economical coastal cruiser

Oyster -thanks for references and links. For the V39 for me some of the cons stand out. I'm definately going to chase up the Bolgers and analogues further. I'll have batteries anyway so I'd not class them as ballast even though they can fill that role. I'll tolerate more weight on the trailer, and displacement, than your solution but clearly will really have to work on CoG. You'll likely win the beauty contest hands down and I'd already figured that I'll have a barge, but it won't be a houseboat. Perhaps it will be called 'the barge', but in French to give it a little bit of cachet, and to perhaps fool some of the marina-dwellers that actually get out of their pens and come near.

 

Pierre - nice trading work. The solution to your fuel issue is to work less, boat more.

But I hear you. I suspect CPP's will be no more frequently found here, and so given our smaller market I probably wont find one locally and so its 'new' or get brave and look more widely afield. And two CPP's on 1 motor is really going to be fanciful, and not make sense unless its almost a 'liveaboard with no fixed address' and lots of motoring. Although I am retired, a lot more annual motor hours in my case are still not going to be a certainty just yet. The dream remains, but it is getting buffeted quite a bit.

 

Thanks, the profile fits my eye but may not fit others when it comes to arrangments and layouts. Capt. Tom thats a regular around here has a beautifull boat which he has cruised on with his lovely and he and I have gone round and round about the walkaround since the world was created though. Of course I am about 100 younger than he is too, so favored and "designed" [I used that term lightly] around this OPTIMUM solution.


I am always about function which there is a lot to be said if you pick the houseboat style designs. In my personal case, fishing has been part of my life and plays into every single hull that I have owned and personally used myself. Its important also that when boating in warmer climates and with the intention to do so, that limiting enclosures has always been the order of the day. Liveaboard for any length of time will not be in the cards either. My house or cabin materials also weighed in with all the wood and the roof sandwich included just a few pounds over 100 lbs. This weight can increase three fold with the wrong materials just in simular sized cabin using many of the big box type woods and framing the top with real 2x4s as some of the plans thats sold for the backyard builder. For me the client is always right, my bride.

 

Regarding budget,
I’m not setting a fixed budget first and then buy/build the best boat I can for the money. I already have a boat and don’t intend to spend any money at all on the wrong boat. Building the boat will take several years so the expenses will be spread over time. When I believe I have a good candidate for my boat I’ll cost it and decide if this is what I want to do, or if I’d rather do something else with the money. Fcfc’s rough estimate €150,000 isn’t a show stopper but I believe it will be less costly. My views on accommodation and necessities are somewhat Spartan which helps to keep cost down without making a sacrifice. Strange as it might sound; my wife has a similar view on comfort.

In a coastal cruiser, compared to the boat I have, I most of all want double space and double speed. 10m - 12m long and 12 knots economical cruising speed fits the bill.

I still believe the stabilised monohull might be the right type of hull. Next step is to fill it with whatever is needed for a coastal cruiser and see if it still is a good choice. For me it’s time to work with the internal layout, make an estimate of the displacement and then revisit the hull form and power calculations.

The general layout envisioned by Brian in post #203 suits me well. I don’t see the need for air condition, but want a heater instead. I don’t consider a shower necessary either. So far I’ve managed without a shower and am not missing it. A head would be nice though.

Next question is the weight of all equipment needed in a coastal cruiser. I’m experienced at backpacking in the Scandinavian mountains. Everything needed for a two week trip, except water, weighs in at 25 - 30 kg per person. Consumables, including fuel for cooking, constitute approximately 1 kg/person/day and the rest is tent, backpack, cooking gear, sleeping bag etc. This weight is achieved with standard backpacking gear, and in my opinion provides good comfort. Spending much more money on equipment can bring the weight down.

It’s necessary to maintain a strict weight discipline when backpacking. Applying some discipline also when boating should help to keep the weight down.

My preferred engine is a small inboard diesel. I’ll see if I can attach a data sheet of one that seems suitable. The engine is heavy and also put some requirements on the width of the central hull.

It’s apparent that there are several opinions on which requirements are most important for an economical coastal cruise. Often they are at conflict with my own ideas. That doesn’t mean they are wrong as long as they apply to at least one person, unless we’re doing a market research to find the boat that best fits the broadest possible market.

I’ve come to the conclusion that long distance trailering isn’t on my priority list at all. I only wish to move the boat short distances twice a year, but can arrange so I don’t have to trailer it at all if it’s too much of a bother. On the other hand I’ve come to realize that the ability to fit the boat into a standard container would be a nice feature. I’m not sure how high on the priority list this is. When I figure out what I have to sacrifice I have to decide.

Fuel efficiency is high on the priority list, and the figures12 knots at 1 US gallon per hour will do as the target.

Ricks calculations show that 3 tons is about the maximum displacement for the required fuel efficiency. The target is less, and a weight budget has to be established to see if something needs to be traded for lower weight.

Erik

 

I understand that there are doubts about the availability of small marine diesels. From what I've seen there are plenty of them. I enclose data from an internet catalogue of a few more diesels. I have more data in paper form on these at home. It's just to ask the seller. I don't see anything wrong with these diesels, or am I missing something?

Erik

 

Erik, its not that small diesels are hard to find. Its that small diesels that actually burn noticibly less g/kWh that outboards are hard to find, and are MUCH more expensive to buy, install, and maintain that outboards.

Again, notice the published numbers from vw-m and volvopenta, and you will see that they only give decent numbers when HIGHLY LOADED, and with a prop, you only fully load up the engine at MAX RPMs. When lightly loaded, the engine will burn about TWICE as much as optimum, which coincidentally is the same amount as an outboard.

So in theory, the diesel is much better, in practice, its much worse.

That's my take on it anyway with the information I have seen. A diesel is just not practical (cheaper to install, own, operate) at these light displacements.

Its not about length, its about the work the engine needs to do. If its 12/12 or anything close to that, the engine is not doing much work.

If you dig around a bit, you'll find that the most efficient engines are the very biggest diesels. A million LW or more gets pretty darn efficient.

Beside pure size, its the complex stuff like turbos, intercoolers, electronic controls, single rail, multiple pulse, direct injection, and ALWAYS RUNNING THE DIESEL FLAT OUT that results in efficient diesel powerplants.

In this feasibility study, its already clear that the power required to move the boat will vary dramatically by condition: 20 knot headwinds will triple the power required to move the boat over calm conditions. Waves, load, ... will also vary the power needs. So a single powerplant running flat out is simply not feasible.

Or again, that's what it looks like to me. I could easily be wrong, if so, please let me know.

 

Rick, with respect to an efficient outboard prop:

"Big Foot" outboards have 2.4:1 reduction, instead of 2:1. Not much of a difference, but something. Some also allow a tiny bit larger prop.

Imagine there is no restriction in prop diameter, because the prop is set aft of the drive leg far enough to clear the cavitation plate, and/or the cavitation plate is cut off. And the engine is a long shaft, set deep in the water.

What would such a prop look like?

 

Are we allowed to also reduce the propshaft RPM? If so, and if we are aiming for relatively low speeds (say 12 knot ballpark) we would likely end up with a relatively large diameter, low RPM prop with a low disc area ratio. Under some conditions, one might get an improvement from, say, 55% to 75% propeller efficiency by doing so.

If we are not allowed to reduce the propshaft RPM (which on an average 30 hp outboard will typically be around 2000-3000 rpm at WOT) then I suspect we won't end up too much different from the props that these outboards come with.

Increasing prop diameter, generally speaking, requires a reduction in RPM for the same power. For outboards, there just aren't any reduction gears available for more than about 2.4:1. It would be great if there were, but the resulting gearset would be too bulky for a lower unit, so you'd probably have to put a second reduction gear in the powerhead. (I do recall hearing about one model- a Honda, I think- that did this a few years ago?)

Of course, for our coastal cruiser here, we do have to consider the cost factor. $5500-6000 or so for a stock 40-50 hp outboard, maybe $6500 for its "big foot" equivalent, in stock condition- add at least several thousand dollars and a void warranty for potentially fuel saving modifications.... the appeal of a stock outboard, even if it does use perhaps 20% more fuel than a "theoretical best case", becomes pretty strong.

 

Erik
The stabilised monohull with a length constraint works better as you reduce weight. The stabilising hulls can be reduced in plan area as the displacement is reduced to achieve the same KMt. This give more scope on what they look like. The aft ends of all three hulls should be aligned longitudinally to avoid flop roll when the main hull is suspended over a trough.

The three hulls provide longitudinal stiffening. With a few well placed beamwise bulkheads the boat will not require framing if built with composite panels.

The fuel consumption data does not make sense to me. The charts indicate it consumes 2lph at 15kW. I doubt that this is possible.

As you go lighter the hull will get shallower so the clearance to install the engine without impinging on the main cabin floor that I envisaged in the shape proposed. I have seen small yachts where the engine box provides the base for a central table. Something similar could be done. Reducing the weight gives greater freedom with the hull shape in terms of rocker and Bwl while still achieving the fuel consumption.

My thoughts on the diesel are very similar to yours. I would not consider an outboard as being suited to plugging away for hours on end.

A little diesel mounted deep in the hull is a good means of getting the centre of gravity down where you want it in a relatively light boat.

Rick W

 

I just messed around with JavaProp, and the result was pretty much as Matt suggested: using 2.4:1 ratio (a bigfoot) running at 3500 RPM (typical torque peak of modern 4 strokes) results in a prop RPM of 1460. 12/12 means 12 HP from a gas engine, and 12 HP means 16000W. 12 knots is 6 m/s. Chose the good foil shapes (the MH series). To start, I assumed standard prop size: 4 inch hub, 15 inch diameter, and two blades.

Turns out the optimum prop with these constraints is over 75% efficient, is a small disk area but not too extreme.

Therefore, it does appear that the high efficiency props do NOT require weird modifications to the physical outboard! The draft will not be altered from a stock unit.

And the custom propellor would not be an odd thing to have made by a prop shop. As I've mentioned before, I've had many props custom made for my boats at a cost of about twice an off-the-shelf prop, so probably $500 in stainless for this application.

 

David
Increasing diameter has the biggest impact on improving efficiency. The limitation with not reducing RPM in concert is that the blades have to work at a low AoA (inefficient) or get too small to handle the thrust. For a 600mm prop at 12kts you would not want to go above say 1000rpm.

You can play with JavaProp to get an idea of how the shape changes as you change RPM and diammeter. It is not particularly hard to use and will give good results for a prop at these speeds and in clean flow.

Rick W



Rick

 

So if we choose a small diesel it will burn 270 g/kWh, which is the same as 0.3 liter/kWh at a diesel density of 0.9 kg/liter. Test data for a modern four stroke 40 hp outboard (Yamaha F40), provided by Yamaha, is 0.52 liter/kWh. Larger outboards are marginally more efficient.

With a small inboard diesel we get 0.3 liter/kWh = 12.6 kWh/gallon = 17.1 hp during one hour per gallon.
With Yamaha F40 we get 0.52 liter/kWh = 7.27 kWh/gallon = 9.89 hp during one hour per gallon.

In addition to the better efficiency for the small diesel it also has a much lower propeller shaft speed and can swing a larger and more efficient propeller.

The fact that a large diesel can be even more efficient doesn’t change this.

The way I twist the figures there’s no doubt that an inboard diesel is more efficient than an outboard. Am I wrong?

Erik

 

David
You have done the HP to W conversion wrong. 12HP is 9kW.

Also check the geometry of the blades. They will probably be about 30mm chord. You will not be able to make these strong enough to take the thrust. You have to reduce the angle of attack or RPM to get the blades to a size that can take the loads.

For the conditions being considered here you should choose the thinnest foil sections for all four stations and the highest Re#, which is 500,000. This is still lower than what is going on but it means the result will be conservative as L/D increases slightly with increasing Re#.

Rick W

Rick

 

There is no way around one tradeoff. Light boats are more lively and will give you a much rougher ride in any kind of wave action.

 

You guys all seem to be forgeting one thing with all this here efficient prop stuff. A fixed pitch prop is the most efficient at one power setting. The setting for which it was designed. If you are designing a prop for max power with wind, waves and maximum load then most of the time this prop will be underpowered. If you design for the lower power the engine will be overloaded a fair amount of the time. That is why props do not absorb max power at the normal cruise settings.

Even with a controllable pitch propeller I do not set it at max loading and efficiency for the engine. Its to easy to overload the engine when things change a bit.