What is "Hull Speed" Anyway

[Kenomac]

Originally Posted by Dockhead

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.

If that’s the case, then I proclaim all internet information regarding our Oyster 53 on the internet to be incorrect. I believe it to weigh 40,000 pounds, which now makes our boat faster than a Moody 54.... on paper. :-)

[Dockhead]

Originally Posted by Kenomac

If that’s the case, then I proclaim all internet information regarding our Oyster 53 on the internet to be incorrect. I believe it to weigh 40,000 pounds, which now makes our boat faster than a Moody 54.... on paper. :-)

I would never be so presumptuous, as to argue with you, or anyone else, about the specifications of your own boat, based on something I read in the Internet.

And I certainly wouldn't waste my time arguing with you about something, if I thought you were a liar who just made up stuff about your boat or your experiences on her.

[jtsailjt]

Originally Posted by Dockhead

I would never be so presumptuous, as to argue with you, or anyone else, about the specifications of your own boat, based on something I read in the Internet.

And I certainly wouldn't waste my time arguing with you about something, if I thought you were a liar who just made up stuff about your boat or your experiences on her.

Based on my own experience, I'd be willing to bet that your boat weighs more than you think it does because all that personal stuff brought on board and all the little things you've added to her and the extra fiberglass used by well intentioned workmen all adds up so almost all cruising boats weigh more than they are advertised to weigh. I'm not calling you a liar, just a general statement that applies to most of us who haven't had our own boats weighed recently.

As to the disagreement regarding the LWL of your boat, I have no idea which of you is right but wonder what makes you think it's over 46'?

Information being on the Internet makes it neither more or less credible than information found on a piece of printed paper. The fact that you own a particular boat makes you no more of an authority on its design than anyone else.

Ken has provided info indicating the LWL or the Moody 54 is 14.02 meters or 46', and the only Moody 54 listed on Yachtworld that includes its LWL as part of the specifications claims to be 14.02 meters LWL as well. Why do you think that both Kens reference as well as this boat on Yachtworld is advertising the wrong LWL? Or is your boat different from this one? Or did something change between when Dixon designed it and the mold was made? There must be an explanation. Whatever your source, there seems to be pretty widespread confusion regarding LWL for your boat. Your ongoing argument about it is getting us all nowhere. How can we determine what the actual LWL of your Moody 54 is once and for all?

[Polux]

Originally Posted by Joli

...

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.
.....
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).

As I supposed your data regarding the ARC is incorrect. I don't know how you have got it. Maybe you have considered corrected times and not real times? And regarding distance sailed not the real miles done but the rhumb line?

Regarding the miles, I did not have consider the 4000nm that TJD had done on that crossing. I had looked at the average courses on this year ARC (they had to come really down this time, near Cabo Verde and due to the winds had a very inconstant and variable course) and considered an average of 3600nm for the crossing.

Then I considered not the corrected times but (obviously) the real times ….and the numbers have nothing to do with the ones you posted:

On the Halberg Rassy 37, a boat with a D/L of 197 and a hull speed of 7.75 the average speed over 20 days 15 hours and 4 minutes was 7.27k and that is 93.8% of the Hull speed, not less than 70% as you say.

Even considering the 21 hours of engine use that is an extremely well sailed boat, I would say raced, even if on the cruising division, under spinnaker almost full time. Not only because 93.8% is a lot but also because even if the traditional hull displacement formula does not apply to a D/L of 197, it is still a considerable D/L for that performance. No wonder it was 1st on compensated time on the cruising division.

Regarding the Dufour 45e that won the Race Division, you say that the average speed is 6.02 knots and the percentage a bit over 70% of hull speed when in reality the numbers for the elapsed time (18 days 14 hours) give an average speed of 8.01kt and that is 94.7% of the hull speed.

However you should know that this edition was a very slow one, with weak winds that slowed down all the boats and had also as consequence a bigger distance done, since the boats had to go far away from the rhumb line to find decent winds.

If we compare the average speed over the water regarding the same boat, a 40ft Pogo S2 (Talanta) that made both races (this and last year), the average speed this year was only 9.07kt (107.1% of hull speed) and last year the average speed was 11.54kt (137.4% of hull speed). A huge diference.

Of course, the Pogo 40 S2 has a D/L of only 74 and therefore its hull speed is much bigger than the one giving by the old formula, the one you and I are using for finding the hull speed on all boats, when it should only be considered on boats with a D/L over 260.

But it would be more interesting if instead of looking at the better sailed boats on the ARC (raced) if we looked in the cruising division, to boats that were not raced but just well sailed by a crew (and did not finish among the first in compensated) comparing on those cases the average speed with the hull speed, the one given by the old formula.

And regarding boats comparing that percentage between boats to whom that formula should apply (heavy boats) and to medium to light modern boats, where according to Dave Gerr, another formula should be applied to determine hull speed.

Light to Medium weight cruisers:

Xc 45 Arauni: D/L 177.39, hull speed 8.67, average speed 7.89kt, 91% of Hull speed. (44.83 engine hours)

Beneteau Sense 50 Pelizero: D/L 153.65, hull speed 9.01, average speed 8.42kt, 93.4% of hull speed. (82.25 engine hours).

Jeanneau SO 509 Region 33: D/L 143.62, hull speed 9.06, average speed 8.33kt, 92% of hull speed (78.10 engine hours).

Heavy boats:

Nauticat 521 Salamander: D/L 368, hull speed 8.48, average speed 5.88kt, 69.3% of hull speed (the boat was raced on the racing division and as the Dufour 45e and all the boats on the racing division has 0 engine hours).

Island Packet 420 Brag: D/L 257.45, hull speed 8.19, average speed 6.45kt, 78.8% of hull speed (engine hours 108.2).

Westerly Ocean 43 Kindred Spirit: D/L 286.69, hull speed 8.03, average speed 6.1kt, 76% of hull speed (engine hours 80.48)

https://www.worldcruising.com/conten...n%20161217.pdf
https://www.worldcruising.com/conten...n%20161217.pdf

You can consider more or less mileage in what regards this Atlantic crossing but regarding the percentage of hull speed the lighter boats will have always a much bigger percentage regarding the hull speed if this formula is considered.

Simply because Dave Gerr is right in what regards hull speed having to do with D/L. If Dave Geer formula was used for the boats with a smaller D/L, semi- displacement ones, the hull speed would be bigger and the percentage in what regards higher hull speed versus average speed would be approximately the same as the one of boats with a D/L over 260, with the traditional formula for displacement boats.

As it is, considering the same formula for all boats, the boats with a lighter D/L will have a bigger percentage, in some cases even higher, than the traditional Hull speed.

And that’s the last post. This really takes too much time…I hope this post helps to clarify the waters.

[TJ D]

Originally Posted by Polux

Regarding the miles, I did not have consider the 4000nm that TJD had done on that crossing. I had looked at the average courses on this year ARC (they had to come really down this time, near Cabo Verde and due to the winds had a very inconstant and variable course) and considered an average of 3600nm for the crossing.

Just for info, our crossing was from Charleston, SC to Ireland via the Northern route. We weren't sailing to the Caribbean, and we had a lot of light winds. Not a trade wind downwind ride for us.

[mitiempo]

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...

Makes sense except a Pogo 40 weighs a bit more - 4800 kg (10,560 lbs).

[Dockhead]

Originally Posted by jtsailjt

Based on my own experience, I'd be willing to bet that your boat weighs more than you think it does because all that personal stuff brought on board and all the little things you've added to her and the extra fiberglass used by well intentioned workmen all adds up so almost all cruising boats weigh more than they are advertised to weigh. I'm not calling you a liar, just a general statement that applies to most of us who haven't had our own boats weighed recently.

As to the disagreement regarding the LWL of your boat, I have no idea which of you is right but wonder what makes you think it's over 46'?

Information being on the Internet makes it neither more or less credible than information found on a piece of printed paper. The fact that you own a particular boat makes you no more of an authority on its design than anyone else.

Ken has provided info indicating the LWL or the Moody 54 is 14.02 meters or 46', and the only Moody 54 listed on Yachtworld that includes its LWL as part of the specifications claims to be 14.02 meters LWL as well. Why do you think that both Kens reference as well as this boat on Yachtworld is advertising the wrong LWL? Or is your boat different from this one? Or did something change between when Dixon designed it and the mold was made? There must be an explanation. Whatever your source, there seems to be pretty widespread confusion regarding LWL for your boat. Your ongoing argument about it is getting us all nowhere. How can we determine what the actual LWL of your Moody 54 is once and for all?

If you had read the previous posts, you would know the answer to this question.

I own the prototype of this type of vessel and possess a set of the original design documents and the as-built survey, and numerous other original documents concerning the development and history of this vessel. I was based for years at the old Moody yard and know the designer, whose office is still there, know members of the Moody family, and know a number of people who were involved in building my boat.

Internet sources concerning everything, including boats, frequently contain errors, and they are worse in the case of the last generation of Moody boats because the company disappeared soon afterwards and there was nobody around to correct them. One of the main errors concerning by boat was caused by dissemination of data relating to the shoal draft version, which differs in several respects, but there are other errors, including a widely published figure for LWL.

Not that one foot of LWL of this or that boat really matters to anyone! But I do really dislike it when people spend a few minutes on Google, start to imagine themselves experts on the basis of this, and then argue with any and all without even attempting to evaluate what someone else might know. Especially when it concerns one's own boat.

A good demonstration of the error-proneness of Internet information on this boat (and many other boats!) is the wide variety of figures on D/L. The last information Ken posted conflicts with earlier information, but in fact is correct for the SHOAL KEEL VERSION -- 201.

As the weight -- none of us who has not weighed our boats knows exactly. I'm sure all of us who cruise and have cruising gear on board weigh a ton or two more than specifications, and more than that if the specs are based on light ship displacement. It makes sense to compare boats, therefore, on the basis of the specified displacement -- it's the closest to apples and apples we'll get. But taking it with a grain of salt. In the case of my boat, I haven't weighed her and presume she's a ton or two over specs, like every cruising boat. But I do know what she weighed when the as-built survey was done, and so I know that the published figure is conservative, based on half tanks and fully equipped with the standard fitment generator etc.


A bit of drift, but note that as TJ said -- bigger boats swallow typical cruising loads with less effect on their sailing characteristics, so if all cruising boats are somewhat overweight compared to their specs, then bigger boats will sail closer to their specifications than smaller ones, on average.

[Jim Cate]

Next time you're on the hard, DH, drop a vertical from each end of the WL and measure it yourself. Be sure to have a friend with a video camera to record the event, and provide the NBS certification on the tape measure, certified copies of your passport and birth certificate and observations of the temperature, baro pressure and moon phase.

Then, maybe Ken will believe you... maybe not.

And while it is fairly easy to measure such lengths, getting similar accuracy with the displacement isn't a slam dunk. In fact, few places have any accurate means of doing so. The usual gauges on the travel lift are notoriously inaccurate, and I'm not familiar with other common means of weighing a 20+ tonne yacht. Maybe a load cell on a single point hoist by a crane... dunno?

I agree that our yachts tend to weigh more than we think... sometimes embarrassingly so, but as long as we all use some light ship sort of data for our calculations, at least we start from a sorta comparable point. I'm sure that we cruise at something like 25-30% over light ship... maybe even more with all tanks full, though we seldom carry full fuel loads.

But no matter what the dimensional analysis says, seems pretty obvious that many of the boats sailed by active CF members can exceed the 1.34 type hull speed. Disbelief does not alter that fact, no matter how often repeated.

Jim

[Dockhead]

Originally Posted by Jim Cate

. .. But no matter what the dimensional analysis says, seems pretty obvious that many of the boats sailed by active CF members can exceed the 1.34 type hull speed. Disbelief does not alter that fact, no matter how often repeated.

Jim

Yes, well, I think very few people believe that more or less modern yachts are limited to the S/L 1.34 hull speed, nor does the theory say that this is a hard limitation. Most of us know the contrary simply from experience, and no, most of us know the difference between SWL and SOG. Paolo has articulated better than I was able to the theory behind it.

Years ago, I experienced the truth of what Paulo wrote when I started chartering Salona 45's in the Adriatic, a nice Croatian (or Slovenian?) cruiser-racer, quite light (SA/D 150 according to the published statistics) and with a lot of sail area -- really fun to sail. Hull speed should be 8.5, but you didn't feel any sort of extra resistance on that boat until well over 9 knots. Doesn't mean there is no hull speed at all -- you can't really get that boat above 9.5 without surfing. But it's a much softer and less limiting hull speed, than what you have on a heavier boat. It's logical if you keep in mind that this very light boat (10 tons) makes very little bow wave, so nothing much to climb.

Weight also influences surfing behavior. On my old boat, I don't think I ever surfed for more than a few minutes at a time. The Salona 45 would surf like crazy; I saw 14 knots one time for the better part of an hour. My Moody surfs pretty readily if there's enough wind, but I never saw 14 knots for even a minute at a time.

I'm guessing your boat is probably quite like that Salona.


This is thread drift, but I had a sudden inspiration thinking about this last night -- concerning bow form. I have never much like the current fashion for plumb bows -- because my many years of experience bashing upwind in windy places, where you really want the extra buoyancy of a raked bow to keep the bow above the waves. I don't even like the moderately raked bow of my present boat, which is already a little old fashioned -- I pretty often -- where I sail -- get the bow pulpit completely underwater, which regularly destroys nav lights or at least the wiring connections. More rake would reduce this problem.

But it occurred to that bow rake works against you when you're trying to push hull speed -- the extra buoyancy pitches you up when you start to climb the bow wave and increases resistance. With less buoyancy at the bow, the bow wave at hull speed has less effect. Don't know why this never occurred to me earlier, but this must be a significant reason for this fashion. A fine entry adds more to this effect.

Likewise, when thinking about hull speed, we should not forget other dynamic effects -- "wedgies", as we discussed, use the wide stern to add buoyancy to reduce squatting at hull speed. But many non-wedgie boats, like mine, have a "bustle" under the transom which will add buoyancy as the boat squats, and also lengthen the waterline. A lot of boats with hull forms like mine, have a couple of feet more hull behind the static waterline, which will be engaged with the water at hull speed. Another reason why different boats, including not only wedgies but also other hull forms, will not experience the strongest hull speed effects right at S/L 1.34 based on the static waterline.

[TJ D]

Plumb bows... Yup, they can be very, very wet! We had to change the way we handle maneuvers involving foredeck work. We pinch up and really slow the boat down, or bear off as appropriate, do our business, and then carry on. We do this even in pretty moderate weather. Never had to do that before.

This mucking around with a foot of green water flying by is best left to young guys who enjoy (or get paid for) this sort of thing.

But, with that in mind- I think that the plumb bow has by now proven itself to help with performance-not too many raked bows out there dominating the race course!

For my part, I'll take the loss off buoyancy and a wet foredeck as a tradeoff for performance. We slow dow a lot less when we thump into a sea than we did on other boats. But, it's a white-knuckle ride if you don't slow down before you head up there!

[valhalla360]

Back in the day when most cruising boats had very similar hull designs, were heavy and rarely if ever planed, the 1.3-1.4 number worked very well. But a few thoughts:
- Anyone telling you the hull speed down to the 1/10th of a knot or worse 1/100th of a knot has no understanding of the concept.
- Throw enough power at it and yes, you can exceed hull speed but each kt extra requires massively more power.
- The standard formula is based on a full displacement hulls of typical shape. Add planing or more complicated, semi-planing into the mix and you can frequently exceed hull speed.
- Length to beam ratio has a major impact. I am no longer able to find the formula but the 1.3 is no a fixed number but related to the ratio. High speed cats and tris have such a high length to beam ratio that hull speeds can reach freeway speeds and beyond and other factors control top speed.
- With some of the modern boats with wide flat bottoms, they semi-plane or if heeled, the water line beam can change significantly.

No magic involved just a change in design and misunderstanding. For typical non-high performance cruisers, hull speed is a good starting point for what to expect in good conditions. You might beat it occasionally if you are overpowered but don't count on it regularly.

[Polux]

Originally Posted by Joli;2543575[B

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

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 don't understand your point. You are comparing a 40ft fast very light boat with a light relatively fast 50ft to arrive at the conclusion that probably the very light fast boat is as fast as the much bigger light cruiser!!!!

That has been the point of this thread, that regarding boats with less than 260D/L there is a proportionality regarding hull speed and the lower D/L.

If you compare the Jeanneau 519 with a Pogo 50, a boat of the same size, the diference will be huge the same way that if you compare the Jeanneau 519 with an heavy Hinckley Sou'wester 50, the Jeanneau would be much faster.

That only proves the point in discussion on this thread.

Regarding weight the reason guys buy very fast boats is because they like to sail fast as much as they like cruising (or even more) and those guys maintain their boats relatively light. if one is the kind of guy that lives with a lot of stuff if and likes to sail fast than it should buy a bigger very light boat that would be less affected by the extra weight.

By the way you are way off regarding the weight of the Pogo 12.50, it does not weight 5400lbs but about the double (12125 lbs).

[Polux]

Originally Posted by TJ D

Plumb bows... Yup, they can be very, very wet! We had to change the way we handle maneuvers involving foredeck work. We pinch up and really slow the boat down, or bear off as appropriate, do our business, and then carry on. We do this even in pretty moderate weather. Never had to do that before.
...
But, with that in mind- I think that the plumb bow has by now proven itself to help with performance-not too many raked bows out there dominating the race course!

For my part, I'll take the loss off buoyancy and a wet foredeck as a tradeoff for performance. We slow dow a lot less when we thump into a sea than we did on other boats. But, it's a white-knuckle ride if you don't slow down before you head up there!

The new designs addressed that problem with the use of chines that deflect the water outside and increase buoyancy.

Regarding bows on the last years there has been a slow revolution. On a near future the tendency would be for slightly rounded bows and slightly inverted for the same reason that lead to the plumb bow (more buoyancy and bigger LWL)



Some cruiser designs are already using some of those charachteristics on the bow design:



You can see here how it works, with the water being deflected outside:

[TJ D]

Originally Posted by Polux

The new designs addressed that problem with the use of chines that deflect the water outside and increase buoyancy.

Regarding bows on the last years there has been a slow revolution. On a near future the tendency would be for slightly rounded bows and slightly inverted for the same reason that lead to the plumb bow (more buoyancy and bigger LWL)



Some cruiser designs are already using some of those charachteristics on the bow design:



You can see here how it works, with the water being deflected outside:

Well, our bow looks like this too, with the hard chine beginning well forward of the mast. It's not as pronounced as 'Safran', but it's there. Anyway, even the chine just turns green water into a wall of heavy spray, which still sucks to be under when you're up forward.

It's still wetter than other boats we've sailed. Even the most modern race boats have foredecks that are seriously wet. If you look at videos of IMOCA boats now, they're tossing at least as much water back over the deck as did older designs.

I think that the move toward a fuller entry helps, but I think that it's pretty hard to say that any of the fine plumb bows are submerging less than more traditional designs with rake and flare.

But, who cares?? I'm perfectly happy to see the bow punching through seas--from under the dodger in the cockpit! I don't really care if we're putting a ton of water down the deck.

We just slow down when it's time to do some foredeck work. We're cruising, so if we're slowing down for a while to make the bow crew (usually me) happy, so be it. I'll take that over hobby-horsing my way along for days!

[malbert73]

This past summer my shift cable snapped while I was trying to engage reverse in a downwind docking situation. My crew was able to jump onto the dock with 4 feet remaining and start to slow the boat (which was going >2 knots in 20-25 kts breeze. But we hit the dock hard with the bow, and the boat rode up onto the dock like a breaching whale, and then slid back. No damage to laminate, just scraped paint. With a plumb or reverse bow the damage would have been impressive with that much momentum and mass. But no doubt my design while quite fast for a cruiser is not as fast for 40 foot LOA as some more modern designs.

I never had considered this benefit of raked bows for real world cruising. While what happened to me is really rare, what about logs, containers, etc? Just thinking about tradeoffs of performance vs real world cruising.