HELP!! NEED A PROP GURU !!!!!

[Donzi56]

Hello all, im new to this sight and would love some input! I recently recieved a gift from my wife that has my head spinning, a 30 ft Donzi ZFC. When I got it the gentalman said it did 74 with more to go.BS! It has twin 225 mercs and he had 27 pitch 15" 4 blade props on it. The firstvtime I ran it I couldnt get it past 1800 rpms. My mechanic and I thought carbs were cloged, waste of money. Long story short (obviously) props were soooooo oversized. So now I'm learning props. A little more info , starbord motor kept overheating. After changing everything in cooling system, twice. My mechanic suggested swapping lowers ( thought this might also be good info when speaking about props) because of prop direction?? So cured my overheating problem but still issues with prop preformance. Heres what ive run, with results. In order
21p 14"3 blade- very difficult to get on plane, cavitates hard. Greatly overrevs past 6k rpms. High 30's mph.
23p 15" 4 blade - couldnt get past 2300 rpm
21p 14" 4 blade - cavitated then ran 6200 rpm, gut got 42 mph
same prop adjusted to 23p. But assuming 133/4 " after bending still about the same.
Now after lower swap so curious if more than one factor at work
22p 14 1/4 3 blade. Ok out of hole, slow throtle to 3k then I can open up. Runs about 51 mph but still hitting 6 to 6200 rpms. Book says 55 to 5800 is optimum.
Boat specs. 2000 Donzi ZFC 30ft. Dry weight 4300lbs. Weight as ive run with fuel and gear around 5300 to 5600 lbs.
Can someone please give me some advice!!!!!!
I'll happily fedex an ice cold 12 pack to the one who cures this headache for me!!!

 

[Sea Rider]

Welcome to Iboats,

- Posted wot rpm test were achieved while light, medium or fully loaded ?
- Both props rotates CW or left engine rotates CCW and right engine CW as seen from behind ?
- Book says 5500 to 5800 is opt, but assume its not the max rpm that will go.
- Which is the factory stated wot min-max rpm range for 225 Mercs ?

Happy Boating

 

[alldodge]

225's and being a 2000 model, I'll guess their outboards

Believe your ratio is 1.75
If your using the boat tach it may be off, need a hand held tach to verify?
Are you using a GPS for speed or the boat speedometer?

 

[Donzi56]

Both props are rotating inward.
reached wot both loaded and lite. Over 6200 rpms
book says 5500 to 5800 but yes, with any of these props I can run over 6k easily.
I was using gps , saw 51.3 ish.
and either tachs are off or one motor may be a little stronger, one blows past 66200 ( port motor)

 

[Sea Rider]

Inward rotation means CW rotation, right ? Single engine props usually rotates CW, when having dual engines installation, left engine should turn prop CCW and right engine CW. Achieves better cruising and boat handling at straight water courses. Don't know if will add issues to lessen boat performance ?

Did you play with trim while testing, made any difference ? Both tachs must be 100% accurate to check each engine health condition.

Happy Boating

 

[alldodge]

I think your tach's are off, hitting that kind of rpm's can blow the motor. Using the calculator the 22 props at 6K rpm and a 1.75 drive gives you 28% slip, and that would be a bunch for a boat that light and twins.

 

[SkiDad]

have you checked compression on these engines ? Have you checked your engine height ? Something doesn't sound right - you "should" be able to plane the boat with one of them. Heck my lowly 125 will plane my boat with 7 people in it (that's about 3000 lbs of boat and people)