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Proof that yacht design isn't rocket science

Discussion in 'Yacht Designers Discussion' started by bmattes, Mar 3, 2010.

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  1. bmattes

    bmattes New Member

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    Let me start by saying I'm new to the world of yacht design so my questions will probably be simple if not annoying to the true professionals on the fourm.

    As to the title of this thread, I'm an astrophysicist by trade but must admit that there are nuances of yacht design that are well beyond me.

    Background: I'm working on my first design for a "down easter" style yacht, and the plan is to produce an extremely efficient hull design that can hopefully cross the pond without sinking or getting beat up too bad in big water.

    Current working specs: 60' LWL 12' Beam ~3' draft ~35,000lbs Disp.

    Now to my "newbie" questions:

    If i'm considering wood/epoxy construction (should I be at all?) what's a reasonable hull thickness?

    Should I be worried about how high the free board and superstructure are on a yacht this narrow for roll? If so how high is too high?

    Will have a couple other questions after I upload a pic for the members to judge my level of lunacy from.
  2. Emerson

    Emerson New Member

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    Astrophysics isn't rocket science either...

    From what I gather superstructure is minimal on Downeaster's not because of the roll imparted (which, when compared to the moment of inertia off of the sails and lines and mast is miniscule) but rather because it's catching wind but not contributing in a meaningful way to the forward momentum of the boat. You will still need free board to stay in the water as you heel over, but you should try and keep the superstructure to a minimum.

    I suspect that a super efficient boat will have a lot to do with how you arrange and stow gear and lines while the canvas is up.

    As a Caveat I am not a yacht designer or a sailor
  3. bmattes

    bmattes New Member

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    very true emerson... they don't let us play with things that blow up.

    Thanks for your input. However I probably should have added that it's a MY.

    Have a quick rendering from some free CAD software that leaves much to be desired. Maybe I'll try and do it in one not designed for ships later.

    But here it is, for what it is...

    edit*
    not even sure if this qualifies as a downeaster actually. And the mast will be for dual booms to handle a dingy and paravanes/flopper stoppers. Can't tell from the pic but the are ahead of the mast is an open flybridge.

    planning on twin screws (would one be better?) but couldn't get the rudders to draw right

    Attached Files:

  4. wscott52

    wscott52 Senior Member

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    Is that axe shaped bow for real? If so, what is the rationale?
  5. 84far

    84far Senior Member

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    wscott52, google Sea Axe. But it is on a planing hull and the claim is it can run at high speeds in some big seas, but also still be fuel efficient. Cheers

    Far
  6. Emerson

    Emerson New Member

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    I misread your first post, thought you wanted a 40' boat, didn't think you could safely make passage in that

    Have you looked into how much HP you want? Just like rockets on a boat you have to take all of your fuel with you. Say you have a 400 hp diesel powerplant that you run at 90% to keep you at your hullspeed of around 10 knots (where did I get these numbers? I pulled them straight out of my ... thin air) Since a marine diesel can get about 60% efficiency and you have about 2000 miles to go to get to the Azores that is 200 hours of 360 hp, which I calculate to be 1.9328X10^8 KJ divide that by .60 to compensate for the efficiency problems and you arrive at 3.22X10^8 KJ of fuel for the trip. Diesel oil is 7.09 lb/gal and 2.049X10^4 KJ/lb. That is 15,645 lbs (7.8 tons) of fuel, or 2,200 galons, ~ 300 cubic feet that's a lot to fit in the bottom of your boat. Granted fuel economy will get a bit better at half load rather than full load (I don't think you need to stay fully loaded for stability) but a margine for safety is nice to have.
  7. wscott52

    wscott52 Senior Member

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    Interesting, I'd never seen it. Thanks.
  8. Henning

    Henning Senior Member

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    For a 60' displacement hull boat (you can't cross the Atlantic on plane without a very specific purpose built vessel) you'll need around 90hp to travel at hull speed which in a 60x12 vessel will be around 11 kts if properly designed and in nominal seas. In expected conditions figure 10. 90hp in a modern diesel is going to use around 3.5 GPH, so if you bunker 1000 gallons, you have the Azores + basically 33% reserve, that's around 3 tons and 4 cu/meters with ullage. That is easily carried in the vessel described.
  9. Emerson

    Emerson New Member

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    Thanks!

    So would he want two engines for this? I can't see the monster he will need to go past hullspeed performing very efficiently at 20% power.
  10. Innomare

    Innomare Senior Member

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    Your freeboard will be determined by a number of factors. To name a few: reserve stability (for re-righting of the boat when heeled at a certain angle), standing headroom inside, dryness of your cockpit, ...

    Take into account also the visibility from the navigation position. Probably not such a big item on rocket design, but very important on boats. There are guidelines for this in the European Recreational Craft directive for boats under 24 m. Not applicable to you if you're just buidling a one-off for yourself, but still it's good to at least try to comply with safety standards.

    The axe-bow concept is terrific, but I don't know if it would work on such a small scale. To work properly, the freeboard in the bow must be very high, because due to the lack of bow flair, the boat goes more through the waves than over it. You do not want to dig your bow under too many waves in the middle of the North-Atlantic.

    As for wood/epoxy construction, build it so it would be strong enough in just wood. Consider the added fiberglass as protection for your wooden structure. The thickness depends on a lot of factors (speed, frame spacing, location on the hull, etc.). There are class rules which you can use as an indication. Have a search on the sites of Lloyd's Register, ABS, DNV, BV, GL, etc. Some of them have quite a lot of information publicly available.

    Good luck!
  11. Marmot

    Marmot Senior Member

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    That would be quite a remarkable marine diesel, particularly for the size and type you have in mind.
  12. bmattes

    bmattes New Member

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    I must say thank you all very much for the info. I half expected people to laugh at the strange design, should have known better on this forum. :)

    To give some more info:

    Yes i have seen the sea axe but was unaware that it was a planning hull. Wouldn't the axe bow still provide a great deal of benefit in reduction of wave making at displacement speeds?

    Really glad for the advice on engine fit out. Just curious if it would be possible to run twin diesels and a genset at ~10kts and burn under 4 USgal/hr

    Also as designed now the forepeak is at 11' above the waterline, with 8' of freeboard in the front and 6' in the rear. Total height at the flybridge is 15" above the waterline. Seams like a lot of leverage length for a 12' wide boat but I have no idea the righting moment or how to find it.

    Also there's no bow flare for better hydrodynamics as far as resistance is concerned, is that a problem?

    Thank you again for the help
    Hopefully the boat will be built in the next year to two.
  13. CODOG

    CODOG Senior Member

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    I too doubt it would work on the round bilged hull depicted in the OP's picture.
    I suggest that a keel is added, running from the lowest point of the 'axe', aft to approx just forward of where the prop / props hopefully will be. The design depicted lacks directional stability IMO, and the lower part of the axe is increasing draft for no benefit. A 'Downeast' semi displacement hull design modified to incorporate an axe bow is one thing, but to take away the inherent stability in following seas that the traditional 'Downeast' keel design provides is something else.
    BTW...love the thread title.
  14. bmattes

    bmattes New Member

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    codog
    Thanks for the advice on the keel. I actually have two alternative hand drawings that both show keels but as I'm new to ship based cad programs and much better with a T-square and pencil, had the same problems with the keel as the rudder.

    I have one with a single keel basically as you recommended from axe to stern, and a dual keeled version (don't know if this is a good idea or not) for the twin engines.

    This is a very amateur question and I know it depends on a lot of variables like bulkhead spacing and glass weight used. But I'm still trying to figure out a ball park guess for the hull thickness...was thinking 3/4" below 1/2" above with three layers of glass/epoxy in and out...any comments?
  15. CODOG

    CODOG Senior Member

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    :D I'm absolutely gob-smacked that an Astrophysicist uses a T-Square and pencil....please reassure us that at least some of the potential earth killers out there are being tracked with something more, er, hi-tech ? At least a couple of valves...maybe a capacitor or two ?
    Single keel is IMO fine for twin props too, in the grand scheme of things thus far.
    As for the scantlings of the hull laminate, no, its not rocket science, but then again, it is a very specialised aspect of yacht design. I'm afraid that there is only a ball park answer to your ball park question, such as possibly, depending upon, and such like.
  16. bmattes

    bmattes New Member

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    I dont know about valves but I'v been known to use a couple super computers and building-sized telescopes ;)

    unfortunately your answer as to my ballpark question is about what I expected. It sounds as if I'm going to have to consult a professional
    (read $$$)

    Shouldn't be too hard here in the annapolis area. They go around calling it the "sailing capital of the world." Anyone have an idea what it would cost to take a set of lines plans, offsets, and requirements list to a pro and have a feasibility analysis done?
  17. Grecko

    Grecko New Member

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    Going to a pro is the right choice, but you need to think more like getting somebody to design it rather than just consulting.

    For example, the axe bow may offer advantages in drag, but the version depicted in the earlier post would result in severe bow steering if not addressed with a healthy keel, and then the resulting hull might still have handling problems.

    There are design standards that serve as guidlines for structural design of ships, but to do this right you need to understand the design of the framing in addition to be able anticipate where and how your design is going to be used, lest you end up in the middle of the pond with water coming in at a high rate. It isn't just the thickness of the hull, it is the entire package that has to be determined to figure out how thick it needs to be.

    While yacht design isn't astrophysics or rocket science, it is a very specialized disicipline that takes into account structural design, hydrodynamics, stability and propulsion. Experienced designers blend all of the conflicting requirements into a compromise that results in a successful design. While it doesn't look that difficult, when compared to the design of something like an aircraft, there are a lot more variables in a boat and the results can be a disaster if the design isn't balanced correctly. And frankly a good bit of experience is required to make those design trades in an intelligent manner.

    I guess what I am trying to say here is that while I am sure that you are a very intelligent fellow, you are probably diving in at the deep end of the pool if you think that you can design a yacht that is a successful design without spending years of research and study on the subject. And no I'm not a yacht designer, I'm only an engineer, but I know enough to know what I don't know, and I probably would not be undertaking such a task unless I did my homework and learned a lot more about the design of a yacht than it appears that you have.

    I would recommend that you get and read cover to cover a few books on design (Elements of boat strength by Gerr might be a starting point) but I'm also sure that the experienced designers might chime in on more recent of better reference books.
  18. AMG

    AMG YF Moderator

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  19. Innomare

    Innomare Senior Member

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    Yes, a professional yacht designer/naval architect will cost you some $$$, but they will be well spent. Don't forget that it will cost you a lot to actually build the boat, operate the boat and maintain the boat. All those costs may spin out of control if you start out on a flawed basis.
    Speak with a designer, and have a budget made both for the design and the entire build. Then decide to go through with it or not. Many people have unrealistic expectations of what a newbuild boat should cost.

    Good luck.

    PS. I can recommend Wooden Boat magazine if you plan to build in wood. They also have a large collection of plans for selfbuilders on their website www.woodenboat.com.
  20. Henning

    Henning Senior Member

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    I don't really agree on going sail. If you design an efficient displacement hull you about the same on 10 year costs and similar port to port mileage, but you spend a lot less time "in between". Plus you have to unstep and haul around your mast cluttering your deck when you want to do most of the inland canals in Europe. Unless you have a keel/centerboard type rig, you also need considerably more draft for the size, even with the board. Plus sailing rigs are a safety liability. The problem with boats less than 120' or so is the 7.2:1 WL length to beam ratio get impractical because the boat ends up too skinny, although, if it's ok for you as a personal boat, that's excellent, just don't be expecting a big resale market. For me, minimum for an ocean crossing boat is 72' WL because that gives you 10 kts cruising speed, and with proper power, 12kts to stay out of most of the severe weather. 15' is about the minimum practical beam for a boat that size although 12 is doable. 10 would be optimal hydro-dynamically but there are some things that can be done to add "virtual waterline length", and if you care for the expense, recapture a lot of waste heat to boost your efficiency. Even without though, 72'x15' in something as below will get you 10 kts on 4-5 gph in nominal sea.

    Southern Ocean 72 LS.jpg
    Southern Ocean 72 LS_Linesplan.jpg

    For me personally though, I'd build something more like this:

    Hennings Boat 127_Linesplan.jpg