Page 4 of 6 FirstFirst 123456 LastLast
Results 31 to 40 of 51

Thread: Hull speed

  1. #31
    mikef's Avatar
    mikef is offline Registered User
    Location : chesham, bucks
    Join Date
    May 2001
    Posts
    24,868

    Default

    Quote Originally Posted by burgundyben View Post
    Not at all, change from 380 to 480 was a software code load.
    No changes to the fuel system or cooling system? I'm guessing the props were changed too? How do you know the engine was actually producing 100hp more after the software change? In the end that proves what I said. You put more power into a boat and it goes faster but in this case, not by much

  2. #32
    mikef's Avatar
    mikef is offline Registered User
    Location : chesham, bucks
    Join Date
    May 2001
    Posts
    24,868

    Default

    Quote Originally Posted by Hurricane View Post
    Here are some real figures from our P67
    9.5 knots -22 litres per hour per engine - displacement speed
    H, have you checked the fuel consumption at minimum in gear idle speed? My boat does about 7.3kts at idle at about 5 lph per engine, equivalent to about 3.3nmpg. If I increase speed to about 9kts, which is still displacement speed, the fuel consumption jumps significantly to about 1.5nmpg. What I'm saying is that we all know there is a big difference between fuel consumption at planing and displacement speeds but there's also a big difference in fuel consumption between slow displacement and fast displacement speed

  3. #33
    Join Date
    Feb 2008
    Posts
    2,065

    Default

    Quote Originally Posted by mikef View Post
    No changes to the fuel system or cooling system? I'm guessing the props were changed too? How do you know the engine was actually producing 100hp more after the software change? In the end that proves what I said. You put more power into a boat and it goes faster but in this case, not by much
    The repower was done by professional yard with properly matched new props to new engines rated speed. Think the old motors were Ford Sabres with 2,500 rpm ratings not sure of output, but many of these boats had 375's. However for various reasons the new engine installation sign off was never completed. It is real easy to pull off very accurate propeller demand curve off the new motors by taking % engine load in 100 rpm increments.

    As to differences between QSB's Ben is kind of correct. 230, 305, 330, 355, and 380 are one CPL therefore ratings purely sortware calibration. 425, 440 and 480 are another CPL with slightly different performance parts e.g. turbo is common with the 8.25 liter QSC motor. Heat exchanger for the 425-480 is higher capacity and larger viscous vibration damper. Think spec sheets are a little out of whack as I cannot believe 480 is same weight as 230 even within stated tolerance band. Other changes are cylinder head, injectors and connecting rods which have no weight implications. I have looked at the rods and see no differences, have been told that that they are balanced to tighter band which calls up another part #.

    Just to add there is little point in comparing Nelson round bilge hull form with true plaining hull. Resistance curves are akin to WWII German E Boats getting speed out of this hull form can be power hungy. I have been aboard Nelsons on sea trails with Scania V8's and CAT C18 and consumption curve has a real scorpions tail for the last few knots.
    Last edited by Latestarter1; 29-04-12 at 16:50.

  4. #34
    Join Date
    Jun 2002
    Posts
    6,246

    Default

    Quote Originally Posted by SpottyDog5 View Post
    Thanks guys,
    I guess what Im getting too, is on a recent trip she toped out at 24knots, 4 months of bottom growth (copper coated) is this the best I'm every going to get ? perhaps even a couple more, with a very clean bottom.

    When new, she had 2 x 300hp Sabres, now re-engined with 2x 480 Cummins, but there must come a point when no matter what amount of HP you give her, she reaches a max hull speed ?
    I'm very happy with her performance, just curious....
    Having built many Nelsons as working boats and assuming this is indeed a Nelson or closely related, what has been said about hull speed limitations is indeed theoretically correct for full displacement hulls. About 7 to 8 knots at most. The Nelson will operate as a FD hull up to that speed with very little power. However a SD type like this is meant to plane to a degree so when she hits that "wall" more power raises the forward sections and she starts to plane while retaining FD stability in her aft sections. From that point a full planing hull will rise well out of the water and actually require less power to keep her there. A SD hull will not. It will require ever more power to go faster but won't be limited by the square root rule.

    Having said this I would expect a Nelson 44 to achieve close to 30 knots under full power with the engines you have under ideal conditions. I have been on sea trails on a Nelson with a total of 1000 HP at 34 knots. I would be happy in your situation to achieve 28. I hope that answers your question.....

  5. #35
    mikef's Avatar
    mikef is offline Registered User
    Location : chesham, bucks
    Join Date
    May 2001
    Posts
    24,868

    Default

    Quote Originally Posted by Latestarter1 View Post
    Just to add there is little point in comparing Nelson round bilge hull form with true plaining hull. Resistance curves are akin to WWII German E Boats getting speed out of this hull form can be power hungy. I have been aboard Nelsons on sea trails with Scania V8's and CAT C18 and consumption curve has a real scorpions tail for the last few knots.
    Thanks for that LS1. IMHO, there is no theoretical difference between a planing hull and a SD hull. If either kind exceed max hull speed, then they are planing. Period. Then the only issue is how efficient that hull is at planing speed in terms of the speed that hull can achieve for a given amount of power. A typical round bilged Nelson hull was never designed to do 30-35kts which is why it takes such a large amount of power to get it to 30kts compared to a normal variable V planing hull. Again IMHO, the reason why the term SD is used is that many hulls which are now termed SD were originally designed as D hulls and when lightweight high powered diesel engines came along, some D hull builders found that, with a little tweaking, they could make their hulls plane by installing these more powerful engines, hence the term SD. Grand Banks would be an example of this

  6. #36
    Join Date
    Feb 2008
    Posts
    2,065

    Default

    Quote Originally Posted by mikef View Post
    Thanks for that LS1. IMHO, there is no theoretical difference between a planing hull and a SD hull. If either kind exceed max hull speed, then they are planing. Period. Then the only issue is how efficient that hull is at planing speed in terms of the speed that hull can achieve for a given amount of power. A typical round bilged Nelson hull was never designed to do 30-35kts which is why it takes such a large amount of power to get it to 30kts compared to a normal variable V planing hull. Again IMHO, the reason why the term SD is used is that many hulls which are now termed SD were originally designed as D hulls and when lightweight high powered diesel engines came along, some D hull builders found that, with a little tweaking, they could make their hulls plane by installing these more powerful engines, hence the term SD. Grand Banks would be an example of this
    Keith Nelson mitigated the penalty by keeping his hull forms narrow gutted, German E Boat thinking, with a sweet buttock angles in order that hulls could be driven fast without large penalty. Remember a Nelson has taken part in the Round Britain Powerboat Race.

    As usual demand for accomodation has led to original Nelson hull forms being diluted and suffering from middle age spread.

  7. #37
    Join Date
    Mar 2002
    Posts
    9,759

    Default

    Quote Originally Posted by mikef View Post
    IMHO, there is no theoretical difference between a planing hull and a SD hull. If either kind exceed max hull speed, then they are planing. Period.
    Well, if you put it this way, also if you install a stupid amount of power on a full D hull, you can bring it on the plane.
    But this doesn't mean that there aren't theorical differences.

    Even between pure P hulls you can find radical differences: those designed for real speed typically have steps, notched transom, padded keel, and other tricks which you'll never see on any plain vanilla P cruiser.

    In other words, if you mean that there isn't a black and white rule to distinguish between D/SD/P, you're right of course.
    But in practice, the differences between P and SD hulls are more than what catches the eye.
    And it shows, where it really matters - i.e. in the rough stuff.
    Last edited by MapisM; 30-04-12 at 09:52.

  8. #38
    mikef's Avatar
    mikef is offline Registered User
    Location : chesham, bucks
    Join Date
    May 2001
    Posts
    24,868

    Default

    Quote Originally Posted by MapisM View Post

    In other words, if you mean that there isn't a black and white rule to distinguish between D/SD/P, you're right of course.
    But in practice, the differences between P and SD hulls are more than what catches the eye.
    And it shows, where it really matters - i.e. in the rough stuff.
    Of course I understand that. My point really was that people use the words P and SD as if P and SD boats were somehow completely different but in terms of their ability to exceed D hull speed, they're not. They both plane.

  9. #39
    Join Date
    Jun 2007
    Posts
    2,702

    Default

    Iím sorry Iím not able to contribute much to this thread, but I am finding very interesting, my thanks to all involved.
    New website - www.rapidmarine.co.uk

  10. #40
    Join Date
    Mar 2002
    Posts
    9,759

    Default

    Quote Originally Posted by mikef View Post
    My point really was that people use the words P and SD as if P and SD boats were somehow completely different but in terms of their ability to exceed D hull speed, they're not. They both plane.
    Yup, agreed.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •