Warped plane vs constant deadrise

It is generall accepted that a constant deadrise planing hull will be more efficient than one of warped plane form. Yet so many of what I'd describe as efficient boats incorporate variable deadrise towards the transom.
Take the very elegant Latitude 46 that Tad referred to in the 'double enders' thread ( http://www.tofinou.com/gamme/index.php?bateau=TOF&chglg=en )
Their largest is 50ft long and manages to cruise at 15K, max @ 20K on a pair of 165hp engines. Pictures and drawings on the website would certainly suggest they have a warped plane hullform.
If you want to run low deadrise at the transom (allowing lower angles of attack) then surely you must use a warped plane hullform - otherwise you'd have a boat with say 5 degrees deadrise until a long way forward, creating a very harsh ride.

So, presuming I want to produce a vessel, with similar performance characterisitcs to the Latitude 46, what;s the best way to go?

 

Will,

... The design process, and within the process a design spiral is applied to each step. There are several variations of design spiral, but the end result is the same. This approach is the foundation from which the majority of the boats we sail and see are developed.

The design process itself can be broken down in to 4 steps. The are as follows:

The Design Statement
The Conceptual Design Phase
The Preliminary Design Phase
The Detailed Design Phase.

With the exception of the design statement a design spiral can be applied to each phase. A design spiral is a series of related analysis which when applied to each of the remaining steps of the design process results in the best fit to the design statement. The same, or similar spiral will be applied to each of the remaining three phases. A typical design spiral for these three phases of the design process might include the following steps.

Hull Definition
Hydrostatics
Weights
Powering
Stability
Structures
Arrangements

A designer will often modify the spiral to suit their own needs or as it applies to a specific design. It is important to point out that a designer does not always follow every step and the exact order of the design spiral in each phase of the design process. For example in the early stages of development, it may be necessary to go from hull and deck design to general arrangements before developing the keel and rudder design. No matter how a designer approaches a particular design, the design spiral will always play an important role in the design process. ...

Dim.

 

sorry WILL, not directly thread related but wow Dim, what spiral you used on english? that looks like a short straight line to me! you write better than me now, compliments! (can i learn russian this good in 1 month following your method?) yipster

 

I shall disappoint you Yipster. It is the citation from one site. I know, that it is not polite to communicate for my part with you in very bad English and consequently I shall try to find citations as far as possible.
I shall continue studying of language as soon as then.

Dim.

 

thanks for the reply Dim, your doing very well.

yipster
(sorry i brought it up, that wasnt very polite of me eighter, that site you use does an exelent translation job i noticed!)

 

Will,

I have in my library somewhere a discussion on this very topic. I have been trying to find it and will let you know when I do. I also think Tad brushes on it some in his article in Passage Maker.

I think a 50 footer would make a nice replacement for your Sea Ray.

Gary

 

Dim: I'm aware of the 'design spiral' concept - and I understand where you are coming from - if I follow the steps, then the hull shape should work itself out. But this is only true to a point. Take C R Hunt & Assoc. for example. All of their boats a deep v's. This is (I suspect) because they have a fundumental belief that the deep v hullform is the best. They may, or may not be correct. But I'll bet their design spiral skips the part about whether they should incorporate a warped plane hullform with low transom deadrise....
Oh - and don't worry too much about your English (it's certainly WAY better than our Russian !!) and it's not impolite to get it wrong, just occaisionally a little hard to decipher.

Gary: Thanks - I'll re-read Tad's article (which seems to mysteriously pop up in any number of magazines Good onya Tad!) and look fwd to your uncovering of the article(If your library is anything like mine then it may take some time!!


If only my budget allowed for a 50ft replacement for the Sea Ray! At this rate I'll be packing the family into one of your canoes

There is (as Dim suggests) a lot more to the story than simply warped plane or constant deadrise - length / beam / displacement / intended purpose. But hey - you gotta start somewhere!

 

:idea:
So as I look for the answer I realize there is none. That is what hull design is all about, the trade offs and compromises that go into every hull design. And that is why before tank testing if you found the right combination you stuck with it. If you change the displacement, the LWL, the power, or any number of other things it is not so easy to predict the end result.

The monohedron is faster than a warped plane, but what dead rise do you choose? 0 degrees is faster than 12 degrees. So do you choose one or the other, or find a way to blend them?

Sorry Will, not much of an answer. This is where Tom comes in and tells me I’m full of it.

Gary

 

First impression

Being under strong impression from beauty of lines " Latitude 46 ", I have decided to try. That has turned out in four hours.

Sorry: water density = 1 t / cub.m.

Displacement 9.81 t
Length OA 50.000 ft
WL Length 49.021 ft
WL Beam 11.499 ft
Draft at LCF 2.005 ft
Prismatic coeff. 0.684
Block coeff. 0.312
Midship area coeff. 0.672
Waterplane area coeff. 0.674
LCB to amidship 6.196 aft ft
tpi 0.896 t / in

Dim.

 

Side view

Dim.

 

And very elegant it is too Dim!
I too was hoping to produce a few hulls for comparisons sake - three hulls of equal length, beam & displacement: a constant deadrise; a hard chine warped plane; and a softer chined variable deadrise.
Unfortunately, I'm falling a little behind in my Westlawn studies, so I had to concentrate on them.....

 

Will;

It is an interesting question, but really deadrise at any particular point on a hull is only a small factor in the overall efficiency of a boat.

As or more important to overall efficiency will be LCB location, CP, LCG, entrance angle, running trim, bottom loading, appendage drag, etc.

Why do you say it is generally accepted that a constant deadrise is more efficient than one with a changing deadrise angle? I think this is another urban legend with little foundation. How much twist is inefficient? Why? In the 1950's Lindsey Lord wildly exaggerated the effect of bottom twist in his book. Because it was in the book it was taken seriously. Lindsay was selling Monohedrons. There is also a series of tank tested hull forms, series 62 and 65, of constant deadrise but otherwise they are not forms anyone would use today.

At the same time other designers had arrived at opposite conclusions. Bill Garden and Ed Monk come to mind. Maine lobsterboats (semi-planing hulls), and Carolina Sportfishermen would also be examples.

I believe the Latitude 46 hulls are semi-displacement or semi-planing and round bottomed below the running strake. I have a vague memory of seeing the smallest one out of water some time back. Their efficiency is gained through narrow beam, light weight, fine entrance angles, and flat sections aft. They are also not trying to go too fast.

If the constant deadrise hull is also of fairly low deadrise, then the boat will plane quickly and with lower power. But she will also be too blunt forward to be any good in a seaway at speed. This is why I believe a twisted or warped bottom is the best compromise. You can make the forward sections fine and the aft sections flatter, this is what is required for all around best performance.

Have a look at the articles section on my website.

All the best for now. Tad

 

Thanks Tad. You've pretty much confirmed my own thoughts on the matter: For what I'd call mid-range speeds, the warped plane makes most sense - with the proviso that displacement, LCB etc are appropriate.

Only because my Westlawn text says so! From memory, I think they quoted the Lindsay Lord stuff as confirmation. As general rule they suggest that it's best not to flatten deadrise more than about 6 degrees from amiships aft.

Nope - most definitely hard chined, take a look here at the back end of their smallest: http://yachtbroker.escapeartist.com/boats/action/viewImage/image/141/boat/17/index.html
- but it begs the next question - is the hard chine better than a 'tight' round chine? If not why have people (like Fexas with the original Midnight Lace series) incorporated the round chine?

I've read your articles - perhaps time I revisited them.... and thanks again for the input.

 

A comment, if I may...

Dim, I would suggest looking pretty hard at that deep, narrow bow. In the first instance, if building in FRP from a female mould it will drive the laminators nuts trying to get down in that deep, narrow hole to lay up the 'glass. In the second instance, a bow that deep and fine on a planing hull will slice through short chop with aplomb but may have a tendency to "bow steer" when overtaking a wave train and burying her bow in the back of the wave she is overtaking. This latter point is, of course, dependant on the anticipated speed of the hull.

 

The answer, in my opinion, lays on a "surface", dear mmd.
Pay attention to photos of "Latitude 46" ‘s boats. I have in a kind a surface of the sea in these photos. In my opinion it also is a natural mode of their operation.
If we want to engage even a little rodeo - we should search for other hull's form. Boats of this firm are boats for reception of maximum quiet pleasure from sea voyages. When I am not afraid to leave from a cabin for a cockpit with a cup of coffee for my wife. The answer also consists in it.
Probably I also am mistaken.