Over Propped?

[RyanB]

When we purchased that boat, I was not able to get full RPM's on the Crusader 350's, only ~3500 rpms WOT. During the trip home we checked throttle cables, replaced plugs, burnt plenty of fuel to rule out water, changed filters etc. and were still only around 3500 rpms.

So this winter I've yanked the carbs and had them rebuilt. Hoping that they were just gummed up a but from sitting with the previous owner, but wont know until we get it back in the water.

While at the at the storage facility today, I noticed that my props were much bigger than every other boat around me, that is somewhat close in size. I mean, almost double in diameter than the others. So now I'm trying to rule out that my 36 isn't over propped, before spring gets here.

The current props are 4 blade 26 x 22 x 1.25 Nibral. She came with spare props, which I can only assume came stock(?), which are 26 x 25 x 1.75. The trans are model 10-18-012, 2.91:1 Ratio.

I know the majority here are diesel guys, but does anyone know if these props seem too large for this set up? Assuming the 3 blades came stock, we're pretty close in size, would a 4 blade drag down the R's that much?

Side Note: I'm fine with it running at 3500ish Rpms, as it cruises at 16-18 mph, which is pretty much all we're going to get/need. As long as the motors keep running for a few more years, being that they're up there in hours (2000 ish), I'm still happy. I just want to make sure I'm not lugging it when its not needed. Once I rebuild the motors, I'll really worry if not getting full WOT RPM's.

Thanks, everyone.

 

[Fanfare]

SAMS/Hatteras/your owner's manual will list the size props originally installed.

 

[dsharp]

Is the 1.25 a typo? Your wheels are much bigger because you’re running a 3 to 1 gear. You need to increase your rpm so those engines will turn 4200 or somewhere close. An inch of diameter is equal to 2” of pitch. Reducing pitch for each inch will pickup approximately 200 rpm. Keep in mind that these are approximations but after changing wheels out for years then running the boat they’re pretty close. I don’t have any experience with nibral wheels since they weren’t that popular in the early 80’s when we sold our yard. Talk to a prop shop or boatyard in the area and get there thoughts or see if they have a set an inch smaller in diameter than your current wheels. First, make sure the bottom is clean and your tachs are correct. It doesn’t take much growth to kill rpm especially on a boat that is marginally powered.

 

[RyanB]

Is the 1.25 a typo? Your wheels are much bigger because you’re running a 3 to 1 gear. You need to increase your rpm so those engines will turn 4200 or somewhere close. An inch of diameter is equal to 2” of pitch. Reducing pitch for each inch will pickup approximately 200 rpm. Keep in mind that these are approximations but after changing wheels out for years then running the boat they’re pretty close. I don’t have any experience with nibral wheels since they weren’t that popular in the early 80’s when we sold our yard. Talk to a prop shop or boatyard in the area and get there thoughts or see if they have a set an inch smaller in diameter than your current wheels. First, make sure the bottom is clean and your tachs are correct. It doesn’t take much growth to kill rpm especially on a boat that is marginally powered.

Its a little worn, but it looks like 1.25 to me. Here is a picture.

 

[Maurice]

I have a 38' FBDC with 24x27 4 blade props as original in 1973. The 1.25 probably means shaft size. My shaft size is 1.75". Running two 427 V8 300hp. Trans are 3:1 ratio, Paragons. Seems okay to me but don't run at WOT ever. Cruise about 2500-3000rpm.

 

[jim rosenthal]

Sounds like your boat is overpropped but I would have them scanned to see what they really are, before you change anything.

 

[RyanB]

I have a 38' FBDC with 24x27 4 blade props as original in 1973. The 1.25 probably means shaft size. My shaft size is 1.75". Running two 427 V8 300hp. Trans are 3:1 ratio, Paragons. Seems okay to me but don't run at WOT ever. Cruise about 2500-3000rpm.

Yea, I honestly don't know about the 1.25. The shafts are definitely 1.75" diameter. I don't run WOT either, just want to make sure they're running properly.

 

[Play'N Hooky Too]

I'm not an expert, but it looks to me like the prop pitch has been changed from 22" to 21.25". The "X" is where they crossed out the second 2 and re-stamped it with 1.25. (ie: re-pitched from 26" x 22" to 26" x 21.25").

 

[jim rosenthal]

They could have been 1.25 bore to start with, and rebored, I suppose, but I wonder if you can take the taper bore out that far. If you have them scanned, you'll know what you really have, and how close it matches the original spec for the boat.
Also, I haven't looked at a set of wheels in a while, but I don't recall that the shaft size is even on the hub. I think the post above is accurate.

 

[RyanB]

I'm not an expert, but it looks to me like the prop pitch has been changed from 22" to 21.25". The "X" is where they crossed out the second 2 and re-stamped it with 1.25. (ie: re-pitched from 26" x 22" to 26" x 21.25").

This make perfect sense now that I'm reading it.... great point.

 

[MadHatter53]

Put on the spares and see. have you tried to break 3500rpm in neutral ?

 

[dsharp]

The third number is the shaft size. As someone noted they took 3/4” of pitch out trying to pick up rpm. I would make sure that your tachometers are reading accurately. I would do a compression test to make sure that you haven’t done some internal damage with it being over propped. Have you talked to the prior owner to see if they ever reached over 4000rpm?

 

[RyanB]

The third number is the shaft size. As someone noted they took 3/4” of pitch out trying to pick up rpm. I would make sure that your tachometers are reading accurately. I would do a compression test to make sure that you haven’t done some internal damage with it being over propped. Have you talked to the prior owner to see if they ever reached over 4000rpm?

The previous owner didn't have a clue on anything with the boat. He more or less just had it sit in a slip as a floating patio, and rarely took it out. He couldn't recall if he ever had it above idle. I'm trying to get a survey from 2015 to see what that noted for performance, assuming it had these props on it.

If Carbs don't help, I'm going to see what the compression is. I know the motors will need work eventually, but hoping I can push that off a few years.

 

[dsharp]

Having too much pitch can lead to detonation. You can’t hear it on a boat and most older marine engines don’t have knock sensors to retard the timing. The engines that we took apart had several intake valves that were tulip shaped. The detonation was pushing the valves up into the head. See if you can find a set with an inch less in diameter. That should increase the rpm by about 400. Any weight that you can take off the boat will help.