In recent months, we have received dozens of letters from readers wanting to know more about boat trim, or more specifically, asking how to trim their boats properly for best performance.
Few articles are written about how to use power trim because most experienced boating writers consider it so obvious that surely everybody knows how to do it! Of course, that is not true. What may seem obvious to the experienced skipper is often pure 'gobblygook' to boating newcomers.
Every weekend, on waterways all around Australia, you will see boats performing badly. More often than not, this is a result of poor trim and the problems can be easily solved. Consider the following example.
During a boat test on Pittwater recently, we watched in disbelief as an alloy runabout 'porpoised' or bunny hopped its way down the bay with the outboard making a dreadful Wooommph, Wooommph, noise as the exhaust came out of the water repeatedly. The skipper, oblivious to the porpoising, appeared to have no idea about how to trim a boat correctly.
But what was the problem ? The outboard on the boat was trimmed up too far, pushing the stern down, and the bow up in the air ? thus creating uncontrollable porpoising.
To get the boat handling correctly, to stop it porpoising, and to get the outboard's exhaust back into the water the skipper only had to trim the motor 'in' or 'down' to change the propeller's angle of thrust, and thus trim the boat so it ran nicely along the water.
Fore and aft trim of a boat is an important safety factor controlled by the skipper. Boats trimmed badly, handle badly, and can be dangerous in anything worse than dead calm weather.
For example, when a small car topper or punt is tiller steered by the skipper in the stern of the boat, all of the weight is resting in the boat's stern, causing the stern to sit low in the water, and the bow to rise up.
In this situation, it is critical the outboard eg be correctly trimmed to compensate for all the weight in the stern. If this weight is not taken into consideration, and the outboard engine drive leg is tilted up, or 'out' from the transom, then the bow will be forced up still further. The boat could well be in danger of capsizing end-for-end if it was caught heading against a strong head-wind and wave combination . . .
The small boat helmsman has a number of options to create a better trim or running angle.
The first is to put more weight in the bow. Fish with a friend, and/or move the fuel tank and fishing gear right forward.
Another is to purchase a tiller extension handle for the outboard, so the helmsman can sit further forward in the boat ? usually on the middle thwart.
As well, drop the outboard leg down a few 'pin' holes so the outboard drive leg is below vertical. It will then tend to drive the stern of the boat up and force the bow down (see drawing) so the boat runs along on an even trim.
With small cartoppers and punts, most people achieve the correct trim angle by using a combination of these three factors.
Larger trailerboats are generally easier to trim underway because most modern outboards above 50 hp are usually available with power trim and tilt.
What's the difference between power trim and power tilt? Easy ? but let's get one thing straight.
An outboard with power trim has automatically got power tilt ? but you can still buy an outboard with power tilt that does not have power trim.
Power tilt is just that ? it's a power mechanism which lifts the leg up for trailering, and down to the pre-set spot where the trim pin has been positioned to give the best all-round performance. It simply goes up ? or down. End of story. No where halfway between.
Power trim is different because it lets you adjust the angle of drive (or 'trim') of the whole outboard leg ? as you drive along. It gives you infinite adjustments ? at the touch of the trim button in the throttle handle. And it costs more !
The modern outboard with power trim and tilt often has a two stage hydraulic ram system.
The power 'tilt' is the tip (or 'up') part of the lifting system, and usually separate to the ram that controls the 'trim' system. The tilt ram usually controls the angle of the drive leg from (say) 35 degrees to about 75 degrees from the transom, or where the leg is deemed fully tilted.
Note also that the 'tilt' hydraulic ram generally raises and lowers the leg much faster than the ram controlling the 'trim'.
The 'trim' hydraulic ram engages once the outboard leg has been lowered to about 35 degrees. It controls the trim from about 35 degrees to full trim down or 'in'.
When lowering a motor from (full) tilt up, to trim right down, the second stage is quite obvious, because there is usually a discernible thump when the second 'trim' ram comes into play.
An outboard with power trim and tilt can be trimmed up or down via the second, more powerful hydraulic ram when the boat is running at planing speeds to improve boat handling, speed and fuel efficiency.
As already noted, most outboards can be trimmed up on the second 'trim' ram until the cavitation plate starts to leave the water (about 35 degrees) and down to about 4 degrees below vertical.
The latter 'tucks' the outboard leg under the transom so the propeller thrust lifts the stern and drives the bow down.
It follows that the power trim facility is used to alter the angle of the outboard's propeller thrust to improve boat handling and speed, and that power 'tilt' is used only to raise and lower the outboard leg from about 35 degrees up to full tilt.
For many common boating applications, power trim is not necessary, but power tilt is indispensable anywhere.
For example, older folk with craft under 4.5 m and powered by (say) 25-40 hp outboards do not need power trim, but they may not be strong enough to lift the outboard leg up each time the boat is beached. In this case they would be better off with just power 'tilt' to raise and lower the outboard leg for them.
Most of today's larger outboards are fitted with standard power trim and tilt, making it very simple to correctly set the fore and aft trim of the boat in any sea state at the touch of a button.
For example, running before the sea (downwind), a boat needs to have the bow lifted up (or out) off the water to prevent nose diving or broaching. To achieve this, the outboard leg should be trimmed up so the hrust of the outboard is pushing the stern down and the bow up. If the trim is used incorrectly, i.e., with the engine leg right down, the boat may bury its bow into a trough, because the thrust of the outboard is forcing the stern up and the bow down, creating a potentially dangerous situation.
Using power trim into a head sea, the opposite is usually true. While some monohulled boats perform better upwind or into a head sea with the bow up, out of the water, most work better with the bow trimmed 'in' to present the sharply vee'd forefoot to the attack.
Experienced helmsmen will trim the outboard leg down, forcing the bow down and reducing the tendency for the boat to jump and leap over waves. This can result in the ride being a little wetter, but this is the compromise that may be necessary to achieve a safe, comfortable head sea ride.
Another important factor worth remembering is that generally speaking, fibreglass, timber or plate alloy boats with a smooth, cleanly finished hull, will have a more effective response to power trim than many smaller alloy runabouts with a 'pressed' and rippled aluminium hull construction.
Smaller pressed alloy hulls have an array of extrusions or strakes stretching fore and aft which causes a lot of aeration in the water under the hull when the boat is running along at planing speeds. This aerated, or bubbled water gathers at the transom, making it more difficult for the outboard's propeller to grip the water.
To avoid this aerated water lying just beneath the hull bottom, and to ensure the propeller has enough solid, clean water to grip, the outboard leg needs to be trimmed down or 'in' further than normal to prevent the outboard propeller tips from cavitating in the aerated water ? especially when the boat is turning.
This aeration phenomenon does not usually affect a skipper's ability to trim the bow of a boat down, and it is still possible to trim the bow of these craft up to some degree ? but not to the extent of a craft with a smooth hull surface ? which allows clean, unbroken water to flow through to the propeller at the transom.
With twin engine craft, it is not only possible to trim a boat fore and aft, but to change the lateral or side ways trim. This facility is particularly useful when travelling beam to the sea in a monohull. In this situation, the wind coming in on the beam will push against a boat's hull, causing it to list into the wind. With twin engines it is possible to compensate for this list by lowering or raising one of the outboards.
For example, to correct a list to starboard, the starboard motor should be lowered a fraction. Alternatively, the port motor could be lifted slightly to bring the boat back on an even keel.
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