How to Synchronize Twin Engines (RPM and Trim)

On a calm day, set both engines to your normal cruise. Listen and feel the deck. A slow, rhythmic throb that speeds up or slows down as you nudge one lever is the classic sign of engines turning slightly different RPM. The closer they get, the slower the beat, until it disappears.

Syncing to two tachs only works if both tachs read correctly. Older tachs, especially ones driven from the alternator, can disagree even when the engines are turning the same speed.

• On engines with a digital display (SmartCraft, Command Link and similar), compare the gauge to the RPM shown in the engine data screen.
• With the engines at idle in neutral at the dock, a technician (or a careful owner standing well clear of moving parts) can compare each dash tach with a handheld optical tach aimed at a reflective mark on a pulley or flywheel.
• If one tach is off, fix the gauge first (see the gauges and tach troubleshooting guide). Otherwise you will "sync" the needles and still hear the beat.

Warm both engines up. In neutral, compare idle RPM. Modern fuel-injected engines set idle themselves, so a big difference there is a symptom, not an adjustment: one engine may have a sensor, air leak or fuel problem. On older carbureted engines, idle is adjustable per the service manual, which is usually a shop job. Write down what you see.

In open water, bring both levers up together to your cruise RPM. Then:

• Pick one engine as the "master" and leave its lever alone.
• Nudge the other lever in very small moves until both tachs match.
• Note how the levers sit. On many boats they end up slightly offset; that is normal and is what cable adjustment (step 11) can tidy up.

Some mechanical twin setups have a sync gauge (a needle that centers when both engines match) or a twin tach with a sync indicator. It compares the engines directly, so it is more precise than reading two separate needles. If you don't have one, finish by ear: after matching tachs, move the slave lever a hair at a time until the throb slows and fades out.

Check your control's manual for how sync works. In general:

• Sync is switched on with a button or happens automatically once both levers are close together and above a certain RPM.
• It turns off at low speed (for docking) and when you move the levers apart.
• An indicator light or display message usually shows when sync is active. If sync never engages, check that both levers are really in the same position and above the minimum speed. A sync feature that worked and then stopped is a question for the dealer, because it often comes with a fault code.

Two engines at the same RPM but different trim angles push differently: one prop is working at a different angle and may even be ventilating. Watch both trim gauges and bring them to the same reading when running straight. Mercury notes DTS has universal and independent trim buttons so you can trim engines together or one at a time. Use independent trim only to correct a deliberate need, such as a list from uneven load, and then re-center. Mercury's general trim advice applies to both engines together: trimmed fully in to get on plane, then out a little at a time once the boat settles at cruise, and back in for turns and head seas.

Twin sterndrives are harder because you can't see the drives under the swim platform, so the trim gauges are all you have. Mercury's MerCruiser advice for a sterndrive: start with the drive fully down, trim up gradually as you accelerate, no higher than about level with the waterline at first, and stop just before the bow starts to bounce (porpoise); that point is usually the most efficient. Bring both drives up together and check that both gauges read the same.

At wide-open throttle, briefly and with a clear course, note each engine's RPM. Both should land inside the WOT range listed in your owner's manual and close to each other. Discover Boating explains that over-trimming lets the prop ventilate: RPM climbs while speed drops. Trim both down a touch and repeat if one engine races.

Syncing hides a problem if one engine simply can't keep up. If one engine needs noticeably more lever to match the other, or tops out lower at WOT, check:

• The propellers. Twin boats usually run counter-rotating props, one left-hand and one right-hand, that should be the same size and pitch. A dinged, bent or wrong-pitch prop changes RPM and load. Yamaha points out that comparing top speed and RPM against your earlier numbers is one of the best ways to catch prop trouble. Yamaha also notes that sand and silt slowly wear down blade edges, so a prop that looks fine can still lose bite.

• Fishing line or weeds on one prop or one gearcase.
• Engine health: fuel filters, plugs and the usual loss-of-power checks on the weak engine. BoatUS notes that with twins, comparing one engine against the other is often the quickest way to spot which one has changed.

Write down the lever position, RPM, speed and trim for both engines at idle, cruise and WOT. Mercury's cruise-speed method (speed divided by fuel burn) works per boat; on twins, log total fuel flow. Next season, compare. A slowly growing gap between the engines is an early warning.

On cable controls, each throttle cable runs from the control box to the engine, where it attaches to the throttle lever with an adjustable end (barrel or clevis). Over time cables stretch and controls wear, so the levers stop lining up. The general idea a technician follows:

• With engines off, each cable is set so the engine's throttle lever rests fully at idle stop when the control lever is in neutral, and reaches full throttle when the control is at full forward.
• Both engines are then set so they respond at the same lever position.
• Shift cables are adjusted separately, with the engine off, so each shifts fully into gear and neutral (see the shift cable adjustment guide). Exact procedures and settings are in the engine service manual. A cable adjusted wrong can leave an engine unable to return to idle or reach full throttle, so if you are not confident, have it done by a dealer.

After cable replacement, a control box rebuild, a prop change or new engines, repeat steps 3 to 8. A short sea trial with a technician aboard is the easiest way to confirm everything matches.