Correct prop size?

[Longrange1971]

Hello all,
I have a new to me C-Hawk 29 pilot house. It has twin 2013 Df 250's with about 300 hours on them. Going to go check prop size tomorrow, however I think they are oversized. I can only get 5100 RPM @ WOT. What size prop should be on this set up. It does have the stern bracket.

3800 Rpm cruises around 31 Mph
5100 Rpm will top out around 45 Mph

Thanks for your help

[Lunchmoney]

Read the sticky above about setting engine the correct height and prop selection. Good stuff in there.

[Moonlighter]

What size, make, and model are the current props?

5100 is bad. The engines are lugging.

These engines should be propped to reach 6000rpm at WOT.

[Longrange1971]

Yes sir, I did read the sticky and it is great info. I believe the engines are at the right height. I have to go check props in the morning but I believe they are 16X21. i will post tomorrow am with the actual size after I verify. Thanks for the responses. I really appreciate it!!!! They are Suzuki props. Stainless

[Moonlighter]

Ok, but first verify the engine height is right as per the guidelines in the sticky thread.

Then you will have a good idea of that change you need to make to get into the right rev range.

I would also suggest that you do the water test and record the revs/speed/economy data every 500rpm from idle to WOT as set out in that article. If you can then show this data to your dealer or prop shop, its going to be a HUGE help in selecting a better set of props for ypur engines.

One thing I would add - given youre running twins : when you do settle on a new set of props, take them off and send them to a reliable prop shop who can scan them, and tune them so they are identical. Mt local Suzuki dealer does this routinely now on motors he supplies, because even brand new props out of the box have been known to vary from their stated size by up to 1" in pitch!

So having them scanned and tuned to be identical will ensure that the twin motors run best and resonance is minimised/eliminated.

[Longrange1971]

The props are Suzuki 3x16x21.5

[Moonlighter]

Well, it looks like you may need to go to a 3 x 16 x 17" to get your motor into a good rev range. Possibly the 18.5 may be an option with engine height optimised. You can do the numbers yourself to make a decision.

Suggest you go to the Suzuki USA website and check the boat tests for the Df250 and DF250AP. You may find a test for a boat similar to yours to give you an idea, although, really, the best approach is to rely on your own data and act accordingly.

I still recommend that you verify again that your engine height is correct. I have a feeling that yours may be a touch too deep. Can you take some photos of the anti-vent plates when running at a good cruise speed?

[Longrange1971]

Yes i can do that. Not sure when I'm going to get out again. Probably be sometime this coming week. Thanks for all your help

[Longrange1971]

Prop issue has been resolved. I took the props to a Jedi Master of Props that works near me. He agreed they had too much pitch for my set up. Dropped them off at 9:30 am. Had them back in hand by 4 PM. put on the next day and boat is now at 5900 RPM WOT. cruise at 4100 still running 30 MPH and dropped fuel consumption by 2 gallons per hour at that RPM (according to my fuel consumption gauge). Much smoother ride and much quieter ride. Night and day difference.

[Moonlighter]

Good result. 5900 is good.

Do you know what pitch they are now?

Still, check that engine height!! You might have more gains to be made!!