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Why would you not want an easily driven hull

Chae_73

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To answer the question of why you may not necessarily want the most easily driven hull, have a dip into "seaworthiness" by Marchaj.

Maybe he's not totally "right", but some interesting stuff in there.
 

Laminar Flow

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Most of what was said above is correct, but it is a little less simple than that.
Low wetted area reduces resistance. This is predominantly an issue at lower speeds where it makes up a significant portion of total resistance. The added resistance due to wetted area is easily overcome by a modest increase in sail area.

Then there is form resistance which is closely tied to displacement - the heavier the boat the greater the frontal projection that needs to be pushed through the water and wave making/form resistance rises sharply and exponentially with increasing speed. This is the reason heavier boats have practical limits to their speed potential.

The next factor that governs resistance in displacement craft is prismatic coefficient. The CP basically describes how full or fine ended a hull is. Interestingly, each CP value has it's own optimal speed range. A fine ended hull with a low CP of say, 0.48, is easily driven in light airs and at slower speeds. At higher speeds it will tend to suck up a nasty quarter wave as resistance rises faster than in a vessel with a higher CP. Vessels with higher CPs (0.60+) and fuller ends excel at high reaching and running speeds. My own boat which is, ironically and even by some of it's most dedicated proponents, considered to be something of a slug has a CP of 0.61. Fitted out with more sail than the factory standard storm rig, she can easily sustain such higher speeds, just not dead to weather.
Designers generally compromise with a CP of 0.56 to 0.58, which favours windward performance in a fresh breeze. Going slow with a high CP is less of a disadvantage, in terms of resistance, than trying to go fast with a low CP.

Induced resistance is caused at the bottom of the keel as water from the pressure side escapes to the low pressure side causing turbulence. The longer the keel, the greater the area of induced drag. On the other hand, high aspect foils are at greater risk of stalling at lower speeds and in turbulent conditions.

Marchaj makes a very convincing case for moderation in all things, but in particular for a long keel.

A longer waterline does not reduce resistance; quite the opposite. It does however increase the displacement wave length and hence the potential speed, given sufficient power (and more of it) of course.

All of the above of course assumes that the hulls are hydrodynamically optimized. This is, unfortunately, rarely the case.
Some of us drag 3-bladed props around and even designers and yards do not always do their best. There are plenty of instances of poor rudder shapes, apertures, abysmal deadwood treatments and blunt "traditional" stems and less-than- optimal keel forms.
 

Carib

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Interesting stuff above - thanks!

Low wetted area reduces resistance. This is predominantly an issue at lower speeds where it makes up a significant portion of total resistance. The added resistance due to wetted area is easily overcome by a modest increase in sail area.
Hang on a minute - so although you always hear 'all that wetted surface area' is a negative in relation to long-keelers, this makes virtually no difference given a little bit of a breeze and a decent amount of sail area?
 
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flaming

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and excluding going on the plane , what defines an easily driven hull ?
The short answer is accommodation.

Some of the most popular boats ever made do not have what you'd call "easily driven" hulls. The Westerly Centaur for example.

The bigger question though is if you actually need what was traditionally known as an easily driven hull to be fast in a cruising context any more....
 
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TernVI

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Interesting stuff above - thanks!



Hang on a minute - so although you always hear 'all that wetted surface area' is a negative in relation to long-keelers, this makes virtually no difference given a little bit of a breeze and a decent amount of sail area?
Except the sail area needs more ballast, so you need more buoyancy to support that, so you get more wave drag as well.
 

TLouth7

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The obvious answer is: when you have unlimited or excess sail area.

Mini 6.5s, IMOCA 60s etc (ignoring foiling) are not limited by the drag of their hulls, they are limited by the righting moment they can generate. A very beamy scow shape can achieve greater righting moment in a given length than a sharp bow, despite having higher drag in most conditions. The increased sail area carried overcomes the increased drag giving a net racing benefit.

The opposite situation is where you have limited power (sail area) and so want to minimise drag. In this case you build an easily driven hull which is very long and narrow, with hydrodynamically optical lines. The classic examples are the Scandinavian square meter classes.

Of course cruisers don't want to pay for a marina berth to hold all that length, nor do we want the cramped accommodation or wet ride, so tend to go for boats that fall somewhere in the middle.
 

Carib

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The opposite situation is where you have limited power (sail area) and so want to minimise drag. In this case you build an easily driven hull which is very long and narrow, with hydrodynamically optical lines. The classic examples are the Scandinavian square meter classes.

Of course cruisers don't want to pay for a marina berth to hold all that length, nor do we want the cramped accommodation or wet ride, so tend to go for boats that fall somewhere in the middle.
I'd guess these two boats illustrate this point quite well (that's a Swede 55 on the right). Very easily driven I suspect, not much accommodation, rather striking!

Swede-55-Deckhouse.jpg
 

Neeves

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People buy yachts for characteristics other than reputedly having easily driven hulls. One example being - if the wife loves one design over another, because she likes the accomodation and galley - who is going to try to persuade her to forget that and instead have a different yacht that might get home 20 minutes more quickly and that has poky and confined accomodation.

People who review yachts do mention that yacht 'X' has an easily driven hull, you will seldom see reviews where the reviewer says a bathtub would be more easily driven than the latest apparition from boatyard 'Y'. Older designs might have 'less easily driven hulls' but then money might be the deciding factor.

Its all a compromise.- - unless you buy a multihull and then they go like the clappers off wind and you stay at anchor if its going to be a long beat to windward in breaking seas :)

Jonathan
 

johnalison

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I wouldn't want a very old-fashioned boat with over 50% ballast and limited sail area, but nor would I want a beamy racer with next to no forefoot that broached to every puff of wind. For cruising comfort and decent performance I look for a fine entry and moderation in other respects. My boat is twenty years old, and therefore a '90s design but it can still outperform many new boats in all winds, though its shorter waterline puts it at a disadvantage at higher speeds downwind. If I were starting again I might go for something more modern, but sparkling performance in light airs is not a cruising necessity.
 

flaming

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I wouldn't want a very old-fashioned boat with over 50% ballast and limited sail area, but nor would I want a beamy racer with next to no forefoot that broached to every puff of wind. For cruising comfort and decent performance I look for a fine entry and moderation in other respects. My boat is twenty years old, and therefore a '90s design but it can still outperform many new boats in all winds, though its shorter waterline puts it at a disadvantage at higher speeds downwind. If I were starting again I might go for something more modern, but sparkling performance in light airs is not a cruising necessity.
The stereotype of the broaching racer is not really accurate any more... The drive towards shorthanded sailing has resulted in a lot of designs that are designed to spend a lot of time under autopilot. A broach happy boat would be no good at all. Sailed even remotely sensibly the modern designs with a fair amount of beam forward, twin rudders and moderate fin keels from Sunfast, JPK, Pogo etc go upwind and downwind as if they are on rails.
It's the previous generation of skinny IRC designs with big bulbs on the bottom of skinny foils and deep skinny rudders, the Corby, Summit, King etc that are more broach happy.
We went from a pretty traditional C/R, an Elan, to a JPK. My stress levels (I'm the helm) have reduced dramatically. It's so, so much easier to drive in gusty conditions.

The point about wetted surface area is that it depends what it's doing. If it's principally in beam and providing righting moment and allowing you to either set more sail or specify a lighter keel, then overall you're increasing the performance potential of your boat in everything but very light winds. If it's just bulk in the water and your cross section is pretty rounded and not providing much form stability then it's just load carrying ability.

Just depends on what type of boat you want...
 

Blueboatman

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I have always lusted after the IDEA of one of those Swede 55 or Skerries cruisers as I know them.
I once owned ( and it ended in tears) 40ft of rotten beautiful metre boat . Canoe body and 2metre draft long-ish keel. Long low and lean, with 3 spreader 7/8 rig..
Accommodations ??😄 Your knees touched when sitting across from another in the saloon .. The forecabin had a loo and a tapering tunnel of sail storage and nowt else .
But .. oh man, as a sailing machine💨💨 it simply flew under sail . Cruised effortlessly under reduced canvas ..
Was very very easily driven . And would surf wave after wave in half a blow. Quite extraordinary , the whole boat would get on a bow wave that came back to the mast and would palpably accelerate like planing dinghies of my youth, the long old iron tiller would tremble, the whole boat vibrate, the deep longish keel felt well enoughplanted , the boat would ‘ surf’ like this for two or three boat lengths (?) at a time then slow,slump a bit then pick up on the next gust of wind and wave and, off again..
Quite extraordinary and , er, did I say very easily driven?

Laminar Flow has summarised Marchaj beautifully , iirc. (Long time since I read his theory though and I’m sure that I didn’t grasp a lot of it then! )

What is left out -and it isn’t much-when considering “why not easily driven?” is the horrendous cost of parking long lean easily driven yachts in popular uk harbours. Ouch! Fleecing like an Aussie sheep shearer on a bonus deal😳.
And the toll of a carp wet summer on their crew wedged all down below .
One might even conclude : Easily driven ?
To drink! 😳
(I will get my sou’wester..)
 
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johnalison

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The stereotype of the broaching racer is not really accurate any more... The drive towards shorthanded sailing has resulted in a lot of designs that are designed to spend a lot of time under autopilot. A broach happy boat would be no good at all. Sailed even remotely sensibly the modern designs with a fair amount of beam forward, twin rudders and moderate fin keels from Sunfast, JPK, Pogo etc go upwind and downwind as if they are on rails.
It's the previous generation of skinny IRC designs with big bulbs on the bottom of skinny foils and deep skinny rudders, the Corby, Summit, King etc that are more broach happy.
We went from a pretty traditional C/R, an Elan, to a JPK. My stress levels (I'm the helm) have reduced dramatically. It's so, so much easier to drive in gusty conditions.

The point about wetted surface area is that it depends what it's doing. If it's principally in beam and providing righting moment and allowing you to either set more sail or specify a lighter keel, then overall you're increasing the performance potential of your boat in everything but very light winds. If it's just bulk in the water and your cross section is pretty rounded and not providing much form stability then it's just load carrying ability.

Just depends on what type of boat you want...
Racers are certainly very weatherly, but reading reviews of AWBs suggests that early reefing and a tendency to broach are not uncommon in cruising designs. Size is a significant factor in stability, and today's 40-footers are inherently better able to stay upright than yesterday's more common 32s. As I understand it, wetted area accounts for a significant proportion of drag up to five knots or more, which I always find surprising.
 

Ado

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Its all a compromise.- - unless you buy a multihull and then they go like the clappers off wind and you stay at anchor if its going to be a long beat to windward in breaking seas :)

Jonathan
[/QUOTE]

As you appear to be based in Australia I'm wondering why the designs of Schioning and Lidgard don't work to windward over there where here they seem well able to see of most monohulls, along with our native Dazcat's and the French Lerouge and Outremer cats.
 

Zagato

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Would or could you factor in not only forward resistance but sideways resistance. Isn,t that where one advantage of a long keel is. You may go faster with a fin keel but you may lose the advantage if your direction isn,t as good!?

I buy my boats on looks first, my wife has no say, she is not interested thankfully... I wouldn,t know a good sailing boat from a poor one but thankfully what to me is a good looking boat happens to be a good sailer apparently. That Swede 55 is gorgeous, never seen one before...

How can you find out what the CP rating is on your boat?
 

flaming

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Racers are certainly very weatherly, but reading reviews of AWBs suggests that early reefing and a tendency to broach are not uncommon in cruising designs. Size is a significant factor in stability, and today's 40-footers are inherently better able to stay upright than yesterday's more common 32s. As I understand it, wetted area accounts for a significant proportion of drag up to five knots or more, which I always find surprising.
We've covered this subject frequently in many threads... There was one recently where the reviewers talked about a boat broaching, and described what they were doing sailing it. And I just thought, "well it will broach if you sail it like that." The techniques used to keep a more traditional boat on its feet, and those used to keep a more modern boat on its feet are a little different. So it's of no great surprise that testers who sail every boat as if it's a Contessa 32 find that more modern boats react differently. And that's more to do with rig than hull, principally that the jib is more important on a modern fractional rig to stopping the boat spinning out upwind than it was on older masthead rigs. But the reviewers in question hadn't touched the jib and were just dumping main in the gusts and complaining that the boat wasn't reacting as their older boats would.

I think the current "evolution/revolution" in cruising designs with every new design pushing the beam further forward and sprouting chines that are just getting longer and longer is extremely interesting. When I saw the pictures of that new Jeanneau that Sunsail are replacing the F40 fleet I sent some er, disparaging, messages to current Sunsail skippers who were similarly not all that impressed with the tubby look of the thing. But with some sailing on it, they report that it's faster upwind and down, and extremely well mannered. Which I find very interesting.
 
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