1972 F31 PROP/RPM/SPEED

[Lou]

Looking to find some reasonable information. 1972 F31 flybridge with twin 233hp ford 351 mercruisers. Cannot seem to get 4000 RPM, only getting 3000. Where it stands now, I'm assuming improper screws. I currently have 14x14 installed.

What I have done:
- cleaned/scraped bottom free of growth
- timing set to 10*
- both two barrel carbs cleaned/rebuild
- new coils (internal resistor) no external ballast (non point distributer)
- fresh clean fuel supplied to engines
- Tach setup for 8 cyl

I'm topping out at 27 MPH at 3000 RPM (port) 2800 RPM (Stbd), cruising 15-17 MPH at 2400 RPM. Won't spin up past that. I currently don't know my fuel consumption at that speed. IMO not concerned since I'm not reaching higher RPM.

Reading from here and there, I'm getting top speed, just not top RPM.

Can anyone chip in their two cents on this?
- too much prop?
- too much timing?
- not enough fuel being supplied?
- etc...?

Who out there has the F31 with Ford Merc and what is their setup?

[prowlersfish]

Your speed is not to bad , but your rpm is to low . Do you have a reduction gear or a 1 to 1 ? If its a 1 to 1 then you have to much prop for sure .

Are you a Dandy Haven ? It looks like it in the photo , I may know your boat .

[Lou]

I am at Dandy Haven. And correction to prop size. They are 16x16. And reductions are 1.52:1. Guessing the boat ran aground since the port strut is bent. After looking back at pics, I had one with the stamp of 16x16. These cannot be the original props for this boat. Let alone the correct size.

[mikeandanne]

Maybe get the props checked to see what they really are, I recently went thru this ,---props were stamped 16x14,one was 13.5 and the other was just under 14---I had too much rpm and not the same side to side so got them repitched to 16x15--now they look like they should be hung on the wall lol---I think stock props on an f32 were 16x16 or 16x15 probably not much different than yours. Maybe your 16x16 are closer to 17 pitch???---Mike

[gitchisum]

My F32 has twin crusaders with 1.5:1 and I am swinging 16x16's so you very well may have the correct props.

I run about 19-20mph at 2800-2900, I would be pretty suprised if you were getting 27 mph at 3000 rpm. Theorectical at that rpm is 45 mph, with no slip or drag. No way you should be getting 27 mph

I would check the shaft with gun and comparfe to your tachs. I suspect you are actually getting close to ~4000.

[prowlersfish]

Won't hurt to check the tachs , But I suspect you need to knock of a inch or so of pitch

I will be over Dandy haven in the spring to haul out

[Lou]

2500 RPM I was cruising at 22.2 mph (wind dependant). I thought I had got the tach in the shot but I didn't. I'll check the tach's but I'd be surprised if all four are bad. I have specked them at idle with my digital meter and they seem to be on point there. Next trip out I'll give it a second look. Boat was previously owned by a couple who kept a log and they used it. Owner after that, well he said he used it a lot as well. When I got it, timing was set to 38* on one motor and the other was 22*, ballast resister wired inline with internal resistor coils (motors shutting down), carbs had screws approx 4-6 turns out. No way last owner had successful trips.

[prowlersfish]

I had a prop spec sheet for Trojans , I will see if I can find it and see if the F31 is on it . With your info and the speeds your getting ,I still feel your over propped some .

[Lou]

That could shed some light on the situation. I appreciate it!

[prowlersfish]

Lou , I found my prop info But its 1975 so no F31 .But I do have the info on the F32 that is only slightly heavier . are with a 1.52 to 1 gear All these are with a 1.52 to 1 gear For Chrysler the 225hp 16x15 for a Merc 225hp (ford) 16x14 for a Merc 255hp (351 4 barrel ford ) it calls for a 16x15 .For the F32s with 1 to 1 gears the props where 14x10 And don't forget Our boats are heavier then when the left the plant as we add out gear and toys .


Based on that info I do believe your over propped