Props? Fresh vs. Salt : Performance Difference

[cww]

When I got my boat it had the factory 28x31 props on it and would do 16.5 knots at +-/ 2000 in salt water. I had the engines gone through and the air side of the intercoolers from the looks of it had never been cleaned. Secondary fuel filters were ancient. The timing got adjusted as well as the racks. Cooling system cleaned. I picked up a lot more power. After that I got 18.5 knots at +/- 2170 in salt water. After the work, no more smoke, no more overheating, and they sound better.

Based on this, I decided to de-prop to 28x29 to get both up to 2300. The boat did not come to me with a spare set of props, and they were slightly bent from getting dusted in NC on the trip down to florida, so I ordered a brand new set of dyna quads from Michigan wheel intending to get the original set fixed and keep them as spares. The new ones came in and I went on my test run yesterday. The boat lives on the St. John’s River which is freshwater. Full fuel, full water, 2 people onboard. It would barely get on plane, took forever to get up, and maxes out at 16.5 knots at 2150 port and 2230 stb. Trying to cruise at 1900-2000 it falls off plane. Full tab on all of these runs. This was obviously not what I was hoping for. Based on the math and prop calculators I should have more power, more speed, and it should spin up to 2300. It has digital flowscan tachs, wot in neutral produces 24-something port and 2500 starboard. Based on that I don’t think it’s the tachs or governors.

I was bitching, and the marina owner told me it’s the fresh water. He says heavy boats always lose speed vs. in salt water. That was news to me. I’m pretty unhappy. My wallet is much lighter just to have the same performance and still can’t make rated rpm, that I had before I messed with it. I wish I’d just had the props straightened and left them alone at this point. I don’t get how I dropped 2 inches of pitch and basically nothing changed. Wtf?

I guess my questions are, is that really a thing with freshwater? 2) how accurate are new props, should I send them off to get scanned and tuned? I figured being new they’d be ok. But maybe not. 3) how big of a difference does tankage make on performance on these boats ? Mine is a 1980’s model so it has the larger fuel tanks. These are the only variables.

 

[SeaEric]

What is the result of the scan and recon of the old props? Are they really the same as they are marked?

 

[cww]

What is the result of the scan and recon of the old props? Are they really the same as they are marked?

That's a good question. I will find out soon they're getting sent off to dominey this week to be scanned and straightened. They were stamped 28x31. The old ones had more bite around the dock and the boat needs another +/- 100 rpm to do 9 knots with the new ones, so there is a difference. Exactly what that difference is, idk.

 

[SeaEric]

It will be interesting to find out what they are vs what they're stamped. In my experience, sometimes they're not even close!

 

[bigbill]

have the new props checked also. i have a friend that got a new set of wheels and they were not even close to what the manufacturer said that they were

 

[Flying Dutchman]

Tankage has a large effect on a planing craft, adds alot of weight.

 

[Captain Ralph]

First, Having run near anything that floats out of Ortega, the freshwater line is BS.
I can imagine who and why this comment was made. Hopefully you learned a lesson here.

The important issue is this area is shallow.
On your chart, way east of the Saddler Point, south of Green 3, is a 20 foot deep stretch.
Some deeper water to run in is out of Doctors Lake..

Past downtown towards the Heart bridge is salt water and deep also.

Second, Was Hatteras or Sam's involved with the Mfg and specs for the new wheels?

What fuel pressures and smoke colors are you reading while trying to get on plane?

Please keep us updated

 

[jim rosenthal]

I would agree with having the old wheels scanned and measured to find their actual diameter and pitch and cup.

 

[cww]

First, Having run near anything that floats out of Ortega, the freshwater line is BS.
I can imagine who and why this comment was made. Hopefully you learned a lesson here.

The important issue is this area is shallow.
On your chart, way east of the Saddler Point, south of Green 3, is a 20 foot deep stretch.
Some deeper water to run in is out of Doctors Lake..

Past downtown towards the Heart bridge is salt water and deep also.

Second, Was Hatteras involved with the Mfg and specs for the new wheels?

What fuel pressures and smoke colors are you reading while trying to get on plane?

Please keep us updated

No smoke of any color trying to get on plane. The only exception is if I’ve been running around at low speed for a long time, then I will get the oil burnoff from wet-stacking that builds up in the airboxes. That clears in +/- 30 seconds, and after that it won’t smoke again until I run it around for days at low rpm. That’s been pretty standard on every DD I’ve ever had. The engines have 1800 original hours and I think are not yet to the stage where they show signs of abuse. None of the former owners ran it fast. Which is lucky because it won’t turn up to rated rpm.

The river is pretty shallow here, you’re right about that. I usually have +/- 2 feet under the boat exiting the Ortega river. I did make sure to go into deeper water around 15+ feet when I was seeing what it would do. That’s about as deep as it gets unless you go over past downtown. They don’t dredge past that area for the oil barges anymore ever since that power plant in Sanford switched to natural gas, so that’s all you get. The depth was comparable to when I first got it back after the engine work in Virginia though. That was also in 15 or so feet. It hit that speed again with no problem outside of kings bay NS when I ran it up, that was in 40 feet.

I asked this question before I ordered the props but nobody knew for sure, what the cup on these props is? The owners manual says the size but nothing about cup. I should have had the original props scanned before ordering the new ones, that was dumb, I was just tired of the vibration. If the 28x31’s had cup on them and now I have 0 cup I guess that could explain it too.

Open to whatever ideas anybody has. I am mystified as to why the boat will produce the factory performance numbers but only when overpropped, and equally mystified at how I can drop 2” of pitch and have almost the same numbers.

 

[31FlyCruzr]

This is always a difficult issue. you said "Based on the math and prop calculators I should have more power, more speed, and it should spin up to 2300." The power is what your engines have it does not change.

Dropping pitch by two inches will lower boat speed not increase it. To make the same boat speed you will need more RPM. If you just use the pitch to calculate speed, you should lose about 1.3 knots of speed going form 31 to 29 at 2000 rpm.

Salt Water is more dense that fresh water so the boat will displace more freshwater, technically it has an impact. It is about 2.5%. For a 30000 pound boat it is equivalent to adding about 100 gallons of fuel

One prop item not discussed is the material, Bronze props are weaker and thicker than NiBral, and if you switch material from Nibral to Bronze you may loose a little due to the added drag of the thicker wheel.

You also made a point that the new prop test was full tanks, was the other data full tanks?

I do not have any idea why you are not making rated engine speed after dropping pitch, unless you can not get over the top and truly on plane. On plane does unload the engine once you are up and allows the engine to increase rpm to WOT.

Regards,

 

[cww]

This is always a difficult issue. you said "Based on the math and prop calculators I should have more power, more speed, and it should spin up to 2300." The power is what your engines have it does not change.

Dropping pitch by two inches will lower boat speed not increase it. To make the same boat speed you will need more RPM. If you just use the pitch to calculate speed, you should lose about 1.3 knots of speed going form 31 to 29 at 2000 rpm.

Salt Water is more dense that fresh water so the boat will displace more freshwater, technically it has an impact. It is about 2.5%. For a 30000 pound boat it is equivalent to adding about 100 gallons of fuel

One prop item not discussed is the material, Bronze props are weaker and thicker than NiBral, and if you switch material from Nibral to Bronze you may loose a little due to the added drag of the thicker wheel.

You also made a point that the new prop test was full tanks, was the other data full tanks?

I do not have any idea why you are not making rated engine speed after dropping pitch, unless you can not get over the top and truly on plane. On plane does unload the engine once you are up and allows the engine to increase rpm to WOT.

Regards,

Both sets of props are bronze, the originals and the new ones I bought. I understand dropping pitch reduces speed all things being equal, but all things weren’t equal, I was running 300rpm shy of rated rpm. Meaning a 6% reduction in pitch would (well - should have if something wasn’t wonky) produce a 14% increase in rpm which results in more thrust. More rpm should also = higher boost, higher airflow, and more power for the same fuel burned. Lugging the engines is both inefficient and bad for them. The run after the engine work before the trip to florida was before I filled the tanks, they took about 200gal to top off, so that first run was with mostly full tanks. New props put on for my disappointing run, the tanks were very full, I had topped all the way off at Fernandina Beach, it took 500+ gallons. When it did 18.5 knots into an oncoming current at kings bay it was shortly before Fernandina, so each tank only had 100 gallons or less in it at that point. Not much difference rpm wise though, it still only turned up to +/- 2100. Boat was much faster though. Just trying to get it to where they turn up to 2300 under full load.

 

[cww]

I'm dumber than I thought.

I had bought new 28x29 dynaquads and was obviously very unhappy when I put them on and nothing changed.

I have been waiting on this DD guy from outside of Savannah to come do the valve adjustment, injector timing, racks, general tuneup on my engines, since I got the boat. He finally managed to fit me into the schedule. I didn't think there was anything wrong with them, they run really well. Just wanted to catch up the maintenance - this stuff is supposed to get done at intervals but nobody ever does because DD's will still run fine with everything in the world wrong with them.

Turns out on one governor the piston and cylinder that runs back and forth inside to control flow were scored to sh*t and sticking up. He honed/polished those and got it working properly. I had 3 dead injectors. They're original to 1981 so a new set of 16 is on order from a shop he recommended that tests and puts them into matched sets before sending them out. There is still a bunch of stuff left to be done.

Since the boat was planing and doing 2150rpm even with all of that going on, it's starting to look like my problem was not the props. Expensive mistake - I should have waited to pull the trigger on the new props.

 

[jim rosenthal]

We've all done it, or something like it. You've got company.

 

[Captain Ralph]

Looking forward to your final run data?

 

[OVERBOARD]

It is not a bad idea to have spare props for your boat. 28x31x2 are not common and your looking at 70-90 days to manufacture. I understand you may have the wrong size, but your original 31 might be primary and new ones your get home props. I lose a couple knots when full fuel and water are on board. Run diesel kleen and keep 1/2 to 3/4 tanks and you will be fine.

 

[cww]

Looking forward to your final run data?

This is almost comical it’s so bad.

This guy is allegedly an expert (recommended by several people on the Detroit diesel FB forum). Waited weeks and weeks to get on his schedule. Comes down, says he only has one day because he has to be in Hilton Head for another job tomorrow. He and his partner work literally all night. Replaces 16 injectors, allegedly does a bunch of tuning.

Take the boat out. Port now has a consistent grey smoke and hunts badly at idle, starboard idles at 650, 100 higher than port, which shakes the boat, also doesn’t respond to stay at 650 when you go in/out of gear, it drops down to 500. The other does the same thing. Makes handling around the dock really unpredictable because I have to use the throttles to compensate or I don’t know how much thrust I’m going to get. He broke - this is a list - my port engine room door latch (missing along with the screws - says his helper did it he doesn’t know where it is), my starboard racor filter ($600 it arrives this week), tried to mount the access panels on the wrong sides so the screw holes didn’t line up and neither fits properly, stripped the screws so at least I can’t get them back off to fix them without bringing my grinder wheel and fucking up the wallpaper. Broke a fuel fitting on the spin on filter, the only way he was able to finish the job at 4am was I let him go take the filter fittings off the detroits in my other boat that’s one dock over at the same Marina (its for sale, buyers will love it when it doesn’t run). Carpet stained, greasy handprints everywhere. Underway there’s a miss in one of the engines that’s so bad between 700-800 the whole boat’s like a 1970’s vibrating motel bed. Performance is worse, I lost power at WOT, the boat does 13 knots instead of 16 now.

I’m still processing all of this. The anger hasn’t even set in yet.

Obviously I need someone else.

 

[dar636]

Ridiculous. I have been there more than once unfortunately.

 

[Finalee]

The realy sad part of this is those guys are likely some of the few people working. Due to demand they can get away with substandard work.

 

[cww]

The realy sad part of this is those guys are likely some of the few people working. Due to demand they can get away with substandard work.

I asked around and there’s an old school diesel shop that’s a detroit parts dealer and highly recommended, they’re local to me. I’m going to talk to them tomorrow. They once handmade gaskets for me when detroit was out of them, the guy at the detroit marine parts counter told me to try them because whenever they’re out of something the other place usually has it. I didn’t realize they did service work, but they do. I think I’m in ok hands at that point. I guess the question now is what else is broken that I don’t know about yet. Just the stuff I’ve seen they broke is $1k plus. Neither engine is right. They took my original injectors with them as cores, so that ship has sailed. What a nightmare.

This whole boating popularity explosion due to COVID thing has really made it difficult to get work done. I’ve had a boat since I was 8 and never seen anything like this.

 

[ingredientmasters]

BIG YES! I have purchased several boats in Florida and brought them home to Cincinnati and I always lose speed when I hit fresh water on the Black Worrier River (fresh water) the salt in the water being more dense gives the boat more lift. Sorry but your wallet won't like your fuel bills either.

Scott
50ft.Sport Deck second Hatteras! 36th boat.