economical coastal cruiser

You will find outboard propellers are close to optimised for their size and intended use. You cannot beat the laws of physics. The limited diameter is a constraint on the efficiency possible at lower speeds. If draft is an overriding factor then you can add more outboards and reduce the thrust required from each. The load is spread over a greater area.

For lower speed application there may be benefit in increasing the number of blades up to 6 but strength limitations then become the constraint.

Rick W

 

The feasibility study I did earlier has given me some insight into the original objective of 12kts at 12nm/USG.

There is a decision tree that can be explored to set the design space based on this single requirement:

1. The basic requirement means we are considering a powered vessel using liquid fuels. It is not going to be sail or solar powered.

2. It will be powered by a diesel or petrol ICE - we will dismiss other options such as fuel cells and steam plants.

3. A diesel can produce 16kW and the petrol 12kW. (If someone has good specific fuel consumption for particular engines it would be good to have more accurate figures than my estimates)

4. There will be house loads to consider. Things like fridge, nav lights. instruments. These could range. Probably from negligible to maybe 2kW. Lets take midway of 1kW.

5. With the diesel we now have 15kW available and the petrol 11kW available.

6. We then have to transmit the power to a shaft. A good transmission will lose no more than 5%. So the diesel will have 14.25kW on the prop. The petrol 10.4kW.

7. The diesel swinging a 600mm prop can provide a thrust of 1800N. The petrol swinging a 300mm prop can produce 1150N.

8. The power limit means the boat is not going to be planing or even semi-planing so we can dismiss those hull forms. This means it will be a displacement hull.

9. A displacement hull to get to 12kts, able to carry the engine and fuel, plus a few people and be structurally sound while achieving 12kts with the modest thrust will need to be long and slender. If it is diesel powered with a 600mm prop it can have a drag of 1800N. If it is petrol powered with a 300mm prop it can have a drag of 1150N.

So that sets the design space based solely on physics and a single requirement of 12/12. It shows what a choice of diesel or petrol will allow in the final outcome. Either is feasible but the petrol driven one with the small prop will need to be about half the weight.

Rick W

 

Or... instead of taking a guess... we could actually consider making a list of the things that are required by the client and get an accurate measure of the house loads....
Hey! I've got a great idea..... why not write a design brief... and SOR....

For those who are interested, here's a link to an article writen by Steven Hollister about the design process. It is just one man's version and everyone has their own take on the process, but it clearly sets out the basic steps that most professional and competent amateur designers would follow to ensure that the optimum design solution is reached:
http://www.westlawn.edu/student_center/lecture1.asp

 

Oh you just don't understand. The proper technical wording is....

Meaning taken from the Oyster Rules of order.
the fine art of coming together to create the perfect union.

 

A well considerd approach no doubt.

On ballance optimise has it.

Optimise is used 39 times. Compromise is used 4. I note now that you have taking to using the term as well. At least I have contributed that much to your education.

Rick W

 

Rick... you have contributed a great deal to my education... in generaly I read everythig that you write and know the opposite is likely to be true.

We should all be open to continued learning. Tel me... what did you learn from this...?
http://www.boatdesign.net/forums/boat-design/compromise-31901.html

 

Or this

http://www.boatdesign.net/forums/boat-design/client-always-right-31899.html

 

Cut the arguing already, boys. All of you are better than that, I know you are.


Looking at Rick's numbers from post #242 above, the goal of 12 nmpg at 12 knots in something big enough to live on for short periods does appear feasible, if we can keep the weight down. Of course, as Will points out, we have to define our standard of living aboard. So, from post #1:

AC and H&C pressure water, plus a fridge? I suppose we're not talking a kilowatt of house loads, but rather several kilowatts of house loads, probably around 5-8 kW with the AC and fridge cranking away in the Florida sun. And a lot of time spent either on shorepower or with an idling engine. But the rest- double berth (is it OK for this to be a folding dinette?), standup head, galley, a layout conducive to single-handing and with a warm place to drive, no more than 3' draught- all of this appears to be feasible within the given limits.

As an example: My partially-designed 2.5 tonne (loaded) trimaran meets Rick's cap of 1800 N thrust at 12 knots, and has sufficient space for the desired amenities. But since it's designed for an outboard, a 60 cm prop on a diesel is not possible. Furthermore, no outboard I'm aware of can handle the kind of house load being talked about in post #1.

So I think we're looking at a long, slender displacement hull, about two tonnes loaded, quite possibly with stabilizing amas, and, if the client will allow it, an inboard engine (if only for the generating capacity and ability to swing a big prop).... and treating house loads separately from propulsion loads when considering miles-per-gallon. Sound about right to you guys?

 

I fear of the very common error of newbee project planner.

Forgotting ALL the implicit statements the customer did not even think he had explicitely to request, because they were so evident for him that it could not be otherwise.

1) The solution should be within customer budget.
2) The solution should have enough comfort and amenities for the reminder of the family accept to climb aboard.
3) The customer and his family should enjoy the ride.

This was not stated, but implicit for the customer. and ANY solution that does not match the 3 clauses above will be rejected.

 

fcfc, you are correct.

I think that in this feasibility study, cost isn't specified, but its easy to find examples of very light boats built by skilled home builders for $10K USD. If anyone is thinking very low prices, they will be thinking home built too. There is nothing so far identified that requires highly expensive construction. On the contrary, it appears that even plywood could be used, which means any fiberglass/core will work too. So I think that the cost can be specified, the more $$ the nicer quality.

I think the feasibility studies don't overly restrict volume -- just weight. So meeting 2 should be easy.

Item 3 (enjoy the ride) also does not seem to be restricted.

In other words, there is plenty of room for a designer to meet those requirements, regardless of what they are, as long as the weight stays on target. To meet the 12/12 requirement, the customer will simply have to live within those weight limits.

 

There is none better in the area of generators for quietness and fuel efficency for any boat that we are speaking about with the basic needs and wants. Even the smaller one runs an AC unit. Charging batteries can even include the simple Mini Kota solid state chargers for house batteries and lights which we also use for the 24 volt batteriy systems for day trolling motors for fishing hulls. All of these units use simple 12 gauge drop cords and the units are tiny and easily stored. We also use the 12-110 refrigeration is we get fancy. Even cooler sized ones works well without all the built in units too. The simple lists is pretty large. KISS

http://www.hondapowerequipment.com/products/models.aspx?page=models&section=P2GG&category=sq

 

I agree with the honda generator. And outboard. KISS.

 

Fair point Matt - especially given the rant I had on the subject a couple of weeks back - but what is one to do when confronted with comments like this?

There is so much misinformation here that it beggars belief. Rick has put forward a proposal, based on nothing more than one theoretical aspect of the design problem. In the process, he has called into question the professionalism of an entire industry. Yet when challenged by those same professionals, he either ignores, patronises, or belittles them.
Further, his solution has some glaring practicality problems - massive draft, high cost as a result of the need to custom build things like props and gearboxes, an absurdly low bridge-deck clearance that will make the boat all but useless in anything more than a surface ripple - just 3 examples.
Everyone has a right to have input here - Rick included - but that input is equally open to scrutiny.
Having said that, I shall endeavour to curb my frustration about the way in which this is progressing and try to post in a more positive manner.

As yest nobody has addressed fcfc's valid question - one of the most fundamental of all - what is the budget for this project(s)?

 

House loads and low power engines.

I can talk a bit about high house loads and low power engines. My boat is a 4 ton full displacement hull driven by a 13.4KW Sabb diesel with a 490mm controllable pitch propellor. I get up to 16 mpg at an s/l ratio of 1.15. House loads that I can have is a 150 amp 12V alternator, a 550watt air conditioner, electrical needs and a 850 watt coffee pot that tends to be on.

The 150amp alternator is driven by a single V belt and probably does not turn up to full power due to the fact that a the Sabb is normally turning at 1450 rpm with a 2:1 reduction.

If the GD coffee pot kicks on you can kiss three tenths of a knot goodby and waste an incredible cup of diesel a day running that power hog appliance. Seems a good price to pay even though its anoying to have the big change in sound and speed just cuz the coffee pot kicks on. Its about the same load as putting the paravanes in the pond to stop the rolling. My fuel bill for 2,985 miles of travel was $432 USD or 7% of my operating budget. I have trailer this boat all over the eastern US and Canada. Perhaps I should look at optimizing the prop eh!.
http://s706.photobucket.com/home/PierreR/index

 

Hush, everyone, HE'S BACK!!?! who? the OP!!!

I've just spent two hours wading through all I've missed, with some regret. I had no intention of starting a cat fight over how many angels could dance on the head of a compromise. That accomplished absolutely nothing.

But in spite of that, some very, very interesting points have been made. And I am pleased that several people with similar interests have helped refine the brief. The suggestion of a very large diameter propeller offering such dramatic improvement in fuel efficiency is at the top of the list!

I know I should put my vote in for some more specific desiderata, so here goes.

I think I would like to self build (with modest skills) a powerboat for exploring the Eastern Seaboard of the United States and Canada in inclement weather. Clear sunny days are for sailing!. I believe that world oil supplies will peak in 2012, making the price of fuel skyrocket to a point where gas guzzling, 30 knot condos will no longer be ALLOWED, or taxed out of business as the Tobacco Industry has been. Recreational fuel will be rationed.

I want my First Mate to come along; thus I require a stand-up shower with hot water, Air conditioning for sleeping in Florida's steamy hot summers, and heat for Coastal Maine's Fall chill. We would like a double bed, 58" x 78", and about two feet of hanging closet, (for winter clothing)with lots of dry storage, sealed off from stinky bilge air. We should have standing headroom next to the bed and in the head, where a fat old man can sit with his knees 2.57' apart.

There need be only one enclosed head (with integral shower), but it should have a 24 gallon holding tank. Since this boat anticipates future events, there should at least be plans for 24 gallons of gray water, plumbed to the head and to topside pump-outs that can be reached and serviced by people who don't bend well.

The Galley should contain a two burner cook-top and a small oven, burning propane or LNG. A top loading DC refrigerator should hold something like 2 or 3 cubic feet of food stuff. Interior furniture should be able to hide a common brand of ice chest, too. Hotel loads will demand 300 Amp hours, and there should be a separate starting battery. The cabin should be able to seat four adults at a convertible table, but there should be an ample single berth available without dropping the table.

There should be forward facing seating for two, with ample foot rests. There should be a sturdy, lockable door to the cockpit, that need only be big enough for two portable chairs and a padded bench for two more. That cockpit should be just long enough to cast a fishing line from. A door to a swim platform would be nice, but not a top priority.

The boat should be tow-able behind a half ton pickup truck. That implies a length of less than 40', a max beam of 8'6", and a weight on a trailer of around 8000 pounds. It should not have so much draft that it would require a steep ramp for launching. The boat should not exceed 3'6" draft when fully loaded. Serious bonus points will be awarded if all this stuff will squeeze into a standard shipping container.

The boat should not require custom castings anywhere but the propeller: mid range off the shelf components will be used for windows, hatches, plumbing, deck hardware, etc. Decks and wall where windows and hatches are mounted will be flat.

There will be enough fuel on board for 600 miles at cruise, with space reserved for additional long-trip bladders.

It should have the look of a 1920's commuter on Millionaire's row, but be built with synthetic materials that mimic fine bright-work. There should be a mast no more than 15' above the water that supports an 18" radome, nav lights, flag halyards and a jib boom capable hoisting a 10 foot dinghy on the cabin roof. Provision should be made for a bow or stern thruster with its own battery, if possible. Extra points for trolley car style windows.

It should be powered to cope with short period 3' waves and 25 knot headwinds; to keep the bow into the wind at least, and to make 3 knots to windward as desirable. The boat will carry a dinghy with a gasoline motor, and a Honda EU20001 (I love mine!) that hides in a vented but spray protected compartment. Propane and outboard fuel storage shall be vented and protected from spray and driving rain.

If an inboard engine is installed, it shall drive the biggest diameter propeller possible within the draft constraints and prudent separation from the hull. The rudder shall be sufficiently balanced to reduce steering loads to the capacity of the least expensive autopilot.

Minutiae: Every available void should provide storage. Other voids will be accessible to a hand and arm length of 28". wiring chases will be supported well above any fluids in the bilges including that resulting from flooding that might occur from an open hatch or a misplaced transducer plug. Bilge frames will have large weep holes at the low point to drain to a single lowest point with sufficient room for two electric pumps and float switches. The engine shall be accessible from all sides, and have a sufficiently sturdy lifting point directly over the center of gravity with the transmission attached. the bilge under the engine shall be hand accessible for cleaning. All floor boards and head liners shall be removable, revealing all plumbing, electrical, control, and air handling parts. Tanks shall be positioned under removable panels to facilitate replacement without bloodshed.

There shall be plenty of elbow room at the helm, coffee cup holders at cruising chairs, room for large instrument displays, and easily accessible, separate chases for data and power connections. Major wiring runs will accommodate a 4 square inch cross-section with no hidden turns. Cockpit coamings shall be high enough to brace against at mid thigh, but not wide enough to invite seating. There should be six large mooring cleats attached to strong points in the deck capable of withstanding a shock load equal to half the vessel's displacement. There will be a crushable rub rail that becomes the widest point of the boat an any frame station.

The previous 12/12 economy requirement is waived, but bonus points will be awarded for approaching or exceeding it.

What did I miss?