Pilothouse Variations -- Boreal

[barnakiel]

It is getting somewhat monotonous, that 'reserve' hum.

Why assume there is any reserve required? E.g. the plumb (or, OMG, reverse!!!) bow can be matched with very narrow entry, or not, built in carbon, or not, matched with tall topsides, or not ...

There is definitely nothing wrong with plumb or reverse bows. We must look at boats as wholes and then we can say that this or that style of the bow works (or does not) on this specific boat.

Let alone that those expensive big beautiful PLUMB bows are most often owned and driven by retired Madame et Messieur from Paris, FR. They will love their petit 50'+ bateau very much and are not likely to push that beautiful plumb bow under l'eau. Jamais! The assurance would never pay the damages!

My boat has an errrr plumb (well, 'nearly') bow and she sails great. I do not see any shortcomings in this specific execution (A.D. 1969!). I do not see any need for 'reserve' anything here.

Tzu Hang burried her bow twice (or was it three times?) up to the point of capsizing over the bow ... neither plumb nor lacking displacement in the bow. How odd.

If you are burying her bow in the sea that's maybe you have specified to big an engine for your boat. Sail more. Speculate less.

b.

[Polux]

Originally Posted by Dockhead

To illustrate what I'm talking about --



The designer deleted that part of the bow shown in red here, with no savings in LOA. With it, he deleted all the buoyancy which that part of the bow would have provided when the bow is depressed in an oncoming sea.
,,,.

And there you come again with that!!!! What matters in a boat is the usable volume and deck surface. That is what makes the size of the boat, not the size of the pole (LOA).

The boat you redesigned with the inclined bow is a bigger boat with a bigger hull length and a bigger deck length. Put the first boat, with a plumb bow or inverted bow, with the hull length of the one you redesigned and it will have a much bigger bow buoyancy.

you must think all the best contemporary NA are silly and don't now what they are doing

By the way, that boat is the new club racing Swan. They clearly don't know what they are doing either.

[Polux]

Originally Posted by hoppy

It doesn't look like the bow shape on Safran matters

You could not be more wrong about that. Just look at the movie and see a couple of times when the boat tended to nose dive at speed. In fact with foils the bow buoyancy has to be increased to prevent that and I am not sure that they went far enough in what regards that in Safran's design even if the bow is also rounded to increase buoyancy.



You can see that on the Rotchild IMOCA the bow seems even more rounded offering more buoyancy.

https://www.youtube.com/watch?v=E3on5CYbZP4

A good look at the bow of the new Banque Populaire explains what I am talking about:

[Polux]

Originally Posted by TeddyDiver

It's actually the increase of water plane area that counts. Plumb bow having none.. And to mention another fault in Polux's reasoning. Plumb bow is worse compared to raked in regards of dynamic stability. Known fact at least since Scene and Herreshoff..

BR Teddy

Sure, that is why on contemporary yacht design plumb bows (or tendentially Plumb bows) are rarely used and the best NA design boats with big overhangs (including the bow).

They work in a sea way like a rolling chair:

Pitching up and down, on a very soothing rocking movement.

Plumb bows and sharp transoms, without overhangs, provide a much steadier and boring motion.

[TJ D]

Hi Dockhead,

Some numbers for you on RS.

Length of fairbody (does not include the bow prod for anchor and housing for some of the retractable sprit)--54'6"

LWL--52'11"
Beam-- 14'8"
Disp--24,000 lbs(she used to be 21,000-this reflects her actual 'cruising' dry disp.
Ballast (most of which in a bulb nearly 10' down) 9480 lbs. This does not include water ballast
Upwind sail is about 1500 square feet, downwind around 3000 with the big asym.

Righting moment is 62kip@ 20deg, 83 at 30, max is 129. IIRC maximum positive stability is 137 degrees.

D/L is 72.

Water ballast tanks are 200 gallons, located in triangular tanks outboard of the cockpit area. There seems to be some misconception among some in the group on this front. These are generally kept empty, and they are certainly not intended to be used as additional FW storage! The displacement hit only takes place at a time of one's choosing, when it benefits the boat. We have them empty most of the time.

She is built to DNV/ABS slamming standards for high speed motor/military craft. This spec calls for the structure to take 7.5 psi. Panel testing was conducted and failure of the hull panels occurred at 54psi. 400,000 cycles at 20 psi produced no damage. The deck will take 34psi.


svrocketscience.com has a very good technical details tab if you're interested further. We plan to be in the UK for a year or so, perhaps our paths will cross.

TJ

[TJ D]

One more thing on the water ballast.

Since these tanks are generally located aft, they change the fore/aft weight distribution to a substantial degree, effectively 'lightening' the bow. This change in fore/aft CG is definitely noticeable on the bow's behavior. We're quite a bit drier forward with a tank filled.

TJ

[Snowpetrel]

Originally Posted by Polux

Bow buoyancy has to do with the design of the bow, the forward hull section, with boat CB and weight distribution but I agree that on a cruising boat a bit of extra buoyancy is the way to go and that's why plumb bows make sense on cruising boats: they have more buoyancy than an inclined bow, considering hull lenght.

Polux, you are not comprehending what I wrote. I said the only length that really matters for a cruising boat is the length the marina and slipyard charges for. and normally they use LOA, including bowsprits and the like. If you make the bow plumb then you need a bowsprit to stop the anchor scratching up the paint, might as swell pull out the bow slightly to give you a slightly drier deck on the same waterline. or you can round the bow, or flair it and chop it off or do some sort of other carry on to acheive the same effect. But its mostly about what looks cool. It is not that a dead plumb bow is the most efficient shape for all hull types and roles. It is if you are racing to a handicap system that measures LOD but not bowsprits, but this has no real relevance to cruisers. By the way this old arguement has been going on for at least a 100 years, and design trends for cruising boats have followed racing rules, regardless of if it made any sense ever since cruising has been around. Its not a good or a bad thing, it just is the way it is.

What makes me chuckle are all the misinformed people who think its all so simplistic and ignore the bigger picture of trends, racing rules and fashion in the design of yachts. The older designs you talk about were measured under a different rule that penalised waterline length.

Also they diminish pitch. Have a look at some movies with old boats with old designed bows and compare the movement (pitch), on movies, of modern boats with modern bows. They increase also LWL and therefore hull speed.

There is a lot more than just the effect of the bow on this. Stern shape, D/L, rocker, longitudonal moment of inertia, rig weight, etc. There are a lot of good things about the new designs. but if we changed the racing rules to measure LWL the shape of the bows and sterns would quickly change and blunt nosed long overhang scows would possibly take over, even now in the mini classes these are doing well, dispite the box rule.

Regarding VOR boats and inverted bows it has nothing to do with looks but with efficiency: Increased LWL, better wave penetration and therefore less wave drag1 and more buoyancy for surfing downwind. You seem to think that the VOR 60 are an exception but the tendency is for all race boats (new designs) to have an inverted bow. Some examplessnip)

This is very wrong, the reverse bows on the VOR boats are all about looking cool. from the the design firms mouth...

“Our goal was to provide a visual link to the world’s fastest boats which are multihulls,” Patrick Shaughnessy, President of Farr Yacht Design, the boat’s US designer, tells Yacht-and-Coast. “Those boats have reversed stems as part of a wave piercing strategy which is arguably ineffective at the beam, length, and displacement of the VO65. In any case, we wanted to share some of that ‘modern look’ to ensure the image we presented was forward-looking and more likely to appear modern in 8 years’ time.”

full link here. http://www.yachtandcoast.com/sail/vo...bow-to-change/

Its now going to be fun watching some of the cruisers copy this latest fashion, and the uninformed commentators prattle on about how efficient this new concept is.

Saying all this, in my opinion too much forward overhang is even worse than not enough because it is more likely to scoop into a steep wave. And its more likely to pound. Better to get the bouyancy with flare or flam, and keep the overhang shorter as long as the anchor doesn't foul it.

[TeddyDiver]

^ second that

[Panope]

^ Third.

[TeddyDiver]

Originally Posted by Polux

They work in a sea way like a rolling chair:

.

Haha! Now you take a saw and cut those "overhangs" away and get rocking

[TJ D]

This is a great thread.

It's interesting to read the different perspectives here. All I can say is that I have been indoctrinated since childhood that heavy boats with small rigs, and lots of overhangs were the real 'sea boats'. Then in 2013, an amazing thing happened. I sold my 43' steel Roberts tank, and switched to the exact opposite end of the spectrum.

We've sailed RS about 11,000 miles since then, in conditions mild and robust, and I can state without reservation that I will never voluntarily embark on a passage on a boat with a waterline substantially shorter than the LOD.

Now, I would also not choose a boat like ours for the kind of expedition sailing that Dock is contemplating. For that, I would incorporate a lot of modern design elements, but would build heavier, out of aluminum, with a lot of watertight compartmentalization and a proper inside watchkeeping station.

For the rest of us, I submit that a well-built modern hull is superior to the older heavy boats in just about every way. (Emphasis on well-built).

TJ

[TeddyDiver]

Originally Posted by TJ D

This is a great thread.

For the rest of us, I submit that a well-built modern hull is superior to the older heavy boats in just about every way. (Emphasis on well-built).

TJ

Agreed!
The conversation has merely been about some design aspects in modern boat building and their pros and cons.
I'm might sound sometimes speaking against some of these characteristics but that's just to bring some reality to the one sided praisal sometimes seen around. In reality EVERY design feature, how well thought and whatever, has their drawbacks and we should recognize them especially when it's what we choose to have.

BR Teddy

[Dockhead]

Originally Posted by Polux

And there you come again with that!!!! What matters in a boat is the usable volume and deck surface. That is what makes the size of the boat, not the size of the pole (LOA).

The boat you redesigned with the inclined bow is a bigger boat with a bigger hull length and a bigger deck length. Put the first boat, with a plumb bow or inverted bow, with the hull length of the one you redesigned and it will have a much bigger bow buoyancy.

you must think all the best contemporary NA are silly and don't now what they are doing

By the way, that boat is the new club racing Swan. They clearly don't know what they are doing either.

No, the comparison is exactly correct -- same LWL, same LOA.


If you think that I think that all plumb bows, or the designers who use them are "silly" then you haven't understood anything which I wrote.

You have chronic difficulty understanding that many design decisions are tradeoffs -- you gain x and give up y, with any given design feature. And that these tradeoffs look different to different conditions, different sailors, etc.

I wrote that plumb, non-flared bows have significant advantages for SOME boats. Those advantages would be:

1. Possibility of greater fineness, which is maintained as the bow is immersed. This gives more speed and reduces hobbyhorsing as the bow "slices" the waves.

2. Less windage and weight (and COST) from chopping off the raked/flared part, for the same LWL.

However, they have a couple of disadvantages also:

1. Less resistance to immersion -- and no progressivity in this resistance.

2. Problems with anchors and some deck gear which often make bowsprits necessary.


As I wrote, some boats work very well with plumb bows -- especially very large and very high performance boats with empty forward sections and very low polar moment of inertia.

Plumb bows may also be ok for boats not used in strong conditions.


So plenty of good naval architects are using plumb bows and for good reasons -- no doubt about it.

Other naval architects, however, I am quite sure, are using plumb bows just because of FASHION, and not because they work better on that particular boat. Fashion plus cheapness -- a powerful combination. However, FASHION, Paolo, is not always the same as PROGRESS. The idea that everything fashionable is automatically better, and is better for all purposes and all conditions, is the Kool-Aid advertisers want you to drink -- it is a shallow way of looking at these questions. Especially with things like boat design where there are always lots of tradeoffs involved.

[Dockhead]

Originally Posted by barnakiel

. . . There is definitely nothing wrong with plumb or reverse bows. We must look at boats as wholes and then we can say that this or that style of the bow works (or does not) on this specific boat.. .

I agree 100%, especially the underlined part.

[hoppy]

Originally Posted by Polux

You could not be more wrong about that. Just look at the movie and see a couple of times when the boat tended to nose dive at speed. In fact with foils the bow buoyancy has to be increased to prevent that and I am not sure that they went far enough in what regards that in Safran's design even if the bow is also rounded to increase buoyancy.

My comment was tongue in cheek