# 12 Trimming devices

Lorne CampbellTechnical

The fastest RIB is no use if you can’t control it.  Assuming a conservative, fairly heavy boat with its centre of gravity (CG ) not too far aft, throttle and steering are all you need. When inboard engines on fixed shafts were normal this was usually it. Nowadays we want to release more of the potential, so adjustability is needed, plus control.


For maximum advantage, trim has to be varied depending on weather and sea/wind direction. Power trim, power lift, trim tabs, ballast tanks and pumping fuel between tanks are used on a craft which is properly set up initially. Other systems include movable weights, wedges, speed rails, and even wings!  Of course, the more adjustability you have the more you need to know how to use the adjustments to advantage – things will be worse if they are used wrongly.
Firstly we’ll look at power trim and lift, and trim tabs.
Power trim adjusts the angle of the drive leg. Tucking in makes the prop lift which keeps the nose down and reduces air time for the prop, thus keeping drive. Trim out and trim increases, reducing wetted surface and increasing speed but there is less control so look out for that solitary wash! Power trim can be jacked out too far, revs may rise but speed may not increase.


With a neutral prop on a correctly balanced boat, the power trim should be aft of vertical by the trim angle of the boat so that the prop is thrusting directly forward and not lifting or digging. If the CG is too far forward you may need to over trim the leg but the reduction in the wetted surface may more than offset the reduced forward thrust.


Power lift is useful for ultimate performance. The prop can be lowered to get on the plane and then be raised to reduce lower unit drag at high speed. This adjusts the pitch handling by changing the height of the thrust line. Sometimes boats run better in a following sea with the prop low and head to sea with the prop high. It is also useful for setting up a boat in the test stages where a number of engine heights can be tried in one test session.


Tabs reduce trim for rough water and help stern heavy or well-laden boats to plane. A RIB with aft CG can use ‘down tab’ to limit porpoising but it would be more efficient to adjust the CG forward instead. Tabs almost always increase cause drag. Tabs are usually mounted near the transom chines with lower faces at the hull deadrise angle and set about 15mm up from the bottom to be clear when lifted. With a fast RIB, however, they may be better further inboard.


Graham Jelley who built the brand Scorpion RIBs says; ” … trim tabs are our first choice for those seeking exceptional handling and performance and they complement the already outstanding qualities of our advanced designed deep V hulls …”


At speed solid water leaves the bottom inboard of the chines, so when running fast widely spaced tabs only work intermittently. They produce great lift at speed and if one makes contact on its own it can kick the hull onto its opposite side. Lighter, quicker boats are most vulnerable. Another possibility is to rotate the tabs about their outboard edges and fit them horizontally. In operation the outboard corner works first, bringing them into effect gradually and, initially, with the low wetted surface. The force is vertical rather than angled inwards so righting leverage is improved. This is good for fast, light RIBs which use tabs mainly for high-speed stability.


There are now some new devices called ‘Interceptors’. These are vertical sliding plates which have replaced tabs on some fast ferries although they have yet to be tested on small craft.


Ballast Tanks, Wedges, Speed Rails and Wings.
Ballast tanks can be useful. They are usually mounted forward and when filled increase hull weight and move the CG which helps rough water handling.


With light, fast RIBs the increase in weight is often the most important aspect. I have known some race boats, theoretically very fast because of low weight, but which spent their time with the prop in the air in a seaway; they had too much lifting area for their weight and in anything other than flat calm they were quicker with ballast on board. Mounting ballast tanks low is good so fitting them forward but before the keel rises much towards the stem is close to ideal.


Adjusting the position of movable items is a cheap way of improving a bad handling boat. Examples might be moving weight forward on a RIB that porpoises or offsetting weight to one side to counter torque on a single prop set up. This would normally be done at the design stage but, nowadays, engine size varies and layout is frequently modified from standard; this means that things don’t always come out perfectly so moving weight around can be good!


Wedges are a cheap way of doing what trim tabs do but are only useful for a craft that needs to use tabs continuously, usually heavy craft operating at low planing speeds.  A ‘wedge’ is an angled addition underneath the transom bottom tapering from zero at the forward edge to a finite depth aft. Their angle can vary from 5 to around 15 degrees.


Speed rails are add on spray rails that can be retrofitted to an existing hull. They originated with Ocke Mannerfelt and increase lift. They can be used to add lift and adjust the trim on the craft that are heavy and/or have a CG problem. Increased lift usually means increased drag, so adding these to the whole bottom would probably slow the boat up, but short lengths in the right place can adjust trim so that the increased hull efficiency more than offsets the extra drag of the rail.


I am not seriously suggesting wings as practical trimming devices but they can have an effect on very fast boats. More to the point is that adding things with an appreciable surface area up in the breeze can adversely affect your RIB when you don’t want it to. Also, bear in mind that the air vortices running up the outside of the tubes and down into the interior of a fast RIB produces both lift and drag.


Concorde relied on these when taking off and landing but has the power to overcome the drag they cause. Filling in or covering over the foredeck will change these vortices and the lift and drag effects. If a stylised wing is added for a show, its aerodynamic effect must be taken into account or you may end up swimming!


Marine Craft Designers and Naval Architects