Full Displacement or Semi Displacement

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I would not at first glance say it was a pure displacement hull. More like a "lobsta" boat hull, with a round stern. And a beautifull boat, if I may add.
 
The "displacement guru" doesn't recall any discussion about performance being the defining line between FD and SD. That is another method that determines this matter by the speed performance of a hull. It goes something like this "if a hull is capable of speeds of 2x the square root of the WLL" it becomes a SD. But who decides it's with that number and not 1.8 x .... . Just as arbitrary as the QBBL angle. Haven't looked at anything like that for a long time and can't remember how power loading got into the picture if at all.

No one would argue that a boat capable of cruising one knot over hull speed is not a FD boat. However given the history here on TF I should know better than to say that.

IMHO .. the difference lies in the way water under the stern returns to the way it was before the boat came along. Significantly submersed transoms need not apply. Too much drag and turbulence as pointed out before. And I still don't see any methods superior to making the call than the QBBL. There are exceptions like the one you posted above but no other yardstick seems to define the deciding line any better or universally.

The first page was good to very good but this is going down hill badly.
 
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kulas44,
Interesting .. The connection to the LB. And I think a straight run aft almost always identifies a planing or SD hull. I didn't make the jump to LB probably due to the very prominent hard Chines. But her performance should be very similar if not overloaded. Should be a very nice running boat at 10 or 12 knots. Judging by the prop in the pic this one's probably powered for slower speeds.
 
SCOTTEDAVIS said:
FD will always displace their weight with water and will always have the same draft (more or less). They will always displace their weight in water.
images


PH will use lift to rise up using the lift provided by the reaction of water on their hull form to increase speed by reduced water drag. at full plane the wake will reduce as the boat rides on top of the water and at this point will require less power to maintain this condition. At full plane they will displace most of their weight with lift.

images


Fast non-planing boats (SD?) will have so much power and some lift built in to there hull that they will achieve a partial plane with slight reduction in surface drag and reduction in draft but with a much higher wake production and with no reduction in overall drag (never over the hump) regardless of use of full power. They will displace a small amount of their weight with lift but most of their weight will be displaced at speed by water.

images


Its like an airplane, the FD has not enough wing (lift) to fly so its really a bus. The SD is like a hover craft lots of power and can fly up to about 3 ft. The planing hull can fly.

images



hull-speed-image10.gif
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post #18 ....posted 2 days ago (page 1)
images 2 and 5 as a reminder about very similar calculations...
 
All this talk about displacement hull or not reminded me of a fellow that came into the marina at Sabine Pass. He had purchased a 65 foot wood ChrisCraft Connie in florida, supposedly Jerry Lewis's old boat. The broker had to him it was a "pure full displacement boat" therefor it was a trawler. Apparently the guy wanted a trawler so he bought it. He was bragging up the fact that the broker told him if he would buy it without a survey he could have it for the deeply discounted price of $120,000. It had been glassed over, quickly and poorly by someone in the very near past. I personally walked thru the boat and watched a river or large stream of water running over the floor timbers on its way to a bilge pump, one of 7. The owner claimed he had over 12000 gallons an hour of pump capacity, no way was this boat gonna sink !! The windows in the wheel house were surounded by mahogany that could literally be taken out by the handfulls. It smelled like wood alcohol. He told me "we lost all hydraulics" meaning a copper steering line had broken. They dropped anchor and when it caught it tore the bow pulpit completely off of the boat, along with all of the rotten decking and support structure. I have never been on a boat in such poor shape that was still floating. The thing that disturbed me more than the owners ignorance was the brokers outright incredible lies. But, full displacement, and trawler, are red letter words for brokers I guess.
 
manyboats said:
Haven't looked at anything like that for a long time and can't remember how power loading got into the picture if at all.

It got into the picture because I for one, think engineering analysis by photograph is nonsense. But I come from the world of engineering flight test.

No one would argue that a boat capable of cruising one knot over hull speed is not a FD boat. However given the history here on TF I should know better than to say that.

Then you reject what Dave Gerr states in his paper.


IMHO .. the difference lies in the way water under the stern returns to the way it was before the boat came along. Significantly submersed transoms need not apply. Too much drag and turbulence as pointed out before. And I still don't see any methods superior to making the call than the QBBL. There are exceptions like the one you posted above but no other yardstick seems to define the deciding line any better or universally.

The first page was good to very good but this is going down hill badly.

Just because you don't agree or wish to dismiss some excellent technical content doesn't mean it's gone down hill. On the other hand, if you want a dock talk discussion, pictures work as well as anything.

Regarding the hull in the photos, the buttock line along the keel is easily a constant 8 degrees all the way to the transom. There's an arch at the bottom edge of the transom which can be seen in the second photo. That arch curves down to meet the chines. So the bottom of the boat does not follow the chine profile inward toward the keel. In other words, it is not flat between the chines and the keel, but curves all the way until it meets that arch on the transom. The "effective flats" are relatively small...and according to the builder, they provide a little lift to limit squat at high power, and also add some stability at rest. The average mid arch buttock line is probably 7 degrees. In practice the chines do what the builder says, but little more. The boat reaches the bow wave in a relatively flat attitude at roughly the calculated hull speed. The one we tested had a very big engine. Adding power...lots of it... added about a knot. We stopped applying power with plenty in hand as the speed was not increasing and wake was humongous. The overall hull showed no evidence of lifting, in fact it appeared to be sinking down in the "hole". At cruise speed the wake emerges virtually calm.
Click to expand...
 
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Displacement hulls (well at least spoon shaped hulls like large offshore sail racers were a couple of decades ago) can and have submerged themselves when surfing along with large waves.

Push a displacement hull hard enough and it just might sink into a hole...now that's a seaworthiness trait that taken to an extreme will surprise you...:D
 
kulas44 said:
All this talk about displacement hull or not reminded me of a fellow that came into the marina at Sabine Pass. He had purchased a 65 foot wood ChrisCraft Connie in florida, supposedly Jerry Lewis's old boat. The broker had to him it was a "pure full displacement boat" therefor it was a trawler. Apparently the guy wanted a trawler so he bought it. He was bragging up the fact that the broker told him if he would buy it without a survey he could have it for the deeply discounted price of $120,000. It had been glassed over, quickly and poorly by someone in the very near past. I personally walked thru the boat and watched a river or large stream of water running over the floor timbers on its way to a bilge pump, one of 7. The owner claimed he had over 12000 gallons an hour of pump capacity, no way was this boat gonna sink !! The windows in the wheel house were surounded by mahogany that could literally be taken out by the handfulls. It smelled like wood alcohol. He told me "we lost all hydraulics" meaning a copper steering line had broken. They dropped anchor and when it caught it tore the bow pulpit completely off of the boat, along with all of the rotten decking and support structure. I have never been on a boat in such poor shape that was still floating. The thing that disturbed me more than the owners ignorance was the brokers outright incredible lies. But, full displacement, and trawler, are red letter words for brokers I guess.
Click to expand...

44 - That brings back recollections to me of a time in late 1990's. My wife and I went to see a late 60's 57' Chris w/ flybridge that had been presented to me as in "remarkable condition" by a broker. He said the only reason the owner (who lived in Mexico at the time) was selling at such a reduced price ($59K) was because he'd just been divorced and desperately needed cash to keep his resort in Mexico open. OMG - The amount of rot and leaking bilge I found was simply astounding! Sounds like the one you account in post and the one we reviewed were "Kissen Cusens"! That event is actually the one that for first time in my life got me to fully swear off purchasing older wood boats again! :eek:
 
Psneeld,
I saw that but didn't study it closely. Should have. Yes that's what I remember from reading books about NA and related subjects. I see displacement hulls go only up to 1.4 don't know w ....... Now I see that chart is for large ships.
I see the upper chart w different numbers. All of this is general and applies to some hulls more so than others and that is the way this whole matter is looked at by most that know and by others that follow like all of us. I wonder why SD was called the "transition zone"? Good description for people like Skidgear that tend to think in the planing mode.

Skid,
How did you measure the BL in the photo? I can't even see most of the bottom. But if the BL alongside or at the keel is in fact 8 degrees then the QBBL will be 4 degrees unless the bottom is concave aft. In that case the QBBL would be more like 2 or 3 degrees. If the hull is convex (or arched as many say) the QBBL would be or could be more than 4 degrees but as I say I can't even see the underside of the hull aft.

By my way of thinking this is not a FD hull for another reason. There are flattish portions of the bottom aft that are placed there to control the tendency to squat and go bow high. Those are not features of a FD hull and since it has these features (much like the Spanish boat w the big flat plane added to it's stern for the same reason. The original FD fishing boat was 100% FD but in my opinion became something else w the attachments (hull modifications).

I am definitely not presenting any of the above in the interest of obtaining followers or believers. It's just my opinion.

Panels I think I mentioned that negative aspect of the FD hull. And being sunk by a stern wave in a second or two could indeed be a surprise.
 
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manyboats said:
How did you measure the BL in the photo? I can't even see most of the bottom. But if the BL alongside or at the keel is in fact 8 degrees then the QBBL will be 4 degrees unless the bottom is concave aft. In that case the QBBL would be more like 2 or 3 degrees. If the hull is convex (or arched as many say) the QBBL would be or could be more than 4 degrees but as I say I can't even see the underside of the hull aft.

Well whataya know, an engineering analysis from photographs is just full of unknowns. I've seen the bottom of the boat we tested many times.


By my way of thinking this is not a FD hull for another reason. There are flattish portions of the bottom aft that are placed there to control the tendency to squat and go bow high. Those are not features of a FD hull and since it has these features (much like the Spanish boat w the big flat plane added to it's stern for the same reason. The original FD fishing boat was 100% FD but in my opinion became something else w the attachments (hull modifications).

You're entitled to your opinion, but once again it appears that you and Dave Gerr, whose blue water displacement boat has anti-squat surfaces aft, are in disagreement as to the definition of displacement. He says his boat would require hull mods to semi-plane. I'm afraid I'm with Dave Gerr on this one.


I am definitely not presenting any of the above in the interest of obtaining followers or believers. It's just my opinion.

Perish the thought. Your motives are pure as the driven snow.
Click to expand...
 
manyboats said:
Psneeld,
I saw that but didn't study it closely. Should have. Yes that's what I remember from reading books about NA and related subjects. I see displacement hulls go only up to 1.4 don't know w ....... Now I see that chart is for large ships.
I see the upper chart w different numbers. All of this is general and applies to some hulls more so than others and that is the way this whole matter is looked at by most that know and by others that follow like all of us. I wonder why SD was called the "transition zone"? Good description for people like Skidgear that tend to think in the planing mode.

Skid,
How did you measure the BL in the photo? I can't even see most of the bottom. But if the BL alongside or at the keel is in fact 8 degrees then the QBBL will be 4 degrees unless the bottom is concave aft. In that case the QBBL would be more like 2 or 3 degrees. If the hull is convex (or arched as many say) the QBBL would be or could be more than 4 degrees but as I say I can't even see the underside of the hull aft.

By my way of thinking this is not a FD hull for another reason. There are flattish portions of the bottom aft that are placed there to control the tendency to squat and go bow high. Those are not features of a FD hull and since it has these features (much like the Spanish boat w the big flat plane added to it's stern for the same reason. The original FD fishing boat was 100% FD but in my opinion became something else w the attachments (hull modifications).

I am definitely not presenting any of the above in the interest of obtaining followers or believers. It's just my opinion.

Panels I think I mentioned that negative aspect of the FD hull. And being sunk by a stern wave in a second or two could indeed be a surprise.
Click to expand...

Study it again I guess....

You musta missed the part of the graph that deals with speedboats, motorboats, fast motor boats, cigarrete boats, fast work boats, fast pilot boats, etc...etc... not just large ships....

Plus the lines on a graph are usually averages of some arbitrary definition "line"...not absolutes...data (real vessels) fall on either side exhibiting and favoring characteristics of above and below the lines....
 
manyboats said:
Psneeld,
I saw that but didn't study it closely. Should have. Yes that's what I remember from reading books about NA and related subjects. I see displacement hulls go only up to 1.4 don't know w ....... Now I see that chart is for large ships.
Click to expand...

Did those books say anything about why towing tanks exist?

If measuring one particular feature by viewing a photograph was adequate to define a hull and project its performance, the work of Mr. Froude and the generations of NAs that followed would have been of less value than politicians.

Size doesn't matter, it is disingenuous to dismiss facts that don't conform to your preconceptions because the examples provided are "big ships."

Big ship hulls are tested the same way as yacht hulls (at least yacht hull designs for people who are willing to pay what it costs to perform tank testing) and military vessels of all sizes.

The thread isn't going "downhill," it is just that more participants are finally seeing that many of the concepts which fuel it are as flawed as most of the preconceptions that started it.
 
Haha I knew you'd say "size doesn't matter" but I didn't change my text. They do test the same way in the tanks but there is then Reynolds Number and other variables that can't be controlled or otherwise accommodated. But it's not the same in many ways. Would the molecular dynamics be the same on a 2' model and a 700'ship. I suspect not.

Psneeld .. I missed something? Been there done that.

My motive in all this was do listen to others (in the beginning) and then try to (once again) express my understanding of the matter as it seemed many were having trouble understanding the dynamics of the QBBL in the stern and the general dynamics of water flow coming out from under the stern.

You three guys are more interested in winning a debate than having an educational experience. There's probably almost no one left reading it now. And by repeating myself I'll admit to some of the blame too. It has gone down hill in that it's been reduced to you guys throwing tomatoes at me and me ducking and probably worse. I took you guys off the ignore list to see how it would go .... and I saw how it went. Would you guys be interested in going from destructive to constructive?
 
Like showing people how QBBL is just part of the design process and not a defining feature?????:thumb:

as shown by many who HAVE TRIED to make it a learning process????:D

sure....:socool:
 
manyboats said:
They do test the same way in the tanks but there is then Reynolds Number and other variables that can't be controlled or otherwise accommodated. But it's not the same in many ways. Would the molecular dynamics be the same on a 2' model and a 700'ship. I suspect not.
Click to expand...

Thanks for confirming my statements about flawed concepts invalidating whatever point you have been trying to make ad nauseum.

I suggest you hit the books for a bit longer and find out how towing tank data is used. The correlation between models and full scale vessels of any size has been used quite profitably for more than a century.

"When a model and its prototype are geometrically similar and their two dimensionless coefficients (Re, Fr) are the same, their resistance coefficients (Ct) should be the same."
 
manyboats said:
Would you guys be interested in going from destructive to constructive?
Click to expand...

Sure, read up on hull design, Froude, and Reynolds and towing tank testing and get back to us.

I'm taking the ignore option. Bye
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manyboats said:
I'm taking the ignore option.

Bye
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Yo, Eric

Please don't let your panties get into too much of a wad! Now, you might not look at this the same way I do... but... Personally, I don't place ignore-feature onto anybody... even though there are a few who I feel are s-disturbers or even dummies that occasionally post on TF. My reasoning... even the lowest of life have some items that not only make sense but that are at times actually interestingly correct. With ignore affixed to their asssses - I just might miss that single pearl of wisdom they at some time may provide!

Important factor here (in this instance) is that both you and RickB are truly a couple of darn sharp intelligences. I know, I know... you are a really nice guy who seldom harshly confronts other contributor's input, and, Rick has a typing finger that lashes out and stings sometimes toward you and/or others, even me too... but so what??!!! To be in most any game of life we need to have thick skins and wear our helmets/shoulder pads while providing input and hoping for agreement by others, but still realizing that stern, even disconcerting rebuttals may be thrown at us from more than one direction.

If you’ll recall... Marin and I were not best of compatriots during many posts concerning our conflicting thoughts, and we’d at times get into a wordsmith shooting match with smoking guns and bullet holes in both bodies. But occasionally he and I would agree and I believe learn at least something from one another. It seemed that in most ways you and he were pretty close. At any rate... I feel that Marin and my conflicting TF relationship carries at very least a glimmer of similarity to yours and RickB’s relationship on TF. That said – I will read and look forward to continue learning from you both! Hopeing that you both can continue TF interaction.

Rick – If I may suggest – Please try to not have your words/messages/cartoons beat some people to a pulp... to me your lashes are just another ho-hum day at BOD meeting. To others you can be crushing!

Come on guys - - > I ask you to make up! Don’t bother kissing (yuck!)... but, at least let us hear a high five that communications can continue!

Now, you can both go back to the stools in your corners for a little R&R before your next bout! :hide: - LOL

Happy Boating Daze! - Art :D :speed boat:
 
Art you can bring a smile to my face no matter where we are. Don't ever loose that ability.

Most of the time we don't need these guys for their knowledge. We're mostly here for the fun of it anyway and these guys dislike me so much it's anything but fun. I had psneeld on ignore for many months and decided I wanted to hear what he had to say again ..... But it's just not worth it

Your post was a pleasant shock and .. If there's such a thing as a pleasant shock .. I appreciate you and what you say. Thank you Art.

But no .. I'm going to keep them in their rabbit holes, doghouses or wherever they go.
 
manyboats said:
Art you can bring a smile to my face no matter where we are. Don't ever loose that ability.

Most of the time we don't need these guys for their knowledge. We're mostly here for the fun of it anyway and these guys dislike me so much it's anything but fun. I had psneeld on ignore for many months and decided I wanted to hear what he had to say again ..... But it's just not worth it

Your post was a pleasant shock and .. If there's such a thing as a pleasant shock .. I appreciate you and what you say. Thank you Art.

But no .. I'm going to keep them in their rabbit holes, doghouses or wherever they go.
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:rofl::rofl::rofl: OK, OK!! I Give Up - Sorta! :rofl::rofl::rofl: :socool: :whistling: :dance:
 
There are no "personalities' involved in this discussion, just reality vs imagination. As far as the cartoon goes, it illustrates what ignoring input does for one and the discussion. It is a shame that it takes a cartoon to get that point across. I seriously doubt that post is or was "ignored" in any event. If it was then once again my point is proven.


Personally, I could care less what fantasies Eric chooses to hold with regard to hydrodynamics and hull design. I chose not to join the angels dancing on that pinhead but the silliness finally overcame my hesitation to try to leaven the rise of absurdity that was beginning to run amuck - again.

Why should I pull any punches with regard to illustrating a point that is patently false or when responding to something like the Swaggart ploy? There is nothing rude or vulgar in my posts - or the ones previous that were removed by person or persons unknown for reasons we can only guess. Nothing I have written violates any rule of this forum as much as some of it may disturb one of the moderators.

Eric is an adult, he doesn't need me or the moderators to shield him from the world or from opposing ideas. If he chooses to "ignore" posts that is his choice alone, I am not responsible for his decisions or his actions. I didn't think that was the purpose of this forum when I joined and I don't believe it should be allowed to become a mutual admiration society for the partially informed or hopelessly thin-skinned either.
 
Life is better on forums when one doesn't repeatedly attempt to convince everyone they're not wrong on a particular issue.
 
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