Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 15-05-2016, 07:22   #151
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,751
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by poiu View Post
As well as going faster by carrying more sail you can choose to sail with the same sail area at less heel for more comfort and less leeway with water ballast.

Just out of curiosity, I looked at my boat's GZ curve and I work out that to reduce heel from 20deg to 15deg I would need about 1.9 tons of water ballast. Certainly achievable in a new build. A fair price to pay in loss of hull volume though.
2 tons of water ballast in a boat the size of yours would also be a significant hit on total displacement -- more immersion and wetted surface, which would slow you down. And 4 cubic meters (!) of volume.

I guess this is another question for the designer of a given boat. Water ballast probably works a lot better in some boats (very light and very wide) than others.

It would be interesting to see the numbers for Rocket Science.
__________________

__________________
"Parce que je suis heureux en mer, et peut-être pour sauver mon ame. . . "
Dockhead is offline   Reply With Quote
Old 15-05-2016, 07:38   #152
Registered User

Join Date: Mar 2011
Location: Caribbean
Boat: Oyster 66
Posts: 973
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Panope View Post
I do not mean to challenge the extremely talented Mr. Dashew, but I think this picture is somewhat relevant to the discussion.

Steve

FPB 64
You can get a cockpit view of that boat pushing through some moderate sized steep waves at the Needles in the video linked below. Gives a fair idea of how a bow will behave with a fine entry, low overhang, lowish sheer, lowish flare and moderate deadrise.

Crikey Baldrick, there a fair few factors at work here and there is no mention yet of how to balance it all with the shape of the stern:

Needles video - wind over tide.
__________________

__________________
poiu is offline   Reply With Quote
Old 15-05-2016, 08:00   #153
Registered User

Join Date: Mar 2011
Location: Caribbean
Boat: Oyster 66
Posts: 973
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Dockhead View Post
2 tons of water ballast in a boat the size of yours would also be a significant hit on total displacement -- more immersion and wetted surface, which would slow you down. And 4 cubic meters (!) of volume.

I guess this is another question for the designer of a given boat. Water ballast probably works a lot better in some boats (very light and very wide) than others.

It would be interesting to see the numbers for Rocket Science.
If you chose to fill the tank with your reserve diesel there would be no extra tankage. Likewise you could use drinking water. You couldn't benefit from running down your tanks though and still retain the ability to keep ballast. With these scenarios you wouldn't need double the tankage, and so 2m3 extra instead of the 4m3 you would need with sea water ballast.

There are clearly many issues. As to adding displacement, I imagine when water ballast is chosen at design stage then less keel ballast is chosen and an acceptance of having a more tender boat if ever water ballast is not used. Water ballast will make the boat less stable for example if broached on the wrong side with water ballast deployed. I don't know how that will factor into the balance. Maybe an emergency ballast release valve is adequate.

My GZ at 15 deg is about .38m and at 20deg is about .48m. Displacement of 38 tons With ballast located at 2.6m from the centreline you have a 6.0x lever arm at 17.5deg heel. To move a quarter (5 deg) of the heel from 20deg you need 38tons/6 *.25 plus a bit for the structure, so 1.8 tons rounded up to 2.0 for good measure and as I'm heavier loaded.
__________________
poiu is offline   Reply With Quote
Old 15-05-2016, 12:26   #154
Registered User
 
Polux's Avatar

Join Date: Nov 2011
Location: Portugal/Med
Boat: Comet 41s
Posts: 5,765
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Snowpetrel View Post
.....
Almost all cruising boats are heavy compared to modern racing boats. Its not just the weight but the longitudinal moment of inertia thats important. Race boats go to extreme's to reduce this by centralising all weight. A cruising boat has anchors, chain, windlass, samson post, fancy interior filled with gear forward. Plus typicaly a much heavier mast. And a bunch of heavy stuff aft like dinghys and all the cockpit locker clutter.

So a cruisng boat generally needs a touch more bouyancy forward to help keep the decks a bit dryer.
.....
Heres a question for you, are the new reverse bows on the latest VOR boats all about efficiency, or looks? should cruising boats adopt this latest trend? If a plumb bow is better than a raked bow, surely a reverse bow is better again? Where do we stop..
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.

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.

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 examples:




















The other modern tendency are kind of plumb bow but slightly rounded. That increases not only hull form stability as it increases hugely bow buoyancy. More than inverted bows (that pose some problems with anchors, I bet that we will soon a lot of cruising boats going that way. In fact there is already one, the new Pogo 36:
Polux is offline   Reply With Quote
Old 15-05-2016, 13:06   #155
Registered User
 
hoppy's Avatar

Join Date: Sep 2009
Location: Melbourne, Australia
Boat: Jeanneau Sun Odyssey 40
Posts: 2,842
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Polux View Post

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

__________________
S/Y Jessabbé http://www.jessabbe.com/
hoppy is offline   Reply With Quote
Old 15-05-2016, 13:29   #156
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,751
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Polux View Post
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.. . .
It seems to me that you are deeply confused about how buoyancy works.

Plumb bow does not give more buoyancy -- all bows have the same static equilibrium -- all boats sit on their lines. The difference is that when you push down a plumb bow, there is no increase in the hull volume as the bow goes down, and resistance to pushing down is constant. When you push down a flared and/or raked bow, the volume increases with increased immersion and the resistance increases. So raked and/or flared bows provide RESERVE buoyancy which plumb bows do not.

If you want to visualize it -- plumb bows have the forward part of the bow which is normally not immersed, cut off, and not available to provide reserve and increasing buoyancy when the bow is pressed down. As someone on here pointed out -- this cut off part is often then replaced, ironically, as someone pointed out, with a long bowsprit in order to make anchors usable and to give a place to rig an assymetrical etc. As is shown on every one of the images you posted but for one. So even if you think you're economizing on hull length, with a plumb bow -- think again. But economizing on hull length, even if it's possible, is not necessarily the point here anyway.

All this is easy to show mathematically.

On some of these boats, like Safran -- rake is clearly not needed, because this is a very long and empty forward section of a pure racing boat.

The problem is that designers imitate this look for cruising boats which do not have the length and emptiness in the forward sections to make this work, with bad results. Designing boats as a fashion statement rather than following sound NA principles is a very bad idea. The reverse bows shown are obviously ridiculous "how cool is this" statements, which will not be good to sail. These are "wave piercing" bows, which on monohulls like these will just be submarine bows.
__________________
"Parce que je suis heureux en mer, et peut-être pour sauver mon ame. . . "
Dockhead is offline   Reply With Quote
Old 15-05-2016, 13:54   #157
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,751
Re: Pilothouse Variations -- Boreal

To illustrate what I'm talking about --

Click image for larger version

Name:	club-swan-50-keel.2730a.jpg
Views:	270
Size:	52.8 KB
ID:	124403

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.

That boat might or might not need the deleted buoyancy. With a long empty forward section it might have a low enough polar moment of inertia that the bow pops up quickly and stays dry enough in any case, and/or the use of this boat probably doesn't care so much about dry decks.

Such a bow will be faster -- it will "cut" through the waves with no loss of fineness. It will also save some windage, and a bit of weight forward. But at some point it will have a tendency to submarine, as there is NO reserve buoyancy.

Cruising boats designed to be used in weather other than balmy, who are packed with gear and do not have empty forward sections, imitate this approach as a fashion statement -- at their peril.
__________________
"Parce que je suis heureux en mer, et peut-être pour sauver mon ame. . . "
Dockhead is offline   Reply With Quote
Old 15-05-2016, 14:10   #158
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,751
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by poiu View Post
You can get a cockpit view of that boat pushing through some moderate sized steep waves at the Needles in the video linked below. Gives a fair idea of how a bow will behave with a fine entry, low overhang, lowish sheer, lowish flare and moderate deadrise.

Crikey Baldrick, there a fair few factors at work here and there is no mention yet of how to balance it all with the shape of the stern:

Needles video - wind over tide.
This is a few miles from my home port and a spot I pass constantly.

These are absolutely typical conditions up here.

And this is an illustration of a number of things discussed on here. For all my tremendous respect for Dashew, his boats don't get this particular thing right, for these latitudes. Form of the bows is all wrong.

What is depicted here is absolutely unacceptable performance. Imagine the water shown there is 2 degrees C -- it's death if you get caught in a sheet of that coming over the bows.

The Dashew videos were shot in very ordinary conditions at the Needles -- not a storm.

But we get caught in storms up here, sometimes, too. Imagine your fru fru reverse bow as it drives into one of these waves:

Click image for larger version

Name:	fishing_ship_caught_in_the_middle_of_storm_08.jpg
Views:	81
Size:	77.9 KB
ID:	124409

You will still be wishing for reserve buoyancy in the bows, even as you are struggling into your life raft.

Yes, I've sailed in conditions like that. That's just a little old F9 in the North Sea which can happen during any month of the year.
__________________
"Parce que je suis heureux en mer, et peut-être pour sauver mon ame. . . "
Dockhead is offline   Reply With Quote
Old 15-05-2016, 14:21   #159
Moderator
 
Pete7's Avatar

Cruisers Forum Supporter

Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 8,570
Images: 14
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by poiu View Post
You can get a cockpit view of that boat pushing through some moderate sized steep waves at the Needles in the video linked below. Gives a fair idea of how a bow will behave with a fine entry, low overhang, lowish sheer, lowish flare and moderate deadrise.

Crikey Baldrick, there a fair few factors at work here and there is no mention yet of how to balance it all with the shape of the stern:

Needles video - wind over tide.
Intregued by that photo of Wild Horse stuffing its nose into a wave I went searching for the photo and last night found the others in the series. When the boat was turned around the waves appeared flat calm with few white waves. Sadly I can't find the website again.

The video of the Needles channel looks spectacular but only because there is a strong ebb tide against a prevailing Westerly wind. When both are strong those conditions are present, short steep waves with a very frequent frequency. What would have been a better test would be taking the boat out into open water in strong winds with larger waves but less frequent.

Quote:
Originally Posted by Polux View Post
Bow buoyancy has to do with the design of the bow, the forward hull section,
And critically the stern. Dockhead mentioned the need to have the whole boat designed as one, so very true. A big fat stern will lead to a vertical stem ploughing straight through a wave as the stern won't allow the bow to rise.

Pete
__________________
Moody 31 - April Lass
Pete7 is offline   Reply With Quote
Old 15-05-2016, 14:28   #160
Registered User
 
TeddyDiver's Avatar

Join Date: Aug 2010
Location: Arctic Ocean
Boat: Under construction 35' ketch
Posts: 1,828
Images: 2
Re: Pilothouse Variations -- Boreal

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
__________________
TeddyDiver is offline   Reply With Quote
Old 15-05-2016, 14:29   #161
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,751
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Pete7 View Post
Intregued by that photo of Wild Horse stuffing its nose into a wave I went searching for the photo and last night found the others in the series. When the boat was turned around the waves appeared flat calm with few white waves. Sadly I can't find the website again.

The video of the Needles channel looks spectacular but only because there is a strong ebb tide against a prevailing Westerly wind. When both are strong those conditions are present, short steep waves with a very frequent frequency. What would have been a better test would be taking the boat out into open water in strong winds with larger waves but less frequent.

Dockhead mentioned the need to have the whole boat designed as one, so very true. A big fat stern will lead to a vertical stem ploughing straight through a wave as the stern won't allow the bow to rise.

Pete
Indeed. Funnily enough Pete7 has been with me in person on my boat at that very spot, in a Force 8 after a fast sail over from Cherbourg, a few years ago. That's a force or two stronger conditions than what is shown in that video. We made very good time (over 9 knots average) and arrived, contrary to plan, when the ebb was still flowing against the gale. I have to admit that in the event we didn't dare go through the Needles with a F8 blowing against the tide -- we turned and went around through the North Channel. Discretion being the better part of valour, right Pete?
__________________
"Parce que je suis heureux en mer, et peut-être pour sauver mon ame. . . "
Dockhead is offline   Reply With Quote
Old 15-05-2016, 14:35   #162
Moderator
 
Pete7's Avatar

Cruisers Forum Supporter

Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 8,570
Images: 14
Re: Pilothouse Variations -- Boreal

Absolutely, I have no problem in running away or turning the engine on if it looks dicey.

Two things stood out for me on that trip, how fast a modern well designed 54ft cruising yacht can travel in nasty conditions and how well the VHF on low power transmitted the best part of 80 miles through using really good quality antenna and coax.

Oh and I really must return the favour by taking you creek crawling by sailing on nothing more than wet grass


Pete
__________________
Moody 31 - April Lass
Pete7 is offline   Reply With Quote
Old 15-05-2016, 15:02   #163
Moderator
 
weavis's Avatar

Cruisers Forum Supporter

Join Date: Feb 2014
Location: SEVILLE - MALLORCA
Posts: 10,140
Send a message via Skype™ to weavis
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Pete7 View Post
Absolutely, I have no problem in running away or turning the engine on if it looks dicey.

Pete
Turning the engine on? I would not have turned it off...

(Motor boat days regression...)

__________________
- Never test how deep the water is with both feet -
10% of conflicts are due to different opinions. 90% by the tone of voice.
Raise your words, not your voice. It is rain that grows flowers, not thunder.
weavis is offline   Reply With Quote
Old 15-05-2016, 15:06   #164
Moderator
 
Pete7's Avatar

Cruisers Forum Supporter

Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 8,570
Images: 14
Re: Pilothouse Variations -- Boreal

Sssshh, we always sail on to the mooring or into an unknown marina in a foreign country
__________________
Moody 31 - April Lass
Pete7 is offline   Reply With Quote
Old 15-05-2016, 15:07   #165
Moderator
 
weavis's Avatar

Cruisers Forum Supporter

Join Date: Feb 2014
Location: SEVILLE - MALLORCA
Posts: 10,140
Send a message via Skype™ to weavis
Re: Pilothouse Variations -- Boreal

Quote:
Originally Posted by Pete7 View Post
Sssshh, we always sail on to the mooring or into an unknown marina in a foreign country
Poser....
__________________

__________________
- Never test how deep the water is with both feet -
10% of conflicts are due to different opinions. 90% by the tone of voice.
Raise your words, not your voice. It is rain that grows flowers, not thunder.
weavis 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
Boreal Sailboats Highwayman Monohull Sailboats 3 25-04-2016 12:43
Jedi's White Bread with Variations s/v Jedi Provisioning: Food & Drink 1 15-02-2011 20:25
gulf 32 pilothouse bearhill Monohull Sailboats 26 06-12-2008 08:58



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 23:36.


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.