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Thread: How common tare the comparatively narrow flats (or reverse deadrise) at the chine?

  1. #1
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    Default How common tare the comparatively narrow flats (or reverse deadrise) at the chine?

    One reason I really like Lathrop's Blue Jacket is the way it transitions from dead stop to top speed while keeping the hull fairly level, and makes little wake at any speed using an outboard of camparatively modest power. A much larger power cruiser featured in a recent issue of WB also incorporated this chine feature. Then yesterday, while looking closely at the bottom of my Albin-25 (avatar) I see that her chines have a 3" flat extending along most of her length. My boat also kicks up only a mild wake, though at much less maximum speed. Are these chines more common than I realized?

    I wish mmd wasn't taking a break.

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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Almost all modern planing craft incorporate a chine flat, which may wind from reverse to flat or be reversed throughout.
    These assist in planing, may provide some stability at rest, as well as offering spray deflection underway. Very wide chines are not normally found aboard any boat designed to plane and go above 18 knots or so. Very wide flats, even with a deep V hull, can induce pounding.

    That's all I got. You are correct: MMD would be the person to address this issue.

    Kevin
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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Chine flats started to appear in the 1970's, and as Kevin states they are standard on planing hulls today as well as on many semi-planing or semi-displacement hulls. They often appear these days on round-bottom Downeast types in the form of spray-rails, strictly to knock down the spray on faster vertical sided hulls. In planing hulls lower deadrise means increased lift with lower drag (resistance), so a small flat out at the edge of the higher-deadrise planing surface can and does have a stabilizing effect in turns or a seaway, they improve the handling of almost any higher speed boat. Sometimes they are added externally to a plywood hull, just to simplify construction. Phil Bolger was a early advocate of really wide chine flats, and did some hulls where they turn into a large section of the bottom aft. Today a few designers are making them wider, especially aft, with good results.
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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Hello,

    I've only been around for a couple of years but I miss MMD too.

    It's sorta like he's several posters.... he could post those photos of the mega-maxi yachts, or fishing boats leaving port, but also answer questions about what it is about boat hulls that make them act the way they do.

    I don't know why he's silent but I hope he begins posting soon.

    Regards,
    Alan

  5. #5
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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    "Fishing boats leaving port": That video mmd posted showing the boats departing a Nova Scotia port on opening day of lobster season is something I resurrect every once in awhile, just for fun!

    Quote Originally Posted by Alan71 View Post
    Hello,

    I've only been around for a couple of years but I miss MMD too.

    It's sorta like he's several posters.... he could post those photos of the mega-maxi yachts, or fishing boats leaving port, but also answer questions about what it is about boat hulls that make them act the way they do.

    I don't know why he's silent but I hope he begins posting soon.

    Regards,
    Alan

  6. #6
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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Chine flats to influence performance in one or several areas are becoming fairly common. I developed the flats seen on Bluejackets for specific reasons. Bluejackets are designed from a different beginning than most boats. The desire was for a boat that would run well and efficiently in the range from the so called "hull speed" up well into full planing. Most designers do this by the usual means of starting with a known displacement hull and making changes to make it run well in that 'never never' range from hull speed to about 16 kts or so. It works but the resulting boat has to have more power because the original was a boat that was a fairly heavy displacement design. The resulting hull is heavier, requires requires more power and fuel which fails to meet my intent.

    In my background of racing lightweight sailboats I knew that weight is critical in getting a boat to plane with limited wind power. I interpret this as all up weight per unit area of the hull bottom in the critical speed range. Not having any limitation imposed by prior art and with the small racing background, my plan was to make a planing run well at low speed instead of making displacement boat run well beyond its natural hull speed. Starting from the opposite of usual assumptions is the crux of the matter.

    As in all boat design, there are limitations to this approach as it does not result in a boat that is the fastest. However this approach results in a boat that meets the design objective better than other approaches for a cruising powerboat with good accommodations and comfort at a speed that is unavailable to most similar boats. This range between about 10 and 18 -20 mph is where I and many others want to run on cruises in fairly rough water when necessary and do it efficiently and comfortably.

    There is more to the basic hull flat design than reported here which would only be offered if there is sufficient interest. Bluejackets have been built in size from 24 to 28 feet and have made cruises to Alaska and several times around the Great Loop of the eastern US.
    Last edited by Sandlapper; 04-25-2022 at 05:01 PM.

  7. #7
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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Aha! So, Sandlapper is T. Lathrop? I appreciate your outline. Bluejackets are my favorite design for a trailerable motor cruiser. As you might guess from the Albin-25 in my avatar, I like economical cruisers. Compared to other current fiberglass cruisers such as Rosburough and Ranger Tug which require much more power (and fuel) Bluejacket is capable of all the speed we need utilizing my favorite engine, the Yamaha HT-50. As a small-boat sailor for most of my life, I also appreciate designs that do not disturb sailboats with boisterous wakes. The video showing Bluejacket starting from stop and transitioning to full speed is something I go back to occasionally for another look.

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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Quote Originally Posted by Nicholas Scheuer View Post
    Aha! So, Sandlapper is T. Lathrop? I appreciate your outline. Bluejackets are my favorite design for a trailerable motor cruiser. As you might guess from the Albin-25 in my avatar, I like economical cruisers. Compared to other current fiberglass cruisers such as Rosburough and Ranger Tug which require much more power (and fuel) Bluejacket is capable of all the speed we need utilizing my favorite engine, the Yamaha HT-50. As a small-boat sailor for most of my life, I also appreciate designs that do not disturb sailboats with boisterous wakes. The video showing Bluejacket starting from stop and transitioning to full speed is something I go back to occasionally for another look.
    Ah yes Nicholas, It is only I, second time around as my computer was hacked and crashed, loosing all kinds of stuff including my original forum data and I also forgot my original three digit number. Sandlapper is a colloquial tag for a person born
    in South Carolina. The sand hills were a playground in youth. Along with building a house following retirement from paychecks, I decided to use my engineering education to design boats. Fortunately engineering is engineering in a big number of related fields and all problems are addressed with the same methods.

    My chine flats result in a hull that is similar to a warped plane but it can be modified fairly easily without building a new hull and so it fits an experimental program much easier. Luckily an good result was obtained in the first (much studied) try and I have seen no need to get into an expensive program in which both money and age are limited. My test models show that a warped bottom can be excitingly unstable at high speed so I opted for this modified version which gives the desired result in the lower speed range I was interested in.

    I make no claim of being the only designer to see or investigate this but mine is almost unique. The first light came on when I saw the claims of Weston Farmer's Trumpet design in 1955 and attempted to duplicate the performance in an easily built plywood hull when my time came. I have since found that a Maine designer also used a basically identical chine design but mine was in the water earlier. I think that, given the same assumptions, the end result will be the same if the designer ignores many past rules about the subject. The first of these is to ignore all established rules of "hull speed" as it is a fiction related to a small group of hull characteristics.
    Last edited by Sandlapper; 04-26-2022 at 10:16 AM.

  9. #9

    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    So how much shorter could the Lathrop/Bluejacket bottom go and still enjoy the benefits of the design? Is there a reason 24' is the smallest so far?

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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Quote Originally Posted by Christopher Morgan View Post
    So how much shorter could the Lathrop/Bluejacket bottom go and still enjoy the benefits of the design? Is there a reason 24' is the smallest so far?
    Christopher,

    I don't know the answer as it involves several variables but I do know that the weight of the boat or really the bottom loading is a strong determinate of how easily it will be to reach planing without the dreaded planing hump or its extension. The longer boats do this better if they follow the same scantlings which can result in even lower bottom loading than the BJ24. At some point drag from wetted surface will become more important than bottom loading though. Of course a short wide boat has other disadvantages as well, such as handling, directional stability, etc. These things and such limitations is why boat designers fret over all this stuff. I am happy that my design hits a good range that allows for quite a bit of variation in these factors and am much too old to do more than think about it.

    Some one mentioned a C Dory earlier and we can see some of these effects. A C-Dory 22 is a nice little cruiser which is very popular. Its about the same weight as a BJ24 but is smaller in all respects and much less commodious for its crew. Looking at the bottom, it is not a greatly different form and the low deadrise and flatness is a plus in early planing, but because of the small waterplane the bottom loading is much higher than a BJ24. As a result dealers will not sell a C-Dory with a 50HP outboard and will insist on power of 75HP or 90HP outboard to insure planing. Now, I'm not knocking a boat that many people enjoy and one I advised a good friend to buy to satisfy his desires. The C-Dory on my website video is this boat and it's easy to see the difference. It is, of course faster and gives 32 mph with the Yamaha 90 while a BJ24 will usually top out at about 22 or 23mph with a Yamaha 50 while pushing a larger and more commodious boat. There is a price or an advantage to just about any design decision.

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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Sandlapper: If you would care to PM me, I am interested in whether your shop would still build a Bluejacket 24 and at what approximate price? If not, can you suggest who might? At this point I am not a serious shopper for a new Bluejacket, though never who the next lottery winner will be. However, I am interested in used boats.

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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Quote Originally Posted by Nicholas Scheuer View Post
    Sandlapper: If you would care to PM me, I am interested in whether your shop would still build a Bluejacket 24 and at what approximate price? If not, can you suggest who might? At this point I am not a serious shopper for a new Bluejacket, though never who the next lottery winner will be. However, I am interested in used boats.
    Nicholas,

    Sorry to say that I don't build boats for sale and never made a business of it. Actually I am old enough with health issues that dictate that I don't do boat building any more. I stopped selling plans on the website a few months ago but some people still have the info on their computers and I have honored a number of orders and will still do so as long as I'm able. Quite a number of Bluejackets have been built around the world but none are for sale as far as I know and I only know of one that was sold. There are at least 6 or seven in Puget Sound and several in Australia. I have collaborated with an aluminum boat builder of fishing boats in Melbourne, Victoria and two BJ25.5s built there by Plate Alloy who plans on building several more of other sizes. They have been operating under very strict rules due to both wildfire and COVID but are moving forward.

    While a few have been built by commercial builders, most are built by family couples for which the design and construction method makes easiest. The boat is built in monocoque construction stitch and glue which offers the easiest and best strength to weight ratio of any other method for a home builder. Another friend who has one of the first BJ25.5s but is not a builder, has offered to take over the plans and we will see how that goes as it has been more of a labor of love for me rather than a money maker.

    While this may appear to be a sales pitch, It is not and has never been as boat design became a second and vastly different career from one with Bell Laboratories and all the various complexities of that time when Bell Labs was one of the foremost R&D organizations in the world.

  13. #13

    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Thanks for such a detailed reply. Sounds like 24-28 is the sweet spot, especially holding the 8' beam constant.

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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Thank you for your reply, Sandlapper.

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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    Quote Originally Posted by Christopher Morgan View Post
    Thanks for such a detailed reply. Sounds like 24-28 is the sweet spot, especially holding the 8' beam constant.
    The BJ28 is actually the max beam at 8'6" and has heavier scantlings although the performance goals are the same for all. My favorite is probably the BJ271 where the functionality seems to be the best in most of what I think are on the wishing list.

  16. #16
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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    The Trumpet design was around 17 feet I think but one problem I found when looking at the design is that today's outboards are significantly heavier than the old which is a seemingly odd fact I can't get over somehow. People often say when you switch from 2 stroke to 4 you should go up 10 hp - which further adds to the amount of iron you are hanging off the transom which will further mess with your hole shot troubles/wake and economy so the Bluejackets are really amazing. So if you want to go shorter it seems like there are going to be some obstacles that require compromise - ie you may need to go wider for the weight of the engine or go with a smaller engine and lose some top end speed and possibly shorten engine lifespan if you make it work harder. Very interesting stuff.

  17. #17
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    Default Re: How common tare the comparatively narrow flats (or reverse deadrise) at the chine

    My wife and I appreciate the Albin-25. We encountered a very nice example in the San Juans, gave contact information to the owners and requested they call us if they ever decided to sell. They must still be cruising!
    We sold our beloved C-Dory 22 Cruiser last year. We used it mostly for two-week cruises.
    The Bluejacket family fits right in my preferred powerboat style.
    B&B Yacht Designs has plans for a couple lovely boats in this range, the Outer Banks 24 and Outer Banks 26.
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