Hull design for a small displacement boat

Leo
I know it is not an absolute result. It was intended to highlight a likely problem with the idea that Martin may not have contemplated.

Rick W

 

Tom28571
"..I hope that that post is not the first salvo leading to destruction of an interesting thread..."
It is intended for those that wish to understand that there is more than one solution and and for the poster to explain the comment to others so they may understand the comment better. Since it is as i noted, for big ship stuff and/or showing poor understanding of how load paths work. If you consider this exploring/clarification "destructive", then i cannot alter your fixed position.

Leo
1. So you object to me using the terminology CFD, but not the ITTC which classifies your work as CFD?
2 It doesn't matter whether i partially or fully know how michlet models transoms,. I know it is extremely difficult to do and with very little success anywhere currently in the world. Even you noted this "..Transom sterns are, as you note, very difficult..."
http://www.boatdesign.net/forums/de...displacement-hull-shape-min-drag-27972-2.html
post #24.
But if you wish to claim that michlet does take transom sterns fully into account and models them perfectly across a wide range of Fn's with 100% quantitative consistent verifiable results, that one can trust and believe for real powering EHP curves, then please correct me and say so.
3. No. Indicative insofar as it provides a trend, for sensitivity analysis for hull design, as used in naval architecture. "What if's"

I'm not referring to one spot on one curve. But endless spots for many curves, as is the role of the naval architect's.

So, not bad for an academic/scientist. Most don't like to pry into the real world, as practice often becomes a little harder to explain and control. So not bad at guessing in the naval architecture world

As I've said many times before Leo, naval architecture is not an exact science. It is culmination of many disciplines, some exact, some near exact and some not at all. It is the role of the naval architect to make sense of all of these often conflicting issues to make the 'design' work. Not many can do this, because their focus is on what they can produce or measure, like from a CFD or similar program, the "results" suddenly become believable once rprinted..why??..go figure. It is just "one spot on the curve" of the design (which must also be corroborated elsewhere to be believed). So, trends and trend analysis is the tool of the naval architect. Exact answers to an "unknown problem" without any form of verification, are not.

PS..I must say, I wondered how long it would take, gets less these days. This is excellent behavior by others that haven't a clue and a very myopic mind. See, this is the problem when pointing out the obvious to those that really need it. Childish people hitting me again for negative points. Boy, the so-called "gurus" and "those wanting advice" only want to hear what they want to hear. They don't wish to learn and educate themselves. Just pat themselves, and others that are signing from the same hymn sheet, on the back.....very immature, childish and pathetic. No wonder those that keep hitting me with negative points are not professionals nor naval architects...keep it going chaps...the sheer ignorance is outstanding!.

 

Oh, forgot to add,

Those hitting me with negative points and comments thus ..Unnecessary and derogative personal comments... haven't got the balls to leave their real name nor the balls to debate this in public. So everyone can see why said person lacks maturity and comprehension.

And for said poster....i think you need to understand the difference between "personal" and "factual"..buy a dictionary perhaps?

 

People....

I am getting sick and tired that in a lot of threads people are getting personal and turn an interesting thread into mud throughing word games. Just pick up the phone and start shouting at eachother, not using the threads to settle an argument. This particular thread was to evaluate a proposed initital design and to help the man out to get to something which has potential and would work. Unfortunately I see a lot of different threads turning into back and forth calling names, etc,etc.

 

ASM

I couldn't agree more.
But this site has been taken over by the "he who shouts loudest" and "don't ever question me" users.... as such real techincal open without prejudice debates never occur. Posters seeking real advice are then side tracked, and often given very poor advice by non-professionals. Sadly...the down side to an "open forum".

But there is also a difference between seeking advice..and just wanting lots of reassuring pats on the back for an idea or dream. Everyone's 'personal' dream boat is just that, very personal. As such many take comments very personally when one appears to criticise the 'dream' or provide comments which were not expected (because said poster is not a professional, what else does one expect, but comments, both good and bad?). So it is a two way street unfortunately.

Unless of course the poster prefaces the posting by saying that only positive and "must agree with me" comments shall be made.....makes for a dull, myopic and very subjective post though! Which is an anathema to real engineering anyway!

 

Well here's my 2c worth:

Low AR keels below a hull are very forgiving from a hydrodynamic perspective, you can get away with a very simple shape tapering to max width between 1/3 and a ½ of the chord then back to a narrower trailing edge with a more gradual transition that you have shown aft.

The leading edges, both the stem and the keel would be better if they were less of a razors edge for the speeds you are targeting, in fact you will find that a flat leading edge will be just as good providing you round the corners with a radius around 8% of the width, this can be a flat bar 50 mm wide with the edges rounded with a grinder, this also gives access for welding and maintenance inside the stem and easy attachment of fittings.

I’d also be more inclined to shift the keel aft , I think you will find that this will have a tendency to steer by the LE of the keel. the way you have designed it. Particularly with the full width transom in anything other than sheltered water and particularly with following waves .


It looks more like a design that wants to get onto the plane (with appropriate rails and fore-body ).

 

Rick noted

"This hull shape has the same problem as yours - a big trench behind it. I played around a little to try to improve it. Made some gain but nothing that I have tried worked particularly well. It probably requires a tunnel of sorts to feed water up to the prop. The Atkins hulls have reverse "V" to achieve this. This might work."

The Atkin setup might have other advantages.

With an inboard the prop shaft might be level, no losses in lifting with the prop.

It has been discussed , but never proven, weather the energy trapped under the hull and dragged along by the keel are of use to the overall propulsion efficiency.

The reverse deadrise on the Atkin boats was designed for a particular speed .

A report of overpowering the boat claimed too much lift caused Bow Steering, which Atkin overcame with chine strakes , simply lifted the bow further.

The use of trim tabs at the rear of the area might overcome this problem at any speed. Allow the most efficient angle of attack to be discovered.

FF

 

For what it is worth.. I am not a naval architect at all, I just look at things and judge it. A propeller should have a right amount of water flowing towards him. In the Noise and vibration world I live in, we strive to have the best (laminair) flow towards the propeller to overcome possible types of cavitation. If a slight V bottom is used I guess the water is splitted in front of the keel, though might 'stick' to the keel a little and other water flows along the bottom of the hull, which is turned away from the vessel. So, if this flow is somehow redirected towards the propeller, and he keel shape is a little sharper at the bow then the original sketched one now, there should be 'lots' of water offered to the propeller. I guess that is what an (local) inverted V bottom would do to the aft of the vessel. If then the tip of the bottomstern is also a little sloped down, then the water is projected against this (from the propeller) and therefor pushes the vessel upwards in the stern (but efficiency ight be down a little ?).

To me just a simple explenation, taken into account only what I know and think.

 

Hi all. I'm back (and happy for it ).

It looks like the thread was hijacked for a while , going off-topic and personal. I hope we can continue now discussing the design, and everybody still feels free to give his or her opinion. It IS appreciated.

Sometimes I get the impression opinions are contradictory, and I try hard to properly understand and weigh them and come to a conclusion. That could be the wrong conclusion. Please don't feel ignored if this happens. At the most, just misunderstood. After all, I am a software architect, not a naval architect.

As far as the design is concerned (we are now back on-topic): the idea was not to design a boat that will (semi-)plane. The idea is a cost-effective (if you like: cheap) boat, that can be built by someone with average skills, but with a way better displacement speed than the common simple small steel boat designs.

I was aiming for 10Kn with 15HP. However, if there are ways to get the displacement speed of the boat way beyond 10Kn, that would be great, as it would mean that at lower speeds the hull will be far more efficient (not producing such a large wake / trench).

I also read some people talking about shaft angles etc. For the record: this design was intended for an outboard, sitting in a sealed compartment just aft of the keel (what do you call these compartments in english nautical terms?). Shaft angles aligned with the waterline are part of the package you get with outboards.

Most steel boats around here have a steel tubing welded along the edges. I don't like that though. I was thinking of making a wooden wale. It's not part of the design just yet however, because at this moment it would be nothing more than a distraction.

I think I understand what you mean. However, this is where "form over function" sets in. It is just the way I like it. I am pretty happy with the lines of the boat above the waterline. A 50mm wide keel would make the triangular shaped stem less pronounced.

What I am thinking about now, is a tri-hull design. With this I mean adding keels (smaller, with less draft than the central keel) to the sides of the boat. For this, the sides of the boat towards the aft will simply be extended below the waterline. This will conduct flow towards the prop, add displacement (meaning less displacement for the main hull), and as a bonus, if I extend them past the transom, they can even support the swimming platform.

Besides that, I want to get more displacement from the central keel, so I will be adding some draft, and reshaping it, also with the idea of preventing turbulance near the prop.

Also, the current design does not have any V shape at the front, only at the aft. This will be reversed, so the main hull will have a very slight V at the bow, and be flat at the transom.

Hopefully, within a few days, I'll be able to show the first renderings.

The main question: do you think a tri-hull design will work?

 

"...Most steel boats around here have a steel tubing welded along the edges. I don't like that though.."

That's called a fender, not a sheerstrake plate.

 

Mwatts

I think the tri (faux tri) will work, mayeb rick could back me up on that, he devellopped the faux tri further. Nice design feature the swimplatform on the extended outer 'hulls'. Although you will not go into form follows function, please keep the shape of the bow the same, this is a nice look (and different indeed from the ones we already have here plenty lying around), although looks are personal, others might prefer different. Looking forward to your updated design !

 

I thought fenders are the soft cushiony things you hang on the side of the boat when docking?

 

Martin
A tri-hull is a better option than the single hull for your target speed. It is not quite as good as the catamaran. However the wave trough is more suited to a centrally mounted outboard. The wave height is about half of what the single central hull will produce.

Long deep slender hulls give the lowest drag. If you come up with the arrangement you like I can check the power requirement.

Unlike the stabilised monohull where the outboard hulls carry little load, this will work best with the outboard hulls having a good proportion of the displacement.

Rick W

 

"...I thought fenders are the soft cushiony things you hang on the side of the boat when docking?.."

Fenders come in all shapes and sizes and materials, but their function is the same.

 

Rick, I think the next design will be a tri-hull then. I'll send you an igs when it's done, so we can get an indication of how well it might do.