posted 06-26-2006 11:22 PM ET (US)        

On my first extended trip out of the harbor yesterday, I found it challenging to develop anything like precise control of the boat's speed. In one throttle position I found the boat would move along consistently at a speed of 10 or 11 mph. It would keep at this speed pretty much indefinitely. If I then nudged the throttle just slightly, to the next barely perceptible change in engine noise, the speed would begin to ramp up. It'd start by getting up to 13 or 15 mph, but then in time keep heading upwards, to 20 mph or even 30 mph. In several tries I was unable to find a throttle position that would keep the boat moving along at a consistent speed of, say, 15 mph.

I know there's a lot I have left to learn about controlling the boat, effects of trim, etc. But is this kind of experience with the throttle typical? Is there a trick here I need to learn?

posted 06-26-2006 11:41 PM ET (US)            

No trick, that's just how planing powerboats are. From idle speed, increasing the throttle will increase the boat speed more-or-less evenly, until the boat gets to "hull speed". At hull speed, the boat is supported at the bow and the stern by the wave that it's creating. On your boat, that's about 7 MPH. This is a very efficient running speed.

To go beyond hull speed, a boat requires a lot more power, enough to climb up its own bow wave. You'll notice that this is when the bow starts to rise and the wake starts to get bigger. Keep adding power, and the boat will slowly accelerate until it is nearly on plane, maybe 12 MPH. This is an extremely inefficient speed to run a boat, but it may serve you well in extremely rough water.

Add just a little more power, and you get "over the hump", or in this case, over the bow wave. The boat climbs up out of its own hole, rapidly accelerates, and begins to skip along the surface of the water. This is called "planing", or being "on plane". Boat speed will suddenly increase to probably the low 20s, and cruising efficiency goes way up again.

Unlike a car, there's pretty much no running in that in-between speed range. Either you're off plane below 10-12, or you're on plane around 20 and up.

Experiment, and enjoy!

posted 06-26-2006 11:47 PM ET (US)            

My little Sakonnet behaves similarly. That is it will plow 6 knots at 2200 or 2300 rpm and plane 15 or so kts at 2450 rpm. There isn't much in between. Sounds like you have similar performance on a grander scale.

You can play with trim to bridge the gap. Or trim tabs. But it sounds like in your case you're going to need a long bridge. Hopefully, someone here with the same boat-motor as you will provide a great answer, and soon.

posted 06-27-2006 01:11 AM ET (US)            

Every boat/motor/propeller combination is different and the planing speed (or "hump") point will vary. What you describe is not unusual. Some, no most, outboard motors have some RPM range where they are less happy. They will tend to either speed up or slow down. If this RPM range happens to coincide with the boat speed where the hull transitions from displacement to planing mode, the transition will be even more abrupt and hard to control.

A single 225 HP motor is going to have a more pronounced "hump" to its power curve. A pair of smaller outboard will be much more "flexible" but there are some things you can do to more smoothly transition from displacement speed to planing speed:

If you want to maintain a minimum planing speed as slow as possible to handle rough water, always trim your motor all the way down. This will help lift the stern and keep the bow down when the hull is traveling at something just less than a full plane. The less efficient thrust angle will also allow the motor to potentially spin up to a higher RPM but still keep the boat speed down.

Trim tabs will also help maintain a level attitude when running in a slightly semi-displacement mode. These are especially helpful with more true deep-V hulls like yours where the transition from displacement speed to a full plane is not as clear-cut.

A change in propeller can have a HUGE impact on how the boat runs at slower speeds. I would recommend you try a four blade Revolution 4 propeller in a pitch maybe less than what you run now. This will act like a "low gear" in a car and allow the boat to hang on at these intermediate speeds with less tendency to shoot ahead and then fall off plane and slow down when you back off the throttle.

posted 06-27-2006 07:12 AM ET (US)            

Once you get her up on plane she does want to get up and go, and can make rough water running tough.

Tom's advice is dead on. Trim the engine all the way down, run the trim tabs full bow down. A Mercury REV4 propeller greatly improved the way my boat handles rough seas, at least compared to the Mercury Mirage Plus or Laser II.

I can run a 17 pitch REV4, the Mirage and Laser 19 pitch. When the boat just gets up on plane, it is a little slower with the lower pitch prop at the same RPMs. The 17 pitch REV4 also planes the boat around 2800 rpms and opposed to 3000 rpms. Overall I can run a couple of miles per hour slower with the REV4 just barely on plane. The REV4 will hold plane much better than the Mirage also.

The Mirage Plus is a great cruising propeller, but caused the boat to be real responsive to throttle when running at slower speeds into the chop. Small adjustments in throttle resulted in relatively large differences in speed, sometimes resulting in the boat launching a bit over waves.

The effect with the REV4 is greatly reduced. The boat really seems to ride alot higher, so it is much more comfortable in headseas and chop. Throttle adjustment is very responsive, but very controlled. I have much more confidence in the handling of this boat with the REV4.

Top speed is identical to the 19 pitch Mirage Plus. Mid range cruising in the 30-35 mph range is a bit less efficient, but the positive change in handling characteristics is well worth the loss in efficiency.

But try running with no engine trim and tabs all the way down. If you don't have trim tabs on this boat you are going to have a hard time correcting your problem.

And I adjust the throttle holding the handle on the side. I find that if I adjust the throttle holding the handle on top adjustments can be a bit jerky. I have found it neccessary with this boat to constantly adjust the throttle in the conditions you mention to maintain a constant speed, so I would say it is normal, but you can smooth it out with some practice and boat set up.

Rich

posted 06-27-2006 08:15 AM ET (US)            

It takes significant power to push the boat along in that mode, and you will find that the fuel economy is not very good. If you look at the wake being made, you can see that a lot of the power of the engine is being translated into wave making instead of efficient forward movement of the hull. Dragging that big wake along with the boat requires a lot of horsepower.

With the typical propeller and gear case, a speed of 10- to 12-MPH will mean an engine speed between 1,500 and 2,500-RPM. For many engines, this speed and load will cause the engine to be "lugging". If you could see the exhaust, it would probably have a lot of smoke in it. Operating the engine at this speed and load for extended periods can lead to problems. As a general rule, I try to avoid as much as possible operating the engine at speeds between 1,500- and 2,500-RPM.

Whether or not this advice is applicable to your boat depends on the engine, the hull, the propeller, and the load, but I think you will find that in most cases the range between 1,500-RPM and 2,500-RPM is not a good region for your engine.

The tendency for an outboard engine to want to accelerate after getting on plane is very common. This tendency is more prominent in boats where the engine horsepower is on the low side of the recommended range. In a boat like that, the engine must be given plenty of throttle opening to get enough power to push the boat onto hydroplane mode. Once on plane, you have to reduce the throttle to a setting what will just maintain the minimum planing speed desired. If you pull back on the throttle too far, the boat eventually begins to settle back into displacement mode. More throttle is need to get back on plane. This cycle can go on for a long time until the operator finds just the right setting of the throttle to maintain the boat on plane. Wave action and sea conditions can also cause problems, and a big wave can knock the boat off plane.