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 with over-propping the boat so WOT is say 4000
> RPM or so? If I continue to run between 2800-3200 RPM won't I get
and
> increase in speed which will result in better mileage? I know the
> engine will be working harder at the same RPMs but there should be
> some gain in gpm. no?
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