My AWB achieved Hull Speed - What did I do right?

[Laminar Flow]

Froude's entire career was based on establishing the concept of "relative speed". It allows us to compare the "relative speed of different size vessels. It is known as the Froude number, a dimensionless ratio. It determines the speed of a free running wave: L = 2*Pi*V^2/g, where Pi is 3.14 and g is gravity. This can be reducted to the formula we all know which is: sqr root DWL x a factor of 1.34, which calculates hull speed, when the movement of a hull through the water generates one wave at the bows and one at the stern with a single trough between.
Other "relative speeds" have different wave patterns and , correspondingly, different factors.
Gratifyingly, displacement hulls behave in fairly similar ways.

A relative speed of 0.9, i.e. of sqr root DWL*0.9, is significant, because ,in the related curves for hull resistance, this is near the point where frictional and form resistance diverge and form resistance rises exponentially.
In other words, beyond this speed it requires exponentially more power to reach higher speeds.

Re flat water: Turbulence, regardless what caused it, has a significant effect on lift over a foil. This is the same for keel and rudder foils as it is for sails, where turbulence can be caused by any number of things, including pitching, yawing, rolling etc.

The angle of incidence of wind over a sail actually varies very little from a reach on up. The real problem is, that the vector responsible for the drive may not be pointing entirely in the direction you wish to be going. Closehauled, a sail is still producing the same amount of lift it did on a reach. Sadly, it is simply simply investing a greater portion of it in heeling the boat. You will actually find that in speed polar diagrams for displacement type craft, typical of cruising boats, the variance of speeds, closehauled, reaching and running, is minimal. In this context, I highly recommend the viewing of Fig. 179 & 180, Pg 293 in Marchaj, Theory and Practice of Sailing.

For my comments on how displacement relates to "relative speed" and how much power it requires, I suggest Dave Gerr, "The Nature of Boats". It also contains a chapter on how to make realistic predictions on sailing speeds.

I have a rather useful book called "Motorsegler" (Motorsailer). As the Title suggests, it's in German. As that may make it a little less accessible your side of the Channel, it has some rather interesting data based on tank tests for folding, feathering (3-Blade) and fixed 2 & 3 blade props.

At a relative speed of 0.9 (see above) the speed loss, in %, for different props is (to put that into context I have added the loss in speed for our 28.25 DWL boat at 4.8 kts):
Fixed 3-blade free to rotate 8% (0.4kts)
Feathering and fixed 2- blade 4% (0.19 kts)
Folding - 1% (0,05 kts)

At hull speed (7.12kts, relative speed of 1.34))
Fixed 3-blade, free to rotate - 4% (0.28 kts)
Feathering & fixed 2-blade, 2% (0,14 kts)
Folding 0.5%

As my boat does not have space to swing a folder, the best possible gain I could expect is about 0.2kts with investing a few thousand in a feathering prop. The real problem is if your transmission will not allow a free spinning prop. In which case you may add another 60% to the free spinning 3- blade figures above.



Best, A.

 

[Daydream believer]

You are quoting research established back in 1880. Things have moved on since then & yacht designers have improved hull design somewhat. I recall reading various articles about the designes of Stephens the Frers, Holland & various wierd hull shapes from the likes of Jones etc. There are of course many more whose names I can no longer recall.
As for the wave from bow to stern, design has developed to confuse the flow & destroy the old idea of the great dip half way along the hull, sometimes followed by the drag of a huge quarter wave. Designers were well aware of how to make use of long aft overhangs. Others did it differently.
As for your comment:-
"You will actually find that in speed polar diagrams for displacement type craft, typical of cruising boats, the variance of speeds, closehauled, reaching and running, is minimal."
Here are the polar figures for a Hanse 37- A typical modern AWB
I would suggest that 7,58 to 9.67kts is more like 22%
or 4.98 to 5.7 kts = 14%
That is hardly " insignificant"
I accept that I have picked some figures showing the wider variance, But I would suggest that it does disprove your assertions.
As for your sail HP it is well known that modern sail design coupled with superior cloth holding its shape better produces greater power. I would suggest that without better proof of supporting data, I would be a little susceptible of the figures presented. You have not commented on the matter of air density.
So I would suggest that your figures are not necessarily accurate. Or more likely founded on out of date data, perhaps

But of course, for your boat they may well hold true & hence, you feel that they are relevant for similar craft. I can well imagine that you easily achieve the speeds you claim They are , in no way excessive for a displacement boat even though heavily loaded. But designs have improved somewhat.

Wind velocity	6kts	8kts	10kts	12kts	14kts	16kts	20kts
Beat angle	44.8°	42.6°	42°	41.1°	40°	39.4°	39.4°
Beat VMG	3.2	3.91	4.46	4.92	5.18	5.27	5.3
52°	4.98	5.95	6.71	7.19	7.43	7.53	7.58
60°	5.31	6.29	6.99	7.39	7.62	7.74	7.82
75°	5.56	6.56	7.21	7.56	7.8	8.01	8.25
90°	5.67	6.81	7.48	7.82	7.98	8.12	8.63
110°	5.7	6.91	7.57	7.96	8.37	8.74	9.19
120°	5.53	6.74	7.47	7.89	8.31	8.77	9.67
135°	4.99	6.16	7.1	7.62	8.01	8.43	9.38
150°	4.23	5.34	6.29	7.03	7.46	7.76	8.36
Run VMG	3.66	4.63	5.45	6.09	6.47	6.98	7.74
Run angle	143.9°	147.5°	147.5°	148.8°	151.1°	172.7°	177.2°

 

[Daydream believer]

Ye of little faith. You’ve not come across a Dragonfly in your travels, obviously.

There is a trimaran that sails around the West Mersea, Brightlingsea area. I have to admit it goes fast & must seem exciting.
However, I have seen it on 3 occasions whilst working our way up the Wallet towards Felixstowe. It does pass us but i note that its pointing ability is hopeless. It does 2 really long tacks across the sand banks & can only gain a 100 yds after 3-4 miles . True they may be only having a blast, but I expect that their angle to the TRUE wind is dismal.

Perhaps you have one of these overpriced Dragonfly toys
But are your sails of the latest cut? I doubt it. Off the wind you will fly away from a well found quick monohull. But upwind in F7? I would not be surprised to see you struggling.

 

[Chiara’s slave]

You might recall we need a new jib. We still point 40-45 degrees true, about 28 apparent. Some people sailing multis don’t realise that the telltales dont stall until you’ve utterly lost the plot going to windward. Sails pinnedin hard at 60-70 degrees, apparent wind no more than 35, it feels like beating, but it’s not.
AndF7, its a 30 foot 2 ton race boat, what would anyone expect? F5, and unless you’ e got a full on 40 foot race boat, you’re history, on any point of sail barring a dead run

 

[Laminar Flow]

Dear me, apparently the laws of physics have been suspended for contemporary yacht design. I'm quite happy to quote any number of contemporary naval architects who still believe them to be relevant, including some who design blisteringly fast ocean racers. Newton defined gravity in 1665. Last time I dropped my glasses, the effect still seemed very much in evidence, just as Froude's laws are for gravity waves.

The biggest single development in yacht design are new materials that have led to significant weight savings over earlier construction methods. Thus we are now able to make all those good "old" ideas and insights stay stuck together when subjected to the rigors of the sea. None of the current go fast concepts in sailing are new, including hydrofoils (1898) or any of the supposedly "modern" rudder and keel foils (1920's) nor high speed planing types.

What determines a boat's top speed is it's displacement/length ratio. Keep that under 150 and provide enough SA and stability, why you can even make the thing plane. Also not a new concept. However, the vast majority of all boats produced today and, for what it's worth, virtually all true cruising boats are displacement craft. As such they are still subject to Froude's law. Even though lighter displacement types have a shallower resistance curve, resistance still rises exponentially, until the vessel breaks through the barrier and begins to plane. Let me know when your cruiser does.
D/L has indeed such an important effect on top speed that marketing departments and designers play fast and lose with the true displacement figures of their offerings. This is not only true for high performance types, but the same goes for companies that traditionally cater to the cruising crowd. In this context, polar diagrams calculated for a stripped out, empty boat with empty tanks and not even a crew on board are not only unrealistic, but, quite frankly, bull sh*t. The difference between light and full load can easily be as much as 30%. At least the creators of older designs still had the decency to factor with half loads.

For the record: I only use traditional "hull speed" as a benchmark number. I prefer to use the term critical speed, which is the point where the stability and resistance curves intersect, which effectively ends any further possibility of achieving more speed.

The real determinant for speed, in all practical terms and given enough stability, is the SA/D ratio. It determines if and when a boat reaches it's "critical" speed. This also means that a heavy boat with a high D/L but also a high SA/D can be as fast or faster than a lighter one, and absolutely so in light airs, until the latter exceeds the former's critical speed. There is now doubt that a lighter boat can exceed what is known as hull speed, it is however so that they rarely do.

Which brings us to the question: are all modern boats always faster than older designs? One might also consider that virtually no classic designs now carry, as has been suggested, hemp rigging and flax or cotton sails.

The German magazine "Die Yacht" sponsored a race between a X-yacht and a two hundred year old design of a Dutch Lemster aak with leeboards and a gaff rig. The race took place over an olympic course. Indeed, the X-yacht won 2 out of 3 races, not because the Lemster was slower, but because the modern boat could tack faster.
It was determined that, in a point to point contest, the bi-centenarian would have come out on top. The difference in pointing angle between the gaff rigged lee board scow and the X was one degree.

Our Dutch friend, with a mixture of national pride and disappointment, reported a similar encounter with a Lemster. He sails a Boreal 43. It wasn't even a contest, he said. The Lemster outpointed and outfooted his boat and when they could bear off, it simply disappeared over the horizon.

Current designs have greater beam which enhances their sail carrying ability, but for everything on one side of the scale, there is a price to pay on the other.
Beam is not conducive to seakindliness, will be much more lively in a seaway, especially when combined with light displacement, it increases roll acceleration which promotes seasickness, increases the risk of capsize in breaking seas and the boat is less likely to be balanced and directionally stable when heeled. Excess beam also increases form resistance and wetted area. Hey, but that's progress I suppose.

Since my boat was brought up.
With a ketch rig and her shallowish draft her windward ability, compared to a fin keeled sloop, will always be compromised. Equally, with a D/L of 360 there is a natural limit to her top speed.
However she has a SA/D ratio of 18, which puts her firmly in the middle of the cruiser/racer pack. This has allowed us to make predictably fast passages and to get going when there is little wind. In light air we have surprised some supposedly much more slippery modern jobs.
Conversely, with her long keel with plenty of drag and a high prismatic coefficient, she will race downhill under spinnaker in 30 kts of wind, in a steep quartering sea with dry decks, little roll and not much for the autopilot to bother with, all the while doing well over her theoretical top speed and more in bursts.
Unlike a "modern" flat bottomed flyer she has enough displacement to support a comfortable life style for her regular crew of two for 5 months, all on a modest footprint of 31 feet and with a centrally heated roof over their heads to boot.
Yes, we've been in a F9, not intentionally, and quite a few F8 and never worried.
She may not be as handy as some contemporary piece of pret a porter, nor is she as delicate, mind. Her 19th century underwater profile may not be everyone's cup of tea, but then she also doesn't burn through her autopilots by the half-dozen (!) and if she were any slower than her modern sisters, then she is only marginally so, in all practical terms.

Best, A.

 

[Daydream believer]

A 51 ft boat modernised & optimised for racing, against a 40 ft boat. ? Some of our club members chartered an X yacht for Cork week & were really disappointed with the performance. Just because it is an X yacht does not make it fast. You have to pick the right model.
Boreal 43? I was not aware that they made one. Have you got your figures wrong ? Or is it an old version? In any event The couple of Boreals that I have seen were bluff bowed vessels. Comfortable, perhaps, but not exactly looking like examples of a "speedy" cruiser. But looks might be deceiving. They certainly lack appeal.
But let's be clear- I was not criticising your boat. Only querying your figures. To each his own

 

[johnalison]

When we used to race in the Blackwater I soon came to the conclusion that most boats go at almost the same speed, given that we mostly went with the tide. The chief determinant of the differences was waterline length, so that although my then modern 26-footer could outsail many larger boats to windward, when it came to reaching or downwind work I was often beaten by clumsy old boats with underwater profiles resembling house bricks.

Laws of physics or not, today’s designs do seem able to ignore their limitations off the wind when there is enough of it. Even my ‘90s design 34 will push itself up to 8kn quite easily and later boats probably a knot or two more, but I think at the cost of speed in other conditions. I have several times in the last few years been surprised to find myself passing modern 38-footers on a close reach when they appeared to be properly trimmed, at around 4kn perhaps.

 

[Daydream believer]

When we used to race in the Blackwater I soon came to the conclusion that most boats go at almost the same speed,

I do not know where you got that one from. In watching the BJRC races they can be strung out for miles. It can happen quite quickly as well.

 

[Chiara’s slave]

The tide makes a huge difference to progress over the ground. Here in the western solent, tide rules all, especially in light to moderate conditions. If it’s wind with tide, the faster boats rocket away never to be seen again by the slower fleet. If your VMG to windward is 4 knots and theres a 2.5 knot tide, that leaves you 1.5kn vmg to the windward mark. The slower boats might have a VMG on 3kn in the same light conditions. Their vmg to the windward mark is 0.5kn. The fast boats are suddenly 3 times as fast as the slow ones. The reverse holds true if it's wind over tide, up to a point. If theres bad chop, the longer boats, usually the faster ones, can negate their disadvantage somewhat. Cross tide, the faster boats sail with less aiming off, and a shorter distance to cover.

 

[Chiara’s slave]

I do not know where you got that one from. In watching the BJRC races they can be strung out for miles. It can happen quite quickly as well.

Any race with a mass start round here, if the start is at Cowes, the fleet will take up to an hour to pass Yarmouth, from 1st to last.

 

[johnalison]

I do not know where you got that one from. In watching the BJRC races they can be strung out for miles. It can happen quite quickly as well.

We would start at Osea and all arrive roughly together at the Knoll. OK, we would spread out once the windward legs were needed, but on all other legs any differences were only a fraction of the differences between handicaps. Even using hull speed as a reference, you would need an eighty foot boat to go twice as fast as my twenty-footer.

 

[Daydream believer]

We would start at Osea and all arrive roughly together at the Knoll. OK, we would spread out once the windward legs were needed, but on all other legs any differences were only a fraction of the differences between handicaps. Even using hull speed as a reference, you would need an eighty foot boat to go twice as fast as my twenty-footer.

Shows that the handicaps are working then
As for 80 ft !! My friend did one race in a Hanse 445 prior to the covid pandemic & says that he was so far in front it was embarrassing.
As I said , one can stand at Stone & see that by then the fleet is beginning to spread out. I suspect that the sailors have improved with experience. The quality of racing is a tad more competitive than it used to be. Marconi YC have spent a lot of time training crews. All it means is that boats now sail to their potential rather than just cruise round.

 

[johnalison]

Shows that the handicaps are working then
As for 80 ft !! My friend did one race in a Hanse 445 prior to the covid pandemic & says that he was so far in front it was embarrassing.
As I said , one can stand at Stone & see that by then the fleet is beginning to spread out. I suspect that the sailors have improved with experience. The quality of racing is a tad more competitive than it used to be. Marconi YC have spent a lot of time training crews. All it means is that boats now sail to their potential rather than just cruise round.

I didn't say that boats go at exactly the same speed. My word was 'almost', which is just a matter of personal judgement - It just seems that boats of very different sizes and ages generally stay almost together on passage, give or take half an hour or so. Training sounds very unsportsmanlike to me. My just cruising around usually seemed to involve me on the foredeck gybing the spinnaker with my 12-yr old on the helm in a force 5.

 

[Daydream believer]

I didn't say that boats go at exactly the same speed. My word was 'almost', which is just a matter of personal judgement - It just seems that boats of very different sizes and ages generally stay almost together on passage, give or take half an hour or so. Training sounds very unsportsmanlike to me. My just cruising around usually seemed to involve me on the foredeck gybing the spinnaker with my 12-yr old on the helm in a force 5.

We have club cruises in company & have done for some years now. There is regularly 4 hours difference between first & last on an 88 mile channel crossing with 5-6 boats. A couple of hours spread from Bradwell to Ramsgate is not unusual. So it would be interesting to see how your definition of "almost" fits into that. I would put it to you that it does not.
As for training for racing being unsportsmanlike- What planet are you on?
A club engaging in such activities encourages more participants & yachts to enter racing. Is that really unsportsmanlike.
Next you will tell us it is unsportsmanlike for our club to teach our cadets how to race.
It is being able to compete that gives all classes of sailors confidence to continue. Surely you realise that if someone turns up, then performs hopelessly, is far less likely to carry on, if some are so much better. You may be happy just cruising slowly. many are. No problem with that. But some of us have always wanted to strive to improve.
Surely it is good if those better ones give their time to help the slower ones.
Or are you one of those comprehensive school types- Give 'em a prize wherever they come But whatever you do , do not have winners.

 

[Tranona]

We have club cruises in company & have done for some years now. There is regularly 4 hours difference between first & last on an 88 mile channel crossing with 5-6 boats. A couple of hours spread from Bradwell to

See post#23. much the same difference in passage times Poole Cherbourg between what I achieved in my Eventide and what was possible with my Bavaria. Passage speed of 5 knots for an older style 7m WL and 6+ knots for a modern 9m WL (and greater sail area plus lower disp/wl)

 

[Daydream believer]

See post#23. much the same difference in passage times Poole Cherbourg between what I achieved in my Eventide and what was possible with my Bavaria. Passage speed of 5 knots for an older style 7m WL and 6+ knots for a modern 9m WL (and greater sail area plus lower disp/wl)

Are you referring to your comment
"Cross channel becomes 11 hours instead of 13/14"
So another 27% longer would suggest that you support my comments. & that is about 64Miles . A Bav 33 should do 64M in under 11 fairly easily in a fair wind, so 11 hours is generous.

 

[Norman_E]

It took me a few years to learn how to get the best speeds out of my Jeanneau 45.2 which had a fairly small masthead rig for a 45 footer. The biggest gain that I made mirrors the point made by Frayed Knot. The original heavyweight dacron overlapping genoa was probably about 135 or 140%. I found it hard to handle so I had a new tri radial laminate genoa made and simply told the sailmakers that I wanted it 18 inches less in the foot. It made a huge difference both to performance and the ease of handling it, much easier to tack, and to wind it away. Second was fitting a MaxProp. With the good genoa and a new full batten main with a bit of roach to replace the old straight leach one that had gone baggy I regularly had GPS speeds over the 8.6 knot hull speed quoted for the boat. This around the Turkish coast where tides are non existent and currents, where there are any are at half a knot or less. The Jeanneau was good upwind, particularly if freed off a bit, but slow downwind, and as coastal sailing where I was often had the winds blowing parallet to the coast the latter was an issue, only solved by buying a cruising chute. If I still had the boat I would now want to fit a short bowsprit and put a code zero on a furler.

 

[Laminar Flow]

A note on hull resistance.
The primary two forms of resistance to a hull (there are others) are frictional and form or wave (making) resistance.
At low speeds, the curve for both are fairly shallow (rise slowly) and paralell, i.e. resistance is low.
At a relative speed of 0.7 - 0.9 (sqr root DWL * the factor 0.7 - 0.9), form/wave resistance shoots up exponentially, whereas frictional resistance continues to only rise slowly.
In other words, at slow speeds, frictional resistance is dominant, but the values are low.

Form/wave making resistance is directly linked to displacement. In fact, given a modicum of hydrodynamic sophistication, a fairly accurate estimate of form resistance can be made from a boat's displacement alone.

What is significant, is that at low speeds, resistances for a light boat and a heavy one are not that far apart.

As sail area is nowadays determined by displacement (SA/D ratio) the heavier boat will have more SA than the lighter one to overcome it's greater form resistance at higher speeds. At slower speeds however, when resistance is more equal, the heavier boat will enjoy an abundance of power, compared to the lighter one.

This explains Johnalison's observation that his heavier, older design outsailed some more contemporary models in light air.
To this point: we have outsailed a Bav in light airs and, to our amusement a Pogo 30.
The various excuses the apologist for the type and brand have made include: badly fouled hull or didn't know what the were doing. As it was a Glenan school boat, both conclusions are most improbable. Additionally it was a broad reach; in naval parlance, a soldier's wind, the historic naval threshold for imbecility.
Turns out, it's simple physics. Mr. Froude was right all along. Until the poor thing has enough wind and is still light enough to fly, it has to play by the same rules as us lesser mortals.



John's other observation, concerning the gaffer's surprising superiority off-wind, has to do with sail planform.

After a hundred and fifty years of obsession with windward ability in yachting, it seems to have been overlooked that the only point of superiority for the narrow, high aspect Bermudan sail lies in a proportionately narrow range to windward.
Fact is, as established in wind tunnel tests, a low aspect gaff rig, on courses below 70 degr., is up to 35% more efficient that the narrow Bermudan.

The major disadvantages of these older craft are their less effective windward abilities and their, comparatively, great weight which curtails high critical speeds. It doesn't mean they are comparatively slow on favoured courses.
Colin Archer's rule for SA on his yachts was 100% to 125% of DWL squared. This, in spite of their high displacements, provides SA/D numbers in the low to mid 20ies, comparable to contemporary ocean racers.
Try that out on you favourite boat. Accordingly, my 31 footer should have a 1000 sqrft and an SA/D of 24.


The argument that a Boreal (a 44, thank you, I checked) lost the race against a Lemster aak for supposedly being blunt-nosed, is ludicrous. A Lemster is virtually the proverbial definition of blunt-nosed. I have joked, that if the Dutch are right about their bow shapes, then the rest of the world is definitely wrong. Looking at the latest crop of scow shapes we are now supposed to be cruising in, perhaps we are and they were onto something ...


In another comparative test done by "Die Yacht" a few years ago, investigated the comparative behaviour of various different types of mono hull sailing boats in waves. ("In der Welle, Langkieler gegen Kurzkieler" available as a video on Youtube in German)
The contestants were:
a 1971 Vindoe 40 with a long Keel
a 1981 Hallberg Rassy with a moderate fin
a 2008 Sun Odyssey 30i as a representative of more modern, beamy design with a flat underbody.

Conditions on the test day were typical of a F5 on the Baltic with short, steep seas of 1m and 20kts of wind.
The boats were evaluated on speed, handling and seakindliness/comfort.

Clear loser on all points of sail, beating, reaching and downwind, was the Sun Odyssey. She was not only the most awkward to control, the most uncomfortable, but also had much trouble maintaining course under these conditions.

The Vindoe was second, exhibiting a very gentle motion while being only slightly slower to weather. The tiny steering wheel and tight cockpit causing some awkwardness.

Overall winner was the comparatively modest Hallberg. She was not only the most comfortable, but also the best sea boat and the fastest.

Much apologizing was made for the Odyssey, while pointing out that emphasis on dockside amenities and space alone does not necessarily make for the a well rounded design, indicating the dilemma current designer find themselves in.

Overall conclusion: progress is not necessarily a linear function.

Best, A.

 

[johnalison]

We have club cruises in company & have done for some years now. There is regularly 4 hours difference between first & last on an 88 mile channel crossing with 5-6 boats. A couple of hours spread from Bradwell to Ramsgate is not unusual. So it would be interesting to see how your definition of "almost" fits into that. I would put it to you that it does not.
As for training for racing being unsportsmanlike- What planet are you on?
A club engaging in such activities encourages more participants & yachts to enter racing. Is that really unsportsmanlike.
Next you will tell us it is unsportsmanlike for our club to teach our cadets how to race.
It is being able to compete that gives all classes of sailors confidence to continue. Surely you realise that if someone turns up, then performs hopelessly, is far less likely to carry on, if some are so much better. You may be happy just cruising slowly. many are. No problem with that. But some of us have always wanted to strive to improve.
Surely it is good if those better ones give their time to help the slower ones.
Or are you one of those comprehensive school types- Give 'em a prize wherever they come But whatever you do , do not have winners.

I am very pleased to see that you are taking me more seriously than my family does.

 

[Tranona]

Are you referring to your comment
"Cross channel becomes 11 hours instead of 13/14"
So another 27% longer would suggest that you support my comments. & that is about 64Miles . A Bav 33 should do 64M in under 11 fairly easily in a fair wind, so 11 hours is generous.

Yes, I was supporting you. Never got the chance to do it in the Bavaria, but yes, 11 hours would be generous, although in reality rarely get a decent wind going south and my best passages have always been the return.