An outboard's shaft length has to match the height of your transom, or the propeller runs too shallow or too deep. Measure the boat and the motor the same way, then compare.
Shaft length is the distance from where the motor's clamp bracket sits on the transom down to the anti-ventilation plate, the flat plate just above the propeller. Honda calls it the motor's transom height. The boat's transom height is the matching measurement on the hull.
Shafts come in classes. Mercury sells its FourStroke 9.9 with a 15-inch short or 20-inch long shaft, and Honda's BF200D, BF225D and BF250D come in L (20 inches), X (25 inches) and U (30 inches).
Measure straight down from the top of the transom, where the clamp bracket will sit, to the bottom of the boat. Do it at the center of the transom, since that is where the motor mounts.
Measure from the top of the clamp bracket, where it rests on the transom, down to the anti-ventilation plate. On a Yamaha, the identification label on the port-side mounting bracket also gives the drive shaft length.
The motor's figure should be a little longer than the boat's, so the plate sits slightly below the bottom of the hull. Honda wants the plate 0 to 2 inches below the bottom of the boat.
With the motor clamped on and straight up and down, Yamaha says to lay a straightedge along the keel to the outboard to see where the plate sits. Raise or lower the motor if needed, then tighten the clamp screws again.
What the makers list for the motor's transom height, model by model.
| Motor | Shaft | Listed length |
|---|---|---|
| Mercury FourStroke 9.9 | Short / Long | 15 in / 20 in |
| Honda BF5A | S / L | 17.5 in / 22.5 in |
| Honda BF25D, BF30D | S / L | 17.0 in / 21.7 in |
| Honda BF200D to BF250D | L / X / U | 20.0 in / 25.0 in / 30.0 in |
| Maker | Plate below the bottom of the boat |
|---|---|
| Honda (BF5A, BF25D, BF30D) | 0 to 2 in |
| Mercury (2.5 to 6 hp portables) | 1 to 2 in |
| Yamaha (portables) | Even with the bottom, or about 1 in below |
Yamaha explains the trade-off: the motor has to sit deep enough for the gearcase to pick up cooling water and for the propeller to work well, but not so low that it adds drag. It warns that the wrong height, or anything that disturbs the water flow such as a ladder or transducer, can throw spray that starves the engine of cooling water and badly damages it.
For its BF5A, Honda adds a check on the water: with the boat afloat and loaded, the plate should be about 3.9 inches below the surface.
Bigger bolt-on motors are set by mounting height, the height of the motor's mounting brackets above the bottom of the transom, and a jack plate lets you change it. Mercury's 150 FourStroke manual says to mount the motor high enough that the exhaust relief hole stays at least an inch above the waterline at idle. For that motor it caps the mounting height at 25 inches for L models and 30 inches for XL models, since going higher can damage the gearcase.
Mercury says raising the motor will usually reduce steering torque, increase top speed and boat stability, and cause the propeller to break loose during planing. Its mounting height chart notes that above 22 inches, a propeller designed for surfacing operation is usually preferred.
T-H Marine says to always trailer with its Atlas jack plate in the lowest position.
Honda, owner's manuals for the BF5A, BF25D and BF30D and BF200D to BF250D; Mercury, 4/5/6 FourStroke manual, 2.5/3.3 TwoStroke manual (8M0133461), 150 FourStroke manual and FourStroke 9.9; Yamaha, Installing a Portable Outboard and Owner FAQs; T-H Marine, Atlas jack plate instructions.