Retirement Cruiser Concept

Discussion in 'Boat Design' started by SAQuestor, Aug 5, 2004.

  1. Tad
    Joined: Mar 2002
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    Tad Boat Designer

    Hi Tom,

    Didn't I write something about that a while back? They put a wave or two in the bottom outboard of the box keel and claimed a 30% reduction in drag?

    Old idea, maybe a really old idea? Weren’t those Japanese boats sort of a wide box on a narrow box? The form makes sense, the narrow section runs easily through the water while the wider section provides deck space, reserve stability, and knocks down waves. Downside is little interior volume in the hull, so topsides must be higher if you need interior volume. That means more weight.

    I recall seeing drawings of the Japanese version of this as used for commercial fishing in Hawaii, Lindsay Lord spent WWII in Pearl Harbor. I have an interview with LL published in 1988 where he talks about one unusual design he did. It was for the Mechanical Equipment Company of Louisiana, which had acquired a bunch of surplus 6-71's that wanted to sell to local fishermen. As it happened the locals liked boats in the 30-35' range, and these engines took up too much space in that sized boat. The company asked LL for a design to make the problem go away. To quote, “My solution was to bury the engines in a hollow keel below the cargo carrying cockpit. The necessary volume of this streamlined keel also happened to provide about one-third of the boat's total required buoyancy. The boats skimmed beautifully." He calls it the Buoyant Keel. Phil Bolger worked for LL, and so we come full circle...maybe?

    I believe Gerr attributes his version to Atkin, who attributes his to the Seabright Skiff. So who the heck was ol' Seabright? I don't know, but Tom Gillmer pans them in his excellent book, A History of Working Watercraft of the Western World. Gillmer claims (from his tank testing) resistance 20-25% higher for the Seabright over hulls of similar shape but without boxed keel forms. So you need a reason other than low resistance to use this form.

    Bolger has used it to create hulls with wide deck beam and fine forefoot’s, done in sheet material. LL's trick of getting the engine down low under a working deck, in a relatively light boat, may be another reason.

    I would want to see some sections of 8's boat before making any statements. But I have an idea the chine forward is too low, (in profile) making the bow very blunt. But I don't know if the boat has curved sections or straight? There doesn’t appear to be enough volume in the ends (especially aft) to support the stated speeds, but it's hard to tell without some numbers. With the twin skegs and the keel forward she would sit upright on any beach without the box. In this case the box only pushes the engines up higher in the boat. How about putting the engines down in the box, one in front of the other, with saildrives at 90 degrees out the sides?

    Bolger says keep the midsection of the box keel parallel and only fairs the ends, and then it has useful volume in the ends of the boat, where you want it.

    All the best, Tad
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    FAST FRED Senior Member

    Getting the beam down the road will be biggest design hassle.
    10ft is easy , after that permits and driving restrictions .

    Don't worry about the weight ,trucks to pull 50,000 lbs are really cheap if 5 or 6 years old ($5000) or so. check at any truck stop for free mags with trucks for sale.

    Many have auto trannys and air ride , so are very civilized to drive. With RV registration insurance is cheap , but you will need a CDL license to drive it.

    Good luck with a great can go to 50ft on leginth , if you can keep the beam from growing into permit land..

  3. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Thanks for the added history of the slender keel, Tad. I still think this hull form has good potential if designed and operated in a way advantageous to the type. The claim of higher hull resistance may have referred to higher speed where the added skin resistance of the buoyant keel iwould be detrimental. My thought would be to use the boat at speeds lower than about 16 - 18 kts. This would mean that the keel would be in displacement mode and the upper hull would be planing. There may be some reason why this will not work but my untutored mind has no problem with it.

    I imagine a L/B ratio for the keel of 8 to 9 so that wave resistance would be minimal at this speed. The keel would carry enough displacement so the the bottom loading of the upper hull might be 30 lbs/ft sq or so. I haven't done enough work to see if this is a practical goal for a boat of 30' to 34' LOA or not. The goal would be to cruise at 12 - 14 kts on low power and low wake in the ICW and inshore waters.
  4. Willallison
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    Willallison Senior Member

    can any of you guys remember the name of that 'revolutionary new boat' that Tad is rferring to - the one we discussed some time back. I've been trying to find the thread, but as yet no luck....
    I'm pretty sure that wherever it was, I posted a (very preliminary) model of a similar hull that I wondered about some time back. Similar in concept to Tom's I wondered at the time whether a beamier version of the 'displaning' hullform that Malcom Tennant incorporates in his catamarans would work in a monohull....what's that they say about great minds? ;)
  5. Willallison
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    Willallison Senior Member

    In regards to 8's boat, I think Tad is on the money as far as engine placement goes - the 'box keel' is a great place to chuck engines etc, but by having twins mounted side by side, you can't do it. Tandem with saildrives is one way to go - but then you have one prop further fwd than the other. What about diesel-electric? Then you can mount the engines wherever their weight is best placed and your drive units can be placed further aft, sticking out eac side as Tad suggests. Further, you then have ability to run both props utilising one or both engines (or even a generator) as speed requirements dictate.
  6. duluthboats
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    duluthboats Senior Dreamer

  7. Willallison
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    Willallison Senior Member

  8. tom28571
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    Location: Oriental, NC

    tom28571 Senior Member


    I can't tell much fom the view you show, but think I had something different in mind. Your keel looks to have a L/B ratio quite a bit less than the 8 or 9 that I'm looking for. I would plan to have the forward keel fair into the main body with an "S" curve from keel to the chine and the upper works would then have flare for more reserve buoyancy. I'd carry the keel as far aft as possible and still keep the prop and rudder post under the hull. I favor a single engine for simplicity, economy and drive protection. If needed, a thruster forward would take care of close maneuvering. The keel should allow a large, slow-turning prop for efficiency at speeds in the teens.

    While I definitely am not in favor of "normal" displacement speed of 6 to 7 kts, I never did understand the rationale for the high speed that most favored for Option 1. It may be justified for a weekend cruiser for busy 9 to 5 folk but, for a real cruiser, I just don't see the need. My interest in this hull form is as a way to get a more reasonable (for me) speed at no more expense than the normal 6 kt displacement cruiser.

    The latest Woodenboat magazine has an article on a 50' Ruell Parker flat bottom sharpie cruiser that he claims 10 kts at 1 gal/hr. Perhaps this goal is attainable but I think this design is limited to very restricted weather conditions.
  9. Willallison
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    Willallison Senior Member

    Unfortunately the view I showed is the only one I have - I think I lost the file in crash here at work.
    As I said the hullform is a direct adaptation of the displaning catamarn hulls - where the 'keel' is a far greater proportion of the hull as a whole. The limitation in terms of shape is that the length/beam ratio must be rather high or the 'keel' becomes rather blunt - still around 9 as you are looking at, but the way I have it modelled, that is essentially the beam of the whole boat - not ver practical. I think your approach is a better one.
    As far as the engines / drives etc go, I was suggesting those for 8's boat as an alternative to Tad's idea of using saildrives.....
  10. Sean Herron
    Joined: May 2004
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    Sean Herron Senior Member

    Motor Sailing...


    I won't judge your profile - because it is obvious we don't think alike - however the rig...

    It would be possible to get a proper or decent amount of sail area on short and collapsible spars by using a sliding gunter rig and a small jib on a self tacking boom...

    Unless that is just to be a steadying sail - and if so - I would rather see it aft so as to affect wind cocking on a hook...

    Any way - I am going to go mix a drink - best of...

  11. 8knots
    Joined: Feb 2002
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    Location: Wasilla Alaska

    8knots A little on the slow side

    Mornin Lads,
    Sorry havent been around much. Tis the busy season for my real job. I dont know anything about bolgers work. But as Will A, was talking a Tennant style "displaning" hull seems like a good option. as for the twins on my boat, Yes it would be a better "efficient" hull with a single screw with a heavy skeg but my intended use was for semi protected waters and lots of tidal/canal service. Those new Westerbeke's are light and small and turn out max hp @ 2500rpm. throttled back in cruise mode I feel would have a perfectly acceptable fuel consumption for the 9 to 5er who could afford a 50' weekend/week cruiser. Yet if the need arose to head before bad weather or get back to the home 20 on sunday night she could provide better than average speed with a sea-kindly way about her ''not pound your teeth out in a 3' chop" I have yet to do sections on her as she is just a concept sketch. The keel is roughly 1/3 the midships WL beam. I had intended to put maybe 800gal in the keel and a small gen along with grey, black and fresh water tanks. This is the only way to keep the profile low and still have full headroom. And for Tad, I know about that atf house.....that thing and I have a love hate relationship. I wanted a protected aft steering station while sportfishing. even in july gets nasty up here on the water sometimes. Ill post the 75 foot version "my charter version" just for a laugh.
    Keep em sharp! 8

    Attached Files:

  12. SAQuestor
    Joined: Sep 2003
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    SAQuestor Senior Member

    Wow! How fast a month goes. Since I first posted the concept of a trailerable Summer Retirement Cruiser just a month ago I've read all the replies here and corresponded with a few folks that took the time to write me. Thanks to all.

    But what really turned my head around was a note from a friend over on a yahoo boat building group ( about Philip Bolger's preliminary design “Sitka Explorer” as published in Mr. Bolger's column in the magazine Messing About In Boats, MAIB. (Available via subscription from

    So based on my perusal of the MAIB article, I wrote to Mr. Bolger and received a return call from Suzanne Altenberger, Mr. Bolger's wife and boat designer in her own right. We spoke for about 90 minutes about various desirable attributes suitable for a trailerable Summer Retirement Cruiser based on the original Sitka Explorer concept.

    Ms. Altenberger told me that the have a commission to take the concept to a finished set of plans and will be working on that this Fall for completion by early next year, (2005). She told me that they will be offering several different hull lengths and interior layouts, from the original concept length of ~11 meters up to perhaps 13-14 meters LOA.

    She invited me to send along my ideas based on the Sitka concept. Attached here is a file that I've converted from AutoCAD with my idea of a Summer Retirement Cruiser.

    Several points of interest.

    1.Bolger Boats are generally easy and (relatively) inexpensive to construct.
    2.IMO a boat this long and narrow, and designed to spend lots of time on a trailer needs excellent support to keep the rigors of trailering from busting it apart, therefore;
    a)A thick plywood bottom – 37mm to 50mm thick.
    b)Extensive longitudinal bottom support is necessary.
    Borrowing from Renn Tolman's Alaska Skiff, I propose to use a network of Versa-Lam beams running longitudinally and wooden I beams (50mm flanges with 12mm plywood webs) athwartships with a (mostly) solid sole. With these beams epoxied and taped to the plywood hull and the sole glued and screwed to the beams, this will provide a extremely stiff monocoque type structure.
    3.Keep the 'heavy stuff' such as batteries, fuel and engine near the center of the boat.
    4.Utilize a Duetz F3L1011F air/oil cooled industrial engine. (
    5.Utilize a Sillette SONIC250 (Heavy Duty) outdrive(
    a)Drive this unit via a ~37mm SST drive shaft supported on pillow block bearings and hidden inside a 'tunnel' atop the sole. (Yes, it is a step-over area in the aft cockpit.)
    By using the Duetz engine and Sillette outdrive there are no through-hulls required. Additionally, by using a composting marine toilet, there is no overboard waste discharge required – no through-hulls for flush water. So the hull is solid with no penetrations to leak or require maintenance.

    Finally, this result is the 5th iteration of this concept. When I first posted the idea a month ago I had not done any real scale drawings. Just some 'napkin' sketches that lead me to believe it was possible and practical. After spending considerable time manipulating ideas and layouts in AutoCAD, I've come to realize that the height and width restrictions placed on this concept by 'legal' trailering requirements are extremely restrictive. How much easier it would have been designing the accommodations to take the beam out 600mm to ~3150mm (10 ½'). The LOA could have been a couple of meters shorter than the current 13.5m.

    But the idea is to be able to trailer the boat without restrictions and that dictates the width and height. :-(

    I'll print this design on a D sized sheet and send it off to Bolger soon – perhaps some of the ideas expressed here will be incorporated into their versions.

    Attached is a jpg of the AutoCAD drawing - not very detailed. I'd be pleased to senf the AutoCAD dwg file (about 130k) to anyone interested. Just drop me a note.

    Blessed be,


    Attached Files:

  13. Ssor
    Joined: Jan 2005
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    Ssor Senior Member

    Have you and SWMBO spent time aboard a boat in less than ideal conditions. I devoted 9 years to rebuilding a sloop with enthusiastic support and participation of my wife who had never gone sailing. She believed that it could be a wonderful way to travel from one place to another and have a place to stay wherever we went. The first time the wind piped up and we had to reef she realized that being on the water was not going to be all blue skies and gentle winds. We still sail but she is not as intrepid as she thought she would be. Allow me to suggest that you plan to charter for a couple of weeks at a time in the places that you want to cruise and see if it is as appealing after the cruise as it was in anticipation of the cruise.

  14. mackid068
    Joined: Feb 2005
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    mackid068 Semi-Newbie Posts Often

    Very pretty boat. 8knot, your idea is very nice as well.
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