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I agree w Fred except the "cruising prop" thing.
 
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Isn't it a common trait among designers to state a boats cruising speed as being a wee bit under hull speed, yet give the boats range somewhere around half throttle? The numbers never really add up...
 
MurrayM said:
Isn't it a common trait among designers to state a boats cruising speed as being a wee bit under hull speed, yet give the boats range somewhere around half throttle? The numbers never really add up...
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Take a builder's range claims with a grain of salt. Seahorse Marine's website says the 14-ton Coot has a range variously of 1400 and 1800 nautical miles for 390 gallons at 7 knots (fraction of a knot less than hull speed). Nevertheless, to go 7 knots requires over 2000 RPM from the JD4045. This equates to a theoretical range (without reserve) of about 1200 miles.

Regardless, my Coot has a fuel capacity of just 318 gallons, for a theoretical range of 980 miles at 7 knots. At half throttle (1600 RPM), the theoretical range is 1400 miles moving nearly 6 knots an hour.
 
markpierce said:
Take a builder's range claims with a grain of salt. Seahorse Marine's website says the 14-ton Coot has a range variously of 1400 and 1800 nautical miles for 390 gallons at 7 knots (fraction of a knot less than hull speed). Nevertheless, to go 7 knots requires over 2000 RPM from the JD4045. This equates to a theoretical range (without reserve) of about 1200 miles.

Regardless, my Coot has a fuel capacity of just 318 gallons, for a theoretical range of 980 miles at 7 knots. At half throttle (1600 RPM), the theoretical range is 1400 miles moving nearly 6 knots an hour.
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Still far better than most boats in that class, no? When I scan "trawlers" on YW it sure seems like everything under 40' has twins with over 500hp aboard. I know this is because most boats in that range are semi-displacement hulls zooming to an anchorage for the weekend, but jeepers small boats with small motors seem to be few and far between - especially new boats! Maybe I'm missing some new build, low HP, full displacement boats? Or maybe I just can't look past those good looking AT Tugs (with the big engines)?

Why the difference between stated fuel capacity and measured capacity on your Coot?

Love the Coot, BTW if I haven't already said so!
 
markpierce said:
Take a builder's range claims with a grain of salt. Seahorse Marine's website says the 14-ton Coot has a range variously of 1400 and 1800 nautical miles for 390 gallons at 7 knots (fraction of a knot less than hull speed). Nevertheless, to go 7 knots requires over 2000 RPM from the JD4045. This equates to a theoretical range (without reserve) of about 1200 miles.
Click to expand...

Mark, wonder if the builder calculations are based upon the Iveco standard engine rather than your John Deere? If I recall the JD was an option last time I looked at the Seahorse site.
 
Some of the HP hype in boats actually come from the engine manufacturer. A builder is buying reliable 240 HP diesels for a few years and suddenly the 315HP version becomes the same price as the 240, and the next year the 240 is no longer available. Eventually, they engineer a smaller block to get 240 HP, and voila, you've got your 240 HP back again, but in a block that is much higher stressed than the old reliable block you used to have. In truck engines, this goes on and on and on, and ultimately, one ends up with something like a 425 HP 3208 Cat, and the limited life that goes with it. The impulsiveness of the retiring baby-boomers, whom rather hear the number 315 than they would 240, will likely keep it that way for the foreseeable future.
 
healhustler said:
Some of the HP hype in boats actually come from the engine manufacturer. A builder is buying reliable 240 HP diesels for a few years and suddenly the 315HP version becomes the same price as the 240, and the next year the 240 is no longer available. Eventually, they engineer a smaller block to get 240 HP, and voila, you've got your 240 HP back again, but in a block that is much higher stressed than the old reliable block you used to have. In truck engines, this goes on and on and on, and ultimately, one ends up with something like a 425 HP 3208 Cat, and the limited life that goes with it. The impulsiveness of the retiring baby-boomers, whom rather hear the number 315 than they would 240, will likely keep it that way for the foreseeable future.
Click to expand...

Ah, I see. Makes sense. Maybe I'm stingy, but I like to hear the number 150!

Is it safe to assume that between two identical 'tugs' the 210HP version will have a lower fuel burn at a given speed than the newer 330HP version?
 
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Animal wrote;

"small boats with small motors seem to be few and far between"

How about the number 40? There are some small boats here w full displacement hulls and a reasonable amount of power. The 30' Willard's came originally w 33 useable hp and managed very well w that amount of power even displacing twice as much as most 30' boats.

But if you aspire to have such a boat there will be few to look at in the marketplace. However they are there and at this time not any more expensive than inefficient boats of the same size.

"Is it safe to assume that between two identical 'tugs' the 210HP version will have a lower fuel burn at a given speed than the newer 330HP version?"

Yes ... all things being equal but of course much of the time they are not. That's why running single on a twin saves fuel ... you cut the size of the engine/engines in half. 100% reduction in cylinder displacement.
 
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Animal said:
Is it safe to assume that between two identical 'tugs' the 210HP version will have a lower fuel burn at a given speed than the newer 330HP version?
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Not really......but there are a bunch of variables.

At a given speed the hull resistance is equal no matter what the power. But the weight (displacement) may have increased with the larger engine (larger cooling and exhaust systems, heavier shaft, larger gear box, larger propeller,etc). Then there's the propeller efficiency, it will change with diameter/pitch and rpm changes. Then there's specific fuel consumption.

If you look at the lowest (#6) curve on the chart below, you see it's called Specific Fuel Consumption and the units are LBS/HP/HR (pounds per horsepower hour) or G/KW/HR (grams per kilowatt hour). Note that below about 1400 and above 2000 RPM the specific consumption increases sharply. If you're operating within that lowest consumption "Bucket", it's good. But if you install bigger power and operate at lower RPM, thus falling outside the bucket, it can be worse than it should be.....

fuel.jpg
 
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TAD has posted the usual almost useless graphs pushed by the engine folks.

Its useful for full throttle operation , after that its all math speculation concerning prop hp ,loading and fuel burn..

The problem is the fuel burn curves are for a fully loaded engine at that operating RPM.

To fully load the engine at every operating RPM impossible to do , even a 2 speed tranny and CP setup would be far from optimum loading at most cruise settings..


Does it matter is a different question , at 2 or 3 GPH , a 20% burn penalty only would matter to a voyager , not an inshore boat.

IF you can access Pro Boat Builder this month there is a rare more accurate fuel / power curve shown , but these are usually kept from the public.

Called a fuel map , they are almost impossible to obtain .
 
I recall that most engines that are found in our old trawlers are most efficient at about 1800rpm or so. Bottom line is that if you run close to full bore you bay a penalty and if you run less than a 50 to 70% load you burn pay even a larger penalty.

I like TAD's comment about weight. A twin engined 36GB running the way most do now (underloaded) you pay two rather large penalties. You've got twice as much power as the single engined boat so running slow (7.5 knots) both engines are at about 25 to 30% load. Far far away from the most efficient speed and load for best efficiency. The single engined boat would run under a load of about 55%. Much better. If propped right that would be (a guess here) about 1900 to 2000rpm.
Also the 2nd engine and all that it has for support equipment weighs over a ton. We frequently talk about the power requirement for a displacement boat as "X # of tons per hp" The only variable there is weight. Hmmmmm Must have a very significant effect on the amount of power required to drive a given hull.

So there is one significant and one large reason that the single engined boat will be more efficient. Of course if the twin engined boat was to have one engine twice the power of the single it would be not far from the same.
 
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Funny...my boat and some who have posted actual floscan numbers for their Lehmans on 36-42 foot trawlers seem to do best (NMPG wise) at around 1500...

Someone posted pictures of a floscan several times that had 5 NMPG at 1500rpm where it dropped to 4 NMPG at 1600rpm and down to 3 NMPG at 1750rpm.

Whether the engine is more efficient at 1800 or not I can't say...but the whole system (actual cruising numbers) is better down around 1500 for a FL120.
 
FF said:
TAD has posted the usual almost useless graphs pushed by the engine folks.
Click to expand...

I disagree, the propeller demand curve is an approximation (as mentioned by myself and others above), and should be taken as such. But it's better than nothing or just using the Max output curve. On that 600 Lugger the difference between max and the prop curve at 1800 is 370 (Prop) and 540 (prop shaft), a huge difference in fuel consumption.

Its useful for full throttle operation , after that its all math speculation concerning prop hp ,loading and fuel burn..

The problem is the fuel burn curves are for a fully loaded engine at that operating RPM.
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If you check at the bottom of the posted table, you'll see that curve #5 is "Fuel use based on prop power draw". Not on fully loaded engine.



Called a fuel map , they are almost impossible to obtain .
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Not impossible....but there's some creative graphics going on with them as well, here's one from MAN and from CAT. They are quite different looking.

Fuelmap.jpg

3208fuel.jpg
 
manyboats said:
I recall that most engines that are found in our old trawlers are most efficient at about 1800rpm or so. Bottom line is that if you run close to full bore you bay a penalty and if you run less than a 50 to 70% load you burn pay even a larger penalty.

I like TAD's comment about weight. A twin engined 36GB running the way most do now (underloaded) you pay two rather large penalties. You've got twice as much power as the single engined boat so running slow (7.5 knots) both engines are at about 25 to 30% load. Far far away from the most efficient speed and load for best efficiency. The single engined boat would run under a load of about 55%. Much better. If propped right that would be (a guess here) about 1900 to 2000rpm.
Also the 2nd engine and all that it has for support equipment weighs over a ton. We frequently talk about the power requirement for a displacement boat as "X # of tons per hp" The only variable there is weight. Hmmmmm Must have a very significant effect on the amount of power required to drive a given hull.

So there is one significant and one large reason that the single engined boat will be more efficient. Of course if the twin engined boat was to have one engine twice the power of the single it would be not far from the same.
Click to expand...

Not just the added weight penalty, there's additional drag from the extra shaft, struts, prop, and rudder.
 
Re TAD's comment again the fuel efficiency may not be load dependent so for our purposes dosn't make much difference. . It may mostly just be rpm dependent. In other words generally the best fuel economy will be found at a given rpm or rpm range and for that to prevail both at high and low loads. Would be nice to know if you were to buy the engine in TAD's example and having read that 1400rpm is best for fuel economy knowing if that's true in the average situation on a boat.

FF you play down the importance of a 20% fuel savings and you've stressed for years about the great benefits of over propping when that only saves 5% or so. ?????
 
Tad Roberts said:
Not just the added weight penalty, there's additional drag from the extra shaft, struts, prop, and rudder.
Click to expand...

A few here have run their own real world tests between single ops of a twin and twin ops...they see something like a 25% or so gain in efficiency on their boats...sound a little high but I have no reason to doubt them.

I'm sure the added drag is most of it...the weight on the smaller boats could be significant but I wouldn't suspect as much as the running gear.
 
And then.....for the fellow who needs a "Dooms Day Boat"....There's Favonius....complete with twin get home engines and 4000 usg of fuel......

http://wolfemarine.com/core/listing/photo_gallery.jsp?slim=broker&lang=en&ywo=wolfemarine&hosturl=wolfemarine&units=Feet&id=2613040&back=/core/listing/pl_boat_detail.jsp&boat_id=2613040

Wow, I love it. My kind of boat. Even the engine room looks...well..sort of less than perfect, and for REAL, just like mine. My only criticism is the pokey ports all round. In this day and age you want better forward vision and views from living areas. An improvement which would make the boat, would be to hack out those small round ports and put in bigger rectangular windows, (quite do-able with steel), but stout ones which would not compromise her ocean worthiness to any real extent. Then she would be perfect.
 
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I thought I recognized Skookum Maru.
Here she is at the LaConner Boat Show not this year but last.
I thought I saw her in the channel this year (while I was working on Willy)right after the show probably going home.

I agree. Near perfect hull form, perfect hull material and perfect engine.

Even for a wood boat it seems like a giveaway price. She must have issues. I think she's been for sale for at least 2 years.

Peter what is it? I have no problems w the windows.
 

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Someone interested in a Doomsday boat will probably give up the big glass high water view motor boat and purchase and aux sail or motor sail.

Doomsday will be world wide , not simply the collapse of one country , so fuel which requires great international trade co operation may be difficult to obtain.

Wind is FREE , and Ocean Passages for the World show the sail routes in great detail.

A custom crunch boat would be set up to survive in the old way,

wood range, sun covers and excellent ventilation , large water tanks with rain catch refill ability.
 
manyboats said:
I recall that most engines that are found in our old trawlers are most efficient at about 1800rpm or so. Bottom line is that if you run close to full bore you bay a penalty and if you run less than a 50 to 70% load you burn pay even a larger penalty.
Click to expand...


What kind of "penalty" are you talking about?

The first thing you need to do is separate engine fuel efficiency from how "efficient" your recreational boat might be. You aren't running that thing in the liner trades and making routine voyages between Seattle and Sydney so what do you really care what the specific fuel consumption is?

To put that into some kind of perspective that might hopefully make sense and get you and others off the "engine efficiency" thing, look at the tables posted by Roberts.

At 2800 rpm that CAT burns 14.2 gph to put 260 hp into the prop. It burns 6.112 ounces of fuel an hour to produce one hp at full load.

It burns 6.368 ounces to produce one hp at 1500 rpm.

It burns 5.824 ounces to produce one hp at 2200 rpm.

Considering that at 1500 rpm the engine burns 2.3 gallons per hour and at 2800 rpm it burns 14.2 gallons per hour and at 2200 rpm it burns 6.5, the difference between highest efficiency and lowest is a whopping increase of .034 ounces per hp to obtain a net reduction in fuel burn of 617 percent.

An 8.5 percent "penalty" in BSFC seems fairly reasonable to obtain a 617 percent reduction in fuel burn, particularly if the boat speed is reduced by only a knot or 3.

Your time might be better spent in finding a way to move a reasonable load between A and B in the time you allot and in the most comfortable manner at a cost you are happy to accept. I don't think you will accomplish that by trying to micro-analyze engine efficiency.
 
manyboats said:
Peter what is it? I have no problems w the windows.
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Favonius only has small round ports about 8-10 inches, (ok, maybe 12 inches), across even facing forwards to steer through Eric. Don't you think a somewhat larger forward visibility potential would be desirable...? One could argue about the side view to look at scenery, I admit, but if I'm crossing oceans, and exploring remote parts of the world, the only reason to own such a vessel, then I would want to see more of what I was paying plenty to see...just sayin'...
 
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Rick wrote;

"What kind of "penalty" are you talking about"

Re Fuel consumption on the GB 36 example boat.

First penalty is having two engines to do the work of one means they will both be extremely underloaded and futher away from the best spot on the efficiency curve.

The second penalty is the excessive weight from the extra engine causes more wetted surface and more wave making resistance.

So the double engined boat is less efficient at trawler speeds for two significant reasons as stated above.

That's all I was saying.

"Your time might be better spent in finding a way to move a reasonable load between A and B in the time you allot and in the most comfortable manner at a cost you are happy to accept. I don't think you will accomplish that by trying to micro-analyze engine efficiency."

Indeed I will be the one who decides how my time is spent and I've always said we obsess about fuel consumption. Too much attention spend on too little. But when something interesting comes up I'll prolly jump in and talk about it just because it's interesting ... not to save fuel. Willy burns 1 gph and I'm certainly not going to make much of any effort to improve on that.

Also is your 617% really supposed to be 6.17%? Couldn't get my head around that.


Oh yes Peter I was looking at the other boat ... Skookum Maru. Sorry.
I don't really care for Favonius.
 
Ultimate trawler

This big heavy Malahide gives a easy 7 knots at 1500 rpm . 260 hp 6 cyl Volvo Turbo , about 3 nmpg , wing engine 150 hp yanmar 5 knots at 1800 rpm.
All in easy weather ! Together full blast about 10 knots , didn't count the nmpg !
Just bombed it home !!
 
FF said:
Someone interested in a Doomsday boat will probably give up the big glass high water view motor boat and purchase and aux sail or motor sail.

Doomsday will be world wide , not simply the collapse of one country , so fuel which requires great international trade co operation may be difficult to obtain.

Wind is FREE , and Ocean Passages for the World show the sail routes in great detail.

A custom crunch boat would be set up to survive in the old way,

wood range, sun covers and excellent ventilation , large water tanks with rain catch refill ability.
Click to expand...

This sounds like you've already given this serious consideration! Every day I open the paper I wonder if its time to start work on an arc! :)
 
RickB said:
Your time might be better spent in finding a way to move a reasonable load between A and B in the time you allot and in the most comfortable manner at a cost you are happy to accept. I don't think you will accomplish that by trying to micro-analyze engine efficiency.
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Amen! As I stated previously, the mental gymnastics that have been posted on the engine efficiency subject don't mean much (if anything) to the recreational boater. There are so many other subjects that affect safety, comfort and cost that are relative to our boats. :blush:
 
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