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  1. #11

    Re: Props? Fresh vs. Salt : Performance Difference

    Quote Originally Posted by 31FlyCruzr View Post
    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.
    Last edited by cww; 08-29-2021 at 11:24 PM.

  2. #12

    Re: Props? Fresh vs. Salt : Performance Difference

    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.

  3. #13

    Re: Props? Fresh vs. Salt : Performance Difference

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

  4. #14

    Re: Props? Fresh vs. Salt : Performance Difference

    Looking forward to your final run data?

  5. #15

    Re: Props? Fresh vs. Salt : Performance Difference

    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.

  6. #16

    Re: Props? Fresh vs. Salt : Performance Difference

    Quote Originally Posted by Captain Ralph View Post
    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.

  7. #17

    Re: Props? Fresh vs. Salt : Performance Difference

    Ridiculous. I have been there more than once unfortunately.
    Matt
    1976 58' YF, Hull #410
    "Time Out West"

  8. #18

    Re: Props? Fresh vs. Salt : Performance Difference

    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.
    Fred
    31 Tiara Open
    "Escalation"

  9. #19

    Re: Props? Fresh vs. Salt : Performance Difference

    Quote Originally Posted by Finalee View Post
    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.

  10. #20

    Re: Props? Fresh vs. Salt : Performance Difference

    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.

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