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Old 11-13-2013, 02:58 AM   #61
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Grantm -
If you are open to a smaller boat, this little beauty really catches my eye.
Arvor Weekender 700 by Arvor Boats Australia

They really pack a lot into a small boat.

It has a single Cummins 150hp diesel shaft drive, does 22 knots, and they are trailerable. They are quite popular with fisherman in SA and seem to handle rough water very well.

They've only been around for a few years, but a few used ones are starting to come on the market. If I was after a newer boat, I'd certainly consider one.
Yes a nice boat but not 'my cup of tea' as they say. I still like the older styles.
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Old 11-13-2013, 08:19 AM   #62
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Why do you suppose almost no other motorcycle manufacturer made boxer bikes? Not a very good design.

Smooth with excellent cooling and simple construction
The BMW was copied by the Russians and made for over 50 years , and in WWII even Harley D had to copy the BMW motor to have bikes for desert use.

EVERY engine has different RPM areas where they will shake from the engine harmonics , regaredles of configuration.The only shake on a flat BMW is from one con rod being seperiated by a main bearing from the other.This can in theory cause a twisting motion for the crank.

From 1964 to 2009 , I was never able to notice it.

Using a single throw crank and having both rods on one bearing (Harley D), or a master con rod and slave rod as on a round motor might cure this in theory twisting , but at the expense of repairability.

The ART of engine building is to place the shake at an RPM that is seldom used.

If the engine idles at 600 , shake of any sort at 450-500 is where the mfg will place it, red line at 2100 , shake at 2300 would be fine engineering.

3 cylinders or multiples of 3 seem to be the easiest to get smooth .

Most trucks , donor engines for large boats have accepted this and an inline 6 is the norm for 200-700 HP diesels
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Old 11-13-2013, 10:31 AM   #63
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Twin 165 HP Pentas may be as economical to use as a single 330 HP. The key is to look at the fuel burn at the cruise RPM/HP. Usually the published curve is in gms of fuel/kW and typically is a shallow curve higher at the ends. The sweet spot is where you're getting the most HP for the fuel used. This is usuAlly in the mid-range RPMs.

You may find that the pair of Pentas use no more fuel/HP than the larger single? Of course a modern, computer controlled diesel will be more efficient that older technology. But maybe not at the HP where you're going to be using her.

Plus, with twins you have better maneuvering and take home power in the event of an engine failure.
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Old 11-13-2013, 10:50 AM   #64
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FF the BMW AC twins are excellent motorcycles .. no doubt. But the're not ideal .. kinda like a stern engined boat.
Three cylinder engines in general aren't as smooth as a four. I avoided the 3s when I repowered Willy. But the K75 3 cyl BMW IS incredibly smooth. Smoother than any four. But the K75s crankshaft dos'nt rotate in the same direction as the flywheel.

rjtrane,
With the dual prop feature that could easily be true. I agree.
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Old 11-13-2013, 11:51 AM   #65
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I had a 500cc BMW motor bike back in 1962 - as I recall, she was quite civilized - smooth and quiet with loads of torque.

I also had an Easthope single cylinder in one of my Mud Hens - it was funky and slow enough to count the rpms visually.

Or how about a one cylinder Sea Gull outboard - rattled my teeth on my Bolger "Black Skimmer" which acted as a sound box for the Sea Gull (later replaced with a sculling oar).
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Old 11-13-2013, 12:23 PM   #66
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"I had a 500cc BMW motor bike back in 196"

Probably the best BMW ever made.

I had a Velocette in 62 in SanDeigo. Even better than the BMW.
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Old 11-13-2013, 01:02 PM   #67
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Twin 165 HP Pentas may be as economical to use as a single 330 HP

Not really possible as the fuel bill to simply operate the 2 nd engine , run 2 fuel pumps , two cooling pumps and all the mechanical friction of 2X as many (smaller ) cylinders exists.

Plus 2 stuffing boxes , two trannys , two sets of bearings and probably with smaller diameter props are going to require more diesel energy.
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Old 11-13-2013, 04:25 PM   #68
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FF - you're missing the point - absolute fuel flow is just that - absolute. If in fact the twins burn the same fuel delivering the same HP as the single, then, assuming the boat is properly propped, it will get the same MPG. The fuel flow/HP curve already takes into account the work the motor needs to create the HP. It does, as you point out, take into account the additional drag after the motor - 2x the bearings plus some additional drag of the additional propeller and rudder. Yes, there is additional energy required after the rear-end of the diesel, but those losses are most likely negligible in a year's worth of cruising.

If, however, the twins need to run well off their optimum speed (where gms/kw are minimum), the twins may use more fuel. Likewise, the larger single engine, may also need to run off its optimum speed, and end up less efficient.

This is actually a good exercise that very few undertake to see if really there is an additional fuel cost in twins vs. singles. There is a lot of prejudice in this area - and I am not trying to convince true believers (whether their religion is single or twin or gas or diesel or wood-burning steam), but instead using published fuel flow figures to make the comparison in order to make an informed decision.

If the fuel cost turns out to be similar, then one can make the twin/single decision based on other criteria - engine room space - redundancy - maneuverability - maintenance - etc.
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Old 11-13-2013, 04:33 PM   #69
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FF - you're missing the point - absolute fuel flow is just that - absolute. If in fact the twins burn the same fuel delivering the same HP as the single, then, assuming the boat is properly propped, it will get the same MPG. The fuel flow/HP curve already takes into account the work the motor needs to create the HP. It does, as you point out, take into account the additional drag after the motor - 2x the bearings plus some additional drag of the additional propeller and rudder. Yes, there is additional energy required after the rear-end of the diesel, but those losses are most likely negligible in a year's worth of cruising.

If, however, the twins need to run well off their optimum speed (where gms/kw are minimum), the twins may use more fuel. Likewise, the larger single engine, may also need to run off its optimum speed, and end up less efficient.

This is actually a good exercise that very few undertake to see if really there is an additional fuel cost in twins vs. singles. There is a lot of prejudice in this area - and I am not trying to convince true believers (whether their religion is single or twin or gas or diesel or wood-burning steam), but instead using published fuel flow figures to make the comparison in order to make an informed decision.

If the fuel cost turns out to be similar, then one can make the twin/single decision based on other criteria - engine room space - redundancy - maneuverability - maintenance - etc.
Not a mech or engineer here..but I have ALWAYS heard everything else equal...the losses in friction, drag, etc are enough in twins of equal power to a single will require as much as 15% more fuel/output....depending on the setup...it's always more...just a question of how much.
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Old 11-13-2013, 04:50 PM   #70
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I can't see what psneeld posted but FF you're not taking into consideration that all those extra parts from the second engine. Transmissions, stuffing boxes, water pumps, oil pumps ect ect are basically half the size and hence half the drag or/and friction so that argument holds no water at all. It's just not part of the comparison. Marin made that mistake for years.

And w twins the rudder dosn't need to be partly deployed to compensate for prop walk causing some extra drag.

But here twins v/s singles is only considered when the twins have twice as much displacement (760 cu in) and twice as much power and their engines at half load re the twins. So of course they are much less efficient. Would be nice to find a boat w twins and singles both having the same power. I submit that they would have the same fuel consumption.
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Old 11-13-2013, 06:15 PM   #71
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Marin made that mistake for years.
Careful. He might start posting again!

I did some sea trials running with just one engine, in the 5 to 8 knot range. The result was that for any given speed, fuel used with just one engine equalled fuel used when running both engines. When running on just one engine I needed higher rpm to get to the speed point. I also needed 7 deg rudder to go straight with just one engine, and there was drag from the idle prop etc.

In my case any efficiency gain of just one engine was not measurable compared to the hull drag. You need x HP for y speed. And x HP requires z gph diesel. So with engines like JD, which have amazing low-midrange rpm fuel efficiency, nothing is gained by running just one.

I suspect that unless you had a very slippery full displacement hull this is quite normal.
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Old 11-13-2013, 06:48 PM   #72
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Careful. He might start posting again!

I did some sea trials running with just one engine, in the 5 to 8 knot range. The result was that for any given speed, fuel used with just one engine equalled fuel used when running both engines. When running on just one engine I needed higher rpm to get to the speed point. I also needed 7 deg rudder to go straight with just one engine, and there was drag from the idle prop etc.

In my case any efficiency gain of just one engine was not measurable compared to the hull drag. You need x HP for y speed. And x HP requires z gph diesel. So with engines like JD, which have amazing low-midrange rpm fuel efficiency, nothing is gained by running just one.

I suspect that unless you had a very slippery full displacement hull this is quite normal.
And there are some on this forum that have tried, documented and posted quite considerable gains running on just one.

I find it hard to believe that many people think just one way or the other.

Each boat model may have similarities between hulls...but to say one boat model has gains therefore all should ...just isn't realistic.

In a totally equal world where 2 identical boats with the same horsepower, one twin and one single...it would be ludicrous to think that the single wouldn't be more efficient...but again...depending the efficiency, it may be in the 1% or the 15+% range.

you just cant drag that many moving parts around and underwater items and think "equal" is gonna happen.
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Old 11-13-2013, 09:12 PM   #73
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Holy cow! You guys will argue anything.
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Old 11-13-2013, 09:50 PM   #74
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Holy cow! You guys will argue anything.


Was thinking most of this thread needs to be merged into that 1,000 post debacle single versus twins thread we had last year.
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Old 11-13-2013, 10:46 PM   #75
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Sounds like sound stuff Insequent.

Don't see how anybody could argue w that.

Insequent wrote "Careful. He might start posting again! re Marin.
No I think he's actually gone. Missed him a few times lately .. actually.


Twins are better of course but if you're going to not run all your engines I think a triple would be even better. It's so obvious I don't even think it's good for 10 posts.
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Old 11-13-2013, 10:56 PM   #76
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you just cant drag that many moving parts around and underwater items and think "equal" is gonna happen.
Not think, measured. In my case.

Apart from hull efficiency, it will depend on the efficiency of the engines installed. Old fuel hungry dinosaurs probably will give a different outcome to what I measured. My point is, dont assume running on one engine will be more efficient. The benefit may quite small or negligable. Do your own tests. Share some results.
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Old 11-13-2013, 11:04 PM   #77
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psneeld why would some commercial boats take of a prop to go all the way up the coast to Alaska to save feul?
Someone talked about that here on TF awhile back.
I remember it varies a lot depending on the boat. Lots of variables.
But it apparently saves considerable fuel on most boats.
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Old 11-13-2013, 11:07 PM   #78
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re Marin.
No I think he's actually gone..
He's not gone....he's lurking.
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Old 11-13-2013, 11:18 PM   #79
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Hello all !



Gents, what are your thoughts on the following, based on say a thirty foot cruiser of some kind

Petrol engine vs diesel
Twin engine vs single engine
Stern drive vs shaft

There are so many options I'd just love to hear the pros and cons !

Thanks !
I think most TF members agree that diesel shaft drives are the way to go. Twin vs single is a personal choice. If you are saying you want the ability to cruise at 10 or 14 knots, then a semi-displacement hull with a 140-300 hp diesel would be needed.
In Australia there aren't too many choices for 30 foot semi-displacement cruisers.

Most of the Grand Banks 32's, Island Gypsy's, and other typical trawlers are displacement speeds although the GB's with twins may have a bit more speed.

There are a few Lobster type boats such as this: Used Mdi 32 Lobster Boat for Sale | Boats For Sale | Yachthub which would cruise in the mid teens.

A Mariner 31 is another option. They often have twin 135 hp diesels Used 31 Mariner Flybridge Cruiser for Sale | Boats For Sale | Yachthub. Good value but it won't handle the rough water as well as the lobster boat.

You can also go to a planing hull such as a Cresta 32 or a Bertram 31 with twin diesels, excellent in rough seas, if you can find one in good shape.
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Old 11-13-2013, 11:31 PM   #80
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I think most TF members agree that diesel shaft drives are the way to go.
A Mariner 31 is another option. They often have twin 135 hp diesels Used 31 Mariner Flybridge Cruiser for Sale | Boats For Sale | Yachthub. Good value but it won't handle the rough water as well as the lobster boat.

You can also go to a planing hull such as a Cresta 32 or a Bertram 31 with twin diesels, excellent in rough seas, if you can find one in good shape.
The 135hp Volvo engines usually seen in Mariner 31s are called "165" models, which can mislead as to hp, Auscan has it right. They are advertised from 55K up, a good entry level twin.
The Savage 33 and Cresta 32 used the same hull, I think. They are one of several boats favored to refit for offshore fishing; another,the Masters 34, usually with a single, looks unfashionable but has an excellent seaboat and build reputation.
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