Reply
 
Thread Tools Search this Thread Display Modes
 
Old 02-04-2019, 04:16 PM   #41
DDW
Guru
 
City: San Francisco
Country: USA
Join Date: Apr 2018
Posts: 1,322
Quote:
Originally Posted by Delfin View Post
In the link above provided by Menzies, the author notes:

"On the other hand, there would be no benefit to powering the Kadey-Krogen used in our example with a 400 or 500-HP engine. Unlike semi-displacement hulls, long range trawlers with true displacement design cannot, in any practical sense, be pushed over the bow wave to get on plane. An over-sized engine would merely add to the weight and purchase cost of the vessel and occupy more space in the engine room than required."
The key is "in any practical sense". There is no magic in displacement hull forms, they simply have low lift/drag, and the power required to climb the bow wave impractical. Very much full displacement hull forms in both power and sail will do this surfing down a steep enough wave. The extra power provided by the wave and gravity. You can make a KK plane. Might take a gas turbine and have a range of 50 miles, making it impractical.

A tire tube doesn't have the look of a planing hull, yet pulled by a ski boat they plane routinely. A human foot doesn't either, yet people barefoot ski. It's pretty hard to define a characteristic or set of characteristics that confines a shape to displacement speeds. Designers tend to pick forms that are efficient at the chosen operating conditions which we have come to recognize as typical planing or displacement types, but these definitions have very grey edges.
__________________
Advertisement

DDW is online now   Reply With Quote
Old 02-04-2019, 05:14 PM   #42
Grand Vizier
 
Delfin's Avatar
 
Join Date: Jan 2010
Posts: 3,459
Quote:
Originally Posted by DDW View Post
The key is "in any practical sense". There is no magic in displacement hull forms, they simply have low lift/drag, and the power required to climb the bow wave impractical. Very much full displacement hull forms in both power and sail will do this surfing down a steep enough wave. The extra power provided by the wave and gravity. You can make a KK plane. Might take a gas turbine and have a range of 50 miles, making it impractical.

A tire tube doesn't have the look of a planing hull, yet pulled by a ski boat they plane routinely. A human foot doesn't either, yet people barefoot ski. It's pretty hard to define a characteristic or set of characteristics that confines a shape to displacement speeds. Designers tend to pick forms that are efficient at the chosen operating conditions which we have come to recognize as typical planing or displacement types, but these definitions have very grey edges.
I don't think that falling down a wave due to gravity quite demonstrates that displacement hulls can exceed hull speed. The basis for asserting that full displacement hulls cannot exceed hull speed is a consequence of the effect of increased drag on those hulls when speeds approach hull speed. Apply more power and drag increases. As drag increases, wetted surface increases as the stern of the vessel is literally dragged into the water, further increasing drag until you reach the hull speed limit. It probably is an asymptotic function, but there is a speed limit that can't be exceeded.

On my boat, if I apply WOT, I go no faster than I do at WOT minus 20% of rated rpm, but my stern is sucked under enough to start to bury it and slop water through the hawseholes. Delfin is, by any definition, a full displacement hull and changes to hull form that provide compensating lift I don't have allow for speeds in excess of hull speed, but for me, it simply isn't possible no matter how much power I employed.

And yes, a person can plane on their feet, and an inner tube can also skim along the surface of the water. That is because those flatish "hull" forms generate lift, something a displacement hull is designed not to provide. Those hull forms are intended to reduce drag, not produce lift which increases drag as lower angles of attack, as in less than planning speeds.
__________________

__________________
Delfin
"Dad always thought laughter was the best medicine, which I guess is why several of us died of tuberculosis." - Jack Handy
Delfin is offline   Reply With Quote
Old 02-04-2019, 06:26 PM   #43
Senior Member
 
Nightsky's Avatar
 
City: Comox
Country: Canada
Vessel Model: 1989 Wellington 57 motorsailer
Join Date: Dec 2016
Posts: 266
I think of FD hull shape as that of a tear drop or wine glass.
Nightsky is offline   Reply With Quote
Old 02-04-2019, 07:08 PM   #44
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,176
I have some more pics that show the FD dynamics well. Had to wait till I got home to my computer.

1. This pic shows a sailboat at WOT (or very close to) bucking a lot of current. You should be able to see the crest of the stern wave about 2' aft of the transom.

2. This shows the stern of my rowboat. The up-turn of the hull bottom aft is very clearly shown. Look closely at her keel and you can see her WL at rest presumably with no people aboard. The transom is well out of the water. With a full load the water level would be close to the transom or a smudge below. Despite her small size she's a good example of a FD hull.

3. Here is Willy at WOT just days after we bought her. It's easy to see she's just starting to outrun her stern wave as it's about 3' aft of the stern. While cruising it would be ideally about a foot fwd of the stern.

SD and Planing boats have a very different following wake. The water boiling out under the bottom at the bottom to transom edge is a frothing mass of extremely disturbed water. Much like the water in a washing machine. Water is literally jumping up and down. FD wakes aft are very different whereas the water could be described as "flowing" w only a small fraction of the turbulence from SD boats.
Attached Thumbnails
Click image for larger version

Name:	DSCF0521 copy.jpg
Views:	62
Size:	143.0 KB
ID:	84960   Click image for larger version

Name:	DSCF2499 copy 2.jpg
Views:	60
Size:	197.9 KB
ID:	84961   Click image for larger version

Name:	CIMG0407 copy.jpg
Views:	59
Size:	116.9 KB
ID:	84962  
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 02-04-2019, 07:36 PM   #45
DDW
Guru
 
City: San Francisco
Country: USA
Join Date: Apr 2018
Posts: 1,322
Quote:
Originally Posted by Delfin View Post
The basis for asserting that full displacement hulls cannot exceed hull speed is a consequence of the effect of increased drag on those hulls when speeds approach hull speed. Apply more power and drag increases. As drag increases, wetted surface increases as the stern of the vessel is literally dragged into the water, further increasing drag until you reach the hull speed limit. It probably is an asymptotic function, but there is a speed limit that can't be exceeded.
Every form produces lift, some not as efficiently as others. Drag increases but not without limit. Wetted area has a limit and is in any case a small component of drag at those speeds. And as I said, if the hole it digs exceeds freeboard and the hatches are open it may sink first. Nevertheless, every shape can be made to plane if enough power is applied. For Delphin, that would be a lot of power . Imagine a dinghy with the same hull form as Delphin. If I tow it at 15 knots it's going to plane if the decks are sealed. It's like skipping stones on a creek: the flat ones work better but a round one will skip if you throw them hard enough.
DDW is online now   Reply With Quote
Old 02-04-2019, 07:52 PM   #46
Grand Vizier
 
Delfin's Avatar
 
Join Date: Jan 2010
Posts: 3,459
Quote:
Originally Posted by DDW View Post
Every form produces lift, some not as efficiently as others. Drag increases but not without limit. Wetted area has a limit and is in any case a small component of drag at those speeds. And as I said, if the hole it digs exceeds freeboard and the hatches are open it may sink first. Nevertheless, every shape can be made to plane if enough power is applied. For Delphin, that would be a lot of power . Imagine a dinghy with the same hull form as Delphin. If I tow it at 15 knots it's going to plane if the decks are sealed. It's like skipping stones on a creek: the flat ones work better but a round one will skip if you throw them hard enough.
I guess we'll have to agree to disagree. If Delfin's 65 tons were hauled through the water at 15 knots, I think she'll get sucked under by the stern. You think she'll put on her dancing shoes and skip across the waves like a paddle board. I think physics are on my side....

For a vessel to plane, lift must exceed drag. For a displacement hull, that never happens, drag just increases, not infinitely, but enough to pull the boat under. If this weren't the case, there would be no such thing as hull speed anymore than there is something called "car speed". Just an endless increase in speed as you add propulsion power.
Delfin is offline   Reply With Quote
Old 02-04-2019, 08:24 PM   #47
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,176
DDW,
I don’t think any shape will plane w enough power.
They go out of control and loose their directional stability and achieve un-godly high angles of attack .... but not plane. Just my opinion though.I’ve never seen a boat like Delfin w 30,000hp.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 02-05-2019, 06:22 AM   #48
Guru
 
Fish53's Avatar
 
City: Pitcairn Island
Country: yz
Join Date: Jul 2015
Posts: 690
Quote:
Originally Posted by Nomad Willy View Post
DDW,
I don’t think any shape will plane w enough power.
They go out of control and loose their directional stability and achieve un-godly high angles of attack .... but not plane. Just my opinion though.I’ve never seen a boat like Delfin w 30,000hp.
I believe you're correct as the sinking sailboat analogy indicates, but really we've gone a bit too far afield from what is practical or common. The picture of your boat is an excellent example of a displacement hulls wave forming. I've played with ideas to make my hull a bit more efficient primarily by sharpening the entrance. I'd like to try a bulbous bow but the technical details are beyond me.
Fish53 is offline   Reply With Quote
Old 02-05-2019, 07:19 AM   #49
Guru
 
psneeld's Avatar
 
City: AICW
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 22,167
https://en.wikipedia.org/wiki/Hull_speed


Hull speed or displacement speed is the speed at which the wavelength of a vessel's bow wave is equal to the waterline length of the vessel. As boat speed increases from rest, the wavelength of the bow wave increases, and usually its crest-to-trough dimension (height) increases as well. When hull speed is exceeded, a vessel in displacement mode will appear to be climbing up the back of its bow wave.
From a technical perspective, at hull speed the bow and stern waves interfere constructively, relatively large waves, and thus a relatively large value of wave drag. Though the term "hull speed" seems to suggest that it is some sort of "speed limit" for a boat, in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.
The concept of hull speed is not used in modern naval architecture, where considerations of speed-length ratio or Froude number are considered more helpful.





https://en.wikipedia.org/wiki/Hull_speed

Hull design implications

Wave making resistance depends dramatically on the general proportions and shape of the hull: many modern displacement designs can easily exceed their 'hull speed' without planing.
These include hulls with very fine ends, long hulls with relatively narrow beam and wave-piercing designs. Such hull forms are commonly realised by some canoes, competitive rowing boats, catamarans, fast ferries and other commercial, fishing and military vessels.
Vessel weight is also a critical consideration: it affects wave amplitude, and therefore the energy transferred to the wave for a given hull length.
Heavy boats with hulls designed for planing generally cannot exceed hull speed without planing.
Light, narrow boats with hulls not designed for planing can easily exceed hull speed without planing; indeed, once above hull speed, the unfavorable amplification of wave height due to constructive interference diminishes as speed increases. For example, world-class racing kayaks can exceed hull speed by more than 100%,[1] even though they do not plane. Semi-displacement hulls are usually intermediate between these two extremes.
Ultra light displacement boats are designed to plane and thereby circumvent the limitations of hull speed.



https://www.boatinternational.com/ya...esistance--635



Plus, of course, the longer a hull is, the greater its top speed in the displacement mode according to Froude’s law which states that displacement speed is directly proportional to the square root of the waterline length, or (1.34 x vLWL). Because of overhangs, this number if often expanded to 1.5 x vLWL.
psneeld is online now   Reply With Quote
Old 02-05-2019, 07:20 AM   #50
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 20,980
"I'd like to try a bulbous bow but the technical details are beyond me."

The big hassle with a bulbous bow is they are optimized for one speed.

The big boys spent big bucks having their shape modified in the last economic slowdown , when slower delivery times became the norm.
FF is offline   Reply With Quote
Old 02-05-2019, 07:43 AM   #51
Guru
 
Fish53's Avatar
 
City: Pitcairn Island
Country: yz
Join Date: Jul 2015
Posts: 690
Quote:
Originally Posted by FF View Post
"I'd like to try a bulbous bow but the technical details are beyond me."

The big hassle with a bulbous bow is they are optimized for one speed.

The big boys spent big bucks having their shape modified in the last economic slowdown , when slower delivery times became the norm.
Precisely, I of course don't intend to operate a one speed so why bother, although it could be said I have one speed, slow.
Fish53 is offline   Reply With Quote
Old 02-05-2019, 09:47 AM   #52
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,176
FF wrote;
“The big hassle with a bulbous bow is they are optimized for one speed”

So is a FD hull. Optimized for
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 02-05-2019, 09:49 AM   #53
Guru
 
North Baltic sea's Avatar
 
Country: Finland
Vessel Model: Nordic Tug 37
Join Date: Sep 2015
Posts: 942


ha, Maybe this goes beyond the laws of physics, but the guy here in Finland with the old FD Hull open Trawler installed a lot of hp (500) earlier about 20 hp. In addition, the hull reinforced steel beams withstand the stress. Previous speed about 7kn now +40kn.


I do not just question the shape of the body as these "trawlers" have a tapered stern and a round base with full long keel and now it is both FD, SD and additionally planing Hull?

Litle trip this video

https://youtu.be/fTHYD_hVAK0




Typical hull profile this series trawler, this boat have litle cabin.

NBs
North Baltic sea is offline   Reply With Quote
Old 02-05-2019, 11:43 AM   #54
Guru
 
Fish53's Avatar
 
City: Pitcairn Island
Country: yz
Join Date: Jul 2015
Posts: 690
Quote:
Originally Posted by North Baltic sea View Post


ha, Maybe this goes beyond the laws of physics, but the guy here in Finland with the old FD Hull open Trawler installed a lot of hp (500) earlier about 20 hp. In addition, the hull reinforced steel beams withstand the stress. Previous speed about 7kn now +40kn.


I do not just question the shape of the body as these "trawlers" have a tapered stern and a round base with full long keel and now it is both FD, SD and additionally planing Hull?

Litle trip this video

https://youtu.be/fTHYD_hVAK0




Typical hull profile this series trawler, this boat have litle cabin.

NBs
While not the same it sort of makes me think of Bartender boats, look like FD but plane. Of course they have a sizable flat section though. I have a 28' seine dory that's flat for most of it's bottom but I'm happy with the 9hp it has now, something to be said for being able to fuel for a days cruising with a five gallon can.
Fish53 is offline   Reply With Quote
Old 02-05-2019, 12:46 PM   #55
DDW
Guru
 
City: San Francisco
Country: USA
Join Date: Apr 2018
Posts: 1,322
Quote:
Originally Posted by Delfin View Post

For a vessel to plane, lift must exceed drag.
No, lift must reduce displacement enough so that power can exceed drag. A planing boat displaces less than it's weight. Even with NO lift, a boat can be made to exceed its hull speed by pushing it hard enough. It simply creates a wave that is longer than it's length. It's inefficient, but quite possible.

Read PSneed's post #49. This is correct, most NA's do not talk about "hull speed" which is pretty much a meaningless number, though ok to compare similar heavy boats. In particular:
Quote:
in fact drag for a displacement hull increases smoothly and at an increasing rate with speed as hull speed is approached and exceeded, often with no noticeable inflection at hull speed.
If you look at drag vs. Froude numbers for hulls with a lot of rocker (what we think of as "displacement") the curve bends upward with speed but does not go vertical - for any shape. If lift is sufficient to reduce displacement significantly, the curve flattens out again.

Yes, it is impractical to think about 100,000 hp in Delphi. A similar hull form of lighter displacement could more easily be made to exceed "hull speed", but there is little practical application. The canoe stern design shown is a poor candidate for planing, as there is likely insufficient floatation in the stern to keep it from sinking before sufficient lift is developed - but there are plenty of planing canoe sterned boats.

A practical definition of a displacement hull is a hull that is designed to operate most efficiently at speeds of less than "hull speed". Typical characteristics are deadrise, rocker, and angled sections aft. Planing hulls are designed to run efficiently on plane, typical characteristics are low or no deadrise aft, low rocker, a flat run aft. However you will see characteristics crossing these lines frequently. The bastard child is the SD hull, not as efficient at either mode, typically has characteristics of planing hull but is over weight and underpowered for that mission. Nevertheless, it is very popular for other reasons.
DDW is online now   Reply With Quote
Old 02-05-2019, 01:38 PM   #56
Grand Vizier
 
Delfin's Avatar
 
Join Date: Jan 2010
Posts: 3,459
Quote:
Originally Posted by Delfin View Post
For a vessel to plane, lift must exceed drag.
Quote:
Originally Posted by DDW View Post
No, lift must reduce displacement enough so that power can exceed drag.
That is just another way of saying that for a vessel to plane or get ahead of its own bow wave, lift must exceed drag. You're simply identifying that propulsive power is what causes lift to exceed drag, but that doesn't negate that physical reality.


Quote:
Originally Posted by DDW View Post
A planing boat displaces less than it's weight.
Well, not according to Archimedes. All vessels displace their weight. I gather you mean that a planing hull on plane displaces less than its weight, and that is certainly true. It is able to pull this trick off because the lift provide by the hull (flat sections) provide more lift than drag getting the boat up and out of the water it would normally displace at rest or at less than planing speeds.

Quote:
Originally Posted by DDW View Post
Even with NO lift, a boat can be made to exceed its hull speed by pushing it hard enough. It simply creates a wave that is longer than it's length. It's inefficient, but quite possible.
Well no, it's not. Without lift, any applied propulsion simply increases drag. With a displacement hull, there is some lift, as you mentioned earlier. However, because it is full displacement, drag increases greater than lift resulting in the existence of the formula mentioned by PSneed in his reference. Speaking of which, why do you think that the "overhangs" change that formula a bit? Answer: Because the faster a displacement hull goes, the lower it sinks in the water and the greater it LWL increases resulting in a higher hull speed. Keep applying power and you will hit the hull speed wall where any additional power simply increases enough drag to prevent the boat getting in front of its bow wave.

Quote:
Originally Posted by DDW View Post
Read PSneed's post #49. This is correct, most NA's do not talk about "hull speed" which is pretty much a meaningless number, though ok to compare similar heavy boats.
NAs use Froude numbers because they provide apples to apples comparisons between hulls, not because the hull speed formula is wrong. It just varies enough between hull forms that Froude numbers are preferred for accuracy.

Quote:
Originally Posted by DDW View Post
Yes, it is impractical to think about 100,000 hp in Delphi. A similar hull form of lighter displacement could more easily be made to exceed "hull speed", but there is little practical application.
Essentially what you are saying is that the physics that apply to Delfin do not apply at smaller scales, even with all other variables remaining constant. Physics doesn't work like that. Which is why the Froude number calculation doesn't include displacement as a component of the formula.

Quote:
Originally Posted by DDW View Post
The canoe stern design shown is a poor candidate for planing, as there is likely insufficient floatation in the stern to keep it from sinking before sufficient lift is developed - but there are plenty of planing canoe sterned boats.
Sure there are. And all of them that plane are not displacement hulls, which cannot be made to plane. Incidentally, my former boat, a 36' Cape George did not have a canoe stern, but it certainly could not plane or ever get ahead of its bow wave. The faster I went, the deeper into the drink the stern went.

Quote:
Originally Posted by DDW View Post
A practical definition of a displacement hull is a hull that is designed to operate most efficiently at speeds of less than "hull speed".
I'm afraid that gets us into the problem of defining efficiency, as AusCan pointed out is a spongey term. Perhaps a better definition is that a displacement hull is one where drag increases faster than lift as you apply power due to hull design. Either such vessels exist, or they do not. But if you would acknowledge that adding 10,000 hp to Delfin isn't going to get her 7'6" draft and 130,000# up out of the water on plane, then maybe we found one.
__________________
Delfin
"Dad always thought laughter was the best medicine, which I guess is why several of us died of tuberculosis." - Jack Handy
Delfin is offline   Reply With Quote
Old 02-05-2019, 01:39 PM   #57
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 17,176
The theories mostly pretain to normal aspect ratio boats.
My 18’ freight canoe will go about 13 knots w an old 6hp Johnson.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 02-05-2019, 01:57 PM   #58
Guru
 
menzies's Avatar
 
City: Jacksonville
Country: USA
Vessel Name: SONAS
Vessel Model: Grand Alaskan 53
Join Date: May 2014
Posts: 6,905
Here's another thought.

Planing hulls - do they use fin stabilizers (for when they chose to go slower) and, if so, what is the impact of those on stability at planing speeds?
menzies is online now   Reply With Quote
Old 02-05-2019, 04:31 PM   #59
Guru
 
Fish53's Avatar
 
City: Pitcairn Island
Country: yz
Join Date: Jul 2015
Posts: 690
There are several sentences that I take issue with but some may be me not getting your point exactly. But this one "The bastard child is the SD hull, not as efficient at either mode, typically has characteristics of planing hull but is over weight and underpowered for that mission. Nevertheless, it is very popular for other reasons.". Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull. I 've had a couple one was a BHM 32 with a 215hp Isuzu that did 18kts WOT and the other a Sisu 26 with a 200hp TAMD41a Volvo that did 25kts.
Fish53 is offline   Reply With Quote
Old 02-05-2019, 07:00 PM   #60
Grand Vizier
 
Delfin's Avatar
 
Join Date: Jan 2010
Posts: 3,459
Quote:
Originally Posted by menzies View Post
Here's another thought.

Planing hulls - do they use fin stabilizers (for when they chose to go slower) and, if so, what is the impact of those on stability at planing speeds?
They do, but the movement of the fins is smaller as speed increases. If I want my system to be less aggressive in deflection in response to roll, I just tell it I'm doing 20 knots rather than the 8 I am doing.
__________________

Delfin is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

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





All times are GMT -5. The time now is 03:23 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012
×