Pocket cruisers

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

  1. Guillermo
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    Guillermo Ingeniero Naval

    Out of size for the thread, but a very nice boat also:

    Designer Arch Logan 1912
    Builder Logan Classic Boats
    Construction Female moulded GRP
    Interior Polished kauri
    LOA 10m, 33ft
    BOA 2.5m, 8ft
    Draft 0.6m, 2ft 2in
    Displacement 3.2t
    Cruising speed 8kts
    Engine Lombardini 1404m 39hp
    Fuel consumption 3L / hour
    Range 600 nm
    Fuel Tank 260L
    Water Tank 260L
    Holding Tank 100L

    A pity she's not containerable only for a mere 4"....

    Attached Files:

  2. so even this:
    34-foot Sea Bright skiff, motorcruiser. Beachable on Sea-Bright-skiff box keel and trailerable at 8 foot 6 inch beam

    LOA: 34 ft. - 2 in. (10.4 m)
    LWL: 31 ft. - 6 in. (9.6 m)
    Beam: 8 ft. - 6 in. (2.6 m)
    Draft: 2 ft. - 9 in. (83 cm)
    Displacement: 7.5 tons
    Diesel: 300 gal. (1135 l)
    Water: 90 gal. (340 l)
    Speed: 8 kts cruise
    Power: Single 46 hp.
    Westerbeke Speed: 8 kts cruise, 9 knots max
  3. Raggi_Thor
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    Location: Trondheim, NORWAY

    Raggi_Thor Nav.arch/Designer/Builder

    I don't know why the shaft is angled.
    But the hull shape is a bit simmilar to the one featured in one of the last isuses of WoddenBoat, also discussed here I think. Quite interesting.
  4. SAQuestor
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    SAQuestor Senior Member

    From: "Of Yachts and Men" by William Atkin. Tiller Publications Copyright 1997 by John Atkin ISBN 1-888671-07-06

    From the treatise on "Hell Diver", a 35' Seabright skiff type power boat. Designed in 1935. 35' LOA, 32.5' LWL 10' 10" beam 2' 9.5" draft.

    Page 129:

    "The engine is off the centerline of the boat, and thanks to the box deadwood has very little angle of shaft - less than 5 degrees. At the flywheel end of the motor, the center of the crankshaft is 7 inches to the port of centerline; but at the deadwood the center of the shaft is on the center of the boat. The result of this is a splay to starboard of the shaft from the deadwood to the motor, the propeller being on the centerline. All single screw powerboats having righthand turning propellers have a strong urge to turn to port, or to the left. Now by spraying the propeller shaft, as accomplished in Hell Diver, this port urge is counteracted by diverting the slip stream from the propeller flowing to starboard, and the boat will hold a true course without correction of the rudder. I have used this arrangement very successfully, and it is a wonder to me that all powerboats do not employ it."

    He goes on to mention that for lefthand turning props that the engine needs to be splayed to starboard.

    But a few pages later he describes another Seabright type boat, the 25' Princess Lena. Lena has the traditional straight centerline prop shaft from a motor in the fore/aft center of the boat.

    Best I can do here is [shrug] and say that both the father and son are passed on and there is no one left to ask for a technical explanation. I doubt John's wife Pat - who still sells plans BTW - would know why one boat has it and one doesn't if splaying the shaft was such a great thing.

    [edit] BTW - the power specified for Hell Diver is a six cylinder Kermath Sea Captain with a 4 3/8" bore by 5 1/2" stroke pulling 50 horsepower at 1000 RPM. The prop specified is 19" diameter by 18" pitch Michigan Wheel.
  5. SAQuestor
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    SAQuestor Senior Member

    Here's some scans of Hell Diver from the book mentioned in the previous message.

    Attached Files:

  6. Willallison
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    Willallison Senior Member

    I could imagine that for a displacement vessel an angled shaft might be of some benefit - particularly for slow speed manouvering. But the Naiad is shown as having a top speed of 17mph, and whilst this is hardly record breaking, you could still imagine some strange things occurring. If nothing else you would have to have the rudder at an angle to counteract the offset shaft.
    Consider the anodes that are placed on larger outboards. They are placed at a slight angle to offset the effects of prop torque. But they are very small and the angles are not large either.
    And if it was such a great idea, why haven't others picked it up over the years?
    Nope - respected or not, I think My. Atkin had this one wrong.

    Getting back to WHIO, when I first saw the lines that Gillermo posted I was struck by their similarity to a boat I saw some years back in Woodenboat. It was a boat called Wood Duck which was based on the old Draketail launches. On closer inspection, the boats are indeed similar, though at 34' 8" LOA and with a beam of 10', Wood Duck is a much larger craft - and forgive me, it's also outside the scope of this thread. She also requires a 250 hp Cummins diesel to drive her to her top speed of 28 knots.
    Where WHIO appears to feature a fair amount of hook in her buttocks lines, Wood Duck shows a little rocker in hers. There is also no keel on the larger boat.


    The boat was featured in the June 1997 Issuse # 136 of Woodenboat

    Attached Files:

  7. Guillermo
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    Guillermo Ingeniero Naval

    Thanks a lot SAQuestor (May I know your name?) and Will. I'm going to try more info on disalignement of propulsion trains from other authors.
  8. Gilbert
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    Gilbert Senior Member

    Atkin, in his comments, says that the offset shaft frees the rudder from the need to compensate for the propellor torque. If we can't accept the word of a man who has designed over 40 different Seabright skiffs and probably used most of them on the water, whose word can we accept?
  9. SAQuestor
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    SAQuestor Senior Member

    Let’s explore these thoughts a bit.

    Will, I’m interested in those “strange things occurring” you mention. Care to elaborate on the specifics you envision?

    Will’s comment further suggests that we should toss out “old” ideas just because others that follow haven’t picked up the idea and used it again and again.

    With that theory, then we should toss many ideas that don’t fit ‘modern’ conventional wisdom.

    Gilbert poses the proper question – whose word can we accept?

    Down another path here for a moment. I want a retirement boat that I can transport from sea to shining sea and with a great lake or two in between. A boat that is suited to a summer in Alaska and a winter in The Bahamas. Next summer in Nova Scotia and next winter in the Sea of Cortez.

    This idea is doable as a home built and owner trailerable concept. Portager’s numerous postings suggest that a 40’ plus boat can be transported across the highways and byways of North America. Option One is another viable concept. Tom Lathrop has posted that he is developing a 28’ version of his Bluejacket and that may be the most practical solution of all. We’ll see when that is done.

    But the exact design of the boat is open to debate and discussion.

    I’ve communicated with several much more knowledgeable people – designers that post on this forum – about adapting Atkin’s Seabright Skiff concept into a transportable boat that will fulfill the above requirements.

    To a man they have pooh-poohed the idea. Not efficient. Too difficult to build. Not modern. The reasons are numerous and I’m not going to fetch the emails to enumerate each reason.

    Regardless of their rationalizations, I can’t understand why a design that was fished in apparent safety for decades off the New Jersey beaches isn’t still appropriate for today’s world. What makes ‘modern’ hull forms superior? I’d suggest nothing except the individual’s opinions and bias toward their own training and body of work. I’d suggest that not many people – boat designers/naval architects notwithstanding – are able to come to terms with ‘borrowing’ another person’s ideas and giving credit for that idea rather than claim it 100% as their own. And no, this is not a slight at anyone that has posted in this thread, but rather an observation of general human nature.

    Why does the Seabright hull form as refined by Atkin appeal to me and my trailerable retirement concept?

    In order to get the most boat into a box that is defined by the rules of the road – 13’6” (4114mm) high by 9’10” (~3000mm) wide without a pilot car in most states – requires some ‘out-of-the-box’ thinking.

    Think of the cross-section of a large trailer. When a “normal” boat sits on a trailer what do you see? If the boat has any kind of a keel, there is usually a huge empty space between the keel and the insides of the wheels/tires/fenders. If the boat has a Vee bottom, then the empty space is much less, but there is still some wasted area.

    Think about a displacement boat suitable for the types of waters I mentioned above. There is certainly a chance that some adverse weather/wind/waves would be encountered at some point. What type of boat would YOU like to have when that happens?

    Personally I’d like to have one that has some weight down low in the hull – low ballast if you will. On a boat that will be towed behind a heavy-duty pick-up it seems impractical to tote along a few thousands pounds of lead just to ensure some low ballast.

    What, to me at least, makes much more sense it to locate the heavy machinery, batteries and tanks as low as possible. How to do that in a conventional hull form? Doggone difficult best describes it.

    But if one takes a box keel ala Bolger (and others) or a Seabright keel from Atkin it is possible to get that space between the wheels filled up with machinery, batteries and tanks. That means that the remaining height can more easily accommodate living space while making the whole boat as stable, safe and seaworthy as possible.

    I know that this is only one approach to hull design – and certainly not the only appropriate one for this sort of use. But it does (at least to me) appear viable.

    Back to the shaft offset in Atkin's designs.

    To dismiss ideas that are 50 - 75 or even 100 years old as impractical or obsolete or not appropriate because one has no personal experience seems to be the height of folly. How much valuable experience or information has been thrown away because it has been forgotten or hasn’t been widely adopted? Probably lots.

    PERHAPS, just perhaps, this slight shaft offset is one of those good ideas that – for whatever reason – got ignored by the main stream of that era and never carried forward to us.

    And granted, it may NOT be a good idea and was rightly ignored. But as Gilbert suggests, should we dismiss out of hand an idea of a man of such stature as Atkin?

    Worthy of a discussion me thinks.

    And Guillermo, here’s my name.


  10. Guillermo
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    Guillermo Ingeniero Naval

    Thanks, Leo.
    As told before, I'll try to find something about not aligned propulsion trains in older boatdesigning books...
  11. SAQuestor
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    SAQuestor Senior Member

    Gracias mi amigo
  12. tom28571
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    tom28571 Senior Member


    I may be in error but I think the original thinking behind the Seabright skiffs was to launch and retrieve these work boats off the beach. Thus the flat bottom and the hidden propeller. Up to moderate speeds, it also looks good from an efficiency standpoint. Also would be more seaworthy than a low deadrise form. I like the form but think it is probably limited to a relatively narrow beam for best performance. The engine and other stuff down low is also good for stability. I don't think Bolger's box keels are this good. This is getting close to Atkin's tunnel hull designs.

    I don't think this form lends itself to trailering in a cruising size too well since it will need be heavier to get adequate stability with that much volume underwater.

    My 40 second evaluation, your mileage may vary.
  13. Zewe
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    Zewe Junior Member


    Can you expand on your comment that you

    'don't think this form lends itself to trailering in a cruising size too well since it will need be heavier to get adequate stability with that much volume underwater'?

    I am sure I don't understand what you are actually trying to say.

  14. Willallison
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    Willallison Senior Member

    1st up, I'm not completely dismissing the idea of an offset shaft - though I will grant that my previous post would suggest that I am. And it's certainly not the case that one should dismiss an idea simply because others have not picked it up.
    But I still have my doubts about this particular application of the idea. If the shaft offset is clearly visible to me in a scanned reproduction of some old drawings, then the angle must be fairly substantial. Just looking at it, it looks like four or five degrees. The boat also has a fairly substantial rudder.
    So, again, just from looking at it, this seems like massive overkill to me. I've driven any number of single engined, inboard boats - both displacement and planing, and whilst the 'prop walk' that is present at idle can be awkward for some, the effects of the propellors torque generally becomes less noticeable as speeds increase.
    That is not to say that the effects diminish as speed increases - clearly they don't - but any required steering input is so slight that it is barely, if at all perceptable.
    Now - as to your question of the strange effects I envisage... With the shaft set at say 4 degrees off centre, I picture the boat doing 17mph with the helmsman hanging onto the tiller for dear life simply trying to keep the thing in a straight line. I know that Atkin says that the angle offsets the prop torque, but I just think its been overdone in this case.
    I may be wrong - certainly happened before!:D

    I would also concur with Tom about the boats suitability as a trailerboat. Under normal circumstances, when it comes to trailering, the lighter the rig the better. If you take lightweight construction and the box keel idea to the extreme, then only the keel sections would be immersed, making for a very unstable craft.
    Also with Naiad you have very rounded sections, so any reduction in weight will lift the boat to a much narrower waterline beam - once again making the craft quite unstable. If it were a hard chined boat with almost the full beam at the waterline, at least you would gain some form stability

  15. SAQuestor
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    SAQuestor Senior Member

    Will, thanks for the thoughtful reply.

    Based solely on the quote I posted above and assuming 6’ – 72” (1828mm) from the front of the engine to the prop, and using the figure of 7” offset at the flywheel (if the flywheel if forward on the Kermath engine that Atkin specified) then the degree of offset is just shy of 4º. If the flywheel is at the aft end of the Kermath engine, then the degree would be even more – perhaps as much a double. And you’re correct, I too can’t see how more than a few degrees would work.

    While I do not doubt your experience and knowledge, I would like to hear from some of the other folks with their thoughts.

    One thing that I failed to note in my screed above is expected trailering weight of the ideal retirement cruiser. Many heavy-duty diesel pickups can have a towing capacity of 14,000 to 15,500 pounds – 6350-7030kg. Subtract a generous trailer weight of 3000 lbs and one arrives at a maximum empty boat weight when sitting on the trailer of 11,000 to 12,500 pounds. In reality I’d imagine something in the 10,000-pound (4500kg) area would be achievable. Add fuel, water and stores for 30 days and cruising weight may increase by two or so tons to (give or take) 14,000 pounds, (6350kg).

    And yes, this is not a boat that one would trundle off to the local lake for a Sunday afternoon cruise. Rather it’s a boat that that one would take to a marina with a straddle lift, set it in the water and go cruising for weeks or months. Then come back and set it back on the trailer for a trip home or to the next cruising area.

    So I hope that mostly satisfies your concerns about weight reduction and stability.

    Back when I started exploring this idea of a transportable retirement cruiser I did some AutoCAD drawings to see if the concept would even work. Attached are two VERY PRELIMINARY concepts – a jpg and a dxf file for your perusal and comment if you see a need.



    Attached Files:

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