Displacement Glider, PowerKeel, etc?

Discussion in 'Powerboats' started by mark424x, Jul 3, 2006.

  1. mark424x
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    mark424x Junior Member

    I've been lurking for a while, taking in the accumulated wisdom and I'd be appreciative to those of you who have experience with modeling drag, power, etc to provide some input on the box keel, sea bright, displacement glider(DG) concept. I'd also love some input on using these hull forms in real ocean conditions. There have been a lot of great threads on efficient hull design, but I haven't found this particular issue addressed.

    When the discussion of efficiency comes up, it seems that everyone agrees that pure planning isn't efficient. That leaves the discussion to go slow in displacement mode or go long and skinny, increasing LWL and/or Length/Beam ratio result in lower required power for a set speed and higher knee in the power/speed curve.

    It seems that variations on this theme pop up in various places.

    (1) Dashew Fast Pilot Boat (FPB) - 83' - L/B = 4.75? (http://www.setsail.com/dashew/dashew233.html)

    (2) Stephens 58 - Chuck Paine's SteadySailor - Does anyone have any numbers on this boat, displacement, nmpg v speed? (http://www.chuckpaine.com/zsteadysailer.html)

    (3) Irens/Rangeboat (http://www.rangeboat.com/index.php)

    (4) Alsphere Displacement Glider - I haven't been able to find any hard data on this one either. (http://www.alsphere.at/dg/performance.shtml)
    Patent # 6,112,687, see www.uspto.com

    (5) Malcolm Tennent's "CS" hull form. (http://www.catamarans.com/news/2006/04/CatComparison.asp)

    (6) Grahame Shannon's "powerkeel". (http://www.aviadesign.com/powerkeel/index.htm)

    To one extreme, the keel is the width of the water line and we use the well known displacement equations, to the other extreme, the keel is razor thin and just increases wetted surface area without adding significant buoyancy leaving us in displacement mode at low speed and planning mode otherwise like most hulls.

    My question is how to model the intermediate values between these two extremes, i.e. the box keel/DG/powerkeel. Qualitatively it would seem that you'd choose some constraining variables, say LWL and design cruise (for illustration I'll use 39' and 14knots from the rangeboat). In order to get an object of 39' Length to go 14knots in displacement mode you'd need a L/BWL of > 12:1?, thus the "keel" would be 3.25' wide. (Lacking any hull speed equation that includes L/BWL, I'm using Bill Roberts' K-Factor http://www.southwindssailing.com/articles/multihulls/CruiseMulti.shtml)

    Choose your boat displacement, Keel depth, prismatic coeff and you could calculate how much of the displacement you'd have to hydrodynamically support with the planning surfaces outboard of the keel. It would seem that this leaves one free to vary the distribution of beam to provide the lift where needed and internal accommodations.

    Interestingly the Displacement Glider patent assumes that the complete displacement of the hull can be supported by the "box keel" and the shoulders/chines are right *at* waterline and serve to cancel the hull wake. They also provide slight shoulders to capture the spray off the center hull forward. In the pocket cruising thread there was talk of a variant on the design having the keel buoyancy support 70% of the displacement.

    I'm curious to see how the power numbers come out. Or will this concept only work in flat water and with any significant swell will the whole concept break down? There are many many "semi displacement" boats that live in the 10-20knots range and drink a lot of fuel, from Mainship trawlers, to all the 40-50' bayliner/meridian/etc cruisers.

    Using rangeboat numbers:
    12m = 39'
    beam - 3.3m = 10.7', presumably much narrower at the waterline.
    5 tonnes
    13kt cruise - 85hp
    16.5 kt max - 240hp
    11000 lbs

    Assume a prismatic coeff of .65 (using Tennant's number), and guessing that with a L/B of ~12:1 I can get hull speed of 2.8 *sqrt(39) = 17.5 knots. That would give me a keel volume of 39 L x(39/12)W x 2.15 D x.65 Cp * 62 lbs/cuft = 11000lbs.

    I'm well beyond my experience here and would appreciate anyone that can help with the Power requirements to push a 39'L x 3.25'W x 2.15'D 5.5ton slender hull through the water. That also assumes 100% of displacement is provided for in the keel, if you wanted some dynamic lift, there is another set of equations required.

    The rangeboat is already a lightweight boat, the same experiment for say a Nordhavn 43 comes out with a 10' deep keel - but if you took the 4000+ lbs of lead out, lightened that baby up a bit, slow down to say 10-12, and perhaps used the 70% number, maybe you could get a reasonable number.

    Of course there are pitch, stability, and see keeping considerations, which are certainly beyond me at this point.

    Thanks all for your help.

    1 person likes this.
  2. fcfc
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    fcfc Senior Member

    Here is an insteresting paper about hullform and efficiency.

    But I think you forgot a very important point : the economic one.

    What is the total ownership cost (design, building and maintenance) of these special hull form with say an ultra classic hull 20 or 50 % longer ?

    If your 28 ft hull is more efficient at 12 kts than a 35 ft len hull by 3 %, but cost 30 % more to design , build and maintain , you will interest about nobody.

    For most people, the limiting factor in boat ownership is not length, it is money :p
  3. mark424x
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    mark424x Junior Member

    Well yes, sort of. As many of the threads on boat design mention, without knowing what problem you are trying to solve, one can't make the tradeoffs. I don’t have a good feel for the cost of a custom boat, but for the above examples.

    (1) The Dashew 83’ FPB is reportedly US$3.5-4.0 M
    (2) Steadysailer - ~$1M
    (3) Rangeboat – reportedly $350K
    (4) N43 - $700-900K

    Does anyone have examples of custom boat costs for smaller coastal boats?

    I would add to that the following
    - the old adage that the boat that is the cheapest to run, gets used the most.
    - there are few “long cheap” production boats on the market – the longer they get the bigger and more complex they get and price goes up exponentially.
    - I’m also after an improved ride (and quiet) of a displacement boat. I would love the feel of a 70’ sailboat slicing along at 12 knots in my smaller power boat.
    - the custom boat of today is the production boat of tomorrow.

    Up until this past winter I had a 32’ planning cat, while I occasionally ran it at 30knots. Often the pacific swells prevented me from going >22knots and if I had guests the ride was rough enough that I was usually at 10-15. I also see tons of express cruisers that claim to be planning boats cruising around with their nose in the air. Once you spend all that money on a boat, it’s obnoxious to spend $100 in fuel to take friends for a bay tour or $200 for a run to the island. While an expensive boat does depreciate over time, fuel depreciates to zero immediately.

    With something like a DG/powerkeel if you could make something like the 40’ Mainship or express cruiser that got 5mpg at 14 knots, I think that would be a viable product. Similarly, if you could build a passagemaker that could do 10-12 in the 40-50’ range at 3-4mpg, you could give Krogen and Nordhavn and run for the money. Look at the underside of the steadysailer - it's basically a mini DG-keel.
  4. fcfc
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    fcfc Senior Member

    For centuries, people have tried to optimize boat efficiency: http://www.btinternet.com/~philipr/froude.htm
    There have been dozain of thousands of hydrodynamic engineers. So unless they were all dumb, there is no remaining unexplored domain.
    I would take with greatest caution someone claiming more than 5% gain on a very limited speed range, without any hidden drawback.

    There are bunch of people paid by big names boating companies to beat competitors. Not speaking of naval forces. Prove the USCG you can save them only 10% of their yearly fuel bill with your new hull form, without any functionnality loss: The next day, you are millionaire.

    What may change is that building technique evolve, allowing now boats that were unbuildable say 20 years ago. That left very specific technology like preimpregnated autoclaved composites. If the building technology existed 20 years ago (steel, alu, plywood, ordinary composite etc ...) , the design would already have been built. And if it was successful, everyone would use it now.

    Now, hullform is a marketing argument. When you look at the Displacement Glider, you learn 1) that they got their patent 5 years ago. 2) They compare to a displacement and planning hull with a colorfull graph. But they do not give a single figure or name on what they compared. = Pure marketing the level of TV ads.

    You spoke of competing Nordhavn, but dont you think the army of engineers they hire do not try to optimize their designs for their customers ?

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  5. mark424x
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    mark424x Junior Member

    I agree with you on some points and am skeptical on others.

    I have a healthy degree of skepticism about this DG/PK concept, which is why I asked the question. It does seem that this would have been tried before. I also agree on the DG marketing fluff, but they maybe going after the commercial market where there is no need to provide data to the consumer types. As the FEYS guys found, the volume in the boating market is so low that it’s hard to make the economics work – which leaves innovation to those who are willing to do a custom boat - and loose money on the deal.

    On the other hand

    (1) take the example of the Glacier Bay cats – certainly something people could have developed and marketed decades ago. Why didn’t someone? They don’t rule the boating market, but make a product that uniquely fills the needs of a certain market segment and are growing like gang busters.

    (2) I believe the boating market is very conservative and full of “me too” products. Occasionally someone takes a chance, but not often. There are so many small players, they don’t have the resources to bring a truly innovative product to market and make it stick. It’s a shame the big boys, e.g. Brunswick, doesn’t have “concept boats” at the shows like the auto companies have concept cars.

    (3) I don’t think Nordhavn has “an army” of engineers, from my reading of (http://www.nordhavn.com/about/team.php4) The have one designer, one naval architect, and one assistant. I sounds like all the other guys are working the systems. My sense is they are using the Beebe approach and doing minor variations on that theme.

    (4) The commercial guys certainly are solving a different problem than the recreational boater.

    (5) The power passagemaker market is relatively new, so has not evolved much. As the market grows and more people are drawn into it, people will either copy the leaders or try to chart their own course. i.e. Chuck Paine’s SteadySailer – a power boat with sail assist, not a motorsailer., or the Dashew FPB.

    (6) The market evolves - another example: it's getting harder and harder to find a quiet lake to go boating and people are aging. I have relatives in their early 70's that live on a lake that have a 19' Sea Ray - the grandkids use the boat to ski at 20-22 mph. When the kids aren't there they often don't take the boat out since it bounces around in wakes of other boats too much. I think if you made a cheaper version of the GB22 with a single inboard that maxed at 25 mph, families could ski, get around, and the old folks wouldn't get beat up in the chop and constantly be going on and off a plane.
  6. Vega
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    Vega Senior Member

    Don't you think that if they put there 4000lbs of ballast it is because it is needed? Do you think that those guys that have been perfecting that type of boat for a lot of years don't know what they are doing?

    Take a look at this thread, we have been discussing about these issues for a long time (efficiency/seaworthiness/consumption/speed/overall costs) :

  7. mark424x
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    mark424x Junior Member

    I wasn't talking literally converting a N43, but a boat to fill that role, range, accomodations.

    Every design has it's tradeoffs, e.g. perhaps using a diesel electric config and a deep DG/PK would allow a different tankage arrangement to lower the CG with less ballast. Actually through the partial diesel electric N72, they are already talking about moving the weight around. Other arrangements use sea water tanks to replace the weight of the burned fuel rather than lead - a consistent displacement would actually be better than just using 50% max fuel load in lead - another Beebe rule of thumb - a good thumb - but ya gotta understand why he did it.
  8. fcfc
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    fcfc Senior Member

    They actually did it two decades ago. http://www.glacierbaycats.com/about_history.html
    I am not sure they could do it earlier. The prototype was built in strip planking red cedar + epoxy. I am not sure this low cost prototyping technology was available in the seventies, or even high power reliable outboards. At that time (in the seventies), some GB were still built in wood. And no computing power was affordable. Even a handheld scientific calculator.

    Your link only lists US team. You forgot the two factories they have in China, where the boats are actually built. One of which is http://www.nordhavn.com/news/pressrelease/south_coast_progress.php4
    You note a four story office in this 680 000 sqft factory. Guess they have 500+ workers (they do not give any figure) , say 10% work in a design office (50 people), and four or five are specialized in seakeeping and propulsion.
    Saying the US naval architect designed Nordhavn boats is a bit like saying Boeing CTO designed the DreamLiner.

    Perhaps, but they have a bunch of money. And if they had found anything useful to recreationnal boaters, the recreational boaters would have used it.

    I suggest you check on google the name "marin marie" and the boat "Arielle 2" (attached picture). He crossed the atlantic on a 40 ft powerboat, with a diesel engine in 1936, a bit earlier than Beebe.

    For the dashew FPB, I do not know when G Buelher designed his Ellemaid 81 http://www.georgebuehler.com/ellemaid.html , the look is radically different from the FPB, but the figures for len, waterline, beam, displacement , power and speed are not that different.

    This just to say that any breaktrough in hull design is not probable.

    Attached Files:

  9. mark424x
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    mark424x Junior Member

    I don't disagree with anything you said, but believe we are communicating past each other. e.g. I commented the the passagemaker *market* is relatively immature. There is no doubt there is a long history of passagemakers. But my understanding is that it's only within the last 20-30 years that you can buy an "off the shelf" power boat that is marketed as offshore passage ready and only the 10-15 years that there have been any significant sales in this market.

    But the real point of my original question was how to take the beam into account when calculating the power required for a displacement hull form. There have been lots of discussion about long and narrow, I just don't know how to model it. The sea bright, box keel, camano keelform et al have been around and DG & PK are some recent variations on a theme. If one cared about categorization, one could argue that the DG is really a variation on a trimaran.

    I'll even accept that it's a completely bad idea and hasn't evolved becuase people have tried it and discarded it. Just trying to understand what its faults are. "Failures teach us more than successes".
  10. fcfc
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    fcfc Senior Member

    You can find resistance prediction with beam :
    http://www.rina.org.uk/rfiles/HISWA...ion of the hydrodynamic forces on a saili.pdf
    http://www.ocp.tudelft.nl/mt/journee/#LINK2 software holltrop.zip.

    Both are regression analysis package. They take a bunch of models, tank test them to measure resistance, and devise formulae to try to obtain resistance from characteristics of the hull.
    From that, they have been validated in their model range. If you go outside their range, they are no longer valid. Typical boat for delph is a 40 ft sailboat. But if you go to a displacement powerboat with a not too big immersed transom, it should be more or less valid. Holltrop is better suited for powerboats, but cargo like.

    You have also http://www.cyberiad.net/michlet.htm A completely different kind, from what I understood. It computes the mathematical wave resistance from the actual hull form. But there is a mathematical simplification done. To be valid, the hull must be thin (L/B better than 6). And you must give the correct hull equilibrium. If you give the hull form at 0 speed, and if you compute the resistance for 10 kts, the hull balance taken will be the 0 kts one. But any self propelled hull at 10 kts will go bow up. So you have to guess what will be the trim at the speed you want to compute resistance, and you will have to enter that trim in the input files to have a better estimate. ALso I do not know how this program deals with planning effect, which is a different mathematical thing than wave resistance.

    Regression, since they are based on actual measures, do include ALL physical effects (planning, trim, squat, etc ...).

    Now, if you find information for resistance prediction suitable for small semi displacement powerboats hulls, say 30 - 40 ft, 10-18 kts, D/L < 100, L/B +- 4 please tell me. I am interested. Heard of Compton and NPL, but would like to see hullform and applicability before buying papers.
  11. fcfc
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    fcfc Senior Member

    The author of the program , Leo Lazauskas , is sometime there. He may correct all falses assumptions and mistakes I have made about it.

    Forgot wave breaking.
  12. brian eiland
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    brian eiland Senior Member

    PowerKeel & SteadySailer, Motorsailer

    Hello Mark,
    Stick to you guns lad, I like this concept of a marriage between the PowerKeel hull concept and Paine's Steadysailer design. It certainly offers the 'sailing alternative' to Dashew's FPB concept.

    And I think we will see more of these long-range passagemaker ideas evolve as the new world order of fuel prices become reality.

    Regrettably I don't have much time to join the discussion right now as I get ready for a summer to Thailand. I'll be keeping track of the discussions.

    And just to add more spice to the discussions I will reference this subject thread on a few other similar discussion threads.
  13. Stephen Ditmore
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    Stephen Ditmore Senior Member

    On the subject of improving semi-displacement efficiency:

    Multihulls are generally lower resistance than monohulls in the semi-displacement speed range. I'm happy to see some trimaran motor yachts finally being built.

    The U.S. Navy has recently made progress in this area - in large measure (where monohulls are concerned) owing to the work of Dominic Cusanelli on stern flaps. See:

    Another guy currently living in Virginia (but originally from England), David Giles, has done work at MIT on semi-displacement hulls in the course of designing the FastShip container ships.

    This 25' launch represents my first attempt to design a better semi-displacement monohull: http://www.boatdesign.net/forums/attachment.php?attachmentid=74&d=1015885847
    The idea is that a narrow forward hull is followed by a perfectly flat aft portion, so the wave being pushed up in front of the flat aft sections cancels the trough behind the bow wave. I see the flat being at about a 4 degree angle of incidence at running trim (If memory serves it's at 2.5 degrees as drawn -- assumed static trim). I've drawn a later version, moving the helm to centerline and locating the engine under. This is to keep the center of gravity farther forward than on a planing boat.

    The jet drive shown would not be the most efficient drive for semi-displacement speeds. I'd like to explore using a Van Cappellen nozzel I saw advertised a while back, but that company appears to be focused on accoustics consulting now... so I don't know if they still make it. Schottel's products seem to be mega-pricey. Perhaps the Vovo IPS drive is an option... but it seems awfully vulnerable.
  14. yacht371
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    yacht371 Yacht Designer

    Flowmocean hull

    Note the similarity in principal to the FlowMocean hull patented by Peter van Diepen. The aft chines in this case actually turn downward. This type of hull operates at low trim angles, planes early and stays on plane at quite low speeds. It has lower resistance than a vee hull up until quite high speeds.


    Grahame Shannon

  15. Stephen Ditmore
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    Stephen Ditmore Senior Member

    Thanks, Grahame. You bring a lot of experience in this field. Glad to have you with us.
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