How To Series: Jet Motor Set-up
August 11, 2018
The most common call into our shop is this: “I have a jet, and it runs like dog sh*t”. Well ok then. Jet motors are known for being a bit more particular, and fine tuning results in HUGE dividends of performance. We asked our fearless leader Frenchy here at the shop to walk us through his best practices when setting up a new jet outboard.
1: Motor Height
The single most critical item on a jet outboard is height on the transom. Period. If your motor was hung wrong, there’s little to no way to recover the full functionality out of your boat. It’s our recommendation that “home brewers” always use a quality adjustable jack plate like the CMC manual plate. It just gives you some wiggle room.
How to set Motor Height with a jack plate:
The “untuned” factory position for us here at Skinny Water Specialists is this: The jack plate 3/4 of the way down (giving lots of upwards travel) and the jet foot leading edge positioned even with the lowest point on the hull in front of the jet foot. That doesn’t always mean bottom of the hull. If your hull has a center strake directly in front of the jet foot, that’s your lowest point.
2: Motor Trim
If your motor has power tilt and trim, congrats. You’ve purchased a motor with the most desirable feature on a jet. It nearly doubles the efficiency of a jet and what’s more important, allows you to adjust your attitude in the water on the fly. Super skinny run? Just trim up. Seriously… it’s that kind of good.
How to set manual motor trim:
I’d suggest you start trimmed all the way in (down). Begin stepping the motor up one trim level until you find cavitation or an undesirable boat attitude. You want a quality jet boat set up to ride as flat in the water as possible. Bow high means transom low, which means less skinny. You’d rather have the jet foot trimmed down 3/4″ than have the transom riding 2″ deeper. Make sense?
3: Deep Water Fins
Here’s where some people will roll their eyes. I use deep water fins on every single jet that leaves our shop. If you’re unfamiliar with them, these aluminum fins mount to either side of the foot and function to actively channel more water into the jet foot. I’ve found that while they don’t always improve performance, they certainly never harm it, and in some instances make finicky setups bulletproof. They’re especially effective on motors with power tilt and trim.
I’ve also found that they function best in shallow riffles and help prevent cavitation in the churned up water over rocky bottoms… you know those spots right in the middle of the sketchy stretch that you fear cavitation the most.
4: Let’s talk technical
When in doubt call the shop and we can talk through more details for your particular set up. There are factors like boat bottom, motor brand, engine set back… the Devil is in the details.
5: Dialing it all in
So motor set up sometimes is as much black magic as it is a regular process to get the most out of a particular set up. Here’s the sequence I use:
- Jack Plate Height: Trim motor all the way in and raise the vertical height the motor using a jack plate until there is minimal “splashing” off the jet foot or it’s not cavitating while on plane. This indicates it’s as high as possible while still efficient.
- For Manual Trim: Begin trimming the motor up one setting at a time until it either sits perfectly flat on plane or you find cavitation coming onto step.
- Change one thing, change everything. Just keep at it until you’ve got it where you want it.