Difference in fuel consumption: Displacement v Planing

[Deleted User YDKXO]

I don't think that's the whole picture mike. On those numbers, the 50kt boat will burn 104% of its no-tide fuel on any given trip, whereas the slow boat will burn 167% of its no-tide fuel burn

At infinite speed the tide has no effect,. At the same speed at the tide, it has infinite effect. Just like airport travelators - the faster you walk/jog the less they help you, whereas if you stand still they help you 100%

There is a speed of foul tide at which it makes more sense to plane at 20knots than pootle. (Not in the Med of course!)

I'm not arguing with the % effect on your SOG or fuel burn. Obviously the faster you go the less will be the % effect on your SOG and fuel burn. My argument was with asteven221's suggestion that a boat could somehow 'fly on top of the water' at planing speed to reduce the effect of the tide, which, with all due respect, is nonsense.
With regard to asteven221's actual situation as to whether its better to maintain 7.5kts against a 2kt tide or speed up to 20kts, that should be easy to work out. He says he gets 2.75mpg at 7.5kts. Lets assume for sake of argument that he gets 1.0mpg at 20kts. A simple calculation tells you that, with a foul tide of 2kts, his mpg drops to 2.0mpg at 7.5kts through the water (5.5kts SOG) and 0.9mpg at 20kts through the water (18kts SOG) so he's still better off maintaining his 7.5kt speed.
To illustrate your travelator point, if you do a few more calcs, you can work out that he is better off maintaining 7.5kts upto to a foul tide of about 5.5kts. Beyond that, he's better off speeding up to 20kts. But obviously that is only the case for this particular combination of speeds and mpg

 

[DAKA]

My argument was with asteven221's suggestion that a boat could somehow 'fly on top of the water' at planing speed to reduce the effect of the tide, which, with all due respect, is nonsense.

If the current is fast enough it does appear to be an experience shared by many although the last time I tried to explain, it was like trying to tell sailors they wouldn't drop off the world if they sailed too far .

At speeds above 20knots in a fast current your boats characteristics change , the boat becomes less stable, very sensitive to steering almost like it is floating on air, perhaps it is turbulence rising , what ever it is ,it is there and shouldnt be dismissed.

 

[Deleted User YDKXO]

If the current is fast enough it does appear to be an experience shared by many although the last time I tried to explain, it was like trying to tell sailors they wouldn't drop off the world if they sailed too far .

At speeds above 20knots in a fast current your boats characteristics change , the boat becomes less stable, very sensitive to steering almost like it is floating on air, perhaps it is turbulence rising , what ever it is ,it is there and shouldnt be dismissed.

I'm not arguing with the fact that a boat's characteristics change when on the plane but the idea that a boat on the plane can reduce the effect of a foul tide is just wrong. Whatever speed through the water you are doing, your speed over the ground is going to be 2kts less if you're pushing a 2kt tide. Boats cant fly

 

[Deleted User YDKXO]

If you calculate your annual running costs and then divide them by the number of 'sunny days' you have on board you will find the daily cost staggering , an 80 nm night passage might save a few euros in fuel but at the cost of writing off the following valuable day as you either sleep through or have an argument with tired kids.
I use my boat for family holidays and the importance of my familys enjoyment is import to me as I want them to join me next year, I suppose if I was on charter it may be different .

I agree with that. If you have limited time on your boat ie a week or two, losing a day sleeping off a night passage probably doesn't make sense. But I think Hurricane's cruising is more relaxed than yours and mine. If you've got more time to spend on your boat than fixed holiday periods, then doing a long passage at d speeds overnight to save fuel can make sense

 

[MapisM]

I'm not sure what point you're making.

Naah, I think you actually got it...
The point is simple: "speed up in huge seas" is a sort of oxymoron, if I've ever seen one.
Of course the faster the boat goes, the less the fins need to flap around to stabilise her. But if the stabs are not man enough to stabilise the boat when they're needed most - i.e. in seas where it's actually impossible to speed up, well, they're pretty useless, aren't they?
In fact, I perfectly understood Deleted User's surprise when he asked about your statement... "to get a decent stabiliser effect in big seas you need 10-12kts of speed to get enough flow over the fins, and in reality 6kt crusing long term isn't feasible for that reason".
Fwiw, my personal reply to Deleted User is no worries, a properly specced stab equipment can and does work at 6kts in big seas. It works harder of course, but you can only notice by looking at the display showing the fins movements, not because you're rolling more.

 

[asteven221]

Whatever speed through the water you are doing, your speed over the ground is going to be 2kts less if you're pushing a 2kt tide. Boats cant fly

Thanks, but just to clarify so I can get my head around this. Are you saying then that a full D hull e.g. an old wooding clinker boat doing 5 knots will lose exactly the same SOG as a RIB at 30 knots, if they both have a two knot tide on the nose? The RIB will be doing 28 knots and the clinker boat 3 knots SOG.

My instinct seems to suggest that the RIB, with only a small (flattish) part of the aft section of the hull immersed in the water (hence my fly description - skimming might be a better description) would create considerably less drag and therefore overcome some of the influence of the tidal flow passing under the hull. If true then the RIB would be losing less than 2 knots from the tide.

 

[DAKA]

Thanks, but just to clarify so I can get my head around this. Are you saying then that a full D hull e.g. an old wooding clinker boat doing 5 knots will lose exactly the same SOG as a RIB at 30 knots, if they both have a two knot tide on the nose? The RIB will be doing 28 knots and the clinker boat 3 knots SOG.

My instinct seems to suggest that the RIB, with only a small (flattish) part of the aft section of the hull immersed in the water (hence my fly description - skimming might be a better description) would create considerably less drag and therefore overcome some of the influence of the tidal flow passing under the hull. If true then the RIB would be losing less than 2 knots from the tide.

It is what the non believers think, they will try to persuade you to imagine you are on a piece of moving flat paper .

They forget the properties of moving water AND benefiting from 2.5mph less head wind !

There is a noticeable difference with a 4+ tide race / River flow, boats take on a whole new set of characteristics, they will try to persuade you it is theoretically impossible but it happens .

 

[jfm]

Naah, I think you actually got it...
The point is simple: "speed up in huge seas" is a sort of oxymoron, if I've ever seen one.
Of course the faster the boat goes, the less the fins need to flap around to stabilise her. But if the stabs are not man enough to stabilise the boat when they're needed most - i.e. in seas where it's actually impossible to speed up, well, they're pretty useless, aren't they?
In fact, I perfectly understood Deleted User's surprise when he asked about your statement... "to get a decent stabiliser effect in big seas you need 10-12kts of speed to get enough flow over the fins, and in reality 6kt crusing long term isn't feasible for that reason".
Fwiw, my personal reply to Deleted User is no worries, a properly specced stab equipment can and does work at 6kts in big seas. It works harder of course, but you can only notice by looking at the display showing the fins movements, not because you're rolling more.

Your describing stab performance as if it were binary - they are either man enough or not. It isn't like that. On any boat you have a compromise between, at one end of the scale, huge fines with huge drag+ cost that can stabilise in huge seas at 3knots, and at the other end of the scale smaller fins that need 10knots of speed to stabilise the same amount in the same sea. And all points in between.

In fact many modern P boats have bigger fins in order to provide at anchor, with the effective size made bigger by proper winglets in the case of Trac and Sleipner. Your boat likely has smaller because the designers knew there would be 6/7kts of speed and didn't plan them to work at anchor

But whatever point on the compromise scale is chosen, the fins are more effective as you speed up. If Deleted User wants to cruise big seas at 6knots he needs to spec his stabs accordingly (which is perfectly possible). But at 6knots with non at-anchor fins there will come a point in huge steep beam seas where another 3-6 (or whatever) knots will improve the stab performance usefully

I don't agree your "oxymoron". As you yourself have said, there are cases when you want to speed up in a huge sea, eg where the sea is behind you or on your quarter. I also find 18kts or so in very big beam seas to be pretty good (of course it depends on the exact wave shape and all those details - you cannot generalise on a web forum)

 

[BGW]

Is this one to submit to Mythbusters?

My view is that the P boat is moving against the same flow as the D boat - The prop is still pushing against a moving target - both craft will lose the same SOG.
The P boat will, however, spend a shorter time pushing the wrong way for the whole journey. The D boat will go through slack, then get helped by the alternate tide thus saving some time and fuel

 

[MapisM]

Thanks, but just to clarify so I can get my head around this. Are you saying then that a full D hull e.g. an old wooding clinker boat doing 5 knots will lose exactly the same SOG as a RIB at 30 knots, if they both have a two knot tide on the nose? The RIB will be doing 28 knots and the clinker boat 3 knots SOG.

Ab-so-lu-te-ly.
We had a similar discussion in the past, IIRC. And Daka already argued against that principle.
But what he's saying, albeit not incorrect, is NOT related to the tide per se, but rather to other effects.
In fact, even in total absence of tide, in very shallow the old wooding clinker thing (apropos, you don't have anything against them I hope..? ) will behave differently, squatting, making big waves and loosing some speed at any given RPM, if compared to cruising in deep water. It happens also to ships, to the point that it might become very difficult to control steering. Otoh, a planing hull can even achieve better results (particularly the biggish, not extremely fast hulls), because they can benefit from an increased lift, which translate in higher top speed.
But that's a different kettle of fish, because it's an effect created by the shallow depth, not by the tide.
If you consider the effect of current per se, your summary above is spot on.
Of course, quite often tides and currents are associated with shallow water, because strong currents are pretty rare in very deep waters.
And this can confuse things a bit. But in order to understand the effects, we can't mix up current and depth, 'cause they're two very different things.

 

[jfm]

Ab-so-lu-te-ly.
We had a similar discussion in the past, IIRC. And Daka already argued against that principle.
But what he's saying, albeit not incorrect, is NOT related to the tide per se, but rather to other effects.
In fact, even in total absence of tide, in very shallow the old wooding clinker thing (apropos, you don't have anything against them I hope..? ) will behave differently, squatting, making big waves and loosing some speed at any given RPM, if compared to cruising in deep water. It happens also to ships, to the point that it might become very difficult to control steering. Otoh, a planing hull can even achieve better results (particularly the biggish, not extremely fast hulls), because they can benefit from an increased lift, which translate in higher top speed.
But that's a different kettle of fish, because it's an effect created by the shallow depth, not by the tide.
If you consider the effect of current per se, your summary above is spot on.
Of course, quite often tides and currents are associated with shallow water, because strong currents are pretty rare in very deep waters.
And this can confuse things a bit. But in order to understand the effects, we can't mix up current and depth, 'cause they're two very different things.

Yup, all agreed

 

[MapisM]

I don't agree your "oxymoron". As you yourself have said, there are cases when you want to speed up in a huge sea, eg where the sea is behind you or on your quarter.

Yep J, all agreed on the principles you're mentioning.
Personally, I wouldn't waste one quid on stabs which aren't effective enough in big seas and slow speed, but of course a P hull requires more compromise in this respect, if compared to slower boats.

Just one thing on your comment above, because it leads me to think that you maybe misunderstood the point I made in the other thread, when I told that in a few occasions, with big quarter seas, I would have wished that my boat could have cruised just a few knots faster: that was related to pitching, not rolling!
I never felt the need to cruise faster because the stabs were not effective enough, and tbh I'd be piXXed if I bought such an expensive equipment, just to realise that 6kt crusing long term cruising "isn't feasible" because the stabs are not effective enough, because in some conditions it can well be impossible to go faster, unless accepting to smash the boat and all its content - crew included - for hours.
I fully agree that the very same sea, when it's on your quarter, can allow for faster speed, hence reducing also pitching, but we can't always choose, unfortunately...

 

[DAKA]

Ab-so-lu-te-ly.
We had a similar discussion in the past, IIRC. And Daka already argued against that principle.
But what he's saying, albeit not incorrect, is NOT related to the tide per se, but rather to other effects.
In fact, even in total absence of tide, in very shallow the old wooding clinker thing (apropos, you don't have anything against them I hope..? ) will behave differently, squatting, making big waves and loosing some speed at any given RPM, if compared to cruising in deep water. It happens also to ships, to the point that it might become very difficult to control steering. Otoh, a planing hull can even achieve better results (particularly the biggish, not extremely fast hulls), because they can benefit from an increased lift, which translate in higher top speed.
But that's a different kettle of fish, because it's an effect created by the shallow depth, not by the tide.
If you consider the effect of current per se, your summary above is spot on.
Of course, quite often tides and currents are associated with shallow water, because strong currents are pretty rare in very deep waters.
And this can confuse things a bit. But in order to understand the effects, we can't mix up current and depth, 'cause they're two very different things.

fairly comprehensive and diplomatic which helps any discussion progress and for what it is worth I dont disagree with your statement either , now in the interests of completeness going beyond the 2 knot tide which I accept is rare..........

You mention squatting in shallow water which is particularly noticeable in channels and narrow rivers where reeds/water lilly can be seen moving above and below the water several meters infront of your boat, its caused by the props drawing the water from the immediate area in front of the props as it cant draw any from the sides or bottom due to the river /channel /depth of water.

That means that under normal conditions, (deep, flat still water) the prop is drawing water from a long way off 5-10 meters, creating a depression in the water that your stern is sat in.

Now put the same prop in a tide flow where there is a fresh supply of 100000000000s of gallons being fed directly to the prop to refill the depression from underneath and we can start to understand why a boat at 20knts behaves differently in a 4 knot head tide , the depression has vanished and the boats stern is higher, bow is somewhat lower than normal and frisky .

 

[Deleted User YDKXO]

Thanks, but just to clarify so I can get my head around this. Are you saying then that a full D hull e.g. an old wooding clinker boat doing 5 knots will lose exactly the same SOG as a RIB at 30 knots, if they both have a two knot tide on the nose? The RIB will be doing 28 knots and the clinker boat 3 knots SOG.

Yes thats exactly what I'm saying. The whole concept of flying or skimming to reduce the effects of the tide is a physical impossibility because any boat, whatever it is, can only achieve forward momentum by the props pushing against the water (not quite the right analogy) and since the water is moving backwards at 2kts, the boat's SOG is 2kts less.
In fact, FWIW, the boat would be slower if it were able to fly or skim because, if the props lose contact with the water, there's nothing to drive the boat forward

 

[asteven221]

Daka and Deleted User, your explanations are interesting, even my pea brain still can't fully understand it! LOL. Usually I am not bad at understanding this kind of thing but this one's pickled my brain I am afraid. Still, I will take your word for it!

 

[jfm]

That means that under normal conditions, (deep, flat still water) the prop is drawing water from a long way off 5-10 meters, creating a depression in the water that your stern is sat in.

Now put the same prop in a tide flow where there is a fresh supply of 100000000000s of gallons being fed directly to the prop to refill the depression from underneath and we can start to understand why a boat at 20knts behaves differently in a 4 knot head tide , the depression has vanished and the boats stern is higher, bow is somewhat lower than normal and frisky .

I don't agree one word of that. In deep water the boat and the sea do not know that the water is moving at 2kts or 4kts relative to the planet surface. The boat does not see that 10000000000gallons/min of water flow, and remember it is all leaving the boat's stern as fast as it is arriving at the boat's bow. The boat behaves exactly the same regardless of the tidal flow, and simply loses 2 or 4 kts of SOG in a foul tide. There is absolutely no difference in the ride or attitude of a 20knot boat in still water compared with 4 knots of foul tide

 

[ian38_39]

I am sure that I read somewhere squatting in narrow channels was nothing to do with the prop but to do with something called canal effect where, because there is less area for the water to be displaced to, the water flow increases in speed lowering the pressure and as such causing the boat to sit lower in the water.
Can't pretend to fully understand it but sure someone does.

 

[DAKA]

I don't agree one word of that. In deep water the boat and the sea do not know that the water is moving at 2kts or 4kts relative to the planet surface. The boat does not see that 10000000000gallons/min of water flow, and remember it is all leaving the boat's stern as fast as it is arriving at the boat's bow. The boat behaves exactly the same regardless of the tidal flow, and simply loses 2 or 4 kts of SOG in a foul tide. There is absolutely no difference in the ride or attitude of a 20knot boat in still water compared with 4 knots of foul tide

I dont have the means to prove this so I dont see any point in us starting one of those ping pong threads where we irritate the hell out of one another so lets agree to disagree.

I dont offer this as an argument , more of an explanation so you can see where I am coming from...........

little to do with this thread anyway in that my experience of fast flowing water is from the Trent tidal bore (numerous occasions) and an inland canal in full flood where I had to move my boat to avoid damage which are extreme compared with anything you find in the med, closest in the solent would be hurst narrows, boats on the plane become frisky.

I dont really mind how you perceive this effect but it is happens.

Your theory is possibly flawed in that you assume the props are acting like a cork screw through cork with out disturbance , my explanation recognises that there is a huge body of water being thrown out the back, this water has to come from somewhere.

 

[MapisM]

my explanation recognises that there is a huge body of water being thrown out the back, this water has to come from somewhere.

Yep, it comes from the water displaced by the boat hull itself. Props have very little to see with that.
It's not so intuitive, and much better underdstandable with drawings.
I've seen some in the past, but I'm afraid I couldn't find any with a quick web search.
Anyway, the point is that any hull, while moving forward, pushes water all around her - not just on its sides (which is the effect we're used to see with the usual wake shots), but also under its bottom.
When cruising in channels, the water pushed under the hull hits the bottom before the whole boat has overtaken its own wave, and in some conditions this mass of water can hit back the boat stern after rebounding on the bottom, creating the effect you're talking about.
There are many components affecting this, but in principle that is more evident with narrower/shallower channels, and heavier/slower boats.

Current also plays a role, but again, it's not the current per se.
There are two reasons why you perceive a stronger effect when going against the tide.
The first is just a perception, due to the fact that you tend to look around you, perceiving visually the SOG rather than the STW. And typically, you would cruise against the current at a much lower STW than when going with with the current.
But there's also another reason, which is indeed real, albeit harder to perceive. In channels, the water doesn't move at the same speed within the whole channel body. Along the sides and the bottom, the water flow is much slower due to drag. Assuming that you're cruising in the center of the channel, where the current flows faster, for any given STW your speed relative to the mass of water flowing along the channel sides/bottom will be higher/lower when cruising down/upstream respectively. In turn, this can and does affect the behaviour of the water displaced by the hull which I just mentioned.
Let's try to make an example with numbers.
In a river with a 4kts current, you cruise both ways, always at 6kts STW - hence at 2kts or at 10kts SOG, when going against/with the current respectively.
According to what myself and others are saying, the boat behaviour should be exactly the same, right?
Wrong, because the mass of water, which flows at 4kts in the center of the channel where you're cruising, could well be flowing at just 1kt along the sides and the bottom.
This means that your STW, when compared to this part of the waterflow, is either 9kts or 3kts, respectively when cruising with or against the current.
And this can make a helluva difference in terms of behaviour of those waves generated by the hull, when they rebound against the channel sides/bottom.
To the opposite extreme of the scale, these effects would be negligible for a catamaran flying along the same channel at 100mph STW.
Mind, it will still make 104 or 96mph SOG, depending on the direction, because there's no way to defeat the force of water, for any kind of vessel.
But the other effects we're talking about won't be relevant, because by the time its own wake is rebounded from the sides/bottom of the channel, such boat will be half mile ahead of them!
If anything, such boat could hit slighly less/more than 104 and 96 (say 103 and 97, frinstance), because at that sort of speed also the air resistance begins to be a factor.
But that's yet another chapter of the book...

 

[DAKA]

thankyou for providing such a thorough explanation which I have digested and learned from.

I am still left with niggles which I will have to live with for now.

Reeds start to move well in front of the hull.

The depth of water in the trent is about 15ft which I wouldnt consider as shallow for my boats.

The water certainly isnt shallow in hurst narrows or the Alderney race either.

I dont ease the throttles back just because I get a tidal push.

My boats appear faster when punching a fast current in that the log is 1-2 knots more at max revs.

Perhaps (but doubtful) my last boat was over propped, but my current boat has an electronic governor which limits revs. so it appears its not just me that is being fooled into thinking its faster against a fast flow but also my instrumentation (paddle wheel log) is also being fooled.

The good news is, it doesnt matter a jot

I'm not an RYS instructor, I dont plan critical passages with tight tolerances so it really doesnt matter if I believe in something that hasnt been scientifically proven (yet).
I also have a 22ct Gold St Christopher at my helm ready to clutch onto