Wednesday, August 19, 2009

Fuel Economy - Part 2 - Boat Attitude

Yes, your boat may have an attitude! Well, at least it should. The “attitude” of a boat, generally, refers to the relationship between the bow and the stern. In other words, the boat can be running with the “bow high” or “bow low”. Adjusting the boat’s attitude can have a significant effect on its’ performance. This performance can be defined by fuel economy, speed, hole shot, even stability in rough water. As this blog is about fuel economy, I’ll limit this discussion to issues that directly relate to maximizing your fuel economy. So as to not get too confusing, I’m also going to limit this discussion to outboard and stern drive powered boats, since these are typically adjustable while the boat is running. Inboard (and some OB and stern drive) powered boats will also have “trim tabs” at their disposal, but we’ll save that for another day.

“Trimming the drive” refers to the angle of the outdrive (or in the case of an outboard engine, the entire engine). More specifically, it refers to changing the angle of the propeller in relation to an imaginary line coming straight back off the keel of the boat. Imagine a straight line being extended past the transom and compare that line to the propeller angle, or “aim”. If the propeller is aimed up (above the line) then the propeller thrust is going to lift the stern. If the prop is aimed down (below the line) then the thrust is going to push the stern down. This is also known as “Trim Up” or “Trim Down”.

Every action has an equal and opposite reaction. When the stern goes down, the bow goes up. When the stern goes up, the bow goes down. So, when you push the “up” trim button at the helm, what’s actually happening is the drive is being raised, pushing the stern down, thereby making the bow rise. Of course, the opposite happens when you push the “down” trim button.

So what does all this mean for economy? Well, more fuel is wasted while trying to get the boat on plane, than when it is actually on plane. If you can spend less time getting on plane, you’ll burn less fuel. While some boats have much better hull designs that have been engineered to provide superior “lift” or “hole shot”, you can decrease your hole shot time by always starting out with the drive completely down. This will provide maximum stern lift, thereby keeping your bow down. If you get the boat “out of the hole” faster, you are increasing your fuel economy.

Now once you get on plane, if you leave the trim down, you will also be keeping a lot of the hull bottom in the water. More hull in the water equals more drag. More drag equals less economy. Once you’re on plane, start trimming up. This will get more of the hull out of the water. You will feel the boat “lighten up” and you should also notice that the spray coming off the hull side will start to move further towards the stern. Another thing you’ll notice is that your speed – without changing your throttle setting – will go up. If you can obtain a higher speed at the same throttle setting, you are increasing your fuel economy. Now if you trim too far up, the prop will lose it’s bite in the water and will start to ventilate. Your RPM’s will shoot up very quickly (possibly over revving the engine) and speed will actually decrease. If this happens, don’t panic. Trim the drive back down and/or pull back on the throttle until the prop regains it’s bite and the RPM’s stabilize.

To wrap this up, you can increase your fuel economy via your boat’s attitude by:

-- Decreasing your hole shot by trimming down

-- Increasing your speed (at the same throttle setting) by trimming up

Once you understand the general theory, it’s actually quite simple. As mentioned above, the difference between one manufacturer’s hull design and another will also play a significant role in the economy game. However, you can increase your particular boat’s economy simply by using the trim wisely. One last thing to keep in mind is that your boat only has one forward gear. You can’t shift through 5 gears for better economy like you can in a car. This means another major factor for economy is… you! Most gas-powered boats will achieve their best fuel economy at around 3,500rpms - some a little more, some a little less as it will also depend on hull design. So if you’re constantly running your boat at high RPM’s and wondering why your fuel gauge needle moves so quickly towards “E”, now you know what (or who) is to blame. I’ll give you a hint… it’s not your boat or the engine! ;)

Bookmark and Share

No comments: