Faster cruising speeds can save fuel

[ QUOTE ]
I'm not even sure that every boat is less economical at hump speed either. I've been recording the speed of my boat over recent months at various speeds and I don't see any decrease in mpg at hump speed which on my boat is at around 12kts.

[/ QUOTE ]

This part of my theory I am not so sure about but here goes

The short distance trials you are doing at hump speed and speeds 12-16 are flawed along with other boat mag tests

Your props draw huge amounts of water passed the log impeller, your log may show 12 knots but chances are you are only doing 10 knots and 20% of your mpg needs knocking off as 20% of water flow is directed to the seabed.

If you can run the tests away from current and ignore the log, using the gps instead then the figures may be more accurate .

When heading into a current ( I have spent a lot of time in the fast flowing Humber, Trent and Ouse) , the faster you go the less effect the current has on you comparing the log to the gps.
This backs up my theory that 17 knots of water under the hull at an awkward angle is not 17 knots through the water , as you increase in speed and the water passed the log becomes speed through the water and the log becomes more accurate.
 
[ QUOTE ]
When heading into a current ( I have spent a lot of time in the fast flowing Humber, Trent and Ouse) , the faster you go the less effect the current has on you comparing the log to the gps.
This backs up my theory that 17 knots of water under the hull at an awkward angle is not 17 knots through the water , as you increase in speed and the water passed the log becomes speed through the water and the log becomes more accurate.

[/ QUOTE ]

What are you on

Can I have some

Sorry - this is Bollox
 
For those that are not too blinkered to understand

Your props draw water from the bow.

For those that have been down a shallow narrow channel or canal at 20 knots they will have seen reeds moving 1/4 mile in front.

In effect setting up a localised current specific to the craft , SOG is reduced as log is increased artificially.

Now that is an effect that is seen 1/4 mile way and in fact if some one is speeding back to the boat club in the dark, mooring chains start to pull as the boat is 1/2 mile away.

Out in open water there will be no need to pull water from such a distance but 10 m from the bow will have a significant effect.

A boat at an awkward angle will not have an accurate log.
 
>Your props draw water from the bow.

No they don't.
They provide thrust from the stern.

However, I can believe that that pushing a lot of water rearwards in a narrow river will affect the boats in front. Your example of a fast flowing, narrow, shallow river is a special case, though, and doesn't really apply to the original discussion.

dv.
 
Is this another tag team get at DAKA day ?

Some have the knack of turning a perfectly sensible suggestion


'16 knots cruise may cost more per mile than 24 knots'

Into an excuse to argue about how their Granny's bath tub handles at displacement speeds.

I will not be digressed anymore today, I have been trolled enough thank you.

/forums/images/graemlins/mad.gif












Not aimed at you S28, and I not really mad anyway,

/forums/images/graemlins/laugh.gif
 
DAKA, my measured speeds were all from the GPS and done in the Med so no current or tide effects and also I did 2 runs in opposite directions to average out wind resistance
I don't agree with your comments concerning log speed. If I understand you correctly, you are saying that the props are drawing water under the hull which has the effect of turning the impellor wheel of the log? There's an easy way to test that. Put your engines in gear when you are still tied to the dock and see whether the log shows a speed. I bet it won't
 
Good reply with a sensible suggest, I believe that the log will register especially if the engines manage to rev to 2000-3000 but I think I would tow my pontoon away, anyone moor against a harbour wall with bollards care to try it ?
 
I could give that a try during the weekend, if I weren't pretty sure that the log will register something, particularly with the boat tied to the dock.
And the difference between SOG and STW will be much higher than the 20% that you previously suggested, because even 0,1 kts is an infinite increase in %, if compared to zero... /forums/images/graemlins/wink.gif
But I'm equally sure that under way the effect you're talking about will be nowhere near 20%. By heart, I would rather bet on something below 2 or 3% in theory - unmeasurable in practice.
 
the 20% was not entirely due to 'prop flow enhanced log speed' , most of the 20% was due to the angle the props send thrust to the seabed.


The further the stern digs in the deeper the water.
A few inchs of water depth have a huge effect in resistance.

(as evidenced with how the paddle wheel effect works)
 
I need to think about that /forums/images/graemlins/confused.gif
In the mean time perhaps you can help me think......

If its the hull affecting the water, which is clearly pushing then the water level preceding the boat would rise as the hull pushes forward not fall as it is sucked away /forums/images/graemlins/confused.gif

Im not arguing, Im not sure on this at all .
 
From my experience I have to agree with DAKA...

I have found that even when fully on the plane and buzzing along nicely at, say, 20kts I get a small improvement in mpg figures by increasing speed... this is against what my instict tells me but it is true. This improvement in mpg carries on up to about 24kts when it then starts to fall back.

As a result of this revalation, when I can, I tend to operate at 24kts rather than the 20kts which I was cruising at. Unfortunately being such a small boat (21ft) I can't cruise at that speed very often as it needs a nearly flat sea so I still tend to cruise around at 15-20kts anyway!
 
[ QUOTE ]
the 20% was not entirely due to 'prop flow enhanced log speed' , most of the 20% was due to the angle the props send thrust to the seabed.

[/ QUOTE ]I've lost you there.
You said that props draw huge amounts of water passed the log impeller, your log may show 12 knots but chances are you are only doing 10 knots.
Now, what has the angle of thrust (compared to the waterline) to see with the water flow under the surface of the hull (which is what affect the impeller)?
That flow is by definition aligned with the hull itself, even when the bow is pointing to the sky, innit?
 
OK, if you think about a narrowboat on a canal, it is a little easier to visualise. The boat moves forward, but clearly can't push it's displacement in water forward up the canal. What happens is that it pushes a little bit forward, and that piles up round the bow before sliding past the boat as a bow wave. In a narrow, shallow channel like the canal, there is a limited amount of room around the boat for the water to squeeze past, so the water is accelerated past (and under) the hull.

This may indeed cause a slight over reading of the log, should you indeed be doing your suggested 20 knots on a canal. It will also tend to suck the hull down slightly - an effect that is well known in shipping, but still caught the captain of the QE2 out some years ago when he grounded her in a channel that should have been deep enough.

What you may well see, if you were daft enough to run Felix at 20 knots up a canal is a surge of water being pushed forward and a counter flow running backwards at a greater rate under the boat probably along the bottom of the canal. This push and pull would set reeds swaying a long way ahead.
 
[ QUOTE ]
From my experience I have to agree with DAKA...

I have found that even when fully on the plane and buzzing along nicely at, say, 20kts I get a small improvement in mpg figures by increasing speed... this is against what my instict tells me but it is true. This improvement in mpg carries on up to about 24kts when it then starts to fall back.

As a result of this revalation, when I can, I tend to operate at 24kts rather than the 20kts which I was cruising at. Unfortunately being such a small boat (21ft) I can't cruise at that speed very often as it needs a nearly flat sea so I still tend to cruise around at 15-20kts anyway!


[/ QUOTE ] same with my Carrera and a mates Searay 230

cheers Joe
 
In the canal situation the vast majority of the flow is backwards .At Torksey which is at the end of the Fossedyke canal you can easily tell if a boat is speeding on the canal by your moored boat being dragged back
The speeding boat may only appear 5 to 10 minutes later
tried planning on it but its too shallow /forums/images/graemlins/laugh.gif
cheers Joe
 
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