10 foot SWATH

Honestly, I still don't understand.

I can tell you that my sim discovers lower resistance with a SWATH hull with 80% of the buoyancy provided by the submerged sections than for any other shape. My sim analyzes shape based on expected displacement - so differences in the practical amount of structural weight required to implement a SWATH design vs a cat are not properly represented - is this the source of your assertion that SWATHs always lose?

Do submarine hulls have less total resistance per displacement than surface vessels? And does not a "bottom heavy" SWATH tend towards the former case?

 

I would build a giant surfer board... Make it self righting by putting engine in a pod deep down like a torpedo thruster. The structure has to be strong and light. Build solar panels on surface of surfboard. Battery can be thin on bottom of hull using lithium flat cells.

 

I think the idea is to avoid the panels and comm electronics from being under constant salt water wash. At least some freeboard is needed.

Just read Bombard story - teh wacky frenchman from 50s who crossed the atlantic in a rubberboat. Besides actual bad weather his boat seemed to stay pretty dry - just floating over the swell. Just a point that a less piercing design might not be bad for small boat.

 

amaurer

OK, just a few pointers.
Your programs that you are using for your resistance, as i assume that is what you are using for your whole concept?

So, even before i delve into some of the whys, ask your self this very simple question (ie no fancy looking software program), how many SWATHs of the type you think is ideal, have you seen?

You're in the same 'boat' excuse the pun, as RW. You're not naval architects and you haven't designed any real boats before, so you're looking for answers from a computer program. Naval architecture is not about pressing buttons.

To design a boat requires many different aspects to be considered, hence my post above above about focusing on your SOR and not the details. Details are only discussed by amateurs and wanna-be's, because it is supported by fancy looking programs to give credibility. Whereas designing real boats is more than the sum of its individual parts. Details as such are only considered once the design has been 'done'. That is why it is called detail design, ie what size are the engine feet and hence where do i place my engine girders, or where can i buy my windows and what fixing arrangement do i need to do to pass rules etc etc.

If you go through the design spiral for the 2 vessels, as i've posted, this shall become self evident.

If you choose not to, that's fine doesn't bother me, i'm only giving you advice as you have asked for.

If you wish, I can send you a paper I co-authored and presented 2 years ago at conference on a unique SWATH we designed recently (not my first). It will give you an idea of what is involved. But i cannot do until next week, as I'm out of my office. Just PM me your details.

 

AdHoc, perhaps your brush is too broad, I didn't think I was getting into details?

I'm not using any software at all - I have a script I wrote in matlab (using the general algorithm I described in my first post) which does nothing more than take displacement, waterline length, and the percentage of the displacement I want the pontoons to represent, and then estimates the wavemaking and friction resistance. Nothing more.

The former is done using a correlation I found in a very old book - basically a plot of Rtotal/Displacement vs. Speed/sqrt(LWL) for displacement hulls. The latter uses the 1957 ITTC friction line. I have not considered the shape of the hull apart from that it has submerged portions, and I don't think I ever implied I'm at the "engine feet"/"girder location" stage.

Perhaps you can suggest, if not here, where you'd start?

 

amaurer

For any design, not matter its purpose nor its innovation, one cannot start without 2 basics being done first

1) A general arrangement. That is to say a lay out of the "things" you wish to have in/on your boat. This is coupled with your specification, if you have one. A spec is part and parcel of a GA. Without this, how do you know what it is you're trying to achieve? It is in essence your SOR (statement of requirements), on paper.
2) A weight estimate. An estimate of everything that will go on board and required that must be accounted for. Even if you're unsure or no clue on some aspects, make an educated guess from various sources. And add a health margin to the total.

Once you have done the above, then and only then can you consider what hull forms to satisfy your SOR. But as posted previously, you still need to go through the exercise of comparing a SWATH with a Catamaran for your SOR. I cannot give you the answer, only your findings will give you the answer.

If anyone suggest otherwise, they simply haven't a clue about naval architecture nor real design.

 

AdHoc,

I'm not sure how we're misfiring here? I already mentioned my weight estimates, as indeed I know what equipment I need to carry for my intended mission. I also mentioned that the calculations I have undertaken require a displacement (estimate) input. I tried to encapsulate it without boring everyone in my very first post: I need to move 1000lbs at 4kts.

Using those estimates, I used a resistance correlation to determine what hull shape can meet my mission requirements - and again, as I said in the first post - conventional designs meet the requirements, as do SWATH designs (and slightly better at that). I started the thread to see if there are factors surrounding SWATH design that my estimates do not properly capture (such as pontoon depth to avoid surface interactions, which would affect my weight estimate if I had to assume a taller boat).

I'm a little confused by your comments, honestly. I can't really tell if I'm so far off the mark as to be blind to it, or if perhaps you're not giving me a fair shake because you're assuming the worst?

 

Regarding the resistance of your proposed boat, can you tell us if the correlation formula includes a factor to account for the depth of submergence of the hulls?

Wave effects diminish exponentially with depth, so more deeply submerged hulls will have slightly less (calm-water) wave resistance.

However, as has been stressed here, there is a lot more to SWATH design than simple resistance considerations.

Good Luck,
Leo.

 

The 3 diameters is not a rule of thumb I have analysed a hull displacing 90kg at various depths and below the three diameters the wave resistance is not signifiicant so you have essentially viscous resistance to contend with.

You need to keep the majority of total weight in the sumberged hull. You need to keep the interconnecting strut or struts small as these contribute to total drag.

Now how you achieve this to support solar panels well above the surface will be a juggling act. Whether you actually get a reduction in drag will depend on how much payload you need to place in the submerged relative to what has to ballanced above the surface.

My speed of interest that I desgned for was a little higher and the minimum drag surface hull was quite long. The wetted surface of the surface hull was 2.1sq.m against 1.7sq.m for a submerged hull of the same displacement. The lower wetted surface and near zero wave drag provided an advantage in drag to the submerged hull. I have attached an image of the boat I built and tested. The challenge with this was getting it to remain stable with most of the weight above the surface. I ended up with hydrofoils on the outriggers to reduce drag of the outriggers.

You can find some information on the submarine Albacore. This was the result of considerable research to determine the lowest drag submarine hull.

This clip shows a large single submerged hull along the lines of what I am suggesting:
http://www.youtube.com/watch?v=kRuUtOmMGR0
Whether the SWATH like this is the lowest drag hull will depend on the submerged payload relative to the above surface solar collectors and controls.

To do the comparison exercise you need to know how much of the total weight can be submerged.

 

Leo,

No, there is no such compensation, it assumes the submerged portions are totally immune from wave resistance. Thats why I was looking for a rule of thumb to ensure the submerged hulls were deep enough for that to be valid.

All of this aside, I have to admit that I didn't analyze a monohull because the solar panels would make it so long as to be difficult for me to build and transport. However, playing with the numbers tonight in order to be rigorous, I'm thinking a 20-foot monohull is a really competitive design. I'd put the chances at 85% that I abandon the SWATH idea entirely at the point in favor of the monohull - easier to build, stronger, lighter, and lower resistance. The only downside is the smaller freeboard... which is a lesser evil, I suppose.

 

amaurer

"I'm a little confused by your comments, honestly. I can't really tell if I'm so far off the mark as to be blind to it, or if perhaps you're not giving me a fair shake because you're assuming the worst?.."

A bit of both.

The fact that you say you are uncertain what hull form in terms of resistance etc is evidence of this. I don't know how you can have a full weight estimate when you don't know what your hull configuration is, for example.

If you don't know which hull configuration you're using how on earth can you have a weight estimate? This just means you have (I assume) a nominal value for the structure weight. This value will change dramatically between monohull, catamaran and SWATH.

So, all I can do is refer you again to my post in #21.

Your post above implies you are not approaching this correctly and you are being derailed by 'chatter' that is not relevant in the first loop of the design spiral to esnure you can meet your SOR.

But that is your prerogative....have fun.

 

By so far I see the "derailed by chatter" point to be the most crucial here. Not all around here are able to give a proper advice, especially not those with the most pictures to show.

Regards
Richard

 

Obviously I have estimates for each hull type (see above where I mention I'll probably go with the monohull because its lighter). But ok, I see you've exhausted the extent of the advice you're willing to give, so thank you for that, in any case.

 

ah, "SOR" means "statement of requierments" witch offcourse can be found in the design envellope

maybe something flat on stilts like this ?

 

Post #7

amaurer,

Did you see my post (#7)?

Was it of interest to you?

Are you having fun yet?

Try not to get distracted from your original post/goal by the counter productive dialogue you receive here.

Stay focused.

Tom