Help With Economical Semi-Planing Designs

Discussion in 'Powerboats' started by SAQuestor, Apr 5, 2007.

  1. kengrome
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    kengrome Senior Member

    I presume that this was a test to determine the highest speeds you could obtain in these two different configuration. Do you happen to know what your fuel consumption (mpg) was in each of these situations?

    It's an interesting test that clearly demonstrates the fact that you can get higher efficiency with more weight in the boat if you ALSO adjust the boat's attitude or pitch angle to improve its planing performance by reducing wetted surface area. It's like testing two different boats when you think about it.

    In a heavier cruising vessel, fully loaded and ready to go on a trip that might last months, adding more people aft like you did might not change the trim angle enough to get the bow out of the water -- in which case the efficiency will undoubtedly be lower instead of higher.

    It would be interesting (to me anyways) to see how your mileage differs at a constant cruising speed of say 15 mph with the two different displacements you mentioned. A test like this might be more interesting to a person like Fred who will likely be cruising for hours at a time at these lower speeds that everyone likes to refer to as "semi-displacement" or "semi-planing".

    They don't mean much to me anyways because they are not real-world numbers like miles per gallon ... but this is my problem not anyone else's ... :)

    I agree that taking the boat's use into consideration is important to figuring out how to develop the most efficient hull for that particular use. After we define all our other parameters such as length, beam, height, displacement, bottom strength, draft, beachability, seaworthiness, acquisition cost, etc. the owner's primary concern is probably going to be:

    "What kind of hull design can I choose that will give me the highest mpg most of the time?"

    So after defining a "concept hull" that meets all the other requirements, fuel efficiency is the one thing I am most interested in learning how to optimize.
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  2. tom28571
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    tom28571 Senior Member

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  3. SAQuestor
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    SAQuestor Senior Member

    Although we've been mostly talking about performance over the past few weeks, a commenter in the Dutch Barge thread said in message 120:

    I think that this is an extremely important point. I've built a house, a barn and several outbuildings and in each case the enthusiasm waned about 2/3 of the way finished and it was a chore to regain the focus necessary to complete the project. Why should it be any different with a boat build?

    And I doubt that I am alone in this. My co-worker just had a house built, but he did the painting (inside and out), landscaping, driveway, built the separate garage/shop and a garden tool storage building. His enthusiasm (and his spouse's enthusiasm) waned when the painting was weighing heavily on the build schedule. He'd drag into work after being up 'till 2 am painting the interior with the prospect of days (and nights) more to go.

    I know, to save a few thousand he volunteered and should have understood what he was getting into. Regardless of that, he wasn't as "pumped up" for the next phase of the whole project as he could have been and that waning of enthusiasm delayed some of the other aspects that he would have liked to have done sooner.

    Perhaps not all folks are like David and I, and can better maintain a steady enthusiasm level for the hundreds (or even thousands) of man hours it will take to see a major home boat build project through to completion.

    IMO this should be something that we all take into serious consideration as we attempt to decide whether and how to proceed.

    As always, YMMV.


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  4. kengrome
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    kengrome Senior Member

    Actually I think most backyard boat builders have the same problem staying focused and making regular progress. Other things in life will 'take over' and become more important if you let them ...

    One guy once told me he just considers it a responsibility like a job. He goes to the garage (shop) on time, works on a schedule, and leaves on time to go home enjoy the rest of his life -- without any feelings of guilt or frustration for neglecting the project. He said he is happier and gets more done this way.

    Even if you did this by scheduling your boat work for 3 or 4 hours every Monday, Wednesday and Friday you would still make regular progress, provided you do not let other things take priority over your boat building time. That's the key to success here -- not letting other things cut into your scheduled boat building time.

    One way to have the exact boat you desire without all that work -- and still keep it affordable -- is to hire a guy who owns an overseas boat building shop and have him build it for you. Materials and labor are so much cheaper in Asia that you probably won't pay any more in the long run, even after shipping costs are included.

    But even if it costs a little more, you could be using your new boat months or years earlier this way -- rather than still building it. For folks who prefer to be on the water rather than than in the shop this seems like an attractive option. There's nothing wrong with having "just the hull" built and finished for you either. Then you can install all your personal goodes and do the final outfitting yourself after the boat is delivered.

    It would be even more interesting to find an overseas shop that would let you go there on a "tropical boatbuilding vacation" in the middle of winter and take part in the construction personally. Or instead of getting your hands dirty, why not just sit in the shade of a palm tree sipping one of those drinks with the little umbrellas in it as a cute local girls fans you to keep you cool ... and simply "inspect and observe" while the local craftsmen do the work of building your new boat for you!

  5. Willallison
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    Willallison Senior Member

    There's really only two reasons why people choose to build a boat. The most obvious is that they can't afford to have someone else build it for them. The other is because they enjoy the process. This latter group are far less likely to fall into the enthusiasm abyss.
    Sadly, for every story one hears about a fabulous boat built overseas, there's another about the horror of discovering that all that cheap teak is full of borers or something. Also, custom builders usually like to be involved in the process, so an overseas build is less attractive. Though visions of the cocktail under a palm tree style oversight sounds good!
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  6. kengrome
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    kengrome Senior Member

    Actually they can in most cases, they simply need to hire an overseas builder. But most people seldom think beyond their local areas so the thought of hiring an overseas builder never even enters their minds. Or they don't trust the builder when he is so far away.

    That's for sure. When you enjoy the work you usually remain interested and motivated.

    I treat my wood -- even the plywood -- with Ethylene Glycol and Copper Naphthenate. American builders could easily do the same and their boats would never rot or be eaten by bugs, but I don't think very many of them bother, and I'm not sure why to tell you the truth ...

    There are always a few greenies around who whine about the dangers of using these and other chemicals, but as far as poison treatments go these two are some of the least likely to endanger you. Once they have been applied to the wood EG and CN are basically benign to humans, yet they are exceptionally effective at doing exactly what they are supposed to do, which is to prevent the "bad things" from destroying your wood.

    Chuck Leinweber at asked me to write up the story/dream of my Tropical Boatbuilding Vacation concept so he could publish it in his online magazine. I haven't done it yet but I probably will some day since it does sound rather appealing, especially in the middle of winder in the snowy and frigid north country.

    There may actually be a big enough market for such a unique niche offering even though it is probably one of the smallest niches in the world. I don't think anyone else is doing it, and it wouldn't take more than one or two clients a year to keep it going if the boats they choose to build are big enough. I'm still considering doing it some day ... :)

    Until then I hope to keep myself busy designing and building new tunnel-stern boats inspired by the Atkin team, mostly because I think there is now -- and will continue to be in the not so distant future -- a growing demand for "go slower, go anywhere" boats like these.
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    FAST FRED Senior Member

    "I treat my wood -- even the plywood -- with Ethylene Glycol and Copper Naphthenate. American builders could easily do the same and their boats would never rot or be eaten by bugs, but I don't think very many of them bother, and I'm not sure why to tell you the truth ..".

    One reason is many US wooden boats get a coat of GRP to keep the worms out which works fine.
    BUT if a boat is slathered inside with diesel , mineral spirits,or antifreez (the vehicle in most preservatives) the stuff will run alongside even "watertight" clench nails and lift the glass bond under the nail head, in time.

    Just wood is almost impossible to sell in the southern US, as the boat literally dissolves before your eyes.
    The wooden canoe folks store their boats in garages , a wood boat in FL water is a VERY temporary thing.

    "I'm still considering doing it some day ..."

    Problem is most folks want to see many completed boats , that are standing up and performing well before plunking down cash.
    A start up is a VERY high risk for a buyer , esp in a country with uncertain courts strange legal system etc.ect.

    "go slower, go anywhere" boats

    Speed is very relative, while some "need" 40+k cruise , most of these boats run out of fuel in 300 miles at best , unless really large.

    I figure high teens will actually get someplace , and 5mpg is loads more reasonable than .5mpg.

    1000 miles is only 200G of fuel , a weight to be sure , but not so large as to be difficult.

    The current boat is powered by a 6-71 giving about 15 or 16 hp for each gallon of fuel.
    Hopefully a new boat would be 1/4 the weight ,6-1 L/B ratio instead of 3-1 , and the engine should be capable of 22hp per gallon of fuel.

    A 2007 engine will probably never have the robustness and reliability of the DD 6-71 ,1936 design , but it should weigh 1/3 the weight.


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    FAST FRED Senior Member

    In the #67 issue of Pro boat builder, I stumbled on an article about water jets. The author is Richard Akers.

    I'm reviewing all the back issues I have looking for construction and other ideas I can steal.

    On page 67, is a discussion of how the water jet works,

    "In simple terms, water flows under the hull, enters the jet inlet, receives a momentum push by the pump and exits through the nozzle. A whole lot of the magic is high performance water jets is in the inlet design. Early jet inlets were just holes in the side or the bottom of the boat. The water would flow under the hull and make a right angle turn into the inlet. Much of the water in the inlet was coming from the still water outside of the boundary layer under the hull. The jet had to accelerate the still water to the forward speed of the boat, then pump it out the nozzle fast enough to go in the other direction, relative to the still water. With this design, the thrust is equal to the water -mass flow rate times the difference between the jet velocity and the ship velocity. We would have been better off with a propeller.

    But the water directly under the boat is not still. The boundary layer of water, which starts at the bow of a high speed vessel and then thickens going aft, varies in speed from the still water outside the boundary layer. Although it is quite thin - at the transom of a 40ft boat going 25 knots it's about 4.2" thick- there is a lot of energy tied up in the boundary layers accelerating water, and we can derive significant benefits from sucking the boundary layer out from under the boat into the inlet (the hull bottom over the inlet itself doesn't have a boundary layer). If we angle the inlet to grab water from the boundary layer, we recapture the energy that would have been lost in a poorly designed inlet because some of the water going into the inlet has already been accelerated to boat speed.

    For mathematically inclined readers naval architects describe the efficiency of a complete hull - propulsion system by defining a thrust - deduction fraction (t) and a thrust-wake fraction (w).

    Hull efficiency - nH = (1-t)/(1-w).

    The mid range values for a typical boat reveal that the propeller driven boat will have a hull efficiency of 92%, while the water jet driven boat will have a hull efficiency of 110%.

    The fast propeller driven boat looses energy because of prop and hull interactions while the water jet driven boat recovers energy precisely because of the inlet and hull interactions."

    As the water jet is only obtaining energy from a square ft of inlet area , and the box keel with reverse deadrise is getting almost ALL the energy from the hill in front of the prop, could this be the explanation for the reported higher efficiency?

  9. SAQuestor
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    SAQuestor Senior Member

    A matter of Perspective

    As most know, there are just shy of a zillion different places where one can exchange information about boats – or any other subject – on the Internet.

    One such place is an email list called “Trawlers & Trawlering”. You can Google it up if you’re interested.

    Reading that list is generally informative because many of the posts are written by folks that are doing instead of wishing. Not all, but many. If I’m not mistaken Fast Fred even posts there. :D

    A recent thread there was about Downeast Style hulls. And earlier in this thread we had a bit of a conversation about those style hull forms before we got off on Seabright skiffs and tunnel hulls.

    The original questioner asked, “Relatively few pleasure boats have been built on this platform and I am wondering why.”

    As we have determined earlier in this thread and in other threads on this site, I wrote back, ”As for why more pleasure boats aren't built on this hull form? Simple answer is they don't do well tied to a dock at the marina. They are great sea boats, but when compared to other 'name brand' boats that offer 2x or even 3x the actual living space you'll find in a proper downeast hull form - there's no contest for most folks that want a boat to entertain friends aboard and not actually go to sea in a northeast'r.”

    Other various conversations took place and then this came across the list-serve.

    “We have an '88 Nauset 35, built on a Stillman hull. (other stuff omitted) Our Cummins QSB 380 repower cruises her at 14-15 knots under 11gph. We do a lot at 8.5-9 kts well under 5gph.”

    Gulp! :eek: I then wrote back and asked if they had ever weighed the boat – the reply was a survey estimate of about 18,500 pounds. For arguments sake here, let’s round up to 10 tons.

    So… A 35’ boat weighing 10 tons consumes (at today’s prices about) $35 US to go 15 NM (28 km). My spreadsheet says that’s about 3 /4 gallon per nautical mile – or $2.33 US/NM.

    So… 9 knots at 5 GPH – That equals about $16 US to go 9 NM (17 km). My spreadsheet says that’s about 1.8 NM/gallon – or $1.77 US/NM.

    Given that these folks are out on the Great Loop and at that 9 knots/hr speed for 10 hours a day they’re moving about 90 NM (103 statute & 166 km) a day. Their cost in fuel alone is about $160 per day. In areas where they can go their 15 knots “cruise” that per day fuel cost JUMPS to $350. And this doesn't factor in any generator running time.

    Holy Cow Batman! The first thing that comes to mind is the difference between having gobs of money versus just being in a “comfortable” economic situation. Personally, I can’t wrap my (raised poor) feeble mind around the concept of spending that kind of cash to pay for fuel for a single day. And yes, I know lots of people drive Hummers and big boats that don’t give this sort of expenditure a second thought.

    But I do, and that’s the primary reason that I started this whole thread. I want a reasonably sized boat that Momma and I can spend several months aboard seeing the same sights as the folks forking out $$$ hundreds per day for fuel while I do the same trip for comparative pennies.

    As an addendum to this, there are apparently some issues with the Trent-Severn waterway wherein there are a number of boats hitting debris and getting out of the channel or having some other calamity befall them. Lots of bent shafts, struts and wheels. So not only does having a large & heavy boat cost in terms of fuel usage, but it is also very expensive when a problem occurs that requires a haul out in the relatively remote areas of the T-SW.

    The point here? Bigger doesn’t appear to share a bed with economical. And yes, I recognize that my definition of economical may not be the same as someone else’s. A matter of perspective I ‘spose.



    P.S. I'm not criticizing the folks that have the big boats and their ability to pay for the boat/fuel/maintenance. It's just not 'me' is all.
    Last edited: Jul 25, 2007
  10. moTthediesel
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    moTthediesel Junior Member

    Leo -- I hear you!

    I feel just the same way and I have been thinking for some time about what boat would serve for that kind of economical cruising.

    What I've found so far is that there is, to my knowledge, nothing available on the market that meets, or even comes close to my criteria. So, for the second time in my life I'm thinking about undertaking a large boat building project :eek:

    It's no secret what kind of boat is needed for that kind of fuel economy, it must be long, narrow, and light. That, of course, means compromises, but all boats are that way, it's just that my compromises are different then those made by the designers of commercially available boats.

    I had been thinking of a beam less than 8', and a LOA of something less than 40'. I had never thought about container shipping until I read FF's idea's about it, but as luck would have it, that's just the size for a "boat in a box" too.

    My first thought was a simple canoe shaped hull, and while that would give the minimum wetted surface for a given displacement, there are some serious drawbacks to that shape. In order to have standing headroom below decks in a ultralight boat of this size, you would need either very high freeboard, or very high crowned decks. One of the things I find attractive about the Atkin tunnel hull SS is that it can get your feet lower in a hull that displaces a very small amount of water. Also, I think that the relatively deep center hull area would give a smoother ride in a chop and better directional stability to boot.

    I'm still just noodeling around, and a long way from lofting a shape, but I sure enjoy reading everybody elses ideas on the subject here, please keep this thread going!

  11. u4ea32
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    u4ea32 Senior Member

    I spent 10 days cruising Alaska in a chartered Nordic Tug 32, single diesel, "semi-planing hull" boat.

    Had a GREAT TIME!

    First tank of fuel, mostly at 2200 RPMs @ 11 knots:
    268 nm, 200 gallons, 1.35 nmpg

    Second tank of fuel, mostly 1800 RPMs @ 8.7 knots:
    396 nm, 200 gallons, 2 nmpg

    664 nm, 400 gallons, 1.66 nmpg

    We had glass smooth water almost all the time.

    My 32 Fountain sportfish cruiser in glass smooth fresh water
    gets the same mileage at any planing speed from 15 to 60 knots,
    with twin gasoline big clock engines.

    In rough water I need to put down my K-planes to have a smooth ride, and I get the fuel economy [!] down to the 1.1 nmpg that the Nordic Tug sees at 11 knots. Of course, I can still go any planing speed at that 1.1 nmpg, up to about 40 knots in rough seas.

    The difference: the Fountain weighs about 10000 lbs, the Nordic 32 weighs about 18000 lbs.

    With the fuel efficiency difference between gas engines (10 hp per gallon per hour) and diesel (20 hp per gallon per hour), its pretty clear that the Fountain is substantially easier to push through the water than the semi-planing Nordic Tug.

    Yet more evidence of my earlier statement: efficiency requires light displacement. All the other stuff is minor compared to displacement.
    Last edited: Jul 26, 2007
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  12. kengrome
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    kengrome Senior Member

    Leo, can you tell us what size this boat might be? Do you have an idea in terms of length or displacement or something?

    If a tunnel-stern Seabright design protects the underwater runnning gear from this type of damage, it seems like a hull that would be good for the Great Loop, provided it meets the rest of your needs too.

    One of my ideas is to develop a very good hull for folks who want to do the Great Loop ECONOMICALLY, then build the boats and lease or sell them. I don't have the funds for such an endeavor, but who knows when an angel might come along to support me? Anyone with half an interest in this concept would certainly recognize the value of having me build the boats overseas, that's for sure.

    I think the tunnel-stern Seabright skiff is the answer here. It protects the prop, shaft and rudder very well, and its (theoretical) efficiency would be great for economical cruising. I want the typical cruising couple to be happy with the boat while living on it for months at a time, but I do not want the fuel costs to be so high that no one can afford to use the boats!

    I have questions about empty displacement, total displacement, maximum speed, cruising speed, dock and lock handling, and much much more . I know I cannot have all the answers at once, but to settle on the basic hull design concept with a rough idea of the length and beam of the boat means I can get started and make some progress while the other answers come to me gradually.

    Sometimes I wonder if it makes any sense at all to go for a semi-displacement hull when pure displacement hulls are much more economical. I even started designing such a boat based on the traditional non-tunnel Seabright skiff bottom. But then I go back to wondering if anyone these days would actually be satisfied to plod along at displacement speeds? If the goal is to "enjoy the ride" rather than to "enjoy the ports of call" displacement speeds might be enough. But I still think people are spoiled enough these days to demand higher top speeds if/when they happen to feel like it.

    Am I wrong about this?
  13. SAQuestor
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    SAQuestor Senior Member

    In general terms:

    Trailerable behind a HD American pickup.

    LOA - 9m -ish
    overall beam <2800mm
    Total height from keel to PH roof <3500mm (to meet federal trailering height restrictions.)
    Displacement - <3650kg
    Permanent accommodations for a couple - arrangements for a guest couple for 2-3 weeks.

    Good-e-nuff Ken?

  14. Pericles
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    Pericles Senior Member


    IIRC there was mention of Down East hull forms. Your last post listed dimensions that reminded me of two vessels with DE antecedents. They are the DE25 and the LB26

    The DE 25 will accommodate two guests, the LB is a couple only boat, but either will provide an easily driven craft with the ability to speed up if necessary.

    There are many other sets of plans for sale and even some with Carolina flare and flair. :D :D

    I've chosen the LB26 for building in BS 1088 Okume and West epoxy, both of which are easily available here in the UK. It's the RCD certification that's going to cause the most grief.


  15. SAQuestor
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    SAQuestor Senior Member

    I'm curious as to your reasoning.

    From the RCD:

    "craft built for own use, provided they are not sold for a period of five years from completion;"

    Perhaps there is more to the RCD that I haven't seen, but the above is a pretty strong statement IMO.

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