How do you feel about plumb bows on cruisers?

[Idlegreg]

Originally Posted by David B

......

I am no naval architect, but the bows that really puzzle me the the reverse sheer bows we are seeing come out now - especially on catamarans. Again, the aesthetic leaves me cold, but thinking about a catamaran in particular, as the bows are pushed deeper, the buoyancy decreases (the effective LWL decreases, rather than increasing). Someone tell me this isn't designers compromising seaworthiness over perceived 'trends' in aesthetics!

No no no - as the bows are pushed deeper the buoyancy INCREASES. It just increases more slowly (with respect to pitch angle) than on a plumb bow and much more slowly than on a forward raked bow. The question is not whether or not the buoyancy increases but how fast you want it to increase and many things might effect what is best in a particular case.


I sail a lovely 1950s sloop with quite a bit of bow overhang and a counter stern. It always amuses me when people say they don't like modern reverse sterns with bathing scoops as they lack the reserve buoyancy of a nice counter. If I moor up in a marina and walk to the next pontoon I see my tiny low and narrow counter alongside modern boats with a stern 3 x as high and 5 x as wide. The modern boats have VASTLY more reserve buoyancy if that is what you are after.


Someone mentioned before in this thread a question often forgotten. What happens below the DWL. You could have a plumb bow which turns 90 degrees just below the water. When it goes down the buoyancy progressivly increases (and with high freeboard on modern designs it carries on increasing for much longer than on my boat). But when that modern bow rises it quickly and suddenly loses buoyancy entirely. I suspect a good effect of this is a resitance to pitching. A bad effect is that horrible slamming when heading into certain waves.


I think they call it an Axe bow when a vertical stem goes deeper below the water. I have an idea that other things being equal that might give the softest ride.

[Mooseboat]

I've sailed thousands of miles in bluewater with the new Scow bow. Two factors 1- much wider bow = more buoyancy. 2- Hard chine the full length of the hull. The boat heels over on to that hard chine and sails like a freight train. Give her an ocean swell and she laughs. Give any sailboat a short Long Island Sound chop and it's gonna take water. Also that ocean racer is making speeds of 30 knots. That's why your seeing so much spray and green water.
Fun discussion.

[skenn_ie]

Bow angle makes very little difference to buoyancy. What IS affected is dynamic lift. Reverse raked bows have negative dynamic lift, counteracts buoyancy which prevents the bow rising up over waves. This is beneficial in for maintaining direction and speed, at the expense of safety and dryness in the cockpit.

[chesapeakesailor]

The most important point on plumb bows made thus far imo is Ann's of JPA Cate - make sure you have an anchor plate on your bow. Lots of anchor dings will ensue if not. I have an anchor plate on mine but not until after I had the dings repaired twice.

I have done some offshore trips on my boat and she was pretty dry depsite the bow line, but then mostly we were reaching. When we were pointing in better than 20 knots we did some 'skipping' over waves to land with a hard thud. We had to slow down to be certain we didn't do any damage as our speeds reached around 11knots from time to time while probably averaging 10 knots. At those speeds the new boats are almost plaining.


If you do alot of ocean passages you likely want the older shape bow and stern for comfort.

J

[rslifkin]

Originally Posted by chesapeakesailor

When we were pointing in better than 20 knots we did some 'skipping' over waves to land with a hard thud. We had to slow down to be certain we didn't do any damage as our speeds reached around 11knots from time to time while probably averaging 10 knots. At those speeds the new boats are almost plaining.

That's a big thing. It starts to become like running a planing hull powerboat at times. Just because it can go fast doesn't mean you should under all conditions. Sometimes you just have to slow down to maintain a good ride and avoid pounding.

[wingssail]

Originally Posted by Na Mara

... Oh the slamming upwind in anything over force 4 or 5 got really old really quickly. Even blasting with her downwind grew old after about 10 hours or so. You couldn't leave her on the auto in those conditions as the hull was so twitchy so all the competent crew had to take turns hand steering her. On that trip over the north sea we had 3 days of force sevens over the stern quarter and we (the four of us) had to hand steer all of it in shifts. I was an exhausted wreck by the time the weather blew through.

But the thing that finally did for me and that boat was her windward performance in a blow. Those rounded fore sections together with the poorly setting self tacking jib and light beamy hull meant that in the kind of seastate you get with a blow of say force 5 and upwards the boat just got stopped when trying to go upwind. You just could not get that boat to go closer that 50 degrees off the wind in the sort of seas you get in a force five. By the time you got to force 7 she just would not go upwind at all. That is not only deeply frustrating, it is also dangerous (lee shore scenarios) and scary for crews unused to the deep blue sea...

This report shocks and surprises me.

I wonder if there is something about that particular design. I say this because boats with similar descriptions are very successful racing even in horrible conditions.

And I race frequently against a similar Hanse (Varianta 44) and occasionally we do it in winds over force 5 with the waves to go with it. That boat is surperb going upwind in those conditions and a rocket off the wind. Yes it is sailed by a professional crew, but still...

Did this Hanse 38 you had have a roller furling mainsail? I could see that with a less effective main and a small self tacking jib it might not have the chops to go to windward, and what keel?

Still, it makes me feel very satisfied that I have an all-around good boat with fin keel and a moderate overhanging bow. Handles upwind in any wind, and sails any direction and any waves with the windvane steering. For example we too sailed from Aruba to Cartegena in very strong wind behind us and big waves and the windvane steered the whole way while we relaxed.

[DMF Sailing]

Originally Posted by wingssail

This report shocks and surprises me.

I wonder if there is something about that particular design. I say this because boats with similar descriptions are very successful racing even in horrible conditions.

And I race frequently against a similar Hanse (Varianta 44) and occasionally we do it in winds over force 5 with the waves to go with it. That boat is surperb going upwind in those conditions and a rocket off the wind. Yes it is sailed by a professional crew, but still...

Did this Hanse 38 you had have a roller furling mainsail? I could see that with a less effective main and a small self tacking jib it might not have the chops to go to windward, and what keel?

Still, it makes me feel very satisfied that I have an all-around good boat with fin keel and a moderate overhanging bow. Handles upwind in any wind, and sails any direction and any waves with the windvane steering. For example we too sailed from Aruba to Cartegena in very strong wind behind us and big waves and the windvane steered the whole way while we relaxed.

Having had similar experiences to Na Mara with a Hanse 415, I can say that the main was fully battened, with slab reefing (my preferred set-up), 7' bulb keel. One reef on a beam reach in a Force Six and she's stable, fast, and fun. Upwind in sheltered racing conditions she's fast and fun despite not being able to shape the jib. Downwind in a Force 7 with decent following seas, she's a lot of work.

[Seabeau]

In Western Europe the first plumb bows were generally seen on fishing boats that had to deal with severe chop in both their fishing grounds and in their return voyage to safe harbors. Later the famous plumb bowed "British Channel Cutter" became the vessel of choice for the Pilot Boats that led shipping to safe harbor, and as usually seen, the first purely recreational boats were simply converted working boats. The later "Plank on Edge" recreational vessels which were a variation of this theme, had very heavy displacement, large sail plan, deep draft, narrow beam and plumb bows to help combat the chop.

[Chotu]

A common misconceptions see in the thread about plumb and reverse bows is they have less reserve buoyancy.

In my case, and hopefully in most cases, the bows were ADDED, not subtracted from the boat. As in, you increase the waterline length to add the plumb or reverse bow, you don’t cut away part of the deck. This adds more buoyancy, not less, but does it in a way that destabilizes harmonic oscillations (hobby horsing).

The ride is absolutely incredible in comparison to an underwater profile shaped like a U fore and aft.

On a cat, it’s really nice because your athwart ships roll is already highly damped and addicting a dampener on your fore and aft pitch makes for a really nice ride.

[Chotu]

You get it.

With the major overhang, pressing the bow into the water acts like stretching a nice, strong spring. Buoyancy goes up higher and higher at a quickly increasing rate, whipping the bow back out of the water and quite possibly, up past the DWL to a negative buoyancy relative to the weight of the boat. This causes oscillation or hobby horsing.

With a reverse bow, the maximum rate of increase in the buoyancy happens in the very beginning of the moment you press it into the water. After that, it increases at a declining rate of buoyancy production. Acting like a stretched out/broken spring. It gives too much and stops the harmonic oscillation by pushing out of phase with the input. Like a stretched out and broken spring. So you don’t get the oscillation (hobby horsing)

Originally Posted by Idlegreg

No no no - as the bows are pushed deeper the buoyancy INCREASES. It just increases more slowly (with respect to pitch angle) than on a plumb bow and much more slowly than on a forward raked bow. The question is not whether or not the buoyancy increases but how fast you want it to increase and many things might effect what is best in a particular case.


I sail a lovely 1950s sloop with quite a bit of bow overhang and a counter stern. It always amuses me when people say they don't like modern reverse sterns with bathing scoops as they lack the reserve buoyancy of a nice counter. If I moor up in a marina and walk to the next pontoon I see my tiny low and narrow counter alongside modern boats with a stern 3 x as high and 5 x as wide. The modern boats have VASTLY more reserve buoyancy if that is what you are after.


Someone mentioned before in this thread a question often forgotten. What happens below the DWL. You could have a plumb bow which turns 90 degrees just below the water. When it goes down the buoyancy progressivly increases (and with high freeboard on modern designs it carries on increasing for much longer than on my boat). But when that modern bow rises it quickly and suddenly loses buoyancy entirely. I suspect a good effect of this is a resitance to pitching. A bad effect is that horrible slamming when heading into certain waves.


I think they call it an Axe bow when a vertical stem goes deeper below the water. I have an idea that other things being equal that might give the softest ride.

[PerfectPirate]

I keep feeling that the people in this thread keep focusing on the wrong issue, probably because of the way the original question was posed. The concern around burying the bows, taking water on the stern, etc. isn't really an issue of the shape of the bow, so much as it is one of the overall hull shape, specifically: where is the centre of buoyancy? That becomes the point around which the hull pivots as the bow rises and falls. How much it pivots becomes a matter of how beamy and full the hull shape is along and below the waterline.

In the more traditional hull shapes, dating back to the 70's, 80's and earlier, the more or less equally tapered ends will place the centre of buoyancy close to the centre of the boat, with the steering position very close to the stern end. This will mean that when the bow goes down by 2 feet then the stern, and the helmsman, will rise by 2 feet. Since the cockpit is all the way aft, crew sitting in the cockpit will also feel the motion more. These traditional hull shapes also typically have overhangs, because that's the way they designed back then. These overhangs, bow and stern, do provide tremendous reserve buoyancy as they sink into waves, which will act to reduce how much either end actually sinks down, or raises up.

As I mentioned in an earlier post, modern hull shapes not only have a plumb bow, but also a long, tapering bow cross-section which moves the centre of buoyancy further back. For simplicity's sake, let's say that it moves from the centre to 2/3 back on the overall length. On a typical Hanse, which has both a plumb bow and a plumb stern, the cockpit and the helm position are still far aft. However, when the bow goes down by 2 feet the helmsman will only be raised up by one foot. For people sitting further forward in the cockpit, the motion is even less. Hence, the crew will have a correct perception that for them the motion is easier than on a traditional design. In a reverse-transom boat, like your typical Beneteau, the helm and cockpit positions are moved even further forward, thereby further reducing the apparent motion. In either case, the wide-bottomed stern possesses tremendous reserve buoyancy which helps to lift it above following waves.

However, the decreased buoyancy because of the narrowness in the forward third of the boat does mean that the tip of the bow is likely to plunge deeper than a traditional design would, making it much wetter. In conditions where a traditional design may plunge 2 feet down, a modern design may go down 3 feet or more. For the crew back in the cockpit this does not matter so much, as they still have an easier motion. It does matter when someone actually has to go forward to fix a problem or do some work. In that situation, the traditional design may well be considered to be more sea-worthy and sea-kindly. However, in general sailing the crew of the modern design may feel that their boat is more comfortable, unless they try to sleep in the V-berth while under way.

I freely admit that this is primarily theory-based reasoning. However, for anyone interested in exploring these concepts I highly recommend Ted Brewer's book, "Understanding Boat Design".

[Clivevon]

[QUOTE=PerfectPirate;3378282]I keep feeling that the people in this thread keep focusing on the wrong issue...The concern around burying the bows, taking water on the stern, etc. isn't really an issue of the shape of the bow, so much as it is one of the overall hull shape, .."

Ok, but there are other related issues. The modern boat hull is basically triangular when looked at from above. So when it heels in strong winds, the buoyancy of that broad stern keeps the aft end up, but the shape of the boat means the bow is down near the sea surface & the centre line of the boat is actually pointing down into the sea, instead of parallel with the water as it will be on a properly balanced traditional hull - plumb bow or no plumb bow. No wonder its wet up there.

So what happens is that your centreline mounted rudder is now being lifted partially out of the water & losing grip - so you cant control the boat which keeps wanting to round up, so you drop the sails & motor home in the gale - not fun.

So they eventually twigged this was a problem & put a sticking plaster on it by fitting TWO rudders - so at least one is always in the water - job done! Sterns can get even wider now - all that room down below. Except now you cant see where you are going from the middle so you have a huge wheel or - I know, we've got 2 rudders, why not have 2 wheels...

All this has nothing to do with seaworthiness but mainly stems from marinas charging by the foot length - so lets keep the boats short but stubby.

Each to their own I suppose, but laugh ?? I could cry.

[wingssail]

Originally Posted by PerfectPirate

...However, the decreased buoyancy because of the narrowness in the forward third of the boat does mean that the tip of the bow is likely to plunge deeper than a traditional design would, making it much wetter. In conditions where a traditional design may plunge 2 feet down, a modern design may go down 3 feet or more...

The narrow bow has much more leverage due to its length from the center of pitch rotation. I think it may be speculation that the bow will plunge deeper, and if the ends are kept light there may be less momentum applied to the rising and falling of the bow.

Originally Posted by PerfectPirate

I highly recommend Ted Brewer's book, "Understanding Boat Design".

This book should carry the title, "Understanding Ted Brewer's view of Boat Design", or "How I justify my Own Backwards Designs".

[DMF Sailing]

OP here: I was going to make it


How do you feel about the handling in a seaway of cruisers with plumb bows, wide beams carried aft, self-tending jibs, drop-down swim platforms, dual Jefa steering systems, fine entry, deep keel, tall rig, relatively light displacement, etc.

But that wouldn't have fit in the poll or the subject line so I cut it off. But it is generally in the original post.

Mid-thread, an astute reader suggested that I should've asked "How do you feel about modern boat design"

But that's spilt milk now

I'm reading lots of interesting things here, so I'm glad this thread has drifted like a rudderless Bristol Channel on an ebb tide in a force 1. Plumb bow and all...

[Idlegreg]

Originally Posted by PerfectPirate

.......


However, the decreased buoyancy because of the narrowness in the forward third of the boat does mean that the tip of the bow is likely to plunge deeper than a traditional design would, making it much wetter. In conditions where a traditional design may plunge 2 feet down, a modern design may go down 3 feet or more. For the crew back in the cockpit this does not matter so much, as they still have an easier motion. It does matter when someone actually has to go forward to fix a problem or do some work. In that situation, the traditional design may well be considered to be more sea-worthy and sea-kindly. However, in general sailing the crew of the modern design may feel that their boat is more comfortable, unless they try to sleep in the V-berth while under way.

I freely admit that this is primarily theory-based reasoning. However, for anyone interested in exploring these concepts I highly recommend Ted Brewer's book, "Understanding Boat Design".

It maybe that a plumb bow with fine waterlines forward would plunge deeper than a traditional overhanging bow when there is some force causing the bow to go down - though I am not sure it will be wetter if the boat has significantly more freeboard (as modern boats tend to).

Of course a common case is when your boat drives into a wave and the bow is levered upwards by the water. In that case the fine, plumb bow might lead to much less vertical movement of the foredeck compared with an overhanging bow.

So the fine, plumb bow might lead to less pitching when driving into a head sea - but even that I suspect depends on a lot else - like stern shape as you said. Or the pitch moment of inertia - the frequency of external forces imparting the pitch - etc. etc.

I think it is very hard to make basic assumptions about the net effects of all these interelated forces except I have an idea that generally the high freeboard, light weight modern boats are drier and suffer less rolling and pitching than older, heavier more banana shaped boats. I believe Hobby Horsing is a feature of old heavy boats (expecially with too much weight in the ends).

I have many fewer miles on light modern boats than I have on older boats but (apart from slamming when on the wind) those newer boats I have sailed are, in the main, less suseptible to excessive motion. But I am very happy with my heavy old boat. After all any small sailing boat is a slow, uncomfortable and expensive way to move around the world but it is still a wonderful activity - maybe some hardship is actually part of the attraction :-|