Twin shafts with different prop pitch

[Jeff.P]

Hi, I'm working on the running gear of my newly restored Island Gypsy 36. It has twin Ford Lehman Sabre L225 engines (intercoolers bypassed so 180hp now) and PRM 402 transmissions. Had the original 3 blade props touched up by a well respected local guy who mentioned they were different pitches. We figured it was related to a reverse gear on a transmission and didn't think much of it. In the water this past summer it was pretty noticeable. My starboard engine easily runs up to WOT of 2600 where as my port tops out at 2200 RPM. I did a simple speed test with one in neutral and the other at 1000rpm and found a solid 1/2 knot difference in boat speed between the engines. Since the intercoolers are bypassed I wasn't expecting to make the full rated RPM's and only intend to run the boat for extended periods at 8-9 knots (1200-1500rpm). Is it normal to run props with that much difference between them? Should I have one of them re-pitched to match the other? I'm 90% sure both transmissions are stamped 2:1 but will double check next time I'm at the boat. I'm leaning toward re-pitching the starboard to match the port but thought I would reach out to people who are more knowledgable first!

 

[jgwinks]

Sounds like a previous owner damaged one and got a deal on one that (wasn't) "close enough". I'd repitch one, a prop shop should be able to do at least a couple inches. Which one I guess would depend on whether you intend to put the intercoolers back on line.

 

[Comodave]

How much difference in pitch is there? We had the same engines in our last boat and I also bypassed the aftercoolers.

 

[Jeff.P]

I'm not sure how different the pitch is the guy at the shop didn't elaborate when he mentioned they were different. Sounds like my best bet is to take them both off and get the shop to either increase the pitch on the starboard or re pitch them both to somewhere in the middle depending on how much they are out by.

 

[Comodave]

I would run the boat up to WOT and see what the max RPMs are. Whichever engine comes up to WOT properly, or close to it, then have the other prop repitched to the pitch of the correct one. If neither engine reaches proper WOT, very common, then go with the closest one and work from there. A good prop shop, maybe not the one that didn’t specify what the pitches are, should be able to tell you how much more of a change is needed to get to proper WOT. And you should probably get a photo tach and use it to verify that your helm tachs are correct. Photo tachs are cheap, probably under $40 or $50. Then you know that you are working with correct RPMs.

 

[SteveK]

Verify the pitch(s) and also the trans ratio(s) and ID which is SB & Port

 

[psneeld]

Do you plan on cruising faster than 7-8 knots?

If not and it isn't annoying having the throttles different to avoid the boat veering than I wouldn't even worry about it.

Also if you can get the engines in sync enough that they feel fine and you don't have any steering issues, again I wouldn't worry about it.

I would get one pitched to the other at some point when the prop(s) are being pulled for a different reason.

Correctly propped or overpropped a little, again if not cruising more than 7-8 knots, either would be fine to me.

The OP noted the prop shop "touched them up a little"....well if the pitch was properly stamped on the props, why would the shop make a note of it or specifically say what they were? Now if a specific report was supposed to be done on each prop....different story.

 

[Jeff.P]

I'll elaborate a little on the prop shop haha. So we have a large commercial lobster fleet here and this is the prop guy everyone swears by. He has been working on props for 30+ years but it is literally a small shop in the woods where at any given time (including morning) there are 3-5 fisherman hanging out in the shop drinking rum and chain smoking. Its almost hard to see through the blue haze of smoke and there is a dog that is comparable in size to a bear....

He is really good at fixing and balancing bronze props and great to deal with. Not the kind of shop thats going to do calculations or print a report for you lol.. The original reason I went to this shop was I was chasing down a vibration on the port side. Boat was purchased off ebay in CT about 5 years ago site on seen as a scrap or complete rebuild project (15-17yrs of dry storage!). I have no history on it and am now about 2000hrs into the project. She is beautiful now and all the systems that have been rebuilt are working great. Just in the fine tuning stage now this winter. I have the vibration narrowed down to the shaft. I wouldn't say I have much skills or knowledge and have made lots of mistakes along the way haha.

Props don't have any marking but are in fairly good shape. Since I have to pull that shaft anyway I think I'll get the prop pitch measured on both then decide which way to go. I do like the idea of verifying the tachs are reading correctly but that will have to wait until boats back in the water next season. With WOT at 2200 and 2600+ I think I'm split (WOT supposed to be 2500rpm). I prefer the way the higher pitch prop grabs the water and runs but since I have no intention of connecting the after cooler I wonder if its a little over proped. The other engine doesn't seem to load up enough as it reaches/passes rated WOT rpm really easily. Again should have verified the tachs but the feel and sound does line up with them. Leaning towards increasing the pitch in the starboard to match the port but thinking of splitting the difference and doing both since the engines are down 45hp. Boat runs really nice between 7.5 and 9.5 knots I mostly run at 8-8.75 but don't have many hours on it yet. Only time it seems to need the extra power is in a big choppy following sea. If its not up to 1800-2000rpm its impossible to keep the bow pointed the right way in a 4-5' breaking chop.

 

[PM]

I know cost is probably a main factor here but.. wouldn't it be nice to buy two new props and keep the present ones for spares? I love the fact that under my settee there sits a spare prop.

pete

 

[jgwinks]

I know cost is probably a main factor here but.. wouldn't it be nice to buy two new props and keep the present ones for spares? I love the fact that under my settee there sits a spare prop.

pete

My 1973 Concorde 41 came with a spare set as standard equipment. Never heard of that before. They came in handy for the previous owner, when I got them out of storage one was mangled up pretty bad, he just never bothered to get it fixed. That was the first thing I did to the boat.

 

[Comodave]

I took the props on our last boat to a prop scan shop. Found out that 1 blade on each prop was way out of pitch. They fixed them and took .5” pitch off each prop. They gave me printouts of each prop before and after showing the pitch on 4 or 5 places on each prop as they turned the prop. It made a noticeable difference in vibration. I would recommend a shop that has prop scan equipment. They scanned the props for free hoping to get the work to tune them up. They have probably found that most props are out of wack so by doing the scan for free they get more business when you see how bad your props are.

 

[rgano]

How much do you know about the condition of the PRM 402 transmissions? While you are deshafting would be a good time to ensure all is well with them. There isn't some kooky thing like a difference in the internals of the two of them as was the case with my Borg Warner trannies behind twin FL 120s which required an inch difference in the pitch of the two props. One tranny had a ratio of 1.91:1 and the other had 2.01:1 as I recall. There was no difference in the ability of either engine to reach WOT.

 

[SteveK]

Rich, my twin 120 has SB 1.91:1 and Port 2.1:1 and the props are Port 24x18 & SB 24x17, so same 1" difference and both engines ran at same RPM's in tune by ear and one set of gauges, the other set was off 150-200 rpm.

 

[rgano]

Rich, my twin 120 has SB 1.91:1 and Port 2.1:1 and the props are Port 24x18 & SB 24x17, so same 1" difference and both engines ran at same RPM's in tune by ear and one set of gauges, the other set was off 150-200 rpm.

Looking at your ratios, I think my port may have been same as yours rather than the 2.01:1 I mentioned. Props were same as yours. My two engines ran at same RPM at cruise with never a tendency to turn in one direction or the other with the rudders centered. The OP has a drive line problem for sure and not just the prop pitch difference. His trannies may indeed be stamped with same ratio, which would lead me to put identical props on the shafts after shaft straightening to see where things settle out. If the trannies are supposed to be the same ratio, I'd re-pitch the more aggressively pitched prop back to the same pitch as its mate and see where things settle out.

 

[Comodave]

First you need to know where you are starting from. Without knowing what you have you are just throwing stuff on the wall so to speak. Go to a prop shaft that will scan them and give you printouts of the props as they currently are. Then you have a baseline to start from, not just guessing. Unless you have a lot of money to just try stuff. Find out what you have and then you can correlate it to engine RPMs and make informed changes. When I had the props rebuilt for my last boat it was $800 each. So I didn’t want to make the wrong decision and have to do it again.

 

[PM]

Way back when many boats in my area were powered by twin 327 Chevrolet gassers it was pretty common to replace one failing 327 with a 350 Chev. Pretty much the same engine but the 327's were getting rare, especially with the camel back heads. Some people changed the pitch of one of the props, some just let it go and figured eventually the other 327 would be replaced by a 350.

pete

 

[Patrick4002]

on my previous bout i had a 1 prop replaced after getting on a sandbank, the suplier told me the new prop was more effecient so the throtle would be a little less on the new prop to go strait, so resulting the sb engine has to run 100 rpm more then the bb engine to go strait, i used the boat for another 5 years without problems, just the handels where a litle different, mostly i even dident need to do a 100 rpm different.
my engines where 180hp detroit diesel

 

[Greg QS]

This is simple
Both prop MUST match
this will split the load evenly to both engines. fuel burn and load will even out. Check out sbma.com Props move boats engine turn props.

 

[kapnd]

I would be concerned that neither prop may be correct!
Work with a reputable prop shop, provide them with the vital statistics of the vessel and your expectation of running speed.
Vic Prop and boatdiesel.com have calculators you can use to get a ballpark number yourself.
Exhaust gas temperature also tells much about how well your motor and prop get along.

 

[SteveK]

This is simple
Both prop MUST match
this will split the load evenly to both engines. fuel burn and load will even out. Check out sbma.com Props move boats engine turn props.

Yes simple.
If both props are the same and are turning at the same RPM and the both transmissions are the same because they are turning the same direction then both engines will be at the same RPM. The boat goes straight.


The transmissions are the same, right?

If they are not then the engines are at a different rpm. OR if the the props are different pitch they can compensate so both engines remain at the same rpm so the boat can go straight. rudders are centered.

 

[C lectric]

This is simple
Both prop MUST match
this will split the load evenly to both engines. fuel burn and load will even out. Check out sbma.com Props move boats engine turn props.

THe site name is

www.sbmar.com