ON TOPIC: Effects of using a low altitude prop at high altitude?
 

My boat is a 17' I/O with a 130 hp Volvo Aq130C and Volvo 270 outdrive

I have two props...a 14.25x17 and a 15x15...I use the 14.25x17 for sea
level and the other for high elevation (4000' and up) operation. These
props have always worked well when using them at the elevations
indicated.

Right now the low elevation prop (14.25x17) is installed and I was
thinking of just leaving it on for this weekend when I will be boating
at high elevation since I am pressed for time to change to the other
prop.

What will the general affects be of leaving the low elevation prop on
while I am boating at 4500' elevation?

Thanks!

What will the general affects be of leaving the low elevation prop on
while I am boating at 4500' elevation?

Thanks!


If your boat has been achieving the proper number of RPM at the two different
elevations due solely to your past practice of swapping out the props, it might
be "overpropped" at 4500 feet with the sea level unit. You will likely notice a
decrease in speed, and a tendency for the engine to run slightly hotter.

Gould 0738 wrote:

:If your boat has been achieving the proper number of RPM at the two different
:elevations due solely to your past practice of swapping out the props, it might
:be "overpropped" at 4500 feet with the sea level unit. You will likely notice a
:decrease in speed, and a tendency for the engine to run slightly hotter.

Thanks for the response. I just found this prop calculator site
(http://www.rbbi.com/folders/prop/propcalc.htm) and put a few numbers
into it:

Engine RPM: 4800
Gear Ratio: 2.15
Prop Pitch: 15 or 17
Prop Slip: 15 (I've heard that is a good average to use)

The speed I get with a 15 pitch prop is 27, which is about what I get
with that prop at 4500' elevation.

If I put in 17 for the prop pitch value and leave all else the same, I
get 30.5 mph.

I'm not trying to be sarcastic here or question your knowledge, but
this seems to contradict your comment about a decrease in speed.
Comments? I'm just trying to understand.

Thanks,
Bob

I'm not trying to be sarcastic here or question your knowledge, but
this seems to contradict your comment about a decrease in speed.
Comments? I'm just trying to understand.

Thanks,
Bob


Bob:

Assumptions include:

1. You are not retuning your engine for the high altitude operation, and will
be running a mixture somewhat lean on oxygen.

2. That you have two different size props for a valid reason, i.e. your boat
needs the decreased resistance of the smaller diameter prop to achieve proper
RPM at the higher elevation.

If your boat runs the same at the higher elevation with the larger prop, why
the H would you bother to screw around changing it?

Without resorting to charts, graphs, theories, and etc:

*If* your engine had been achieving proper RPM at the high altitude only
because you had previously been swapping out the props........(please read the
first paragraph of my response more carefully), running the larger prop could
create an overpropped condition and the results would have been as I described.

In article ,
Gould 0738 wrote:
:I'm not trying to be sarcastic here or question your knowledge, but
:this seems to contradict your comment about a decrease in speed.
:Comments? I'm just trying to understand.
:
:Thanks,
:Bob
:
:
:Bob:
:
:Assumptions include:
:
:1. You are not retuning your engine for the high altitude operation, and will
:be running a mixture somewhat lean on oxygen.

That is correct.

:2. That you have two different size props for a valid reason, i.e. your boat
:needs the decreased resistance of the smaller diameter prop to achieve proper
:RPM at the higher elevation.

Actually, the smaller diameter prop is the one I use at low elevation.

I inherited the boat from my Dad and he always switched between the two
props, so I can only assume he did that for valid reasons. I
personally have not done any testing to verify that though, which is why I
posted the question.

:If your boat runs the same at the higher elevation with the larger prop, why
:the H would you bother to screw around changing it?

See above. I've always run with the larger diameter (15), but smaller
pitch (15) prop at high elevation and the smaller diameter (14.25) but
higher pitch (17) prop at low elevations, and performance has always
been fine...WOT right where it should be, no cavitation, etc.

One thing just occurred to me. is the first number of the prop spec
the diameter or the pitch? On the low elevation prop, it is stamped
14 1/4 17 which I assumed was diameter and pitch, respectively.

One thing just occurred to me. is the first number of the prop spec
the diameter or the pitch? On the low elevation prop, it is stamped
14 1/4 17 which I assumed was diameter and pitch, respectively.



Props are normally referred to a (example)
14 x 17, or (14 by 17) The first number is the diameter, the second is the
pitch.

Gould 0738 wrote:

:Props are normally referred to a (example)
:14 x 17, or (14 by 17) The first number is the diameter, the second is the
:pitch.

OK, that's what I thought...thanks.

Engine RPM: 4800
Gear Ratio: 2.15
Prop Pitch: 15 or 17
Prop Slip: 15 (I've heard that is a good average to use)

The speed I get with a 15 pitch prop is 27, which is about what I get
with that prop at 4500' elevation.

If I put in 17 for the prop pitch value and leave all else the same, I
get 30.5 mph.


Of course you would get a higher speed using the 17" pitch prop in the
calculator versus the 15" pitch prop, leaving, as you say, all else the
same. The problem is that outside of the calculator all else is not the
same. When you tow your boat to the higher elevations, the simple fact is
that your engine will not produce as much horsepower as it will at sea
level. This is due to lower air density at the higher elevations. This is
why your dad wisely compensated for the loss of power at high elevations by
dropping to the 15" pitch prop. It is somewhat analogous with dropping a
gear in your tow vehicle when climbing a hill.

So, if your boat is properly propped with the 17" pitch prop at sea level,
meaning that the engine reaches its proper maximum RPM at wide open throttle
(WOT), it is perfectly reasonable to expect that you will need to compensate
for the loss of power at high altitudes by changing to a lower pitch prop.
This will allow the engine to continue to reach proper maximum RPM at WOT,
because you essentially have reduced the gearing in your drivetrain (see
hill climbing analogy above). This will also probably result is a lower top
speed than at sea level, but you will have that with the 17" prop at high
altitudes as well.

The consequences of not performing the prop switch could range from the
inconvenience of reduced performance of the boat to potentially damaging the
engine. Much depends on the current health of the engine, and how well the
low altitude prop is dialed in for the boat at sea level. Personally, if
the boat runs correctly at seal level with the 17" pitch prop, I would most
definitely swap to the 15" prop for the high altitude trip. You're only
working with a four cylinder engine with a modest amount of power and
torque, so putting the extra workload on the engine at high altitude would
be ill-advised, in my opinion. Changing a prop is not all that time
consuming, and doing so will make the boat more responsive and possibly
prevent running your engine outside of its recommended operating range,
which is not a good thing to do. This a clearly a case of Father Knows
Best. You would be wise to follow his footsteps in this situation.

RG wrote:

majority of RG's excellent post snipped

:Best. You would be wise to follow his footsteps in this situation.

I had already decided it was best to follow my dad's footsteps, my
prior experience, and others' advice, and made the time to swap props,
but none the less, thank you very much RG, for your very informative
and easy to understand explanation and advice. As it turns out, I am
not going boating/fishing this weekend after all due to the uncertain
weekend weather forecast for Northern California. Oh well, maybe next
weekend!

Thanks again and thanks to you as well, Gould 0738, for your replies.

Bob