In the previous article we took a look at the operation of the trim tabs and the benefits arising from their use. Let's now see, in practice, the differences that come up in measurement and performance of our boat when we use the trim tabs.
- Getting on plane using trim tabs
We went for a ride with a Marvel 930 aiming to record the effect of trim tabs in planing. From the below data it seems that the rib stays on plane at a much lower speed without the fuel consumption increasing actually.
Watching the stern wake in the two pictures below, we can clearly see the effect of trim tabs. In the left picture we were riding keeping the engine’s rpm at 2700. Lowering the trim tabs by 30% the rib got on plane directly as shown in the right one.
- Comparing the drag of trim tabs with the lower unit’s drag.
We were riding at 4000 rpm, our speed was 29.5 knots, the fuel consumption 40 lt/h, adjusting the engine trim slightly negative, at 40% (our neutral trim is at 50%).
We chose these settings because these are the ones that we usually prefer when traveling.
Lets suppose now that we are running into a head sea in a sea state about 4-5bf. Under these conditions, we often trim our engine too far down (trim 0%) in order to lower our bow so as to achieve the most comfortable ride.
So, this is what we also did in the first test. We trimmed the engine completely down taking the following measurements:
In the second one, we did not trim the engine down, while maintaining the engine trim at 40% we lowered the trim tabs by 30%. In this adjustment we got the following measurements:
Surprisingly we saw that in both cases we had a loss of 100 revolutions per minute, reducing the speed by 2.2 knots and increasing the fuel consumption by two litters per hour.
In other words, the load that was applied to the engine was the same either lowering the trim tabs or trimming the engine.
However with the use of trim tabs we had:
- the bow slightly lower
- our propeller running at the better angle related to the water surface
It is worth noting here that even when we lowered the trim tabs completely down, the fuel consumption increased further by 1 litter per hour while the speed reduced by 0.5 knot, numbers certainly very small if we consider that in these conditions the bow was almost submerged in the water.
- We continued our ride testing the effects of trim tabs in speed and fuel consumption at 3500, 4000 and 4500 rpm.
Raising now the engine by one inch higher (using the power lift) and the engine trim at 50% (which means that the A/V plate was parallel to the water surface), we recorded the following measurements:
Without using trim tabs
Using trim tabs
Comparing the above numbers we see a slight increase in the boat speed at 3500 rpm when we lowered the trim tabs by 30%. Maybe because at low speeds the stern of the rib is too submerged and when we lowered the trim tabs the stern raised and its wetted surface reduced.
We also note that the higher the boat speed, the greater the drag of the trim tabs is and the lower the speed gets. But once again we see that the use of trim tabs affects only by little the fuel consumption.
- By using trim tabs we can keep the boat on plane at lower speeds, a major benefit when we try to cover long distances through rough seas.
- When we are travelling into a head sea, it is better to trim down the trim tabs (and not our engine) thus maintaining the optimum angle of the thrust-line with the surface of the water. In other words, it is preferable to use the trim tabs to trim the hull and the power trim to trim the propeller.
- As shown in the above measurements, the load for our engine is greater when we are lowering the trim tabs at high speeds. This means that the combination of lowered trim tabs-reduced wetted stern’s surface-increased wetted bow’s surface appears more drag than the combination of increased wetted stern’s surface- reduced wetted bow’s surface without using trim tabs. Still, in bad conditions we prefer the first combination in which our ride is impressively improved while the negative effects on fuel consumption and our speed are not of great importance.