economical coastal cruiser

[Brian@BNE]

Rick - your prop size expose was news to me and I'm much obliged. Intuitively I felt that an outboard at low-cruise-type rpm was going to be inefficient, but I didn't realise how big the difference could be. And I'm past listening to an outboard at say 80% of max rpm where it will still be ok on fuel but not particularly efficient at the prop either. There probably isn't a big enough market for high thrust versions to warrant the neccessary drive shaft/gearbox mods required to get into the bigger props we'd like to see for 'our' cruisers. But this could change, and it mightn't be that hard. How about a 70HP powerhead on top of a 200HP outboard leg, and a cavitation plate twice the distance from the prop shaft? Also a gear reduction just below the powerhead. All to fit under that 200HP cowl...

Pierre, would you mind posting costs of your 490mm CPP setup? I'm ready to say Ouch! But I might still want to find the budget rather than compromise.

[Guest625101138]

Originally Posted by Brian@BNE

Rick - your prop size expose was news to me and I'm much obliged. Intuitively I felt that an outboard at low-cruise-type rpm was going to be inefficient, but I didn't realise how big the difference could be. And I'm passed listening to an outboard at say 80% of max rpm where it will still be ok on fuel but not particularly efficient at the prop either. There probably isn't a big enough market for high thrust versions to warrant the neccessary drive shaft/gearbox mods required to get into the bigger props we'd like to see for 'our' cruisers. But this could change, and it mightn't be that hard. How about a 70HP powerhead on top of a 200HP outboard leg, and a cavitation plate twice the distance from the prop shaft? Also a gear reduction just below the powerhead. All to fit under that 200HP cowl...

..

Brian
The results are sometimes surprising - although the difference can be deduced easily, I had to work through the whole exercise before that realisation popped out.

The outcome does not rule out the use of an outboard but it does show how it will impinge on possible solutions.

There seems to be interest in more fuel efficient boats so the market pull may yet have manufacturers working on more fuel efficient options for outboards. Increasing the prop size is simple way to get big gains. Some of the very large motors have high thrust legs that are designed for survival under high thrust, low speed conditions. Maybe we will see this idea get into smaller outboards. At this stage you can get low pitched props for low speed applications but diameters are still small so efficiency ordinary.

As you point out all the parts are there in other combinations but not available in a single package.

Rick W

[fcfc]

Originally Posted by sandy daugherty

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.

...

What did I miss?

1) Budget. In every case.

If homebuilding :

2a) Time available for building the boat. That the hidden budget part for homebuilders.

2b) Material preference/skills. Special building constaints. ie workshop size, special expensive tooling available or you have access to.

[fcfc]

Originally Posted by Brian@BNE

Intuitively I felt that an outboard at low-cruise-type rpm was going to be inefficient, but I didn't realise how big the difference could be.

The difference :

A yamaha T60 hi thrust outboard weigh 260 lbs, and cost around 6000€.

A diesel volkwagen marine TDI4-75 cost 14500 € and weight 450 lbs. You need to add a ZF 45-1 1:3.74 gearbox, price 2500€ weight 132 lbs. and at least a prop 22" for 1000€. I omit details like shafting, exhaust, sound proofing.

The diesel setup is at least more than 3 time the price of the outboard.

The TDI4-75 is the best small efficient diesel (208gr/Kw). But you need to understand it comes at price of turbocharging, intercooling, and full electronic common rail direct injection. Maintenace price is on par.

[Guest625101138]

Originally Posted by fcfc

The difference :

A yamaha T60 hi thrust outboard weigh 260 lbs, and cost around 6000€.

A diesel volkwagen marine TDI4-75 cost 14500 € and weight 450 lbs. You need to add a ZF 45-1 1:3.74 gearbox, price 2500€ weight 132 lbs. and at least a prop 22" for 1000€. I omit details like shafting, exhaust, sound proofing.

The diesel setup is at least more than 3 time the price of the outboard.

The TDI4-75 is the best small efficient diesel (208gr/Kw). But you need to understand it comes at price of turbocharging, intercooling, and full electronic common rail direct injection. Maintenace price is on par.

Both are bigger than required for the nominated fuel consumption of 1USg/hr. The diesel would be operating well outside its best efficiency so the stated fuel consumption for the diesel is going to be worse. (I would be interested in a fuel map if you have one.)

Also the comparison is not one-for-one. The 55kW diesel matched to a suitable prop will be about twice the muscle of a 45kW outboard with its puny prop. As noted earlier the equivalent power for the diesel swinging a 600mm prop will be 30kW.

Rick W

[fcfc]

Originally Posted by Rick Willoughby

The diesel would be operating well outside its best efficiency so the stated fuel consumption for the diesel is going to be worse. (I would be interested in a fuel map if you have one.)

Rick W

See http://www.vw-m.de/fileadmin/PDFs/en...75-4_e_m_i.pdf

It is the only small diesel manufacturer that dares to publish figures, not a tiny curve where you cannot read something useful.

[Guest625101138]

Originally Posted by fcfc

See http://www.vw-m.de/fileadmin/PDFs/en...75-4_e_m_i.pdf

It is the only small diesel manufacturer that dares to publish figures, not a tiny curve where you cannot read something useful.

The specific consumption is below 230g/Kwh from 1000 to 3000rpm but only if it is heavily loaded. If it was the engine choice then it would have to be overpropped or have a variable pitch prop to get below the 230g/kWh when delivering 14 to 15kW consistent with the fuel consumption requirement.

The variable pitch would give it plenty of reserve. The chart also highlight why a variable pitch prop merits serious consideration.

Rick W

[fcfc]

Originally Posted by Rick Willoughby

Also the comparison is not one-for-one. The 55kW diesel matched to a suitable prop will be about twice the muscle of a 45kW outboard with its puny prop. As noted earlier the equivalent power for the diesel swinging a 600mm prop will be 30kW.

Rick W

Yes, but there are no low power good efficiency diesel. A 30Kw diesel will be no turbocharged, mechanical indirect injected, no multivalve.

Expect 260 gr/KW at engine best torque, and over 500 gr/kw at lower settings.

[fcfc]

Originally Posted by fcfc

The difference :


The diesel setup is at least more than 3 time the price of the outboard.

And I do not even speak of the man hours necessary to put the diesel engine and all details (shaft, exhaust, etc ...) in place.

[fcfc]

Originally Posted by Rick Willoughby

If it was the engine choice then it would have to be overpropped or have a variable pitch prop to get below the 230g/kWh when delivering 14 to 15kW consistent with the fuel consumption requirement.

Rick W

See http://www.volvopenta.com/volvopenta...ges/d2_40.aspx

A volvo D2 40 29.1 kw engine cannot deliver better than 11 kw at the prop for 1 gph. and 28 kw for 2,5 gph.

[Brian@BNE]

fcfc - so no real surprises. Outboard best for small boats. Diesel for larger cruisers. 'Our' (Sandy's) boat is close to cross-over point. So, short/infrequent day/weekend trips its obvious. If we seriously intend coastal cruising (=extended periods) and by the time we leave the jetty are sitting pretty low in the water (the comforts and supplies all add their bit to displacement) then the diesel is worth considering. But not obvious, agreed. And impact on draft could count against it (Oyster's 8" draft is what he desired and is impressive). I don't think I need to go that shallow, and I'm guessing but think a modified Topaz could accomodate the diesels additional weight plus the 'modern comforts' and not sink, although I know I wont get close to their loaded draft of 11". So its going to absorb the power of that VW (thanks for the link). Rather than a smaller diesel (you may be right, there may be no hgih efficiency ones that are smaller) specify a larger outboard for the comparison?

[Brian@BNE]

I suspect that for boats under about 9m outboards will have an edge, and if I recall correctly thats the way you're going? As did Oyster. Over that and bearing in mind Oysters earlier comment about shallow draft, narrow beam 'Bolger type' boats ((eg Topaz) then the weight of the diesel might be a blessing and the cost penalty not that severe, all things considered. But I've made a mental note that stability needs to be assessed and worked on before I commit. eg minimize superstructure height and weight along lines I have posted earlier.

[fcfc]

Originally Posted by Brian@BNE

Rather than a smaller diesel (you may be right, there may be no hgih efficiency ones that are smaller) specify a larger outboard for the comparison?

I would work the other way.

Put what you expect for your intended budget in your layout.

Fancy a hull around it.

Estimate costs.

If budget less hull cost = negative amount : rethink layout and expectations or extend budget. Generally it is easier to rethink layout/expectations.


If budget less hull/layout cost = small amount : put a outboard.
If budget less hull/layout cost = big amount : put a diesel engine.

[Oyster]

Originally Posted by Brian@BNE

I suspect that for boats under about 9m outboards will have an edge, and if I recall correctly thats the way you're going? As did Oyster. Over that and bearing in mind Oysters earlier comment about shallow draft, narrow beam 'Bolger type' boats ((eg Topaz) then the weight of the diesel might be a blessing and the cost penalty not that severe, all things considered. But I've made a mental note that stability needs to be assessed and worked on before I commit. eg minimize superstructure height and weight along lines I have posted earlier.

Bolger is pretty famous for using batteries as balast in some of his houseboat barges mounted in the bilge areas. For me, I want a boat that stands on its own merits with any power boat. If a power boat needs balast, persay, then you have attempted to put fifty pounds in a five pound bag. I think the issue with many of the Bolger hulls is the materials in the massive super structures that most people use and can be made better with some attention to details. In many of the older Elco boats and some of the older Rum Runner type commuter hulls, they were narrow in their beams but did have big power plants and round chines. There is another guy that has done some accomadating hulls for coastal running . His personal boat used a small outboard and he personally lived on a 39' for several years.


http://www.craftacraft.com/mark_van_abbema_v39



http://markvdesigns.tripod.com/siteb...0913bridge.jpg

[Pierre R]

Originally Posted by Brian@BNE

Pierre, would you mind posting costs of your 490mm CPP setup? I'm ready to say Ouch! But I might still want to find the budget rather than compromise.

Well sorta, here is the killer. I found the 18hp Sabb 2H online with controllable pitch propeller for a mere $1,200 USD. I sold the 47hp Perkins 4-107, gears and prop for $1,700.

The 18hp Sabb setup is nearly the same weight and pushes the boat better. The 4-107 had 2:1 reduction gears and a 350mm three blade prop. The Perkins burned 0.8gph to push the boat and the Sabb requires about 0.5 gph to push the boat or a decrease in fuel burn of about 40% right about where Rick predicts things with his large fixed pitch prop ideas.

What does all this mean? Well it means a 40% increase in range from 450 NM to a range of 720 NM or enough range increase to turn the fuel gunky at the amount I use the boat now. I use the boat about 180 hours a year so my savings is about 54 gallons or about $150 USD here in the states. A whopping savings of some 4% of my yearly costs. Enough for a few nights in a marina. If we used the outboard situation and paid retail for an outboard or a CCP setup I think that if you put the difference in the costs into the market you would get more in interest on the initial cost differences than you would save in fuel. That is why these exercises are so silly but hey, one can dream. Maybe that is why there are so many outboards and virtually no CPP's in small boats. At least here in the states.