Online article regarding the seaworthiness of the 5.80

[scratcher]

Hello everyone. My name is Peter, and I have just joined the forum.

I am very interested in the whole idea of a small, trailerable sailboat that can literally go to "any blue bit of the map".

Especially if a lift keel version becomes available, which someone else asked about on this forum. That would definitely be my dream boat. :-)

However, my question revolves around this article I recently stumbled across on the internet:

interestingsailboats.blogspot.com/2020/03/class-mini-580-inexpensive-boat-to.html

The author basically says that a 5.80 is a nice coastal sailer, but not a blue-water / open ocean boat.
Has anyone else come across this article?

I am not posting this to insult anyone, least of all mr. Maderski. However, I am new to the "dark art" of boat design, and I would like to know what makes a sailboat a good open ocean boat. Or not, as the case may be. Even in the comments to the article, there are a lot of contrary opinions regarding seaworthiness.

I would appreciate anyone's thoughts on the matter, especially with regards to the sea trials of the 5.80.

Regards,

Peter

[Daniel]

You may have noticed that in that article, there were a lot of generalizations about stability, and some generalized comparisons to other vessels that are largely spurious, but there was no mention of the actual stability of the 5.80. The stability curve can be found here:

www.classglobe580.com/specification/

The point of vanishing stability for the 5.80 is very high and exceeds the requirements for Category 0 stability.
I would suggest that the blog you point to there does not have much useful information to offer.

[Daniel]

Also, the blog author is fundamentally incorrect about how stability equates to safety. The stability curve of the 5.80 is such that it is essentially self righting. Many of the beamy mini 6.50s that the blog author suggest are safer, are in fact also somewhat stable when upside down.

[Admin]

This guy was really interesting LOL!! and anyone who understands the most basic aspects of design and safety will realise very quickly that he shot his mouth off too early, before anything was released to give substance to the general statistics of the boat he is referring to, ...he simply made the wrong assumptions...he should have waited till we told the whole story just a few weeks later...so everyone is entitled to their opinion..and I am glad this guy is not building a 5.80 cause he would have to eat his words once he got sailing ....and he would be really upset about that

[pete]

Feb 25, 2021 16:56:27 GMT scratcher said:

Hello everyone. My name is Peter, and I have just joined the forum.

I am very interested in the whole idea of a small, trailerable sailboat that can literally go to "any blue bit of the map".

Especially if a lift keel version becomes available, which someone else asked about on this forum. That would definitely be my dream boat. :-)

However, my question revolves around this article I recently stumbled across on the internet:

interestingsailboats.blogspot.com/2020/03/class-mini-580-inexpensive-boat-to.html

The author basically says that a 5.80 is a nice coastal sailer, but not a blue-water / open ocean boat.
Has anyone else come across this article?

.....

Hi, I was actually the person who made this anonymous reply to that blog you state above (I was not any of the other replies below this one on that thread):

<

Anonymous May 20, 2020 at 10:15 AM

Not sure i follow your logic regards stability above.

...

>

The blog writer is confused about a lot of aspects of yacht stability.

Resistance to rollover from breaking waves (found offshore in extreme wind speed increase events or wind-over-tide etc) is related mainly to beam, and while the 6.50s are beamier, the overall difference is minimal when it comes to real world wave height. (even 50+ foot yachts have been rolled 'relatively' easily in breaking wave modelling).

For some actual science and yacht design facts I recommend the following books:

'Desirable and undesirable characteristics of offshore yachts' - Ed - John Rousmaniere (loads of good ex-Fastnet analysis here)

'Principles of Yacht design' - Eliasson, Larsson - (heavy, but includes every aspect of stability, down to calculating moment of intertia for carbon vs ali masts etc)

'Elements of Yacht Design' - Skene (old school, but obviously still highly relevant)

You are of course free to make up your own mind regards mini stability but I'd strongly recommned reading the theory from experts in the books, not taking the opionion of one blog writer with no qualifications visible (or in fact from me, who has no qualifications, but wanted to check the design was ok before shelling out a lot of money on building it). In my opinion I'd rather be in a boat that will right itself after a knockdown, over one that needs help (either requiring keel canting or water ballast shifting) to encourage it to re-right. Not to mention one that is extremely strongly built (i.e. not one with structures made down to a weight spec so it is light enough to fly).

Thats why I'm building one.

[scratcher]

Hello everyone.

Thanks for the replies. The thing is: I am from an aviation background, and for a (light) aircraft, a designer can make all the calculations by hand if necessary. And unless you have some weird configuration of wings and tail(s), you can be reasonably certain of the aircraft's behaviour. But with boats, I get the idea that there is no way to quantify the concept of seakeeping.

I did find this article: www.kastenmarine.com/beam_vs_ballast.htm
This article also contradicts the other author's fixation on beam for stability, and it also warns against too much ballast. Apparently the secret is a balanced amount of roll moment of inertia, but how much that is, they do not say. (Well, they wouldn't, would they?)

To my eyes, the 5.80 seems more than seaworthy enough, especially with the huge AVS value. But the first author insists that that is not the whole story. Hence my original question.

Regards,

Peter

[scratcher]

And thanks for the homework, Pete. I will definitely have a look at the books you suggest!

[pete]

Mar 1, 2021 18:28:41 GMT scratcher said:

Hello everyone.

Thanks for the replies. The thing is: I am from an aviation background, and for a (light) aircraft, a designer can make all the calculations by hand if necessary. And unless you have some weird configuration of wings and tail(s), you can be reasonably certain of the aircraft's behaviour. But with boats, I get the idea that there is no way to quantify the concept of seakeeping.

Hi - it is possible to make a number of not overly complicated calculations to figure out a yacht's general performance or stability characteristics. It doesn't need complex CFD or other modelling to achieve this initial assesment. The Principles of Yacht Design book takes you through these calcs step by step.

The key thing is to then compare the results you get to a known yacht with what has generally been accepted as the characteristics that you want - yacht design is mostly developed by iterating the designs to tweak certain features that are desirable for your use-case, and not sudden step changes in design.

The key thing to note is that almost every design change or tweak comes with a compromise (which is presumably very similar to aircraft design). i.e. you want to carry more sail for a given ballast, then you need more form stability (wider hull generally), but this comes at a cost of lower AVS, so you extend the keel to provide more righting moment if you want to maintain the AVS, but then your wetted area and overall structure weight increases, needing more sail area etc....

Your point above though is about seakeeping or seaworthiness, which is the kicker - defining what 'seakeeping' or 'seaworthiness' actually is, is perhaps the hard bit.

If you take it to mean safety, then maybe a boat that self rights easier is best, or is it a boat that doesn't roll over at all? - both of these are at the opposite ends of the stability spectrum - i.e. a boat shaped like a tube will not have any upside down equilibrium, and will always self right, but a boat that resists capsize in the first place will almost always have an upside down equilibrium (like a catamaran for example). You cannot have the best of both - hence the compromise point above.

Likewise, if you retain weight at the top of the mast to resist a breaking wave capzise (adding to the roll moment of inertia) - i.e. using an aluminium mast over a carbon one, or using roller furling jibs etc, then this is at the opposite end of the compromise spectrum from having a high AVS is you do actually capsize etc... so it becomes a spiral down to the characteristics you want the vessel to have.

If 'seakeeping' for you means having a slower pendulum motion in a seaway (better for crew comfort - i.e. is have a more rested, awake crew safer?), then having more roll moment of inertia is good (heavier mast) - again, sacrificing AVS, and so it goes on...

In summary for me the mini 580 has -

Not blistering mini-transat-esque performance, but sufficient for my requirements given its short waterline.

A high AVS keeps the boat upright if i screw up and we get knocked down (as i'm not a professional, and will be sailing alone, this is more important to me than outright speed)

A very strong hull and deck structure that i can make in a garage (not with fancy composites etc).

Somewhat finer ends than a purely planing hull - nicer fore/aft pendulum motion than a flat planing hull that would slam (also reduces forces on the hull structure).

Lightweight keel (relatively) so i can make it and move it when dismantled (i.e. feasible for DIY construction).

Hard chines add to roll resistance and are easier to build.

Enough space inside to be usable.

Cheap.

Each of the above is compromising something else, but in my mind the design makes sense for my personal use-case and ability.

[jimschofield]

Well summarised Pete,

From building the 580, I can see it is stronger than some production 30 ft boats I've sailed. And I know exactly what has gone onto it, so I trust it. If my Globe 580 carries me across an Ocean, safely and in relative comfort, it will have fulfilled all my needs.

Jim

[martellmarine]

There is a follow up article here (about the 5.80 and the Mini Globe Race) to the one in the original post by the same author. The 5.80 relevant bits are toward the end. You all should read it. 

Thoughts?

[Joe - Sail No 46]

Reads like he thinks it sounds like fun boat to build and adventurous to sail and is probably building one himself hoping to get onto the leader board !!

[Igor]

Mar 2, 2021 23:19:47 GMT jimschofield said:

Well summarised Pete,

From building the 580, I can see it is stronger than some production 30 ft boats I've sailed. And I know exactly what has gone onto it, so I trust it. If my Globe 580 carries me across an Ocean, safely and in relative comfort, it will have fulfilled all my needs.

Jim

+1!!

[scratcher]

Thanks, everyone for your replies. Especially you, Pete. I have since bought the book you recommended, and I am (slowly) working my way through it.
Looking at the pictures on the Class Globe 580 website, I have no doubt that the 580 is more than strong and stiff enough for the job, and I wish everyone a lot of fun with their new boats!

MartellMarine, your link somehow redirects me back to this page. Could you post the URL? I am curious whether the author has changed his mind about the 580.

Regards,

Peter

[peterbrumby]

You can find a post on his blog where he reviews “cheap” ways of doing ocean races.

He’s actually a little more complimentary about the 580, but still says he probably wouldn’t take it for long ocean passages.

That’s fine, his choice. Personally I’m following Jim’s Mantra!

[brucel]

interestingsailboats.blogspot.com/2021/03/5-vendee-globe-for-poor.html?m=1

Towards the end of the article.