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  #1  
Old 02-11-2010, 08:11 PM
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sandy daugherty sandy daugherty is offline
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economical coastal cruiser

Forgive my ignorance; I'm a sailor looking for powerboat to snowbird the US Eastern Seaboard. Is there a design that will carry the accomodations of a 16' travel trailer at 12 knots in coastal waters at 12 nmpg? Since gasoline is currently cheaper than diesel, does that affect the traditional power choices?
Could this be done in a barely trailerable outboard driven vessel? I'm looking for a warm enclosed helm, 15' bridge clearance and very easy access to bow and stern for single-handed docking in moderately adverse winds and currents. I've made a half-hearted attempt to search this site and would be glad to be directed to a previous thread.

The accomodations I would like to find are a double berth, standup head and shower, galley and dinette for 4, stove and oven, H&C pressure water, AC and Heat, 3' max draft, refrigeration, and a comfortable place to read.
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  #2  
Old 02-11-2010, 08:23 PM
Boston Boston is offline
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Diesel is going to be your fuel of choice for the time being. Gas would break the bank in about a split second.

barely trailer-able is a mater of extremes
your in the US so 8 is easy 10 is legal in most states with a wide load sign and a light, 12 is doable but you start getting into hassles

in the end trailer-able is a relative term

you could drag a 60' x14' hull down the road but would you want to is the bigger question

define your idea of trailer-able and maybe we can get some of the designers to chime in and begin to pin things down
cheers
B
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Old 02-11-2010, 09:20 PM
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marshmat marshmat is offline
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I wouldn't be so quick to dismiss gasoline there, B.... not for a relatively small boat in coastal waters. Yes, the range will be better with diesel, but the capital cost difference must be considered- and the difference between a $6000 gas engine of 40-50 hp or so and its $12000 diesel counterpart buys an awful lot of fuel.

Dave Gerr's 5200 lb "Offshore Skiff" ( http://www.gerrmarine.com/power_30.html ) or his substantially larger, 7-ton "DR Northwest Cruiser" ( http://www.gerrmarine.com/power_50.html ) might be interesting starting points- both are trailerable without special permits (the latter just barely so).

12 knots at 12 nmpg is a pretty tall order. To meet that will require a very long, slender hull optimized more for efficiency than for accommodation. (Even getting 6 nmpg at 10 knots is pretty tricky if you want enough room for a couple of people to sleep aboard.)
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Old 02-11-2010, 09:45 PM
Boston Boston is offline
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there are some screaming deals to be had on used diesels these days
seems like something could be found to fit the bill and at substantially less than new prices

that and a diesel will loaf along and get way better millage

my two cents

if Sandy is looking to transfer the space available in a 16 x 7 foot box he is going to be looking at a boat in the 25' range with a raised deck. I know some old school designs that are within that range assuming you ignored his speed requirement but I'm sure some of you folks can come up with something more economical and modern.

Rick is working on something like this but its all still in the theoretical stage and his is ultralight and electric
that and so far it looks kinda like a death trap

cheers
B
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  #5  
Old 02-12-2010, 01:40 AM
TollyWally TollyWally is offline
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The short answer is probably not.
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Old 02-12-2010, 08:24 AM
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sandy daugherty sandy daugherty is offline
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Great responses, excellant points. Lets limit beam to 8.5 ft, and displacement to 8,000 pounds, to keep the towing issue practical. I'm completely open to the fuel question. Outboards appeal to me because of their weight advantages, and because I could get the running gear out of the water much of the time.

If it were easy,Tolly, I might have figured it out by myself. But I would like to point out that the Nimble 32 has the accomodations, and comes close to the economy while hauling around a ballasted shoal keel. Thirty two feet LOA doesn't bother me, I've hauled a fifty foot sailplane trailer through the Rockies.

And thanks for the lead to Dave Gerr. I had not seen this site before, and at a glance it looks like the rest of the day is going to be spent there!
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Old 02-12-2010, 08:35 AM
rasorinc rasorinc is offline
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Here is just one Glen-L design that fits your design parameters. They have others.
http://glen-l.com/designs/hankinson/coastalcruiser.html
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  #8  
Old 02-12-2010, 11:36 AM
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marshmat marshmat is offline
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The Glen-L there would fit the design parameters except for the fuel consumption- you won't get 12 nmpg in that thing, not at 12 knots.

Coming anywhere close to this kind of efficiency at 12 knots in a vessel with the described accommodations is going to mean going long, light and slippery- think of a Nigel Irens power tri, for example. A slender monohull of 40-50 feet or so, displacing maybe two tonnes max, might come close.
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Old 02-12-2010, 01:56 PM
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Tad Tad is offline
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A simple parametric study can pin the magnitude of the problem fairly quickly.

We are offered two fixed numbers, 8000 pounds displacement and 12 knots speed. So run some resistance numbers around length which gives you S/L (speed length ratio). Below are bare hull resistance numbers for an 8000 pound hull of various lengths, all running at 12 knots. These are representative, not exact.

At 25' s/l = 2.4, required HP is 94 and gph (diesel) is 4.5.
At 35' s/l = 2.02, required HP is 53 and gph (diesel) is 2.54
At 45' s/l = 1.82, required HP is 38 and gph (diesel) is 1.82
At 55' s/l = 1.61, required HP is 28 and gph (diesel) is 1.34
At 65' s/l = 1.48, required HP is 21 and gph (diesel) is 1.01
At 75' s/l = 1.38, required HP is 17 and gph (diesel) is .82
At 85' s/l = 1.30, required HP is 14.5 and gph (diesel) is .69

The specific power output of gas is less than diesel, so more gas (gph) will be used to produce the same HP. For the 85' requiring 14.5HP, you're already using more than 1 gph of gas. So that's not going to happen as building an 85' boat at 8000 pounds will require it be rather tiny in cross section.
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Old 02-12-2010, 03:42 PM
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sandy daugherty sandy daugherty is offline
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Those are some lovely vessels, Tad. What would you consider a more reasonable set of parameters? This level of accomodation is available in a 6 nmpg spec, at something like 6 knots. Can we split the difference? Eight at eight? I think it would be prudent to have a higher top speed available that disregards fuel consumption, such as fighting adverse wind to get across a rough section of water. So a pure displacement hull fine tuned to a fixed speed might not be optimal.
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Old 02-12-2010, 06:40 PM
TollyWally TollyWally is offline
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Sandy,
Not trying to be dismissive. I don't have the answer and haven't really seen one despite being attracted to similar but more modest goals myself. Someone like Tad can explain the minutia far better than me, but it is sort of like trying to get 30 mpg out of an RV.

I will be following your thread and paying attention to see if anything new turns up. You are on the right track asking Tad for reasonable parameters. There is a book that always keeps getting mentioned. The Nature of Boats by Dave Gerr. It will get you up to speed on some basics without having to pester people for the elementary stuff. It's sort of like Naval Architecture for Dummies. Once you've grasped some of this book you will be able to ask better more tightly focused questions and will learn more much faster. Best of luck!
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  #12  
Old 02-12-2010, 07:17 PM
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sandy daugherty sandy daugherty is offline
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Thanks Tolly. Amazon says it will be here Wednesday! I'm fairly knowlegeable about outboards, thanks in part to Gerr's prop book so I was thinking about Honda's venerable B50 propped for 12 knots. I got a gallon per hour out of one at low rpm (for 7 knots) on a previous catamaran. I'm not adverse to running small outboards at full throttle; we get 1500+ hours out of Yamaha high thrust 9.9s doing that (with good care) but they are noisy.

I'm looking forward to hearing from Tad: he has some pretty sweet looking designs on his site. I like the lean and narrow look, as purposeful as a WW II Destroyer.
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  #13  
Old 02-12-2010, 08:32 PM
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Tad Tad is offline
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Sandy,

Thank you for your kind words concerning my work.

If we work backwards a bit....this is very (rough) ballpark.....

One (1) gph is our goal. One US gallon of diesel is 7.485 lbs of fuel, .36 pounds of diesel produces approximately 1 horsepower/hour. So 7.485/.36 = 20.8 HP.

A 30' waterline at a speed/length of 1.45 is running at 8 knots. A properly shaped hull running at S/L 1.45 might require approximately one HP per 375 lbs of displacement. 20.8 * 375 = 7800 pounds, close to your 8000.

An 8000 pound 30' boat is quite achievable, but it will be smallish, beam of about 8'-8'6".
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Old 02-12-2010, 08:39 PM
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Willallison Willallison is offline
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Sandy,
In essence, you have laid down your most basic design parameters - which is good. The trick now is to find an appropriate design to meet those goals, or to commision a designer to come up with one. That may seem like stateing the bleeding obvious, but as often as not, it's done the other way around...." I like this boat that does 40 knots and gets 0.5 mpg... can you make it 10 feet shorter, I only want to do 20... oh, and I'd like it to do 10mpg..."

We have:
Tailerable coastal cruiser
Max 8000 lb
Max cruising speed approx 12 knots (negotiable by the sound of it...)
"double berth, standup head and shower, galley and dinette for 4, stove and oven, H&C pressure water, AC and Heat, 3' max draft, refrigeration, and a comfortable place to read."
.... and economical.

I've attached the fuel flow data of a recent design of mine, Graphite, which is a planing cruiser with a top speed of 32 knots, so is not appropriate for your application, but otherwise meets most of your goals. You can see that at a speed of around 7 knots she can manage 6.5 nmpg, and that's from a hull that's intended to travel at between 10 and 25 knots and is powered by a 260hp diesel. The point being that your goal should be achievable without too much trouble... though 12 mpg might be pushing things a bit, at least with anything resembling a 'normal' boat.
In order to manage it, however, you will need to achieve a relatively low D/L (light weight relative to its length), so complications like air con may make things a bit tricky. Removing weight is going to be the surest way of reducing fuel consumption
Attached Files
File Type: pdf Fuel Flow Chart - Rev 1.pdf (135.7 KB, 309 views)
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  #15  
Old 02-13-2010, 10:41 AM
Boston Boston is offline
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I been loving this question and these answers
always an interesting read guys
B

oh
and I also like the idea of an outboard
back in the day it was the cheapest thing for a kid to get ahold of
and I could always make the bastad run

even an old beat up one can generally be brought back to life somehow
that and changing out an outboard is no big deal
so you could get one that works and stick it on there
while you find the one you want and work on it till it runs like a top

my two cents
B
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