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Thread: Sailing/rowing/motor sailing dinghy decisions.

  1. #36
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    I've been doing a lot more thinking, making up sail plans, researching and I think I've settled on a sail plan. Well, sort of.

    First off, I decided to just keep the mast as short as possible, but if it winds up to long to store inside the boat then that's just how it will have to be.

    In regard to boomless lug sails, There are a lot opinions about them running the gamut from "they're great" to "they suck". Most of the "they suck" opinions seem to come from those who desire the utmost performance from their boats, and the "they're great" crowd favors ease of use and are willing to sacrifice a little performance. I guess I don't mind sacrificing some performance, and see enough of the pros that I want to give one a try. I may not stick with boomless, but I at least want to experience it.

    The 3 sail plans below all use the same mast, yard, and mast location. The plan on the right uses a scaled version of the boomless standing lug sail Todd Bradshaw designed for an Argie 15 builder with the addition of a small jib. I'm going to start with this rig initially to decide if I like it. If I do it ends there.

    12 Dinghy 11-3-18.jpg

    If after sailing the boomless lug I decide it isn't for me, I can move to one of the two plans on the left. The only real difference between these two is the main sail area. I think for a little boat like this, the 67 sq ft main shown in the center should be enough, but the a main in the one at left is a little bigger.

    As for the jib, I'm going to try running it without mast shrouds at first. I think it may work based on the laser pico which has a similar sized jib on an un-stayed mast. Time will tell. If it doesn't work I'll just add dyneema shrouds.

    My plan is to first build using lumber store plywood so that if this thing is a turd I don't have too much invested. For that one I'm going to use a centerboard box that allows me to shim the centerboard either forward or aft within a 1 ft range to allow for the different CE from each of the sail plans and to find the best location for it. I'll be able to hack on it and try it out with the motor, and oars, etc.

    Getting back to the hull, I originally designed it with the V-hull cross section in the picture below, but am considering re-designing it with a flat bottom cross section similar to the one shown. I would use the flat bottom all the way to the transom. I'd like to hear opinions on which one would be better. Some experimenting in Freeship indicates that I should be able to get a better shape on the bow.

    cross section.pngFlat bottom.jpgFREE!ship linesplan.jpg

  2. #37
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    The flat trapeze bottom is the best choice. I use it on all my plywood designs.

    JS
    sassdesign.net
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    I'm not lost, I'm just uncertain of my position.
    I'm still confused, but on a higher level

  3. #38
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    I like everything in the plan except the fact that the leech on the jib will forever tangle with the yard. A longer bowsprit?
    -Dave

  4. #39
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by Woxbox View Post
    I like everything in the plan except the fact that the leech on the jib will forever tangle with the yard. A longer bowsprit?
    I was so focused on getting the main worked out that this slipped through the cracks. I took a quick jab at it as shown in the picture below. I lowered the head a bit, extended the foot, and put some hollow in the leech but left the bowsprit the same length. I think this would work and only lost about 1 sq ft, which I could get back by extending the bowsprit just a few inches.
    jib mod.jpg

  5. #40
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    The five panel boat looks dope. Five panel boats work pretty well, for how simply they are made.

    If the loose footed sail doesn’t work out, you can always just clap a sprit boom on her, like a sharpie sail. Nothing to adding one except a snotter...

    Peace,
    Robert

  6. #41
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by Andrew2 View Post
    The Drascombe range mostly have boomless mains and also have a reputation for poor windward ability.
    Having fitted a Drascombe boat with a proper boom, I think the poor windward ability can be attributed to the flat steel plate centerboard and rudder.
    That and they are handicapped by waterline length.

  7. #42
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by WI-Tom View Post
    [...]also easy reefing (which the spritsail doesn't do).[...]
    I have yet to see an explanation of how that is so.
    It is often stated as a fact, but I have, despite explicidly asked for it, never seen any proof.
    Fact is that one reef can be taken in on a sprit sail quite easily, under way; I have done it several times and it takes less than a minute.
    As for simplicity, there is a huge misconception that a sail needs to be hoisted, with a small boat the sail can be laced permanently to the mast and the mast and sail taken down together. With a larger boat/sail the sprit sail can be belayed to the mast instead of taken down.
    Personally I can not see how it can be any easier.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  8. #43
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    I should add that I am genuinly interested in why lugs are supposedly so much easier to reef/handle than sprit sails. I am by no means advocating that sprit sails are superior.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  9. #44
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by marujo.sortudo View Post
    With a boomed sprit there is a simple emergency reef where you remove the sprit and it top of the sail flops over in the lee
    A boom is not needed for this.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  10. #45
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by mohsart View Post
    I should add that I am genuinly interested in why lugs are supposedly so much easier to reef/handle than sprit sails. I am by no means advocating that sprit sails are superior.

    /Mats
    I've sailed both, but have far more time with lugs and they sure do reef easily. I think the problem with reefing spritsails is that as the sail is reefed, the heel of the sprit eventually gets low enough to hit the foredeck or otherwise cause problems. You can rig it to get around this, but it's an added complication compared to lugsails. I can see how taking one reef in may not be enough to cause that problem, but it's not unusual for me to sail with 2 or 3 reefs in.

    I'm aware that spritsails don't need to be hoisted; I always kept mine laced to the mast as you describe.

    It can also be a little pesky to keep the tip of the sprit attached to the sail while reefing or deploying the sail.

    I have never heard of anyone who has sailed both spritsails and lugsails who finds spritsails easier to handle. But I like spritsails. I just like lugsails a lot more.

    Tom
    Last edited by WI-Tom; 11-04-2018 at 08:23 PM.
    You don't have to be prepared as long as you're willing to suffer the consequences.

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  11. #46
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    So now that I have settled on a sail plan, I am working more on the hull design. I'll be going with a 5 panel design similar to the last one I posted. I'm contemplating the beam width. As I said before, there are 2 primary uses for this boat. First is sailing of course, but also fishing with an outboard leaving the sail behind. I currently have 2 outboards, a 4 hp and a 15 hp. I would like to use the 15 hp if possible and based on safe horsepower calculations It looks like I'm going to need about a 6 foot beam on a 12' boat. That seems a bit wide for a sailboat. I was initially planning on 5' beam. What do you guys feel about a 6' beam on a 12' sailboat? Too much?

    12 ft dinghy Linesplan 2.jpg
    Last edited by 109jb; 11-07-2018 at 10:25 PM.

  12. #47
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    That's catboat propirtions. You'll need a bigger sail, bigger rudder and bigger centerboard to make it work. And heavier scantlings in the hull. This is the upward design spiral at work, driving cost and complexity in a vicous circle.
    Have you tried a 4 hp motor on a very light boat? It can be very quick.
    -Dave

  13. #48
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    What exactly do you mean by "make it work" ? So does that infer that the sail plan I had proposed earlier it won't work, and then what would that mean. I understand that performance under sail would suffer, but would increasing the beam like this really take it from liking everything in the plan except the jib cut to it not working?

    Please don't interpret this wrong. I'm not trying to argue, but trying to really understand what the ramifications to sailing performance would be.

    BTW, yes I have used a 4 hp on a small light boat. Not my idea of quick. My last fishing boat was 16' and had a 115 hp on it, 52 mph flat out with just me in it I don't expect anywhere near that kind of performance, but if I can use the 15 hp I'd like to. Not mandatory, but desired.

  14. #49
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Adding beam to any boat adds a huge amount of drag. If you widen your boat to 6' it will have similar shape and dimensions to Phil Bolger's Bobcat. This boat is said to sail quite well, but look at how much sail it takes. 110 square feet. On the power side, to lift up and plane a hull needs a long, straight run aft. So even if you did make it wider and clamp on the 15, I don't think the boat would perform the way you'd want it to. The stern would squat a lot, for one thing. I think you understand that what you're trying to do is get a multipurpose boat that doesn't really exist. The hull shape that sails well is not like the hull that planes under power.



    On the plus side, you've pretty quickly and independently come up with a hull shape that closely matches the work of an extremely talented naval architect.

    Last edited by Woxbox; 11-08-2018 at 09:47 AM.
    -Dave

  15. #50
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    I'm not an expert on boats but everything that have read in books and absorbed from this forum indicates that a boat that will plane with a 15hp will sail like a dog (so boring that you probably will give up in less time than it takes to read all the responses on this post).

    A 12 foot boat that sails reasonably well will put its nose skyhigh and dig its tail in with the 15hp, and maybe even with the 4hp. See post on the Dyer 12.5 above. There are pictures on the net showing that an upturned table with an outboard on the back will plane better than that. The displacement speed of ~4mph that a 12' sailboat can be run at can be achieved with 0.5hp.

    If you want something even reasonably quick, you need two separate boats. If you think a small light powerboat with a 4hp is too slow, you might not like sailing at all because it will be guaranteed to be slower.

    The boat you have drawn is closer to a sailboat, even if narrower would be better. It's bottom isn't wide enough and it has too much rocker for planning under motor.

    I hope that more people with real life experience at dual-purposing a 12' boat will present their data points on this, because otherwise this is all just empty theory.

  16. #51
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by WI-Tom View Post
    Tortoises have four legs and also have a reputation for poor land speed ability.

    Tom
    I wasn't going to bother, but the reason they don't point well is the boomless main. Compriende?

  17. #52
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    15 HP is way too much for that little boat. 5 HP is more than enough. Even that much can rattle her to pieces.

    Fast skiffs are capable of working in both planing and displacement modes. To plane, move aft to make use of the long flat run. At low speed, move forward to get the butt out of the water a bit.

    However, I would not worry about designing an efficient planing hull that much. There is so much power available from an outboard, it isn't really necessary.

    Read ALL of Phil Bolger's books to get an idea of how boat hulls work, and read Frank Bethwaite's book to see how modern fast sailboats work.

  18. #53
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Thanks guys.

    I'll just keep the beam to 5 feet. I can always sell the 15 hp and buy a 9.9, or just use the 4 hp I have, although my 4 hp is an import and is more like 3 hp in reality. I don't know how many of you are fishermen, but droning across a lake to reach a fishing spot in an under-powered boat is a pain I would like to avoid to as much extent as possible.

    I'm not too concerned about bow rise for a few reasons. First my primary reason for this boat is to sail, so the bow rise will be what it is. Next, having been a power boater my whole life there are many things that can be done to reduce bow rise and I have played with many of them. In addition, I have seen several videos of similar designs and they do have some bow rise but it doesn't appear unmanageable. Actually in a couple of Argie 15 videos it appears they do quite well in terms of bow rise and my design is quite similar. Finally, the foolproof way to keep the bow down is to roll off the throttle which goes back to my first point which is that the bow rise will be whatever it is and that's ok.

  19. #54
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by Andrew2 View Post
    I wasn't going to bother, but the reason they don't point well is the boomless main. Compriende?
    I understood what you meant. I just disagree with you. Boomless sails do quite well to windward--it's off the wind where they lose some performance.

    Tom
    You don't have to be prepared as long as you're willing to suffer the consequences.

    www.tompamperin.com

  20. #55
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by 109jb View Post
    the bow rise will be whatever it is and that's ok.
    Correct me if I'm wrong (I'm not a power boater) but isn't the bow rise what happens in-between when the boat is going at the theoretical maximum speed and when it starts planing, without any significant increase of speed?

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  21. #56
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Yes technically bow rise is the bow coming up during the transition to planing. It is just easier to say that than "bow attitude during planning". Bottom line is the aft up-turn on the hull of a sailboat will cause the bow rise to be high, and also the bow attitude during planning. But whatever it works out to I'll just have to deal with.

  22. #57
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    109,

    The price of having a boat that can sail, motor (and row a bit) will be using a light low power engine on the transom, and going at low to moderate speeds. At 12ft (also relatively shorter than optimum for the best results here), you could look at an Oughtred Shearwater for a planing hull, with some fore and aft heeled balance that will still enable sailing performance with the lug rig and even the odd semi inefficient row (the Guillemot will row better but not plane).

    Ignoring this, your design objectives will become suffocated by compromise and incompatibility, if not the weight of that '15'. If you cannot accept lower speeds and a light engine, you will have to forget sailing and rowing, and build something power focussed to bolt to the front of that engine, and have bigger, better and faster inshore fishing boat: a 'classic' like a Jericho Bay Lobster Skiff or something more contemporary from the Bowdidge Marine catalogue.

  23. #58
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    My point is that you cannot really deal with the bow raise.
    You either have to go at a relatively slow speed, or put enough power to get it planing.

    /Mats
    My blog about my time as a boat building student, a rigger apprentice and Journeyman http://kaptenmohsart.blogspot.se/

  24. #59
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    For me the interesting part of the thread title is 'motor sailing dinghy', which strikes me as a relatively unexplored area. Given that the boat may have up to a 15hp motor there doesn't seem to much point in pursuing sparkling light air sailing performance.

    Maybe the ideal rig is a small one optimised for motor sailing - where the apparent wind comes forward and the rig provides some drive but also steadies the boat?

    I am sure it will offend all kinds of purist but I think Russel Maxwell's Endorphin Tri offers a glimpse of something fascinating - pairing a 3.5hp outboard with a sailing rig:


  25. #60
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by mohsart View Post
    My point is that you cannot really deal with the bow raise.
    You either have to go at a relatively slow speed, or put enough power to get it planing.

    /Mats
    Having been a power boater, I would disagree that there isn't anything you can do about the bow rise during transition or the bow attitude while planing. There are many things, including but not limited to transom angle, trim tabs, wedges, wings, ballast, and more. I'm already envisioning things I can try once the boat is done.

    Having said that, as I already stated, I am going to finish designing the boat with its primary purpose, sailing, driving the design. If the motoring or rowing performance suffers I'll deal with that later if I need to.

  26. #61
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by Clarkey View Post
    For me the interesting part of the thread title is 'motor sailing dinghy', which strikes me as a relatively unexplored area. Given that the boat may have up to a 15hp motor there doesn't seem to much point in pursuing sparkling light air sailing performance.

    Maybe the ideal rig is a small one optimised for motor sailing - where the apparent wind comes forward and the rig provides some drive but also steadies the boat?

    I am sure it will offend all kinds of purist but I think Russel Maxwell's Endorphin Tri offers a glimpse of something fascinating - pairing a 3.5hp outboard with a sailing rig:
    I think you are misunderstanding the design criteria. This will be a sailboat, OR a motor boat, OR a rowboat. There will be very little combined mode use. Although I may take my little 25 pound 4 hp outboard along while sailing in case the wind dies, it is not intended to be a motor sailer. It is intended to be primarily a sailboat, and when I want to go fishing I will leave the sail, mast, etc behind and use it as a sail-less dinghy with an outboard.

  27. #62
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    Default Re: Sailing/rowing/motor sailing dinghy decisions.

    Quote Originally Posted by 109jb View Post
    I think you are misunderstanding the design criteria. This will be a sailboat, OR a motor boat, OR a rowboat. There will be very little combined mode use. Although I may take my little 25 pound 4 hp outboard along while sailing in case the wind dies, it is not intended to be a motor sailer. It is intended to be primarily a sailboat, and when I want to go fishing I will leave the sail, mast, etc behind and use it as a sail-less dinghy with an outboard.
    Oh well, that is what the syntax of your thread title seemed to suggest. Best of luck with the design.

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