What makes a "Blue Water Boat"?

Discussion in 'Boat Design' started by alex folen, Feb 5, 2009.

  1. Loveofsea
    Joined: Jan 2007
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    Location: Southern California

    Loveofsea New Member

    Landlubber, I wish! (check your math--)

    i don't run like i used to, but my trips average over 170nm per..

    Here is a satellite pic of my stomping grounds:


    See that island that sits all by itself, the farthest offshore? I've made that run 131 times and have spent 213 nights anchored around that god-forsaken place that is affectionalely referred to as Jurassic Park--more commonly known as San Nicolas Island, The Island of the Blue Dolphins...

    I do underwater photography as a hobby, so for me it is all about being able to get to those offshore destinations. I searched far and wide for a real blue water boat that would fit into my garage, but to no avail..

    As you boatbuilders already know, that's why we build our own~!
  2. Tcubed
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    Tcubed Boat Designer

    Apex, That jet pump drive is news to me. I do read german so maybe you can fill me in on its pros and cons?

    I presume the jet nozzle collar is rotatable 360 deg so you can aim the thrust any way?

    However it seems to me that with all the accelerations that the water needs to undergo through the unit it must not be so efficient. By efficient i mean energy imparted to fluid/ shaft power.
  3. apex1

    apex1 Guest

    You are right, it is not as efficient as a shaft prop arrangement, but if one counts every single point as previously mentioned, it comes very close. Main advantages are cutting the hull friction by approx. 40%, and reducing drag by eliminating the prop and prop well, another (different to measure) is the reduction of hull induced resistance due to steering the vessel. (You must not stem the whole ship against the waterflow when changing course).
    It´s just a few % here and there but sums up noteicable. As does the cost! All in all I calculated twice the price as for a single engine CPP arrangement.

  4. Tcubed
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    Tcubed Boat Designer

    OOps i meant to say do not read german.

    40% hull friction reduction ? How are you arriving at that number? I am not being judgmental , just want to know. Seems to me the parasitic drag of a well designed hub+shaft is an order of magnitude less than what you say.

    As for forcing laminar flow a normal prop at the stern favorably affects the pressure gradient as well...

    If it has 360 deg thrust capability then why are you putting them midships? Is it so you can more or less move sideways? Control would be almost impossible though. Seems to me more advantageous to put them aft (maximize benefits to BL) for control or even since you have one either side, staggering them one about 25% LWL aft and the other at say 85%. It would be asymmetrical but that does not matter much and it would give you excellent maneuverability.

    Maybe i'm just not understanding very well. Perhaps we should start a new thread for this topic as well...
  5. apex1

    apex1 Guest

    Yeah, sorry, I was´nt precise enough, i reckon. You completely missed the subject. Please go to the website again (is completely in English) and have a deeper look.
    You´ll understand then, the flux LEAVING THE PUMP is in contact with the hull about 1 mtr behind the outlet, forming a laminar flow, wich naturally reduces friction to about zero.
    And yes there are two of them, if not, how would it function?
    I do´nt like to annoy you, but please try again to get the "Pump jet" principle in.

  6. mydauphin
    Joined: Apr 2007
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    mydauphin Senior Member

    One thing I ran into in using a jet drive in small boat I modified was the lost of displacement, by that I mean the water in drive made my boat heavier and sit in water more than just weight on drive. Here are some weird numbers for you. On a 14' boat, the jet drive weight 35lbs, water within drive 35lb, positive displacement if drive wasn't there 35lbs. Yes, I save a rudder drag, a prop and less draft. But I made boat handle like it had another person, didn't turn as well, accelarate as well and I believe the air intake adds drag .... I think jet drives are not as great as many proponents would like like you to believe.
  7. apex1

    apex1 Guest

    Yeah...you´re right.
    But we are´nt talking about Jet´s here. We are talking about "Pump Jet", a complete different world, and about a 80 ton vessel! The few cups of water in the pump are not worth talking.

  8. marshmat
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    marshmat Senior Member

    Most data sheets I've read on jet drive units, from most manufacturers, give both the weight of the pump itself and the weight of the entrained water.

    I had a look through the entire English side of Schottel's pump-jet pages and their application brochure. They indicate that the jet outlet flow is angled 15 degrees downward from the hull, so I'm not sure where the idea of "flux leaving the pump is in contact with the hull about 1 mtr behind the outlet" comes from. Could you clarify?

    Another thing- if the flow does bend upwards and roughly follow the hull (which it very well might, if the boat is moving somewhat), and if the jet outlet velocity is 10 m/s, the Reynolds number of that flow a metre behind the jet is on the order of 10^7. So even at fairly low power, the boundary layer of the jet wash along the hull underside is most definitely fully turbulent. At higher speeds it would become more turbulent.

    I can see it being a very versatile unit for workboats that are often manoeuvring at low speed, in shallow waters, or with awkward loads. And, except for its 360 kg weight, the SPJ15 looks like it could be very appealing for a sailing auxiliary (no grounding damage! no shaft/strut drag under sail!). But I'd have to see a lot more test data in order to believe that it would have an efficiency advantage.
  9. apex1

    apex1 Guest

    Hello Matt,
    thank you for your valuable input.
    First I should straighten out something:
    I guess I have not expressed clear enough in one of my former posts that I am NOT a advocat of this system. It seemed to be of some advantages if compared with a classical single engine fixed prop, thats all. I still think I´ll give it another look in the future, but gave up for now, to follow the idea a year ago, for some very simple reasons, it´s at least twice (I reckon even more) the price as a single engine CPP system, and some of the parameters are still just estimations.
    The figures, facts and estimations I posted, have been collected over a period of about 12 years.The sources are, the manufacturer, the HSVA Hamburg, a protocol of a two years operation of a 85 mtr. Coaster, some studies which I made in collaboration with a shipyard in Germany.
    The effect of a noticeable reduced friction (although astonishing) was discovered by HSVA. I have no reason (and shure not the knowledge) to doubt their test results.
    And, yes the outlet is 15° downwards, but the flow lays laminar at the hull after about one meter, even at slow speed.
    The advantages of a eliminated drag of prop and well, as of rudder function should be clear and self-explanatory. As is the maneuverability and the fact, that a genny diesel never sees any underload. Alltogether still not unattractive, but... see above. And it´s not that I refuse to reply to any further question coming up about this subject, it´s just too time consuming. And as long as I have no access to my collected data (I´m in Turkey for a while), I feel unable to provide proven data. And it would make me dubious to guess.
    So instead of developing this system further on here (which for me is riding a dead horse at present) I would prefer to come back to topic.

  10. TeddyDiver
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    TeddyDiver Gollywobbler

    Maybe starting another thread when you return to Germany? It opens whole bunch of possibilities.. like having multiple much smaller units around the hull to maintain flaminar flow constantinously..
  11. apex1

    apex1 Guest

    Why not, or posting in the "New propulsion systems" thread. But at present, I think, it´s easier to disgrace myself, than to provide proven info.

    So, what makes a "Blue Water Boat" ?

  12. GTO
    Joined: Jul 2007
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    GTO Senior Member

    Posting proudly, based upon total ignorance aside from armchair reading and analysis, I've decided a "blue water boat" is whatever "boat" makes it across some "blue water".

    Boat in this case being defined as any thing someone can at least hold on to and survive crossing "blue water".

    From Hugo Vihlen's 6 ft April Fool, to the 10' Yankee girl and upwards, it has occured to me that a "blue water boat" is whatever anyone wants it to be.

    If you are a tough sailor, you can sail across the atlantic in a 6 foot boat, or around the world in an open 19' Drascombe lugger. I've pompously decided that it is up to the people on a boat to determine if it's "blue water" or not, or even if it "is" a boat. How do we define "is"?

    Say I build a "boat" with a LOA of 4 feet, but a draft of 16' to hold all the necessary provisions, and drift with the gulf current from the USA to the UK. Is that really a boat or ... ?

    So in summary, I've decided that the sailor in question must define their "boat", determine if said boat, being built, actually meets their specifications, and then determine if their specifications were correct!

    I would say the Titannic was not a blue water boat, as it was designed to be unsinkable, yet it did indeed sink! Having failed its design specifications, it therefore failed to be a "blue water boat" as its owners, designers, and builders specified one to be.

    On the other hand, Webb Chiles' Drascombe Lugger was a perfect "blue water boat" as it carried him, mostly, around the world in the manner he expected it to carry him, i.e., half swamped on several occasions, and at sea level for one 2 week period.

    So there you go, there is the definition of a "blue water boat".
    Send me your gifts of gold, wimmins, and beverages (cold), and I will declare your boat a "blue water boat". :D
  13. alex folen
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    alex folen Flynpig

    Probably the most definitive and correct definition of a “Blue Water boat”, ..significantly eloquent and scientifically and spiritually exact in speaking. But every and ALL other definitions of a blue water boat have equal weight and value to me. I tend to agree if you can survive the seas on a floating log then that’s a blue water boat. Thanks fellows and gals.
  14. alex folen
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    alex folen Flynpig

    Whoops, Thanks GTO.

  15. alex folen
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    alex folen Flynpig

    Alabama! GTO? Yeah, right by the sea you is.
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