What is "Hull Speed" Anyway

[StuM]

Originally Posted by Don C L

Just for fun I went looking for that 1.34. Wikipedia gave me:
The speed of a deep-water wave may also be approximated by:

{\displaystyle C={\sqrt {{gL}/{2\pi }}}} C = \sqrt{{gL}/{2\pi}}
where g is the acceleration due to gravity, 9.8 meters (32 feet) per second squared. Because g and π (3.14) are constants, the equation can be reduced to:

{\displaystyle C=1.251{\sqrt {L}}} C = 1.251\sqrt{L}
when C is measured in meters per second and L in meters. Note that in both formulas the wave speed is proportional to the square root of the wavelength.

So when using meters and meters/second, the constant is 1.251 times the square root of the waterline. length.

So my 22.5' waterline is 6.858 meters and if I did my math right that came out to 11.7939301371 km/hour which is 6.368212822510335 knots

Now with the other formula I get 1.34 x square root of 22.5 ft = 6.35617809694 knots

So this clearly explains why all the European boats are faster. They are all measuring with the metric system.

Don't know where they got that 1.251 from.

(Oh, that's right - it's Wikipedia, the font of all accurate knowledge )

Sqrt( L x 9.8/(2 * pi)) = Sqrt(L) x Sqrt(9.8/2/pi) = Sqrt(L) * 1.249

Using 1.249 gives 6.358 knots (which is lot closer to the Imperial 6.536 knots - in fact, given the vagaries of that 9.8, the best that can be said is that both give 6.36).

[Don C L]

Originally Posted by StuM

Don't know where they got that 1.251 from.

(Oh, that's right - it's Wikipedia, the font of all accurate knowledge )

Sqrt( L x 9.8/(2 * pi)) = Sqrt(L) x Sqrt(9.8/2/pi) = Sqrt(L) * 1.249

Using 1.249 gives 6.358 knots (which is lot closer to the Imperial 6.536 knots - in fact, given the vagaries of that 9.8, the best that can be said is that both give 6.36).

Again! I stand corrected! I had trusted Wikipedia's numbers. Never again. I too get 1.249, well, no, actually: 1.24888688131

which yields: 6.357455996367134 knots
as opposed to: 6.35617809694 knots using that 1.34 constant and measuring in feet....

which all goes to prove that given two identical boats, the Frenchman, or woman, will win every time.....

[StuM]

Originally Posted by Don C L

Again! I stand corrected! I had trusted Wikipedia's numbers. Never again. I too get 1.249, well, no, actually: 1.24888688131

which yields: 6.357455996367134 knots
as opposed to: 6.35617809694 knots using that 1.34 constant and measuring in feet....

which all goes to prove that given two identical boats, the Frenchman, or woman, will win every time.....

It's a bit pointless quoting a number like 6.357455996367134 (16 digits) when the gravitational constant is only taken to 2 significant digits. Especially considering that it varies geographically quite bit depending on location. (at sea level it varies between 9.780 at the equator to 9.832 at the poles)

The standard SI defined g is actually 9.80665 , not just 9.8, so that 1.249 would be around 1.24931 in the given formula, resulting in 3.27167 m/s or 6.35962 knots.

And that of course assumes that your mesurement of LWL is equally precise. i.e. accurately measured to the nearest millimeter or 3/64".

So in truth, somewhere around 6.4 knots is about as accurate and precise as we can realistically get.

Hmm, actually, that g variation between poles and equator is significant.

A couple of calculations for a 12m waterline gives a theoretical hull speed of 8.7 knots at the equator and 8.4 knots at the poles using the metric formula involving g.
Using the imperial/empirical 1.34 formula, it's 8.4 knots.

[TJ D]

You guys are making this waaaay too complicated. All you need is 1.21 gigawatts of power (prounounced jiggawatts), and simultaneously reach 88 miles per hour. Sheesh.

Not sure where the gull-wing doors should go in the cockpit, though. I guess they could do double duty as weather cloths...

[Pete7]

Originally Posted by Joli

The ARC "Cruising Class" winner this years was an HR 37 with a 34 foot waterline. Theoretical hull speed is 7.81 knots, they won the class averaging 5.32 knots or a bit under 70% of hull speed.

The ARC "Racing Class" winner this year was a Dufour 45e with a 40 foot waterline. Theoretical hull speed is 8.47 knots, they won the class averaging 6.02 knots or a bit over 70% of hull speed..

Didn't this years ARC spend the first week almost becalmed? if so what would that do to the numbers on a 3 week trip?

Did they then sail at fast displacement speeds for the rest of the voyage due to better wind conditions and carrying a weeks less provisions?

Pete

[TJ D]

Say, here's a screen shot summarizing a transatlantic trip last year of nearly 4000 miles. This was with everything we own on board, full tanks, etc. Our 'hull speed' is 9.7 knots, so here is some empirical evidence that Polux is pretty close to the mark with regard to performance on faster designs.

The second one is a 6 hour average, in excess of 'hull speed'. This was conservative, short-handed sailing. We absolutely do not sail like racers!

Current was minimal.

[Joli]

Originally Posted by TJ D

Say, here's a screen shot summarizing a transatlantic trip last year of nearly 4000 miles. This was with everything we own on board, full tanks, etc. Our 'hull speed' is 9.7 knots, so here is some empirical evidence that Polux is pretty close to the mark with regard to performance on faster designs.

The second one is a 6 hour average, in excess of 'hull speed'. This was conservative, short-handed sailing. We absolutely do not sail like racers!

Current was minimal.

So you averaged 80% of hulls speed, that's good but your boat is a carbon fiber, water ballasted semi planing hull. Not your typical cruiser. Curious, what is the PHRF rating? -50 or so?

Those who won their classes in this years ARC managed 70% of their theoretical hull speed, handicap ratings are immaterial.

Again, managing 70% of hull speed is pretty good for the average cruiser out there today.

And since everyone likes numbers let's look at a few more;

The new Jeanneau 519 rates in the mid 50's whereas a late 80's Jeanneau 52 rates in the mid 50's, same speed. Some may argue they like the features of the newer boat (Ikea interior, wide transom, chines, twin wheels...) that's fine but the argument cannot be made the boat is faster.

Another?
A newish Swan 66 rates -30 but a Swan 68 from the early 90's rates -30 and our 61' boat designed in the 60's rates -21. Again argue features and decide what is best for you and how you cruise but don't be deluded that the newer designs are faster or better in a seaway. (again carbon shells excluded)

Polux, you are very mistaken about our old heavy boats light air sailing ability. It's quite easy to sail wind speed or better in the lighter stuff. Upwind/downwind we build apparent very quickly. Blowing 5~6 we can set an assy and easily sail deep at 7.5 (we pull app forward quickly), upwind we tend to match wind speed to about 10 knots.

Anyway, it's good people buy new boats so those who want older boats have a variety to choose from.

[Dockhead]

Originally Posted by Kenomac

There seem to be many owners of displacement boats who don’t understand that the ocean and seas have currents which add and subtract boat speed. We travel through these currents and eddies all the time, and when I see the speedo go up over hull speed.... I know how to interpret the data. It doesn’t mean I have a boat which can magically go 2-4 knots above hull speed... Oh boy, I now have a racing boat!

The ocean water is in constant motion with currents, it’s not just sitting there stationary as your displacement boat travels through it. It’s hard for me to believe the boaters on this forum are having difficulty understanding this concept. Your boat travels against the current... it goes slower than you’d expect; your boat travels with the current... it goes faster than it would otherwise. This isn’t rocket science...

Planing hulls, hydrofoils and unusually narrow hulls is another game.... they can go faster.... but not displacement hulls which need to move the H2O out of their own way.

So are you saying that the many people in here who have reported sailing above hull speed are confusing speed through the water with speed over ground, and don't know about currents? I don't think I've ever met a sailor who was unaware of such basic concepts, and the people on here certainly deserve more credit than that, I think.

You should read Paulo's excellent post for the theoretical explanation for it, if you don't believe the many people who have described their experiences. The fact is that any displacement hull can exceed hull speed with enough power, and that lighter boats don't require all that much power to do it.

And "lighter" is not racing boat light -- Paulo says:

"When it has clearly established that there was not not a single hull speed for the same LWL, but a fixed one for boats with a D/L of 240 and over and another one, a variable one, for boats with a D/L lesser than 240, one that for the same LWL is increasing with the lightness of the sailboat . . . "

He's not talking about planing, or surfing, he's talking about sailing in normal conditions. He's saying much the same thing as I said earlier -- which was that the power vs. speed curve turns up less steeply when the boat is lighter.

I also like the way Paulo explains it -- not "breaking hull speed" but that hull speed is higher for lighter boats. That corresponds well to my experience. That's also what naval architects talk about -- they say that hull speed is really somewhere from 1.2 to 1.51 for displacement monohulls, depending on weight, fineness, etc.

Of course the physics of wave-making don't change, what changes is the amount of waves which are being made. The lighter the boat, the less waves are made, and the less power needed to climb the smaller bow wave. The point at which the bow wave starts to become unreasonably steep comes later and later.

[Dockhead]

Originally Posted by Joli

I'm pretty sure I've asked you before if you have raced and was never given an answer. Race course numbers tell the tale and teaches you how to look at the real average speed of a boat. And pressing a powered up racing shell is not indicative of what the average sailor sees for daily runs or even what the average racer sees for daily runs with a racer cruiser.

Let's look at a couple numbers.

The ARC "Cruising Class" winner this years was an HR 37 with a 34 foot waterline. Theoretical hull speed is 7.81 knots, they won the class averaging 5.32 knots or a bit under 70% of hull speed.

The ARC "Racing Class" winner this year was a Dufour 45e with a 40 foot waterline. Theoretical hull speed is 8.47 knots, they won the class averaging 6.02 knots or a bit over 70% of hull speed.

I think the ARC record held by a Rambler 88, a no holds barred racing maxi is 16.8 knots, the theoretical speed of that boat is 12.6 knots. But who would want to cruise an empty carbon fiber black hole with a crew of 20?

Our heavy old boat designed in the 60's and built in the 70's has a phrf rating or -21. Hull speed is around 9.5 but we've pressed the boat into the high teens when racing. We plan on an average of 7 knots for cruising, it's no faster or slower than modern 60 footers we've sailed against (empty carbon racing shells excluded).

I think you are both right in your way, so perhaps I could try to reconcile these positions.

Paulo is absolutely right about weight vs. hull speed, and I think the simplest way to explain it is to say that lighter boats produce less of a bow wave (and less of a stern wave, so less of a trough between then). The physics don't change -- the length of these waves is the same for all boats heavy and light -- but with a lighter boat, you can get further up that wave before the power required to go faster goes through the roof.

And as a result you will see short-term speed around hull speed to be much higher, with lighter boats.

But you are talking about the ARC -- a different kettle of fish. Here you will be sailing a lot well below hull speed because it doesn't blow 20 knots every day. Once you are sailing much in light conditions, then SA/D becomes more important than D/L. In lighter conditions, the magic speed is not hull speed or hull speed plus one or whatever, but something closer to economical motoring speed -- where very, very little power is required. Ships sailing for economy mostly sail at 0.9 Speed/Length which is -- yep! -- around 70% of hull speed. That's because the increase of resistance due to wave making starts far below theoretical hull speed. So for boats with enough canvas to keep moving over the range of conditions experienced, average speed will start to congregate around that number, which represents that speed which can be reached with a small amount of power. To reach an average speed much above that for a whole ocean crossing requires a lot of SA/D, a good variety of canvas, and a lot of work, for anything like a normal cruising boat or racer-cruiser. It's easier in a lighter boat, but averages speeds through a wide variety of conditions do gravitate towards that number for a variety of boats.

[Joli]

Originally Posted by Dockhead

I think you are both right in your way, so perhaps I could try to reconcile these positions.

Paulo is absolutely right about weight vs. hull speed, and I think the simplest way to explain it is to say that lighter boats produce less of a bow wave (and less of a stern wave, so less of a trough between then). The physics don't change -- the length of these waves is the same for all boats heavy and light -- but with a lighter boat, you can get further up that wave before the power required to go faster goes through the roof.

And as a result you will see short-term speed around hull speed to be much higher, with lighter boats.

But you are talking about the ARC -- a different kettle of fish. Here you will be sailing a lot well below hull speed because it doesn't blow 20 knots every day. Once you are sailing much in light conditions, then SA/D becomes more important than D/L. In lighter conditions, the magic speed is not hull speed or hull speed plus one or whatever, but something closer to economical motoring speed -- where very, very little power is required. Ships sailing for economy mostly sail at 0.9 Speed/Length which is -- yep! -- around 70% of hull speed. That's because the increase of resistance due to wave making starts far below theoretical hull speed. So for boats with enough canvas to keep moving over the range of conditions experienced, average speed will start to congregate around that number, which represents that speed which can be reached with a small amount of power. To reach an average speed much above that for a whole ocean crossing requires a lot of SA/D, a good variety of canvas, and a lot of work, for anything like a normal cruising boat or racer-cruiser. It's easier in a lighter boat, but averages speeds through a wide variety of conditions do gravitate towards that number for a variety of boats.

Light boats are kind of a yes or no thing, as in; faster Yes! slower Yes!

Let's explore a little bit. Take a Pogo 12.5 or 40' for us Yanks, it rates 9 in PHRF, so it's bloody quick. It weighs in at a little over 2 metric tons or 5,400 lbs for us Yanks again.

When empty and sailing off the wind, that nice assy is pulling it along and it's gonna roar past most anything else on the water. Empty, upwind it's still quick but you better have your fillings checked by your dentist because falling off waves that nice flat bottom that is oh so quick when planing is gonna pound (everything is give and get). And to top it all off, this is a light easily driven hull so the sailplan can be small and easily managed.

Now add cruising gear: people, tools, sails, ground tackle, dinghy, outboard, spares, extra batteries, water for when the watermaker quits, clothing, scuba gear, fins and masks, canned goods, refrigeration, autopilot... The list is quit endless and can easily add up to the weight of the boat. Now this light boat is down on it's lines, the easily driven fun boat is sluggish and has a small sailplan that is trying carry around two boats worth of displacement. The PHRF rating of 9 is no longer valid but a more traditional racer/cruiser can swallow those stores and continue to sail at it's designed phrf rating.

So at the end of the day are you better off with a Pogo 12.5 or a Jeanneau 519 that on paper looks like a slower cruising boat? In my mind the bigger Jeanneau is a better cruising platform that will be just as fast or even an old Alden 52 for half the price and an even lower PHRF rating then the new Jeanneau...

[TJ D]

Originally Posted by Joli

Light boats are kind of a yes or no thing, as in; faster Yes! slower Yes!

Let's explore a little bit. Take a Pogo 12.5 or 40' for us Yanks, it rates 9 in PHRF, so it's bloody quick. It weighs in at a little over 2 metric tons or 5,400 lbs for us Yanks again.

When empty and sailing off the wind, that nice assy is pulling it along and it's gonna roar past most anything else on the water. Empty, upwind it's still quick but you better have your fillings checked by your dentist because falling off waves that nice flat bottom that is oh so quick when planing is gonna pound (everything is give and get). And to top it all off, this is a light easily driven hull so the sailplan can be small and easily managed.

Now add cruising gear: people, tools, sails, ground tackle, dinghy, outboard, spares, extra batteries, water for when the watermaker quits, clothing, scuba gear, fins and masks, canned goods, refrigeration, autopilot... The list is quit endless and can easily add up to the weight of the boat. Now this light boat is down on it's lines, the easily driven fun boat is sluggish and has a small sailplan that is trying carry around two boats worth of displacement. The PHRF rating of 9 is no longer valid but a more traditional racer/cruiser can swallow those stores and continue to sail at it's designed phrf rating.

So at the end of the day are you better off with a Pogo 12.5 or a Jeanneau 519 that on paper looks like a slower cruising boat? In my mind the bigger Jeanneau is a better cruising platform that will be just as fast or even an old Alden 52 for half the price and an even lower PHRF rating then the new Jeanneau...

I think that you're spot on with regards to boats smaller than, say, 45-50 feet or so. Bigger boats can swallow weight better. I think if you compared a Pogo 50 to a Jenneau 50, the differences would be substantially smaller. The Pogo would outsail the Jenneau handily carrying the same weight, I would suspect.

A lot of the gear that we're lugging around has very little difference in weight, regardless of size of boat (a sewing machine or a scuba tank for a 30' boat weigh what they do for a 60' boat-even anchors/chain become a smaller part of displacement very quickly with size), so the impact is smaller with a big hull. Exclusive of fluids, and stuff that's not bolted down, we're probably carrying about 10% of our boat's displacement in non-working sails, anchors, dink, outboard, clothes, food, tools, etc. We're not racing, anyway.

One also has to re-think what's really necessary for gear. On our previous, heavy boats, I carried HUGE amounts of crap. Way more spares than needed, tons of tools that I never used, the list goes on and on.

The minimalistic approach that a light boat calls for is actually pretty liberating. When we sold our last boat, we already owned Rocket Science. Probably 80% of the weight that was on the old one didn't come along with us on the new one, and here we are, 4 years and a half a world later, and we don't miss any of it.

BTW the last time RS was rated PHRF, she was -39. Since then, she's been refitted into more of a cruising boat, so that number is no longer correct.

The purpose of my post with the stats, though, was to support the idea that performance boats can do well loaded, on average outperforming their heavier counterparts, and that 70% is easy to exceed, even with a long-distance cruising weight aboard.

Over the nearly 4000 miles that we averaged 8.0, a lot of it was motoring at 7 or so, and some of it was sailing slowly. So, we had many days at near 100% of 'hull speed', and quite a lot of time exceeding that. This was the original premise of the thread, I believe. On that crossing, our best 24h run was just over 260nm, 10.9 average speed, shorthanded, with our one crew complaining that we were sailing the boat like an Island Packet-way too conservative for his taste (funny how the guy not writing the checks always wants to push harder...).

I don't think that we sail differently than any reasonably competent mom and pop. I've never raced, and care very little for getting max speed out of the boat.

That run was early in the crossing, with over 200 gallons of fuel/water.

OK, RS is special-I'll grant you that, but she's also a 20 year old boat-surely more modern rigs have caught up by now?

It's a good discussion. I've been on both sides of this question, my previous 3 boats having been traditional full-keeled 'bluewater cruisers' and I will absolutely never go back to a more traditional, heavy boat.

Thanks for the input, gents.

[Joli]

Originally Posted by TJ D

I think that you're spot on with regards to boats smaller than, say, 45-50 feet or so. Bigger boats can swallow weight better. I think if you compared a Pogo 50 to a Jenneau 50, the differences would be substantially smaller. The Pogo would outsail the Jenneau handily carrying the same weight, I would suspect.

A lot of the gear that we're lugging around has very little difference in weight, regardless of size of boat (a sewing machine or a scuba tank for a 30' boat weigh what they do for a 60' boat-even anchors/chain become a smaller part of displacement very quickly with size), so the impact is smaller with a big hull. Exclusive of fluids, and stuff that's not bolted down, we're probably carrying about 10% of our boat's displacement in non-working sails, anchors, dink, outboard, clothes, food, tools, etc. We're not racing, anyway.

One also has to re-think what's really necessary for gear. On our previous, heavy boats, I carried HUGE amounts of crap. Way more spares than needed, tons of tools that I never used, the list goes on and on.

The minimalistic approach that a light boat calls for is actually pretty liberating. When we sold our last boat, we already owned Rocket Science. Probably 80% of the weight that was on the old one didn't come along with us on the new one, and here we are, 4 years and a half a world later, and we don't miss any of it.

BTW the last time RS was rated PHRF, she was -39. Since then, she's been refitted into more of a cruising boat, so that number is no longer correct.

The purpose of my post with the stats, though, was to support the idea that performance boats can do well loaded, on average outperforming their heavier counterparts, and that 70% is easy to exceed, even with a long-distance cruising weight aboard.

Over the nearly 4000 miles that we averaged 8.0, a lot of it was motoring at 7 or so, and some of it was sailing slowly. So, we had many days at near 100% of 'hull speed', and quite a lot of time exceeding that. This was the original premise of the thread, I believe. On that crossing, our best 24h run was just over 260nm, 10.9 average speed, shorthanded, with our one crew complaining that we were sailing the boat like an Island Packet-way too conservative for his taste (funny how the guy not writing the checks always wants to push harder...).

I don't think that we sail differently than any reasonably competent mom and pop. I've never raced, and care very little for getting max speed out of the boat.

That run was early in the crossing, with over 200 gallons of fuel/water.

OK, RS is special-I'll grant you that, but she's also a 20 year old boat-surely more modern rigs have caught up by now?

It's a good discussion. I've been on both sides of this question, my previous 3 boats having been traditional full-keeled 'bluewater cruisers' and I will absolutely never go back to a more traditional, heavy boat.

Thanks for the input, gents.

Your boat is certainly a very unique boat and must be fun to sail.

I agree completely that as boats bigger stores become a smaller percentage of total displacement and the penalty of loading a light boat is diminished. But take your boat as an example, carbon with ballast tanks, it still, per your earlier post, only cruising at 80% of hull displacement and I don't mean 80% disparagingly. If you compared it to a Swan 66 that rates -30, so similar to your boats rating, would it be much slower on a passage? Probably not and it could carry any stores the couple wish to carry.

What I'm getting at is the cruising sailors is not going to see speeds like racing shells see regardless of the boat they sail for lots of reasons. If you sail a passage at 70% of theoretical hull speed you're doing pretty darn good, if you see 80% you have done fantastic.

There are no right or wrong answers for cruising boats, only choices.

[Kenomac]

Originally Posted by Dockhead

So are you saying that the many people in here who have reported sailing above hull speed are confusing speed through the water with speed over ground, and don't know about currents? I don't think I've ever met a sailor who was unaware of such basic concepts, and the people on here certainly deserve more credit than that, I think.

You should read Paulo's excellent post for the theoretical explanation for it, if you don't believe the many people who have described their experiences. The fact is that any displacement hull can exceed hull speed with enough power, and that lighter boats don't require all that much power to do it.

And "lighter" is not racing boat light -- Paulo says:

"When it has clearly established that there was not not a single hull speed for the same LWL, but a fixed one for boats with a D/L of 240 and over and another one, a variable one, for boats with a D/L lesser than 240, one that for the same LWL is increasing with the lightness of the sailboat . . . "

He's not talking about planing, or surfing, he's talking about sailing in normal conditions. He's saying much the same thing as I said earlier -- which was that the power vs. speed curve turns up less steeply when the boat is lighter.

I also like the way Paulo explains it -- not "breaking hull speed" but that hull speed is higher for lighter boats. That corresponds well to my experience. That's also what naval architects talk about -- they say that hull speed is really somewhere from 1.2 to 1.51 for displacement monohulls, depending on weight, fineness, etc.

Of course the physics of wave-making don't change, what changes is the amount of waves which are being made. The lighter the boat, the less waves are made, and the less power needed to climb the smaller bow wave. The point at which the bow wave starts to become unreasonably steep comes later and later.

I’m still waiting for an explanation on how your Moody 54 came to be 18 inches longer and 1,132 pounds heavier than all othe Moody 54s. Separate molds?

[TJ D]

Originally Posted by Joli

There are no right or wrong answers for cruising boats, only choices.

Absolutely. The main point is to be out there doing it. But, too often, sailors convince themselves that a very heavy, traditional boat, with a small rig to boot, is the only really proper way to go offshore.

We get told frequently that our boat is far too big and powerful for just a couple to sail safely. We generally just smile and nod and sail away.

There's an old saying, from Mark Twain, I think- "Man as he ages, tends to become more resistant to change, particularly change for the better." I feel like this is very much in play with the folks who are stuck thinking that a Westsail 32 or the like is the only appropriate offshore boat.

Obviously, that's not you, but the sentiment is alive and well, that's for sure.

[Dockhead]

Originally Posted by Kenomac

I’m still waiting for an explanation on how your Moody 54 came to be 18 inches longer and 1,132 pounds heavier than all othe Moody 54s. Separate molds?

Do you see the words "shoal keel" there? Are you aware that shoal keel versions of boats use more ballast and are heavier to make up lost righting moment?

But the main point is don't believe all data you read for free in the Internet. There are many errors, versions of boats are confused, etc.