How much does trim tabs regulation affect your cruise speed

[MapisM]

I would be particularly interested to hear about experiences with flybridge P boats, whose cruise speed is typically in the low 20s.
But also those with faster boats are welcome to contribute.
Let's restrict the field to boats on shafts though, 'cause outdrives are in a rather different league in too many ways.

So, what is your normal cruising speed?
And how much does such speed increase/decrease by setting trim tabs to their "best" and "worse" position respectively, without touching the throttles?

Btw, before anyone mentions this, I'm well aware that trim tabs position (hence hull attitude) can also be chosen depending on sea state, rather than optimal speed. But the thread is meant as focused just on efficiency, nothing else.

 

[Portofino]

First of all low 20,s is not good for me 14/1500 rpm turbos just starting to whistle .
Even 22knots feels like semi planing , the bow is high and stern down .
However you did “ welcome faster boats “
So for me 1780rpm is 27/28 knots this is our normal cruise - engines just under 80 % load about 78 %
L/hr about 90each side ,
Interesting I’ve been cruising in company lately and being the gent accepted my mates pace of 22 knots ( 3000rpm D6 IPS600 )
At this speed our load is 75 % and consumption 75 L/ hr @1530rpm which is why I suspect we are moving water about a lot and not quite planning in my book .
Terribly inefficient speed for me .

Back on topic our tabs go from minus 2 up to zero ( inline with the hull ) to plus 5 down .
At 28 knots fully down they knock off about 1-1/2 knots in flat calm .
We usually run on O , no tab .
How ever if fully fuelled then we run at +1 down , this adds about 1-1/2 knots on to 0 .Just brings the planning angle to within 2-4 degrees optimal angle .
This is because our tanks are behind the engines as the engines are actually in the middle of the boat rare these days .

1900rpm is over 30 knots and we hardly go there - no need .Faster you go the less tab needed .

 

[jrudge]

I cruise at 20 knots most of the time.

Trim down gives something under a knot - say .7/.8 ish.

The s65 tab made no difference to anything and I never used it. ( I did check they worked!)

 

[kashurst]

my sealine T50 would go 1 to 1.5 knots faster from 20 to 25 knots with @ 80% tabs, cruised at 23 for best economy there after no difference
the Rodman 38 gets @ 1 knot faster from 17 to 24 knots with 100% tabs - cruise @ 18knots
the fairline targa 43 on shafts the tabs didn't make much difference

 

[Hurricane]

We really only cruise at 10 or 25 knots.
I try to keep away from low 20s because our second stage turbo switches in/out at that speed and after a few hours the electronics will report a turbo not up to speed.
Best (and more economical) to go through that speed and keep the turbo spinning.

So, at 25 knots, I don't really notice much effect of the trim tabs.
I really only use then to trim left/right rather than fore aft.
They don't seem to make any difference to the speed.

 

[PowerYachtBlog]

Recently I cruised a clients Sunseeker 53 Portofino Mk.II version with Mans R6 800hp, 2008 build.
Sea was calm for that kind of boat, waves of around two feet etc.
But I had this dilemma with it, at a speed the engines felt sweet and the cruise also.
At 1820rpm I was doing 25.5 knots tabs closed and could not see a thing, go to bow down and she cruises at 24 and 24.5 knots and the boat feeling like a dragger more then a powerboat.
Consumption at that speed was 90 lph and load about 74%.
I also tried going a bit higher but the bow really does not go down and with the tabs down it even felt more like a dragger.
So the dilemma with the Sunseeker 53 Portofino Mk.II is this, drive her with tabs closed and you really do not see a thing and the boat handles and feels much sweeter, go tabs down and it feels like a dragger and you see something.
Flybridge want have the visibility problem I guess.
It is a pity that no boat review goes into detail about such things especially when it is a big brand like Sunseeker.

 

[Portofino]

Reinforces the need to have a proper test drive first .
Walk away if not happy .
No Brit stiff upper lip deposit subject to survey malarkey. They are not merchant vessels ! Supposed to be leisure .

 

[MapisM]

Thanks guys for some very interesting feedbacks.
The last P boat I've been very familiar with (i.e. a Fountain 27 Fever, aka my "lake toy", capable of 72mph) was, unsurprisingly, EXTREMELY sensitive to trim tabs settings.
To the point of being unable to go up on the plane without lowering them, and losing up to 8mph when not raised.

But I was wondering what's a more common behavior with normal, civilized boats.
And based on your indications so far, it seems that my DP is not much different than other similar boats.
When cruising in the low 20s, she gains almost one knot by lowering the tabs, with an attitude which is VERY close to her static position, i.e. essentially flat. Funnily, it's rather obvious that the wet surface increases a bit, with the bow wake starting more upfront compared to the slightly higher AoA which she takes with raised tabs.
But since that results in a higher speed, it's equally obvious that hydrodynamically the hull works better with tabs down.
That begins to change in the mid/high 20s, where the max efficiency is achieved with just a little bit of tabs down, and the efficiency improvement becomes less relevant too - say half a knot max. At 30+ kts, it's definitely fully raised tabs territory, but that's practically irrelevant for real world cruising.

I'm a bit surprised to hear of boats like the Sq65 or the Prin67 where trim tabs don't make any noticeable difference though, because the theory matches nicely the above observations, i.e. that a P hull should be designed for max speed/efficiency requiring no tabs, and at anything below, tabs should help keeping the same ideal attitude, compensating the inevitable progressive loss of dynamic lift.
And this theory doesn't really change a lot with size - it's more dependant on speed...
Did anyone else with 20m boats experience the same? Deleted User for instance?

 

[MapisM]

Terribly inefficient speed for me.

You aren't very kind with your own boat, Porto.
How can you call "terribly inefficient" a speed where you are burning TWO percent more from A to B (and with a lower engine load & LPH, anyway!), I wonder...? Sounds like a very usable speed to me!
And you can save the effort to explain that she's more comfortable at 28 rather than 22 knots in any sea conditions, because Amati did build good boats, but he couldn't get around physics anyway...

 

[baylabayla3288]

My Ferretti 480 behaves similarly, for low 20s half tabs gains up to 1,5 knots. But I do like the possibility to rise th bow in big following seas at medium speeds despite the draw back in efficiency. Of all my fly bridge boats, Bayliner MY 32, Princess 34, Fairline phantom 38 and Ferretti 480, only the Phantom 38 did not gain any speed and actually slowed by deploying tabs.

 

[Deleted User YDKXO]

My current Ferretti 630 behaves very similarly to the 2 other Ferrettis I've owned. At low planing speed, say 14-15kts, it needs 100% trim tab down to maximise speed. Without any tab it loses 1-2kts. At moderate planing speed 20-22kts, the hull needs about 50% tab down to maximise speed. At this speed with 100% tab down it will lose about 1kt and it will also lose about 1kt with 0% tab. Above 25kts, the hull needs 0% tab for best speed. With tabs 100% down above 25kts, the boat is 1-2kts slower

For me this is all logical because at the normal cruising speed of 20-22kts and 50% tab, there is the option either to use more tab to bring the bow down in a head sea or the option to use no tab and bring the bow up in a following sea

 

[Portofino]

You aren't very kind with your own boat, Porto.
How can you call "terribly inefficient" a speed where you are burning TWO percent more from A to B (and with a lower engine load & LPH, anyway!), I wonder...? Sounds like a very usable speed to me!
And you can save the effort to explain that she's more comfortable at 28 rather than 22 knots in any sea conditions, because Amati did build good boats, but he couldn't get around physics anyway...

Eh !
The thing is configured ( prop / gearbox / torque plateau -etc ) to run @ 28 knots -all day .
If you review my earlier post and Hurricanes, you will discover a reference to turbos .
As I said it’s actually “ unkind “ to leave them in limbo , partially spooling , not giving any significant boost wavering about @14/1500 rpm .
They whistle away at 14/1500 rpm .Those in the U.K. who travel by rail a lot will be familiar with the noise when the rail car pulls away from the station , same engines laid flat under the cars .

N of 1500 rpm that they are happy the whistle stops .
Actually I quite ( and guests sat above them ) like the spool up whistle.

Remember I set my cruise not on knots but on EGT and load only .
75 % load is what they do @ 22 knots due to the quote Liam above PYB it feels “ dragger “
Transition bow down is @ 24 knots - this is as slow as I go .
The AoA feels wrong bow high / stern not up enough = bit low in dynamic lift @ 22 knots .
How ever push the throttles to a shade under 1800 rpm the bow drops , enough so no tab needed , boat levels out thus drag reduces speed increases to 28 knots but the load settles at 78 % .cos now it up and off the bow naturally drops as the speed increases further - It’s truly planning now .
No tab needed

So for me an extra 3% of load = an extra 6 knots .
importantly EGTs right in the zone too .

Oh and like Deleted User correctly says we full control of the bow at optimal running speed ,as I,am either at 0 or if fully loaded with fluids +1 .
This means I can move tabs to control the bow depending on sea / wave state/ running dir relative to wave etc .
Which is what they are there for I though ——- not to keep it up !!

As far a “ comfort “ you refer to actually there are times believe it or not going faster using the tabs to dip the bow ( more V slicing ) and crucially crossing waves as opposed to riding up and down them in a rollercoaster manor - is Infact a comfortable ride .Ask my Wife .

At 1800rpm or a shade under ,thereabouts we do around 6 L/nm

 

[rafiki_]

I don’t really need to use trim tabs at all on Rafiki. She seems happy with the neutral setting.

 

[MapisM]

I don’t really need to use trim tabs at all on Rafiki. She seems happy with the neutral setting.

Just to avoid misunderstandings, also with my DP the cruising attitude doesn't change radically. Maybe a couple of degrees max, if that.
But by playing a bit with different tabs level, and keeping a constant eye on speed, I can see the effects.
It's an exercise that you should make purposely, in case you didn't already.
In fact, a sub-optimal tabs setting doesn't make the boat attitude patently "bad" - as can happen with more extreme and fast boats.

 

[MapisM]

My current Ferretti 630 behaves very similarly to the 2 other Ferrettis I've owned.
...
For me this is all logical...

Agreed. That's why I was a bit surprised to hear that the Sq65 and P67 seem to be unaffected by tabs position.
Hurric/Jrudge, did you specifically check that as I mentioned in my previous reply to Rafiki?

PS: I can't remember, have you got a gauge indicator showing the actual degrees?
For some reason, most trim tabs gauges show generic numbers like +1/-5, which are obviously meaningless for a comparison.
Fwiw, the excursion on my DP is from 2 deg above neutral to 12 deg below.

 

[Deleted User YDKXO]

Agreed. That's why I was a bit surprised to hear that the Sq65 and P67 seem to be unaffected by tabs position.
Hurric/Jrudge, did you specifically check that as I mentioned in my previous reply to Rafiki?

Its an odd one this because my impression is that British Olesinski designed boats tend to ride more bow high than typical Italian boats like mine and yours, I guess, and you would expect therefore for them to be more responsive to trim tab setting but it seems not. Like your DP, my F630 runs at pretty much the same angle wherever the tabs are but the tabs certainly make a difference to speed. I dunno maybe its as simple as the tabs fitted to our boats are larger?

 

[Deleted User YDKXO]

I forgot to say that the MAN MMDS engine display on my boat has trim tab position indicators on one page and there is also an indicator which tells you the optimum trim tab position at every speed and it does change with speed. I have no idea who set this up (MAN or Ferretti) but what I can say is that it is not very accurate as I always need more tab than the display says I need to maximise the speed. It could be that this system was set up with the boat light and no cruising gear on board but actually its not very useful!

 

[JerryC]

...my impression is that British Olesinski designed boats tend to ride more bow high than typical Italian boats like mine and yours, I guess, and you would expect therefore for them to be more responsive to trim tab setting but it seems not.

I know my Olesinski designed Prin40 is a lot smaller than the subjects of this thread, but I was surprised to find it rode very (imho) bow-high when planing on the sea trial, compared with my previous much older boats. Surveyor was dismissive saying that's how they are nowadays (I had a few years break from boating). I guess the Fairline 40 behaves the same? I thought maybe it's to do with trying to squeeze more room into the fwd cabin, demanding a less pointy bow. On getting to know my boat, I found that the trim tabs were partially located in pockets in the hull, so their action is very limited. Anyone know why did BO do that?

 

[baylabayla3288]

I know my Olesinski designed Prin40 is a lot smaller than the subjects of this thread, but I was surprised to find it rode very (imho) bow-high when planing on the sea trial, compared with my previous much older boats. Surveyor was dismissive saying that's how they are nowadays (I had a few years break from boating). I guess the Fairline 40 behaves the same? I thought maybe it's to do with trying to squeeze more room into the fwd cabin, demanding a less pointy bow. On getting to know my boat, I found that the trim tabs were partially located in pockets in the hull, so their action is very limited. Anyone know why did BO do that?

One characteristic of BO’s designs seems to be a very full and deep forward section of the hull. When these boats are at rest the keel has a negative angle pointing downward while the deck-hull line is angled slightly up. I’m not sure what the reason is, maybe the space needed for the forward cabins together with the deep v needed for the good head wind ride, however this contributes fore sure to the bow up attitude of these boats at speed. Here is a link to he side profile of a P42 where this can be seen.

https://goo.gl/images/RizJAv

 

[MapisM]

One characteristic of BO’s designs seems to be a very full and deep forward section of the hull. When these boats are at rest the keel has a negative angle pointing downward while the deck-hull line is angled slightly up.

I believe that this is related to how much variable the deadrise is along the hull length, which is what I suppose you mean by "deep" fwd section. Not sure about "full", though.
Regardless, it surely ain't specific of BO design: my DP has exactly the same static attitude, but while under way she rides almost as flat as she is at rest.
Another relevant factor in BO hulls (though I'm not sure if ALL of their hulls are built like that) are the semi-tunnels, which have their advantages obviously, but significantly reduce the stern lift, AOTBE.