Help With Economical Semi-Planing Designs

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

  1. Pierre R
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    Pierre R Senior Member

    I am just not sure I buy anything said about semi planing hulls anymore. I have not seen anything yet that lives up to the hype. They seem like motorsailers to me. Not good displacement boats and not good planing boats. I have not seen fantastic fuel savings in the hump range of speed over properly designed planing hulls. The only thing semi displacement seems to offer is a good ride and controllability in the hump range of speed so most people with a semi displacement hull operate in that speed range.

    Convince me that I am wrong.
     
  2. SAQuestor
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    SAQuestor Senior Member

    Nah. I'll pass. Thanks for the offer.

    As an aside, this is certainly an interesting way to enter a conversation that has been on going for over 2 1/2 months and has a 150 pretty informative posts from folks that at least have an open mind.

    But, different strokes for different folks.
     
  3. kengrome
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    kengrome Senior Member

    Hi Leo,

    I checked all the pix in the links you posted, and this the best view of the bottom that I could find:

    http://newimages.yachtworld.com/1/6/2/1/3/1621393_1.jpg?1168964118000

    ... but of course this shows absolutely nothing useful to a person who wants to learn more about designing boats that run efficently at low-end planing speeds, or semi-displacement speeds, or whatever you want to call it when a boat runs at 12-20 mph or so.

    It looks like I'm not going to learn anything new by trying to see the bottom of this particular boat. That's fine, I'm happy to work on new designs based on Atkin's tunnel-stern Seabright skiffs anyways. At least I can understand how they work and how to build them ... :)
     
  4. Pierre R
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    Pierre R Senior Member

    You are assuming that I have a closed mind and that in itself signifies a closed mind. The question was genuine.

    I have two degrees in engineering and make my living as an inventor. I cannot afford closed minds. I am in the process of thinking out what I want for my next boat.

    The statement that you made that raised and eyebrow was "The genuine semi-planing hull, however, performes as well at dead slow as at full throttle."

    Performs well means what to you? It tracks nice or delivers the goods on efficiency at S/L ratios of 1.0 to 1.25? What does that mean. Performs well at full throttle. Does that mean that its great in downwind swells to the equal ability of a properly designed planing hull or does that mean it delivers good efficiency on fuel?

    You hang the shingle out as Naval Architect and therefore have some sway with me. Most boats sold today have hull designs as an afterthought. The marketing department hates hulls and naval architects. They get in the way of great interiors. Those same folks market these as semi displacement all the time. It means nothing to me. You come along and claim that there is a true semi displacement hull just like a true displacement or planing hull and didn't appear from under a rock.

    My guess is that 98% of the hulls out there are damn poor designs as far as what I would consider delivering performance.

    I looked at the Saga web site. Frankly there are things I liked very much about the boat and things I think were more for marketing an interior with safety as an afterthought. I realize you have to sell boats and I realize the general public thinks they must take their house afloat with them.

    If I want all the convinences of a house I will stay in my house. First and foremost I want a boat that performs well throught its designed range of speed in all conditions that is pure fun to operate. Damned all the convinences of home. Let me move anywhere on the boat and work on anything on the boat with ease and give me a fun quality built boat to operate.

    I have actually got to be a naval architect's dream client. Are you sure you don't want to take another shot at it instead of dismissing me as another critic?
     
  5. Pierre R
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    Pierre R Senior Member

    On the contrary. I think this picture does raise some eyebrows with me.

    I don't see much of a keel under the forward two thirds of the boat yet I see a substantial short keel and skeg protecting the prop and rudder. I see a rudder that is large by semi displacement standards and looks well balanced.

    These attributes would indicate to me that this hull would be more nimble at planing speeds than most semi displacement boats on the market and that translates into fun. I don't want a boat that handles like a 1974 U-haul at planing speeds. This is certainly different from what I am use to seeing.

    Given what I see as reasonable but high AB ratios up front I would opt for a bow thruster to take away the frustration in tight marinas with wind and current.
     
  6. SAQuestor
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    SAQuestor Senior Member

    Pierre - in the quote above I have highlighted 3 words that are erroneous. I DID NOT make that statement - I merely quoted from the Saga web site.

    Actually, when a newbie comes into a forum with "guns ablazing" making demanding statements like "Convince me that I'm wrong" and then tops that by attributing something that was not written by me... and then adds high octane gas to the conflagration by spouting about engineering degrees and supposed "inventor credentials". Seems like all we have is an individual that likes to set up straw man arguments to prove their own "superiority".

    I once worked with a fella that was a legend in his own mind. Seems like another one has revealed their true colors.
     
  7. SAQuestor
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    SAQuestor Senior Member

    I can't disagree with you Ken. As I said in my initial post, about all the information I have found is mostly marketing hype.

    But it appears to me that the hull form is a deep vee in the forefoot and then the bottom shallows out to a moderate vee from midships back. The deep keel seems to be kept with a bit more protrusion to protect the wheel and rudder.

    As other folks have pointed out, that forward section seems to be a key element in getting these sorts of hull forms to be sea kindly.

    But still, these Saga boats appear to need those several hundred horsepower engines just like many others of their modern ilk. So please do keep on exploring your Seabright hull forms. Maybe we'll get away from mere speculation and informed commentary to where we actually have some empirical data. Gosh, what a concept. Data. How's that go? My Kingdom. My Kingdom for some data... :D Oops, maybe not quite that way. ;)

    Best,

    Leo
     
  8. Pierre R
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    Pierre R Senior Member

    Well SAQuestor it seems that I may have missunderstood your post. Perhaps I should have wadded through the 10 pages of posts before responding.

    I am sorry to have fanned your flames and will not request you opinions again.
     
  9. kengrome
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    kengrome Senior Member

    Maybe some of the hype is true, but it's hard to tell these days when everyone's boat seems to be the answer to everything -- at least according to the marketing talk.

    Right, and this is common to what ... hundreds or thousands of planing power boat designs? Nothing new here so far ...

    I wouldn't even call that thing a keel myself, although that's apparently what the marketing guys are calling it. To me a keel is a very long protrusion from the bottom of the boat, not a short thing like that "skeg" they call a keel. Then again, maybe there really is a keel under there somewhere -- but without decent pictures it's all just theory and conjecture.

    My take on this is that you need to keep the forefoot "in the water" if you want to deal with rough or choppy conditions effectively. By insuring that the bow cleaves the water nicely, the bottom will never present itself to a wave, and it will be impossible for the boat to "pound".

    The problem with this approach is excess spray when compared to a planing boat that runs bow-high and skims over the surface on the aft half of its hull bottom. But I think spray can be mostly controlled by the intelligent use of spray rails located in the proper position to prevent water from climbing too far up the hull sides.

    I intend to do just that. At least with the Seabright hull form I can understand how and why it can be efficient. I'm not claiming that these hull forms are efficient -- although Robb White certainly thought so -- but if they are I won't have any question why, that's all.
     
  10. fcfc
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    fcfc Senior Member

    Searching with google, I have found the powering option of their lastest offering (the 310, available as semi displacement only ), is a volvo D4-225 or D4-260.

    I find this model surprisingly close to http://www.nimbus.se/modelPageCoupe2007.aspx?pageid=686 . Same powering, about same external size, about same layout.

    Top speed of the 320 coupe is advertised as top speed 24 kts with 225 hp and 26 kts with 260 hp.

    In one page the Saga 310 is advertised as cruise speed 20 kts, on the other page 14-24 kts.

    The 320 coupe is nowhere classified as a "semi displacement" or "semi planning".

    So I guess this term for the Saga 310 is just pure marketing blurb to differenciate from competitors.


    BTW, when you try to read in D GERR propeller handbook entry for 10 000 lbs / 260 hp (38 lb/hp), you are outside the scope of displacement/semidisplacement boats (1000 lb/hp to 50 lb/hp) . But you are a bit below the middle of planning boats (60 lb/hp to 2 lb/hp) .
     
  11. FAST FRED
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    FAST FRED Senior Member

    "When accelerating at intermediate speeds, however, most planing hulls experience a rather pronounced "resistance hump" combined with steep trim angles and unpleasant wake wash. The consequence is that the driver normally will try to avoid the "hump" by passing it as quickly as possible either by speeding up or slowing down.:

    This Hump is the product of the stern going down as the bow begins to rise .

    With the ATKIN inverted V aft , the pressure created by the prop fills the aft section , and the boat remains level.

    The bow stays level as the ENTIRE boat rises , as it gains speed.
    The only "problem" seems to be when too much power is used , and the pressure raises the stern too high , depressing the bow , causing wonky steering.
    As stated the solution seems to be forward chines to lift the bow as much as the stern , or perhaps my , suggestion of shrinking trim tabs to lessen the accelerated water lift astern , maintaining the stock trim.

    The great sea worthy ability reported for these boats probably comes from the fact that the full hull is still mostly immersed at speed, so slamming is eliminated.

    There might be some extra fuel burn from the extra wetted surface (compared to a true plaining boat) but the soft ride would allow higher speeds at lower G loads . An OK trade to me.

    There might NOT be extra fuel burn as all the water the prop gets was accelerated before the prop increasing the prop efficiency to pump already accelerated water aft. The boat might NOT be plaining on its stern wave as some guess, but simply more efficient at recycling energy already paid for.


    FF
     
  12. tom28571
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    tom28571 Senior Member

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  13. kengrome
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    kengrome Senior Member

    Hi Fred,

    You've mentioned this before but I still do not understand what you're talking about. Can you please explain how the water gets accelerated BEFORE it reaches the prop?

    Are you referring to the suction created by the propeller, which is nothing unusual since all propellers do the same thing?

    Or is something else going on here -- something totally independent of the propeller's suction, and possibly unique to this particular hull design -- that accelerates the water in front of the prop?
     
  14. SAQuestor
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    SAQuestor Senior Member

    Tom did something strange to hide his reply from being quoted directly, but copy and paste works real good too.:p
    Indeed. As much as I'd like to think that one of these Atkin Seabright hulls will be built by someone, reality does rear it's ugly head.

    In this thread, fcfc slaps me around soundly with: "But there will be no comparison possible for resale value in future time."

    This concept of resale value was reinforced today when perusing the YachtWorld listings. How's this one? I don't know who the builder is, but I would suggest that the build quality and fit and finish is comparable to what I would be able to produce. I've gotta believe that the builder has a lot more toil, trouble and sweat into that boat than $39k (eventual actual sales price probably much lower) will ever return. The sales price may just about cover the materials used. Sad. And yes, I know it's a sailboat.

    But for the same $39k - how's this semi-displacement power boat?

    And since I am comparing wooden vessels here, how about this selection at about $39k asking price?

    But going back to fcfc's admonition, how about these fiberglass power boats at about $39k? Certainly not a lot to choose from, but representative of what is available at that price point.

    My point in this post is to reinforce Tom's point and fcfc's point.

    Tom's is that we don't have any data. And fcfc's is that even if we did have good data and someone built one, the resale value would be hard pressed to allow us to recoup our materials costs.

    So we can speculate all we want - and sometimes that's interesting too - but if and when the $$$ fly from the bank account, we'll be able to see what there is to see regarding actual performance of boat bigger and heavier and much more expensive than Robb White's Rescue Minor. Until that time, I agree 100% with Tom.

    Best,

    Leo

    Then I run across these for a few $$ more than the $39k mentioned above. These also fit in with fcfc's idea.

    32' Vinette Custom Trawler

    40' Vinette Steel Hull Trawler
    29' Prairie Coastal Cruiser Sedan
     
    Last edited: Jun 24, 2007

  15. Excalibur
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    Excalibur Junior Member

    Hey All,

    I live behind a VERY restrictive firewall. I can see US based YachtWorld pics, but I can't see any that are on the server that carries the Saga hull pic. I don't know why the Navy blocks it, but they do. Would someone please send me an email with the pic attached? I'd love to compare it to my 40 year old Chris Craft hull. My Sea Skiff is a true semi displacement hull. The best pics I have of it are Here and Here . You can almost make out the keel that extends 2/3 of the length of the hull from the point where the hull curves up to the props. She's 38 feet, and I've seen the displacement written as anywhere from 17,000 pounds to 22,000. I'm pretty sure it's over 20,000. 700 horsepower. It cruises (3,000 RPM) at 14.4 knots, and tops out at 24.8. What makes me characterize it as semi is the fact that bow rise is undetectable from either helm and it cruises with the forefoot still in the water. I personally like the characteristics of the hull; a very smooth ride, no "hump" and an ability to cut through chop without pounding. But she is certainly wet! Spray to the flybridge in 2 foot chop. A maxim in the aviation community is that there is nothing more worthless than altitude above you or runway behind you. I would say that the ability to run 30 knots only in smooth water runs a close second. SA, one of my winter projects will be the addition of a good fuel flow meter to my gauge set. Engineering data will be taken! In addition to an FF meter and a GPS, what other data gathering instruments are needed for a decent report? Will empirical observation and published reports coupled with backup runs be enough for weather and tides? How many dimensions should the graph have?
     
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