Monohull Speed Secrets

Speed Secrets for Monohulls

Today Monohulls reach speeds of 65mph on the racing circuit and do it with more control than ever.Twenty years ago monohulls were labeled “rock boats”because they rocked uncontrollably from side to side down the straightwayat top speeds of about 35mph.The evolution from rockboats to high-speed racing machines is a result of a team effort in which modelers have worked hand in hand with boat and hardware manufacturers.Engines that are hot right out of the box and greater selections of racing propellers have taken speed to a new level that demands properly trimmed boats.

Deep-Vees

The reality of heat racing is that after two laps,the water gets rough no matter how calm it was before the race started.The boats have to run well in rough water,so deep-vees are the models of choice.They have a deadrise angle of 16 to 26 degrees,which is great for rough water ; at top speeds on calm water,however,they can also “air out”. The hot deadrise numbers today are 21 to 24 degrees.

I have tried up to 26 degrees but have found that rudder action from slow to high-speed transition is not linear.Anything less than 21 degrees and you stand a good chance of rolling over in choppy water because the boat bounces rather than cuts through it.

Trim Tabs

Trim tabs are used to adjust the ride angle of the model.

They are usually mounted on the transom about halfway between the keel and the chine.The inside edge is usually about 1,5 inches from the keel.Some trim tabs are single-tab designs (one on each side of the hull);others are split into two adjustable plates.

The double tabs allow two separate adjustments for each tab.With a two-tab setup,the innermost tabs are adjusted up and down to raise or lower the bow.Adjusting both tabs downward pushes the bow down because the water leaving the tab’s trailing edge lifts the transom.Raising the inner tab’s trailing edges reduces this lift and allows the boat to run higher (lighter) on the water.

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The inner tabs have a second function as well.They can make the model lean left or right depending on the “right-vs-left”adjustment of the tabs.If the boat leans to the right because of engine torque (and it will),the right (starboard) tab should be adjusted downwards.This will lift the right side of boat.Keep in  mind that lowering the right tab not only lifts the right side of the boat but also causes the bow to be pushed down.If the boat is leaning to the right and also running very wet (bow down),it would be better to raise the left inside tab to both loosen the bow and lessen the lift on the left side.Most hulls that are set up properly for heat racing are adjusted with the left tab level with the bottom of the hull and the right tab about 1/16 inch down.If the bow rides too light at this point ,both the left and right tabs should be adjusted downward the same amount to push the bow down.The right tab would still be down 1/16 inch more than the left one to compensate for torque.While running straight ,the outer tabs have little effect on the boat’s ride.They do,however,adjust the boat’s ride through the corners as the hull leans left or right.A monohull slides as it turns ,and if the bow digs in,the boat will turn more sharply.Pushing an outer tab downward makes the model turn tighter on that side of the hull because it lifts that side of the transom.This in turn, pushes the bow into the water with more authority but only during a turn.The right outermost tab adjusts the hull’s ride for right-hand turns and the left outer tab adjusts the ride in left-hand turn.Raising the outside tabs makes the boat less sensitive in the corners because it’s bow will ride higher (lighter) on the water.

Fiberglass Hulls

Many fiberglass models I have worked with are too sensitive in the corners because the hulls tend to warp as they cure.This causes a cup or “hook” to form at the transom’s upper outer edge close to the chine.The hooked surface of the hull bottom acts like a trim tab and will cause the boat to turn more sharply as it leans into the corner.Some boats don’t use trim tabs at all because they use a hooked hull bottom and a more forward CG position to balance the boat’s ride.

You should definitely use the manufacturer’s hardware designed for a non-trim-tab equipped boat.Most manufacturers tune their hardware to make the boat work with it’s natural hook and CG placement.Generally speaking , the  CG placement for non-trim-tab boats is from 34% to 36% of the hulls length I front of the transom and between 28 to 31% for boats equipped with trim tabs.

Top-secret tip

There is an area of the boat’s hull that,to my knowledge,has rarely.if ever,been mentioned in any publication before.It has the biggest influence on trimming a mono at today’s higher speeds.It is the area on the bottom rear portion of the hull at the transom that extends from the keel to about 1 inch up the transom toward the side of the boat at the very last ¼ inch of the transom of the boat.It is the place where there are no trim tabs.It is a super-critical area that not only controls how high or low the bow rides but can also control the left to right balance of the model.The tricky part of this area is that it works totally opposite to the trim tabs.A hook on the ride side of the hull in this area does not lift the right side of the boat like the trim tab does but rather “trips” the boat into leaning to the right.If you want to see how this works, trtm your boat normally,then put a couple of drops of cyanoacrylate glueon the rear portion of the hull at the transom to form a 1-inch strip running outward from the keel toward the chine.The boat will then lean to that side and run “more wet” at the bow.

Article from RCBM by John Finch