economical coastal cruiser

I agree with the risks of gear accumulation. One must be extremely diligent

 

An interesting approach, but I have my doubts about the numbers and the validity of the Crouch method in this case.

A Crouch number of 250 is what one might apply to a 3-point racing hydro; using a more realistic 200 and the figures above, we get 23 hp to do 12 mph. Still an underestimate, if you ask me, because we haven't considered wind, waves, or the planing hump yet.

As a rough empirical estimate, the bottom loading to skip the "hump" altogether and be on plane at 12 mph would be on the order of about 100-150 kg per square metre or about 20-30 psf- so 16 to 23 square metres of planing surface. This suggests a very long, wide boat for the weight- almost like the proverbial kitchen-table-with-an-outboard.

I don't think Crouch's method is very accurate at these speeds- it's an empirical formula for boats well above the planing hump. And at 12 mph / 23 hp, headwinds and waves are going to sap proportionally more power than they do for a 400 hp boat at 40 mph.

Can we build a 40-footer that weighs less than 2.3 tonnes with full tanks, all gear, engine, and a full crew on board?

To get 12 mpg at 12 mph, I think, would- in a planing hull- call for a boat about half this weight.

***

The points made earlier about the risk of gear accumulation, and the minimum standard of luxury desired, are critical.

The point made earlier that the actual cost savings from such stinginess might be pretty small is also worth considering. In the smaller, relatively powerful boats I use, trips to the gas pump are one of the big expenses- as you get to cruiser size, though, this becomes secondary to insurance, maintenance and mooring. The balance seems to stay that way- fuel cost being a relatively small fraction of TCO- until you get up to commercial craft such as light freighters and fishboats.

 

Yes, I totally agree with all your statements, except the 250 number: modern long narrow deep V stepped bottom designs by Fountain, Donzi, etc are 250 to 265.

The point I was making was that one could explore a planing hull rather than restricting the design to only a displacement hull.

Certainly, the Crouch method works well for craft that are fully planing, and 12 knots or so is only a planing speed for very light bottom loads.

And I also agree that one would probably need to go to about half the design weight -- 2500 lbs instead of 5000 lbs -- to get there. And at that weight, carrying much of anything gets tough.

However, just because the problem is tough, does not mean its impossible. By focusing on such numbers, perhaps a design is there. Perhaps we will see such a design in the current Wooden Boats magazine contest. I'm looking forward to those submittals!

Oh: And there are existence proofs of such boats, but you won't find them in a showroom.

 

This discussion is nudging me dangerously close to buying an old Marinette.

Entirely too much common sense and practicality; we're talking BOATS, remember?

 

Graphite will burn a little over 1 litre per nautical mile at anything from 10 to about 25 knots, depending on how much is on board. Like all boats, she is a compromise - a middle ground between amenities and the ultimate persuit of light weight. Without too much difficulty, I could shave quite a bit off the overal total.

Pierre - I have to say, you are the 1st person I've actually heard suggest that owning a boat could be considered to be being green! Bravo!
Not that I agree with you mind you

 

Sandy,
Reread your original post and the best boat to fit that bill that I know of is an Albin 25 8.5 x 25 x 2.30'. 1/2 gph at 6 knots .. 1 gph at 9 knots. I know where there is a trailer for sale and several boats for less than 30K. The best value is in the higher priced boats.

Easy Rider

 

OK, here's some more data. Graphite, designed by our friend Will Allison, gets about 4 nmpg at any speed from 9.5 to 23 knots. So its fully planing at 9.5 knots: planing boats exhibit this flat "nmpg" performance when fully planing. Probably about 7000 lbs as tested (two people, 40% fuel, 70% water, some gear).

Since planing boat performance is pretty much linear with weight, all you've gotta do is go to a third the weight to get three times the mileage.

An example of this is:

http://www.biekerboats.com/Bieker_Boats/25_Footer.html

which, using an outboard and so about half the fuel efficiency of a diesel (about 10 hp/gal/hr for the outboard .vs. about 20 hp/gal/hr for the diesel), gets about 6 nmpg. About a third the weight, about three times the energy efficiency.

By the way, both Graphite and the Bieker Shearwater have warped bottoms, which are supposed to have Crouch numbers of 125 to 150. Plug those numbers into here, and you'll see the proper results. Therefore, the Crouch numbers do in fact still work at low planing speeds, IF the boat has light enough bottom loading to fully plane at those speeds. (try this again: http://www.go-fast.com/boat_speed_predictions.htm)

So, the key to achieving the high target efficiency numbers are light weight (like 2500 lbs all up), diesel, and a planing bottom.

Unlike a displacement boat, the length is pretty much irrelevant for a planing boat efficiency. However, the longer it is, the slower the motion, the smoother the ride. Since 40 feet on a trailer is generally no problem whatsoever, might as well shoot for that length target. Especially as weight of a box 8.5 feet wide and high enough for comfort is simply linear with respect to length.

The Bieker boat is 2250 lbs including power, yet is built of wood. So a glass/nomex/epoxy boat would weigh about a third as much -- about 650 lbs. So double that for a 40 foot, and the boat is still lighter than Bieker's Shearwater.

Hence, I think its not tough to reach these targets, but you've got to go LIGHT.

Fortunately, that way one also ends up with a boat on trailer of about 3500 lbs, and you can tow that with almost any small SUV, like a RAV-4. So you might see high teen's for MPG on the road too. That's sweet!

 

David,
Some of your logic is a little way off reality there, but the main problem is that it's not always feasible to simply build lighter. For Graphite, for instance, the hull is a strip-plank composite structure - ie a timber core, in this case kirri (or paulownia). I investigated using foam-cored construction techniques for the hull (as are used in the bulk of the remaining structure) but in order to achieve the necessary puncture resistance, the skins would have to have been substantially thicker. So any weight savings in the core were offset by the skins. In fact the hull itself would have weighed almost the same with either method. The cost of both materials and labour thus 'weighed' heavily in the favour of the strip-plank.
Sure, I could have engineered it to be somewhat lighter, but almost certainly at a cost in terms of durability and definitely at a cost in terms of $.

Likewise, I could have shaved a considerable amount off the total by reducing the amenities on board. But the the boat I would have ended up with would have been a far more spartan affair and not in line with the design goals that were set for it.

 

Well, I'll respond the same way, Will. Your logic is a little way off reality.

There do exist much, much larger yachts with substantially lighter displacements than can be obtained via a strip plank hull. For example, as I mentioned earlier, the STP 65 Rosebud, a 65 foot racing yacht with a 20,000 lbs lead bulb on the end of a 14 foot carbon fin. That keel juncture absorbs astronomical loads, far higher than the boat we are proposing here. To say nothing of the tens of tons of rigging loads. And the broad flat bottom needed to be designed to absorb tremendous pounding loads at sea, far more vertical accelerations than anyone would chose to experience while cruising. Yet that 65 foot by 18 foot yacht, with 6 foot of headroom throughout below, weighed 2200 lbs: that's hull, deck, and all internal structure and floors.

Now that's an expensive carbon/nomex/epoxy structure, carefully constructed. But its MUCH more area, and many times as strong, as required in this case.

That's reality, Will. Your chosen construction method aint close to optimal for weight. Its a nice cheap way to build perhaps, but its very heavy compared to the state of the art. Light compared to some other methods, true. But there is no reason to lock into outdated heavy technology.

Its all part of the engineering trade space.

You traded, and High Modulus traded, and two different solutions were found. Sorry, Will, but in engineering, that isn't unrealistic logic, its just different weightings for value functions.

 

The state of the art is not always the correct answer for a given engineering exercise. Perhaps the term unrealistic logic is the wrong one here. Different weightings for value functions doesn't neccessarily imply one set of weightings is logical and another is not.

Expensive bleeding edge technology is often cost prohibitive and unavailable as a viable solution for a specific engineering exercise. I think that may be the case for a carbon/nomex/epoxy structure as an engineering solution when Economical coastal cruiser is the defining mission statement. Graphite, while not the lightest conceivable structure available, seems an elegant, relatively cost effective solution .

 

Wow David... glad to see you edited out the last line of your post.... there was nothing personal intended about my comments.
As I said, I could have engineered Graphite's structure to be lighter, I have no argument with you there. And yes, in the recreational field, any designer that doesn't consider cost as a part of the design spiral is not doing his/her job properly. She is light by production standards, but is conservatively specced - intentionally. The BJ24 is evidence that you don't need to go high-tech in order to save weight.
I never suggested that you couldn't build lighter than using strip plank, and the larger you go the easier it is to achieve substantial weight reduction using composites, but to suggest that it's nothing more than outdated technology is missing the point.
We are not talking about a 65 ft racing yacht that will likely be outdated, valueless and probably scrapped in a few of years. It will almost certainly undergoe repairs during that time. We are looking at a recreational coastal cruiser that will get smacked into jetties, driven onto its trailer and the like every weekend. Owners of racing yachts are prepared to shave safety factors in order to improve performance. Not many weekend boaties would do the same.

As an example of where your comments are a little off target, Graphite's bottom shape is not warped, it is a monohedron for about the aft 40%.

By way of another example, let's look at the MJM series of boats. The 29Z is probably the closest in size compared to Graphite. She incorporates some fairly high-tech construction and the designer has gone at least some way towards keeping things simple ( at least to the extent that their market segment will permit)"

The result of all that effort? A dry weight of 7600lbs & a bit over 3 nmpg at cruise speeds.
So whilst you may poo-poo strip-plank epoxy composite construction as being out-dated technology, I'll await for you to provide a comparable example that weighs significantly less.

 

TollyWally - bang on.

I've designed and built ultra-light carbon/nomex composite structures (solar cars, in my case); I can say with confidence that while these are amazing materials, they will not work miracles.

Will is quite correct that when you have to consider such loads as running aground, or ramming a dock at four knots in a nasty crosswind, many of the "advanced composite" weight advantages are nullified. A racing boat built to overall safety factors of 1.2 or so is a very different animal than a cruising boat built to safety factors of between 3 and 10.

Multiaxial S-glass over strip plank wood is, indeed, one of the most structurally efficient construction materials available for situations where both large global loads and large, random point loads must be considered (ie, boat hulls). Holger Danske, arguably the lightest (for her length- D/L of 40, of which 43% is the keel) bluewater racing monohull around, was built in this method.

Outside of NASA and the big racing syndicates, cost usually gets a pretty big weighting. This alone rules out most "advanced" materials for the vast majority of projects.

 

An old hot rod maxim comes to mind;

Speed costs money, how fast do you wanna go?

 

I still don't see why a long narrow displacement hull wouldn't fit the bill, a faux tri, stabilised displacement hull or whatever they may be called. I also count "displacement gliders" like the BJ Glider Tom is working on as basically a stabilised narrow monohull.

OK, there are yet no ready plans to buy and I know of no suitable production boats, so a comparision with existing reasonably efficient boats is unfair. Sandy asked us to not be so down to earh practical so why not explore possibilities? If he isn't going to buy or start building a boat this spring there's no reason to go for existing conventional boats yet.

My understanding is that a 12m long narrow (1m) displacement monohull will meet the requirement on fuel consumption at 12 knots for 3 - 4 tons displacement if using a diesel inboard. There's a challenge to make a hull like that into a practical boat, and not looking like something from Star Trek. I'm not convinced that it cannot be done though.

Erik

 

A comparable example that weighs substantially less than Graphite.

Well, the Bieker boat is only 2 feet shorter and weighs a third as much, using a very similar construction approach. So one does not even have to leave wood core to get a much lighter vessel.

But for these comparisons, I assume Graphite without engine is about 4500 lbs (about 1200 lbs for the engine, outdrive, essential bits (pumps, batteries, engine mounts, fluids, ...).

How about an Olson 30, without a keel. Same length and beam, less freeboard, but 1600 lbs without a keel instead of 4500 lbs without an engine. Olson 30 is just glass, polyester, and balsa core.

Or a 1D35: longer, about same headroom, more beam. 4000 lbs without keel but with engine, mast, rod rigging, ... and the additional strength and structure for the keel and rigging. Glass, foam core, epoxy.

Or a Farr 40: a substantially larger boat, a bit less that 5000 lbs again with all the structure for a sailboat, with mast, engine, ... Again, just glass, foam, and epoxy.

Or even a crazier example: a 30 square meter, 40 feet long, 6500 lbs including 3500 lbs of lead. Built of wood planking 70 years ago.

So lighter structures are pervasive, even when they need to be designed to MUCH higher loads than a cruising powerboat, and even when built using cheap materials (plain glass, cheap cores).

So again, my point is that to achieve dramatically different performance, some things must be dramatically different. In the specific case we are discussing, the desired performance is high fuel efficiency at moderate speed, and trailerability. Both are clearly greatly advantages primarily by weight. Weight matters more than any other aspect of the design.

Low weight is the essential enabler to achieve the 10 to 12 nmpg of the original query. Without light weight, the goal can't be achieved. Only with very light weight (like 2000 to 2500 lbs) can the goal be achieved, and then it can fairly easily, regardless of hull design: planing or displacement.

Obvious, correct?