Thank you, John, You're a real treat to read - although I did doze
off for a moment about half way through. So, all the trails I've
followed lead back to home, that is, I need to relax, enjoy my boat,
and keep it well maintained. My Chebbys are twenty-five years old
with about 1200 hours and run great.
The best thing I can do for fuel economy is to take a lesson from
sailboaters and pay attention to current, wind and weight. I'll take
you advice and not try to fix anything that ain't broke.
Tha again, I really did enjoy the information.
Rocco
On Nov 24, 3:26 am, john hamilton <[EMAIL PROTECTED]> wrote:
> The original question was what's wrong with overpropping. The short answer
> is, it gives you no better (and usually worse) fuel economy, it cuts your top
> end and it works your engine too hard at all speeds.
>
> The next question was how I determine what prop will work best given my
> cruising speed and most efficient engine speed. The answer is I'll prop my
> boat the same way everybody else does, to reach the peak power the engine
> will produce. The key is to make the engine as efficient as possible in the
> first place and that will give me better performance all all rpm's. A side
> benefit will be a higher top speed because of the steeper prop my more
> powerful and efficient engine will swing at all rpms.
>
> Some chatter:
>
> The only way to accurately measure BSFC is on a dynamometer. You have to
> have accurate measurements of horsepower at all rpm's and at the same time
> put an accurate fuel flow meter on it. I had to use general rules of thumb
> rather than specific numbers because I can't put my engine on a dyno in
> Alaska. I do have a Floscan fuel flow meter and it's very valuable. I did
> use a computer engine dyno program to get approximate numbers for horsepower,
> torque and volumetric efficency when designing my engine. BSFC is expressed
> as pounds of fuel/horsepower x hours. The reason diesel's get better fuel
> economy is their much better BSFC numbers, due mostly to the higher energy
> content of the fuel and the much higher compression ratio of the engine.
> BSFC also varies with throttle opening and temperature. In general, higher
> compression engines get better BSFC numbers than lower compression engines
> do.
>
> (The differences in BSFC numbers for gas engines is slight, there's not much
> improvement to be had with gas. I did the best I could by increasing
> compression, reducing exhaust back pressure, improving flow in the intake and
> heads, and carefully choosing a camshaft to match the use of my engine. If
> you want real improvement, you have to go diesel, but you can make a
> significant difference in economy by adding torque and lowering rpm. You
> can't do just one, you have to do both.)
>
> To answer your questions in more detail, if I can (I'm no rocket scientist),
> I started with the limitations of the hull and the fact that in general a
> boat gets it's best fuel economy at a speed where it is just on full plane,
> no faster. For my boat (and probably yours) this is around 13-16 knots. My
> goal was to put together a power package that would produce enough power to
> go that fast as efficiently as possible, with plenty of reserve power.
>
> I designed my engine around very high torque rather than high horsepower,
> although if you increase one you'll usually get more of the other. I bored
> and stroked my small block Chrysler to 410 cubic inches and I have 10.5/1
> compression Keith Black pistons with aluminum edelbrock heads. The camshaft
> was custom ground to maximize torque between 2500 and 3500 rpm. The torque
> peak is 500 lb/ft @ 2500 rpm and the horsepower peak is 410 @ 5000 rpm. My
> small block is producing more power and more torque than your big blocks
> are. On your Chebby big blocks the torque peak is likely at a bit higher rpm
> than that and the power peak is likely at a bit lower rpm than that due to
> the camshaft profile. My engine produces more than 450 lb/ft of torque from
> 2000 rpm through 5000 rpm. My whole idea was to produce as much torque as
> possible at the speed the hull cruises most efficiently at, given the
> transmission ratio I have. I'll then prop the boat
> to allow me to reach about 5000 rpm at full load. Whatever that does for my
> top end, I really don't care. I'm not focusing on speed, I'm looking at fuel
> burn @ 2500-3000 rpm, vs torque output. I NEVER exceed 18 knots, because
> it's just not that kind of boat. If I wanted to go fast, I'd get a speed
> boat again. Now that I'm older, I'd rather cruise comfortably. It's easier
> on the back.
>
> I don't have my prop figured out yet, I'm still working on the engine. I did
> some work with some prop calculators and I am starting with two inches more
> pitch than I had with the original engine. I may have to settle for just one
> inch more pitch, but I won't really know where I'm at until I'm done and the
> engine is broken in. I am in the process of completing this project now.
> Theoretically, by doing it this way I'll be able to cruise at the same speed
> I used to with a stock, smaller displacement engine while turning a 500 rpm
> slower engine speed and burning less fuel.
>
> Of course, It may turn out I have no idea what I'm talking about, but I do
> have a couple of past experiences to draw from. I've repowered one boat of
> my own and one boat of my employer's. In my own boat I replaced a 150hp
> outboard with a 225 hp outboard. In the employer's boat I replaced twin
> 305/225 chebby small blocks with twin 350/300 chebby small blocks. That
> employer's boat also had twin 160hp Volvo TD's for a while. In both cases,
> by substantially increasing the horsepower I was able to prop the boat much
> steeper and I got much better fuel economy at the most efficent cruising
> speed because the more powerful engine was turning much slower. The fact
> that I was much faster in the top end was a side benefit, not the main goal.
> As a reference, I went from a 15.5x15 prop with the 150hp outboard to a
> 14.5x19 prop with the 225 outboard. Speed went up 30% and fuel economy went
> up 15%. Cruising rpm dropped from around 4000 to around
> 3000. I don't remember the numbers on the bigger boat, it's been almost 20
> years now.
>
> (The big boat did get much better fuel economy with the Volvo TD's but we
> were very unhappy with the performance, durability and the maintenane costs
> vs. the gas engines in that particular application. I'm a fan of diesels for
> most boats but that boat had to go really fast and it got used very hard for
> short bursts and idled around at 5 knots 90% of the time. Diesels {at least
> those ones} didn't like that)
>
> All of that said, your boat would probably work best with 300hp Cummins 6BTA
> engines and down angle transmissions (I'm assuming you have Velvet Drive 71C
> transmissions like most older Uniflites). You might also improve a bit with
> more modern, fuel injected and higher horsepower big blocks like the new 420
> horse 496 Chebbys. Either way it's a big chunk of money. If you want to go
> faster, you will need more power/torque. If you want better fuel economy,
> you will need more power/torque. With the stock engines you have you're
> propped correctly. I'd leave well enough alone until they blow up, and then
> decide whether or not to convert to diesel or add power. That's what I did.
> When my old engine blew, I added power (and reduced weight by about 300
> pounds) because I couldn't afford to convert to diesel. If you don't already
> have them, install a pair of Floscan fuel flow meters. They're worth it.
>
> This advice is worth every bit of what you're paying for it. Stay tuned.
>
> John
>
> --- On Sun, 11/23/08, Rocco <[EMAIL PROTECTED]> wrote:
>
> From: Rocco <[EMAIL PROTECTED]>
> Subject: [UnifliteWorld] Re: Prop sizing
> To: "UnifliteWorld" <[email protected]>
> Date: Sunday, November 23, 2008, 7:22 AM
>
> My engines are Crusader 454/350s.
>
> I think I understand what you're saying. It leads me to the question
> of how one measures BSFC vs prop configuration? When you say you're
> propping your boat to cruise around 2800-3000 rpm, what do you mean?
> In other words, are you determing which prop will give you the
> greatest speed at that rpm since you have establihed that as you best
> BSFC?
>
> On Nov 22, 3:53 pm, john hamilton <[EMAIL PROTECTED]> wrote:> It's more
> complicated than that. The boat is going to use x amount
>
> of horsepower to go y fast. It doesn't care what rpm that is. The
> engines will use x amount of fuel to produce y horsepower. What you want is
> to produce the amount of horsepower required to go y fast as efficiently as
> possible. The term for this is BSFC. Best BSFC (brake specific fuel
> consumption) numbers are usually at or around the peak torque RMP. Where that
> is depends upon your camshaft, but with a stock marine cam it is probably near
> your current cruising RPM. You want your boat propped so that the best BSFC
> number is the rpm where you cruise at, assuming your hull and transmission
> ratio
> allow it. I had a camshaft custom made so that my peak torque comes in @ 2500
> and the torque curve is very flat, particularly between 2500 and 3500 rpm.
> I'm propping it to cruise at around 2800-3000 rpm or so, even though the way
> I built the engine it achieves peak horsepower @> 5000 rpm. I think the
> torque peak is more important than the
>
> horsepower peak and the torque curve is what I focused on when I designed the
> engine. I expect you're propped correctly, assuming your engines are stock
> Chrysler Marine Engines. With those fuel flows you're running twin
> 440's?
>
>
>
>
>
> > John
>
> > --- On Sat, 11/22/08, Rocco <[EMAIL PROTECTED]> wrote:
>
> > From: Rocco <[EMAIL PROTECTED]>
> > Subject: [UnifliteWorld] Prop sizing
> > To: "UnifliteWorld" <[email protected]>
> > Date: Saturday, November 22, 2008, 2:25 PM
>
> > Here it is again, the ever so often discussion about prop sizing.
> > I've read lots of info on this subject but I can't find the one
> piece
> > I need.
>
> > My boat is propped properly - so I'm told - and achieves 4600 RPM @
> > WOT. However, except for the occasional whoppie moment when I run it
> > up to full speed - 26KTS and 60GPM - I consistantly cruise between
> > 2800-3200 RPM. At these RPMs the boat cruises 12-13 KTS @
> > appoximately .5/gpm.
>
> > I think my hull is very efficient considering its top speed. Speed
> > goes up quickly above 3200 RPM but then secondaries open and the fuel
> > consumption rises even faster.
>
> > So, what's the problem
>
> ...
>
> read more »- Hide quoted text -
>
> - Show quoted text -
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