 50-50 miles.

We have a modern design 40 ft yacht, lifting keel and plumb bow and stern.

Our critic has a more traditional design with encapsulated keel and a waterline length about 10 ft less than the loa.

Does hull shape really affect the boats motion in swell that much?nam happy to accept tat the bow of his yacht will cut trough waves better and not slam as much, but we accept that.

We both are medium displacement yachts,

He was adamant that his boat was able to deal with swell on the beam much better than ours.
However I've watched lots of other yachts dealing with the swell inter island and they all seem to be getting knocked about.

Thoughts?
 
I don't think that an older design with rounded hull sections will necessarily be better, and other factors could come into play. One is the depth of your keel. I have a shallow option of 1.5m fin and sailing on company with identical boats I have occasionally observed that we roll a little more. Another factor might be the height and weight of the mast, and how wide your stern is. I suspect that it is quite a complicated subject.
 
I shouldn't lose any sleep over his pronouncements if I were you.
All boats move with a beam sea.
The best way to deal with bores of this kind is to agree with them immediately and change the subject to something else.
Then you remove the argument by doing so, and it leaves you unencumbered to do exactly as you please, including walking away if persistent nonsense is repeated, period. PCUK may have pinned it.:cool:
 
I'm with PCUK and VO5 on this one, but when changing the subject dont mention dogs as his will be blacker than yours.
Hull shape, keel depth and mast weight/height will all have an effect, just what effect and which is best, that is best left to engineers with wave tanks (and cat owners)
 
Every owner says their boat is better than yours. And as an owner you tell everyone else yours is better than theirs. Its part of life. Live with it and move on (......to the next pub or some such place...)
 
Your friend is correct that a narrow beam boat (usually older boats) with deep fixed keel will be more comfortable in a swell than a modern beamy design, particularly one with internal ballast like many lifting keel designs.

Most modern boats are very beamy partly to maximise interior space but also to give form stability (resistance to heeling), allowing less ballast to be carried and therefore reducing the overall weight of the boat (which improves light wind performance).

The drawback of a lot of form stability is that the boat is very "stiff" and so lies flat on the surface of the water as swell passes underneath, resulting in a lot of movement. A design with a narrow beam and low down ballast (usually in a fixed keel) will be more "tender" and so tend to stay more upright as the swell passes under the boat.

The designer Ted Brewer developed a "comfort index" which predicts how comfortable a boat will be in a seaway - boats with high ballast ratio, narrow beam, low freeboard, and high reserve buoyancy (i.e. high LOA/LWL) come out with high comfort indexes.

Unfortunately most boats are sold on the "marina comfort index" which favours boats with huge beams (particularly at the stern to accommodate a huge cabin), no sea berths, huge saloon table occupying the entire mid-ship near the centre of gravity of the boat, and massive freeboard.
 
Ric is correct.The reason for the beamy boats and wide aft cabins is sailing was a male preserve so the manufacturers decided that if women were invloved in buying a boat sales would go up and they did, women like the wide aft cabins. At the other end of the scale BA were advertising types of planes and routes, then Saatchi and Saatchi told them they fly people and sales rocketed.
 
Yes I agree it is very much a function of form stability. In a swell from the side the slope of the water (wave) dictates that the boat be parallel to the water (wave) level. Hence she will roll with the wave.
Now the pendulum effect of the mass of the keel will tend to resist any roll. But it's effect is only significant at a large degree of deflection from keel vertical.
So a narrow boat with minimum volume in the chines will rely on keel pendulum so tend to roll less in the swell. The down side is that it will not be stiff against heel until the heel angle increases.
Your more modern wider flatter bottomed hull will be stiff against heel from small heel angle but will roll much more with the waves. I think I prefer the more modern design. olewill
 
Ric said:
snipped
The designer Ted Brewer developed a "comfort index" which predicts how comfortable a boat will be in a seaway - boats with high ballast ratio, narrow beam, low freeboard, and high reserve buoyancy (i.e. high LOA/LWL) come out with high comfort indexes.

Unfortunately most boats are sold on the "marina comfort index" which favours boats with huge beams (particularly at the stern to accommodate a huge cabin), no sea berths, huge saloon table occupying the entire mid-ship near the centre of gravity of the boat, and massive freeboard.
Click to expand...

All true, the other point is that a narrower more traditional long-keeler can be "driven harder" by sail pressure without losing control. Slight over-heeling may be fractionally slower than an optimum heel, but it means a greater wind-steadying effect. Most modern AWBs cannot be "over-driven", for as soon as the combined wind and swell heel angle exceeds a certain amount, commonly quite small, it will lose control and round up.

For most people's usage, with more time actually spent at anchor or in a marina, or sailing in nice weather, the modern beamy boat wins hands down. But at sea in bad weather the older narrower designs can be better.
 
Oscarpop said:
Last night over drinks I was told that I had the wrong boat for our current sailing .

I was complaining that the swell at 2m on the beam was bloody uncomfortable especially combined with upwind sailing, for 50-50 miles.

We have a modern design 40 ft yacht, lifting keel and plumb bow and stern.

Our critic has a more traditional design with encapsulated keel and a waterline length about 10 ft less than the loa.

Does hull shape really affect the boats motion in swell that much?nam happy to accept tat the bow of his yacht will cut trough waves better and not slam as much, but we accept that.

We both are medium displacement yachts,

He was adamant that his boat was able to deal with swell on the beam much better than ours.
However I've watched lots of other yachts dealing with the swell inter island and they all seem to be getting knocked about.

Thoughts?
Click to expand...

He is both right and wrong. Old fashioned short waterline long keel boats roll like pigs because they favour ballast over form stability. That said your boat is likely to behave badly because its lifting keel which implies ballast inside the hull, a lower roll inertia and likely a reduced keel area giving less damping.

Any small boat will roll unpleasantly across a reasonable swell
 
Dougal Tolan said:
I'm with PCUK and VO5 on this one, but when changing the subject dont mention dogs as his will be blacker than yours.
Hull shape, keel depth and mast weight/height will all have an effect, just what effect and which is best, that is best left to engineers with wave tanks (and cat owners)
Click to expand...

My cat seems to roll much less from side to side when broadside to the waves. We often look at the masts of monohulls swinging from side to side and thank the Lord that we are not on board a monohull. However, as Dougal says, it is not that simple. Sometimes, if the wave frequency is just right, we get a sort of reasonance where I assume that one hull is down on one wave and the other is up on the next. It has only happened a couple of times but when it does it's like being on a roller-coaster.

I don't know whether monohulls also have a "resonance" roll or whether they just always roll?

Richard
 
You have not experienced rolling until you have sailed a Colvic Watson. First time crossing the Thames Estuary in Rhoda Rose she rolled so much my son and I laughed at it. You soon get used to it and having a sail or two up reduced it, but she really enjoys her self in a beam sea. Trawlermen has nothing on it.
 
Lots of factors involved - one often forgotten is the weight of the rigging, etc. aloft. That will affect the rate of roll which can be more important than the amount of roll. Also what's comfortable for one person may not be for another. I can cope with large amounts of roll as long as it's fairly slow. A rapid roll rate and change of direction from rolling one way to the other and I'm out in the cockpit, staring at the horizon and wishing for a oak tree.
 
Don't think that a larger monohull will necessarily roll less than smaller ones. We watched an Oyster 125 (Twilight) rolling far further than any of the (much) smaller monohulls in Lower Bay, Bequia yesterday. They eventually gave up and moved to the north side of the bay behind us. They still rolled an awful lot. Maybe its the huge rig acting as a pendulum?

I would imagine underway their comfort factor would be higher though!
 
The OP does not say what the helmsman was doing with the swell.
When I had such a sea I turned into each wave at about 45 deg. and then straightened up.
For 50 miles down to Brittany.
Bit of a nuisance, but we had a good passage.
 
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