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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