I have a Boston Whaler 13-foot boat, powered by a 1984 Evinrude outboard engine of 40-HP. When I throttle up to plane, starting at about 3000-RPM, and around [a boat speed of] 16-nautical-miles-per-hour, the speed engine wants to continue to climb. If I then throttle back slightly, the engine wants to also slowly continue to decrease engine speed and then [the boat will] come off plane.
During all this time the throttle does not move in my hand but the engine RPM does. I am constantly adjusting the throttle to maintain speed. Is there something wrong with my cable? Or what is worng? Any ideas?
Older Two-cycle Engine Throttle Behavior
Re: Throttle speed floats
Our 13 Super Sport acted exactly the same with our 2000 2-stroke Johnson 40. Once the boat lifts to plane and resistance is lowered, it always wanted to go like hell, and you needed a surgeons hand to feather the throttle back just enough to find a "sweet spot." I really hated that!
The world changed when I replaced the engine with with a computer controlled 30 E-TEC, and added to the engine a Sting-Ray Jr. Hydrofoil. The hydrofoil allows a more pronounced trim, with a keen ability to lower and raise the bow, plane at lower speeds, and the E-TEC had much better adjustability with the throttle. BTW, I used the old controls from the 40, so it wasn't that.
That little 13 ran perfectly and was very well balanced with the 30 E-TEC, and we loved it. But more grand-kids dictated us move up to the new 150 Montauk.
The world changed when I replaced the engine with with a computer controlled 30 E-TEC, and added to the engine a Sting-Ray Jr. Hydrofoil. The hydrofoil allows a more pronounced trim, with a keen ability to lower and raise the bow, plane at lower speeds, and the E-TEC had much better adjustability with the throttle. BTW, I used the old controls from the 40, so it wasn't that.
That little 13 ran perfectly and was very well balanced with the 30 E-TEC, and we loved it. But more grand-kids dictated us move up to the new 150 Montauk.
Re: Throttle speed floats
Sounds normal to me. You're trying to hold an in-between speed and your boat, motor, and prop combination just won't do that. Every planing boat I have every piloted has that behavior, although not at the same speed, of course. Since this is the opposite of a sweet spot, I am going to invent the term "sour spot" to describe it. Some boat-motor-propeller combinations have a very narrow sour spot, and some wider. In general, I really liked the old Johnson 70 on my 15, but the sour spot was really wide; you needed to be either just putting along, or going fast, no in between.
The problem is that, as you're transitioning on-or-off a plane, there is a huge change in drag, with the maximum drag occurring right before you get on plane. That combines with the fact that engines make more power at high RPM to create a positive feedback situation: go a little faster, you take off, go a little slower, you fall off plane. There are three potential solutions:
--don't do that, that is, stay out of the sour spot by going faster or slower;
--change to a lower pitch propeller, possibly with more blade area. This will put your motor into an RPM range where it will provide more power at that transitional speed, and shift your sour spot to a slower speed--but probably not by much. You'll lose some top end speed, and, if you don't have a rev limiter you'll need to be careful not to over-rev;
--put some type of foil appendage on your motor. I have never tried this, but user's report the ability to hold plane at lower speeds. You'll likely lose some top end speed with this as well.
I would advocate simply avoiding that in between speed.
Tim
The problem is that, as you're transitioning on-or-off a plane, there is a huge change in drag, with the maximum drag occurring right before you get on plane. That combines with the fact that engines make more power at high RPM to create a positive feedback situation: go a little faster, you take off, go a little slower, you fall off plane. There are three potential solutions:
--don't do that, that is, stay out of the sour spot by going faster or slower;
--change to a lower pitch propeller, possibly with more blade area. This will put your motor into an RPM range where it will provide more power at that transitional speed, and shift your sour spot to a slower speed--but probably not by much. You'll lose some top end speed, and, if you don't have a rev limiter you'll need to be careful not to over-rev;
--put some type of foil appendage on your motor. I have never tried this, but user's report the ability to hold plane at lower speeds. You'll likely lose some top end speed with this as well.
I would advocate simply avoiding that in between speed.
Tim
Last edited by flymo on Tue Jan 19, 2016 11:27 am, edited 1 time in total.
Re: Older Two-cycle Carburetor Engine Throttle Behavior
The throttle on older carburetor two-stroke-cycle engines does not set the engine speed. It just sets the amount of restriction in the air flow into the engine. In turn, this sets the amount of fuel the engine can receive as the air flow Venturi effect pulls fuel from the fuel metering jets of the carburetor. Recall that the word throttle means "to choke or strangle someone; to compress the throat." This is precisely what the throttle does in a carburetor engine; it chokes off the airway into the engine.
The engine crankshaft speed varies in the boat operating conditions you describe because the load on the engine is varying. When the load on an engine increases, and the throttle is not changed, the engine speed will decrease, and vice versa. If you want the engine to maintain a constant engine speed under varying load conditions you must adjust the throttle to compensate. This is not at all unique to your particular boat and engine combination.
As noted, maintaining a boat on plane at slower planing speeds can require a lot of adjustment of the throttle due to the varying loads on the engine at those speeds. This tendency can be reduced if the engine has much more horsepower than the minimum needed to get onto plane. Larger engines will be able to develop sufficient power to turn the propeller under its load over a wider engine speed range. The actual force needed to turn the propeller is TORQUE. A larger engine will typically have a wider range or band of torque being produced, and this helps minimize the tendency for engine speed to drop under increasing load.
Modern engines control the amount of fuel the engine will be getting by electronic methods, based on the throttle position indicator and the fuel mapping algorithms contained in the firmware of the engine controller. Engines controlled by such electronic systems tend to be able to maintain a particular engine speed under varying load better than traditional carburetor engines. In particular, they won't tend to accelerate on their own as the engine load decreases because the electronic control of fuel supply prevents this.
The Evinrude E-TEC direct-injection modern two-stroke-cycle engine is particularly outstanding in the relationship between throttle position and engine speed. When operating an E-TEC the throttle control behaves as it were just an engine speed control lever, not at all like the throttle control on a carburetor engine.
The engine crankshaft speed varies in the boat operating conditions you describe because the load on the engine is varying. When the load on an engine increases, and the throttle is not changed, the engine speed will decrease, and vice versa. If you want the engine to maintain a constant engine speed under varying load conditions you must adjust the throttle to compensate. This is not at all unique to your particular boat and engine combination.
As noted, maintaining a boat on plane at slower planing speeds can require a lot of adjustment of the throttle due to the varying loads on the engine at those speeds. This tendency can be reduced if the engine has much more horsepower than the minimum needed to get onto plane. Larger engines will be able to develop sufficient power to turn the propeller under its load over a wider engine speed range. The actual force needed to turn the propeller is TORQUE. A larger engine will typically have a wider range or band of torque being produced, and this helps minimize the tendency for engine speed to drop under increasing load.
Modern engines control the amount of fuel the engine will be getting by electronic methods, based on the throttle position indicator and the fuel mapping algorithms contained in the firmware of the engine controller. Engines controlled by such electronic systems tend to be able to maintain a particular engine speed under varying load better than traditional carburetor engines. In particular, they won't tend to accelerate on their own as the engine load decreases because the electronic control of fuel supply prevents this.
The Evinrude E-TEC direct-injection modern two-stroke-cycle engine is particularly outstanding in the relationship between throttle position and engine speed. When operating an E-TEC the throttle control behaves as it were just an engine speed control lever, not at all like the throttle control on a carburetor engine.
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Huckelberry145
- Posts: 16
- Joined: Sun Dec 13, 2015 2:00 pm
Re: Older Two-cycle Engine Throttle Behavior
Could [the cause of the engine speed varying] be [due to] a propeller bushing?
Re: Older Two-cycle Engine Throttle Behavior
Thanks everybody. I'll try a little faster rpm and speed. Your insights were great.