Why would you not want an easily driven hull

[doug748]

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..)

I once sailed on a 57 footer called Silkie, designed for singlehanded ocean racing, beam under 10ft and a tiny aft double cabin. Looking back I am sure it must have been designed on the lines of those skerry cruisers.
Terrific sailing experience, I don't know how fast because very little on board worked! But a great trip, Plymouth - La Rochelle in short order.

.

 

[LONG_KEELER]

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?

"What's love got to do with it " ?

Everything...................................?

 

[Laminar Flow]

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?

Frictional resistance due to wetted area rises only very gradually as speed increases. At a relative speed of 0.7 (x sqr root DWL) the curves for frictional resistance and form/wave making resistance begin to diverge and at a speed factor of 1.00 wave making rises sharply and exponentially.
Generally, a boat is designed to reach "hull speed" in a F4. These are the conditions in which most recreational sailing takes place. To achieve this requires a certain SA/Displ. ratio. Comparing two designs of equal SA/Displ ratio, one heavy and one of light displacement, the heavy one will outperform the other in light winds. The reason for this is that the heavier boat carries proportionately more sail, resistance at this point due to wave making is less significant and of course sail carrying ability in light conditions is usually not a problem.
This explains the phenomena why some supposed slug can outsail a racer in light going.
Both boats should, theoretically, reach "hull speed" at the same time and it is at this point that they now part company. The lighter boat will have an extended range of speed as it has a less steep curve of form resistance and a boat with a length/ displacement ratio of under 100 and an SA of 400sqrfeet/ton will, at this point (F4), start to plane.

 

[johnalison]

Frictional resistance due to wetted area rises only very gradually as speed increases. At a relative speed of 0.7 (x sqr root DWL) the curves for frictional resistance and form/wave making resistance begin to diverge and at a speed factor of 1.00 wave making rises sharply and exponentially.
Generally, a boat is designed to reach "hull speed" in a F4. These are the conditions in which most recreational sailing takes place. To achieve this requires a certain SA/Displ. ratio. Comparing two designs of equal SA/Displ ratio, one heavy and one of light displacement, the heavy one will outperform the other in light winds. The reason for this is that the heavier boat carries proportionately more sail, resistance at this point due to wave making is less significant and of course sail carrying ability in light conditions is usually not a problem.
This explains the phenomena why some supposed slug can outsail a racer in light going.
Both boats should, theoretically, reach "hull speed" at the same time and it is at this point that they now part company. The lighter boat will have an extended range of speed as it has a less steep curve of form resistance and a boat with a length/ displacement ratio of under 100 and an SA of 400sqrfeet/ton will, at this point (F4), start to plain.

There is no doubt that the latest boats will slip along faster at near-planing speeds. I remember sailing with a poled-out jib and reefed main in about 30kn true and enjoying myself with the occasional push up to 10kn, peaking at 11.5 in my 34. We were passed by a modern-looking boat with full main alone. It was too far away to identify but looked something like a Dehler38 or maybe about 40. I wasn't surprised to be passed but what impressed me was how little wash and spray she threw up, compared to the relative fuss that my boat was making, though I never felt out of control in any way.

 

[Buck Turgidson]

Easily driven you say?

 

[LONG_KEELER]

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.

Re: Marchaj book.

Read it some years ago but found it very hard going.

The book I think majored on the Fastnet disaster and made some very useful comparisons between older and newer designs.

He made a big thing of dangerous rolling in modern designs which don't have much in the water and rely more on form stability. Large breaking waves in particular. You don't hear much about this now . Perhaps the RTW flyers
have found a technique to minimise the problem.

 

[Laminar Flow]

Easily driven you say?

Easily driven you say?

It is interesting to look into the numbers of these skerry cruisers. In light airs they are very fast and so they should be with a SA/Displ ratio of 18.5 (30m skerry cruiser by A Neesem). They are actually very light boats, considering. A 39' skerry cruiser displaces a mere 5000lbs and has a very flat run. If you look at the effective waterline length (see above pic) their Length/Displmt ratio is only 155. Ballast ratio is near 44%.
So, all in all their performance is not exactly a miracle, getting any accommodation in those 39' x 6.5' is.

 

[Laminar Flow]

Re: Marchaj book.

Read it some years ago but found it very hard going.

The book I think majored on the Fastnet disaster and made some very useful comparisons between older and newer designs.

He made a big thing of dangerous rolling in modern designs which don't have much in the water and rely more on form stability. Large breaking waves in particular. You don't hear much about this now . Perhaps the RTW flyers
have found a technique to minimise the problem.

Agree, some of his book, "Seaworthiness, the Forgotten Factor", is not immediately accessible. I keep it on my nightstand in the hope that repeated application might help me fall asleep more easily and perhaps assist me in overcoming my natural density.

What I like about it is, that he very carefully and scientifically constructs his case for what he considers are desirable features and qualities in an oceanworthy small boat. How relevant that may be is another question, as 95% of sailors are not very likely to experience conditions that might make such aspects a necessity.

I believe the technique found by the RTW flyers to minimize the problem is to employ modern telecommunications for weather routing, so as not to be where the proverbial hits the fan in the first place.

 

[TernVI]

Easily driven you say?

You call that 'easily driven'?
Maybe it's esily driven at 2/3 of that speed, but in the picture it appears to be driving some big bow wave. That takes a lot of power!

 

[Zagato]

Similar to this, powering through waves down wind on or past the limit at times...

 

[Laminar Flow]

You call that 'easily driven'?
Maybe it's esily driven at 2/3 of that speed, but in the picture it appears to be driving some big bow wave. That takes a lot of power!

These boats were never designed for and are unsuitable for the open sea - they simply do not carry enough buoyancy in their ends. This will also add to the impression of being over pressed in hard going and finally, they are still displacement hulls and simply do not have sufficient surface area versus displacement to get up on a plane, but that was not the original OP's premise either.
Within their own parametres they still compete successfully in many Skandinavian races (under Skandicap rules) - it really all depends on how soon, compared to other designs, that these boats reach their "2/3 of that speed".
Since we neither know wind speed nor the actual boat speed of the vessel in the image, any comment on how easily driven she might be is pure conjecture. I would guess that she has probably reached her practical limit - perhaps around 9-10kts?

 

[TernVI]

These boats were never designed for and are unsuitable for the open sea - they simply do not carry enough buoyancy in their ends. This will also add to the impression of being over pressed in hard going and finally, they are still displacement hulls and simply do not have sufficient surface area versus displacement to get up on a plane, but that was not the original OP's premise either.
Within their own parametres they still compete successfully in many Skandinavian races (under Skandicap rules) - it really all depends on how soon, compared to other designs, that these boats reach their "2/3 of that speed".
Since we neither know wind speed nor the actual boat speed of the vessel in the image, any comment on how easily driven she might be is pure conjecture. I would guess that she has probably reached her practical limit - perhaps around 9-10kts?

Indeed. I wasn't intending to be critical of the boat.
The people who race boats like that will drive them as hard as they can.
However much power is in the sails, that's how hard the boat is driven.

For a lot of racing, and UK sailing generally, 'easily driven' in light airs is more often important than high speed performance in a good breeze.
Some fleets in that non=planing genre deliver excellent racing.

I wonder if some of the more modern cruiser types are copying some racer 'styles' to the detriment of light weather performance?
Maybe that doesn't worry the 'stick the engine on if we can't sail at 5 knots' brigade?

 

[anoccasionalyachtsman]

These boats were never designed for and are unsuitable for the open sea - they simply do not carry enough buoyancy in their ends.

You've said this before, but the owners of the Swede 55s that won the Transpac and Bermuda races in 'bad weather' years might beg to differ.

 

[Laminar Flow]

I wonder if some of the more modern cruiser types are copying some racer 'styles' to the detriment of light weather performance?

There is no doubt that racing has had a powerful influence on many cruising designs, regardless of the fact that many racers where designed to beat a rule as much as the next boat. The Skerry cruiser is a point in case. So you limit SA and waterline length, what do you do? You can't really increase SA, but you can at least make it as efficient as possible, to weather, with tall, skinny foils. On the bottom end you add these sexy long overhangs and pick up another 20% of (displacement) speed as the thing goes on it's ear and you make it as skinny as possible to reduce frontal projection and form resistance. In the end it produces a relatively fast, but not a well-rounded, seaworthy boat. IOR went on to produce it's own crop of of rule influenced cruising and handling cripples.
In some ways the current rules are perhaps better, since there is less punishment for hull features that enhance ultimate speed. But, for a boat to be fast it must be light and in cruising terms that is not easily achieved, particularly in in the smaller sizes.

Once one comes to terms that a displacement hull is a defined system in terms of speed, it makes more sense to make a boat go faster at the bottom end of the speed scale, i.e. improve light weather performance by reducing low speed drag and increasing light weather sail.

 

[Laminar Flow]

You've said this before, but the owners of the Swede 55s that won the Transpac and Bermuda races in 'bad weather' years might beg to differ.

A Swede 55 is a long way along the evolutionary path from the 1930ies Skerry cruisers by Anker and Neesem and indeed the one in the picture posted and in terms of seaworthiness: size does matter.

 

[anoccasionalyachtsman]

A Swede 55 is a long way along the evolutionary path from the 1930ies Skerry cruisers by Anker and Neesem and indeed the one in the picture posted and in terms of seaworthiness: size does matter.

But here

Swede 55...a very sleek beauty

You said.

Direct descendant of the Schaeren Kreuzer (Skerry cruiser). Fast & weatherly and never really intended to go offshore.

 

[Laminar Flow]

But here

Swede 55...a very sleek beauty

You said.

Designed to sail in the sheltered waters of the Schaeren/Skerries. Fact.
Slocum's spray, as an inshore oyster dredger, was also never conceived to sail around the world, nor the Wayfarer dinghies that have been cruised across the open sea or the folding kayaks that have crossed oceans. People are always happy to push the envelope. Hell, I even heard of some who crossed the Atlantic in retired river barges and a fellow who circumnavigated in a Drascombe Lugger .

 

[jimi]

Its all about the spreader to porthole ratio. The higher it is the faster the boat.