nameplate HP vs. HP to shaft on marine diesel?
1.1557
(due to pump losses, etc) only about 2HP to the shaft.
Are there any engine-knowledgeable folks out there that can tell me if
this estimate is reasonable? I had thought that the nameplate HP
rating would indicate the actual HP (at stated max RPM) to be delivered
to the shaft to do work. I looked a bit at some of the engine mfgr
sites but never saw this subject discussed.
Thanks.
Glenn Ashmore
power at the output shaft with the auxillary equipment needed for the
engine to run (including the base alternator I believe). You can't run at
that power for long so they are also rated for maximum continuous output
which is usually about 20% less. You will loose another 10 or 20% of that
in the transmission and shafting. So most of the time the prop will only
see about 60% to 65% of the "Nameplate HP".
Unless you have a big honkin' alternator and engine driven fridge, you are
probably getting 7 or 8 HP to the prop at normal cruise.
"1.1557" wrote:
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.mindspring.com/~gashmore
Larry W4CSC
>I have been told that my nameplate rated 13HP diesel engine puts out
>(due to pump losses, etc) only about 2HP to the shaft.
WOW! 1HP = 746 watts X the 11HP = 8,206 watts wasted! SOMETHING is
getting awfully hot in that bilge! I'd have the fire extinguisher
close at hand.
The water pump probably sucks up 1/2hp tops, unless something's all
plugged up. The engine IS under the water line so it doesn't have
much of a lift. The alternator pulls on the belts, too, but only
until the battery is recharged, unless you have a big load on it.
You're probably losing 1HP tops to run it all.....2HP isn't enough
power to turn that screw of yours at those RPM levels!
>
>Are there any engine-knowledgeable folks out there that can tell me if
>this estimate is reasonable? I had thought that the nameplate HP
>rating would indicate the actual HP (at stated max RPM) to be delivered
>to the shaft to do work. I looked a bit at some of the engine mfgr
>sites but never saw this subject discussed.
>
>Thanks.
The estimate isn't reasonable at all. I always love those who think
you lose half the power in the transmission. See any smoke from the
transmission? No? If the power is lost anywhere IT HAS TO GET HOT!
Even boat crap must conform to simply physics...(c;
larry....
edgar cove
s.net>, 1.1557 <pio...@pdq.net> writes
>I have been told that my nameplate rated 13HP diesel engine puts out
>(due to pump losses, etc) only about 2HP to the shaft.
>
>this estimate is reasonable? I had thought that the nameplate HP
>rating would indicate the actual HP (at stated max RPM) to be delivered
>to the shaft to do work. I looked a bit at some of the engine mfgr
>sites but never saw this subject discussed.
>
>Thanks.
You do not say what else was on the nameplate-did it not say 'maximum',
'intermittent', or'continuous', for example?
The concept of 'maximum' rating is used by manufacturers in USA for
sales purposes and what is not clear to many people is that this so-
called maximum horsepower is calculated with many items necessary for
operation of the motor disregarded. For example the power to drive the
generator, water pump and certainly any gearbox will not be taken into
account. If however the rating is stated as BHP (brake horse power)
then this is the power (intermittent or continuous) that the complete
motor can actually produce at its flywheel with all its auxiliaries
connected but still without any gearbox fitted.
So then you get SHP (shaft horse power) which is the useful power (again
continuous or intermittent) that the engine with its gearbox can put
onto the propeller shaft, and this is the only rating that the user can
make use of.
If your engine is producing 13 bhp then the shp will certainly not be as
low as you say. The power consumed by the marine gearbox will depend on
the type of box-ball/roller bearings or plain for example-and will
typically for a small diesel be between 5% and about 8% of the power
transmitted. So assuming 13bhp your available shp should be about 12.
In setting the horsepower ratings for continuous or intermittent duties
the engine manufacturer will take into account the use to which the
engine is likely to be put. For example they will assume that a small
engine coupled to a marine gearbox will be going as an auxiliary in a
yacht or small pleasure boat which will not run many hours per annum and
will rate the engine higher on that account. If you run such an engine
24 hours/day and 7 days per week at that rating you would get rapid wear
and short life between overhauls. Engines that are used in say
commercial fishing boats or tugboats would be set to run at lower speeds
and cylinder pressures at their continuous ratings so as to produce
acceptable life between overhauls. Some engines which by their nature
may be used in a variety of applications will bear different ratings
depending on the application.
Finally of course, whatever power the engine is capable of producing is
not necessarily the power that will be produced in the boat. That
depends on the propeller fitted, the condition of the hull etc. ...
--
edgar (remove nospam from return address for e-mail reply)
Matt & Mary Colie
You were misinformed, or the question was mis-interpreted.
Now the Real Story.
Engines sold as "marine" or for marine service and complying to the
applicable standards must be rated as continues duty propshaft power net
(required auxiliaries must be included) at sea level with standard intake
air temperature and humidity (I can not lay hands on the book with the
numbers for these just now) and minimum exhaust back pressure including .
I could end the writing there, but it would leave many with more questions;
so here goes.
Your Case:
Your 13 horsepower diesel is 13hp propshaft net at rated speed and maximum
fuel delivery. This is with the raw water pump, circulating pump, fuel
pump, the alternator supplying any load the engine requires to run (not
charging the battery) and an some exhaust back pressure.
You are probably not running the engine at rated speed. All modern diesel
engines use a fuel controlling governor. If you have not tampered with the
fuel control, you can always run the engine at the maximum speed it will
go. Most owners are afraid to do this. Say you are comfortable at 80%,
well that is actually about 70% (9hp) if the propellor load curve has not
fallen too far. It probably has, if you have the correct prop on the
shaft; so you are really using about 2/3 of that (6hp). This is all prop
shaft stuff. Boats don't care about that, they are looking for "tow rope
horsepower". We all take a beating on propellor efficiency. Fifty
percent for a small boat with a fast turning prop would be good. Guess
What? We just got to 3 Horsepower.
Get out on smooth water with another boat in tow and push it to max and if
you hear the engine lose a little speed on turning or wave action you will
be delivering 13 horsepower to the shaft. You propellor efficiency will
still only get you to 6 or 7 hp
Back to the ratings thing.
This rating is why a passcar engine that is rated at rated at 253 SAE Gross
will maybe rate at 200 Hp marine (like a Chevy 350). The SAE standard is
flywheel gross and uses a lower intake air temperature, minimum back
pressure, no auxiliary load (fan, alternator, p/s, a/c), and it is not a
continues duty rating (all it has to do is hold it long enough to take
data).
This is also a fight American outboards had with Japanese imports for many
years. Americans were using a marine rating at the propshaft, and the
imports were selling a SAE gross rateing. This cause a lot of hard
feelings.
Matt Colie
Lifelong Waterman, Licensed Mariner (documantation by request only)
Evan Gatehouse & Diane Selkirk
Glenn Ashmore <gash...@mindspring.com> wrote in message
news:3A252F05...@mindspring.com...
> Engines are generally rated in shaft horse power. That is the Maximum
> power at the output shaft with the auxillary equipment needed for the
> engine to run (including the base alternator I believe). You can't run at
> that power for long so they are also rated for maximum continuous output
> which is usually about 20% less. You will loose another 10 or 20% of that
> in the transmission and shafting. So most of the time the prop will only
> see about 60% to 65% of the "Nameplate HP".
Glenn,
Things aren't that bad!
I don't have my Skene's or Larrsson's here (they are at work) but from
memory, straight gearbox losses are around 1-1.5%. You might lose another
1% for the stuffing box friction, and 1% for the cutless bearing but that's
about it for transmission losses.
For example, Yanmar quotes for their 2GM20, 18 HP Maximum at 3600 RPM, 16 HP
continuous at 3400 RPM at the crankshaft. 1.5% losses for their straight
gearbox, and 2% for their V-drive gearbox (pretty good for a V box by the
way).
So if you multiply the losses (cause they are muliplicative, not additive),
you would get 98.5% (gearbox) x 99% (stuffing box) x 99% (cutless bearing)
= 96.5% delivered power.
Evan Gatehouse
1.1557
I gather these are reference books on marine engines?
Thanks, Rob
Evan Gatehouse & Diane Selkirk
1.1557 <pio...@pdq.net> wrote in message
news:F5AFD521DF3991ED.4990597E...@lp.airnews.net...
> Evan, can you give me a bit more reference on Skene's or Larrsson's?
> I gather these are reference books on marine engines?
>
> Thanks, Rob
"Skene's Elements of Yacht Design" edited by Francis S. Kinney - now out of
print but still has a few good items of use in it.
"Principles of Yacht Design" Lars Larsson & Rolf E. Eliasson - a great book
on yacht design. Pretty complex for the amateur or non technical type. A
nice reference for the naval architect or marine engineer who deals with
yachts.
Evan Gatehouse