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  1. #1

    Props? Fresh vs. Salt : Performance Difference

    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.

  2. #2

    Re: Props? Fresh vs. Salt : Performance Difference

    What is the result of the scan and recon of the old props? Are they really the same as they are marked?
    Eric
    41TC 1966 Hull #53 "Requisite"
    Kent Island, MD/Ft. Lauderdale, FL

    "Though she creaks - She holds"

  3. #3

    Re: Props? Fresh vs. Salt : Performance Difference

    Quote Originally Posted by SeaEric View Post
    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.

  4. #4

    Re: Props? Fresh vs. Salt : Performance Difference

    It will be interesting to find out what they are vs what they're stamped. In my experience, sometimes they're not even close!
    Eric
    41TC 1966 Hull #53 "Requisite"
    Kent Island, MD/Ft. Lauderdale, FL

    "Though she creaks - She holds"

  5. #5

    Re: Props? Fresh vs. Salt : Performance Difference

    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
    Mis b' havin
    1972 58 yachtfish
    58yf352
    pass christian, ms.

  6. #6

    Re: Props? Fresh vs. Salt : Performance Difference

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

  7. #7

    Re: Props? Fresh vs. Salt : Performance Difference

    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
    Last edited by Captain Ralph; 08-29-2021 at 10:20 AM.

  8. #8

    Re: Props? Fresh vs. Salt : Performance Difference

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

  9. #9

    Re: Props? Fresh vs. Salt : Performance Difference

    Quote Originally Posted by Captain Ralph View Post
    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.

  10. #10

    Re: Props? Fresh vs. Salt : Performance Difference

    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,
    Bill

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