Suzuki DF 90 (2006) performance

[Valiant Knight]

My Valiant PT 520 RIB (5,2 m) is equipped with a Suzuki DF 90 from 2006.
I had a 17" propeller installed but the engine ran "too fast" (6.2 k rpm).
I moved to a 19" propeller and the RPM at WOT fell to 5.5 k.
Recently the max RPM fell to 5 k and the cause was identified as a faulty fuel injector.
Having replaced the fuel injector, the RPM at WOT returned to 5.5 k, and the speed of the RIB at WOT is around 55 km/h (30 knots) with 2 people on board plus 70 liters of fuel.
With 6 adults + fuel it will go easily onto plane at 3.5 k and will continue above 4.5 k (I decided not to go faster, it was not an engine limit)

I would appreciate any comments about the performance of the RIB (+ engine).


Best regards


Antonio Carrapatoso

[jeffstevens763@g]

I had a a ribquest 585 with 2008 Df 90 two up 70 ltrs of fuel 14x 19 Ali prop flat calm 45 mph at wot forget the rpm seem to think 5850

[Limecc]

Originally Posted by Valiant Knight

I would appreciate any comments about the performance of the RIB (+ engine).

Using a prop calculator for a 2006 DF90 (gearbox ratio 2.59:1) and max revs of 6300 with best case 0% prop slip we arrive at 43.8 mph for a 19" pitch and 39.2 mph for a 17" pitch.

The 19" pitch at 5500 rpm will do 34.4 mph with a realistic 10% prop slip, yours is 10.56% at 55 kph (29.69 kn) so everything checks out.

I'd be inclined to fit a 17" four blade.

[Valiant Knight]

Originally Posted by Limecc

Using a prop calculator for a 2006 DF90 (gearbox ratio 2.59:1) and max revs of 6300 with best case 0% prop slip we arrive at 43.8 mph for a 19" pitch and 39.2 mph for a 17" pitch.

The 19" pitch at 5500 rpm will do 34.4 mph with a realistic 10% prop slip, yours is 10.56% at 55 kph (29.69 kn) so everything checks out.

I'd be inclined to fit a 17" four blade.

Thanks for the info.
Just one comment.
The recommended RPM range for the DF 90 is 4.5 k - 5.5. k.
Using a 17", 3 blade propeller the RPM at WOT was around 6.2 k, above the recommended range.



What is the expected result in moving to a 17", 4 blade?


Best regards


Antonio Carrapatoso

[Limecc]

Originally Posted by Valiant Knight

Thanks for the info.
Just one comment.
The recommended RPM range for the DF 90 is 4.5 k - 5.5. k.
Using a 17", 3 blade propeller the RPM at WOT was around 6.2 k, above the recommended range.

What is the expected result in moving to a 17", 4 blade?

I understood from Google the max rpm was 6300 on that motor and expect an electronic rev limiter will prevent over revving.

If it can pull max rpm with a 19" then stick with that, a four blade will give you more stern lift and better acceleration because of less prop slip which varies throughout the rev range. At high speed the water behaves more as a solid and slip is reduced, so it's at low speed and mid range where most benefits are felt. Your prop slip could be as high as 25% at lower rpm's so reducing slip could improve fuel economy and give you the better holeshot that a 17" prop would give you.

You would do well to note the precise speed and rpm every 500 or 1000rpm throughout the rev range and this will help enormously to understand how your current prop is behaving. A prop has a power curve like the engine has a power curve, ideally both are matched.

After talking to Steele Developments and looking carefully at Solas four blade props it seems they adjust the diameter and/or prop design to compensate for the extra blade area over a three blade prop. Therefore the usual advice to go down a prop size when fitting a four blade is incorrect regarding Solas. Stick with the 19" pitch. Welcome comments from others to confirm this with real life experiences.

[jeffstevens763@g]

using a 4 blade prop of the same size will bring top end down but give better acceleration and load carrying, its not an exact science what works for one hull might not for another. they usually say a drop in pitch of one inch is 150 - 200 RPM increase in RPM but can be as much as 400 rpm your at 350 per inch you might find changing diameter might help 1 inch of diameter = 2 inch of pitch

[Limecc]

Originally Posted by jeffstevens763@g

using a 4 blade prop of the same size will bring top end down but give better acceleration and load carrying, its not an exact science what works for one hull might not for another. they usually say a drop in pitch of one inch is 150 - 200 RPM increase in RPM but can be as much as 400 rpm your at 350 per inch you might find changing diameter might help 1 inch of diameter = 2 inch of pitch

Not necessarily. This is the conventional wisdom that I mentioned doesn't apply with Solas. You don't get to pick the diameter, except by choosing a different model of four blade and then the pitch and profile will be different and it will have totally different slip characteristics.

Steele Developments can adjust a prop one pitch up or down for a modest fee by adjusting pitch or adding cupping, they can also reduce the diameter to increase rpm by reducing load on the engine. In general terms increased diameter or blade area reduces slippage but there's a limit according to available engine power.

[Limecc]

Originally Posted by Limecc

You would do well to note the precise speed and rpm every 500 or 1000rpm throughout the rev range and this will help enormously to understand how your current prop is behaving. A prop has a power curve like the engine has a power curve, ideally both are matched.

I forgot to mention that the most fundamental thing is to ensure you have optimal engine height (which is the highest possible position without suffering prop ventilation) before working out if you have the expected performance or not.

A performance RIB ought to have the cavitation plate flush with the bottom of the hull or up to 2" above. Certainly not lower than flush.

[Orwell boy]

Do as many sums as you like ,doubt if it will do you alot of good ,, check out the Ribcraft 5.3 m /df90 information live test on the r/c website ,sure they know how to set up a boat /outboard combination better than most . they have curves across the rev range