Warped plane vs constant deadrise

[Willallison]

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?

[Dim]

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.

[yipster]

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?) :D yipster

[Dim]

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.

[yipster]

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!)

[duluthboats]

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. :eek:

Gary :D

[Willallison]

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:D !!) 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!!
:D

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!:D

[duluthboats]

: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 :D

[Dim]

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.

[Dim]

Dim.

[Willallison]

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.....

[Tad]

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

[Willallison]

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.
Why do you say it is generally accepted that a constant deadrise is more efficient than one with a changing deadrise angle?
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.

I believe the Latitude 46 hulls are semi-displacement or semi-planing and round bottomed below the running strake
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.:D

[mmd]

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.

[Dim]

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.

[Willallison]

mmd: This has been a comment I've seen any number of times - and on several of the hullforms that I've posted myself over time.
So what's the solution? If we want sharp sections fwd to slice through chop and flat aft sections to provide an easily driven vessel that doesn't poke it's bows skyward at the 1st hint of speed, then surely we are stuck - at least to a certain extent - with a shape somewhat like Dims. The addition of a small keel would help somewhat no doubt......

[mmd]

I will agree that bow steering will not be an issue at moderate semi-displacement speeds and in calm to moderate seas. I was basing my comments on first hand experience gained through supervising the construction of FRP hulls with a fine, narrow bow such as that shown by Dim and having to deal with the quality assurance problems created by trying to laminate into such a deep, narrow recess. I subsequently operated these same hulls in seas of 1 to 2 metres and found that speed needed to be reduced to avoid bow steering well before the ride became uncomfortable due to pounding.

Also, I have designed several high L/B hulls that operate in the range of 20 - 24 knots in coastal waters of the north Atlantic with a similar underbody form but with a bow profile considerably more cut away below the waterline. These hulls are able to maintain a rapid cruising speed in a seaway with minimal pounding (fine entry sections forward) and no tendency to bow steer or broach when entering a large wave (minimal lateral area in the underwater bow profile).

I'm not trying to say that I have all the answers, just trying to make a suggestion based on my experience with these types of hulls. As with most boats, the form develops to accomodate the conditions and useage of the boat as well as the tastes of the client and designer. C'est la vie.

Will, my solution in past designs has been to (using Dim's model as the example) maintain the upper bow profile and hullform from station 2 aft, but to substitute an elliptical profile form for Dim's circular shape in the forefoot area of the bow below the waterline. This allows reserve buoyancy to be realized smoothly without pounding as the narrow bow submerges in a wave, but doesn't present the lateral area of the minimally-buoyant sharp circular forefoot that can be the cause of bow steering.

[Willallison]

Ok, I take it you mean change the generally circular shape to generally eliptical one, when looking at the profile of the forefoot? But in doing so, you must reduce the deadrise here - or do you reduce the chine beam correspondingly to maintain the higher angles?
You make a good point about the difficulties of construction: not something that we (...well me) student designers would always consider.

ps. I think you and Dim are coming from the same view point. Deciphering his message, I think he intends the hull only be used in relatively calm waters....;)

thanks for the input

[Dim]

Hi, Will and mmd,

If I have correctly understood mmd and if we speak about more correct reconstruction of "Latitude 46" boat's hull - I completely agree with opinion mmd. I as completely agree with opinion mmd that narrow form of the bow parts of the hull create more problems at manufacturing the hull and accommodation of the various equipment. It is indisputable. But I as want to tell the following. If I had opportunity to maintain such boat I would try to do it in the conditions, similar that are represented in photos "Latitude 46".
I have tried still to work with the hull's model and for me that is represented in a picture has turned out.

water density = 1.025 t / cub.m.

Displacement 10.16 t
Length OA 50.000 ft
WL Length 49.187 ft
WL Beam 11.501 ft
Draft at LCF 1.774 ft
Prismatic coeff. 0.757
Block coeff. 0.355
Midship area coeff. 0.662
Waterplane area coeff. 0.713
LCB to amidship 4.047 aft ft
tpi 0.976 t / in

Dim.

[Guest]

I've run into this issue at work recently. We had an existing hull form which had a constantly decreasing deadrise angle all the way to the transom. We updated our molds with CNC tooling, and following our waterjet manufacturers advice, left the last 15% of hull length at a constant deadrise angle.

The resulting dynamic center of effort is considerably further forward as a result of the change. Though I knew the theory going into this excercise, I did not expect such a dramatic effect from such a seemingly minor change.

[Dim]

Hi All, again.

I am sorry for irrepressible enthusiasm. It's just very interesting for me. I have some free time and continued work above the model and have carried out as a first approximation the express - analysis of resistance. Calculation of resistance was made with the help of one of methods for planing hulls.

water density = 1.025 t / cub.m.

Displacement 10.42 t
Length OA 50.000 ft
WL Length 49.180 ft
WL Beam 11.499 ft
Draft at LCF 2.004 ft
Prismatic coeff. 0.725
Block coeff. 0.316
Midship area coeff. 0.602
Waterplane area coeff. 0.722
LCB to amidship 2.471 aft ft
tpi 0.988 t / in

Dim.

[Dim]

Dim.

[Willallison]

No need to apologise for your enthusiasm Dim!
It's difficult to make out the differences between the 3 hullshapes represented in your resisitance graph. It appears that No.3 has flatter aft sections and bow sections which are more cut away - as mmd suggested. Is that all? There's quite a difference in the resistance for No. 2 as opposed to the other 2....

Incidentally, what did you use for the resistance calcs?

[Dim]

You are completely right, Will.

In this small work I tried to simulate the hull similar in the sizes "Andreyale 15m". Therefore these models should be considered not as three different models, but as development of one hull.
Basic difference exists between the first and second hull's forms (in the bow frames form).

( Many thanks mmd for his posts.)

But at me it has turned out small a draught of the hull on the second model. Therefore the curve of resistance for this model has passed little bit below. I managed to return to "normal a draught" on the third model. I have gone little bit further in development of the exterrier on this ( third ) model. For calculations of resistance I usually use methods to which me learned at university. This method is very close to method Lahtiharju.

Dim.

[Dim]

It's the last model with deck.

Dim

[Willallison]

Well - now that's an impressive little presentation Dim!:D
How will the amount of 'rocker' that you have in the fairbody shape affect this boats performance? I was always under the impression that it is best to keep the fairbody and buttock lines as straight as possible. This will also make your waterlines more parallel - another desirable feature I believe.....

[Dim]

If I correctly have understood you, Will,
we speak about a bend of a keel's line downwards in a aft part of the hull. My opinion on for what it's made on "Andreyale 15m" and it's make on other boats. When the boat goes, there is a running trim. And the aft part of the boat's bottom starts to carry out a role anti-trim surface. Thus, the boat can receive an optimum running trim on the move, that will affect very favorably his going performance.
An other business - that up to elegance "Latitude 46" to me "as on foot up to the Moon" - as at us speak.

Dim.

[Willallison]

I guess that at the likely speed at which the boat travels, the 'hook' in the bottom will have more positive benefit than negative. Though I would expect that it would make the potential bow steering problems that mmd referred to even worse....

An other business - that up to elegance "Latitude 46" to me "as on foot up to the Moon" - as at us speak.


Sorry, I can't understand what you are trying to say here Dim..... maybe that you think the Latitude 46 is very elegant? If so I most certainly agree. Though for me, I would need something a bit more practical - can't see two dalmatians, a 6 month old baby, diving and fishing gear all rattling around in something so elegant...:p

[Dim]

Yes, Will,

I wanted to tell, that I think "Latitude 46" is more elegant, than my hull. :)

Dim.

[terhohalme]

Hi Willallison,

Andreyale 15 m is a semiplaning hull, her Froude number is only 0.64 at cruising speed. At semiplaning speeds boat will almost never jump out of the waves and therefore you need only make the hull "sharp" at the bow. Aft amidships just few degrees deadrise is necessary.

There is no point to use deep V hulls under Froude number 1, while deep V hulls need much more power and are akward to steer at lower speeds compared to semidisplacement hulls. Deep V surely makes jumping in the waves more tolerate at planing speeds (Froude number <1).

Dim: What a beaty...

I have designed something similar few years ago, she is only LOA 9 m

http://kotisivu.mtv3.fi/terho.halme/salmon.htm


Terho

[Dim]

Thank you Terho.
There can be this combined image will help us with a question about "hook"? Now I can't understand about what you speak, Will.

Dim.

[Dim]

I have understood, Will.

All began that I have not understood that you want. And you have not understood above what I work. I have put before myself a task to simulate the hull on the philosophy, if so it's possible to tell, very close to "Andreyale 15m". Simply to copy is not interestingly and silly.
But any other boat which answers your inquiries is necessary for you. In it the reason of all your questions. I just now have understood it.
To me it is interesting "Andreyale 15m" and I continue above it to work.
The most interesting, that "Andreyale 15m" really accelerates speed in 20 knots on two engines on 165 hp! On how many I can judge it.

Dim.

[Willallison]

No - I think we've both understood each other all along - I know you've been trying to re-create the "Andreyale" as close as you can - I'm simply interested in how the various parts of the design affect performance.
I too am drawn to this boat because of it's performance.... though if you go back to the boats built 40 + years ago, this kind of efficiency was not uncommon. I guess that the consumer demand for more and more accomodation space has resulted in fat, heavy and very inefficient boats. When one like this comes along we are reminded that speed need not require huge quantities of fuel....

Terhohalme: Thanks for your comments. And I concur with your reasoning.... it makes me wonder once more why so many boats designed to operate at these sorts of speeds incorporate constant deadrise, deep vee hulls.....

[Dim]

I want to thank everyone who participates in this discussion.
Many thanks Will for a very interesting design task.
Many thanks mmd and Terho for very interesting comments.

Dim.

[Dim]

Comparison of images of two hulls and dimensions of engines which are possible for establishing on my boat have led me to ideas, that it's necessary for me to increase height of the hull's board.
As a first approximation it is accepted:
engine's model: "Deutz TD229-6", 109kW ("c"-mode),
transmission's model: "ZF 45 A", 2.435:1.
Images of two views of the engine were vectorized from the PDF-document. The vectoring has turned out not so high quality, but sufficient for preliminary arrangement.
The shaft's angle is equal 12 degrees from level.

Dim.

[mmd]

Dim, you should check with the engine manufacturer to find out at what maximum angle the engines can be installed. Twelve degrees is a bit extreme. A down-angle gearbox (usually 7 degrees) will make the engine installation angle better and will lower the deck height needed to clear the engines.

[yipster]

Hi guy's,
Its a very interesting subject i've been thinking about also for many years.
yes, the hulls board up a little more and downscaling everything -i'm fantasising again- it fits O1, you may have some competition there Will ;-)
Dim, i'm not a pro, can you roughly estimate economy gains of a warped plane slender semi planning design over a wider body constant deep V or must i make time and start calculating your numbers?
This not so elegant hard chine patrolboat (http://www.boatdesign.net/gallery/showphoto.php?photo=164&password=&sort=2&thecat=500) follows the same design priciples i belive. The keel on this one prevents the stern from sliding away in a following sea i asume? I -personally- like this modern deckhouse, even the up front bridge model -with sometimes steering cables running over the deck backwards- better than a nostalgic deckhouse. Allways did like those boats, till looking inside one and seeing the giant engine and was told about its consumption.
Just letting you know i'm following this with interest. yipster

[Willallison]

'Andrayale' doesn't incorporate prop tunnell's , but this too would allow you to reduce the shaft angle. And as Yipster refers to, I think the addition of a small keel would improve down-sea handling.

Yipster - this is a much prettier design than the hull I posted way back for O-1, but the biggest problem would be all the complex curvature in the surfaces. Remember O-1 must be achievable for the 1st time home-builder and be built out of flat panels of one kind or another - foam core or plywood....

Comparison of images of two hulls and dimensions of engines which are possible for establishing on my boat have led me to ideas, that it's necessary for me to increase height of the hull's board.


But you're overlay image suggests that you have the same freeboard as the 'Andrayale' - and they manage to fit the engines in Dim....

[Dim]

In the first, let to thank All You for your comments once again.

Thanks mmd and Will ( the end of his last post ) for the very correct remark. I think only about this, when wrote the message to Friday.
I have no information on possible angle of an inclination of the engine at present. But at present this information is not necessary for me. Why?
I always try to visualize a constructive problem if it is possible for making quickly. In this situation I wanted to receive the maximal height of a "engine room". If the engine will have a smaller install angle I through transmission can sustain 12 degrees necessary for me. I not deeply studied a question of profitability of use of this or that engine at the given stage. Therefore I am not confident, that this model of the engine will be used by me at the further stages of work. The corner in 12 degrees takes place to be on a boat " Andreyale 15m ". I have measured this angle on combined to pictures.
I on it to week want to engage in study of design of superstructures. It now the most interesting to me. The idea has come to me when I went home from work on Friday. It is necessary to start to model it.

Dim.

[Guest]

mmd,

I have consulted with our expert about engines's installation.
His opinion is those.
The majority of engines type without problems are established with angle of inclination of 5 degrees inclusive. If a angle - more - it is necessary to contact to firm - manufacturer.
( I in the given situation do not think possible to disturb such firm dear by me, as "Deutz" ).
But "ZF 45 A" has an inclination of a output shaft to equal 8 degrees. 8 + 5=13.
The problem can be solved.

Dim.

[Dim]

Sorry, I forgotten login again.

Dim.

[Dim]

That at me it has turned out. It to not like me, but I have decided to show it to you.
Angle of the engine's inclination - 4 degrees from level. Propeller shaft - 12 degrees from level.

Dim.

[Dim]

The general view.

Dim.

[Willallison]

I see you have chosen to move away from the styling of the 'Andreyale' for your cabin styling Dim.
I like the steps built into the transom. I'll wait 'till we see the final images before I pass judgement on your styling....;)

[Dim]

Ok, Will.
Some, very, very small progress.
From next week I shall have the rest for two weeks.
Probably, at me time will think of domestic conditions. :)
Let's meet in two weeks.

Dim.

[Tom Lathrop]

Hello all,

I have been away for over 6 weeks cruising in the Bluejacket 24 "Liz" in Ontario, Canada's Trent Severn Waterway and the North Channel of Georgian Bay. Sorry to have missed out on this discussion since it is very interesting to me. I learned a bit more about the boat from the trip also.

In getting back to the original subject of monohedron versus warped plane hull efficiency, I am firmly placed in both camps. My experience with my own boat and in towing tests show some advantages to both types in different conditions.

Briefly, I believe that the warped plane bottom offers less resistance in achieving planing mode and that the monohedron bottom is both better and more stable at higher speeds.

Since we are not talking about deep V hulls here, the monohedron hull has constant deadrise only over the aft planing sections and the deadrise in the forward sections is variable, rising to a fairly sharp entry forefoot.

In the model used for towing tests, only the aft bottom sections were were varied from constant deadrise to a warped shape by raising the keel toward the stern while keeping the chines parallel to the waterlines. At low speed, the warped bottom gave a slightly lower towing resistance. At high speed, where the monohedron was always stable, the warped bottom exhibited a tendency toward yaw instability. This got very bad when the speed was increased further and eventually resulted in foundering of the model and destruction of the towing rig.

Whether this was due to bow steering, stern steering impulses, both, or something else, I can't say, but I have some thoughts on it. I do know that many of the Carolina Sportfisher boats that Tad referred to have a bit of rocker in the aft bottom in addition to the warped bottom. When I asked a builder abour this, he said it was to keep the bow higher at sea. So it might be that they have one factor, warp, holding the bow down and another opposing factor, rocker, holding the bow up. I don't think this is the best way to do a design, but the local fishemen are happy with it. I do suspect that these are macho fishermen who want to drive a macho boat for macho customers and that the designs are evolved rather than being designed from scratch.

What speeds are involved? Perhaps up to 25 or 30mph, the warped hull has the advantage and the monohedron is better above that. Other factors are also involved, so this is just a gross guess. I was working with low bottom loading and an aspect ratio of about 0.34 with a CG about 37% of LWL forward of the transom.

[Willallison]

Ahh - now we're getting somewhere: so as a general rule of thumb, one could say that the warped plane is more efficient at planing speeds to approx 25 mph, the monohedron is the way to go at speeds over 30.
I, of course, am chasing the impossible - a hullform capable of maintaining efficient planing down to around 12 - 15 knots, yet capable of cruising @ 30 + in surface chop!

Since we are not talking about deep V hulls here.....
Well actually, I was thinking of deep V's too when I posed the original question. Where, for instance would you place the Hunt 33 ( www.crhunt.com )? At 10000 lbs it is relatively light and with a single 370 hp diesel manages around 2 nmpg @ 20 knots. The boat can manage 32 knots flat out and I can see that if you wanted to charge around at that speed regardless of conditions, then the deep V hullform might make sense. But to my way of thinking - and based on your comments, Tom - you would have to assume that the boat would be better with a warped plane hullform......

The trip sounds terrific Tom - very jealous! I'd be interested to know what you "learned about the boat from the trip" too.....

[Tom Lathrop]

Originally posted by Willallison
Ahh - now we're getting somewhere: so as a general rule of thumb, one could say that the warped plane is more efficient at planing speeds to approx 25 mph, the monohedron is the way to go at speeds over 30.
I, of course, am chasing the impossible - a hullform capable of maintaining efficient planing down to around 12 - 15 knots, yet capable of cruising @ 30 + in surface chop!
____________________________________________________

Will,

The speed ranges that I mentioned are just a generalization but I think it is safe to say that a monohull will plane earlier with low deadrise in the aft sections. A deep V hull is not the best for low speed planing.

____________________________________________________

Well actually, I was thinking of deep V's too when I posed the original question. Where, for instance would you place the Hunt 33 ( www.crhunt.com )? At 10000 lbs it is relatively light and with a single 370 hp diesel manages around 2 nmpg @ 20 knots. The boat can manage 32 knots flat out and I can see that if you wanted to charge around at that speed regardless of conditions, then the deep V hullform might make sense. But to my way of thinking - and based on your comments, Tom - you would have to assume that the boat would be better with a warped plane hullform......

____________________________________________________

The Hunt 33 looks like an improvement over most of the powerboats that I saw on my cruise. It is not only handsome but shows an effort to avoid the 'quart in a pint pot' route taken by so many builders. I saw one boat with an aft cabin and highly sloped foredeck pick up anchor on a damp morning. The poor wife was on her hands and knees trying to crawl up the foredeck to safety and having a hard time of it.

I don't know what the minimum economical planing cruise speed of the Hunt 33 is but doubt that it is as low as you would like. I am not able to say that the Hunt 33 would be a better boat with a warped hull. Their use of wide aft chines drive the design in that direction anyway. I'm sure that the positively angled (relative to waterlines) chines of my boat make it somewhat like a warped hull.
____________________________________________________

The trip sounds terrific Tom - very jealous! I'd be interested to know what you "learned about the boat from the trip" too.....
____________________________________________________

I learned that "Liz" is very satisfactory for two people to make extensive cruises in comfort.

Trailering makes it possible to cruise in remote areas in limited time without having to tread "old water". We drove almost 3800 miles and cruised about 350 miles in the boat.

The Trent Severn Waterway and the North Channel are both excellent places to enjoy a cruise in a small boat.

"Liz's" top speed dropped about 3mph from the normal 23-24mph when lightly loaded near home to about 20-21mph with full cruising load aboard. Since I have had more weight than the cruising load aboard before and not had this much drop in speed, I attribute most of the loss to the added load forward. This increases the wetted surface and is most likely responsible for the speed drop. The good news is that she handled chop even better than before with the waves striking the higher deadrise sections forward first. Were these results also affected by the deeper immersion in freshwater?

A sun shade over the cockpit would be a plus and I'm looking at that possibility.

Fuel mileage for the cruise was 6.8mpg, which is about 1 1/2mpg less than we usually get on day runs. Average speed for the trip was 10.1 mph, including the locks and many no-wake areas on the Trent Severn.

Such cruises cost a lot of money but MUCH less than the larger boat people spend.

At the risk of boring you all too much, that's all for now.

[Willallison]

At the risk of boring you all too much, that's all for now.

Never happened yet!

Such cruises cost a lot of money but MUCH less than the larger boat people spend.

Almost 7mpg! - That's almost as good as most American cars isn't it!!!:D It just goes to show what can be accomplished when a little thought - rather than a lot of unnecessary junk - is put into a design....

[Guest]

http://cnczone.com/forumdisplay.php?s=&forumid=5

[gsdickes]

Hi Guys:

I've done some model testing on this too and it was clearly demonstrated that warped planes are more efficient at lower speeds and monohedron or constant deadrise forms are more efficient at higher speeds. The question then, of course, is what's "lower" and what's "higher" in terms of speed? Our tests were not terribly comprehensive, but we did enough of them that I feel comfortable having placed the dividing line in my own subsequent work at a Volume Froude Number of about 3.3 (or a V√L of around 5.0, if you prefer). Faster is monohedron/constant deadrise territory; slower is for warped bottoms. In general, the slower you go, the more warp you want too, until water is no longer breaking free of your transom (somewhere around a Froude Number of 1.0), at which point you're going too slow to even have chines.

That all said, I would most heartily second the comments of others to the effect that there's a lot more to achieving a good overall result than picking the bottom shape. Most efficient isn't necessarily best.

[tom28571]

I had intended to do more testing of the model for but one reason or another have been tied down with other stuff. I have looked into the Carolina Sportfisherman design a bit more. The reason for the aft rocker seems to be that they want to make more speed safely downwind. It keeps the sharp entry, which they like for going into waves, from digging into the wave ahead and causing broaches or at least discomfort (terror) to the customers.

I would think that the crossover speed between warped and monohedron hulls would be affected by length, bottom loading and aspect ratio. Length might be a factor because in a short boat the warping would require more curvature in the buttock lines and therefore more vortex to the wake.

Looking closely at the lines presented earlier by Dim, I think that what has been interpreted as hook in the hull bottom may not be that at all. It looks to me like the aft buttock lines are straight and the "hook" is the forward keel sections being lowered. This will keep the bow more in contact with the water at speed and make for easier wave entry. Such a design was used by Weston Farmer in a little boat called "Trumpet" many years ago. Trumpet also appeared to have a hooked bottom but closer inspection showed that this was not so. A possible downside may be a tendency to bow steer, especially when running at an angle to the waves. Or maybe not.

[Willallison]

I have a ski boat with exactly the features that you describe Tom. She's rather beamy for her length and is pretty shallow on the deadrise front - it is a competition ski boat after all! - but the deepening of the forefoot for around the fwd 3rd of the boat improves it's performance in many respects. It doesn't pound quite as much when running up-slop as you'd expect (it's no offshore racer either...), it also truns like no other outboard powered boat I've driven, and it tracks better than many inboard, skeg equipped, boats that I've driven too. All in all it's a very impressive package - I'm yet to discover a handling vice of any kind

[tom28571]

Hi Will,

That checks out. Would you say that the deep bow acts like a skid fin and that the boat rotates around it in a turn? What is the aft deadrise?

[Willallison]

yep - the boat handles a lot like a good inboard ski boat, which are usuakky eqipped with 1 or more skegs located under the ski pole. It turns like it's on rails - at any speed!
aft deadrise is probably somewhere between 10 and fifteen degrees...

[artemis]

While not a professional (or even semi-professional) designer, I've "been around boats" (having been raised in the Puget Sound country of Washington) most of my life. I've done my share of boatwright work (some I even got paid for).

For the past thirty-plus years I've also had an interest, and participated in "hobby steamboating". This, of course, brings to many people's mind the classic fantail launch (ala Weston Farmer's Diana to name but one) with a slow (300rpm) turning, open engine and a large boiler, with a S/L ratio of 1. Or, if one thinks of high rpm, one thinks of the "buzz" of a 5000rpm hydroplane model.

Somewhere between these two is an area which appeals to me (and a lot of other hobby steamboaters). I have access to a company (well, I just designed a new website for them) which has designs and castings for a series of single acting, high speed engines in the up to 2000 rpm and up to 150HP ("TRUE" horsepower) range. However, I am a "tradionalist" in small (under 24' LOA) appearance and don't want to create a "look-a-like" fiberglass hull with a lot of flam at the bow and milk carton design (no offense intended to any participants in this thread as it's obvious y'all aren't into that either). Most of us would like a boat that can be "run up on the beach" without damage (you know, like what we used to do with dad's 18' outboard hull - going at 20mph and tilting up the outboard at the last minute. Gee, Dad, I don't know how that big ding got in the propeller?). Well, I'm certainly not thinking of anything that'll do that speed, but 10 - 12mph would be nice. And an easy way to allow for beaching is to not have a fixed shaft and prop coming out of a skeg. I'm planning to use a Mercruiser Alpha 1 sterndrive. With something in the 1.5 reduction ratio range and a 16" dia prop, there are a number of pitch sizes that will work nicely. So with this sterndrive and the engine I have in mind there'll be about 275 lbs. in the transom area. The boiler - as it is only 16"W x 24"L x 30"H and 300lbs - can be placed just about anywhere. This seems to lead one to the "double wedge" type design. So...

There are a number of designs I've looked at. Bill Atkin's Sea Bright skiffs have some good ideas and I've sent for study plans of his Sea Hawk (LOA 21' LWL 19'6" Beam 6'8+") http://www.atkinboatplans.com/. She'll drive up on a beach nicely and stay upright when the tide goes out. But I don't think she has enough buoyancy at the stern to support the sterndrive and engine. I've also looked at outboard motor driven "Santoise" design used by Caribbean fisherman around Guadelope. It appears to be very similar to the lobster boats used in Nova Scotia (figures - both came from Breton and Normandy fishermen). I am well aware that one should be very cautious in taking a "snip from here, a snip from there" in hull design, but there are features in each which will lead me to what I want.

Hard chine is also not the most desireable. I plan on using lapstrake, plywood planking using Ian Ougthred et al techniques. Any thoughts, suggestions, pointers will be welcome

Maybe this should be a new thread. If so feel free to move it.

Ron Fossum

[Richard Petersen]

I think I have solved all the hull design problems. Buy 1 boat of each type. I am beginning to beleive that all types of stern drives will always be "squirrely" because they have all their weight at one end. The boat has to "blow over- lift and dive stern first" . Simple laws of physics- throw one of those folded paper delta wings we made in school hard, they always lifted stalled, and fell tail, stern, first. Our boats will always try to flip once we get the bow up. All are balancing the boat beyond it's natural nose stays down during water or wind caused lifts. The old Hackers, Garwoods of that time period would lift their bow slightly when going into a wave taller then them. The cleaver ( vertical ) bows punch straight straight thru with little or no hull shock or passenger lurch at 40 mph. Wind or blow over is non existant because CG and MAX. speed CG are almost the same place. Thats why they are so stable when repowered with 800 hp . Ask a Geezer owner- simple big 3 blade and he's out there racing in a small circular course, no sweat.The HP EFF. of those boats is staggering compared to our sterndrives wasting HP to artifically lift the bow. Their only problem was rudders that were 6 sizes too small. Fix that and they will compress your spine into the seat. If speed is more important than your life- do it in a stern drive- you will always blow away any other type of present day drive.