Judging by the number of badly trimmed planing powerboats you see on our waterways, it seems that there are still many boaters who don’t understand what the word “trim” means, and apparently, don’t even realise when it’s properly trimmed or not! Leisure Boating provides an in-depth look at the correct trimming of your motors in a two-part feature.
Ultimately, trim is a safety issue. It’s certainly possible to capsize a craft because it’s badly trimmed. However, the more you cruise in open water, the more important it is to understand trim and how to adjust it to suit different conditions.
The difference between a well-trimmed boat and one that isn’t, is quite dramatic. But before we get into the effects of the dreaded trim button, it’s necessary to understand trim — and that starts with weight distribution.
It’s a skipper’s responsibility to ensure weight is distributed appropriately and evenly, before a boat leaves it’s mooring. The positioning of passengers, iceboxes, portable fuel tanks or any heavy gear should be reviewed every time a boat is loaded.
Passengers can be asked to move and anything that’s not bolted down can be moved around to redistribute weight evenly. Remember, you may need to move a fuel tank or icebox as fuel is consumed, or an icebox empties.
Common sense suggests that a boat needs to sit level or “square” in the water, yet while that’s essentially true, it’s not always the case with planing powerboats.
Bow down is never a good idea. A boat with a heavy bow won’t steer well and takes longer to get onto the plane. What’s more, a bow-heavy boat will use excessive amounts of fuel.
In contrast, it’s occasionally good for a craft to be slightly stern-heavy. In case of an emergency with an anchor deployed, moving weight aft keeps the bow high, allowing it to deal with oncoming seas.
Similar to when under tow, redistributing weight aft minimises any tendency or tension of the anchor rope to bury the bow into the towing boat’s wake.
Stationary trim is about common sense and about balancing a boat by distributing weight to keep the hull at floating level in the water. Except that sometimes it may be better to distribute some weight aft.
Once the hull moves, nothing changes. However, speed does introduce the effect of the hull’s hydrodynamics while planing speeds magnify both these and the interaction between hull and surface conditions, such as wind, chop, swells, etc.
At speed, trimming becomes a little more complex because appropriate trim for travelling in one direction (relative to a prevailing sea) may not be appropriate in another direction.
This is why easy trim adjustments at the touch of a switch are provided on planing powerboats — because it’s necessary to adjust the trim as the conditions or direction changes.
The often-cursed trim tab is actually a wonderful concept. There are two common means of adjusting tabs mounted on the transom. The other is by adjusting the angle of an outboard or sterndrive relative to the transom.
Small outboards adjust manually by moving a pin along a series of holes in the mounting bracket. The trim ’n tilt fitted to larger outboards and sterndrives adjusts the angle of the drive leg electric or hydraulically at the touch of a switch.
It either alters the angle thrust from where the propeller is delivered, relative to the water surface and the hull, and the attitude (or trim) of the boat to the water accordingly.
Trimming in and out
At speed, with an outboard or sterndrive, adjusting the propeller closer to the transom — commonly referred to as trimming in — raises the transom and pushes the bows down. Trimming out, adjusting the leg away from the transom, lowers the stern and raises the bow.
Trim tabs work by adjusting the tab away from contact with the passing water, or down further into the water. Adjust the tabs down, into harder contact with the water and the stern lifts and the bow lowers. Adjust them up, easing contact with the water, and the transom squats lower and the bow rises.
Trim tabs are usually independently adjustable making it possible to adjust both bow and stern trim and lateral trim. With independent controls, one tab may be set harder than the other to correct a lean to one side or the other.
It’s important to stress that while at speed, trim adjustments will, to some extent, compensate for uneven weight distribution, they should never be used for that specifically.
Side-to-side (lateral) trim adjustments are there to compensate for the effects of strong winds on one side of the hull, and not to straighten up a boat leaning over from all the people sitting on one side.
Similarly, a bow-heavy boat that’s re-trimmed with a set of tabs is still a bow-heavy boat. The problem is that being bow-heavy, the boat is still waiting patiently for an opportunity to bite.
An encounter with a bigger than average wave, or even a bigger than average boat wake and suddenly you find out how violently a bow-heavy boat can broach.
Getting static trim right is always the starting point before the motor is ever put into gear. From there, at speed trim changes to a planing hull are about trimming to suit changing conditions.
To be continued…
Catch our November 2011 edition for PART II on Boat Trim. Next month, we’ll look at different trim angles, trim gauges, steering and changing sea conditions.
PULL QUOTE: “A bow-heavy boat that’s re-trimmed with a set of tabs is still a bow-heavy boat”.