RT Firefly
Enigma
??
Definition Of Full Displacement
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If you have answers, please help by responding to the unanswered posts.
If you have answers, please help by responding to the unanswered posts.
Delfin
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By the way Baker, dues this Hobie really look like a FD hull?
https://www.google.com/imgres?imgurl=https%3A%2F%2Fi.pinimg.com%2F736x%2F32%2F40%2F38%2F3240380b870dfdb70a6bf6497396f9e3--sail-boats-sail-away.jpg&imgrefurl=https%3A%2F%2Fwww.pinterest.com%2Fpin%2F322077810824513869%2F&docid=pUst1eELhu7ueM&tbnid=XNBRiRJrnAmCOM%3A&vet=1&w=415&h=600&client=ms-android-google&bih=714&biw=393&ved=2ahUKEwjCkcGtoLfgAhVpxlQKHbU5Az8QxiAoAHoECAEQHQ&iact=c&ictx=1
https://www.google.com/imgres?imgurl=https%3A%2F%2Fi.pinimg.com%2F736x%2F32%2F40%2F38%2F3240380b870dfdb70a6bf6497396f9e3--sail-boats-sail-away.jpg&imgrefurl=https%3A%2F%2Fwww.pinterest.com%2Fpin%2F322077810824513869%2F&docid=pUst1eELhu7ueM&tbnid=XNBRiRJrnAmCOM%3A&vet=1&w=415&h=600&client=ms-android-google&bih=714&biw=393&ved=2ahUKEwjCkcGtoLfgAhVpxlQKHbU5Az8QxiAoAHoECAEQHQ&iact=c&ictx=1
RT Firefly
Enigma
Astounding. 12 pages, 238 posts about...and no, it isn't dead yet.
What do you expect, we haven't determined who won the pissing contest yet?
A boat hull is resting/settled in and moving/pushing through water at hull speed... be it D SD or P
Or, a boat bottom is sliding/gliding on-top-of/over water in planning speeds... be it SD or P
To speak about multi thousand HP pushing or pulling a D hull fast enough for it to at least try to get on top of water is simply silly.
No real boater will ask a real boat to perform other than the hull design qualifies it for.
Or, a boat bottom is sliding/gliding on-top-of/over water in planning speeds... be it SD or P
To speak about multi thousand HP pushing or pulling a D hull fast enough for it to at least try to get on top of water is simply silly.
No real boater will ask a real boat to perform other than the hull design qualifies it for.
Nomad Willy
Guru
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Delfin
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Eric, if a Hobie is a full displacement hull, the folks at Nordhvn are going to have to redo their website.
https://www.nordhavn.com/35/overview_hull.php4
What is the difference between a full displacement and semi-displacement hull design? Essentially, the full displacement hull remains completely in the water at all speeds. It virtually "pushes" the water away from its bow. The water then travels down the length of the entire hull, finally leaving a wake behind the stern. This "bow wave" creates a resistance to the hull, from which it can never escape [/I]
markpierce
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Pushing water at 6.3 knots:
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psneeld
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sunchaser
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Actually, Nordhavn's full displacement marketing blurb is closer to the truth than any other boat builders blurb out there. They have backed it up with many hundreds of builds in the past 30 years. Their starting point was to do an update of Beebe's written work and run with it. Trivializing their contribution to safe blue water recreational cruising is off the mark, by far.
But, the beauty of the internet is those that want, can attempt to drag down. Not successful in any sense for this case.
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Delfin
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So, does that mean that a boat builder who states physical reality with respect to hull design can't be trusted because you can buy their boat? Interesting perspective, Fish.
Yeah OK I'll genuflect to Nordhavn and beg forgiveness for assuming they had any interest in promoting their product. Does that work for you and Nordhavn? I believe it's quite a stretch to imagine I had any intent to "drag down" Nordhavn with my comment, advertising is not normally where a person goes for detailed technical information. I go to books on the subject in question that address acknowledged facts and how they're arrived at. Sure advertising may actually have valid information however the bias toward selling a product puts the majority of it up for scrutiny.
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Delfin
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You are quite right, it is a complicated subject. However, one doesn't need to get too far into the technical weeds to make the observation that there are, in fact, full displacement hulls that have a maximum speed, regardless of how much power you throw at the problem.
Nordhavn's statement pretty much says it all:
"Essentially, the full displacement hull remains completely in the water at all speeds." Simple.
So, if you have a vessel that planes, or lifts up out of the water enough that the water displaced weighs less than the weight of the boat because it can get ahead of its bow wave, be it a catamaran, oil tanker or inner tube you can call it a full displacement hull if you like, but that is effectively the same as saying that all squares are circles. They just are squares with a constant radius.
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You can just share my previous response to this issue.
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sunchaser
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Fish
A read of the voluminous Nordhavn website can prove illuminating, plus the hundreds of available pages of first hand blogs from world cruisers. Don't forget to browse their in house written and then published "update" to Beebe's Voyaging Under Power. Then direct your curiosities and questions to them in well thought out written correspondence that should elicit a cogent reply.
A read of the voluminous Nordhavn website can prove illuminating, plus the hundreds of available pages of first hand blogs from world cruisers. Don't forget to browse their in house written and then published "update" to Beebe's Voyaging Under Power. Then direct your curiosities and questions to them in well thought out written correspondence that should elicit a cogent reply.
BruceK
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As distinct from the "pissing people off" contest. A clear winner there!
Nomad Willy
Guru
The laws, rules of thumb and other functions of hull design don’t address extreme aspect ratio hulls.
The Nordhavn description of FD hulls is a little weak but they are pretty busy building boats so I’ve heard. For example FD boats actually start to climb their wave at about a knot below hull speed. Prove it to yourself by slowly increasing your speed until the bow rises a bit and observe your speed. It will be below hull speed and quite a few will start rising long before hull speed is reached. So many variables. Trim, balance and ballast can lead observations astray too. It isn’t black and white. Many boats are FD specifically but very close to SD. They will be way different in hull design than a deep FD design but both will be FD.
And boats that are very close to FD that are SD may look way different than SD boats that are very close to planing boats. Many SD boats have soft chines but many have hard chines.
Some people have great dificulty with things that can’t be positively identified and classified. This is one of those things.
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Delfin
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While I can't attest that this fellow on boatdesign.net isn't trying to sell me something, so may fail the Fish test, it does seem to be a fairly straightforward description of what characteristics the three type of hull design discussed in this thread, and so useful.
"Displacement hulls are designed to travel in the water at a typical hull speed of 1.34 *(square-root LWL) in knots. This formula applies to the distance between bow and stern wave crests. At faster speeds, wave making resistance increases exponentially because the vessel is trying to climb on top of the bow wave - meanwhile the stern is being sucked down by the dynamic forces from the "hole" created in the water as the vessel moves forward. Displacement hulls tend to have pointed bows and sterns because this form poses the least wave making resistance at "displacement" speeds. It takes a relatively small amount of power to push a displacement hull at its "hull speed."
Semi-displacement hulls tend to have wide, flat aft sections - like a New England lobster boat. These hulls are designed to partially climb on top of the bow wave and separate the transom from the stern wave. Semi-displacement speeds are usually in the area of 1.5 to 2.5 *(square-root LWL) in knots. It takes a *lot* of power to drive a hull in the semi-displacement speed range. The flat wide stern sections help to provide additional lift in the stern to partially overcome this problem.
Planing hulls are designed with straight sections aft. A typical deep-V bottom hull has the same angle to the 'V' (the same "deadrise" angle) from midship to transom. They are designed to climb completely out of the water at high speed and "hydroplane" on top of the water. At planing speeds, water is breaking cleanly from the transom and the hull is riding on its straight aft sections. The greatest resistance at planing speeds is frictional resistance. It takes more power to climb out of the water over the bow wave than it does to maintain planing speed once this is achieved. At very high planing speeds (>25kts) any change in deadrise angle in the aft sections of the hull can adversely affect performance. Hulls with a "variable deadrise" angle in the aft sections (where the angle of the 'V' decreases and flattens toward the transom) are a further modification of the semi-displacement hull form. They are more easily driven at speeds < 25kts, but at higher speeds tend to push the bow down due to the higher dynamic lift in the aft sections. Variable deadrise hulls can actually become unsafe at very high speeds >30kts because of this tendency - it is possible for the bow to dig in and cause the boat to broach at high speed."
"Displacement hulls are designed to travel in the water at a typical hull speed of 1.34 *(square-root LWL) in knots. This formula applies to the distance between bow and stern wave crests. At faster speeds, wave making resistance increases exponentially because the vessel is trying to climb on top of the bow wave - meanwhile the stern is being sucked down by the dynamic forces from the "hole" created in the water as the vessel moves forward. Displacement hulls tend to have pointed bows and sterns because this form poses the least wave making resistance at "displacement" speeds. It takes a relatively small amount of power to push a displacement hull at its "hull speed."
Semi-displacement hulls tend to have wide, flat aft sections - like a New England lobster boat. These hulls are designed to partially climb on top of the bow wave and separate the transom from the stern wave. Semi-displacement speeds are usually in the area of 1.5 to 2.5 *(square-root LWL) in knots. It takes a *lot* of power to drive a hull in the semi-displacement speed range. The flat wide stern sections help to provide additional lift in the stern to partially overcome this problem.
Planing hulls are designed with straight sections aft. A typical deep-V bottom hull has the same angle to the 'V' (the same "deadrise" angle) from midship to transom. They are designed to climb completely out of the water at high speed and "hydroplane" on top of the water. At planing speeds, water is breaking cleanly from the transom and the hull is riding on its straight aft sections. The greatest resistance at planing speeds is frictional resistance. It takes more power to climb out of the water over the bow wave than it does to maintain planing speed once this is achieved. At very high planing speeds (>25kts) any change in deadrise angle in the aft sections of the hull can adversely affect performance. Hulls with a "variable deadrise" angle in the aft sections (where the angle of the 'V' decreases and flattens toward the transom) are a further modification of the semi-displacement hull form. They are more easily driven at speeds < 25kts, but at higher speeds tend to push the bow down due to the higher dynamic lift in the aft sections. Variable deadrise hulls can actually become unsafe at very high speeds >30kts because of this tendency - it is possible for the bow to dig in and cause the boat to broach at high speed."
Nomad Willy
Guru
Delfin,
Boat builders are business people. Sure they hire engineers and designers but they have a lot to say that they don’t pass through the engineering department. And they are primarly in the business of selling boats. And the boat building business being what it is few have no worries about making money. So what boat builders have to say about boats may not be something to take to the bank.
Quite so and I appreciate that it's appreciated, just getting a laugh out of the scrum at my own expense, as is appropriate.
Delfin
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Those people wouldn't include boat designers, apparently.
And yes, a full displacement hull can "start to climb their wave". Problem is, it can't ever get to the top. If it can, it isn't a full displacement hull because it is no longer travelling at displacement speed.
But if it makes anyone feel better, we can define full displacement as whatever one wants it to be. Kind of like gender these days. You get to make up whatever definition you like, and you don't have to get pinned down to a definition, classification-wise.
I'm actually not very likely to take you up on your invitation, while it is very gracious of you to offer I'm in fact not the subscriber that brought the hallowed name of Nordhavn into the "conversation".
Delfin
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So Nordhavn doesn't know the difference between a FD and SD hull? If they don't, I guess we shouldn't be buying their boats. If they do, I guess I'm not sure of your point.
Nomad Willy
Guru
Eric - Sorta looks like your boat... all grown up! No wonder you like her.
OP
OP
menzies
Guru
It was such a simple question.
Seems the answer is somewhat along the lines of: "if you ask a hundred doctors..."
psneeld
Guru
Because that formula doesn't take into all the factors that affect the 1.34 factor and obviously it applised to vessels in "the displacement mode"...not necessarily "just" full displacement hulls.
Exceptios to theory such as a full displacement hull with trim tabs as someone mentioned... might have been altered enough to now be a displacement hull (maybe not full displacement by definition) that now can travel in the semi displacement mode.
The catamaran hull. like the Hobie 16, even though it may have the characteristics of a full displacement hull, it has a prismatic coefficient and draft to beam ratio off the normal design scales...and when it lifts a hull and presents a nearly flat side to the surface...the wave making formula obviously no longer applies.
I have never seen what exactly the Hobie in hydrodynamic terms is all summed up as.... but like many simple concepts..... change but one of a few elements and new formulas now apply.
That said....Delfin repost of the boatdesign comments is pretty much what I have always heard and understood....and like my comments...the more I learned (like pretty much everything in life), exceptions exist.
The question ultimately is, does a basic definition include or exclude these exceptions?
Exceptios to theory such as a full displacement hull with trim tabs as someone mentioned... might have been altered enough to now be a displacement hull (maybe not full displacement by definition) that now can travel in the semi displacement mode.
The catamaran hull. like the Hobie 16, even though it may have the characteristics of a full displacement hull, it has a prismatic coefficient and draft to beam ratio off the normal design scales...and when it lifts a hull and presents a nearly flat side to the surface...the wave making formula obviously no longer applies.
I have never seen what exactly the Hobie in hydrodynamic terms is all summed up as.... but like many simple concepts..... change but one of a few elements and new formulas now apply.
That said....Delfin repost of the boatdesign comments is pretty much what I have always heard and understood....and like my comments...the more I learned (like pretty much everything in life), exceptions exist.
The question ultimately is, does a basic definition include or exclude these exceptions?
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Delfin
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Folks can make it as complex as they wish to, but a FD boat is one that cannot exceed a speed close to ideal hull speed because the lift generated by forward motion is more than offset by the drag created by the waves generated. In other words, they can't get ahead of their own bow wave. The reasons why this is so are complex. The phenomenon is not.
Their advantage in an ocean cruising is fuel efficiency at a high percentage of hull speed and most will after, a more comfortable motion while underway.
