So, how fast will she go?

[Laminar Flow]

Hi, everyone. I thought I would post an interesting approach to predict hull speed developed by Dave Ger, author of several books on boaty things and Prof at the venerable West Lawn School of Yacht Design.

It goes beyond the traditional method of determining hull speed, which is square root of DWL multiplied by 1.34, by introducing Displacement/Length ratio as a relevant factor. His approach recognizes that light displacement vessels can exceed what used to be once regarded as an impermeable barrier.

The new formula is: square root DWL x factor (SL)
Whereas SL is: 8.26 / DL ratio to the power of 0.311

For my boat with a 28.25' DWL and a DL of 322 SL works out to be about 7.3kts.By the traditional method it would be 7.12kts.
(It should be noted that we have frequently sustained speeds of 7.5kts and occasionally 7.8kts. This, I suspect, has more to do with our high prismatic coefficient (0.61) which is fine on a reach, but gives us a bit of a blunt nose for going to weather (fast) in any kind of sea)

Now, if I could put my boat on a diet and, just to say, miraculously lost 3 tons, I would then have a DL of 244 and could expect a nominal top speed of nearly 8kts (7.97).

Just a word of caution on DL ratios: the displacements given in brochures and in publications such as Sailbota Data need to be taken as empty boat values at best.
I just recently helped a friend haul his boat and that was listed in Sailboat Data as weighing 21500#. In fact, it came out at 33000#, almost a third heavier. Interestingly he didn't seem to have that much stuff on board, beyond the regular cruising gear and no provisioning. It did set me to thinking and I'll be looking to rent a locker at our marina to store all the things we do not constantly need on board.

So, borrow your kid's scientific calculator and have some fun.

 

[bitbaltic]

The traditional method would give me 6.96 knots. We’ve got some polar software onboard which I don’t particularly trust and haven’t run in ages but gives max recoded speeds a bit over 7kn which sounds about right by your new formula.

in practice the boat does 6kn or a little higher easy enough but I doubt my ability to get it much higher by anything other than luck so max speed is not a replicable achievement, margins of a quarter of a knot or so probably academic.

max speeds prob done in flat water in Cardiff Bay and not at sea anyhow.

the boat’s displacement is listed at 3 tonnes but in cruising fit and with full tanks it was just under 4 tonnes on the marina hoist (however accurate that is). Interestingly enough we just removed a load of junk prob adding up to 100-150 kg (spare chain and such) so maybe we’re ready for our fastest season ever ?
And tbh she’s such a light boat that much would be achieved by ditching the crew and putting myself on a diet!!

 

[TernVI]

How fast your boat will go depends on two things:
The drag/speed curve
The power available.

Did anyone knowledgeable ever think hull speed was a impenetrable barrier? AIUI in Nelson's day they understood it was just a guide and some hulls were faster? In those days, for a British warship, the formula was more like 1.0 x sqrt (lwl in feet).

All these slight tweakings of the sqrt(lwl) formula are playing with two things. Firstly how much power shall we use to decide how far up the drag/speed curve to look and secondly different hull shapes suit different factors.
It's slightly blurred in a sailing boat, because more weight may or may not give you more power.
The OPs boat has a hull speed in the range 7 to 8 knots, depending on how you define it. Fly a bigger kite in more wind, it will go a bit faster.

I think the concept of 'hull speed' is vastly over-rated. Having a 'pitcure' of the drag/speed curve is much more enlightening.

Dave Ger of course is something of an authority on propellors, so I suspect his concept of 'hull speed' for a yacht is fairly focussed on design targets for propellors? Of course over the decades, yacht shapes have evolved. And generally engines have got bigger. (Have props improved too?)
So it's no surprise that the fudge-factor in a rule of thumb has changed. What the OP is talking about is really making the fudge-factor a function of some characteristics of the hull shape. The dimensions you are prepared to process, the better an answer you might hope to get?

 

[Rum_Pirate]

This might help

Express 0.311 in scientific notation

 

[Gwylan]

Hi, everyone. I thought I would post an interesting approach to predict hull speed developed by Dave Ger, author of several books on boaty things and Prof at the venerable West Lawn School of Yacht Design.

It goes beyond the traditional method of determining hull speed, which is square root of DWL multiplied by 1.34, by introducing Displacement/Length ratio as a relevant factor. His approach recognizes that light displacement vessels can exceed what used to be once regarded as an impermeable barrier.

The new formula is: square root DWL x factor (SL)
Whereas SL is: 8.26 / DL ratio to the power of 0.311

For my boat with a 28.25' DWL and a DL of 322 SL works out to be about 7.3kts.By the traditional method it would be 7.12kts.
(It should be noted that we have frequently sustained speeds of 7.5kts and occasionally 7.8kts. This, I suspect, has more to do with our high prismatic coefficient (0.61) which is fine on a reach, but gives us a bit of a blunt nose for going to weather (fast) in any kind of sea)

Now, if I could put my boat on a diet and, just to say, miraculously lost 3 tons, I would then have a DL of 244 and could expect a nominal top speed of nearly 8kts (7.97).

Just a word of caution on DL ratios: the displacements given in brochures and in publications such as Sailbota Data need to be taken as empty boat values at best.
I just recently helped a friend haul his boat and that was listed in Sailboat Data as weighing 21500#. In fact, it came out at 33000#, almost a third heavier. Interestingly he didn't seem to have that much stuff on board, beyond the regular cruising gear and no provisioning. It did set me to thinking and I'll be looking to rent a locker at our marina to store all the things we do not constantly need on board.

So, borrow your kid's scientific calculator and have some fun.

I hope your boat is impermeable, the barrier might be impenetrable

 

[LONG_KEELER]

Interesting. In say a f3, how much HP, or percentage of power generated by the sails is
required to overcome the drag of propellers and stern gear ?

All yachtsmen hate giving this away and apart from improved propellers, which are very expensive and arguably delicate at times , very little has been down about this in yacht design.

I would be very happy to do away with cockpit lockers if there was some kind of opening, retractable bomb doors arrangement for engine power. Prop cleaning , less chance of fouling etc are additional benefits apart from the drag savings. And why not have a leg each side which would assist berthing. Cars do it with a diff.

It would probably affect the popular aft cabin, depending on the size of the boat, but it would be nice to have the options.

 

[Laminar Flow]

Interesting. In say a f3, how much HP, or percentage of power generated by the sails is
required to overcome the drag of propellers and stern gear ?

All yachtsmen hate giving this away and apart from improved propellers, which are very expensive and arguably delicate at times , very little has been down about this in yacht design.

I would be very happy to do away with cockpit lockers if there was some kind of opening, retractable bomb doors arrangement for engine power. Prop cleaning , less chance of fouling etc are additional benefits apart from the drag savings. And why not have a leg each side which would assist berthing. Cars do it with a diff.

It would probably affect the popular aft cabin, depending on the size of the boat, but it would be nice to have the options.

That would depend very much on how fast your boat sails in a F3 and the resistance of your hull.

At a relative speed of 0.9 the loss in speed would be 8% with a fixed prop left to rotate, at a relative speed of of 0.55 it would be 11%.
At 4kts a fixed 3-blade will cause a drag of 12kg. That is roughly 50% of the total hull drag of a medium displacement 32'er.
In a F3 one sqft of sail produces 0.015hp.

 

[Laminar Flow]

How fast your boat will go depends on two things:
The drag/speed curve
The power available.

Did anyone knowledgeable ever think hull speed was a impenetrable barrier? AIUI in Nelson's day they understood it was just a guide and some hulls were faster? In those days, for a British warship, the formula was more like 1.0 x sqrt (lwl in feet).

All these slight tweakings of the sqrt(lwl) formula are playing with two things. Firstly how much power shall we use to decide how far up the drag/speed curve to look and secondly different hull shapes suit different factors.
It's slightly blurred in a sailing boat, because more weight may or may not give you more power.
The OPs boat has a hull speed in the range 7 to 8 knots, depending on how you define it. Fly a bigger kite in more wind, it will go a bit faster.

I think the concept of 'hull speed' is vastly over-rated. Having a 'pitcure' of the drag/speed curve is much more enlightening.

Dave Ger of course is something of an authority on propellors, so I suspect his concept of 'hull speed' for a yacht is fairly focussed on design targets for propellors? Of course over the decades, yacht shapes have evolved. And generally engines have got bigger. (Have props improved too?)
So it's no surprise that the fudge-factor in a rule of thumb has changed. What the OP is talking about is really making the fudge-factor a function of some characteristics of the hull shape. The dimensions you are prepared to process, the better an answer you might hope to get?

The drag/speed curve
The power available.

Drag for a hull can, for most part, there are other components, be separated into frictional drag and form drag.

Frictional drag is a function of hull surface area and it rises linearly and in a relatively gentle curve with increasing speed.
Form drag, on the other hand, rises exponentially and the heavier the boat the steeper the curve. A lighter boat has, conversely, a shallower curve. This explains why a lighter hull with less resistance will have a higher top speed potential.

Up to a relative speed of about 0.7 frictional and form drag are virtually identical; thereafter form drag rises sharply until you reach a point of ever diminishing return for any amount of power fed into the system.

Form drag is a direct function of displacement ( in fact it is precisely on this basis that form drag is calculated) and this is what Gerr's formula is based on. It actually makes a lot of sense. It's not just a fudge factor, it's physics.


The amount of power available to a sailing boat is limited by it's sail carrying ability and stability. The displacement or weight of a boat has not necessarily anything to do with it's power to carry sail, but it has a major impact on form drag and hence it's higher speed potential.



As can be seen from the curve: above 1.34 it becomes pretty pointless to increase power input and after all a sailboat does not have infinite stability, bigger spinnaker or not.

Re Mr. Gerr: you seem to have a very limited knowledge of him and his accomplishments. He has designed numerous sailing boats, including high performance types as well as powered craft and is a Prof for yacht design at Westlawn. He likes his numbers and he quite obviously has an excellent handle on it, that much is clear from his numerous publications.

I'm quite sure the British war ships of Nelson's era were incredibly fast, although, arguably, the French built the better (faster) ones. It just happens the British crews were better trained and hadn't shortened the majority of their (aristocratic) officers at the neck.
On square rigger speeds:
Santa Maria (Columbus) average speed 3.5kts, relative speed 0.45 - good passage. Length 70'
Clipper ship average speed 6kts, relative speed 0.38, mean figure, Length 250'
(average speed of 420 documented passages by wool clippers from Australia to England, 6.5kts)
Parma, average speed 9kts, relative speed 0.49, length 330', best passage.
Preussen, average speed 7kts, relative speed 0.35, mean figure, length 410'

Various yachts by comparison: average relative speeds 0.84-0.99

 

[TernVI]

....

View attachment 116147

As can be seen from the curve: above 1.34 .....

Looking at that graph, the point at which 'diminishing returns' sets in, in terms of 'hull speed' is completely arbitrary.
You could say hull speed is at 1, or 1.1 or 1.2, all equally justifiable.
You could say the curve looks asymptotic to something like 1.4.
Going to two decimal places is false precision.
How many people can even credibly measure their speed through the water to better than 0.1 knot anyway?

If you look at more modern yacht shapes, the curve may be quite different.

 

[fisherman]

I have found that maximum available speed in any condition of weather or lading is often half a knot less than that required to get back to my drying berth before the tide leaves it.

 

[Laminar Flow]

If you look at more modern yacht shapes, the curve may be quite different.

Yes indeed, it is shallower, somewhat. Which, incidentally, is the whole point of Gerr's formula.

There is a diagram of comparative curves (light/heavy) in Eliasson/Larsson, Priciples of Yacht Design.

The Delft Institute did an exhausting study tank testing various hull forms and the single most important factor governing speed is displacement.

 

[Buck Turgidson]

So there is zero induced drag from the keel?

 

[Laminar Flow]

So there is zero induced drag from the keel?

Induced drag occurs at the tip of the keel. A longer keel has a longer vortex and hence, more drag. In the big picture though, induced drag plays a minor role.
The Delft series all used the same keel.

 

[LONG_KEELER]

That would depend very much on how fast your boat sails in a F3 and the resistance of your hull.

At a relative speed of 0.9 the loss in speed would be 8% with a fixed prop left to rotate, at a relative speed of of 0.55 it would be 11%.
At 4kts a fixed 3-blade will cause a drag of 12kg. That is roughly 50% of the total hull drag of a medium displacement 32'er.
In a F3 one sqft of sail produces 0.015hp.

Thanks for doing the figures .

Interesting what that extra knot does to drag.

My current boat has an outboard in a well. On longer trips it's fun to remove the outboard
and fit the bespoke insert that blends in with the hull shape. The outboard weighs 26kg which is quite hefty. Apart from the leg and prop drag, there is a fair bit of turbulence in the well itself which must also affect speed. I also have a large DAR ratio for the saildrive prop.

It's not always easy to determine how much faster the boat is travelling across a wide range of sailing conditions. You would have to take the outboard it in and out frequently to make definite comparisons. 3/4 knot is not uncommon on a 26' boat . However, you feel the change much more in how the boat is sailing rather than definite numbers.

 

[Davy_S]

I had an interesting comparison with feathering and standard propellers, this was two Albin Vegas, my friend had a Vega with a Beta 13 engine and a standard fixed prop, i had the Volvo MD6, this used a combi drive, as you opened the throttle the pitch on the two blade prop changed, in flat water it would do 6.8 knots top speed, when you switched off the engine, you moved the throttle lever back and it feathered the prop so the blades were flat, like a knife edge, if you did this when sailing, you could feel the boat surge forward, if you sailed the two Vegas side by side and i feathered the prop, it would walk away from the fixed prop Vega, it was a good knot faster, i would not have believed the difference had i not experienced it first hand, so certainly feathering or folding props are well worth a consideration.

 

[westernman]

On square rigger speeds:
Santa Maria (Columbus) average speed 3.5kts, relative speed 0.45 - good passage. Length 70'
Clipper ship average speed 6kts, relative speed 0.38, mean figure, Length 250'
(average speed of 420 documented passages by wool clippers from Australia to England, 6.5kts)
Parma, average speed 9kts, relative speed 0.49, length 330', best passage.
Preussen, average speed 7kts, relative speed 0.35, mean figure, length 410'

Various yachts by comparison: average relative speeds 0.84-0.99

Here is a square rigger going at a decent lick.

 

[Laminar Flow]

Thanks for doing the figures .

Interesting what that extra knot does to drag.

My current boat has an outboard in a well. On longer trips it's fun to remove the outboard
and fit the bespoke insert that blends in with the hull shape. The outboard weighs 26kg which is quite hefty. Apart from the leg and prop drag, there is a fair bit of turbulence in the well itself which must also affect speed. I also have a large DAR ratio for the saildrive prop.

It's not always easy to determine how much faster the boat is travelling across a wide range of sailing conditions. You would have to take the outboard it in and out frequently to make definite comparisons. 3/4 knot is not uncommon on a 26' boat . However, you feel the change much more in how the boat is sailing rather than definite numbers.

Propeller turbulence does a lot more than just slow down the boat. It also destroys a good portion of lift over the rudder and can have a detrimental effect on the steering and contribute to weather helm which in turn increases drag.
A common observation is that installation of a folding or feathering prop also solved or alleviated problems with weatherhelm.
A normal and properly profiled rudder of 5 sqrft will cause 23 lbs of drag at 10 degr incidence and 6kts, or as much as a fixed 3-blade prop.

 

[Laminar Flow]

However, you feel the change much more in how the boat is sailing rather than definite numbers.

While it is gratifying to achieve an improvement in performance, I like to know beforehand whether the expected improvement, if any, is going to be worth the effort, time and expense. This it what numbers are good for.

In the case of our boat: as designed and built she had serious steering and performance issues under sail. This was due to a 5''-7" wide blunt deadwood, a flat steel plate rudder and an unbalanced sail plan. By looking at the numbers and making the calculations I was able to analyze the problems, design suitable countermeasures, minimize the risks and make some qualified predictions as to the probable outcome. It was a success.