Pocket cruisers

Discussion in 'Powerboats' started by Guillermo, May 13, 2006.

  1. tom28571
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    tom28571 Senior Member

    Will explained it very well. It's a bit like deep V hulls. It's not coincidence that they are heavy, they have to be heavy in order to get enough of the boat in the water to make them fairly stable. At that they are not very stable at rest or at slow speed. Just looking at the lines of the Seabright skiffs, you could estimate the displacement required to set it on its waterline.
     
  2. SAQuestor
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    SAQuestor Senior Member

    Tom, that’s my understanding also.

    Since I don’t think that a planing or semi-displacement boat of this size and weight would be economical to run, a hull form like this would of necessity, be displacement.

    Your Liz design is nearly the opposite in that it’s light and planes at a low speed and thus is pretty efficient in its fuel usage. But these two types are really apples and oranges.

    Please see my response to Will and the attached jpg and dxf files.

    In my mind the whole idea of either a box keel or a Seabright hull form is to get the fixed machinery weights as low as possible and to utilize all the space between the wheels of the trailer.

    Best,

    Leo
     
  3. Willallison
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    Willallison Senior Member

    Leo - All this gets back to a debate that's been had a number of times on these boards - indeed Tom & I have discussed it on more than a few occaisions!:D
    It really depends on two things
    1. Whether you are after a boat that is trailerable or a trailerboat
    2. Whether you're confident in handling a very substantial boat trailer combination (and whether you want the hassle that comes with it).

    My family own a 26ft deep vee offshore fishing boat. It has a pair of 225hp o/b's and a combined boat trailer weight of around 4500kg dry. We have a Holden (Chevy) Suburban 6.5lt turbo-diesel to tow it with. Having towed this rig and others like it, from Tasmania to Queensland and back (about 5000 kms) on a number of occaisions, I can assure you that you don't just bolt the thing on the back of the car and take off. Tyres, bearings, brakes etc etc are under much greater strain than normal trailering loads and require fairly regular replacement / service. Not only that, you are somewhat restricted in where you can go with such a large rig.
    Having said that, we quite often use the boat as a day boat - launching and retrieving as you would any other trailerboat, but there is definitely more hassle involved in doing so. To the point where we have now had the bottom coated with a copper-based epoxy antifoul so we can leave it in the water for months at a time. (Interesting stuff too - meant to last 10 years... but that's for another time..)
    Not inconsequential in all of this is the investment required. In our case, the Suburban was purchased specifically to tow the boat.

    It also depends on what kind of boating you do. Tom's boat was designed to specifically meets his needs and objectives. It does that very well - so well in fact that others pestered him to the point where he now sells the plans to her. But her success depends to some extent on remaining light. I've no doubt the boat would still perform well in a heavier state, but with a comensurate loss in efficiency. That's not a criticism of the design, it's simply a fact of life with planing boats.
    Now if you are happy to doddle along at 6 knots or so, then you are quite correct, weight becomes less of an issue (setting aside the trailering thing). And for a 30ft boat 6 - 7 knots is about as fast as you'll be able to go before you enter the realm of diminishing returns. Naiad, with a top speed of 17 knots, is not a displacement boat.

    As far as keeping weight low goes, if you travel light, then there's less weight to worry about...;) I would be surprised of the initial stability of Naiad were any greater than Tom's Bluejacket 24. In fact I would expect quite the opposite.
    Standard trailers in Oz are built somewhat differently to those in the US. Here we just run an axle thru with a couple of hanger springs either side, so your boat would in fact be higher off the ground with a box keel.
     
  4. Zewe
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    Zewe Junior Member

    Thanks for the responses, Will, Tom and all. Perhaps I understand the point about trailerability now.

    In stepping away from the question of trailering facility, I come to another group of questions.

    Although they offer somewhat similar accommodations, the design Whio offers much lighter displacement than my estimate of the displacement of another of Atkin’s seabrights, a design called Jersey Blue:

    http://www.boat-links.com/Atkinco/Cruisers/JerseyBlue.html

    What are the pro’s and con’s of a heavier boat, within the context of this thread? I am assuming for these questions that both boats are built to form and spec’s that meet their design goals successfully.

    The lighter boat will be more fuel-efficient, but what is being given up by choosing the lighter boat?

    With regard to seaworthiness, would the crew be taking a larger risk in the lighter boat or the heavier boat, say, if they decided to make a crossing that would keep them out longer than the latest reliable weather forecast?

    Are the two boats actually so different that it’s apples to oranges? It seems like the design goals for each are not dissimilar, so these should be fair questions.

    The idea of using modern, relatively lightweight components and construction techniques to build an older design that was intended to weigh more seems to offer some room in the weight calculations for extra tankage, stowage, ground tackle and/or the occasional extra passenger. I wouldn’t suggest building the older design without the appropriate scrutiny/redesign by a qualified source, but it’s attractive to imagine some slack in the weight calc’s for a change. Am I naïve in letting my mind run in this direction?

    I hope I don’t come across as being flip or bombastic. I am learning a fair amount while musing about the content of this thread.

    Will - I typed this offline and posted it, and then read your most recent post. Your comments as they apply to my questions are duly noted...
     
  5. tom28571
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    tom28571 Senior Member

    Several others and I have been intrigued with a similar keel structure to the one you show. While I will never build such a boat, it does open up some possibilities. For me, one of the most interesting attributes is that it may be possible to cruise at speeds up to 20 or a bit more with high efficiency, low power and low operating expenses. I blue sky the thought of a very long canoe body (very high length/beam ratio) that assumes about 70% of the displacement. The upper body is of wide beam and only slightly immersed for form stability. The underbody could operate at the desired speed beyond the normal “hull speed” like a catamaran hull. The upper hull’s waterplane would be so lightly loaded in pounds/sq ft that planing would be possible with low power. The prop would be either tucked behind the underbody as you show or two props to either side, which would allow an even longer underbody. None of Bolger’s box keels seem to pursue this line of thought but Atkin may have had it in mind with some of his designs.

    One issue is how to fair the bow smoothly into both structures for good rough water performance. Your lines look unfair and suspect to me. I might expect a lot of noise along with some other negative characteristics.

    Such a hull might be a good choice for cruising but might also be too heavy to fit my weight limits. The launching ramps we encountered in Canada on a trailer/cruise there would have defeated the bigger and heavier boat and made some areas off limits without much time, bother and big bucks.

    In short, the compromises I made in designing the BJ 24 still make sense to us and I don’t envy the bigger boats at all.
     
  6. SAQuestor
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    SAQuestor Senior Member

    Will, you are exactly right. And I thank you for recognizing the difference.

    Being exiled here in south Texas (Really, I like the 5 or 6 months of 95° to 105° (35° to 40°C) temps.) we see thousands and thousands of wintertime “snowbirds” – seasonal residents as one friend calls them – that drag their trailers behind their ¾ or 1 ton PU’s from their frigid homes in Indiana or Wisconsin or ??? These folks might have a trailer as small as ~25’ and go up to behemoths that are 40’ or a bit more long and nearly 13’6” tall. Like this one as an example.

    All of these largest trailers seem to have 5th wheel hitches and tow just fine thanks. I had to go to our corporate office a few days ago from the R&D facility where I work – on the way back I passed a convoy of 3 of these enormous PU/trailer rigs. Each combo had a ~35’-40’ 5th wheel trailer being towed by a turbo-diesel PU – two Dodges and one Ford – and the couples in each were at least in their 60’s and perhaps, if the amount of white hair and wrinkles are any indication, maybe in their 70’s. The “suggested” speed on this portion of the freeway is 65mph (105kph) and all three were tooling right along at that speed.

    My point of this little story is that if they can navigate a 50’ long trailer/truck combo through city traffic what’s the difference if it’s my retirement boat on a trailer instead?

    Certainly, in matters of detail there are differences, but not in the concept – at least as I can envision.

    What I can see as differences are:

    1. Movable load on a trailer versus a fixed trailer load.
    a. Must be tied down securely and frequently checked.
    2. Over-width up to 16” (400mm)
    a. Need permits, signs and maybe flashing lights to legally travel.
    3. Trailer needs to be engineered for induced stresses.
    a. Tires, wheels, brakes and frame members of correct size with added safety factor for load.
    4. Tow vehicle rated for GCVW.
    a. Maintained in good condition to avoid breakdowns.

    Can anyone else see any other significant differences that I’ve overlooked?

    I know. And Tom’s Bluejacket is another possibility – so much so I bought a set of plans a few weeks ago. :)

    Obviously Bluejacket – or even the 28’ version that Tom is working on – is a much different concept than my Starsinger. Apples and oranges once again. Having said that, Bluejacket is a viable alternative if my health goes further south, if my investments tank or if the wife ultimately doesn’t want to invest her time and money in a larger project. Bluejacket was designed for a specific set of circumstances and in those circumstances the boat performs superbly!

    And that once again illustrates why minimal overhangs and long water lines are a good thing for getting the most speed at efficient fuel consumption rates in a displacement boat. A reasonably efficient speed is ~6 knots. The key – at least in my mind – is being able to do that 6 knots on less than a gallon of fuel, hopefully significantly less. It’s doable, but lots of details have to coincide for that to happen consistently.

    Best,

    Leo
     
  7. SAQuestor
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    SAQuestor Senior Member

    I understand your point and I want to make sure everyone understands that these were proof of concept doodlings rather than any effort to actually design something viable.

    And yes, that box keel idea has seen the light of day in an Italian boat with the added feature of some "wavy" hull forms that (supposedly) minimizes wave making resistance, improves efficiency and yada-yada. I forget the name but it has been discussed here on BD.N about a year or a bit more ago.

    Best,

    Leo
     
  8. tom28571
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    tom28571 Senior Member

    Ah yes Leo,

    That was/is the "Slider" that was on the forum. I can see that my comment needed more clarification. I knew that your sketch was just that and there had to be some way to blend the structures. I have not figured out how to do that most effectively either. It's easy if you don't mind otherwise useless length but my mantra is always efficient use of space and material. The idea of what I call the "two phase hull" was also discussed here on the forum a couple years ago.

    Liz and I have been traveling about North America in a Dodge campervan since I retired. It has all the necessary features but at 19' is tiny compared to the big rigs like the one you showed. The one big difference between the two kinds is in the way they are used most effectively. We mostly travel, and camp in order to make the traveling economically feasible. The big rig folk stay in one location extensively and travel much - much less than we do. We have traveled to all the lower 48, Alaska, all Canadian provinces except Newfoundland and also in Mexico. We did this and still have a pretty normal home life where I can work on boats and make a nuisance of myself here.

    We wake up and the weather is bad. So we travel. Takes about 5 minutes to get underway or set up. The big rigs take hours to get away and are limited to "improved" campsites while we can stop in rough spots that appeal to us. This same sort of reasoning applies to easily trailerable versus transportable boats.

    As was said, it's all about personal choices and decisions. The more stuff you have, the more effort and bother there is to keep it fed and happy. I don't want to disparage the other point of view, it's just not for me. Many of my boating friends have much larger vessels, sail and power. Most seem to enjoy themselves although their toys consume much more of their time, money and effort than ours. Now, some have enough money that they just let someone else do all the work. That works too.
     
  9. Guillermo
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    Guillermo Ingeniero Naval

    I've been searching for disaligned propulsion systems and found nothing till now. Just the usual stuff about offset shafts and the like.
    Maybe this Atkin's idea didn't work because cruising boats really do quite a lot of maneouvering, so an angled shaft adding side force to the propwalk when in astern wasn't a desirable effect...? Or, at the end of the day, propwalk was not such an important issue when going forward...? Just aesthetics...? :confused:
     
  10. SAQuestor
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    SAQuestor Senior Member

    Yes!:!: Exactly what I have in mind.

    One of the first inspirations for doing this came from these folks.

    The sort of traveling that they do is amazing. But if you read the entire site you'll soon see that they have been traveling all their lives.

    Best,

    Leo
     
  11. Corpus Skipper
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    Corpus Skipper Hopeless Boataholic

    Something like this, Leo?
     

    Attached Files:

  12. SAQuestor
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    SAQuestor Senior Member

    Kinda-sorta, but not exactly.

    More like this sort of styling with a bit more room.
     

    Attached Files:

  13. this is a Devlin?
     
  14. Guillermo
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    Guillermo Ingeniero Naval

    Nice boat Leo.
    Talking about not trailerable boats, I'd like to further explore the cat concept, like the Grahame ones (http://www.aviadesign.com/) or these Motorcats (http://www.motorcat.com/tec.html) from Poland. I do not like their modern styling (I'm a conserrvative guy, more in the line of this Tomcat 255 http://www.c-dory.com/24 description.htm . Not precisely this particular one but you know the idea...), but the concept is appealing to me. Provided with the minimum possible power to fully plane under load, just to go let's say up to a maximum of 23-25 knots. Any other interesting cats?
     

  15. new power catamaran by AB

    Here our new day cruiser power Catamaran under design by Albert Nazarov. A cabin cruiser version will follow.
    Lenght 9,5 mt
    beam 5 mt
    3500kg of displacement
    The first will be a day cruiser more as a “Floating Sala” with a rigid roof. Will follow a cabin version similar to the Prowler 9000 (with similar specs) by Shionning design.
    Max speed with 2x90h.p 24 knots. For 2 per 75 h.p. 20 knots.
    To be build economically in plywood epoxy (ply core between two layer of epoxy with GRP).
    .
     

    Attached Files:

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