Engine size vs gearbox
Sander Wissing
The designer specified 80hp with a 2-1 reduction.
What are the tradeoffs of going with a smaller engine and larger reduction?
If I go with 40hp and
4-1 reduction, what do I loose? It is typically much cheaper here.
If this is really viable, what are the ratios, eg. If I go with xhp, what
reduction ad prop size do
I need to have the same performance as a 80hp with 2-1 reduction?
Thanks
Sander Wissing
Bob Walters
power.
Remember when your Mom told you there's no free lunch? Well, she was right.
Otherwise, why not go with a One hp engine and a 160 to 1 reduction?
Sizing the engine, gearbox, prop diameter, prop pitch, number of blades, and
prop blade area is often done by guess and by golly when attempted by the
uneducated simply because the calculations required are beyond the
capability of many. But, guessing isn't a very good method for this sort of
thing.
If you're interested in this area of study, I suggest you begin by reading
Dave Gerr's book "Propeller Handbook" and his very interesting book, "The
Nature of Boats".
Mr. Gerr doesn't say anything you can't read in other books, but he has the
ability to put it in easy to understand terms so that the non-naval
architect can get a grip on the subject matter.
I'd suggest you stick with the recommendations of your designer. Chances are
he knew what he was talking about when he suggested 80 hp with a 2:1 gear
box.
Bob Walters
PERDIDO DESIGN
check out the web page at:
http://home.att.net/~bobandching
Sander Wissing wrote in message <7f4fdg$5f2$1...@hermes.is.co.za>...
Mel Haylock
>
> Hi all,
>
> The designer specified 80hp with a 2-1 reduction.
>
> What are the tradeoffs of going with a smaller engine and larger reduction?
> If I go with 40hp and
> 4-1 reduction, what do I loose? It is typically much cheaper here.
>
> If this is really viable, what are the ratios, eg. If I go with xhp, what
> reduction ad prop size do
> I need to have the same performance as a 80hp with 2-1 reduction?
>
> Thanks
> Sander Wissing
Keep in mind that when your 40hp (4:1)engine is at 4,000rpm, it turns
the prop at 1,000rpm. The 80hp (2:1) is only at 2,000rpm for 1,000rpm at
the prop. If you have the same prop on both, they will behave more or
less the same until the 40hp runs out of rpm's (wide open). At this
point, the 80hp would be loafing at (around) 1/2 throttle and would have
the reserve (all other things being equal)to go to twice the prop rpm's.
The simple answer: you would lose top speed (assuming 80hp at 2:1 was
correctly sized for the hull). Best advice: listen to the designer. If
you have ideas for changes, give the designer a call to check. They are
usually very helpfull to those who have purchased their plans.
Mel Haylock
Stephen G. Lusardi
The other answers you received are very much correct, but please consider
that the hull design demands a certain prop diameter (clearance), the
anticipated speed determines the prop pitch and the overall anticipated load
(resistance) determines the blade area ratio generally. This combination
presents the shaft torque demand (shaft diameter). The speed of the this
prop, times the required torque to spin it determines the horsepower
requirement necessary. The chosen engine of the correct horsepower will have
with it a torque production curve (delivered torque at various engine
speeds). Since you know the required prop shaft torque, multiply the engine
torque by the proposed gear reduction to suit for best fit. There are other
factors that also need to be considered as well, but this should give you
the idea.
Steve
Sander Wissing wrote in message <7f4fdg$5f2$1...@hermes.is.co.za>...
Bryon Kass
to get the desired speed 40hp will give obviously less.
A 4:1 is not available but 3:1 is. The added efficiency
will not give you much more speed but more thrust
for a give hp output. The normal rule of thumb is that
for an increase of twice the hp you can figure 50% more
speed up to the limit of the hull. You must also figure with
sailboats and displacement hulls that you should have a
25% to 50% power surplus for safety. If you look at the
pricing of available engines doubling the output does
not add that much cost. For example a 40hp Isuzu marine
sold through us would cost about $8000 where a 80hp would
cost about $9500 new plus options and shipping.
Bryon Kass
webmaster and
Custom Design
150 Mechanic St.
Foxboro, MA 02035
508-543-9068 or fax 508-543-5127, Foot yard 508-384-2415
in THE ENGINE ROOM http://getit.at/engineroom
Sander Wissing
<Mel_H...@NortelNetworks.com> wrote:
>Sander Wissing wrote:
>>
>> Hi all,
>>
>> The designer specified 80hp with a 2-1 reduction.
>>
>> What are the tradeoffs of going with a smaller engine and larger reduction?
>> If I go with 40hp and
>> 4-1 reduction, what do I loose? It is typically much cheaper here.
>>
>> If this is really viable, what are the ratios, eg. If I go with xhp, what
>> reduction ad prop size do
>> I need to have the same performance as a 80hp with 2-1 reduction?
>>
>> Thanks
>> Sander Wissing
>
>Keep in mind that when your 40hp (4:1)engine is at 4,000rpm, it turns
>the prop at 1,000rpm. The 80hp (2:1) is only at 2,000rpm for 1,000rpm at
>the prop. If you have the same prop on both, they will behave more or
>less the same until the 40hp runs out of rpm's (wide open). At this
>point, the 80hp would be loafing at (around) 1/2 throttle and would have
>the reserve (all other things being equal)to go to twice the prop rpm's.
>
>The simple answer: you would lose top speed (assuming 80hp at 2:1 was
>correctly sized for the hull). Best advice: listen to the designer. If
>you have ideas for changes, give the designer a call to check. They are
>usually very helpfull to those who have purchased their plans.
Thanks to everyone who replied. It was unanimously a bad idea. Just
a few things left to clear up. In the example above, with the 40hp
(4-1) at 2000rpm and the 80 hp (2-1) at 2000 rpm I can run a bigger
prop on the 4-1, to get the same thrust. Well, that was basically the
original question. How much bigger does the prop have to be?
Obviously one down side is that a bigger prop means bigger drag on a
sailboat.
I did ask the designer, Bruce Roberts, who replied a one liner that
basically said "small engine, big reduction = huge prop", at the same
time drawing a gap for the prop at 1.6m on the plan...
Thanks all
Sander Wissing
Jaguar 22 #4039 - Dreamfar
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