Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 24-11-2008, 22:38   #31
Registered User

Join Date: Sep 2007
Location: Colombo
Posts: 1,059
Quote:
Originally Posted by Hiracer View Post
I never said steel makes for good racing boats. What I said is that in a cruising context steel need not be meaningful slower than the next cruiser.
You did not twig to the context of my comment. If what you say is correct that increased displacement can be made up by sail area then one has to wonder why open class race boats are not built of nice strong steel (or heavily built of something else in an extravagent manner to ensure they will never break) and the weight penalty made up just by adding sail. That is not done and that for good reason.

It would be tiresome for me to get into a lengthy discussion of the reasons why this is so but anyone not knowing but interested should look out descriptions of Froude numbers, the effect of displacement on wave making resistance at slow speeds and the way "displacement speed" (aka "hull speed") is decreased as boat displacement increases. That will get one to finding that wave making resistance (which to windward is likely around 35-40% of total resistance) increases approximately linearly with displacement (so for two boats the same except for one being heavier than the other by 10%, total resistance (aka "drag") will increase by around 3.5-4%) and "displacement speed" is significantly reduced for a heavier boat (as it cannot reach the same Froude number as lighter ones of same hull form).

If you take two sail boat hulls of same design and all else being equal (eg surface roughness on underwater surfaces the same) except one is heavier than the other then the heavier one will be slower at all speeds. For two such hulls the main difference will be in resistance in wave making - as the boat proceeds energy is taken away by these waves. Without even any analysis I think it is intuitive that a heavier displacement boat will "displace aside" more water than a lighter one and so have more energy taken away from it in those waves.

I suspect that for vessels much under 40 foot it is possible to built a strong composite boat, for example, considerably less than half the weight, excluding ballast, of a best build in steel. I know of composite boats (one done multiple Cape Horns) over 50 foot that are lighter than our light for steel 40 foot steel boat (that comparison again less ballast, but including cruising fitout such as freezer, good accomodation, etc). I leave out ballast in these as ballast, as opposed to the weight of the rest of the boat, is offsettable against sail area. It is easy to see that a small steel boat built as light as possible may suffer a resistance penalty of more than 30-40% over a lightly, but still strongly, built composite one if both carry a sail area appropriate to their ballast ie the small steel boat will be a "slug" compared to the composite one.

Now as you say, you can just add more sail but that will never increase the "displacement speed" as that is inversely proportional to displacement and the energy hump is too high to get over by adding sail for cruising boat designs. But a further fact is that if one can add sail to the heavy boat one to get speed up at slower speeds then one can add the same sail to the light one too and it will be faster again. In fact one can put more extra sail on the lighter boat because without ever giving any weight advantage to the heavier boat one can add displacement in the lighter boat's keel as ballast enabling it to stand up to more sail than the heavier hulled boat can.

So the argument of adding sail to the heavy boat to beat weight is a spurious one and furthermore no matter what you do once hull speed is approached the heavier boat, of the same hull form, will always be slower no matter how much sail you put on it. The heavier boat can only be faster than a lighter one if the lighter one is disadvantaged in some way such as by less efficient hull lines, or smaller rig than such a boat could otherwise carry, or in less efficient crewing.

Adding sail to defeat heavier displacement also introduces the disadvantages of carrying more sail than would otherwise be necessary on a cruising boat, especially one intended for blue ocean cruising.

There are though situations where the performance of two similar boats that are only differentiated by their displacements may wander away from the above - one situation that comes to mind is in heavy pitching conditions where the heavier boat will have a larger longitudinal mass moment of inertia than the lighter one and so the two boats will have different resonant frequencies in pitching. This may mean that in some pitching conditions the heavier boat gets an advantage and in others the lighter boat gets an advantage.

There are crew factors too, it may turn out that the distribution of mass in the heavier boat (by way of increasing longitudinal and roll mass moments of inertia) makes for lower accelerations in pitch and roll than the lighter one. That is especially important in smaller vessels in heavy weather and so may mean the heavier boat is faster in heavy conditions but that only from increased crew performance not from inherent boat speed.

But, overall, the lighter boat will always be faster and we know that as fact without even boring ourselves about all the above just by being aware of the quest for light displacement in open class race boats, and where whatever discretionary displacement there is in the weight budget goes into ballast in the keel or crew on the rail.

Note I am not making any case that slow is bad for a cruising boat. I am just refuting the claim that my contention that because of the impossibility of building steel sail boats much under 40 foot that are not very heavy compared to what is possible in other constructions they will always be slugs in comparison, is wrong. I know from the great care that went into keeping the structure of our own 40 foot steel boat as light as reasonably possible that even at 40 foot you will come out with a boat that can only be described as very heavy compared to a composite one, for example, that equal care in weight has been taken.

In the end we mostly all love our boats but that should not make us blind to their deficiencies.

Off to my glass of wine now and no more from me on this refutation .
__________________

__________________
MidLandOne is offline   Reply With Quote
Old 24-11-2008, 23:20   #32
Registered User

Join Date: Jun 2008
Posts: 128
Steel other advantages

Wow, I have been away for a couple of weeks. On return I see many really quality replies to my original post. Thank you all. A very interesting topic and at times heated! It seems most of the band width has been taken up talking about the speed of steel hulls vs. composite. While speed is an issue it seems few have mentioned safety. One of the most compelling reasons to consider steel in my case is the safety issue.

I dream of cruising off the beaten track. Does steel offer more safety? Almost anyone can weld steel if needed. Even in underdeveloped places. If I except that I will become grounded at times does steel not offer more?

It is true, I would like a modicum of speed. I gather a well designed, well built steel boat may still offer more peace of mind in some circumstances.
__________________

__________________
Solosailor is offline   Reply With Quote
Old 25-11-2008, 11:02   #33
Registered User

Join Date: Oct 2006
Location: The boat lives at Fidalgo Island, PNW
Boat: 36' custom steel
Posts: 992
Quote:
Originally Posted by MidLandOne View Post

It would be tiresome for me to get into a lengthy discussion of the reasons why this is so but anyone not knowing but interested should look out descriptions of Froude numbers, the effect of displacement on wave making resistance at slow speeds and the way "displacement speed" (aka "hull speed") is decreased as boat displacement increases.
That's just about what I said, only with fewer words. I mentioned the ease of escaping hull speed, but you are right that hull speed for light vessels is slightly higher than for heavy boats.

But, if you compare hull speed between two equally sized vessels, one with a D/L ratio of 180 (about as low as you can reasonably get in a cruiser under 40') compared to one with a D/L of 300, you will find the difference in hull speed to be essentially meaningless. It's a small fraction of a knot.

Quote:
Originally Posted by MidLandOne View Post
So the argument of adding sail to the heavy boat to beat weight is a spurious one and furthermore no matter what you do once hull speed is approached the heavier boat, of the same hull form, will always be slower no matter how much sail you put on it.
Hull speed is hull speed. Light boats have a higher hull speeds, but the increase in hull speed is so slight that in the cruising context it's meaningless. Heavy boats need big sails to reach hull speed. Big deal.

As I said the first time, light boats accelerate faster and can exceed hull speed easier. Both of these performance attributes are important in racing. Neither is a major benefit in the cruising context.

And as I previously stated and you have pointed ignored, displacement is a major aid to carrying a heavy loads, something cruisers should very much be concerned about, certainly more concerned that the incredibly small incremental increases in speed you are referencing.

Trying to argue that what's good for racing is good for cruising doesn't strike me as the most effective argument. Racers are not load bearing vessels. If you look at the people actually cruising with sub-40' boats, you will be hard pressed to find many such boats that are akin to racers, for the simple reason that sub-40' racing sailboats can't handle the load. Most cruisers under 40' are mid-weight boats. A properly designed steel boat can keep with those boats just fine, as those of us with first hand experience can attest.

The real blow to your argument, which I will deliver at a later time, is to take two boats the same size, same SA/D ratio, but different displacements. After you load them with 4,000 pounds of gear, the lighter boat will have a resulting SA/D ratio that is significantly lower than the heavy boat. Meaning, the lighter boat will be slower, as a matter of physics, simply because the lighter boat has less power from its sails compared to the heavy boat. The addition of the cruising load adversely decreases SA/D much more for the light boat than the heavy boat. Which goes a long way to explain why you don't actually see many light sub-40' boats out there cruising. After you load them up, they are too slow.
__________________
John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.
Hiracer is offline   Reply With Quote
Old 25-11-2008, 12:18   #34
Registered User

Join Date: Oct 2006
Location: The boat lives at Fidalgo Island, PNW
Boat: 36' custom steel
Posts: 992
OK, here is the math on how weight more negatively affects SA/D for light boats than heavy boats.

If you take two boats at 35' with the same empty SA/D ratio of 17, the one with an empty displacement of 17,000 pounds will have a better final SA/D ratio than the other boat with an empty displacement of 10,000 pounds after you load each of them up with 4,000 pounds of cruising stores. Assuming reasonable design quality for each boat, the heavier boat will be the better sailor fully loaded--period. Its SA/D after loading will be 14.8 compared to only 13.6 for the light boat. That is a meaningful difference and will be readily apparent in the real world.

All the talk in the world about Froude numbers is irrelevant when comparing a SA/D of 14.8 to 13.6. Horsepower rules. This in a nutshell is why a fully loaded steel boat, if properly designed, can keep the pace with plastic cruisers.

Also, bear in mind that the stability curve for the heavy boat will be less adversely impacted by this load than in the case of the light boat. Further, the heavy boat will have the room to store it, while the light boat might well lack the space.

On the race course, of course, it's different set of physics because racers don't carry heavy loads.
__________________
John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.
Hiracer is offline   Reply With Quote
Old 25-11-2008, 12:30   #35
Registered User

Join Date: Oct 2006
Location: The boat lives at Fidalgo Island, PNW
Boat: 36' custom steel
Posts: 992
Dave Gerr, N.A., spends a chapter or so in his book, The Nature of Books, analyzing the allege speed difference between light an heavy sailboats, IIRC. If somebody wants an expert's view on this question, I can suggest his book.

The Nature of Boats, Dave Gerr, Book - Barnes & Noble

Over all the book is excellent on variety of both motorboat and sailboat issues.

* * *
Biography


Dave Gerr (rhymes with “bear”) is a naval architect who designs both yachts and commercial vessels. Gerr Marine has been in business since 1983 and has designed everything from dinghies to 60-foot around-the-world racing sailboats, an 82-foot aluminum voyaging motoryacht, and currently, redesign work on the U.S. Navy’s nuclear submarines. Gerr is a contributing editor with Boatbuilder, Yachting, and Offshore magazines. HOMETOWN: New York, NY
__________________
John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.
Hiracer is offline   Reply With Quote
Old 25-11-2008, 12:33   #36
Registered User

Join Date: Sep 2007
Location: Colombo
Posts: 1,059
Quote:
Originally Posted by Hiracer View Post
All the talk in the world about Froude numbers is irrelevant when comparing a SA/D of 14.8 to 13.6. Horsepower rules.
This is absolute rubbish, with respect to displacement boats. It would belittle me to waste my time taking this argument seriously enough for me to comment further.
__________________
MidLandOne is offline   Reply With Quote
Old 25-11-2008, 12:37   #37
Moderator Emeritus
 
GordMay's Avatar

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 31,573
Images: 240
Quote:
Originally Posted by Hiracer View Post
... If you take two boats at 35' with the same empty SA/D ratio of 17, the one with an empty displacement of 17,000 pounds will have a better final SA/D ratio than the other boat with an empty displacement of 10,000 pounds after you load each of them up with 4,000 pounds of cruising stores. Assuming reasonable design quality for each boat, the heavier boat will be the better sailor fully loaded--period. Its SA/D after loading will be 14.8 compared to only 13.6 for the light boat...
But, why would you compare a taller-rigged, heavier boat, to a shorter-rigged lighter boat?
Wouldn’t you typically expect the lighter boat to have a higher “empty” SA/D ratio than the heavier boat, of otherwise comparable features?

Dave Gerr’s “The Nature of Boats: Insights and Esoterica for the Nautically Obsessed” is also available to read (free) on-line.
http://books.google.ca/books?id=yc6Q...um=4&ct=result

Gerr is also the author of the “Propeller Handbook”, and “The Elements of Boat Strength”, all published by International Marine/McGraw-Hill
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"



GordMay is offline   Reply With Quote
Old 25-11-2008, 12:44   #38
Registered User

Join Date: Oct 2006
Location: The boat lives at Fidalgo Island, PNW
Boat: 36' custom steel
Posts: 992
Quote:
Originally Posted by GordMay View Post
Wouldn’t you typically expect the lighter boat to have a higher “empty” SA/D ratio than the heavier boat, of otherwise comparable features?
Bingo. Precisely correct. Most light boats have, in fact, higher SA/D ratios. Which is why they are faster--not because of the lighter weight. What we have here are confounding effects and the misattribution of causes.

But there is no law of physics that says heavy boat must have lower SA/D ratios. It is harder to design a decent SA/D ratio for a heavy boat, for sure. Most times it's draft that is sacrificed. You need deep draft on a heavy boat to have the stability to stand up to a higher SA/D.

I used the above example to illustrate what happens to speed when SA/D is equalized. I think it's an eye-opener for some people, but I readily acknowledge that it does not represent what is typical. I do maintain, however, it is possible. My steel 36' has a SA/D of nearly 17; OTOH, it's draft is a tad over 6'. Thus, when I sail against other boats of similar size, I fair OK, unless the other boat has a much higher SA (which typically is a lightweight boat to boot). I am faster than plastic boats if they have lower SA/Ds; no surprise there.

I wish I had more draft, because my keel-to-total-draft ratio is less than lighter boats so I may not point as well, depending on the other boat. Obviously my hull, being heavier, sponges up more of my total draft--so for any given draft I have more hull and less keel than my lighter brothers.

The worst combination from a pure speed perspective, is shallow draft and heavy boat. That design will insure a low SA/D. So, when you combine the low horsepower from a low SA/D with the inability to point on account of the stubby keel, you end up with a slow boat. That model, however, describes many a steel sailboat, unfortunately.

To be sure, I don't like most steel sailboats. I don't think they have been designed well. They are undercanvassed. As a consequence they do poorly on speed metrics.
__________________

__________________
John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.
Hiracer is offline   Reply With Quote
Reply

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
steinless steel fittings on steel boat Gregoris Construction, Maintenance & Refit 5 14-03-2009 07:03
steel hull boat prices? RRR Monohull Sailboats 7 25-09-2008 17:03
Putting New Zincs on a Steel Boat - How to do it? ssullivan Construction, Maintenance & Refit 9 11-08-2007 19:47
A question of steel boat painting anglooff Construction, Maintenance & Refit 6 31-07-2007 01:42
steel boat kraker Construction, Maintenance & Refit 1 15-04-2006 14:39



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 19:57.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.