Nomad Willy
Guru
Does the displacement change when the bow is riding higher than it does at rest?
Definition Of Full Displacement
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Does the displacement change when the bow is riding higher than it does at rest?
No, any lessening of displacement forward is met by a corresponding increase aft until an SL ratio of 1.34 is exceeded.
Giggitoni
Guru
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- 1986 Grand Banks 42 Classic
Ok, you win... 121 meters per second
Nomad Willy
Guru
But a lessening of displacement fwd means the bow has risen. And an increase aft means the boat is at a positive angle of attack and to some degree climbing the bow wave. Thus a FD boat in this attitude could, must, should be said to be semi-planing.
What you're interpreting as the bow rising is not actually the bow rising it's the stern squatting as the propeller draws buoyancy from under the stern, this is why a sailboat being driven to near hull speed by the wind doesn't experience the same alteration of fore and aft attitude. Have you ever noticed that the stern squats more in very shallow water? It's the same concept as bank cushion and bank suction when operating parallel to shore in say a canal, a pressure wave forward that pushes the bow away from the bank and a reduction of buoyancy that draws the stern toward the bank. It's all buoyancy effects and not the hull being forced up on top of the water as in semi-displacement and planing.
Nomad Willy
Guru
OK good,
I understand most of what you say.
But is it flyst*t or is it important?
I feel we/I got lost in a murkey swamp.
Re the author Gerr,
I haven’t got nor have I read this man’s book.
Does he classify P, SD and FD by their Speed/Length ratios? Long ago (in this thread) I mentioned classification by SL ratios. It works quite well as per getting the labels right. It is speed dependant and that introduces many more irrevlant variables like power. Do you think definitions are independant of hull form? Just if it goes X fast for it’s WLL it’s X hull type?
That’s throwing out the question. The question is about hull type. Did you notice the word hull? The question is asking about a hull ... not about speed. Speed is an element but it is a result of the hull form in question.
Gerr’s book seems to talk about the result of the hull form. Not what the hull form is. The statement “it goes this fast so it’s this kind of hull form”. It really is “it’s this kind of shape and should/will run at these speeds if powered appropriately”.
My own thought on the matter is that one should use all the information available and come to supportable conclusions. As to hull form type the hull itself can only dictate it’s type. It is what it is.
I understand most of what you say.
But is it flyst*t or is it important?
I feel we/I got lost in a murkey swamp.
Re the author Gerr,
I haven’t got nor have I read this man’s book.
Does he classify P, SD and FD by their Speed/Length ratios? Long ago (in this thread) I mentioned classification by SL ratios. It works quite well as per getting the labels right. It is speed dependant and that introduces many more irrevlant variables like power. Do you think definitions are independant of hull form? Just if it goes X fast for it’s WLL it’s X hull type?
That’s throwing out the question. The question is about hull type. Did you notice the word hull? The question is asking about a hull ... not about speed. Speed is an element but it is a result of the hull form in question.
Gerr’s book seems to talk about the result of the hull form. Not what the hull form is. The statement “it goes this fast so it’s this kind of hull form”. It really is “it’s this kind of shape and should/will run at these speeds if powered appropriately”.
My own thought on the matter is that one should use all the information available and come to supportable conclusions. As to hull form type the hull itself can only dictate it’s type. It is what it is.
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It's probably best if you read the book to get the definition as I'm not sure I have the writing skills to present the same information in yet another form to simply say the same thing again. I believe part of the impasse is while the definition is simple in practice with the myriad hull forms that definition gets picked apart, yet the definition remains rather simple. As you brought up elsewhere it all revolves around the SL ratio a hull is intended to operate at, not one it's capable of operating at with ridiculous power applied or inadequate power to attain the designed speed.
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caltexflanc
Guru
He's a noted naval architect, there are other books by other naval architects on various approaches to hull design, but his is the most layman friendly I have found. I gave you all those definitions by the way and there is more detail as to how they are arrived at in the book.
You obviously are in need of reading the book, as witnessed by your musings here. I think you would really enjoy it, and not just for this particular subject. Start at the beginning, don't skip ahead to the pages relevant to this thread because you will just find yourself thumbing back a lot to understand the terminology used.
He is a pretty good writer, he keeps what could be sort of dry technical material from being too boring as most of my textbooks on naval architecture do. I do still struggle with the math though, my failing not the authors. Have you also read his Boat Mechanical Systems Handbook?
caltexflanc
Guru
No, I haven't. May have to take a look, though I've found Calder to be pretty user-friendly for that, at least for my purposes.
Nomad Willy
Guru
HaHaHa......
A resounding recommendation.
Don’t know if I will as I’d need to find it first. Only know of one place to look and I’ll do that. Need to go to town today and get a car part so I’ll be in the neighborhood of the big used book store. Need another Dick Frances anyway.
I think there was a discussion on hull types in boatdesign.com where I’ve been for years and read most all the opinions on the matter. Quite sure Geer’s was in there as well.
But at this point I think if you say “that’s a displacement hull” you’re talking about the subject of the scentence ... hull.
Thanks for not throwing tomatoes.
A resounding recommendation.
Don’t know if I will as I’d need to find it first. Only know of one place to look and I’ll do that. Need to go to town today and get a car part so I’ll be in the neighborhood of the big used book store. Need another Dick Frances anyway.
I think there was a discussion on hull types in boatdesign.com where I’ve been for years and read most all the opinions on the matter. Quite sure Geer’s was in there as well.
But at this point I think if you say “that’s a displacement hull” you’re talking about the subject of the scentence ... hull.
Thanks for not throwing tomatoes.
Eric I find you to be an intelligent and engaging fellow member, I don't think anyone wants to be "throwing tomatoes" certainly not me.
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Nomad Willy
Guru
OMG ..
Find the humor button quick.
Twas 100% humor Fish53.
Find the humor button quick.
Twas 100% humor Fish53.
RT Firefly
Enigma
Well there was a sort "tongue in cheek" element to it, I'm not professing love or anything. I do enjoy your posts though even when we aren't in total agreement.
Hull Shape Definitions - In Meaningful, Useful Reality [i.e. hull movement through or over water]:
Full Displacement: No go too fast through water... regardless of power applied
Semi Displacement / Semi Planing: Go somewhat faster than FD... via more power applied so through water becomes at least somewhat over water circumstance
Full Planing: Go faster and faster over water depending on power applied
It's all really pretty easy to define and understand! For simple minded pleasure-boaters like me, as well as some 90% + of other boaters on the water!!
Full Displacement: No go too fast through water... regardless of power applied
Semi Displacement / Semi Planing: Go somewhat faster than FD... via more power applied so through water becomes at least somewhat over water circumstance
Full Planing: Go faster and faster over water depending on power applied
It's all really pretty easy to define and understand! For simple minded pleasure-boaters like me, as well as some 90% + of other boaters on the water!!
Nomad Willy
Guru
Something that’s been missing from this discussion is observations.
When I repowered Willy I kept track of all the boats I saw as I went about on the floats, on TF, in magazines ect ect.. I even observed the numbers posted on a plaque on the bulkhead of an Alaska State Ferry. And when I didn’t know how much power the boat had or what the WLL was or the boat’s displacement I tried to find out and usually did. What I paid attention to most was how much hp a boat had per ton of displacement. I quickly found out there were quite a number of boats that had considerably less power than most. And observed their performance. This led me to determine how much power per ton was required to push a FD hulled boat. The average was a wide range of 4-7 .. for boats that I thought were powered well. The KK42 has 7. Once and awhile I’d find a boat w only 2hp per ton. Most ofthose were large. I chose for Willy a 37hp engine that delivered 5hp per ton of disp. I knew 4 would do from my calculations and observations but I was too chicken to risk being underpowered. But now I know I’d have been fine w 33hp. Interestingly that’s what Willard gave the 30’ boats. It was a 36hp engine but they overpropped all the boats so only had about 33hp “on tap”. But I’d probably be running at 24 to 2500rpm instead of 2300. Or gone slower. So I’m glad I choose to get close to the edge but not too close.
But it was observation that led me to what I needed to know.
When I repowered Willy I kept track of all the boats I saw as I went about on the floats, on TF, in magazines ect ect.. I even observed the numbers posted on a plaque on the bulkhead of an Alaska State Ferry. And when I didn’t know how much power the boat had or what the WLL was or the boat’s displacement I tried to find out and usually did. What I paid attention to most was how much hp a boat had per ton of displacement. I quickly found out there were quite a number of boats that had considerably less power than most. And observed their performance. This led me to determine how much power per ton was required to push a FD hulled boat. The average was a wide range of 4-7 .. for boats that I thought were powered well. The KK42 has 7. Once and awhile I’d find a boat w only 2hp per ton. Most ofthose were large. I chose for Willy a 37hp engine that delivered 5hp per ton of disp. I knew 4 would do from my calculations and observations but I was too chicken to risk being underpowered. But now I know I’d have been fine w 33hp. Interestingly that’s what Willard gave the 30’ boats. It was a 36hp engine but they overpropped all the boats so only had about 33hp “on tap”. But I’d probably be running at 24 to 2500rpm instead of 2300. Or gone slower. So I’m glad I choose to get close to the edge but not too close.
But it was observation that led me to what I needed to know.
Nomad Willy
Guru
Fish53,
We vary widely to the degree of knowledge we have about this. Probably isn’t a good subject for this forum as many (perhaps most) seem “snowed” and don’t take part. Others interupt the the thread w jokes and some get onboard and try to understsnd. But that’s really hard when those that seem to know most about it and have the knowledge to argue ... do just that. And disagree on even basics. So most are frustrated and probably don’t read any of it.
I have had an attitude that thought those that are interested would take part and others would go elsewhere. Most subjects on TF don’t interest me so I tend to get involved in those that do.
So no ... I disagree w myself. Not a poor subject and we’re all learning .. even while arguing. And those that dislike the subject can go to others.
2hp per ton is the traditional number and many sailboats have less than that. I powered my 3 ton boat with a Lister rated at 20hp@3000rpm and adjusted the fuel stop at 2600rpm for 18hp so I could go up an inch in propeller diameter so now I have the most diameter that will fit the aperture, 18 inches. I'll share something from the UN of all places that you may find interesting.www.fao.org/3/i2461e/i2461e.pdf
This is interesting as well.....
How to “Right Size” the Engine. Most displacement-hull commercial fishing vessels in Alaska are overpowered; that is, their engines can produce more power than is needed to propel the boat at its “hull speed” and do the required work. Hull speed is the rate through the water at which a displacement hull vessel starts to encounter excessive wave resistance forces and requires disproportionately more power. Hull speed (in knots) is calculated as 1.34 times the square root of the waterline length in feet. (The 1.34 multiplier applies to a typical hull with a length to beam ratio of approximately 3:1. A lower multiplier would apply to a beamier hull, whereas a slimmer hull would have a larger multiplier.) For example the hull speed of a boat with a waterline length of 36 feet would be calculated as follows: the square root of 36 is 6. Multiply 6 x 1.34 and the result is a hull speed of about 8 knots. Hull speed for an 80 foot hull would be 12 knots based on the following calculation: 80 has a square root of approximately 9. Multiply 9 x 1.34 and the product is about 12 knots. Hull speed for a typical boat in calm sea conditions (that is, steaming power demand only) requires about 4.5 hp per displacement ton. Increasing speed by one knot increases horsepower and fuel requirements by about 50%, and at speeds above hull speed the increase is even steeper. At a speed:length ratio of 1:1 only about 1 hp per displacement ton is required. Add a 15% horsepower “sea margin” to overcome adverse wave conditions, and a 36-footer that displaces 12 tons needs only 62 hp to achieve an economical 8 knots. Since a diesel engine is most efficient running at about 80% of its rated horsepower, the
A seiner is steaming at about hull speed. The “bone in its teeth” shows that it is using more fuel than necessary.
7
nominally correct size of engine for this vessel would be 77.5 hp. Most Alaska fishermen are unwilling to settle for such a small engine—they like the feel of additional power, they believe that it’s easier on the engine to run it well below its 80% output rate,1 or they simply feel they need to go faster. But additional power comes at the cost of greater fuel consumption. Right-sizing an engine at replacement time or during new construction can save in both capital and operating costs. This was from a booklet about fuel savings for Alaska fishing vessels from Seagrant. This all pertains to fishing vessels but is nonetheless pertinent to our discussion.
How to “Right Size” the Engine. Most displacement-hull commercial fishing vessels in Alaska are overpowered; that is, their engines can produce more power than is needed to propel the boat at its “hull speed” and do the required work. Hull speed is the rate through the water at which a displacement hull vessel starts to encounter excessive wave resistance forces and requires disproportionately more power. Hull speed (in knots) is calculated as 1.34 times the square root of the waterline length in feet. (The 1.34 multiplier applies to a typical hull with a length to beam ratio of approximately 3:1. A lower multiplier would apply to a beamier hull, whereas a slimmer hull would have a larger multiplier.) For example the hull speed of a boat with a waterline length of 36 feet would be calculated as follows: the square root of 36 is 6. Multiply 6 x 1.34 and the result is a hull speed of about 8 knots. Hull speed for an 80 foot hull would be 12 knots based on the following calculation: 80 has a square root of approximately 9. Multiply 9 x 1.34 and the product is about 12 knots. Hull speed for a typical boat in calm sea conditions (that is, steaming power demand only) requires about 4.5 hp per displacement ton. Increasing speed by one knot increases horsepower and fuel requirements by about 50%, and at speeds above hull speed the increase is even steeper. At a speed:length ratio of 1:1 only about 1 hp per displacement ton is required. Add a 15% horsepower “sea margin” to overcome adverse wave conditions, and a 36-footer that displaces 12 tons needs only 62 hp to achieve an economical 8 knots. Since a diesel engine is most efficient running at about 80% of its rated horsepower, the
A seiner is steaming at about hull speed. The “bone in its teeth” shows that it is using more fuel than necessary.
7
nominally correct size of engine for this vessel would be 77.5 hp. Most Alaska fishermen are unwilling to settle for such a small engine—they like the feel of additional power, they believe that it’s easier on the engine to run it well below its 80% output rate,1 or they simply feel they need to go faster. But additional power comes at the cost of greater fuel consumption. Right-sizing an engine at replacement time or during new construction can save in both capital and operating costs. This was from a booklet about fuel savings for Alaska fishing vessels from Seagrant. This all pertains to fishing vessels but is nonetheless pertinent to our discussion.
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53 - I agree wholeheartedly... with your serious doubts mentioned above!
Reason I enter this confab is due to my desire for TF to remain my adjunct of boating knowledge and reading pleasure during hours of my busy and often fun filled life!
TF enables me to somewhat take the edge off... while my mind still plays with other items of which I'm involved.
Regarding boating and many other circumstances in my life... I usually try to keep taking the helicopter view. I like to see the forest... not just the trees!
caltexflanc
Guru
I think everyone pretty much agrees about the behavior of different hull types. But it became apparent to me that almost no one here save two, maybe three other posters, understood what those hulls could look like, and the design elements that lead them to look that way and as a result behave (or have the ability to behave) a certain way. And equally, what design elements in and of themselves have little to do with it.
Is it draft? No, look at a Great Harbor.... Is it nice rounded chines and cross section? No, look at the hauler... is it an above waterline transom with a rounded, tapering bottom, no flattish sections? No, look at the Nordhavn 62.... and so on and so on.
Certainly, most rec boaters and not a few pros, don't care, they just want to know what the boat's range of behaviours are not so much what causes them. Nothing wrong with that, I'm certainly the same way about most contraptions.
Is it draft? No, look at a Great Harbor.... Is it nice rounded chines and cross section? No, look at the hauler... is it an above waterline transom with a rounded, tapering bottom, no flattish sections? No, look at the Nordhavn 62.... and so on and so on.
Certainly, most rec boaters and not a few pros, don't care, they just want to know what the boat's range of behaviours are not so much what causes them. Nothing wrong with that, I'm certainly the same way about most contraptions.
Your response to the quoted comment of mine leaves me wondering how exactly you are interpreting what I said? However I agree with much of what you said, I share your disinterest in curtain colors, countertops and sundecks. The subject at hand is one that throughout my professional career was one of great interest and importance, hence my enthusiastic, opinionated and to some even annoying participation.
Nomad Willy
Guru
Fish53,
You have no idea how glad I am that you came along. For the most part you’re helping me propagate much of the things I’ve been talking about for about ten years. Went to town but not the bookstore. Not feeling well .. came home. Hold the fort.
PS. What’s Gerr’s first name? Dave?
You have no idea how glad I am that you came along. For the most part you’re helping me propagate much of the things I’ve been talking about for about ten years. Went to town but not the bookstore. Not feeling well .. came home. Hold the fort.
PS. What’s Gerr’s first name? Dave?
clyde
Senior Member
reply to fish53...pitcarin island, where bounty ended up! back in the later 60`s, i`d had my ham call for a coupla yrs+, was on one afternoon, i lived near bremerton, wa, in my [inherited] mom`s house, right on the water, had a 70 ft rohn tower, with a small antenna farm on it, ran collins gear, with a 1000 watt heathkit linear...early in an afternoon, winter time, was on 15 meters i think, there was a pile up, swung my beam around to east/south est, to improve my signal, heard lady with a qst request, turned till i had her good, went back to her, she answered, she was on pitcarin, was a decendent of the bountcrew, i was her first ever contact!! never forget...clyde
