Help Choosing


A look at propellors, shaft-length options and more... to help you to decide the right outboard engine for you.

The goal in propeller selection is to determine what style and size will maximize your boat's performance, while allowing your engine to operate in the recommended r.p.m. range. The correct propeller will prevent the engine from over-revving, yet allow it to reach the minimum r.p.m. at maximum horsepower.

If the engine over-revs beyond the maximum recommended r.p.m., you may need to increase the pitch of the propeller. Increasing the pitch increment by 1" will result in approximately a 150-200 r.p.m. reduction at wide open throttle (WOT). If you cannot reach maximum r.p.m. at WOT, then pitch should be decreased. These recommendations apply to single engine installations only. For most twin engine installations it is necessary to increase pitch by 4". Adding diameter will typically decrease rpm, while reducing diameter will raise the rpm.

Propeller selection will depend on your usage. Ski boats need more top end speed, and should choose a prop with a higher pitch. Cruisers and houseboats need more performance at displacement speeds, and should use a prop with a lower pitch to achieve low-end power. It is essential, however, that the WOT r.p.m. falls within the range specified by your engine. If your engine is not able to reach this r.p.m. range, it's operating under an extremely loaded condition.

If the propeller supplied with your engine is not providing desired performance follow these rules to help choose a different prop:

1. Consult owners manual to find recommended wide open throttle RPM range for your engine.

2. Determine current WOT RPM and speed using existing propeller. Vary trim to achieve maximum speed.

3. If existing propeller doesn't achieve RPM within specified range at WOT select larger or smaller pitch to achieve this.

-Adding 1 inch of propeller pitch will reduce WOT RPM by 150 to 200 RPM.

-Subtracting 1 inch of propeller pitch will increase WOT RPM by 150 to 200 RPM

4. A 4 blade propeller will usually generate 50-100 RPM less than an equivalent pitch 3 blade propeller.

Most engines come with Aluminium propellers as standard as they are inexpensive and suitable for all round use. Stainless steel propellers are much stronger, more expensive but also offer greater performance. Upgrading to a stainless steel propeller however is a relatively cost effective way of achieving increased performance including improved acceleration and top end speed.
If the propeller is likely to remain submerged at all times, a three-bladed prop usually provides better top speed performance. Most standard propellers are three-blades. A four-bladed propeller can provide better acceleration and is more widely used in craft that pierce the surface or the propeller is very shallow.
The shaft lengths on standard Tohatsu engines are 15" short shaft, 20" long shaft and 25" ultra long shaft (only available in 6hp as factory fitted model but other models can be made into UL shaft with a UL kit (see accessories page). Shaft length on an outboard is measured from the top of the mounting clamp bracket to the bottom of the cavitation plate.

To determine what shaft length you need for your boat measure the height of the transom of your boat at the mid point where the engine would be mounted.

As a general rule:-

Short shaft : 15-17" transom (small dinghys, inflatables, sailing day boats with moveable mounting brackets).

Long shaft : 18-21" transom (day fishing boats, aluminium boats etc)

Ultra long shaft - 22-27" transom (sailing boats or very high transoms)

The cavitation plate should be flush (as close as possible) with the bottom of the boat.

We recommend that you consult with your local dealer to determine which is the correct shaft length for your particular boat.
There are many factors which must be taken into account, most importantly is to never exceed the horsepower rating of the boat.

Other factors to be taken into account are the weight of the boat, weight of passengers/gear, and your expected use of the boat (do you want to water ski, troll around inland waterways or use commercially etc?).

To find the best "bang for your buck" please consult your local dealer and discuss your needs with him/her. They will be more than happy to help you find the best outboard for your particular needs.

 

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