Feedback desired for unusual powercat

Discussion in 'Boat Design' started by Dayneger, Jul 3, 2008.

  1. Dayneger
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    Dayneger Junior Member

    Hello everyone! I’d love have some feedback about a concept I’ve been working on for a powercat for use in Maine. Sorry for the long post, but I wanted to give sufficient information in order to aid responses. :cool:

    While I’ve designed lots of smaller products as a professional and have boated and sailed all my life, I know very little about actually designing great boats. I’ll start by detailing what design goals I have in mind, followed by a description of the current concept. Any thoughts and helpful comments you might have would be greatly appreciated!

    Specification Summary:

    Design a very fuel-efficient, stable, ocean capable powerboat that can be home-built in under 1000 hours for <30k including engines. Pure operational functionality and fast, inexpensive construction vastly outweigh aesthetic considerations.

    Environment:


    The boat will be used for exploring the hundreds of islands and coves along the Maine coast. The water is very cold and the weather is rarely particularly warm, with fog and rain frequent occurrences, so it’s very desirable to be able to enjoy the beautiful environment through large windows from the protection of a comfortable cabin. The tides range from 10 to 16 feet (3-5m), meaning that a small dinghy is required for shore landings. The hulls must have multiple watertight compartments to ensure unsinkability.

    Usage and capacity:

    Most of the time the boat will be cruised by 2 active seniors in their sixties and early seventies, with up to 10 people possible for day outings in good weather. A double berth, a porta potti and very simple galley facilities (cold water, sink and a double burner gas stove) are required. Cruises will usually be for a maximum of 2 weeks duration. It should be possible to move around the boat safely even in large seas.

    Dimensions:

    Trailerability is not required, although the boat should be demountable for transport with a 10 foot (3.05m) road permit. The mounted beam should be narrow enough to allow access to standard marina slips. The length shouldn’t exceed about 30 feet.

    Performance:

    The boat should be able to hit at least 18-20 knots (34.5-38 km/hr) in a sprint while being able to cruise at 10-12 knots (19-23 km/hr) (by far the most frequent usage) at < 3 gph (11.3 L), yielding a range >200 miles (330 km). It must effortlessly cut through sloppy 2-3 foot (60-90cm) wind-driven waves and be able to take a serious beating should it get caught in a storm.

    My Kennecat concept was inspired by some of the Kurt Hughes designs, such as these two:

    32limoferry and 32ferry.

    The concept presented here calls for a 29’ by 14’ (8.84 x 4.27m) powercat composed of 3 components--2 hulls, a frame and a cabin structure. The long, slender hulls shouldn’t require more than 2 outboards of max 50hp each to hit 20 knots. The fine entry and hull length will cut through the chop, with enough reserve buoyancy to pop them back up in bigger waves (no idea what the actual shape is, though, or whether 29’ is actually longer than necessary!). The hulls are currently a round bilge design and hopefully could be built with a fast process, maybe cylindrical molding? (Ideas welcome!). The hulls contain almost nothing other than several watertight compartments, a 50 gallon (189 L) tank per side, and motor control cables. The hulls have massive mounting brackets to attach them to the main frame.

    The reversed bows help reduce windage and weight for a given waterline length, but they’re mostly there because I thought it looked cool. It’s very possible they’re a bad idea! A nearly plumb bow would be fine if the performance and seakeeping were better.

    The main frame ties the two hulls together and forms a solid base for the cabin. It’s currently conceived to be welded together out of 4x6x0.25”(100x150x6 mm) aluminum extrusion. Four beams run the length of the boat, with lateral members out of the same extrusion welded across the beam. The handrails are out of 2x2” or 3x3” (50 or 75 mm) extrusion, depending on the location. They’re firmly mounted to the cabin structure and help create some of the torsional stability of the frame (which I’m still unsure would be sufficient). The frame also integrates support for the front and rear deck areas (surfacing out of wood slats or plywood?). The front includes 2 lockers for rode and fenders, plus enough room to handle the anchor. The lockers also double as seats. The rear deck is big enough for deck chairs and a small barbecue. The integrated davit allows very easy launching and retrieval of the tender, which is crucial in that environment.

    The cabin is mostly a wooden construction out of 0.5” (12 mm) okoume plywood or similar, with a protective layer or two of fiberglass on the outside. There would be stiffening ribs in the corners and along the roof, and all of the built-in furniture would be bonded in to provide structural support. To keep costs and weight down the side and rear windows would be big sheets of Lexan screwed into a thickened frame on the cabin sides, while the front windows would be treated glass. The downward-angled front windows are challenging for the aesthetics, but certainly functional in all that rain and spray. There’s enough headroom everywhere you can stand for a 6’3” (191 cm) person wearing boots.

    Keeping the cabin boxy is poor for windage but excellent for interior volume and ease of construction. Of course, the frame and cabin body could all be integrated as one lighter, highly-cored and ribbed fiberglass structure, but I’m assuming that would massively complicate the construction process to the point that the project would never be finished, especially if I succumbed to the temptation to start making everything swoopy. :rolleyes: Please correct me if I'm wrong on this point.

    The lowest point of the bridgedeck is a full 36” (914 mm) above the water, and the floor of the cabin and rear deck is 6” higher at 42” (1067 mm). The front deck slopes up so that the lower edge of the leading cross beam is 4’9” (1448 mm) above the water. It should be pretty hard to get any slapping or pounding with this design.

    The accommodations are very simple and are similar in spirit to a van I converted into a lightweight camper a while back. The galley has a sink, a 2 burner stove with gas canister, and a cooler. The water supply comes from a few plastic containers with a little plunger pump and soft-sided grey water storage. There’s a small wet zone for hanging jackets and boots, a hanging clothes locker, and a small enclosed head compartment with a porta potty. The dinette converts into a nearly queen-sized berth, with storage below both seats. 3 captain’s chairs grace the front from a raised platform, with an excellent view forward and to the side. A small door by the captain provides easy access to the bow, while the door to the rear platform also allows for a very short and safe route from dinghy to cabin. A battery system and a few lights round out the package.

    The final bonus is that this design makes it pretty easy to bring along sea kayaks! Really beautiful area for them. :cool:

    On the one hand, it seems as though the Kennecat could be an extremely effective vessel for the described environment and user needs. On the other hand I’ve never seen anything like it on the water before and wonder why! What do you think?

    Again, I’d love to hear your feedback about any aspect, from the size and construction of the hulls, to the layout, to the seaworthiness, to whatever comes to mind.

    Thanks in advance :).

    Dayne

    PS Sorry for the hand sketches, I no longer have access to a good CAD system!
     

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  2. messabout
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    messabout Senior Member

    Dismiss the swept back bow idea. That will be most unpleasant when running in a sizeable chop. The hulls look a bit skinny. Have you done the displacement calculations? I think you'll need a lot of power to get to 20 knots, if that is achievable at all with this design..
     
  3. kach22i
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    kach22i Architect

     
  4. brian eiland
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    brian eiland Senior Member

    Hulls are definitely too fine.

    Bows are 'interesting'...sort of trawler/tug-est looking.
     
  5. kengrome
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    kengrome Senior Member

    Hi Dayne,

    Fuel efficiency and high speed are opposites. The faster you need to go, the higher the engine cost and the more limited your hull options ...

    Two small engines always cost more than one large engine so you'll have to go with a single engine for the greatest savings. If you stick with a catamaran style boat instead of switching to a trimaran configuration you'll probably want a center mounted pod for the engine too.

    You may have to use planing hulls which are fortunately easy to build using developed flat panels, but you're not showing this type of construction in your drawings. Round bilge hulls do not plane and they cost a lot more to build, but they are minimally more efficient at your stated cruising speeds.

    Your hulls will probably have to be 50-100% wider for the boat to ride at your design waterline while hauling 10 people around on day trips.

    It looks like you're wasting a lot of useful space by not extending the cabin over the hulls. I wonder why you don't use the full width of the hulls for enclosed living space?

    The swept back bows are probably a bad idea unless you're trying to snag the boat on trash, nets, etc. There are good reasons why most boats have forward sweeping bows -- a dry ride and reserve buoyancy are two you're eliminating by using the bows you've drawn.

    The hull-to-deck connections look pretty weak to me. Unless I had a better look at them and was convinced they could handle the substantial stresses they might experience, I'm not sure I would want to be farther from shore than I could swim in such a boat. It won't matter how many waterproof bulkheads you put in the hulls if the hulls get broken away from the cabin in a seaway. Tying the deck into the hulls continuously eliminates this issue and gives you far more living space too because then you can extend the cabin sides laterally. It also makes the engineering much easier and the cost to build probably much lower as well.

    I think two large glass panels forward might be nicer to look through than the 6 you've drawn. All those mullions will make viewing rather frustrating if you want my opinion ... and if I'm not mistaken you did ask for opinions, right?

    :)

    The fastest and easiest one-off boat building method is usually on frames, and for minimal maintenance and a stronger composite structure the plywood can become the core of a ply / epoxy / fiberglass sandwich.

    It's an interesting design.
     
  6. Dayneger
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    Dayneger Junior Member

    Thanks to all four of you for your initial comments! And yes, critical questions and alternative suggestions are indeed very welcome. This is immensely helpful!

    I'll respond to your inputs in the order written.

    I haven't done any displacement calculations yet. This was simply a first stab at the general layout and I wanted to see if it made any sense before investing more time. The two designs I mentioned above from Kurt Hughes list speeds of 20 knots with 2x 35hp motors, so it seems as though my Kennecat should be able to attain comparable performance (if I get the hull design right ;)).

    I was reading elsewhere on this forum about hull speed and its relevance for very slender hull forms in displacement mode. If I understood the discussion correctly, having hulls with a L/B of 11:1 or more should allow the 1.34*(WL)^0.5 formula to look more like 4*(WL)^0.5 before the resistance curve rises too steeply. If this is true the displacement hull speed should be around 21 knots with these hulls.

    I'm sure the comments about my hulls currently being too narrow are correct, though. I'll fatten them up. Should I also shorten the main hulls a few feet?

    The swept back bow idea was a nod in the direction of wave-piercing styles that are showing up in sailing catamarans and some power ferries. Jeff Schionning used this design for his recent very fast G-Force sailing cat and here is the Nacra Infusion F18. The form combined with very narrow sections for the first 25% or so of the hull is supposed to help a boat punch straight through chop rather than porpoise over it. I'm certainly not wedded to the style, though--just trying it on for size. If it doesn't make sense I'm perfectly happy with more traditional bow forms! :)

    That's an interesting point about one engine on a pod. I'll have to look at how the additional cost for building the pod and adding a strong enough kicker compares to 2 motors. Do you think the boat would still steer ok with only 1 motor, or would I have to add rudders on the hulls?

    I hadn't thought about a trimaran configuration. No idea what it would look like, but I'll give it some thought!

    I definitely need some help with the hull design and construction techniques. Cylindrical molding sounded like a possibility for making round bilge hulls quickly, but I know almost nothing about it. Developed plywood could be a better choice.

    Good question. The cabin plus hand rails as sketched is 10' wide, which I decided to hold maintain in case they would want to have the boat taken apart and trailered somewhere. I also felt that adding more cabin than necessary would just make it heavier and more expensive. I essentially drew the cabin around the minimal desired interior in the interest of keeping down weight, cost, and building time.

    That's very true, and it's hard to see what I intended there from the sketches. The basic idea I was entertaining involved having 2x 6' extrusions welded onto each arm and glassed into the hull bulkheads to distribute the force. The arms themselves would be sized appropriately for the forces and bolted onto the main frame. Since I'm a mechanical engineer I'd probably err on the overbuilt side for the arms. :cool:

    I'll look at the impact of tying the hulls to the frame along more length, as suggested. I'm concerned about reducing the overall beam to 10' in the process, as well as losing the easy walkaround aspect that the current exposed hull surface area allows (at the moment you could actually follow a fish around the whole boat outside the cabin). But, maybe the integration would make the most sense. Or perhaps I'll need to throw out the trailering requirement.

    Is a 14' beam about the maximum for most standard marina slips? I thought I read that somewhere.

    I did ask for opinions, and this is awesome!! Please keep them coming! I agree whole-heartedly about all the mullions. I drew them in because I was concerned about the having large expanses of glass exposed to possible frontal wave impact. I would be delighted to have 2 or 3 large windows across the front if it can be done without compromising safety.

    Could I also use Lexan across the front as well as for the sides? That would significantly simplify the construction and lower costs (although I'd probably have to replace the Lexan every few years).

    If I use aluminum for the frame, do I need to have it painted/coated in some way to protect against corrosion, and which process or product would be the most effective?

    Do you think it would be possible to build the boat minus engines and electronics for $15k?

    Thanks again!

    Dayne
     
  7. kach22i
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    kach22i Architect

    I like the swept-back bows and the example links posted.

    Something crazy in me wants to hook a kite sail up to this design.

    I think sails are going to be added to a lot of fuel burners, why not make it part of the design now?

    Thread on kite sails:
    http://www.boatdesign.net/forums/showthread.php?t=20319&page=7
     
  8. kengrome
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    kengrome Senior Member

    Longer hulls are driven more efficiently, they give a better ride, and they deal with choppy conditions better.

    It should steer well enough. It won't turn on a dime, but I doubt you'd ever need such a boat to turn that radically anyways.

    The Bolger Bantam is a simple trimaran design you might want to look at, if for no other reason than to get some ideas:

    http://www.duckworksmagazine.com/07/projects/bantam/index.htm
    http://www.google.com/search?q=bolger bantam trimaran

    For a one-off design plywood is almost always fastest, cheapest, easiest, etc.

    How likely is it that the owners will ever do this? If it's not very likely, you're designing and building in a lot of complication and expense that won't ever be needed.

    Makes sense to me, but people always want more space too.

    There's nothing wrong with a 10' beam on a 30' power boat. The main reason sailing catamarans have wider beams at this length is to stand up to the overturning forces of the sails -- which you do not have -- so 10' overall beam would work fine and be completely safe with 30' hulls.

    I would make the overall beam almost 10' then install fold-down planks along the sides for walking to the ends of the boat outside. This provides several feet of additional width and better access outside the boat while on the water, yet the planks can be easily folded up against the sides of the boat to comply with the 10' wide load limit.

    Or you could go with the hulls a full 10' wide then install removable planks that extend over the water on each side of the boat. If you ever decide to trailer the boat some day, just remove the planks and you're back to a maximum 10' trailering width ... :)

    You can always build the boat to accept interior mullions that are installed manually if/when the waves threaten to get that big. Two or three mullions made of little more than 2x4's could be installed and locked into place only when you need them. I would think this might eliminate your concern about big waves while allowing full viewing the rest of the time.

    You could, but it seems like a hassle and not the cheapest solution in the long run by any means. I would go with laminated safety glass like that used in auto windshields. It's heavier but it also lasts forever and gives you a much clearer view all the time.

    If you don't count your labor, it might be possible ... but you would be wiser to come up with a final design first, before you try to guess at a price.

    Go join the [tolmanskiff] Yahoo group and ask those guys how much they invest in their home-built Tolman Jumbo skiffs if you want an idea of the cost of a smaller monohull that would serve a similar purpose. There are dozens of builders of these boats and they are not shy to tell you what they spend on everything that goes into their boats. Just keep in mind that your boat will be bigger and it will have two hulls to build instead of one, so yours will cost more (quite a bit more I suspect) in materials even if you value your labor at $0.00 per hour.

    Personally I think you should stick with a 30' overall length, a 10' overall width, and build the cabin as wide as possible -- possibly with those folding side walkways I mentioned if you really think the boat needs them. But if you ever hope to buy the materials and components (other than engine and electronics) for $15,000 you'll probably have to do one of two things:

    1- Have the boat built overseas where materials and labor are so much cheaper that even with shipping to the USA your total will be less than if you had built the boat yourself, or ...

    2- Make cost savings your primary focus during the design phase, and eliminate every feature you don't really need -- because every item that goes into a boat raises the price a little bit more, and it all adds up.

    One good thing about building the boat in one piece and keeping the overall width 10' or less is that you can 'easily' trailer it on a simple flatbed trailer if you hire a crane to get it up there, and you'll never have to hassle with disassembling anything before you transport the boat over land. Boats built in one piece cost less to begin with too of course ...

    :)
     
  9. Dayneger
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    Dayneger Junior Member

    That's a pretty funky idea!

    I read up on it some and it's a very intriguing trend. I'm not sure it's the best idea for this user situation, especially issues of launch, retrieval and control while weaving between islands. However, your comment did get me to think about the possibility of adding wind propulsion to the concept!

    The best layout I came up with to meet the needs of low center of effort, good efficiency and extremely simple sail handling would be to use the rig Garry Hoyt (designer of the wonderful Hoyt jib boom) has developed. While not perfect for all sailing situations, it could be a great fit for adding on to this kind of vessel. You can read more about it here: http://www.garryhoyt.com/id19.html

    The attached sketch shows vaguely what the boat plus rig could look like. The sail came out a bit small as shown, with only 150 sq ft of sail area, but it could be made larger, ideally by lengthening the booms rather than extending the mast. I'm curious how low the height to length proportion of the sail can be before performance falls off dramatically? Anyway, it should be possible to get the boat moving along at a few knots on a reach if the winds are above 10 knots.

    To make this concept work I'd have to integrate a stayless mast and rig into the structure, along with a daggerboard / swingboard in one of the hulls and one or two rudders. The two rudders concept would work well with only having one outboard, as mentioned earlier. I also played with ideas for making the amas collapsible and extendable to get more beam. As far as I can tell it would work to have the beam collapse down to the 10 foot trailering limit and then extend out to about 18 feet maximum beam.

    The down side is that by the time I added all of the necessary structure and blocks, cleats, sheets, sail, etc. I'd probably have added about 200 lbs and a lot of extra time and money. Probably only really makes sense if the likely percentage of sailing (and performance under sail) would be much higher. Hmmm. Still, fun to think about! :)
     

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  10. Dayneger
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    Dayneger Junior Member

    Under way it's very true, there's no real reason to turn that rapidly--no slalom courses here! :) I was concerned more about situations such as trying to maneuver the boat in a tight marina on a windy day, when the widely-spaced twin hulls and high-windage superstructure could be hard to handle.

    Thanks for the link about the Bolger Bantam! Interesting design work there.

    That's an excellent question and one that's well worth considering carefully. The owners will be retiring very soon and given their love for water, I can easily imagine their desiring to have the boat in warmer climates than Maine for some of the year. I'll confirm this with them, though. What I know little about is how much of a hassle it is to have something 10' wide trailered. If anyone has some experience with this, please let me know!

    I appreciate the thoughts about keeping the overall beam down to 10', but I think I'll stay with the ability to walk around the boat on the hulls. I also like the more stable platform resulting from the 50% increase of center-to-center hull beam and the modularity of having the boat out of 4 large pieces.

    What a great idea! I think I'll work with 3 windows across the front, with provisions for mounting your storm braces should things get Really ugly on the water.

    Where do people get laminated safety glass, and is there a best-practice way of mounting them?

    I'm sure I'll need to very much take that to heart, more so than I currently realize. It's so easy to get big eyes and start wanting to throw in all kinds of expensive goodies. :rolleyes:

    Which building materials are less expensive in low-wage countries?
     
  11. kengrome
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    kengrome Senior Member

    Wouldn't they want to cruise to the warmer location?

    It is very litte hassle if you hire it done. Otherwise you need a really big truck and trailer and wide load permits for the states you'll tow the boat through. Then with your current design you'll also have to hire several people at one end to take the boat apart and load it onto its trailer before you leave, then hire several people at the other end to get it off the trailer and put it back together again.

    Well, it's your boat. I wouldn't appreciate the inconvenience or the expense myself, but I'm kind of a simple-minded designer ... :)

    Go visit a glass shop, they should be able to help you.

    All the basic raw materials that gets used in volume: plywood, lumber, cement, flat glass of all kinds, fiberglass, epoxy, steel in all shapes and sizes, etc., etc.
     
  12. FAST FRED
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    FAST FRED Senior Member

    "hauling 10 people around on day trips."

    If the folks are going to pay , even just a "contribution" for fuel,or snacks, you will need a USCG license and the boat will need to be built to USCG Sub-chapter "T".

    That would require inspections of the plans , inspection during construction , and pass inspections when afloat.

    The local charter folks with their short short season will turn you in in a NY second, the first time you pull out of the harbor.

    FF
     
  13. Dayneger
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    Dayneger Junior Member

    Perhaps they will. But, it's an awfully long way from Maine to Florida or to the West Coast! :)

    One of the links from a Tolman builder answered my question about the marine glass. The Wynne company seems to have reasonable prices: http://www.wynneinc.com/Models/models.html. Any thoughts on which windows should be tempered glass and which ones laminated safety glass?

    Good point. Fortunately I was only referring to bringing friends and family along, with no commercial application. No "contributions"! :cool:

    Going back to the wave piercing concept, I sketched out an alternative bow with geometry analagous to those from Gold Coast and Craig Loomes. I find myself oscillating back and forth about the aesthetics, but if it would work the it'd be pretty cool!

    The piercing bows as drawn would make for a more complicated construction, but it could also make for a very smooth ride. It does seem as though 20 knots is fast enough for their effect to be desirable. Is there a reason why wave piercing shouldn't work with only 30' hulls? Could the hulls still be designed to behave well in larger seas?

    Thanks again for your inputs!
     

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  14. kengrome
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    kengrome Senior Member

    Laminated glass is for windows you don't want 'open' even when the glass breaks. I would use it on all my boat windows where I don't want to use acrylic.
     

  15. FAST FRED
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    Location: Conn in summers , Ortona FL in winter , with big d

    FAST FRED Senior Member

    Any glass that may see a wave over the bow ,should be laminated.

    You must use glass on any window that will have a windshield wiper.

    Cheap glass or plastic is fine where there are no loads or need to keep clear to navigate.

    FF
     
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