Hull speed

[Latestarter1]

I'm now feeling inadequate :-
http://www.dalenelson.co.uk/classic-45/specification.asp

Roy,

Merely words on paper.

I note #'s are almost two tonnes lighter with clean bum etc.

Cannot remember if we discussed this before but are you seeing between 3,460/3,480 @ WOT??

On the back of a fag packet @ 20 knots you should be pulling a tad over 300 kW out of your motors. However when I plug in 24 knots using same hull resistance #'s power requirement rises by extra 100 kW just for extra 4 knots. Just the way these Nelson hulls are.

The other issue is your engines were never signed off. For example, no clue as to simple matter like adequate ventilation leading to excessive turbine inlet temperatures which causes performance to go off the edge of the cliff. Ventilation provided for previous motors unless modified will clearly be inadequate. The surveyor does sound like a bit of a nob head regarding engines, however seems to part of their DNA.

 

[burgundyben]

I'm guessing though that the 480hp QSBs were heavier engines in which case there is a self defeating element to fitting larger engines

Not at all, change from 380 to 480 was a software code load.

 

[SpottyDog5]

Morning Paul,
Like a plumb I never looked at the rpm, just had the throttles hard down.
I'm not sure on the ventalation,seems ok,with large apertures and two noisy blowers, I guess I could do some performance trails with and without the engine room door open ?
What I really need is YOU, when do you fancy a nice trip down to the sunny south coast ?

 

[Latestarter1]

South Coast.........Sunny!

 

[SpottyDog5]

South Coast.........Sunny!

I know, I know, was cringing as I typed it.....
Just need to come up with a cunning plan to incise you down do you like beer ?

 

[Latestarter1]

I know, I know, was cringing as I typed it.....
Just need to come up with a cunning plan to incise you down do you like beer ?

Actually no!
However bottle or maybe more of red wine, Burgandyben will give clue to how much I bunker.

 

[SpottyDog5]

Ill treat you to a wine box.... Country manor or Rosemount castle, your choice?

 

[vas]

Actually no!
However bottle or maybe more of red wine, Burgandyben will give clue to how much I bunker.

LS & SD,

don't really want to interfere, but LS, once you finish with the foul weather you have up there, sort out SD and fancy some decent sunshine, I'd love to host you down here for a few days!

I'm talking proper sun, a few crates of red and lots of fish or meat or whatever you fancy

cheers

V.

Don't want to go too much OT, so a few ventilation Qs to come on MiToS thread next week

 

[SpottyDog5]

LS & SD,

don't really want to interfere, but LS, once you finish with the foul weather you have up there, sort out SD and fancy some decent sunshine, I'd love to host you down here for a few days!

I'm talking proper sun, a few crates of red and lots of fish or meat or whatever you fancy

cheers

V.

Don't want to go too much OT, so a few ventilation Qs to come on MiToS thread next week

Hey hands off ! I saw him first
But can I come as well? sounds just what we all need.

 

[Hurricane]

This might be a stupid question, but here goes... I am new to boating and am in the process of purchasing a falcon 27, she is a sports cruiser with a planing hull. Based in the water line length of roughly 24ft , if I chugg along at 6.5 knots will it be more economical than getting her up on the plane and cruising at 20 knots?

Clearly I would get from a to b quicker but I guess I want to find out which method will give maximum range.

Here are some real figures from our P67
9.5 knots -22 litres per hour per engine - displacement speed
24.5 knots 175 litres per hour per engine - cruising planing
If you work out the costs of the two speeds you will see how much cheaper it is at speeds below your max displacement speed

 

[Deleted User YDKXO]

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

 

[Deleted User YDKXO]

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

 

[Latestarter1]

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.

 

[boatmike]

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.....

 

[Deleted User YDKXO]

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

 

[Latestarter1]

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.

 

[MapisM]

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.

 

[Deleted User YDKXO]

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.

 

[SpottyDog5]

I’m sorry I’m not able to contribute much to this thread, but I am finding very interesting, my thanks to all involved.

 

[MapisM]

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.