Bow Oscillation

Optimizing the performance of Boston Whaler boats
jimh
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Bow Oscillation

Postby jimh » Fri Dec 04, 2020 9:02 am

Oscillation of the boat bow up and down when a planing-hull boat is on plane typically results from the boat's weight distribution creating a static trim that results in a natural bow-high trim at rest and a generally "light" bow that is easily raised. Bow oscillation is usually excited by crossing waves. In crossing a wave the bow rises and is momentarily sustained in the higher bow trim, but eventually falls. To stop the oscillation the remedy is to alter the trim on the boat, which can be done in two ways: altering the engine trim position, which can raise or lower the bow, or altering trim tabs (if used) which can only lower the bow.

Generally in small planing hull boats the effect of engine trim on the boat trim angle is quite pronounced, and oscillation of the bow can be corrected by adjusting engine trim. Often this adjustment must be made carefully, as an equilibrium must be found that provides enough bow rise to promote good planing trim yet not so much that every wave induces oscillation.

Regarding use of an engine set-back bracket: the movement of the engine weight farther aft results in the boat static trim increasing to bow-high and creates the light bow condition that might induce oscillation. However, the effect of the engine trim on overall boat trim tends to increase when he engine is moved aft by the set-back bracket, so changes in engine trim become more effective on bow height. Again, finding the engine trim setting that results in an equilibrium condition instead of an oscillation can require careful adjustment.

Use of trim tabs set to reduce bow rise can help avoid oscillation because they will tend to limit the bow rise when crossing a wave, and thus reduce the tendency to excite the oscillation. The advantage of trim tabs is that they can be set so that at normal planing attitude they are not causing any effect, but when the bow rises in crossing a wave the trim tab is then forced down into the water stream and takes immediate effect.

But existence of trim tabs does not guarantee that a boat will never enter into bow oscillation. Let me use my own boat as an example.

I have Boston Whaler REVENGE 22 W-T Whaler Drive, and the Whaler Drive is, in effect, a giant trim tab that is always inhibiting the bow from rising. However, it is still possible to produce bow oscillation in certain conditions of waves and engine trim. Whenever bow oscillation occurs, the remedy is always the same: the engine trim is altered to bring the bow down. Additionally, there are certain boat speeds (typically very high boat speeds) and certain wave patterns (of particular wave height and wave spacing) when my boat is on plane, and there is no setting of engine trim that will completely eliminate bow oscillation. The remedy in that case is to reduce boat speed.

Also, installation of trim tabs adds a further degree of complexity to boat operation. For many boaters, setting proper engine trim is a sufficient demanding task for them as boat drivers, and adding the complexity of setting the position of two trim tabs is just too much complexity for the helm.

Propeller design can also affect the tendency to have bow oscillation. Propellers with blade rake seem to be more effective at influencing the boat trim with changes in engine trim. Such propellers are often described as "bow lift" propellers, a recognition of their better ability to affect boat trim and hold the in a raise position.

Engine power band also affects bow oscillation. Some engines do not have very good reserve power at mid-range engine speeds. When the bow rises, the boat is now trying to propel itself uphill more than a moment before when on more level trim. This will require more engine power to sustain. Engines operating at an engine speed where they have little or no reserve power will tend to decelerate when the boat bow rises. The reduction in engine power causes the bow to fall. The boat is now not trying to go uphill as much, so the engine speed increases, driving the bow up. This cycle is repeated. The remedy is to use an engine with a wider power band or an engine with higher maximum horsepower so it has plenty or reserve power. In this regard, the Evinrude E-TEC engine is an outstanding example. I continue to be impressed with the rock-steady engine speed it produces even as the boat bow rises significantly while operating in large waves. The E-TEC runs like a diesel, and much differently that the prior engine of the same power rating that was prone to engine speed variation in waves.