Modular Cruising Catamaran

Discussion in 'Multihulls' started by ImaginaryNumber, Jul 19, 2009.

  1. groper
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    groper Senior Member

    If you edit the michlet file with a text editor, you can change the hull spacing to whatever you like, its located in the "second hull" parameters and the value needs to be in meters, youll see teh value is set to 4.0 atm.

    I changed the spacing to 2.4m and ran it again, it justs get much worse as i suspected... now you have a peak in interference wave drag of 3.3kN @ 11kts - almost doubled it to 25.74 engine kilowatts. Its a poor configuration...

    Ok, so youd like to see how much difference modified vs unmodified will be? Then you can decide if its worth doing, correct?

    ok... so the existing design is simply the one ive already modelled, with the aft run chopped off to form a full depth transom correct?
     
  2. groper
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    groper Senior Member

    Hull with proposed modifications - > View attachment in.mlt
    Your Existing hull -> View attachment Deering current hull.mlt

    The difference is considerable at hump speed - between 9-13kts. 17kN total resistance peaks @ 11kts vs 13kN for teh modifed design... thats about 31kW less engine power required for the same speed @ 11kts. This is also a very inefficient speed for both hulls as it occurs at peak wave making resistance - which is almost 3 times the viscous resistance at this speed. I havnt looked at the wave profile michlet can generate, but i know its going to be huge...

    You also need to remember that you loose the ability to plane and much higher speeds with the hull modification - it wont plane at all anymore...

    So i guess you could work out the grams of diesel per hour required for a difference between hulls of ~30kW - i think its about 230grams per kW from memory, so thats 6.9kg = 8.1litres per hour.

    Then divide the $50k for the hull modification by price of diesel*grams of diesel, then youll know how many years it will take to pay for itself based on the number of hours you do per year. Over here diesel is $1.60 per litre, so youd be saving $12.80 per hour in todays fuel prices - this means you pay for teh modification after 3908 hours usage, although in reality it would be less as the price of fuel inflates...

    An efficient displacement catamaran design (a completely new boat) could be designed to perform much better than the either of the above. It would have more slender hulls, more seperation between hulls, round bilge or radius chine Beam draft ratio of around 2:1, and maximum waterline length.
     
  3. Ad Hoc
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    Ad Hoc Naval Architect

    It isn't about being the most efficient or totally new from scratch, it is about what he has. Naval architecture isn't about what "we" as NA think is the best, it is how we can maximise the requirements for the client/requirement given the knowledge of what are the changes and their effects on the basic parameters. Whether 1 or 2 aspects are not ideal to begin with is irrelevant, all design are like this, none are ideal to begin with nor finsih ideal. What is the SOR...and can you achieve it, yes or no? You're totally missing the point of what design is and how to achieve it and the role of the NA. Naval architecture and more importantly a finished design, is greater than the sum of its individual parts.
     
  4. groper
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    groper Senior Member

    Understand that fully, and no im not missing the point, i appreciate that design is a set of compromises. I think you are failing to realize what it means to be at the other end of the story... - the client sitting opposite you at the desk who owns and runs the boat and needs to consider other options besides modifying the existing boat.

    Like i said earlier, a better option is to simply sell the boat as is and assume he gets fair value for it. Then buy another used or build a new design which is more inline with his requirements - which is an identical SOR, but with a lower design speed.

    We can assume the original SOR has changed because it was originally a planing powercat... now he realizes that fuel efficiency and range is not acceptable at planing speeds, hence the new SOR. This new SOR would be well suited to a new displacement catamaran design, with good efficiency above the hump speed - where he has stated he would like to run it. For this to occur, the waterline beam of each hull needs to be reduced and the hull separation increased in order the reduce the massive wave train generated by the current hull at his desired cruising speed. The current hull cannot be easily or cheaply modified to reduce hull beam or hull separation - thus transom extension improvement is the only viable option (as you recommended) although it doesnt address all the inadequacies of the new SOR, only part of it. The economics of the decision, is a completely seperate issue from the naval achitecture ie. is it worth it to him, and only he can decide that... i appreciate that also, and why i offer my contributions here...
     
  5. Deering
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    Deering Senior Member

    I have most certainly evaluated the option of selling this boat and replacing with one more optimally suited to my current objectives. The original SOR has not changed dramatically, other than the desire to run at a more moderate speed as opposed to either 8 kts or 25 kts that a planing cat offers. The expectation of this boat was to have a higher planing fuel efficiency, as indicated by the builder and observed in similar hull builds. But the builder failed to estimate/control weight on this hull, and the resulting fuel economy was significantly worse than hoped for. The builder went out of business shortly after taking delivery, so there was no recourse.

    The two questions that must then be answered are:

    1. Aside from the cruise speed and fuel economy, does the current boat meet my needs, and would the extended boat meet them better? Answer: Most definitely. There is very little that I would change about the functionality, durability, or configuration of the current boat, and the extended hull would only make those features better. This boat was custom-configured for me and it would likely be impossible to find a used boat that met most of the features that were designed into this boat for me.

    2. Could I sell this boat and buy a replacement with the performance features that you are suggesting for only $50K more? Or $200K more for that matter? Answer: Unlikely. I've looked at the resale market for similar boats. I've looked at the used market for boats as you suggested (they don't exist here). I've explored new custom construction. Multihulls are nowhere near as ubiquitous here as they are down under - the options on the market or from builders are slim and nearly all of them are planing hulls or sailing hulls.

    Two other notes:

    1. Right now this boat is massively overpowered as a displacement craft. That adds significantly to the inefficiency of the entire system, both through suboptimal engine utilization as well as excessive engine weight. My plan includes significantly downsizing the engines.

    2. Losing the ability to plane - entirely acceptable with me. I have no desire to run at 20+ kts. Too many very large things in the water up here with whales and logs... If I can run 15 kts as necessary I'm great with that.

    Many thanks for the analysis, but so far you have not convinced me that there's a better option if I wish to retain my other objectives with this boat. $50K is a considerable sum of money, but other people have spent far more for far less in return. I'm OK with it.
     
  6. Deering
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    Deering Senior Member

    The distance is actually 2.8m, though I guess that won't change things dramatically.
     
  7. groper
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    groper Senior Member

    Thats perfectly understandable. ... only you can weigh up the potential benefits and what they are worth in your area. And I have no idea of the used market there either...

    15kts is still a highly inefficient speed for your modified hull. You will burn considerably more fuel than your current hull @ 8kts... can you see the result s using michlet?
     
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  8. groper
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    groper Senior Member

    Sorry got results mixed up... disregard my last comment above.

    The biggest gains to be had from your new hull modification, occur in the range upto 9knots - the savings are considerable. Above 9kts upto 15kts, is where the worst of it happens... 15kts old vs new = 16kN vs 13Kn or circa 20% difference.

    However, @ 8kts, you have 3.8kN vs 9kN = less than half!

    So if improving teh efficiency below 10kts is your goal, then its well worth it id say... but doing it so that you can cruise @ 15kts seems hardly worth it... just my 2c...
     
  9. Deering
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    Deering Senior Member

    Sorry, can't see Michlet here - I'm in a Mac house. Will need to take the file to a different computer.

    Improved efficiency at 9 kts is a welcomed improvement and I'll utilize that when I'm not in a hurry. If I can cruise with reasonable efficiency (relative to the original hull) in the 12-13 kt range I'll be very satisfied. If I can run at 15-17 kts with my available power (say 150 kW per side) when I'm in a hurry, that would be nice.

    I'm confused. You're comparing power usage between the hulls at 15 kts, but I don't think I can even run at that speed on the current hull, at least not without enormous power input as I'm nearly planing. You're implying that the extended hull will require similar power input at that speed?

    What happens if you compare the same hulls, but just a single hull, at those speeds? How much of the drag is a result of the wave interference vs just wave and resistance due to the hull form? I'm trying to validate the very significant improvement in displacement speed I saw in the performance of my half-sized hull model (for a single hull) when I added on the bow and stern extensions. Speed increased from 6 to 11.1 kts with the same power input on an identical hull, scaled at .5 after attaching the extensions.

    Incidentally, I also plan to add a small 1.25m bow extension, sort of a wave-piercing bowlet, to provide a finer entry. Between the two extensions the WL length will be maximized - no overhangs.
     
  10. Deering
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    Deering Senior Member

    Looking at the numbers...

    13 kN for the new hull at 15 kts compared to 9 kN for the current hull at 8 kts... or about 135 kW compared to 50 kW...so a 170% increase in fuel consumption for a 66% increase in speed. That might be an acceptable tradeoff in some circumstances.

    Am I interpreting that correctly?
     
  11. groper
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    groper Senior Member

    Basically yes that is correct. One thing that is not sitting correctly is the transom drag on the existing hull doesnt appear to be estimated by michlet - usually it does, im not sure why it has ignored the transom for this model, id need Leo to troubleshoot the reason for that. Obviously it doesnt apply for the modified hull as it doesnt have an immersed transom.

    So the gain is probably even better than shown.

    Ive read off the numbers on the graph incorrectly aswell, pulled the data from the out file and you have 12kN @ 8kts for the current hull and 2.7kN for the modified hull - a huge, very favourable difference, ~4 times more efficient @ 8 kts.

    Go upto 14kts and its not quite so great, 12.6kN for the modified hull vs 15.64kN on the existing hull... All teh exact figures are in the out.mlt file...

    All i wanted to highlight, is where the gains are... Doing the modification to run @ 15kts or higher seems pointless to me. However if your doing it to run @ hull speed, then you will use 1/4 the fuel for the same speed and quadrouple your range... This seems well worth it to me. So just be sure your doing it for the right reasons is all...
     
  12. Deering
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    Deering Senior Member

    So let's look at those numbers through the lens of your client on the other side of the table. You're approaching this as a NA, and I would suggest that is blinding you a bit. As your client-cruiser, I don't care about michlet or kN or kW...what I really care about is fuel economy at increased cruising speeds - nautical miles per liter of fuel used. That was my stated objective.

    Current hull at 8 kts = 0.45 nm/liter
    New hull at 8 kts = 2.07 nm/liter

    Current hull at 14 kts = 0.35 nm/liter
    New hull at 14 kts = 0.43 nm/liter

    Current hull at 15 kts = 0.34 nm/liter
    New hull at 15 kts = 0.42 nm/liter

    This perspective tells me three things:

    1. You are correct. The efficiency of the new hull below hump speed improves dramatically. More than I would have ever expected, and that's a welcomed surprise.

    2. If my fuel economy on the old hull at 8 kts was .45 nm/l (which isn't far from the observed economy) and the new hull gets .42 nm/l at 15 kts, I've nearly doubled my cruise speed while maintaining almost the same fuel economy and range. Please explain to me how this effort will be "pointless" based on my stated objectives.

    3. The predicted fuel economy numbers for the current hull at the higher speeds (11 kts, 14 kts...) bear no relation to the numbers observed in the real world. Something is wrong with the Michlet predictions. Perhaps the fact that it's not accounting for the submerged transom is having a larger impact at those speeds? If Leo can troubleshoot I would be very interested in the results.

    groper, thank you for taking the time and effort to help me with this analysis. This has been most enlightening.
     
  13. groper
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    groper Senior Member

    No it's more complicated... I always knew this, but didn't want to complicate things too much where we couldn't get accurate answers regardless.

    No account was made for sinkage and trim. This would have been significant in the speed range around hump speed as the vessel tries to climb over its wave system. I cannot offer a ballpark figure for sinkage or trim, on either your modified hull or your current one.

    The results in this range could vary quite considerably because of it. You should realise this before making any serious decisions. Consultation with an experienced naval architect would probably offer the best guess at what the values might be for sinkage and trim. Perhaps adhoc would offer something here? I'm not a naval architect deering - in case you thought otherwise... Treat my analysis accordingly...
     
  14. Deering
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    Deering Senior Member

    Oh, sorry about the naval architect misapprehension - I assumed from your dialogue with Ad Hoc that you were both NA's.

    Regarding the predictions you have generated from Michlet, they are largely useless without a great deal of additional information applied to them. For all you know, the extended hull could have significantly superior sinkage/trim properties...which my real-world, half-sized model testing has already confirmed.

    Your assessments about the extended hull's performance are about as lacking as your computer projections. Perhaps you should provide that disclaimer when you first engage so no one takes your pronouncements seriously.
     

  15. tomas
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    tomas Senior Member

    ... ???
     
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