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Thread: Dead flat bottom boats.

  1. #1
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    Default Dead flat bottom boats.

    Hello to all fellows.
    One of the designs wich most attract me is the Carolina Dory, for its low power requirements and ability to break the chop at displacement speeds, as easy planing when conditions allows it. Aesthetics and easy of construction plays a rol too.
    I see other boats with great reputation as Bolger's Diablo have a straight bottom, or Gardner's flat iron in a smaller version.
    My experience with flat bottom boats is positive, except I found the boats with good flare and a bit of rocker up front tends to pount a bit more and to be a bit more affected by windage in the bow.
    My question is if this boat, the Carolina Dory is dangerous in swells, if it has some tendency to pitch pole. To me and theoretically, the flare will do all the job (think as a Vee bottom with an exagerated delta pad, where the bottom is the whole boat, just to imagine what I mean).
    I seen positive and negative comments about in the net, but generally comes from people who is enamorated of his boat -as everybody else-, or haters wich never been in a flat bottom, or specially a straight run flat bottom.
    Wich more disappoints me is all the actual plans for flat bottoms -including Diablo copys- includes a rocker in the forefoot. Except the plans of the Carolina from Glen L and Spira.
    Thanks to all.

  2. #2
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    Default Re: Dead flat bottom boats.

    2nd boat I built was a Carolina Dory. Used it on a month long Baja trip and many trips on Puget Sound. Last I heard it was commercial fished with hand gurdies out of Eureka Ca. - open Pacific, big swells...It was too small for that use but the guy survived a season without incident. It started with a 20hp which was just adequate - the 40hp was much better for any kind of load. I never noticed any tendency to pitchpole which I assume you mean it buries the bow in swells. It was a 10kt or occasionally slower boat in rough conditions - faster meant you'd drop off a wave & rattle your teeth but it was much more a case of the boat could take it but not the crew. It's a very safe small boat - very small for a 19 footer - but easy to launch off a beach or sketchy ramps. Main danger was going too fast in bad conditions. Easy to build! Easy to tow too...The relatively narrow bottom & relatively long length made for less pounding than a design shorter or wider. Highly recommended.
    I should do a head to head comparison with Bolger's bigger Diablo...Moe


    "My question is if this boat, the Carolina Dory is dangerous in swells, if it has some tendency to pitch pole." No.

  3. #3
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    Default Re: Dead flat bottom boats.

    Hmmm, that plan view looks familiar, kinda like this view in real life...



    I successfully and painfullly piloted that flat-bottom bitch in seas for more than 20 years. Never again. She was fine in a head or quarter sea but miserable in any following sea. In following seas she would surf and crash in to the trough, nearly capsizing.

    She is now a semi-vee.


  4. #4
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    Default Re: Dead flat bottom boats.

    "Hmmm, that plan view looks familiar, kinda like this view in real life..." That hull's shorter & wider...the Carolina would pound much less - the greater length would better bridge the waves in closely spaced waves. Interesting your report of poor handling in a following sea - when first built I'd put external chine guards running full length - so there was a flat section on top of the guard maybe 3/4 inch wide. That 3/4 inch flat would catch the water in a following sea & scare the crap out of me as the boat would yaw to one side or the other. I ground off the flat several feet back from the bow & the bad behavior stopped. Great early lesson in hull design!

    "I successfully and painfullly piloted that flat-bottom bitch in seas for more than 20 years" ... Or to put it another way..."That boat was so bad I didn't put up with her nasty character for a day over 20 years!"

    Yes, a boat with a flat bottom will pound in some conditions - worse than other hull shapes (except concave - in San Francisco Bay summer chop the Boston Whaler 17' Montauk was the worst pounding hull I've ever experienced - so bad we only kept her for 6 years!).

    At the time I built the Carolina we had a 20hp Merc, a thousand bucks, minimal skills & wanted to go to sea. The Carolina did the job. We were young though & comfort was a relative thing. I can definitely say that 6 hours in the Carolina Dory was more comfortable than 6 hours on a motorcycle or riding a horse...Moe

  5. #5
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    Default Re: Dead flat bottom boats.

    I am not familiar with flat bottom poser boats. That said, flat bottom sail boats and rowing boats should have enough rocker so that the bottom is out of water at each end, and most of the curvature to create the rocker in the center third of the water line length.

    Does this make any sense for power boats?

  6. #6
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    Default Re: Dead flat bottom boats.

    Quote Originally Posted by ahp View Post
    I am not familiar with flat bottom poser boats. That said, flat bottom sail boats and rowing boats should have enough rocker so that the bottom is out of water at each end, and most of the curvature to create the rocker in the center third of the water line length.

    Does this make any sense for power boats?
    It might do at the bow but the stern should be straight and "flat".
    My problem with flat iron power boats is that it is so much easier to produce a comfortable ride in real lumpy seas with deadrise and a lifted chine forward.
    It really is quite difficult to build an ugly wooden boat.

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  7. #7
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    Default Re: Dead flat bottom boats.

    "My problem with flat iron power boats is that it is so much easier to produce a comfortable ride in real lumpy seas with deadrise and a lifted chine forward." True enough - but at the cost of easy beach-ability (assuming you don't put a keel on it...) , some decrease in initial stability and ease of building. Personally I'd only go for a flat bottom if I had to handle the boat on & off beaches.Moe

  8. #8
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    Default Re: Dead flat bottom boats.

    This is probably a question that should be answered by an experienced boat designer and/or NA.

    Years ago there was a thread that Steve Redmond joined, and explained in detail how one of his small cabin sailboat designs came to have a completely flat bottom -- all based on some very complex calculations and physics that were (and still are) far beyond my modest understanding of hull shape and design.

    Here is that thread with Redmond's response partway down - http://forum.woodenboat.com/showthre...ighlight=elver

    "Can't help answering the flat bottom comments, since the reason behind it may be of interest to design-oriented folks.

    If you add rocker to an extremely light boat like Elver, or worse, rocker and round sections below the bilge, you must increase the fairbody depth to have her sit on her lines at the same displacement.

    In simpler terms, everything you pull up must be compensated by another bulge that is pushed down to maintain the same volume. What this does is increase the body draft (by about double).

    Then to get enough board-up lateral tracking stability beneath the body, you need to increase the depth of the skeg. You have now increased also the weight of materials used. You've also brought the bottom of the bow up so that it will likely slap at anchor.

    You've also depleted the form stability, since the hull is now barrel shaped with the metacenter approaching the center of gravity. Probably you will need to reduce sail area and/or try to add enough ballast to compensate. But unfortunately ballast increases the displacement of the boat, necessitating a further change in the lines to adjust.

    The new ballast unfortunately doesn't have much effect until heeled , and now makes the boat sinkable. That being a worry, you probably need to add enough ballast to make her self righting in the form of a more or less deep keel, and a self bailing cockpit. The displacement has now doubled or tripled, trailering is probably difficult with a conventional low trailer and standard automobile.

    Costs, building complexity, and time to build have all now tripled. Sail area has doubled. Rigging complexity and masting requirements have increased as well.

    In fact we now have a conventional small cruiser, one of thousands like her, which certainly can give pleasure, but not exactly what I had in mind. The reason I know all this is because I originally thought to design Elver round bilged, but was rapidly pushed by scribbled numbers and succesive alterations into flattening and flattening her, until it made no sense to imitate in her bottom anything other than an ironing board. That being a requirement, there was no reason to make her ugly just because the bottom was flat, so I did my best to avoid that, and stick with my original mental picture of her

    Sad to say, I didn't design the flat bottom on purpose. I just couldn't escape it once the displacement was set.

    Best Regards,

    --Steve"
    "The enemies of reason have a certain blind look."
    Doctor Jacquin to Lieutenant D'Hubert, in Ridley Scott's first major film _The Duellists_.

  9. #9
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    Default Re: Dead flat bottom boats.

    I had one of Steve Redmond's Elvers, the subject of the thread that Thorne quoted. In the several years we had it, we sailed it only on some fair sized lakes. I never got the chance to take it out on Chesapeake Bay. It was very stable and sailed better than you would expect if you were used to tall marconi rigs on round bottomed hulls with ballast keels. I've sailed a lot of different boats, and I was not disappointed with the Elver. Flat bottom and round strip planked hull sides, even a bit of tumble home in the stern. The flat bottom was actually fairly narrow relative to the overall beam, and the angle between the bottom the hull side at the chine was not too hard. It was very easy to launch and retrieve and rig compared to other keel boats of the same length.

  10. #10
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    Default Re: Dead flat bottom boats.

    This is from Phil Bolger's description of his 18' Clam Skiff in Boats with an Open Mind.
    "Flat-bottomed boats with pointed bows get into trouble if their bows cut solid water. The sides push the water out harder than the bottom pushes it down. The water rushes from the high pressure at the sides toward the lower pressure underneath, and breaks into powerful eddies as it goes around the chine corner. The turbulence is unstable.. The rush of water lets the bow drop, making the erratic flow still stronger, and the boat will yaw and broach. A garvey bow, like a toboggan, is much better in this way, but it has to have a long, sloping overhang at the bow to carry it over wave crests. If the slope of the wave is steep enough, a garvey can put the top of its bow into the crest before it starts to lift. The garvey is also noisier when it's working at a standstill."

    My Clam Skiff is now 22 years old. I have learned not to stuff it into the back of a wave heading downwind. The resultant wallowing of the bow is unsettling. Fortunately my location is free of swells but is prone to steep chop. The flat bottom is a delight to work from when hand-hauling traps and is easy to drive when there isn't too much chop.

  11. #11
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    Default Re: Dead flat bottom boats.

    Quote Originally Posted by MoePorter View Post
    "My problem with flat iron power boats is that it is so much easier to produce a comfortable ride in real lumpy seas with deadrise and a lifted chine forward." True enough - but at the cost of easy beach-ability (assuming you don't put a keel on it...) , some decrease in initial stability and ease of building. Personally I'd only go for a flat bottom if I had to handle the boat on & off beaches.Moe
    Simply not true. We launch and fish off this beach.


    And would not dream of using a flat iron.
    This


    and

    are quite popular amongst our club members.
    It really is quite difficult to build an ugly wooden boat.

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  12. #12
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    Default Re: Dead flat bottom boats.

    "Simply not true. We launch and fish off this beach." Fair enough for that beach but in any kind of surf the bow V digs in the hull pivots uncontrollably. The Oregon dory type is what developed for routine beach launches on our west coast. Our regional experiences explain our contrary opinions. See - https://www.youtube.com/watch?v=Ssm4C53MQUg

  13. #13
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    Default Re: Dead flat bottom boats.

    Quote Originally Posted by MoePorter View Post
    "Simply not true. We launch and fish off this beach." Fair enough for that beach but in any kind of surf the bow V digs in the hull pivots uncontrollably. The Oregon dory type is what developed for routine beach launches on our west coast. Our regional experiences explain our contrary opinions. See - https://www.youtube.com/watch?v=Ssm4C53MQUg
    That is not flat in the same way that the OP describes.
    Is that as rough as you fish in? Not a lot different than our waters if so.

    There is enough flare and reserve buoyancy in the two boats that I posted than pitch poling/broaching is not an issue
    It really is quite difficult to build an ugly wooden boat.

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  14. #14
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    Default Re: Dead flat bottom boats.

    "That is not flat in the same way that the OP describes." Correct, the typical Oregon Dory has rocker in the front sections. My point being the dead flat sections aft allow predictable beach handling - particularly in the presence of surf. Thus a flat bottom is an asset in handling a power dory on beaches - with any water at all you can spin them around.

    The OP's question was about whether the dead flat bottom of the Carolina dory is prone to pitchpole/erratic handling due to the effect Phil Bolger so aptly describes & cyclone helpfully quoted. The forward rocker in the Oregon Dorys is one way to get around the handling vice of flat bottom power boats.

    As I posted, I built a Carolina Dory and it exhibited the behavior in a limited form (annoying but not dangerous) until I beveled the small flat upper sections of the exterior chine guard. I designed & built a 21 ft power skiff with a dead flat bottom & round sides which also had no handling vices in following seas. What these two boats had in common was a relatively narrow bottom with the flat section at the bow much smaller than Phil's clam skiff. My theory is the relatively small front flat triangle doesn't generate enough low pressure to cause the effect Phil describes - "The water rushes from the high pressure at the sides toward the lower pressure underneath, and breaks into powerful eddies as it goes around the chine corner." To answer the OP's question - what I can say is I've built two relatively long narrow flat bottom power skiffs that handle well in following seas. Moe

  15. #15
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    Default Re: Dead flat bottom boats.

    Quote Originally Posted by MoePorter View Post
    "That is not flat in the same way that the OP describes." Correct, the typical Oregon Dory has rocker in the front sections. My point being the dead flat sections aft allow predictable beach handling - particularly in the presence of surf. Thus a flat bottom is an asset in handling a power dory on beaches - with any water at all you can spin them around.

    The OP's question was about whether the dead flat bottom of the Carolina dory is prone to pitchpole/erratic handling due to the effect Phil Bolger so aptly describes & cyclone helpfully quoted. The forward rocker in the Oregon Dorys is one way to get around the handling vice of flat bottom power boats.

    As I posted, I built a Carolina Dory and it exhibited the behavior in a limited form (annoying but not dangerous) until I beveled the small flat upper sections of the exterior chine guard. I designed & built a 21 ft power skiff with a dead flat bottom & round sides which also had no handling vices in following seas. What these two boats had in common was a relatively narrow bottom with the flat section at the bow much smaller than Phil's clam skiff. My theory is the relatively small front flat triangle doesn't generate enough low pressure to cause the effect Phil describes - "The water rushes from the high pressure at the sides toward the lower pressure underneath, and breaks into powerful eddies as it goes around the chine corner." To answer the OP's question - what I can say is I've built two relatively long narrow flat bottom power skiffs that handle well in following seas. Moe
    Fairynuff. Useful input in order to avoid taking this

    out in anything but a millpond.
    It really is quite difficult to build an ugly wooden boat.

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  16. #16
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    Default Re: Dead flat bottom boats.

    Thank you all for sharing all your experience and knowledge, I will read again all this answers.

    Edited: I had a 19 feet double chine flat bottom, with plumb sides and a bit of rocker towards the bow, narrow with the maximum beam rear of amidships, very similar to a Carolina in some aspects. And it was very comfortable with a beam sea or following sea. I was in real rough conditions and never felt unsafe. Was 20+ miles offshore in that boat, north Atlantic. The only problem it had was wondering of the bow because the wind, and into very small waves the bow lifted too fast. My tought was the rocker is more a disvantage than an advantage in a very light powerboat. But as I don't have much experience and didn't found info I asking because next time I like to build a better boat but must be a fast and easy build. Or may my problem be too much of a rake in the stem? I used a 4.5 Merc. at displacement speed all the time, except sometimes at full throttle it skimms over the water. Was used for cruising and live onboard, three years, sculled and downwind rag over bulkhead one. The advantage of this boat over a C.Dory is more elbow room with the same beam, two can sleep in the cabin, if are friendly. The disvantage is I used okume and stitch and glue, something I don't like to repeat. Did much beaching and docked in tidal areas, sleeping inside. Very economic but people tought was too small of a boat.dorlns.gif
    Last edited by VGTL; 10-29-2018 at 09:55 PM.

  17. #17
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    Default Re: Dead flat bottom boats.

    Quote Originally Posted by
    [IMG
    https://cache.apolloduck.com/image_oem/16815.jpg[/IMG]
    are quite popular amongst our club members.
    I like this kind of cuddy cabin, looks like the Orkneys and OSM I was watching.

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