Hi Dennis,
A comment regarding "Increased fuel economy? - Maybe"
With both engines properly maintained... the only situation of not
having increased fuel economy is if acquiring gasoline at nearly half
the price of diesel. The reasons are twofold, diesel engines are
inherently more efficient due to higher compression ratio and diesel
fuel is 15% heavier than gasoline so each unit contains more hydrocarbons.
A third factor in sailboat fuel efficiency is the size of wheel you
swing in the water. As you mentioned below, the direct drive A4
propeller is too small to be useful in a chop.
For the above reasons I changed out the A4 to a used diesel and
fitted a 16" KiwiProp . In part due to long motoring sessions
sometimes required when cruising our coast and needing to transit 75
miles and get back to work on time. (I know, that in itself is
overated :) My fuel consumption went from 4 litres per hour to less
than 2 litres per hour @ 5+ knots and I don't drop to 3.5 knots in the chop.
Cheers, Russ
Sweet 35 mk-1
At 04:37 PM 29/01/2014, you wrote:
Explosivity of gasoline? - No
Increased fuel economy? - Maybe
Torque/horsepower? - Yes
The AT4 is rated at 28-30 hp at nearly 3000 rpm. The AT4 torque/hp
curve is fairly steep. That is, at 1400 rpm, the AT4 is only
generating 12 hp or so. At 2400 rpm, it's about 20 hp. Torque
follows hp closely.
Most marine AT4's have a 1:1 tranny. That means, in order to
generate 30 hp, the prop is spinning at 3000 rpm. That's why AT4
props are usually pitched around 6. Exclusive of the Indogo prop
which has a somewhat flexible pitch, at 3000 rpm in heavy seas, most
likely, the prop will be cavitating and not efficiently transferring
the horsepower to the water.
On the other hand, diesel torque curves are fairly flat. A
Universal 25XPB reaches max torque at 2000 rpm. With a 2:1
reduction tranny, the prop is only spinning 1000 rpm. Diesel props
are pitched around 14-15. You get a lot more bite with a higher pitch prop.
In practical terms, imagine you are punching into 2-4 foot
seas. With an AT4, you might make 2-3 knots and a LOT of
bubbles. (Been there, done that. Touche' had an AT4 when I bought
it.) With a diesel, you might be making 4-5 knots. BIG difference.
Let me relate another experience. One day we were struggling
against 20-25 knots and 1-2 foot seas in Touche' (35-1) with the
AT4. We were making 2-3 knots. My friend came by us in his
35-1. He was making 5-6 knots. His 35-1 had a Universal 25 XPB
with a 14 x 14 2 blade folding Martec prop. Even with the folding
prop, he was waaay faster than us.
After I repowered Touche' I had to motor off a lee shore in a
tropical storm. 45 knots and 2-3 foot seas. I cringe at the
thought of having to do that with an AT4.
Before the repower, I had to carry extra fuel or stop halfway to
refuel on my way to Pensacola from Lake Pontchartrain. Now I can
make it all the way motoring if I don't run hard.
Please don't take me wrong. I like the AT4. I respect the
AT4. I've owned 2 of them. I think it's a great engine for
motoring in and out of a harbor. I don't think it's a great engine
for certain white knuckle situations or for motoring long distances.
Dennis C.
Touche' 35-1 #83
Mandeville, LA
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