economical coastal cruiser

I'm sorry, but this is all rather absurd - how one can begin to develop even the basics of hullform when there is not even the most basic list of design objectives - an SOR as it is often called - I find quite extraordinary. It is THE most important document in the whole design spiral process. Without a well written SOR, the project is doomed to failure.
I tend to agree also that there is probably enough boats in this discussion to be worthy of at least 3 separate threads... it all becomes way too confusing otherwise.

 

It's really quite easy. All one needs is a computer program to generate hull forms and presto, you can scrap one hull after another as "constraints" become known. I believe it is called optimisation.

 

Repetition

I am repeating myself.

What is the price tage and allowed yearly cost of the intended boat ?

Because I think it is a primary issue.

We are speaking here at design stage, so NEW boats.

And when I see people advocating trailer boats 40 ft long, IMHO at least 150 000€ built as new , on a 10 000€ trailer, towed by a 60 000€ SUV (either dedicated to trailering the boat only, or daily used for commuting). I forecast heavy problems. The guy who can afford to spend a 150 000 + 10 000 + 60 000 = 220 000€ for purely leisure or pleasure, and at least 15 000€ for yearly maintenance of the boat, trailer and SUV, will never drive himself the thing. Waiting for the crane to finish the previous operation before loading/unloading his boats, making half a turn on a crowded parking with his 40 ft trailer is not for him. He is largely wealth enough to subcontract this to a professional trucking company, or conveyors, or even renting a boat where he wants to sail. And trucking or conveyors have a completely different limitations in beam and weight than that guy driving his SUV/light truck.

So I think a Statement of Requirements is needed, and even more, a coherent one, one that makes a balance between the money needed to buy and maintain the boat, and the boating expectations of people who can spend this amount of cash strictly for leisure purpose.

 

I don’t agree that we’re working without a statement of requirements. The topic of the thread is also the draft statement of the requirements: “Economical coastal cruiser”. More explicit requirements can be found in Sandy’s first post.
…”12 knots in coastal waters at 12 nmpg”
…”the accommodations of a 16' travel trailer”
...”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.”
Some posts later Sandy amended the requirements:
…”Lets limit beam to 8.5 ft, and displacement to 8,000 pounds, to keep the towing issue practical.”

I have currently stolen the role of the client, but tried to stay true to the original requirements. They are very close to my own anyway, so that didn’t involve any trade.
Acting as the client I have chosen to prioritize economy and put a restriction on the length as well. The max 12m requirement is motivated because increased length would drive cost. I also believe that the requirement on single-handed docking puts a constraint on length, but exactly what limit is debatable.
Right now we have made a design assumption on a 3 ton stabilised monohull and will see where this leads.
Accommodation is left as the variable; the success of the concept will be judged by the amount of accommodation we can achieve while meeting all other requirements, of which some will be decided along the road.
There’s nothing wrong with making another design assumption and see where that leads, but why not pursue this one first?

Regarding wind, 20 knots headwind (plus own speed) is a good value for maintained 12 knot cruising speed where I live. It should be possible to make progress into a headwind of 30 knots. At higher wind speeds we will have to let the course depend on the wind. I have so far avoided being caught out in my boat in winds above 30 knots so my hands-on experience is limited. I don’t have a boat suitable for coastal cruising yet so I’ve never been far away from a suitable port.
I’ve read up on transport container size. Door width is 2.34m and door height 2.28m. I fear that would be too restricting for the concept and suggest that we don’t include container size as a requirement. Limiting to container size is otherwise a realistic way to get ocean-crossing capability. Personally I’m satisfied to explore this side of the ocean with this boat.
Regarding trailer width and length, in my opinion it’s a usable feature. I would like to have the option to trailer the boat every year to winter storage. I can rent a boat trailer for that, and can borrow or rent the car to pull it. I don’t have the proper driving license for such a heavy load so I would ask a friend to drive for me. I don’t see the need to invest in trailer and car for this.

I’ve been working with product development for many years. A key to success is to prioritize the requirements. In the best of worlds we should put weight factors on every requirement, and thus have the input for an optimization. I understand that some prefer to call this compromising, but English isn’t my native language so I’ll keep out of that debate. I know that we have problems with the word “compromise” when we translate to and from Swedish. The flavour of the word is different in Swedish and (British) English.
I’ve been involved in competitions where the customer has put weights to the requirements, sometimes with absurd consequences. Once we had a situation where we concluded that we would get the highest score if we made the land vehicle in question immovable. We would lose all the points on mobility, but gain much more in other areas. Somehow we figured we wouldn’t get the contract with such a product, even if we got the highest score.

What we’re doing now on this thread is what I would call a concept study. We do them quite often where I work, financed from the marketing budget, R&D budget or by a customer. A detailed SoR isn’t needed for such a study; the SoR can be the outcome of the study.
For a concept study, I think we’re attacking the problem in the right way by making a design assumption and then investigate the consequences.

Erik

 

Everyone has his own approach to design....all are valid....this is the cool thing about boat design. Give a concept to 6 folks and everyone comes back with unique questions.....

1) Eric has a list of requirements
2) Will wants a list of requirements
3) Pierre wants to know what it will look like
4) Fc wants a budget
5) Rick wants to explore the hull form and power reqirements
6) I want to make quick pencil sketches

Every designer has his own method of designing a boat...this is the part you can't teach, it is arrived at via experience.

 

This deviates from the basic utility of a trailerable coastal cruiser. The inability to trailer the boat to far destinations with a personal vehicle severely limits what can be done in the way of cruising these distant locations. The presumed comfort of the large and heavy boat begins to fade when it must be either motored on its own bottom to all locations and returned, or a commercial carrier hired to do the job at great expense. These are very real concerns and make the smaller boat much more attractive to many who have actually done much of this kind of cruising. Certainly those who need to operate within a limited budget or a limited time schedule need to think about the ease and costs of trailering and storing their boat.

The above reasoning is based, not on work at the design table, but from years of actually using both sizes of these boats.

 

No. You have not investigated the problem.

At least in france, but I fear in Europe it is the same :
It is very unfrequent to find for hire a boat trailer above 25 ft (7m50).
It is next to impossible to rent a car or SUV with towing ability. As a trailer is dependant of the towing vehicule insurance, most if not all rental companies prohibit towing. You need to hire professionnal equipement (trucks) at professionnal prices under a professionnal contract to have legal trailering capabilities.

 

Ahhh... so that's how it's done...

Tad - I agree that everyone has there own way of going about things, but surely you would agree that everything else must stem from a well thought out design brief....? I too like to start scribbling very early on in the process... it is the most fun bit afterall

It is true that Sandy listed some of the basics of his SOR, but also agree with fcfc - the cost is one of the most important elements of that list.
Erik, to suggest that you can 'act as client' for someone you've never met is almost as absurd as Rick designing a hull for which the constraints are yet to be determined. There will be a myriad of decisions that need to be made - decisions that only the real client can make. I agree that there are a number of people who are on the face of it after a similar thing, but already everyone's comments would suggest that there are at least 3 quite different boats on the go here.

The other point to consider - one which Tom has quite rightly alluded to, is that there is a big difference between a trailer boat, a trailerable boat, and a transportable boat. Each has its advantages and disadvantages. Each 'client' must think carefully about exactly how he/she is really likely to use their boat and make a decision about which camp to pitch his tent in. And, yes - that will involve a number of compromises in itself...

 

Again for france, legal towing limitations with car license are 12m 3500 kg. But the practical towing limit is around 7m50 (25 ft) 2000kg (4500lbs) trailer included. This can be done with the smallest SUV. Beyond that, the price of the car + trailer + boat and all the hassles involved in trailering make people prefer a trucking company.

Just look at big boating companies at what size they stop advertising trailerable boats.

 

Erik
My objective was to get as quickly as I could, with some specific and maybe realistic requirements, to a first pass of the required thrust to get an idea of the optimum prop.

We are really close enough for that now.

We have a total power to drive the 3t boat into a 20kt wind at 12kts of 13.5kW. Unless you are in sheltered water there will be waves. Power loss to waves peaks at about 30% for waves of the worst length relative to hull and a reasonable average allowance is 20% - so 13.5 gets to 16.3kW with wave allowance. Determining added wind wave resistance is complex and requires the shape above the waterline as well as under.

With a small allowance for rudder drag we arrive at 2700N (16.7kW) as the design thrust for the propeller.

I have attached the power absorbed by nearly optimised props for the nominated diameter ranging from 0.9m to 0.3m. The hull draft I have in this iteration is 550mm. So up to 900mm 2-bladed prop can be fitted without reducing static draft and trailering. Of course it will need at least 1m to operate in. Below 400mm the blade number can go up without impinging on static draft. The curve shown makes this allowance.

It is clear to me that it would be remiss of a designer looking at the most economic cruiser not to consider the prop and simply accept what is available on an outboard.

The optimum diameter needs to take into account such things as hard draft constraints; method of prop protection; the required range; available engine selection.

In terms of arriving at the engine power you need to make allowance for house loads under way, transmission losses and engine parasitic loads to arrive at the engine power. Then you get into fuel BSFC maps of the engine range to see what size is going to be near the optimum. I expect something around 30kW for the 0.9m prop and 45kW for the 0.3m prop.

Of course the bollard pull from 0.9m prop will be more than the 0.3m prop even with the higher installed power on the smaller prop.

Prop losses have a huge bearing on the engine power required. The 0.3m prop will burn up 12.64kW to produce a useful output of 16.32kW - almost as much lost as it delivers. The 0.9m prop burns up 3.52kW to get 16.32kW of useful output.

With the 0.3m prop the motor has to be almost 50% bigger; the fuel consumption almost 50% more and these factors are going to increase the displacement. So going forward with a small diameter prop we enter the slippery slope where everything has to get heavier, hull drag goes up and prop losses get even worse.

Hopefully this gives you and others an idea of why the prop should come into calculation. It is an essential consideration for the optimum "efficient coastal cruiser".

I will do a rough boat shape of where we are so you can get a better idea of the envelope you have to play with based on the requirements and constraints you have established so far. I will explain a couple of the design optimisations that are still required for that envelope. You may be able to encourage someone who likes to make quick pencil sketches to get you something that looks good within that envelope and also satisfies your accommodation requirements.

My expectation is that as you iterate around the design loop the large diameter prop will result in weight savings and you will easily get under 3t - right now the proposed boat will get about 9nmpg at 12kts into a 20kt wind with associated wind waves. Reducing weight and windage are needed to get to 12nmpg. An interesting outcome of reducing weight will be that prop losses go down as well and something less than 0.9m may prove to be the best.

My boat shape will give you the plate area for the main bits. You can get engine weights from motor specs. You can now determine fuel load to suit the desired range from published engine data. You should then narrow down on the weight by listing all the things you want in it.

Rick W

 

Barely trailerable was i think noted by Sandy somewhere early in the thread and is the 1 of the 3 boats on the thread that i think is of most interest.

Erik - you may well be right that containerable is too restricting. Such a boat had a thread here http://www.boatdesign.net/forums/motorsailers/containerable-motorsailers-11797-10.html
but was more on sail than power and thread seems to have died, but still worth a read.

I hear fcfc on budget and hassles, but think that firstly trailer boats to 7.5m (2nd type of boat on this thread) are already on the market in large numbers. But few if any would meet the 'coastal cruiser' spec of Sandy or Erik and i doubt that such a limited LOA will enable it. The hassles part can be overcome, read on....

So i'd still kinda like the containerable aspect - ideally take road wheels off boat trailer, slide trailer and boat into container on some HDPE rollers and truck to/from house to destination. Trucking a standard 40' container is going to be much easier and cheaper than craning a slightly wider boat on and off transport. Drive SUV there by itself. At destination slide boat out and refit road wheels and use SUV to launch at various localities. At 'home' store boat in container, perhaps even at commercial facility. This way, a small SUV can handle the 'freight yard to waterfront' part - a large expensive tow vehicle is not required.

Boat might need to have a partly demountable coach house to fit in container. If so, it shouldn't be a huge task to refit - folding, just a few elements or perhaps pop-up.

Tad - i thought you already had something like this (from your posts of a few years ago) but couldn't find it on your website.

Will - can graphite go on a diet? Thin down a bit to slide into a container? It should be easy - graphite is a good lubicant) What we are talking here needn't go as fast as Graphite either, so Yanmar 4BY2, and not the 6? I think Graphite is barely trailerable in the sense that you go from home to your local boat ramp OK. But its getting towards the 3rd type on this thread - transportable - if any significant travel around our long Australian coast in planned, so i reckon that 1st type is best. Barely trailerable but just containerable gives maximum flexibility.

 

I'm not going to re-enter the to-and-fro foolishness of a few days ago - I still have a headache from bashing my head against that (very thick) brick wall... but on the prop front, I have just two questions... how does a 900mm prop not impinge on a 550mm draft? Or is the operator required to keep the prop blades horizontal?
The 2nd, is that since "it would be remiss of a designer looking at the most economic cruiser not to consider the prop and simply accept what is available on an outboard", how would you propose that one fits this 900mm prop onto said outboard?

Brian - simple answer, Yes - you could put Graphite on a bit of a diet. But one has to ask, at what cost in terms of facilities and durability. The more sensible approach would, IMHO, be to start from scratch if you want to fit into a shipping container.
I disagree, however, that Graphite is firmly in the transportable category. Clearly with a 2.8m beam and an on-trailer weight of 3500kg, she's would be regarded as somewhat of a heavyweight in the trailerboat segment too. But, from the outset, the boat was designed (from a well considered SOR, I might add too ) to be trailerable. In Oz, the restrictions for going to the overwidth beam are not onerous, and the gains in interior volume were considered to be of sufficient value to make the compromise (uh-oh...) worthwhile. There are an increasing number of large trailerboats getting around these days - particularly in the recreational fishing boat sector. Personally, I would regard her as lying at the top end of the trailerboat segment... but then I've had quite a lot of experience in towing quite big boats up and down the East coast of Oz, so don't consider the propect to be at all daunting. To someone who's never trailed a boat before it would be nothing short of terrifying. Again - an example of how the boat needs to be tailored to a specific individuals need... and of why the SOR is so important.

 

Will

I trailed a 28' Shark Cat a bit some years back. Triaxle trailer probably on the limits - thankfully never got weighed by the authorities (fuel was cheaper at roadside servos than at marina, and 2 x 200HP meant large tanks...). I agree it can be done. But isn't fun, and so the boat was mostly left in the water. I'd like easier options to get beyond home port. And for periods of non-use (altogether too long and too frequent!) hardstand somewhere s preferable for both cost and maintenance.

I'm sure you are right re: starting from scratch. Container doors (plus room for lubricant) means a beam of 2.3 max, a long, long way down from 2.8. But even on a trailer, inside a container with enough clearance for towbar, LOA can be close to 11m, I think. The idea is to trade a chunk of beam for a lot more LOA to get even greater interior volume, albeit not as easily utilised.

Couldn't a 900mm prop be raised, eg into a box near the stern (perhaps under a seat) for slow rpm shallow water operation? Just leave 400mm actually in the water for that? Good for on the trailer too.

 

All sorts od things are possible, of course. But with every added complication you tend to add cost. So, let's say it costs you an extra 5 - 10K to engineer a solution that would allow for a very large diameter prop. A guess, of course, but probably not an unreasonable one. For arguments sake, we'll use Ricks numbers - 40hp for the big prop, 60hp for the smaller one. How long will it take you to recouperate the difference in operating expense between the two? And does the complexity of the arrangement force compromise in other areas.... It's all a balance... a compromise as one wise chap pointed out....

 

Reading through this thread I also went back to the original posting to reply with some common sense. Towing on a trailer along the eastern seaboard restricts the beam for sure. But you also do not need an 8000 lb boat to do what you are requesting to do at all. There are numerous trailable outboard motor boats fully cabined which are shallow draft and powered by an outboard that allows you to poke in some really neat coves with the advantage of the trim feature in lieu of an inboard engine setup. I have just finished construction on a 24 footer that weighs approx dry at under 2,000, power included.

The cabin roof for is 90" from keel to the standup cabin interior, and drafts 8 inches with the motor trimmed up. There is another fellow posting here that has a proven runabout simular to these numbers with speeds in the neighborhood of 15 cruise with full accomadations. A three foot draft does nothing for you but truely restrict your access along the ICW since you also do not need that much and only requires more hp and fuel to push it. I have a 90p E-Tec outboard engine on another boat 22 in length that also burns at cruising speeds 2 gph.