Reply
 
Thread Tools Display Modes
 
Old 12-04-2007, 05:08 PM   #1
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,721
Design

John,
Thats where the planing surface is supposed to be.What part of your boat is still in the water at 25knts? A full planing hull has a flat or at least a straight run aft with sharp hard chines. It also has fairly low weight and high power. A semi displacement and a semi planing hull ( SD @ SP ) is harder to classify. The word " partly " is used to define " semi ". So an SD hull is like a planing hull but partly not. An SP hull is like a disp. hull but partly not. So Carey's lobster hull is a SD hull. All it would need to become a planing hull is to have hard chines aft. An SP hull is one that partly planes. It is slower than an SD hull and faster than an FD ( full displ hull ). A FD hull is usually defined as a hull that operates at or below 1.34 x sq. root WLL.If this is a speed / length ratio of one then FD hulls operate below an SL of 1.4T. Here are three ways to identify a FD hull. It will not be possible to drive it to a SLR of 2 ( about 15 knts for a 30' boat ). Also at about 75% power it's bow will not have begun to rise. This happens at about a SLR of about 1. With a good working load 100% of the transom will be out of the water.. as in not submerged.The geryest area is between SD and SP hulls. Some boats that I think are in this zone are: Eagle 32 @ 40, Nordhaven 35 @ 47, some Devlins and the Selene Archer 36. The GBs and NTs are in the narrow grey area between FP andSD hulls.
Eric Henning
30'Willard
Thorne Bay AK
__________________
Advertisement

Nomad Willy is offline   Reply With Quote
Old 12-04-2007, 06:00 PM   #2
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Design

Erik---

Probably largely semantics as you say, but from what I have read from boat designers on the topic, semi-displacement and semi-planing mean exactly the same thing in terms of what the hull does, only semi-planing is the more hydrodynamically correct term.

Lobsterboats need more than just hard chines aft to be able to plane completely. Actually, they have very tightly radiused chines aft so they are "almost sharp" as it is. To fully plane, they need to lose the keel, obviously, but the big requirement would be to extend the lifting surfaces much farther forward than they are now. They would need a hull more like a scaled down WWII PT boat in order to be able to achieve a full plane. The rounded front half of the typical lobsterboat hull, plus the weight, plus the normal power on these boats, prevents them from achieving a full plane (assuming the keel was gone already).

But then you'd have a very "poundy" boat, so the forward part of the hull is as it should be for this particular application.

But in terms of how his hull acts, Carey's boat is semi-planing, as is our GB. He just has way more power and considerably less weight so he can take more advantage of his hull shape than we can.
__________________

Marin is offline   Reply With Quote
Old 12-04-2007, 09:22 PM   #3
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,721
RE: Design

Mike,

This is about as simple as I can get and and still be definitive.
Looks like you should know a great deal about design. What identifiable feature of a planing hull distinguishes it as a planing hull?
What part of my post didn't make sense to you?

Eric Henning
30'Willard
Thorne Bay AK
Nomad Willy is offline   Reply With Quote
Old 12-04-2007, 10:02 PM   #4
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,721
RE: Design

Marin,

Are you following me around? Good. I'll always have somone to talk to. Your'e very dependable.
In calm weather who cares what the bow of Carey's boat looks like ...it's completly out of the water! Most lobster boats are flat on the bottom like a canoe with the same rounded chines. A planing hull needs hard chines to break the out board flowing water free so it dos'nt suck the hull, down. Water tends to follow a curved surface. Your GB has the wide flat aft section and the hard chines. The keel is fine as long as one has the power to drag it along. The GB is a planing hull with way too much weight and way too little power. You just won't admit it. Your'e right about stability though. But if you're high on the face of a wave broadside, hard chine will get you capsized where the rounded chines will suck the high chine down, the other chine will ride high allowing your boat to slide down the face of the wave instead of tripping on on the sharp edge of t5he hard chine.
Good night Marin

Eric Henning
30'Willard
Thorne Bay AK
Nomad Willy is offline   Reply With Quote
Old 12-05-2007, 12:45 AM   #5
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Design

Actually, the front of a lobsterboat is not completely out of the water, calm or not. Take a look at this website http://www.ellisboat.com/hullDesign.php which clearly shows the difference between a planing hull and a lobsterboat hull. They use the term "semi-displacement," where I prefer the term "semi-planing" but in terms of function they're the same thing. The lobsterboat hull does not have enough lifting surface to truly plane the boat even if you replaced the soft chines with hard chines unless you put a tremendous amount of horsepower in it. With enough power, you can plane anything, even a rock And in fact, I do agree with you: a GB hull could be planed with enough power and enough weight reduction. But since it has neither, it is "semi-planing."

And while I have nowhere near the experience with hull shapes as Mike does. I've been around a lot of planing hulls for a couple of decades in the form of floatplane floats. I have also done a lot of reading and research about PT boats in preparation for the book I am now writing, and we own a small planing boat in the form of a 17' Arima. From what I have observed regarding what defines a true planing hull, they have lifting surfaces that extend almost to the front of the hull. As power is added, enough hydrodynamic pressure must be generated to lift the boat out of the water, thus reducing drag, which in turn enables the hull to be driven faster, which creates more hydrodynamic lift, which lifts the hull farther, which reduces more drag, and the cycle continues until the hull is riding on the minimum possible wetted surface.

The forward lifting surfaces are what's missing on a hull like a lobsterboat, Grand Banks, etc. So they cannot efficiently generate enough hydrodynamic lift to get the forebody up and start reducing drag enough to get onto a plane. The only way they can overcome this is to use a huge amout of power to generate sufficient hydrodynamic lift on the aft lifting surface to get the whole hull up. But most boats of this type are not fitted with engines this powerful, so you start to get up onto a plane but then can't go any farther because of the lack of sufficient lifting surface and power. Hence they are "semi-planing."

So my opinion is the most identifiable characteristic of a true planing hull is the lifting surfaces that run from the front of the boat, or very near the front, clear back to the stern. A PT hull is an excellent example of this because it's so big (80') and the configuration of the lifting surfaces is very obvious. Another very obvious example of this is a floatplane float.



-- Edited by Marin at 02:15, 2007-12-05
Marin is offline   Reply With Quote
Old 12-05-2007, 10:23 PM   #6
Member
 
Join Date: Dec 2007
Posts: 7
RE: Design

Airplane designers say you could fly a brick to paris and back if it had enough thrust. I imagine my displacement hull would plane if I had three thousand horsepower. It wouldnt plane well, but it would eventually lift itself out of the water (unless of course the hull collapsed under the stress).

My flat bottomed mastercraft ski boat is a good example of what a real planing hull looks like.

-- Edited by holohonu at 23:25, 2007-12-05
holohonu is offline   Reply With Quote
Old 12-06-2007, 07:35 AM   #7
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,633
Design

I totally agree with Marin on this subject although power or weight has absolutely no bearing on hullform or what we call the hullform.....it only has a bearing on its performance. What needs to be remembered is that there are advantages to a SD hull. One of the big ones is a smoother ride in the range of planing speeds. Whereas a planing hull has flatter hullform forward(as Marin stated) which can cause pounding in the chop, a SD hull neither pounds nor does it even lift.....it just goes right through the water cutting thru the chop. A smoother ride for sure but sometimes a wetter ride for the same reason. Chine shape has really nothing to do with it either although I think Willy might be refering to chines as a totally rounded bilge aft. A chine really is just the edge of the hull. You can have a totally flat no deadrise hull(max lift) and still have soft chines. You can also have a full disp hull with hard chines.

The reason why SD hulls are used in trawlers unable to achieve planing speeds(due to be underpowered) is for form stability or static stability as we call it in airplanes. Due to keeping the flotation all the way out to the edges(chines), the boat has the initial tendency to resist rolling. This flotation is also a disadvantage as once the boat is upset, it has a tendency to snap back quickly whereas a full displacement boat does not have this flotation at the chines so it does not snap back(although its initial stability is not as good so initially, it rolls easier).

Willy, my boat and boats like it are planing hulls from about midship back and FD hulls forward. Obviously there is a gradual transformation along the length of the hull. I have some good pics of my boat outta water that I will put up....
Baker is offline   Reply With Quote
Old 12-06-2007, 07:44 AM   #8
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,633
RE: Design

Look at this pic.* Notice how from the bottom of the boat up to that spray chine.* If you drew a line from the bottom to the chine, there would be "air" in between the hull and that line.* On a planing boat the hull would at least meet that line and most likely "bulge out beyond it.* It would also not be so deep of a "V" providing even more dynamic lift.
Attached Thumbnails
Click image for larger version

Name:	pa313099 (medium).jpg
Views:	87
Size:	91.0 KB
ID:	262  
Baker is offline   Reply With Quote
Old 12-06-2007, 10:59 AM   #9
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Design

John makes a good point about semi-planing boats tending to have wet rides. If you want to experience a wet ride, try a GB quartering into wind waves with a good 15 knot breeze blowing everything the hull kicks up back at you. A lot of trawler-type boats have significant bow flare to help direct water out and even down a bit. But not a GB. Bellingham Bay is rough more often than it's not, and our windshield wipers are often the most imporant components on the boat.
Marin is offline   Reply With Quote
Old 12-06-2007, 01:16 PM   #10
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,721
RE: Design

I still disagree with you guys about fwd lift. See if this makes sense. When a boat reaches a speed length ratio of 1 the hull is riding right in the middle of the trough. The bow is on the backside of the bow wave and the stern is on the face of the stern wave. Both ends of the hull are equally supported and the boat moves forward in a level attitude ( neither bow up or down ). Now we try to go faster. The boat moves forward and in doing so assumes a position on the backside of the bow wave because the center of the boat is supported by the bow wave and the stern drops into the trough of the two waves. The stern droping and the center of the boat rising is what causes the bow to rise. The bow is just along for the ride.

Eric Henning
30'Willard
Thorne Bay AK
Nomad Willy is offline   Reply With Quote
Old 12-06-2007, 02:38 PM   #11
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Design

Eric---

I think you're wrong about this. I've had the whole hydrodynamic principle of floatplane floats--- which are no different in form and function that a planing boat hull--- explained to me in painstaking detail when I was writing my floatplane instructional book back in the '80s by the guy who ran Edo floats for decades. Without that lifting force in the forebody of the float as well as farther back, there is not enough hydrodynamic force to lift the float onto a full plane (the "step" as it's called in seaplane flying.

The stern down-bow up squat you are talking about is not the kind of force needed to lift a hull. The stern drops in part because the prop is "digging" water out from under it. I (and Carey) have experienced this in canal boats where the boat can go over very shallow spots at idle, but if you put in power, the prop pulls the water out from under the rear portion of the hull, the hull squats, and it hits the bottom. Since you're standing on the stern with these boats, you can feel this happening. And canal boats are about as full-displacement as you can get with a boat

Merely supporting the bow with the bow wave will not provide enough upward force to lift the hull and reduce the wetted surface enough to reduce drag and allow the boat to accellerate and enter the "more lift = less wetted surface = less drag = more speed = more lift" cycle I mentioned before.

Planing is not about the angle of the hull, it's about the reduction of drag. The only way to do that with a boat is to reduce the wetted surface, and the only way to do THAT is to generate enough hydrodynamic force--- lifting force-- to get as much of the hull out of the water as possible. To do that efficiently the hull must be shaped in such a way as to generate as much lifting force as possible with the the minimum amount of power.* Ideally you would want the ENTIRE hull out of the water for true efficiency, which is what a hydrofoil is all about.

The most efficient planing surface is a sheet of plywood, but obviously that introduces all sorts of control and stability problems. The ubiquitous "John boats" used by duck hunters, fishermen in the bayous, etc. have absolutely flat bottoms and they can be put on a full plane with surprisingly small motors. ANd of course the flat bottom is what you want when operating in shallow water like most of them do.

Flat bottoms are not what you want to be riding around in when there are waves, so you start getting compromises with sharper entries, keels, and so on, all of which make it harder to generate the lift you need to get up onto the plane.

But the bottom line (literally) is that if you want to plane, you have to get that hull up out of the water and minimize the wetted surface. Doing that with any degree of efficiency requires getting as much of the hull as possible to contribute to the generation of lift, and that means the front as well as the back. Once you're on the plane, the front is no longer a factor as it's pretty much out of the water. But that forward lifing surface is essential to get the boat up onto the plane. Without it, you either need gobs and gobs of power, which often isn't practical, or an amazingly light boat, which in a trawler isn't practical either.

Since a boat like a trawler, lobsterboat, etc. doesn't have the lifting surface at the front like a true planing boat--- ski boat, fast sportfisherman, etc.--- you end up with a boat that starts getting up onto a plane but never makes it because it's too heavy, too low-powered, and doesn't have a hull design that generates a lot of lift as speed is increased. In other words, you have a "semi-planing" boat. It tries, but it just doesn't have the right combination of weight, power, and hull shape to get away with it.


-- Edited by Marin at 15:45, 2007-12-06
Marin is offline   Reply With Quote
Old 12-06-2007, 02:58 PM   #12
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,633
Design

>


Marin, again, I agree with you. But weight and power have absolutely nothing to do with a semiplaning/displacement hull.....only it's performance. I have a boat with a semi-planing hull and it does plane. It is still a semi-planing hull even though it has more power and less weight which allows it to operate in the planing range. If you put more lift forward on my boat I would get more speed/efficiency with the same weight/power combination but it would also not be as sea-kindly.....that is what the trade off is.
Baker is offline   Reply With Quote
Old 12-06-2007, 04:09 PM   #13
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Design

John---

Perhaps the difference stems in how we each define a boat. If you have a boat that planes--- achieves a full plane, not plowing along partway up--- then to me, it's a planing hull regardless of what the hull shape happens to be. I have always categorized boats by what they can do--- plane, partial plane, not plane at all, and so on.

So by my definition--- and I'm not saying it's the one everyone should follow--- if I have a rock and I can strap a great big engine to it and get that rock up onto a plane, it is a planing rock as far as I'm concerned. By my definiton, a planing hull must have the right combination of weight, power, and hull shape to get onto the plane. They all contribute to the boat's ability to plane. To me, a semi-planing boat is one that doesn't have the right combination of weight, power, and hull shape to get onto a plane.

However....*if you are using the term semi-displacement to define a particular hull configuration, then I have no argument with your statement.* *If your boat has a sharper entry, deeper and more rounded forebody to cut through waves rather than slam into them, and then a flatter cross section aft to allow it to achieve faster-than-displacement speeds (and better stability in some situations), and that is how we define a semi-planing hull, then yes, I would say your boat has a semi-planing (or semi-displacement if you prefer) hull even though you can get it up onto a full plane.

So if we accept that I define a boat type strictly by what it can achieve in terms of performance*and you define a boat type by its physical configuration and how it behaves, then we're both right.


-- Edited by Marin at 17:21, 2007-12-06
Marin is offline   Reply With Quote
Old 12-06-2007, 05:45 PM   #14
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,721
RE: Design

Marin,
Your'e dealin with apples and zucchinis! Ever heard of a boat whith the thrust line 22 ' above the keel. I'll bet you'd need some fwd lift or you'd go right down in the water. Planing IS all about angle of attack. No angle of attack .... no lift...none zip nada. Whats fore and aft on an airplane float may be confusing to you also as the effective transom at 10 knts becomes the step.

Eric
Nomad Willy is offline   Reply With Quote
Old 12-06-2007, 05:53 PM   #15
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,633
Design

You are correct, Marin. It is how we define the hull. Anything will plane given enough power....that does not mean it has a planing hull. The rock might be planing, but it does not have a planing hull nor does it have a hull at all. We are in agreement. I was talking about the hull configuration....not what that particular hull was doing at any particular time.
Baker is offline   Reply With Quote
Old 12-06-2007, 06:07 PM   #16
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Design

Eric---

You're correct but in a planing hull the angle of attack is built into the angle of the forward lifting surfaces. Just saying "the bow comes up" is not a very accurate explanation of what's creating the lift. In a truly planing hull, the forward part of the hull begins developing lift before the bow begins to rise because of the upward angle of the lifting surfaces. As soon as the lift starts to build the bow starts to rise, but the bow does not have to rise first in order to develop lift in a planing hull. It does in a semi-planing hull, which is why a semi-planing hull either can't achieve a full plane at all or if it can, it does so relatively inefficiently in comparison with a true planing hull.

As to floatplanes, the parallel is apples and apples. The center of thrust is higher than the floats, but only about five feet in the case of the Beaver I fly. And the height of the center of thrust is more than offset by the use of up-elevator at the begining of the takeoff run, which is why you use up-elevator.

Once a floatplane begins to accelerate, the afterbody of the float--- the portion aft of the step--- is irrelevant. The only reason it's there is to keep the plane level and on top of the water in the displacement mode Once the speed starts to build, the rear half of the float is just along for the ride. It contributes nothing to the lift needed to get the plane onto the step. The only part of a floatplane float that is important when getting onto and staying on the plane or step is the forward portion, from the bow to the step. In this regard, the float acts exactly like the hull of a planing boat, which in fact, it is.



-- Edited by Marin at 19:24, 2007-12-06
Marin is offline   Reply With Quote
Old 12-07-2007, 05:18 AM   #17
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,531
Design

If you want speed in rough water ,

http://www.cutfingerwatercraft.com/

Not suitable for bloat boat dockside cottage living tho.

FF
FF is offline   Reply With Quote
Old 12-07-2007, 07:56 AM   #18
Moderator Emeritus
 
dougd1's Avatar
 
Join Date: Oct 2007
Posts: 300
RE: Design

I'm bored.* Let's get out of our underwear and go boating!
dougd1 is offline   Reply With Quote
Old 12-07-2007, 09:27 AM   #19
TF Site Team
 
Baker's Avatar
 
City: League City, Tx
Country: Texas
Vessel Model: Carver 356
Join Date: Oct 2007
Posts: 5,633
Design

Let's go boating in our underwear!!!
Baker is offline   Reply With Quote
Old 12-07-2007, 11:20 AM   #20
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
Design

Wait....! Is there a way we can use underwear to increase the ability of a semi-planing/semi-displacement hull to get on the plane? And what are the best methods for attaching underwear to the hull?
__________________

Marin is offline   Reply With Quote
Reply

Thread Tools
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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Anchor design and performance Nomad Willy Anchors and Anchoring 30 11-03-2011 06:49 PM
Help with design cgoodwin General Discussion 39 07-30-2011 11:01 AM
Design Nomad Willy General Discussion 61 11-09-2010 10:53 PM
Design Nomad Willy General Discussion 1 12-30-2008 04:13 AM
Design, old and new Marin General Discussion 10 03-01-2008 12:51 PM




All times are GMT -5. The time now is 01:26 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012