I've decided to buy Dick Morland's spare 23x23 props, which led me to wonder what the ideal size would be for my boat. 

I read this from Jim:

As to your question as to the correct propeller size, the 1966 through the 1968’s came with 23” x 22” x 4 blade Michigan Dynaquads.  The 1969 through 1972 models came with 23” x 21” x 4 blade Michigan Equiquads.  These were used on all installations with the Detroit Diesel 8V53N and the Cummins V8-300M engines riding on 1.5” shafts.  Difference in the pitch is attributed in that the early models only carried 275 gallons of fuel (the 1968 model was upgraded to 350 gallons) and the 1966 and ‘67 were equipped with the short fiberglass top.  The 1968’s were equipped with the extended wood hardtop.  The 1969 to 1977’s had a very heavy extended fiberglass hardtop.

I also read that Chris tried a couple of sizes and settled on 23x22 (I think), running n60 injectors.

I don't know exactly which injector I have, only that the previous owner told me they are "one size up" which makes me think n55. The PO also increased the props to 24x23. I can only think this was for a faster dinner cruise speed.

My no load speed is 2800 and 2850, so I'm thinking the high speed governor needs to be adjusted (or is the lower number correct for a larger injector?). Even so, it feels like the engines are working hard to move the boat if I try to bring the boat up on plane (which it doesn't do). I can hit around 2400 rpm and 16 miles per hour.

My boat is heavy @ 43,000 lbs. Many of the 1968 and older are closer to 35,800 lbs. Based on Jim and Chris' comments, I have to think that my current 24x23 setup is completely wrong.

We typically run @ 1300rpm and 9 mph. I realize I will drop speed at a smaller prop size for a given rpm, but I would rather not overload the engines and would love to see this boat get on plane. It sounds to me like I might want to re-pitch Morland's 23x23 to 23x22. That's possible, right?

The engines run great, with no smoke to speak of (except when I pushed them up to WOT, but that mostly cleared quickly after a couple of minutes, likely due to our typical slow cruise speed).

I know I touched on this last winter, but now that I have new props coming, I'd like to re-visit and get ideas from those that have been through this. I'd rather get the size right the first time so I don't have to pull the boat to try another.

Thanks!

Steve.

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Steve,

Just as a data point.  We have a 1967 47 with the short hardtop, no bridge, no aft enclosure, running Volvo Penta 8.1 gas engines.  Boat weight from the travel lift is 32,000 lbs.

-Darin

Interesting that the boats can vary by 11,000 lbs! The fuel and hardtop sure make a difference.

After re-reading the 9 page thread from last winter, I'm pretty sure the answer to my question above is to re-pitch the 23x23 props to 23x22. Jim/Chris/others, please confirm or deny this as a good idea. Also, if any of the data points above (mostly just the no load rpm is new) change anything, please let me know.

"Rule of Thumb":  A boat engine should turn the Max rated RPM for the engine at WOT with a full load of fuel and passengers. This would be considered the correct Prop size.

So what is the Max RPM recommended for your Detroit 853's?

Next, what RPM to they turn now at WOT? 

If they do not reach Max recommended RPM, they are "over propped". If they exceed the recommended Max RPM, they are "under propped".  

What do your 853's turn at WOT now? What is the Max rated RPM?

I agree - so I'm looking for guidance such that I can make one change to what I have instead of hauling the boat several times this summer as I try different sizes. WOT should turn 2800 and I believe I saw that no load should be 2970. So I am definitely over propped. 

Joe Marino said:

"Rule of Thumb":  A boat engine should turn the Max rated RPM for the engine at WOT with a full load of fuel and passengers. This would be considered the correct Prop size.

So what is the Max RPM recommended for your Detroit 853's?

Next, what RPM to they turn now at WOT? 

If they do not reach Max recommended RPM, they are "over propped". If they exceed the recommended Max RPM, they are "under propped".  

What do your 853's turn at WOT now? What is the Max rated RPM?

I would suggest is to first check which injectors are in the engines. This will give us a up baseline then you can start to make some informed decisions. At this point engine to is an unknown.

Can I check the injector type without removing it? Can I see a label by just removing the valve cover? I plan to replace the valve cover gaskets this spring, so I could look then. The PO also left a spare injector on the boat so I can look at that (assuming it is the same as installed).

I have to believe I have N60 injectors. I was told one size up and reading the chart of injectors vs engines the N55 I mentioned above doesn't exist.

That said, would some extra HP mean that spinning a 23x22 would be a bad thing? Or would I just not be taking advantage of the added HP? I can see that more HP would allow me to run a larger/higher pitch prop, but can I have too much HP for a smaller prop? I wont be getting back to the boat for a couple of months so I'd like to understand as much as possible before then. 

Ronald Zick said:

I would suggest is to first check which injectors are in the engines. This will give us a up baseline then you can start to make some informed decisions. At this point engine to is an unknown.

Yes, you just remove the valve cover and look at the injector. Every injector has a tag specifying the cup size.

The N55s very much exist. Not sure what chart you where looking at but the 55s should be listed. The 55s were/are the normal injector for the 8V53. The 60s are often dropped in and push the SHP to 300. If you do have the 60s, I would expect a slight haze at WOT.

Your no load engine speed is correct and I definitely think you are over propped.

Your idea of de-pitching Disk's props is sound and should put you in the sweet spot but at the cost of a slightly noisier cocktail speed.

Hello Everyone:

Our 1971 47' Commander had the following settings on its 8V53N engines with N50 injectors (no typo):

1) Michigan Wheel 23"x22" bladed props riding on 1.5" shafts;

2) No load speeds set to 2970 RPM;

3) Underway WOT 2800 RPM;

4) No flybridge.

Be very careful about getting too much power out of these engines because the Twin Disc 506 Marine Gears are at this point at the limit of power input into them.  Much more, BANG!!

Thanks Jim and Ron. I’m hoping for no transmission destruction. I didn’t install the larger injectors, but hope to be able to safely live with them.
It’s settled then, I’m going to rework the props to 23x22. I’ve seen different definitions for cup type - is there a specific type or size cup that should be used?

Steve,

Cupping acts like adding another inch or so of pitch. If you repitched to 22" and had them cupped, they would perform like 23 x 23 !!  :-)

Ah..didn't know that. I thought they were all cupped, but to different amounts. So no cup. Thanks for the clarification Dick.

Dick Morland said:

Steve,

Cupping acts like adding another inch or so of pitch. If you repitched to 22" and had them cupped, they would perform like 23 x 23 !!  :-)

I'm bring this thread back alive...

When I installed these new props, I left them 23x23 and had one of them re-worked to make it true. I didn't leave them on long enough to check the WOT, because they had no bite around the dock. I mean none. Why would that be when everyone else is running 23x21 or 23x22? Does having an extra inch of pitch make that happen? My marina says going down an inch (24 to 23) is what did that, but I can't believe all the other 47 owners are using a lot of throttle around the dock to maneuver the boat. Could it be because our boat is so heavy at 43000 lbs?

The marina's suggestion is to re-pitch my 24x23's (which are great around the dock) to 24x21 or 24x22. They suggest that the slow speed bite will be the same as now, but of course WOT should go up 150-300rpm.

Thoughts on this?

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