10 foot SWATH

Hello all. Looking through this forum, I bet my thread title has already raised the alarm - seems like you guys get the occasional nut asking about SWATHs. Allow me to be the latest... bear with me!

Here's my problem - I'm building a small autonomous boat, a robot basically. My project is solar powered so the the deck area needs to be ~8 x 10 feet and the more separation between the deck and the Deep Blue Electrolyte (sea), the better. On the face of it, I'd choose a catamaran for simplicity, but in the event of a capsize the vessel needs to be able to right itself using active ballast tanks in the hulls. With 600 pounds of batteries in the keels, this is difficult - I've written a few scripts to simulate the recovery plan and cats are realllly problematic .

Now SWATH, on the other hand, rights quite readily, especially with a little ballast helping it.

Thats straightforward enough, but as you might expect power is at a tremendous premium on a solar boat like this, so I've really been struggling with the "SWATH efficiency problem". The reason I'm calling it a "problem" is while there is consensus that SWATHs are less efficient than conventional designs, my design is hanging out in the 4kt max speed regime, and my calculations are coming out... inconclusive... as to how handicapped a SWATH design would be.

I'm an aeronautical engineer, so while I like to consider myself bright, shipbuilding appear to often take a very different approach to some of the concepts I'd otherwise be familiar with.

So, question one:

-How far do SWATH "pontoons" need to be submerged to be considered well free from the effects of wavemaking drag? I'd imagine there is some rule of thumb out there like "2 diameters", but this probably isn't common knowledge. Anyone know? At present I've been analyzing 1foot dia. pontoons with the upper surface 2 feet below the waterline.

Question two:

-What I've been doing to estimate the total resistance of my designs is to calculate the viscous (skin friction) drag of the pontoons based on their surface area and the 1957 ITTC Friction Line. Then I've been calculating the drag of the remaining structure (pylons, etc) using an empirical Speed/Length Ratio to Resistance/Weight correlation, taking the displacement of the pontoons as a deduction to the total weight of the craft. Valid? I realize it neglects form drag and such, but my pylons are VERY fine, just 4" wide where they meet the pontoons, and I'm doing trades at this point.

Question three:

-In the end, I need ~1000lbs to achieve 4kts on 30lbs of thrust. My calculations for a SWATH tell me I might juuust make it, while similar calculations for conventional (say, monohulls) say its a little hairy. From experience with trolling motors on my old Capri 16.5, this seems ambitious. Has anyone else heard of a SWATH being more efficient at slow (<4kt) speeds, or do I have a systematic error in my approach?

 

just curious - is the 8x10 set dimensions or is the surface area the main issue?

I would think that by going 4x20 deck proportions you could make a mono with pretty skinny hull - battery ballasted. It would be the easiest to make stay upright - low weight alone could do it or some kind of float mast of modest dimensions (not to shade too much)

My idea for solar drone was a mono with moderate beam and sort of traga bar (whatever they are called) for righting - you could have your cameras and comm equipment in this bar. My speculative boat idea is to have a personal drone cruising the seas of the world and send me images every day

Sorry for not being able to help with the swath issues - and congrats for working on such interesting project.

 

They need to be submerged, so there is a problem of scale. Their efficiency shows when there are waves and they don't get as much resistance. In a boat so small, I don't think you can get it to move without pitching unless the waves completely submerge it. There is a monohull design which does that. I don't recall the name, but the bow is at the waterline lever and there is a breakwater type deck where the water lifts and separated to the sides.

 

A single SWATH hull will give the lowest drag displacement hull. Better than any surface hull. The supporting strut needs to be as small as structural competence allows.

I expect it can be made practical if the vast majority of the weight is in the submerged section.

You need to place the submerged portion at least 3 diameters below the surface to reduce the influence of surface waves to negligible level.

A fineness ratio of 8 works out to give the lowest drag.

You could try a Carmichael laminar flow hull shape. It may have an advantage at such low speed. An eliptical forward section that flows into a parabolic aft section is a commonly used shape.

Rick W

 

Random thought here.

What if you designed a Cat pontoon system to be top/bottom symmetrical? You could design your panels on offsets that can rotate, rather than 'righting' the craft, you just right the only part that needs to, the panels. This would of course make for heavier panels and such, but would require far less energy to just unlock and swing panels back up into place.

 

No, 8x10 is not a hard requirement - that is just one arrangement of the solar panels. I found it to be an attractive one, but perhaps I'll talk it out:

As you propose, an alternative would be to place them in a line - the result would be a deck area around 16 feet x 3 feet. This is attractive from a wave drag standpoint and would be easy to make self-righting, but I don't have an elegant solution for raising the solar panels - if indeed the boat is rolled the panels will make an excellent hydrofoil/sea anchor and will need VERY robust support structures. I also have some instruments that would be best elevated from the water, but again not higher than the panels so that shadowing is not an issue.

I settled on SWATH (for now anyway) because my sims showed it wasn't bad for my needs, and was extremely attractive from a layout and construction standpoint. But I need some validation.

Rick, thank you, what you say actually meshes well with what my scripts have been telling me. You've made my day.

I actually have a code I wrote for sizing the diameter of the submerged sections, and I run it for diameters of zero through whatever diameter is large enough to float right up out of the water: In all reasonable cases, the trend is that the total resistance is greater for a SWATH hull up until the buoyancy of the pontoon supports about 60% of the total weight. After that point the SWATH hull has slightly lower resistance, with in some cases a 25% total savings in drag when the pontoon accounts for 80% of the total displacement.

I also found that this trend does not hold for higher speeds - I'm mentioning that because, to my untrained eye, the SWATH info on the web consists of a lot of wild claims and I don't want to be "part of the problem".

If I recall correctly (I'll rerun things again tonight - I stopped because it wasn't clear if I was on the right track) the savings were largely mitigated when switching to a classic two-pontoon SWATH design, but "breaking even" is OK with me - I need a hull that works, not one that sets records.

Thank you very much for the fineness ratio info, and the "3 diameters" rule of thumb, that is EXACTLY what I was looking for.

EDIT: Regarding the "laminar hull" - that shape may be difficult for me to achieve. I was planning to use my 4'x8' CNC router for plywood-and-fiberglass construction, and I think such a complex profile would need to be molded - thats probably beyond my capabilities.

 

Swath+

amaurer,

I think you may find this site of interest:

www.ise.bc.ca/dorado.html

It was featured for a second time on Daily Planet last night.

Go to their web-site for ~8 minute video of Durado operating in Vancouver Harbour recently.

Tom.

 

It seems that such a small boat would be constantly being swamped by waves, no matter the depth of swath. Such a large platform would be hard to make stand on a monohull. It is too short to be stable, or to provide a stable platform. You want a solar boat, with big batteries to propel it, but consider instead a vertical windmill powered boat. The windmill will produce a lot more power, need less batteries, and easier to have just a mast sticking up. The you can put bigger motors to go faster... Just another idea to kick around...

 

You're right, I should have made it clear - I have no interest in the "stability" advantages of the SWATH design; I'm interested only in the hull efficiency.

As for the wind turbines, I considered that, but the "constantly swamped" condition made me fear for the mast. Its not feasible for me to create systems to react autonomously to large incoming waves - its a certainty that my craft will get hit, and rolled, because I wasn't smart enough to reset that day's waypoint so its not getting hammered broadside. On top of that, the turbine complexity goes up significantly when you figure that, at some point the prevailing winds will likely exceed its capacity and it will require feathering, etc etc.

Solar panels are a huge handicap, but any way you slice it the reliability advantage is in their corner. I've sized things such that the boat will be underway only during the daytime -> flat out at whatever the day's sun can support. The goal for the battery system is station-keeping only, to wait out the nights and bad weather days until sunny days return to recharge and continue on. I work in the commercial spaceflight industry, the philosophy is similar to that used on communication satellites to survive eclipse seasons.

 

Would be nice to have Ad Hoc chime in here, he has done very much on Swath design and refinement (on a professional basis). But I fear he will not, he is in Singapore for a week.

 

Do you need to be on surface or your problem is power only?

 

I'm not sure I understand?

I need to be on the surface during the day for the solar panels. I also have instruments that need to be on the surface, but the solar panels are by far the more demanding requirement.

I was thinking about it on the way home, I might need to take a step back and look at the bigger picture. The long narrow monohull would probably win a trade study handily - a slow SWATH might have an efficiency advantage, but even a super-long monohull is probably going to require less structural weight and have lower overall displacement and resistance.

I'm going to twirl the numbers and report back.

Thanks guys, I like it here, intelligent group.

 

amaurer

Interetesing little dilemma and exercise.

You have selected a SWATH, for your SOR, yet in later posts you say that you're on interested in "hull effeciency".
Can you define what you mean, by hull effeciency. Since you're first post and post #9 are contradicatory in terms of why one wishes a SWATH design to do and what it can do.

As for what depth to be free from wave making drag, you can't. The depth has nothing to do with the wave making drag, per se. The depth of the submerged body is related to the effects of decoupling the motions of the vessels, from wave excitation, not Cw. A SWATH is a very very draggy hull form. Can't get around it.

 

I'll be the first to admit I'm out of my element here, I'm sure I'm playing it fast and loose with some terms with precise meanings.

By "efficiency" I meant with respect to total resistance in my particular application. I would define "higher efficiency" as accomplishing my mission with lower total resistance than another/other hull shapes.

So the crux of the issue is: can I design a SWATH hull that carries my loads at 4kts with lower total resistance than a monohull. My sims and Rick say yes, are you saying no? You say SWATH is draggy which I understand, but conceptually, I think a SWATH with the displacement heavily concentrated in the pontoons is more akin to a submarine, which IIRC submarine hulls generally have lower resistance than similarly sized surface ships... which makes me think there are operating regimes where SWATH is not the draggier option.

 

Ok, you have define effecincy in terms of resistance. Understood. You will not get an equaivalent catamaran nor monohull as draggy as a SWATH. A SWATH is purely about seakeeeping, everything else is irrelavent. A SWATH is the most stable platform with regards to seakeeping bar non.

But the pentalty is the resistance. These are all very draggy hull forms, no matter your configuration of hulls, struts etc

Since you're doing SOLAR power, it is a fair bet that the resistance envelope of your design SOR is crutial for the concept to work. This being the case, your selection for a SWATH is flawed, as this is the most draggy hull form you have selected.

However as with everything in design, when designing real boats, not printouts on a computer, it is all a compromise. So, what you need to do, is to design the boat with:

1) a SWATH hull form, meeting your SOR
2) catamaran hull form, the self righting aspects etc are easy to design into, if you understand that basics in terms of naval architecture, meeting your SOR.

Once you have gone through the first 'design spiral', for both vessels, your answer will "pop" out.

Designing boats isn't....problem, press this button, out pops the anwser. Every design is unique and has its own unique solution. So, ignore the chatter about small details etc, focus on the SOR.