Suzuki df250 prop change

[ahmincha]

Hello everyone.
I have a 22 foot stratos walk around 2250 with a Suzuki df250 with a 3x16x 21.5l and I am about 600 rpm shy of target.
I have a chance to buy a 4x15x21L at a great price but don't l ow if this prop will help or hurt my rpm shortage?
Any help would be appreciated

 

[Sea Rider]

Could modify current 3 blade 3x16x 21.5prop at a prop shop to a 3 x 16 x 19.5, if pitch formula works prop should achieve + 400 wot revs, only checkable on a water wot test.

Second option has pitch to close to current prop ,will need to go at least 2 less sizes to a 4 blade 17 pitch to pull wot revs up for combo to perform much better..

Happy Boating

 

[jimmbo]

If you add a blade, you are adding more load to the engine. Since you are already below your target rpm, you will have to drop at least down to a 17. There are some good 3 blade props, like Mercury's Enertia that have very good Acceleration and good top speed. It has a tunable Venting system. But in any case, some testing/trials may be needed. I have never believed in repitching props, especially aluminum. Tweaking, as in adding/adjusting cup, adding vent holes is fine, but trying to bend a blade to a different contour, and 3 or 4 consistently, No. End result is almost always poorer than a prop originally made at the final pitch

 

[Sea Rider]

I've re pitched at a prop shop who has great experience doing so 2 props in the 0.5 to 1 full pitch with excellent results, re pitching is done over a metal mold while banging hard on prop blades contour, alum ones are way easier to work than SS ones. Once done prop is well ballanced and re painted.

Sometimes you need to re pitch where you only need 0.5 pitch on a 1 pitch prop and 1 pitch on the 2 pitch prop. Isn't re cupping a form of changing which was factory original made ?

Re pitching is necessary when + 200 wot per pitch formula stayed home and your engine is wot over revving say + 400-500 rpm way over fomula on a light, medium light application, this happens often, just need a tach in healthy cond to check that out as currently loaded.

Happy Boating

 

[ahmincha]

ok so I did go with a 17 pitch 4 blade and my rpms are way high amd I lost about 20 mph whats next option

 

[Sea Rider]

As you have seen it's very difficult to dial top wot revs and good speed by plain theory recommendations on a second attempt. Personally I'm not a big fan of 4 blade props, always go for 3 blade prop maximizations with OEM props and just playing with usually less pitches to pull wot rpm up.

Something to know :

What's the min-max wot rpm range factory stated for your OB, was combo lightly, medium or fully loaded when tried the 4 blade prop which is a OEM one, any other particular prop brand ? How much are "way high revs", losing 20 MPH seems out of this world.

Happy Boating