Maximum simple trailerable power cruiser - looking for ideas

Discussion in 'Boat Design' started by Steve2ManyBoats, Sep 15, 2010.

  1. u4ea32
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    u4ea32 Senior Member

    I second this observation. 10K or 12K lbs is not that hard, but 3K or 5K lbs is very easy.
     
  2. cthippo
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    cthippo Senior Member

    Something to consider with the canal boat design is that they are designed to sail on, well, canals. By and large, canals don't get a lot of waves, so the narrow, shallow design works fine. Get out on more open water and it may be a different story.

    RE: definitions.

    Is it possible to set up a thread to try to come to some reasonably solid definitions and post them in a sticky? It seems like we spend more time trying to explain what we mean by that word than we do actually discussing things.
     
  3. u4ea32
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    u4ea32 Senior Member

    While recent experience on ocean going narrow recreational powerboats is hard to come by, such vessels were the norm before WW2. After WW2, fuel and powerful engines became much cheaper and widespread, enabling the current fleet of fat, heavy, and inefficient vessels.

    Long, narrow, and light displacement boats dominate the modern fishing fleet in the third world, where pangas and similar are the norm.

    Increased beam leads to more rapid rolling, and more abrupt vertical accelerations. The roll rate is directly related to beam, e.g., a boat with twice the beam will roll at twice the rate. The vertical acceleration in a seaway is related to the cube of the beam, so twice the beam results in eight times the vertical accelerations.

    This is the primary reason that military ships, which must traverse heavy sea conditions speedily and safely, have length-beam ratios on the order of 8:1, similar to the length beam ratios of the maximum trailerable boats being explored here.
     
  4. Steve2ManyBoats
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    Steve2ManyBoats Junior Member

    Roll rate is also affected by Longitudinal CG...

    As long as you can keep the weights centered as much as possible around the longitudinal axis, rolling is minimized. A more common problem in powerboats is ompimizing the lateral CG - Cg far aft planes most efficiently but provides suboptimal performance in displacement and planing transition zones. Cg farther forward reduces drag in displacement mode and reduces the planing hump but reduces top end speed. With a long, skinny boat, displacement speed should be pretty high and the planing hump in the transition from displacement to planing speeds shouldn't be too pronounced. That's one of the reasons I'm considering a big 4 stroke as a powerplant for a long skinny hull, and one of the reasons to opt for a long, skinny hull in the first place.

    One of the probems I've observed on power boats (many times on many boats) that are of more conventional dimensions (3 to 1 LB ratios are very common today), is that you can't drive them at the speeds you want to. Most production planing designs don't perform well at other than planing speeds. Heading dead into a square 4 ft. chop in a 25' production powerboat at more than 15 - 20 mph is downright uncomfortable. However, there are not many 25 ft. production powerboats that can be driven at 15- 20 mph without wallowing in a semi planing mode, wasting a lot of fuel and dragging a big wake. However, my 24' pilothouse fishing boat can do that very well. It has a sharp 50 deg entry but a nearly flat hull at the transom. It will plane nicely at 13 mph and runs nearly flat above 15 mph. Now that hull will never be able to go into even a 2 ft. chop at 30 mph like a 24 deg deadrise boat will but I reagularly pass these boats into a 4 ft chop wallowing in a semi-planing mode while I make steady and efficient progress at 16 to 18 mph, letting the bow do it's thing cutting the chop and the flat stern do it's thing supporting the hull. And I can do this at 1/3 throttle using little fuel and keeping my fillings in my teeth.

    To me, at least, it's all about having reasonable expectations about what I want my boat to do. I won't expect a long, light, skinny boat to carry a load of people or provisions. I won't expect it to cross oceans or maintain a high speed in truly adverse sea states. I would expect it to be relatively comfortable and safe in some pretty nasty conditions as long as I don't overload it and don't overdrive it.

    Just my $0.02.

    Steve2ManyBoats.

    Thanks...Steve3ManyBaots
     
  5. u4ea32
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    u4ea32 Senior Member

    The older I get, the more enjoyment I find in going slow. I don't want a planing hull or even a semi-planing hull, the slamming forces and loads are just not pleasant. A long narrow (say 6:1 to 8:1 L:B) light displacement hull will cruise at 8 to 15 knots without needing to plane, while still getting much higher efficiency that a semi planing hull.

    Its the journey as much as the destination. Once retired, what else are you going to do with the time saved? Sit on the sofa watching TV?

    The shape I'm most attracted to is mostly like a canoe or ocean going kayak: rounded bottom amidship, deadrise fore and aft, double ended, with narrow waterline angles fore and aft. If you were going to move the boat with muscle, you sure would not want a flat bottom over a round bottom -- wetted surface and structural weight is worse. And you sure would hate dragging along that water behind a transom.

    Extra power costs in every way: more to purchase, more to feed, more structure, more complexity, more repairs, more noise, more vibration, more weight, ...
     
  6. tom28571
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    tom28571 Senior Member

    In my estimation, heading directly into a square 4' chop at more than 15 - 20 mph is downright uncomfortable in any 25' powerboat. Either I underestimate the height of "chop" or others overestimate the same stuff. Around here, a 4' square chop is pretty rough stuff in any boat. There seems to be a tendency to talk about chop as if it were a generic "3 to 4 feet". That is serious chop and not seen every day.

    I get asked often if my boats will run comfortably at speed into 3 to 4' chop. If I say no, they think my boats are inferior. If I say yes, I am lying, so I have to say that I think they are over stating their local conditions. I can say that my boats run well in the NC coastal sounds which have the reputation of being as rough as any on the eastern seaboard.

    I don't mean to denigrate anything that has been said but feel that this point needs to be brought up.
     
  7. u4ea32
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    u4ea32 Senior Member

    Tom, you are clearly correct.

    As mentioned earlier, the limit to trailering is more about weight than length.

    Perhaps the real touchstone should be load carrying ability. That's really the point, right? To take you and yours, all the necessary stuff, someplace. The weight of the people and gear desired can be calculated.

    So, for a given amount of load, what would get through rough chop comfortably at whatever speed the owner selected?

    Let's say the speed is 10 to 15 knots (personally, I'm targeting 8 to 10, but for this discussion lets crank it up some).

    There is substantial research that shows beam has a cubed effect on harshness of ride: twice the beam is 8 times the vertical accelerations.

    And we all know that deadrise has a big impact, but it turns out to be more or less linear in the ranges one would consider (up to about 45 degrees) for a planing or semi planing hull.

    Of course, a non-planing hull with a fine entry and nearly plum bow approaches 90 degrees of deadrise at entry.

    Let's assume that water plane area dominates load carrying -- that load carrying really means how much the boat increases depth for a given additional displacement. Not strictly true, but true enough at these low speeds.

    Just for a concrete example, let's say you wanted to carry the amount of stuff that is easily carried on one of Tom's beautiful Bluejackt 24s. This video shows how nice these boats are: http://www.youtube.com/watch?v=6OzDpbMJM0c

    One could get the same waterplane area by doubling the length and halving the beam. Keep the hull depth the same for the same displacement. Sure, the hull and deck surface area would be greater (the minimum for a given volume is a sphere, so the further from a sphere, the more surface area). So one may have to go to foam-glass instead of ply to hit the weight target, but that's not hard.

    The result would be a boat whose hull speed is 1.4 times as fast, and so that much of the semi-planing speeds would become displacement speeds with dramatically lower power requirements.

    And of course, the vertical accelerations would be one eighth as much, so a much smoother ride in that same chop.

    While a 4 foot wide 48 foot boat might seem weird today, L:B ratios of 10:1 were not unusual in the early 1900s.

    For example, a successful racing powerboat from 1903-1904 (including winning the first Gold Cup race), the Standard, was 60 feet LOA, 58 feet LWL, 7.5 feet of beam on deck, an even 6 feet at the waterline, 0.6 feet deep canoe body, with 100 HP. She won 20 to 100 miles races averaging 21 to 24 mph, easily pacing the fleet. Her top speed was apparently about 30.

    A racing displacement of 5800 lbs, with the 3/8" planked mahogany hull and deck being just 1500 lbs, but the engine was 3200 lbs. Remember, pre-WWI, so these were early, slow turning engines: at 420 RPMs, driving a 3 blade 36" diameter prop with 6'6" pitch!

    A displacement length ratio of 12.08! This put the speed where wave drag dominates to the same degree of a typical boat (i.e., the "Gerr Hull Speed") up to 29 knots! It got about 8 nmpg at 10 knots, 3 mpg racing.

    Its worth remembering that all was not wonderful with such boats: at their maximum speeds (SL of 3+) the roll stability was nearly non-existant. Remember those IOR pigs rolling on beam ends downwind? Same problem: the boats would be balanced on their very narrow ends, and would happily roll dramatically to the side.
     

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  8. srimes
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    srimes Senior Member

    Also not much usable room. If the boat is 4 feet wide what will you do in it? You'll be living in a hallway.
     
  9. tom28571
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    tom28571 Senior Member

    There are many varying attributes competing for levels of importance in any boat design. Smooth riding in chop is only one of them. The sine quo non of any cruising boat is that it must provide an adequate level of comfort to its passengers. Cruising amenities and interior room to not only move about but to feel like you are not being squeezed all the time. No matter how it is sliced, this requires some minimum waterline and deck level beam. Sacrificing these qualities for all out running efficiency or chop handling ability makes no sense to me.

    I would put the minimum WL beam at about 6' and the deck beam at about 18" greater than that. 4' is just not practical to me. Bolger might use vertical slab sides and a deck beam equal to the WL beam but since the deck beam has very little effect on running qualities and a lot of effect on shoulder room, why do that? A long skinny boat may be great underway but not so hot in the slip, stored in the yard or when building it in limited space or paying for it in many of the ways we have to do that.

    I find that a waterplane aspect ratio of about 0.34 or so makes for pretty good harmony with all the competing factors for home built cruisers up to about 30'. Above that, a reduction in aspect ratio can be in order which will make u4ea32 happier. We need a better name than u4ea32, give us something usable, please.
     
  10. brian eiland
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    brian eiland Senior Member

    Multitasked Space

    I like this sort of multitasked space on Chris White's Powercat
    ...reminds me of a Bertram 31....or here's another nice space but on a wide cat
    http://www.boatdesign.net/forums/multihulls/weekender-picnic-powercat-33751.html


    ....lets see, the stock vessel is 12.5 feet wide, so we need to knock 2 feet off either side
    ;)
     

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  11. Willallison
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    Willallison Senior Member

    Steve,
    I'm sure that Tom would agree that these are the fundamental thoughts behind both of our designs - Graphite and the BJ series. I've been fiddling with ideas for a larger version for some time now - I don't really think there's an obvious upper limit to which the main principals behind the two boats performance can be applied....
    Vessels that can modify their speeds to suit the conditions are indeed a rarity, but as long marina's continue to charge by length and people only rack up 50 or so hours per year, then that's unlikey to change.

    David,
    Nigel Irens is a strong advocate for the sort of vessel you are talking about - though not with the extreme narrow beam. He has produced a number of lovely boats that slip along beautifully. Low displacement is fundamental to their success however - it allows them to be pushed to SL's of around 2.5 with good economy as long as the displacement length ratio is kept under 100 and preferably less. The problem as I see it though, is that if you load them up - as cruisers are want to do - they no longer perform well. Boats like Graphite and Liz, however can happily contend with heavier loads, albeit at a minor fuel cost.
     
  12. Steve2ManyBoats
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    Steve2ManyBoats Junior Member

    I truly appreciate the contributions to this thread...

    All of the thought and effort that have gone into this post have been most informative. However, I'd like to try to get back to the original thought benind this thread. Here's where we started...

    " My idea was to leverage that mileage and speed and buy/build a 40 ft by 8.5 ft (legal limit without permit) power cruiser, get one of those nice aluminum triple axle trailers the offshore powerboat guys use and drag my boat to a different destination each summer and winter and spend a month in each place just farting around getting to know each place. In general, here are the specifications of what I think I'll need:

    length - 40 - 48 ft.
    beam - 8.5 ft.
    power - 200 hp 4 stroke outboard
    displacement - 10,000 lb (upon further reflection, 7,500 lb)
    construction - ply with xynole and epoxy covering
    accmmodations- 2 adults
    cruise - 12 with a max of 18
    water - maybe 200 gal
    fuel - 100 gal
    no bridge but photovoltaics on the cabin top
    generally a lot of room and minimal gadgets

    For this design to work, there's no need to worry about building space. Assume there's lots. If you have the room to store a 50 ft+ boat and trailer combo, assume you have the temp space to build it. Also no need to worry about marina docking or berthing of any kind, other than to get fuel and some minimal provisions. My intent was for the boat to live on the trailer, move most long distances on the trailer and be local cruising and as "off the grid" as possible while in the water. Most provisioning can be done in the driveway and I hope to never see a marina other than to get gas. So no worries that marina slip pricing - which discourages length and encourages beam - should dictate any design parameters. There's also no need to make the boat narrower than 8.5 ft. That beam restriction is solely driven by US federal highway trailering restrictions (the maximum width object you can trailer without a permit of some kind.) If the trailering restrictions allowed a 10 ft. wide boat, then that's probably what I'd build. Tom is right on point with his observations that spaciousness is a key ingredient in cruising. Heck, that's one of the key reasons why I'm still considering a roomeran. It's not because they sail so great. Most sail OK, but nowhere near a true performance cat like a Reynolds 33 or even an old MacGregor 36. It's because they have reasonable performance but tons of room and the sight lines actoss the bridge deck make them seem even bigger.

    I'm trying to use length as a way to overcome a (relatively) narrow beam dictated by the desire to voyage by trailer without permits or hassle and as a way to increase volume and overall efficiency and seakeeping. Seakeeping is one of the goals but not the only one. I'm well aware that, in cruising, 90+ percent of time afloat is at anchor. In a boat like this, where most actual voyaging is done by diesel pickup and trailer and only the cruising part is done afloat, maybe 95 to 98 percent of the time will be spent at anchor or nosed up to a quiet beach (another benefit of a shallow light boat even if it's long.) Since so much time will be spent at rest, I don't want the need for efficiency and wave penetration to drive me to a design that will feel like living in a tunnel. Similarly, those hard chines that may be less efficient to move at displacement speeds sure will come in handy stabilizing the motion of the boat both underway and even at anchor. Ever see a round, slack bilged boat with a short keel or CB at anchor in a busy harbor? Rolly polly and not much fun.

    It still seems to me that, with an eye towards a more traditional asthetic, I keep coming back to a simple V hull with a nice sheer and a deadrise that runs from sharp forward to flat or nearly so aft. There needs to be some flare forward to keep the bow from burying and to give the forward sections a little volume but the aft portions of the topsides need only enough flare to avoid looking boxy.

    Regarding the cabin, there's an interesting option to consider. Both Benford in his packet cruisers and Bolger in his Wyoming run the sides of the cabin to the sides of the hull. In many of the Bolger designs, it's just one panel with a paint line to mark the transition from hull to cabin. The presumption is that you go forward in the boat through the cabin, not around it. Although it sounds like a kind of crude solution, it does maximize livable volume and will offer the widest cabin possible. In a long, skinny boat, with the helm about in the center fore and aft (for lots of reasons I like that postion best) going forward through the boat to drop anchor or handle a line might be workable as long as there is a clear, open path through the cabin, fore and aft.

    Keep the great ideas coming.

    Thanks...Steve2ManyBoats.
     
  13. tom28571
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    tom28571 Senior Member

    Steve,

    You lay out your case very well and are sticking to it. That is good. It means that there will need to be good reasons to change things, rather than flop willy-nilly about as others suggest variations. I have to run right now but may have a couple comments later.
     
  14. u4ea32
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    u4ea32 Senior Member

    Drawing of 60x7.5 "Standard"

    Found it
     

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  15. Willallison
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    Willallison Senior Member

    I too venture back to my initial post...

    Steve - you have the bare bones there, which is more than can be said for many, but your SOR is lacking in some areas (eg, budget, area od operations, etc) and too detailed in others (eg, 200hp outboard)...
    I suggest that before you can move any further forward, you need to prepare a comprehensive SOR... It's clear that you've spent a great deal of time thinking about this boat, so you almost certainly have it all in your head, but if we are to help you, then it needs to be there in black and white...
     
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