A4 with a 2:1 reduction gear

[Grampian30]

I have just acquired a Pearson 35 with a direct drive A4 and a two blade prop.

It pushes the boat a 5.9 knots in calm water but only develops 1500 rpm.

I have an A4 with a Paragon 2:1 reduction gear. If I swap it will I get up to the hull speed of 6.7 knots or better ?

Existing engine needs to come out for a rear end oil seal and re-paint.

Selwin

[Don Moyer]

A 2:1 reduction model with a properly sized prop will usually outperform (at least slightly) a direct drive model with a properly sized prop, simply due to the fact that a larger prop turning at half the speed will be somewhat more efficient than a smaller high revving prop.

This theoretical difference notwithstanding, you could probably improve the performance of your current engine simply by downsizing the prop slightly. At 1500 RPM your current engine is only able to produce approximately 12 HP. Reducing the pitch on your prop from whatever it is now by at least one pitch number (or 1" of diameter) would increase your available RPM by approximately 200 to 300 RPM and allow you to pick up another couple HP.

If you do decide to convert to the 2:1 reduction model, you will have to raise the engine by 1.042" to make up for the fact that the center line of the output coupling is that amount lower than a direct drive. You will also have to shorten the prop shaft by approximately 5.5" to allow for the fact that the reduction unit protrudes out the back of the engine by that amount.

Don