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Old 08-20-2013, 01:47 AM   #161
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Originally Posted by Tony B View Post
The waterline of my sailboat, as with most full displacement sailboat hulls, comes to a point at the stern at the waterline. Sailboats are basically canoe shaped at the waterline.

So does this mean that since I have no width, I cant expect any speed? Like, just stand still?
I have had so many laughs this morning....
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Old 08-20-2013, 01:55 AM   #162
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Old 08-20-2013, 02:07 AM   #163
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Here's a shot of Bucky (Krogen Manatee) when I pulled her for a bottom job and boot stripe in Daytona last year. For sure, the corners of the stern are out of the water when at rest, and indicates what Eric is saying about the buttock angle. The design makes for a wonderful experience in a following sea, and up to 7.5 knots or so, the wake is almost flat. So many of the PWC's running around here chase me down to jump my wake, and their so disappointed.
Interesting simple bottom.....little wake indicates some efficiency to me
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Old 08-20-2013, 03:20 AM   #164
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and appendices.
The pitching is not a problem at all we 'concentrate the weight but the the roll it is not the same because we have a ( too ?!) great stability (like a sail boat ) and the period of roll is also like a sail boat ...without mast
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There is something I see about this vessel that I don't like too much,...what appears to be the extreme down angle of those prop shafts. Maybe it has something to do with the angle of this photo shoot, but it appears to be a big down angle.

This can cause big differences in the loading on any single propeller from its top blades to its bottom blades.
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Old 08-20-2013, 03:33 AM   #165
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Atkin's Seabright Skiff, Shannon SRD, Dave Gerr's Tunnel Drive

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Originally Posted by longcours62 View Post

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And , may be one day ,the next evolution :
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That second illustration would seem to indicate that something akin to the Atkin's Seabright Skiff, or Shannon's SRD hull shapes were under consideration, reverse deadrise concepts ??

Dave Gerr also did development work and a few vessels that utilized his tunnel hull concept. In fact Longcour's vessel design reminds m a lot of Dave Gerr's designs? Power - Up to 50 ft.

I was actually looking thru this subject thread to see if those might be mentioned,...which they were not. So no one really has first-hand experience with these hull forms??
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Old 08-20-2013, 06:14 AM   #166
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The Atkin Seabright boats are very different from the others mentioned.

A simple tunnel or pocket is far different from a boat that carries much of its displacement in a box keel and is built with reverse deadrise.

Reverse deadrise is used to pressure the aft section of the boat with both the prop and water already accelerated by the hull .

The box keel engine location allwos a zero shaft angle.

When done properly it seems the hull at speed is a single hull catamaran that gains its stability from the hull shape and aft plaining surface of the reverse deadrise section. Sadly this action is limited in speed and after a SL of 3 a std plaining boat becomes more efficient , but at 2.5 it is reported as very efficient.

I have often thought an Atkin Seabright would be an ideal cruiser because of its ability to take the ground .

Most cruising grounds are chock a block full so in tidal areas the ability to anchor in mostly unused water would be a big plus. Arrival and departure times would have to be planned in advance.
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Old 08-20-2013, 11:08 AM   #167
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Well Brian Marin's even more opinionated than me and in this case he's dead wrong. PI.

And here on the forum probably over 90% are SD. There are probably more planing hulls than FD.

Brian I agree about the prop shaft angle but think that boat has redeeming features.

The Shannon SRD is great I think.

What on earth does vanity have to do w hull types????
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Old 08-20-2013, 11:31 AM   #168
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There are plenty on here dead wrong...all you have to do is sift through the true NA's, people who drive and work on boats for a living and it becomes apparent pretty quick who has read a lot and those with experience.

If you really want to understand not only hull design but how the designs are used in everyday work..and how they actually perform....look though mags like Workboat and Professional Mariner and Maritime News mags...then follow links or ideas to sources that are actually relevant.

Listening to guys who walk around a few boat yards admiring boat shapes is hardly factual ...maybe somewhat if they study with an open mind..they make good points but only from usually a relatively narrow framework.

Other than the guy or two on here that has a Sea Ray, I would say most trawler hulls are designed at the absolute lower limit of being semi-displacement with more displacement in mind. Sure the "new wave" tugs that go 15 knots and above are certainly semi-displacement..and if you load them up...you'll pay the price...but calling them planning is well....what do you REALLY think?????
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Old 08-20-2013, 01:20 PM   #169
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I would think that the distinction between a true full displacement hull and any other hull is fairly simple. A full displacement hull creates virtually no "lift" as it moves through the water, thus the formulaic "hull speed" limit. Any hull that creates lift to allow it to get past the theoretical "hull speed" limit is some version of a "planning" hull. At that point it is only a matter of degree. The degree to which the hull design creates lift to allow the hull to achieve higher speeds. As to SD trawlers, I agree that SD moniker is probably a marketing tool. To me, such hulls are an attempt to retain the interior volume and liveability of the full displacement hull while moving them beyond the hull speed limitation. 14 knots sounds a lot better than 7 knots, and in fact it is, it just comes with a substantial cost in engine size and fuel costs. As is often noted here, every hull design is a compromise.
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Old 08-20-2013, 03:01 PM   #170
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I am of the school that there is really no such thing, other than a marketing ploy, of a semi-displacement hull. I think that a "semi-displacement" hull is really a slight modification of a planning hull. It allows one to market a planning hull with either a less than optimal planning hull or a planning hull with not enough power on board for a true 'plane'.
From some of the data I have seen gathered by folks while on long trips, the semi-planing hull does not quite match the data of a full displacement hull at theoretical hull speed.
At hull speed or just Below it, a FD hull will show a really small difference of speed increase if you almost double the RPMs. All of my sailboats, when run at hull speed will almost double the fuel consumption to increase speed 1 kt above hull speed.

On my Mainship Aft Cabin Semi-Displacement hull, the increase of fuel, RPMs and speed remain fairly linear before and some beyond the theoretical hull speed. Then somewhere before planning, the fuel economy gets better because unlike a FD hull, the SD hull starts to squat and then rise slowly into a plane creating less water to push and then into a plane with greater fuel economy as not much of the boat is pushing water.

I believe that hull speed calculations are relatively meaningless on anything other than a full displacement hull. I say 'relatively' because they have some merit.

AS THD said, every hull design is a compromise. Even amongst FD hulls - wide vs. narrow.
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Old 08-20-2013, 04:02 PM   #171
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Old 08-20-2013, 07:32 PM   #172
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And I'm quite sure yours (SCOTTEDAVIS) is more efficient than my Willard.
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Old 08-21-2013, 12:43 AM   #173
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Well Brian Marin's even more opinionated than me and in this case he's dead wrong. PI.
Are you saying that Marin is wrong, or his references to how Tom Fexas describes SD vs FD ??

Trawler Forum - View Single Post - Your hull type



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The Shannon SRD is great I think.
I wonder just yet? Sound like a great sales pitch but....
Quote:
Originally Posted by Ad Hoc
It is more than just looking at hull shape and saying...hmm..flat bottom, with a Vee, must be a planning boat...or wow, boat is fast so why is it not called a planning boat.

Any "body" moving through the water, ie at the air-sea interface, will create pressure variations around the 'body'...these pressures variations manifest themselves as waves. These waves are a measure of energy and hence drag.

The shape of a hull can affect this pressure distribution considerably. In a nut shell to cut a long story short, the stern experiences suction pressure fields. Speed then also begins to play a part....ie trim/squat, the faster one goes.

For a "normal" boat, the faster one tries to go, the more trim and the greater the power required for little gain. The back is sucked down and dragging creating a lot of wash/waves. Too much energy is being used in making waves. The reason is the hull shape and its length displacement ratio. This is seen in the resistance curve by humps, and the main prismatic hump. A hull must over come this main hump to go faster, ie make "lesser waves" or better still no waves. Long thin hulls, hydrodynamically, behave differently to short fatter ones.

However, just making the aft section flat with/out a vee, doesn't mean that is all that is required just to go fast, or get over the "hump" in the resistance curve. That is just hull shape.

Making the hull longer and thinner, reduces the 'near vertical' curve in resistance of a 'normal' hull form...ie its length displacement ratio. Not only does the resistance curve become less step, but also the main hump is much less pronounced. The longer and thinner one makes it (that is light for its total length), the curve slowly approaches a smooth curve and almost no discernible prismatic 'hump'. This is why fast ferries, for example, go fast, have a high froude number, but are not planning. They are long and thin.

The length displacement ratio (ie long and thin) allows the hull to be driven faster than would normally be the situation. The wave making résistance gets less and less, the longer and thinner the hull becomes.

The down side is, one of these long thin hulls is generally far too unstable to be used effectively. Solution, put two side by side, ...a catamaran. Utilizing the benefits of the long thin hydrodynamic effects, but providing a stable platform for use in almost any application safely.
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Old 08-21-2013, 06:29 AM   #174
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Attachment 22268

There is something I see about this vessel that I don't like too much,...what appears to be the extreme down angle of those prop shafts. Maybe it has something to do with the angle of this photo shoot, but it appears to be a big down angle.

This can cause big differences in the loading on any single propeller from its top blades to its bottom blades.


After read your post it’s appear you misunderstood what you see in the photo.
But It could be happen to people who have not habit of looking at boats .
On the photo you can’t see the real angle between shaft and the water line.
Because the shafts pass thru this (named by another peoples) bilges keels.
And , except if your are Superman with X-ray view, it is absolutely impossible too see this angle
In reality the angle is 5°, of course it could be better but people who built boat know , always we must make compromise .
The post you made before show you take time to look at some info ,just take time also,to understood what you see and you get the answer at your question ( i don't know in Englis but in French it is the difference betwen : Voir et regarder or entendre et ecouter)
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Old 08-21-2013, 11:40 AM   #175
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Dear Longcours,
I saw those 2 drawings you referenced just now, but they are from the end view. That makes it very difficult to determine the shaft angle. Do you have a profile (side) view of the internals,...engines/shafts?
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Old 08-21-2013, 02:25 PM   #176
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Dear Longcours,
I saw those 2 drawings you referenced just now, but they are from the end view. That makes it very difficult to determine the shaft angle. Do you have a profile (side) view of the internals,...engines/shafts?

Sorry , i am thinking you have the habit to take a look at drawing .
I will explain to you :
- early on this threat I already wrotte than our boat is 19,06 m lenght and I said the boat drawing is "cut" in 41 section, 19,06 divised by 41 it mean 465 mm betwen each sections
- first reference above section 28, second reference above 32, 32 less 28 = 4
4x 465 mm = 1860 mm
- on each view not too difficult to check the distance betwen the axis of the sterntube to the waterline , make deep at 32 - deep at 28
- after that just apply a basic trigonometric formula and you got the angulation made by the shaft or easier for people who don't like trigo make a drawing and check the angle with a "rapporteur" (in French).
With this aproximation very easely you can find 6°
But one thing I can't understand it is people who wrotte : "I don't like "that" , when they don't know what "that" is ? It is very curious

For the longitudinal view , sorry ,I can't scan one because they are at 1/20 and now ,we are on board , and no scanner of this size
But if you want I could make an approximative "croquis" (it s means bad drawing ) of the longitudinal view of shaft.
On our motor sailing boat we could put the shaft at 0° angulation, probably better, but it was easier for (at less) 3 raisons the hull was 20 cm deeper, the diameter was smaller (just 90 hp ) and the aft part of the hull "raising " quiker (sorry for my English).
On our actual boat for 0° not possible exept with very very long sterntubes
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Old 08-21-2013, 05:16 PM   #177
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(sorry for my English).
No need to apologize for your English mate. You are doing a whole lot better than most of us could do in French
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Old 08-21-2013, 08:04 PM   #178
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Brian,
I wasn't referring to the Tom Fexas definition or Marin's posting of Tom's theory. Never heard of Mr. Fexas. Two days ago he sent me a PM w quite a description .. wasn't bad at all.

Marin's note to me,

" Eric--- Semi-displacement a marketing term, like "trawler." You can have many degrees of planing because planing is all about having some, most, or just about all of the boat's weight supported by the hydrodynamic forces created the hull's movement through the water. The faster the speed the higher the hydrodynamic pressure, the closer the hull becomes to be planing. So you can be fully planing, partially (semi) on the plane, on the hump, or everything in between. Seaplane floats and hulls act exactly the same way and use the same terms.

Therefore there can be no such thing as semi-displacement because displacement by definition is NOT having any of the boat's weight supported out of the water by the hydrodynamic pressure generated by forward speed against the underside of the hull. So there is nothing about displacement to be "semi" about. You either are or you aren't. As I said a long time ago, it's like being dead. You are or you aren't.

Full-displacement is the same kind of incorrect term. Again, displacement is the definition of that type of hull. Saying full displacement is redundant because by its very definition, displacement is all it can be. If it's not displacement, and it's not a submersible, that it must be a hull that can be lifted out of the water to one degree or another by hydrodynamic pressure generated by forward speed and so be able to go faster than its displacement speed. At which point it is a semi-planing hull, not a displacement hull. So saying "full displacement" is like saying "displacement displacement."

It may just be semantics, and the marketers have touted these terms so long they've become part of the language. But that doesn't make the terms correct or accurate, it just makes them accepted.

Cheers,

Marin

I think he's full of it. Marketing hasn't ANYTHING to do w the definition of the semi disp hull form. He's trying to mix science and business. They are on different planets. Science is for finding the truth and business marketing is for leading or misleading people in any way possible to get them to buy something.
I've had quite a number of theories myself. I had the notion that a FD boat needed to be a double ender or have it's transom out of the water. That held up well until Seaton's designs came into view. I think the buttock line on the bottom quarter is most revealing. Somewhere on BD.net it's pinned down ... the angle.
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Old 08-21-2013, 09:40 PM   #179
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Displacement, semi or planning???? you decide...only a bazillion out there...no doubt in my mind when it's carrying a quarter or half to a million pounds of something. So many designs and some still think they know what they are talking about.

Raked at one end, immersed transom at the other...must be a planning hull....
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Old 08-21-2013, 11:35 PM   #180
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Displacement, semi or planning???? you decide...only a bazillion out there...no doubt in my mind when it's carrying a quarter or half to a million pounds of something. So many designs and some still think they know what they are talking about.

Raked at one end, immersed transom at the other...must be a planning hull....
I own pretty much a “Full” Planing boat when its pushed with enough power-force to lift itself more on top of the water than at the times when it fakes us out by acting somewhat like as a “Full” displacement hull by setting deeply in the water and simply gliding through the water at or below its calced “hull speed”. Of course, for considerable inefficient fuel use, I could keep her at speed that makes her act like a “Full” semi planing boat, or, if you like, a “Full” semi displacement boat. IMO, “Full” is not relevant in most cases mentioned. Except, I believe “Full” is relevant in the term “Full (i.e. Fully) Displacement” because that design is the only hull that has no chance of reaching any other parameter than displacement movement through the water and therefore is limited to displacement speed. Indeed those boats’ hull designs are not partially or semi... but rather... they are Fully Displacement!

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