Most efficient speeds

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asteven221

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On a previous thread, I got some great advice on getting to know the most efficient speed on my Sealine F44. Next time out on a calm day, I am going to do some experiments to see what is the max speed the boat starts to lift, plus the least speed I can keep the boat planing. That got me thinking about trim tabs and where they come in.

In a nutshell - should I aim for the minimum planing speed with the trim tabs in a raised postion where they have no affect, or should I lower them? My instinct says that trim tabs cause drag which is less fuel efficient, however maybe a little trim and staying on the plane at a slower speed is more efficient i.e. better MPG. Hmmmm not sure really.

All advice appreciated.
 
asteven221 said:
instinct says that trim tabs cause drag which is less fuel efficient
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I tend to agree with your instinct in principle, but I'm afraid the only way to be 100% sure on a specific boat is to have some accurate (and real time) fuel measurement system.
 
It isnt a simple answer.

We have a range of 300 miles at 25 knots but less (about 270) at 22/23 knots.
Due to the engines having a second stage turbo that works more effeciently at the higher RPM.

I'm not convinced that slower speed planing is saves much either.

Now take it off the plane - thats another story.
Displacement speeds significantly increase the range.

As you say - you need to do some tests.

I'm also not convinced that trim tabs make much difference either.
I normally set them about mid way and then trim them to keep the boat level (that is the port and starboard sides of the boat at the same level).
Generally, I dont use the trim tabs for much else.
 
Hurricane said:
I'm not convinced that slower speed planing is saves much either.
Click to expand...

I was wondering a similar thing. When I read the tests on full planing boats in magazines they suggest (not that I've done any calculations) that:

Up to hull speed = good consumption, but obviously slow
Climbing up to plane = bad consumption, lots of drag, also slow
On the plane = even at a range of speeds not much difference in fuel consumption
WOT = gives a small increase in speed, consumption rapidly gets worse

My V48 doesn't seem to climb over a hump as such, it stays fairly level while getting faster, lifting gradually. The feeling is of going vertically up, like in a slow lift, not of the bow climbing skywards and the rest of the boat eventually falling over the bow wave onto the plane.

With 50% tabs as Hurricane says, I reckon it is planing at 16 knts, maybe earlier. WOT is 38.5 knts with a clean hull.

I don't have fuel flow measurements, but I wondered if there is a rule of thumb that would give me an idea where the band of speeds that don't much affect consumption might be (if indeed there are any)?

I cruise quietly at 24 knts, but 30 knts doesn't seem to be stressing anything, and crossing the Channel this weekend the difference between those speeds will alter the time I start on the beer quite noticeably :-)
 
kcrane said:
... WOT is 38.5 knts with a clean hull.

I cruise quietly at 24 knts, but 30 knts doesn't seem to be stressing anything, and crossing the Channel this weekend the difference between those speeds will alter the time I start on the beer quite noticeably :-)
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experts feel free to shout me down... but I think to make that decision it would be better to know what rev's your engines are pulling at your top speed of 38.5knts and at 30knts. 30kts is 78% of your top speed, if the rev's have come down by that %'age as well I would think 30knts would be a half sensible (if not a ridiculously fast) cruise! ....and if it leads to an early beer...all the better! :)
 
Firefly625 said:
to make that decision it would be better to know what rev's your engines are pulling at your top speed of 38.5knts and at 30knts. 30kts is 78% of your top speed, if the rev's have come down by that %'age as well I would think 30knts would be a half sensible (if not a ridiculously fast) cruise!
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Ah, now I can do those tests with the instruments at my disposal!
 
I think "just on the plane" is not the best speed. "Just on" must also mean "trying to fall off", hence you're using fuel to keep it up in the air, so to speak. Comfortably on the plane would be better, IMO. From my own observations and non too technical tests (keeping details of fuel used, speeds, RPM, conditions etc) my boat seems to be pretty much the same at anything between "nicely on the plane" and a couple of hundred RPM short of WOT. It doesn't seem dramatically more economical at displacement speeds. Sure, it uses much less LPH, but it's doing much less SOG, so MPG isn't that much better. Certainly not enough to entice me into plodding across the sea at 6 knots (river cruising not withstanding).
 
I'm afraid that the most efficient speed for any mobo is the one where it's tied up.

It's downhill all the way from there.
 
Im kinda new to all this, I only just discovered that if I raise the trim (lifting the bow) about 3-5 degrees up from all the way down (where I usually keep it), I can achieve greater speeds at the same revs! DOH im sucha noob
 
benjenbav said:
I'm afraid that the most efficient speed for any mobo is the one where it's tied up.

It's downhill all the way from there.
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Ahmen to that...If it's nigh on dead flat I'll give her a blast...anything less than perfect I like to pootle along...

Then again I've got an under powered, in-efficient petrol engine trying to get a heavy old girl moving...sometimes it's just better to sit back, put on some music...have a cold beer in hand and pootle.
 
I'm a raggie so anything over 6 kts gives me a nose bleed; however, I notice that some boats create enormous wash & some almost prop-ride with virtually no wash. Now my basic physics (1963, O-level, failed) tells me that to make wash you have to move a lot of water & that takes a LOT of energy, which uses fuel without propelling you forward.

So I would be inclined to add into the mix of indicators that a nice flat wash may be important. Maybe the trim tabs can help you reduce your wash - even if the speed isn't affected, it seems probable to me that fuel consumption must be.
 
Searush said:
I'm a raggie so anything over 6 kts gives me a nose bleed; however, I notice that some boats create enormous wash & some almost prop-ride with virtually no wash. Now my basic physics (1963, O-level, failed) tells me that to make wash you have to move a lot of water & that takes a LOT of energy, which uses fuel without propelling you forward.

So I would be inclined to add into the mix of indicators that a nice flat wash may be important. Maybe the trim tabs can help you reduce your wash - even if the speed isn't affected, it seems probable to me that fuel consumption must be.
Click to expand...

That's a good point and i think you may be right. It does tie in with my post above, in so much that at all the speeds that seem more economical with my boat she also makes the least amount of wash.
 
I was on the CI Condor Ferries fast cat last year & that has 2 enormous jet propulsion units that throw a rooster tail 30' up in the air. But the actual wash is quite little & the wake is just a broad flat line of bubbles. Compared to the Holyhead/ Dublin cats which throw a 6' high wall of death into the harbour about 20 mins after passing. I can't help wondering about the fuel consumption of the Irish boats.

It makes me feel a lot better when some ignorant twazzock blasts past at an angle of 45deg & throws me to the floor while trying to set sails if I remember that it is costing him a fortune to be so stupid!
 
Searush said:
I'm a raggie so anything over 6 kts gives me a nose bleed; however, I notice that some boats create enormous wash & some almost prop-ride with virtually no wash. Now my basic physics (1963, O-level, failed) tells me that to make wash you have to move a lot of water & that takes a LOT of energy, which uses fuel without propelling you forward.

So I would be inclined to add into the mix of indicators that a nice flat wash may be important. Maybe the trim tabs can help you reduce your wash - even if the speed isn't affected, it seems probable to me that fuel consumption must be.
Click to expand...


There are two things going on. One is to get the boat to the right attitude, the other is to get the engines to their most efficient. If the boat is set up right, this should be achieved around the same time, a little adjustment with the trim tabs should help, because weights are never the same.

But the main use of trim tabs, is to get the boat into the right attitude for dealing with the sea. Down, bow, low in to head seas, but more importantly up and high bows for following seas. This will help you stop "broaching" or cart wheeling, in big following seas.

Only if there is none of this, can the trim tabs be used to to trim for efficiency. Which is easy, Revs set to most efficient, then move trim tabs till more speed is shown.
 
Just how much can you save on a, say, 4 hour trip? After making the obvious adjustments of getting up on the plane at a comfortable cruising speed, adjusting for balance and now and then a fiddle with the trim just to pass the time, and I enjoy myself. (And don't forget the enjoyment to be had by just pootling along at raggie speed close to the shore and enjoying the views)
I do recommend looking at the wake as a good indicator of efficiency as it makes sense and accords with my limited experience. Like an elephant, it is hard to describe but you'll know it when you see it!
 
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