#### dnauhei

##### Senior Member

- Joined
- Jun 16, 2015

- Posts
- 216

- Likes
- 31

- Location
- Charleston, SC

- Boat Make
- '78 Sisu 22, Power: inline 6 cyl diesel Chrysler Nissan SD-33 (aka CN6-33)

Short version: For optimal speed on a 22" full displacement hull, should I do a 14" prop with a 1.1 gear? Or a 15" prop with a 1.51 gear?

EDIT: My prop is a 14 L 12, so not a 15". And I don't think I have space to swing a bigger prop.

Long version:

I have a '79 Sisu 22 with an Nissan Chrysler SD33, a straight 6 with a 15". RPM range is 2600 cruise - 3200 WOT (although the high end of that RPM may have been set lower, but I have had no tach up unitl now).

I did an engine swap for a better condition Sd33 last winter, but it had a 2.67 different gear. My speed dropped by about 30%. So I am changing the transmission ration this winter. I don't know what the old ratio was, but I believe it was 1:51. So . .

With the 1:15 I got 9 kts cruise - 11.5 WOT

With the 2.67 I get 5 kts cruise - 9 WOT (high end of throttle may be set higher)

Others are reporting higher top speeds out of this hull. Another '79 Sisu is getting 9 kts cruise - 16 kts WOt with a 14" prop and 1.1 gear. That with a Perkins 4.236, which is a straight-4, 85 HP. 2000 cruise, 2700 WOT I think. Yet another is reporting 13 kts at WOT with a 65 hp engine.

I would like to get more speed out this hull with my 85 hp engine.

Or . . . I have noticed that the cruising speed for the comparison boats is the same as mine with my old gear (9 kts); maybe I am already at the optimal gear vs prop ration and the only reason my WOT speed is not higher is the high end of my throttle was set too low.

And is the fact that the boat can swing a 15" prop say the designed thought that was the correct prop size?

Thank you!

-dn

EDIT: My prop is a 14 L 12, so not a 15". And I don't think I have space to swing a bigger prop.

Long version:

I have a '79 Sisu 22 with an Nissan Chrysler SD33, a straight 6 with a 15". RPM range is 2600 cruise - 3200 WOT (although the high end of that RPM may have been set lower, but I have had no tach up unitl now).

I did an engine swap for a better condition Sd33 last winter, but it had a 2.67 different gear. My speed dropped by about 30%. So I am changing the transmission ration this winter. I don't know what the old ratio was, but I believe it was 1:51. So . .

With the 1:15 I got 9 kts cruise - 11.5 WOT

With the 2.67 I get 5 kts cruise - 9 WOT (high end of throttle may be set higher)

Others are reporting higher top speeds out of this hull. Another '79 Sisu is getting 9 kts cruise - 16 kts WOt with a 14" prop and 1.1 gear. That with a Perkins 4.236, which is a straight-4, 85 HP. 2000 cruise, 2700 WOT I think. Yet another is reporting 13 kts at WOT with a 65 hp engine.

I would like to get more speed out this hull with my 85 hp engine.

**Should I try a lower gear ration with a small prop?**Or . . . I have noticed that the cruising speed for the comparison boats is the same as mine with my old gear (9 kts); maybe I am already at the optimal gear vs prop ration and the only reason my WOT speed is not higher is the high end of my throttle was set too low.

And is the fact that the boat can swing a 15" prop say the designed thought that was the correct prop size?

**Should I try 1.1 w 14 prop or stick with 1.57 w 15" prop?**Or if it ain't broke . . .Thank you!

-dn

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