Bow shape and characteristics under sail: Overhang v straight stemmed

[silver-fox]

I have a 20 year old Moody which has the traditional overhang at the bow. When browsing a yachting mag recently I began to wonder what the difference in sailing characteristics would be compared with a more modern straight stemmed bow.

Presumably the primary effect is a longer waterline and top speed - or is it? I have always been told that the overhang delivers extra waterline length when heeling and extra buoyancy when beating upwind into a sea

Can anybody enlighten me on the pros and cons of the two designs?

 

[Tranona]

The modern style of short overhangs is to maximise the sailing length of boats that tend to sail at lower angles of heel. The long overhangs came about originally partly because rating formulae used LWL and beam as key factors - the shorter and less the better the rating. So you get long slim boats that sail at high angles of heel and the waterline effectively increases without a rating penalty. Racing tended to emphasise windward ability so such boats were at their best in those conditions, but often poor off the wind. That shape also allows long bases for the sail plan so you can maximise sail area, compounded by fact that foresail area behind the mast does not count. Look at a Contessa 32 compared with an equivalent size modern J boat and you will see the difference.

From a cruising point of view there is little real advantage to the short waterline model when load carrying and easy rig handling (plus lower angles of heel) are arguably more important. Does not mean that old style boats cannot be used successfully for cruising as many people can only afford older boats so have to use what is on offer. However, most of the latest purpose designed cruising boats have shorter overhangs and more easily handled rigs and get their power and stability from greater beam and lower ballast ratios, but carry the ballast lower.

 

[silver-fox]

Thanks for that information Tranona, which all makes sense. I am still left wondering if there aren't additional factors that affect bow design for example a trawler's bow has an overhang and they have never been subject to racing regs!

Can I ask a supplementary questions?

What about sea keeping or sea kindliness?
Doesn't having a straight stem lend to having a flat hull section forward and slamming?
Doesn't an overhang lead to a rapid increase in buoyancy as you hit a wave, causing the boat to rise more quickly, which in turn keeps the boat dryer?
Why don't yachts and small boats have the torpedo shape bow below the waterline like large ships?

Ok Ok I will quit asking questions as this is already becoming obsessive

I guess the answer will be like many other aspects of yacht design - a compromise - because every feature bring benefits and problems

 

[Tranona]

Vertical stems in themselves don't necessarily lead to slamming - that is more to do with the forward sections. My Bavaria for example is very full forward with flattish sections and tends to slam. Boats with finer bow sections - some of the Dufours for example which have very little forefoot and vertical stems tend to slam less, but lose out in interior volume.

Modern boats tend to have more freeboard and flair above the waterline which reduces the amount of water that comes aboard. Low freeboard, long overhangs and large angles of heel result in wet boats! Not sure about the use of bulbs - people have experimented with them on yachts and motor boats, but guess they offer no advantages otherwise somebody would have made them work.

As to seakindleness and seakeeping there has been masses written on the subject and "good" does not seem to be the sole preserve of any particular hull shape. The Ted Brewer comfort formula seems to confirm what makes some boats better than others but good scores are driven by length, modest beam and particularly displacement, hence the preference of many cruising people for older, more traditional heavier designs. On the other hand many modern boats seem just as capable and comfortable even though they score poorly on the formula. Maybe its a case of a formula that reflects what people observe - that is explanatory rather than predictive.

 

[outtametree]

Isn't modern cruiser hull design more about accommodation, rather than sea kindliness?

 

[Barbican]

No idea if this is right but read through it the other day when looking at their new design

http://www.gtyachts.com/files/PlumbStem_FashionOrPhysics_Jan2013.pdf

 

[Roberto]

The vertical stem provides two appropriately placed hull sides to stop those annoying anchor oscillations when one weighs anchor in a chop

 

[dunedin]

So many other design factors that bow incline is probably not a major one. And many traditional gaff types had plumb bows, albeit with bowsprit
One factor though is anchoring, often the plumb bowed boats end up with an ungainly and perhaps weak metal outrigger to hold the anchor out so it doesn't damage the bows. I always think this is a bad idea, and a slight extension of the hull ahead of the forestay to provide anchor platform and now asymetric furler would be better

 

[silver-fox]

No idea if this is right but read through it the other day when looking at their new design

http://www.gtyachts.com/files/PlumbStem_FashionOrPhysics_Jan2013.pdf

Barbican

Thanks for the link. Written by an MRINA qualified designer, it tells me exactly what I wanted to know

 

[CC@GTY]

Silver-Fox

I am he. If you need any deciphering please fire away. I like your use of the highly raked fishing vessels example. I probably could have used that in the article as a contrast. The long fine bow of a trawler is to maintain a fine entry whilst also rapidly increasing reserve buoyancy.

I have other articles in the pipeline which also seek to unpick fashionable hull characteristics.

Conrad.

 

[silver-fox]

Silver-Fox

I am he. If you need any deciphering please fire away. I like your use of the highly raked fishing vessels example. I probably could have used that in the article as a contrast. The long fine bow of a trawler is to maintain a fine entry whilst also rapidly increasing reserve buoyancy.

I have other articles in the pipeline which also seek to unpick fashionable hull characteristics.

Conrad.

I really would like you to expand on your comments on your web site .....which I have cut and pasted below. (Obviously you have already covered the bow design)

"Well, although there are some aspects of modern styling which do not necessarily affect performance (and can therefore be adopted), there are other aspects which have found their way into modern cruiser design which affect performance greatly. Lack of a back stay, full beam waterline aft to accommodate twin aft cabins in small yachts, fully vertical or plumb bow, 'T' keel to name but a few."

 

[CC@GTY]

OK no probs.

Lack of back stay. Well, with modern design tools and materials it's pretty straight forward to find the load paths and engineer the appropriate structure to accommodate it, for the vast majority of situations. The problem with having no back stay is that in order for your mast to stay up in the vast majority of situations, the shrouds must come aft, closing off the available boom angle. Two issues here; the main sail cannot open as wide as you'd like resulting in (without an arrestor) an increased likelihood of a gybe and your main gets more of a beating from the spreaders. Taking out the back stay is not the result of an analysis to increase sailing performance. It is the result of somebody wanting a clear transom and aft deck when parked.

Waterline beam. Growing up in rainy Cornwall, I used to surf as a youngster. A good surfboard is flat. That's what Volvo ocean yachts do - they surf around the world on a beam reach. You don't. You also like to go up wind! As you heel with full beam a long way aft, the stern gets lifted and the bow will plunge. It's just a lot harder to find your groove but if fighting it is your thing, then fine.

'T' Keel. A racing fin tends to posses a higher aspect ratio (ratio of span to chord) due to the associated lower drag. For a given draught, the fin becomes narrower and if the longitudinal position of the ballast must remain in the same place, the leading edge of the fin moves aft and exposes the nose of the bulb. Hence you have a protrusion resulting in the characteristic T keel. Fine if you're racing. But you're not. You're wafting around trying to find wind and if you haven't got any keel plan area you'll get pushed sideways due to low speed producing no lift. Cruising keels need area to get more lift at lower speeds.
The other obvious point here is the safety one. I find that however many times I'm able to remember to shorten up the painter, I often find I've forgotten. It's in this state that the emergency shove into reverse will result in the (now slack) painter going straight for the prop. If this happens when I've hooked next-door's anchor chain round my T keel then we're in for a long morning. Could be worse of course.

Conrad.

 

[geem]

So many other design factors that bow incline is probably not a major one. And many traditional gaff types had plumb bows, albeit with bowsprit
One factor though is anchoring, often the plumb bowed boats end up with an ungainly and perhaps weak metal outrigger to hold the anchor out so it doesn't damage the bows. I always think this is a bad idea, and a slight extension of the hull ahead of the forestay to provide anchor platform and now asymetric furler would be better

The other thing I suspect is its just fashion. You have a fashionable vertical bow so to stop anchor contact you put one of those little bow extensions on to move the anchor away. You pay for length overall in the marina so it cost you as much as the older boat with the raked bow but dont have the advantage of reserve boyancy.

 

[maby]

The other thing I suspect is its just fashion. You have a fashionable vertical bow so to stop anchor contact you put one of those little bow extensions on to move the anchor away. You pay for length overall in the marina so it cost you as much as the older boat with the raked bow but dont have the advantage of reserve boyancy.

Not sure I understand your final comment - the vertical bow is very much a marina thing - I pay for every foot of overall boat length and the vertical bow means that every foot I pay for is useable accomodation.

 

[silver-fox]

OK no probs.

Lack of back stay. Well, with modern design tools and materials it's pretty straight forward to find the load paths and engineer the appropriate structure to accommodate it, for the vast majority of situations. The problem with having no back stay is that in order for your mast to stay up in the vast majority of situations, the shrouds must come aft, closing off the available boom angle. Two issues here; the main sail cannot open as wide as you'd like resulting in (without an arrestor) an increased likelihood of a gybe and your main gets more of a beating from the spreaders. Taking out the back stay is not the result of an analysis to increase sailing performance. It is the result of somebody wanting a clear transom and aft deck when parked.

Waterline beam. Growing up in rainy Cornwall, I used to surf as a youngster. A good surfboard is flat. That's what Volvo ocean yachts do - they surf around the world on a beam reach. You don't. You also like to go up wind! As you heel with full beam a long way aft, the stern gets lifted and the bow will plunge. It's just a lot harder to find your groove but if fighting it is your thing, then fine.

'T' Keel. A racing fin tends to posses a higher aspect ratio (ratio of span to chord) due to the associated lower drag. For a given draught, the fin becomes narrower and if the longitudinal position of the ballast must remain in the same place, the leading edge of the fin moves aft and exposes the nose of the bulb. Hence you have a protrusion resulting in the characteristic T keel. Fine if you're racing. But you're not. You're wafting around trying to find wind and if you haven't got any keel plan area you'll get pushed sideways due to low speed producing no lift. Cruising keels need area to get more lift at lower speeds.
The other obvious point here is the safety one. I find that however many times I'm able to remember to shorten up the painter, I often find I've forgotten. It's in this state that the emergency shove into reverse will result in the (now slack) painter going straight for the prop. If this happens when I've hooked next-door's anchor chain round my T keel then we're in for a long morning. Could be worse of course.

Conrad.

Thanks for passing on your thoughts. It certainly has filled in a few answers for me. I know I am guilty of thread drift here....but I am allowed as the OP!

Can I ask what impact does having a broad flat bottomed stern have on the motion of the boat with a sea on the quarter? Will it roll more or less than a more traditional cruising design?
Secondly am I right in assuming these designs based on the "surfers" are relying on hull form rather than ballast for stability? That being the case aren't they also likely to be equally stable when inverted? And if that is true aren't modern designs vulnerable in the same way that the fleet in '79 Fastnet race? (I have a hazy memory of reading about this problem in one of the books about the race?)

 

[maby]

...
Secondly am I right in assuming these designs based on the "surfers" are relying on hull form rather than ballast for stability? That being the case aren't they also likely to be equally stable when inverted? And if that is true aren't modern designs vulnerable in the same way that the fleet in '79 Fastnet race? (I have a hazy memory of reading about this problem in one of the books about the race?)

Not sure that the two go together - we have one of the widest, flattest on the market currently but it still has a very large amount of cast iron hanging off the bottom. I think there are very few boats in production that will self-right if fully inverted.

 

[silver-fox]

Not sure that the two go together - we have one of the widest, flattest on the market currently but it still has a very large amount of cast iron hanging off the bottom. I think there are very few boats in production that will self-right if fully inverted.

You are right. You can have wide and flat plus ballast. The problem I was describing (badly as it turned out) was that wide and flat can mean very stable either way up. A narrower more traditional design is unlikely to stay inverted for long in the conditions that will roll it in the first place. This is not the case <say> with a catamaran.

This then gets us into the classic debate about which design to you prefer...... 1) a raft or 2) a tree trunk with a nail in it as ballast?

The tree trunk will always self right because of the nail, but has no usable stabilty as it can be inverted using little energy. Conversely the raft is very stable but even when inverted - which can be inconvenient too! Somewhere between the two is a compromise designers have to make. My question was an attempt to get an informed opinion on the modern hull shapes currently produced by many of the major boat manufacturers.

To my eye current designs seem to be driven by a certain set of "quayside instant appeal" characteristics that will drive sales at boat shows, which is quite understandable. Typically these characteristics are driven by "Med style" requirements. For example:-

1. Easy access via stern of boat
2. Bathing platforms
3. Maximisation of accommodation leading to wider, flat bottomed higher hulls with vertical stern and bows
4. Lightness, presumably to reduce costs and deliver speed in high winds
5. Large cockpits for the charter market and for entertaining
6. Large windows
7. Raised cabin soles to enable Deck saloons
8. No back stays
10 Double steering positions, double rudders
11 Use of spade rudders unprotected by either keel or skeg

Some of these modern features will stand the test of time, some may be down right disadvantageous once you cast off. Some, like a large cockpit, may suit depending on the sort of sailing you do. One design cannot suit all.

Boat design is evolving, stimulated by new materials and new ideas. I am just trying to learn more from a designer what the effect (or cost if you like) of each of these design decisions really is.

 

[alant]

I have a 20 year old Moody which has the traditional overhang at the bow. When browsing a yachting mag recently I began to wonder what the difference in sailing characteristics would be compared with a more modern straight stemmed bow.

Presumably the primary effect is a longer waterline and top speed - or is it? I have always been told that the overhang delivers extra waterline length when heeling and extra buoyancy when beating upwind into a sea

Can anybody enlighten me on the pros and cons of the two designs?

If your talking about the old Moody knuckle bow, always thought this was supposed to deflect the bow wave & keep the deck drier.

 

[silver-fox]

If your talking about the old Moody knuckle bow, always thought this was supposed to deflect the bow wave & keep the deck drier.

No that was the earlier Moody 40 and preceding designs. The Moody 425 has a "conventional" bow - no knuckle, but for all that a very dry boat.

See link

http://www.moodyowners.net/Moody_Archives/32_pic.jpg

 

[Tranona]

You are right. You can have wide and flat plus ballast. The problem I was describing (badly as it turned out) was that wide and flat can mean very stable either way up. A narrower more traditional design is unlikely to stay inverted for long in the conditions that will roll it in the first place. This is not the case <say> with a catamaran.

This then gets us into the classic debate about which design to you prefer...... 1) a raft or 2) a tree trunk with a nail in it as ballast?

The tree trunk will always self right because of the nail, but has no usable stabilty as it can be inverted using little energy. Conversely the raft is very stable but even when inverted - which can be inconvenient too! Somewhere between the two is a compromise designers have to make. My question was an attempt to get an informed opinion on the modern hull shapes currently produced by many of the major boat manufacturers.

To my eye current designs seem to be driven by a certain set of "quayside instant appeal" characteristics that will drive sales at boat shows, which is quite understandable. Typically these characteristics are driven by "Med style" requirements. For example:-

1. Easy access via stern of boat
2. Bathing platforms
3. Maximisation of accommodation leading to wider, flat bottomed higher hulls with vertical stern and bows
4. Lightness, presumably to reduce costs and deliver speed in high winds
5. Large cockpits for the charter market and for entertaining
6. Large windows
7. Raised cabin soles to enable Deck saloons
8. No back stays
10 Double steering positions, double rudders
11 Use of spade rudders unprotected by either keel or skeg

Some of these modern features will stand the test of time, some may be down right disadvantageous once you cast off. Some, like a large cockpit, may suit depending on the sort of sailing you do. One design cannot suit all.

Boat design is evolving, stimulated by new materials and new ideas. I am just trying to learn more from a designer what the effect (or cost if you like) of each of these design decisions really is.

You could have drawn up a similar list of undesirable characteristics when Angus Primrose introduced the Moody 33 and 36 onto the scene in the mid 70's. It would have been drawn up by people who had been brought up in deep long keels, small cockpits, gaff and ketch rigs, transom hung rudders and so on. Indeed the list would have featured in all the reviews of the boat at the time.

However it set the scene for a style of boat that dominated the market for the next 30 years or so, reflecting the fact that it suited the main market, that is North European coastal waters sailing, very well. Buyers wanted boats that could stand up well in cold wet environments and had plenty of warm below decks space. Such characteristics also turned out to suit liveaboards when that became popular - even though there were significant shortcomings when used in warmer climates. No different really from the previous generation that used workboat derived hull forms for the same purpose - because they were available.

Things have moved on, though and now the main market for cruisers is warm water orientated, particularly the Med, so unsurprisingly builders tend to design boats suited to that environment because that is what buyers want. But just as in previous generations such boats get used in other ways - look at the ARC entries to see what I mean.

The thing about ultimate stability is in practice overrated in my view. The number of times a boat gets into a situation where this is an issue is tiny. Most boats are used well within their capability and owners will chicken out long before capsizing becomes an issue. More important is being able to sail the boat comfortably within its capability and modern designs with easy handling are just as good in this respect as older boats.

In 20 or 30 years time, cruisers of all sorts will be happily buying today's style of boat when the price comes down to affordable levels and probably moaning about the unsuitability of what will then be "modern" boats - but actually a bit like today it might reflect the unaffordability of such boats as much as their unsuitability.

Funnily enough having been brought up on "traditional" boats - in my case a heavy long keeler, I had the same view about modern boats until I could afford to buy a new one and then realised what I was missing!