A new Raidboat and Cruising Dinghy Design for Home Builders

Discussion in 'Boat Design' started by luckystrike, Jun 29, 2019.

  1. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Hello!

    In this thread I will start to design a new Raidboat (or Fast Cruising Dinghy) for home- and professional builders alike. It will be on the simple, fast and cheap side of construction with as few parts to be built as possible. But it will be but not a „simple as can be“ design to allow all the features that make sailing and cruising live safe and comfortable.

    On the sailing side it will be a fast and lively boat with a good overall performance and balance, able to sail many miles a day. Easy Daysailing with your partner and one kid is also a design target as well as racing on club races.

    For me Lightweight construction and -displacement is the key to reach the targets and also allows easy trailering and -handling on the beach.

    Safety is another important topic in this design. Recovery from capsize, self bailing cockpit and getting back aboard ... I will turn attention to these topics

    Concept Catalogue

    General
    This boat is meant to be a singlehander for cruising and cruising raids and can take up to two crew and a kid for daysailing. When sailing in club races it shall not automatically earn the red lantern.

    Length over all will be two sheets of plywood long, meaning a little under 16feet. So it will be a long and light boat for a single hander and therefore a fast one due to the long waterline.


    Safety:

    A raised floor for a self bailing cockpit after recovering from capsize, no bailing out water when sailing in strong winds and waves, no need for Elvström bailers, no wet sleeping bag when sleeping in the boat.

    No side tanks (a la 470 or 420) to allow the boat to sink in a little when capsized. This helps to reach the daggerboard from swimming to re-right the boat. It also makes the boat roll back more slowly, so that you can climb back into the boat in one rush as the boat becomes upright. The boat will stay at 90° heel when the mast is nearly parallel to the waterline and will have no tendency to become 180°.

    A safety ladder for getting aboard after capsize as well for taking a bath when anchoring.

    A sea anchor keeps the boat with the bow into the wind while reefing or capsizing.

    Water ballast will calm the boat down when sailing in strong winds and inceases the stability. It also helps to re-right the boat. Off course the ballast tank is empty in light winds and regattas.

    Performance and Balance

    Sharpie hull with flaring sides. Sharpie hulls offer good performance when designed with care. In a modern design like this one they have a slim waterline for low resistance and good planing abilities. The center of flotation is slightly aft of the middle to allow an effective trim from the aft sitting skipper. The flaring hullsides increase the sailing stability when sailed heeled and dampen the tendence to capsize. They also make hiking out more effective.

    Lightweight construction. The lighter the boat, the faster it is … so simple. Even if this design is a cruising dinghy and has to carry a lot of stuff beside the crew, for me performance is important. Take out all the cruising stuff and have some fun racing on club regattas. Overall boatwheight ready to sail but without cruising gear should be well under 90kg.

    Effective sail plan. Cruising Dinghys need a sailplan with a low center of effort and the ability to reef easily to make sailing safe. Singlehanders are suited best if they have only one sail to handle. I have chosen to use the balanced lug sail with a large sail area as the standard rig for my design. It can be trimmed very good to balance the boat upwind and is very neutral and effective when the wind is coming from the side or behind. Mast will be free standing without the need for shrouds.

    I know that a lot of sailors prefer the yawl rig for cruising so this rig will be an option in the plans. On the downside of this rig I see the building effort of 3 to 4 more spars and the cost of two sails instead of one. And the loss of efficency of the departed sail area, the increased wind resistance and the wheight of the aditional spars.

    The rig of the OK-Dinghy or other singlehanded dinghys is another option. Even the sails from a windsurfer can be rigged, but this would be a experiment on your own risk. Windsurfer sails cannot be reefed.


    Comfort

    Side decks for comfortable sitting and hiking out over a long time.

    Sleeping inside the boat is possible on the uncluttered raised floor and unter a custom deck tent. This is important in my home country as „wild“ camping is not allowed in Germany.

    Dry stores are under the raised floor in the forward part of the boat, reachable through watertight hatches in the floor. Here are fixed places for the anchor, water supply and other heavy gear that will double act as ballast. A step upwards in the raised floor brings here a height of more than 30cm (1').

    High sides for dry sailing in the sitting area midships


    Construction

    Construction will be sitch and glue, the fastest and most effective construction method for plywood. Every part you build will also be in boat later. No need to waste time and money on a buiding jig or temporary moulds. Okume marine grade plywood with 6mm, 5ply will be used for all parts of the hull. 4 frames/bulkheads plus stem and stern will form the transvers members. Only one stringer will be in the hull sides, mainly to be the base for the raised floor.

    The Sharpie Hull is the fastest and simplest hull to build and with it's three panel layout also the cheapest one to put together. There is nearly no waste in the panels

    The round Mast will be made from wood in the birdsmouth technique and rectangular sections for boom and yard.

    As far as I can see it now you will need 7 sheets of plywood and 12 – 15kg of multi purpose Epoxy for glueing, sheeting and laminating glasfibres.
    Opposite to 5 sheets of ply for a „simple as can be boat“ of the same 16 feet lenght. In my oppinion it is worth the extra effort.


    That's it for now!

    I'am open for comments and incitements. If you have other topics and ideas to be minded let me know and I will try to incorporate them into the design.

    Have fun! Michel
     
    Last edited: Jun 29, 2019
  2. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Here is a first impression of the hull, no raised floor and side decks yet ... hydrodramatics first!
    Raidskiff Radical hull and deck4.jpg

    Perspective view with side decks ... and still no raised floor
    Raidskiff Radical hull and deck2.jpg

    Have fun! Michel
     
  3. Waterwitch
    Joined: Oct 2012
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    Waterwitch Junior Member

    I will have to see your design of the raised deck forward. But what caught my eye was the idea of stowing heavy objects forward on such a small bow as ballast; and not making the boat trim bow down as a result.
     
  4. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Hello Waterwitch,
    first to say ... sorry! English is not my native language and I became a little confused in this very long post and have chosen the wrong term. Raised floor is the correct term., not raised deck. I corrected that in the post.

    second ... a cruising dinghy has to carry equipment, clothes, water and food. A all this stuff this has to be stowed somewhere. A designer can calculate this to be sure his design is in good trim. In german this is called "Stabilitätsberechnungen zum Längstrim". So stay tuned and follow the development of this design. ... and of course I will not place heavy equipment in the bow of the boat. The first metre is a separate flotation tank filled with helium to keep the bow up ... *smile*

    Regards, Michel
     
  5. Squidly-Diddly
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    Squidly-Diddly Senior Member

    reverse rake on bow says "wave piercing" but top view says "not thin enough to pierce waves" and would be sending lots of water over the bow.
     
  6. messabout
    Joined: Jan 2006
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    messabout Senior Member

    If you mean to have some flare forward near the bow, you will need to rake (slope) the stem. Yes that will shorten the waterline a little bit. Alternatively you could let the stem taper outward as it approaches the stem head. The tapered stem might not be the best alternative when you encounter some serious chop or wakes.

    I suggest that you place the lowest part of the bottom, with respect to the base line, farther forward, something like 45% of the waterline length +/-. Meanwhile move the widest part of the chines aft to somewhere past 50%. The idea here is to keep the run at the lowest angle consistent with the needed aft rocker. Basic trig. The reason for moving the beam aft is to have your area distribution transition smoothly. Construct an area distribution curve to tinker with the section sizes and placements.

    I have a boat very similar to the one you have described. It is a nominal 16 foot Sharpie constructed of 6mm Ocumee for the sides and 9mm at the bottom. It weighs 59kg including a glass clad bottom and a lot of saturation epoxy and paint. The chine widths of the boat is one meter at the widest point. The boat will plane readily with one person and minimum gear. All up weight in planing mode is about 150 kg. It goes to windward well enough, particularly if you let it heel to about 10 degrees more or less. Sail area is about 7 square meters which is modest and easily manageable. It has a sprit boom rig. Your idea for a lug rig has considerable merit but may not be the fastest one you can imagine. A lot of that depends on the conditions at the regatta venue. The lug has a lot going for it as a cruising sail.

    Your design has some compromises to make. You want it to be fast in single handed mode but still capable of accommodating two adults and a child. You can have one of those aims but not both at the same time. For example if you design the boat to perform well with extra weight of passengers and gear, then it is likely to suffer some speed loss when lightly loaded. The evil Gods of boat design forbid us to have all our best intentions become reality.

    Aside from all that nay saying, I wish you the best of all outcomes.
     
  7. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Hi Squidly Diddly,
    this will be no wave piercer, the reverse rake (just 5cm) is just for looks to underline the modern appearence of the hull. If you don't like it you can have a vertical bow, no problem.
     
  8. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Hello Messabout,

    you are no "nay sayer", I asked for comments and suggestions and this it what you did. Your answer shows that you know what you are saying. I thank you for your suggestions, I will consider them in the next round of the design spiral.

    Just to be sure that I understand you correctly ...
    1. your percentage ist meant to be from bow to stern? Here in Germany I have been teached it the other way round, we also number our stations from stern to bow.
    2. Do you suggest that there is no curvature in the rockerline at the deepest point of the hull parallel to the baseline?

    Yes, there will be some taper outwards in the stem. Otherwise the upper panel would be narrower towards the deck and result in less volume. I don't want this.

    Boatdesign is always a compromise. My focus is on having a comfortable, well performing, light displacement cruising dinghy for a singlehander with all the features mentioned above. Having achieved this succesfully, why not take all that cruising stuff off the boat and your partner and kid in for a nice daysail. Racing on club races ( we call it "wednesday regatta") is last on the list and just to have a little fun. My new design is in no way a raceboat and I will not incorporate details typical for a racer (or a daysailer).

    Seems that its time for a working title for the boat ..."my new design" sounds not so good. ... suggestions? ... *smile*

    Is your sharpie your own design? 59kg .. Whou! hull wheight only or rudder, board and rig included?

    Regards, Michel
     
    Last edited: Jul 1, 2019
  9. messabout
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    messabout Senior Member

    Michel, the percentages that I mentioned are only approximate. They are subject to some graphic and numeric experimentation to have the underbody of the boat (the normally wetted part) comply with what you/me/we display characteristics that we think appropriate. The operative word is ; "think". It is well to consider the experience of others to arrive at some idea of what works well, and why it works well.

    Uffa Fox, for example, was an early and successful designer of fast dinghies. His designs often had the deepest part of the hull well forward of the mid point. He also moved maximum beam aft of the midpoint. In answer to your question, yes, I was referring to the locations as measured from the front of the boat. Please note that these design details are not carved in stone.

    The lowest part of the bottom would, it is claimed, be advantaged if one or more of the maximum displacement areas are the same..... Some catamaran designers use this feature. The general idea is that the water is being pushed outward in the front part of the boat and then presumed to be returned to some exit point near the back of the boat, that contracts into a more narrow exit. The argument is that we should give the water some time or space to transition from expansion to contraction of surface pressure. That is getting a little too technical and will surely get some arguments from knowledgeable forum people.

    None of that works for a flat bottomed boat like a sharpie if it is sailed flat. The exit point is wide and almost perpendicular to the direction the boat is moving. A well designed sharpie is a whole other beast when it is heeled appropriately. The waterlines become rather like a foil shape, the wetted surface decreases and the section centroids line up into a near straight line. I do not mean to become overly technical here because we are only discussing possibilities.

    My little sharpie is named Goldilocks. That is because she is too narrow to use a powerful sail but too wide to make an ideal rowing boat. It does row quite nicely but is not equal to a narrower boat. In any case I like it and recognize her limitations. The bare weight of the boat is about 59 kg and the rudder, dagger board, and sail rig come to about 12 more kg. I am a lightweight at about 73 kg so the all up displacement is not a lot. This boat was designed as a single hander and the rocker reflects that aim. If the anticipated weight had been larger I would have given it more rocker in order to raise the transom clear of the waterline when carrying more weight. Goldilocks behaves reasonably well with an additional person in the boat but the difference in behavior is noticeable. The original design had the forefoot barely touching the water. I did that to ....hopefully.... make her tack more readily. Sure enough when it is loaded more heavily it is not as happy when tacking, lightly loaded it tacks as if on ball bearings.

    I do like the simplicity of the flat bottomed sharpie but there are other configurations that can save a bunch of wetted surface and also improve the structural rigidity of the bottom. The so called trapeze bottom is a case in point. It is still built with flat panels of plywood but the wetted surface is decreased by a significant amount. That makes a difference in light air. A trapeze bottom is simply an arrangement that uses a flat section in the mid line and has some upward tapering parts that approach the upper chine............example.... A mid section of perhaps 48 inches (excuse me for lapsing into the Imperial measure mode...my mind works in that obsolete way) might have a middle plank of 30 inches width at the widest part and a port and starboard bevel that makes a triangle 9 inches wide and rises 5 inches. Such an arrangement also mitigates the eddy making of the plank intersections, reduces the bottom width and therefore the need for excessive framing, and also reduces the wetted surface by a favorable amount.

    I have gotten carried away with opinions and a few actual facts. I had no original intention of making this into a federal case. Apologies. I will stop typing and go away for now.

    P.S. Wednesday afternoon informal regattas are often called "Beer races" here in the US.
     
  10. messabout
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    messabout Senior Member

    After all that typing it has occurred to me that there is already a thoroughly proven boat that is close to your specifications . It is the Goat Island Skiff. It is a lug rigged sharpie that goes like a bandit and is simple and moderately economical to build. It has become a very popular boat and it has many capabilities.

    The GIS is wildly popular for good reasons. The Australian designer/builder?sailor, Michael Storer, knows his business. Take a look at any of the numerous videos on You Tube. You will see a lot of happy people sailing a good little boat. Here is one of them....


    Designing your own boat is appealing to some but doing so is to expose oneself to disappointment. Some say that amateurs who design their own boats are embarking on a fools errand. I have been on those errands more times than I care to recall. It is much safer to pay the few dollars for a proven and well established design that has had all the bugs worked out of the build process.
     
  11. luckystrike
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    luckystrike Power Kraut

    Messabout,

    yes, we are talking about possibilities and I will carve nothing in stone of that what you said. It's my design and I will do it on my own, respecting my own practical experiences. But its good to hear a second opinion.

    I like sharpie hulls too, mainly bebause of their simple build and fair hydrodramatics. I draw area distribution curves for the underwater body too (at 0° and 10 to 15° heel). Even if I have to do it by hand, because my design CAD program makes it just for the whole station. But even then I make model tests with a radio controlled model before I start prototyping. Just to be sure the the sharpie lifts its forefoot out of the water without wheight shifting (for example).

    Of course I know the Goat Island Skiff, I have built one for a customer and sailed it a dozen times after the build. But it has its shortcomings, mainly because it is designed as a daysailer and not as a cruising dinghy. (See my first post).

    The I550 sportsboat is another good example for a powerful modern boat with a sharpie hull.

    Have fun! Michel
     

  12. luckystrike
    Joined: Feb 2010
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    luckystrike Power Kraut

    Round 3 in the design spiral.
    Further devellopments in hydrodramatics and the double bottom is in.

    Raidskiff4.jpg
     
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