Information on trim tabs please

[BlueHeaven]

I have been watching on the Forum for several years now, but this is my first post, so please be kind!

I have a 38 foot boat Aquastar and I am in need of any advice that the learned forumites can give me about trim tabs.

I know the theory of how to use them but my question is, how do you know whether they are up or down? There is no readout of position, its just guesswork-is that normal? Am I missing a trick here? They were made by Insta-Trim.
Many thanks for any help offered.

 

[Bandit]

There is normally a pair of graduated displays (lines that light up) from bow up to bow down , one for port one for starboard.

 

[spannerman]

Generally you get a feel for your boat (a bit like the wife) and you know how its responding, if the flaps are down you will lose a few knots and the bow will be lower in regard to the horizon, if only one flap is down then the boat will be level in a side wind, if its the wrong flap you will soon know about it.
The rule is to use as little as possible which will be apparent if you try to retract them past the point where the boat really needs them.
My boat is very sensitive to lateral weight distruibution, and so I fitted a Mente Marine auto flap system which takes care of trimming the boat level for me, I am normally a great advocate of 'hands on' driving but this relieves me of a tiresome chore and lets me concentrate on driving and enjoying the scenery.

 

[stillwaters]

You obviously have the standard set up which only comes with a pair of 'bow-up/bow-down' rocker switches. They also sell a simple add-on electronic tab locator which consists of simple to fit plug-in connectors to a dual diode light display unit of similar size to the rocker switch - I think they are available in white or black from memory. www.saltwater-marine.co.uk are a possibility for supply. Best of luck.

 

[Nick_H]

It's not uncommon to have trim tabs fitted with no gauge. As Spannerman says, you play with them until you get the best mix of speed and trim. Different hulls respond in different ways, for instance I get no real speed penalty between zero and full tabs at cruising speeds, so I choose the trim that suits me best, which in my case is usually full tabs because I like the bow down.

 

[KevB]

Obviously the thing to do is always start your journey with them fully up (unless required down to get on the plane) then adjust as required. In an ideal world they would always be up as they cause drag the more they are down.

 

[benjenbav]

Hi BlueHeaven, and welcome. I can relate to your question and there is a subtlety to it which others might have missed. I say this as I have a similar hull. My Aquastar is a 33 and I think the 38 is just an extended version of essentially the same design. I have trim tabs which appear large and well made. Indeed they are. But, honestly, they don't make a great deal of difference!

On some planing boats of similar size you can, of course, steer using the tabs alone whereas on mine the effect of the tabs is much less pronounced. Nevertheless, when you get used to their operation you start to appreciate their value, albeit in an almost binary way. Up at displacement speed. Open the taps and down with both tabs to bring down the bow; maybe up a smidge as she settles into the groove. Unlike a deep vee hull there's not much lateral effect and I can't ever remember having one up, one down.

How to check where they are? Look out the window! Get someone else to steer and look over the stern to see whether the rams are extended or not.

Might be telling granny how to suck eggs, but leave them up when the boats's in the water but out of use - else you get barnacles growing on the rams and then they won't move.

 

[epervier]

I have been watching on the Forum for several years now, but this is my first post, so please be kind!

I have a 38 foot boat Aquastar and I am in need of any advice that the learned forumites can give me about trim tabs.

I know the theory of how to use them but my question is, how do you know whether they are up or down? There is no readout of position, its just guesswork-is that normal? Am I missing a trick here? They were made by Insta-Trim.
Many thanks for any help offered.

I have a similar craft, semi displacement hull, albeit shorter, I have tabs fitted and can say at displacement speeds they don't do anything for the handling at all, but at max speed, a head turning 15.5kns the tabs are essential to gain that max speed. Without them being deployed she just digs her arse in deeper, with tabs in use to adjust the attitude of the boat to lift the stern the difference (for my boat) is 1.5kns although doesn't seem much but as a %age is huge.

 

[MapisM]

I get no real speed penalty between zero and full tabs at cruising speeds, so I choose the trim that suits me best, which in my case is usually full tabs because I like the bow down.

If you have electronic displays for your engines, did you check if/how that affects the load (hence the fuel burn)?
That's actually the best indication (rather than speed) of the optimal tabs position at cruising speed.
I mean, the position which minimizes the engines load, of course.
Then again, it could well be that on a boat like yours the difference is not worth giving up your personal preference on the boat attitude...

 

[Nick_H]

If you have electronic displays for your engines, did you check if/how that affects the load (hence the fuel burn)?
That's actually the best indication (rather than speed) of the optimal tabs position at cruising speed.
I mean, the position which minimizes the engines load, of course.
Then again, it could well be that on a boat like yours the difference is not worth giving up your personal preference on the boat attitude...

Surely if I set the engines to cruising revs, 2000 rpm in my case, then play around with the tabs to get max speed, then that has to be lowest engine load for that speed doesn't it, or am I missing something?

 

[benjenbav]

I have a similar craft, semi displacement hull, albeit shorter, I have tabs fitted and can say at displacement speeds they don't do anything for the handling at all, but at max speed, a head turning 15.5kns the tabs are essential to gain that max speed. Without them being deployed she just digs her arse in deeper, with tabs in use to adjust the attitude of the boat to lift the stern the difference (for my boat) is 1.5kns although doesn't seem much but as a %age is huge.

Us semi-d owners are a desperate bunch, aren't we?

Everyone else talks of their top speed to the nearest 5 knots. But we know it down to one decimal place and we've all got evidence in terms of the gps read out.

Like this casual shot:

which, of course, is only here to show the large number of AIS units in the Solent. But what's that on the right of the screen? 15.2, y'know. And I wasn't really trying. And don't imagine I couldn't tell you exactly the rate and direction of the tide flow... if I wanted - but you might have to waterboard me before I let on.

Oh, and here's a picture of one of the trim tabs on my Aquastar:

 

[jfm]

Surely if I set the engines to cruising revs, 2000 rpm in my case, then play around with the tabs to get max speed, then that has to be lowest engine load for that speed doesn't it, or am I missing something?

Yup you are missing something. The throttle sticks merely tell the ECUs (or governors, on a mech engine) "I want 2000 rpm". The ECU therefore squirts in enough fuel to deliver 2000rpm. It adjusts the fuel squirt in rate, according to load. This is utterly different from a petrol engine, where the throttle actually is just that: a throttle

So lowering the tabs may well have nil effect on speed, because the engines just take more fuel as ordered to by the ECU (or governor), and keep spinning at 2000rpm, and keep the props turning at 1000rpm or whatever. you feel that as "no effect on speed", but you could be burning 5% more fuel

As you say, it varies according to the boat. Some boats might burn less fuel by having the arse lifted up a bit. There are no universal rules and you need to experiment, if you have fuel flow or % load data. Also it might be a perfectly good trade off as MapisM says to have more fuel burn but a nicer trim angle

 

[MapisM]

All 100% correct.
I would only add, just in case Nick doesn't have the load numbers on his displays, that the easiest way to check how much tabs affect his boat, is playing with them at WOT.
At WOT, engines are giving all they can anyway, so the tabs effect (if any) surely IS reflected on speed.
At a guess, in a boat like his, there should be a perceivable difference, possibly up to a couple of knots.

 

[MapisM]

Us semi-d owners are a desperate bunch, aren't we?
Everyone else talks of their top speed to the nearest 5 knots. But we know it down to one decimal place and we've all got evidence in terms of the gps read out.

Desperate bunch? You are privileged to see two figures speed!
With my trawler, whenever cruising below 10 kts (=99.99% of the time), I've got TWO decimals on the GPS...

 

[Nick_H]

Yup you are missing something. The throttle sticks merely tell the ECUs (or governors, on a mech engine) "I want 2000 rpm". The ECU therefore squirts in enough fuel to deliver 2000rpm. It adjusts the fuel squirt in rate, according to load. This is utterly different from a petrol engine, where the throttle actually is just that: a throttle

So lowering the tabs may well have nil effect on speed, because the engines just take more fuel as ordered to by the ECO (or governor), and keep spinning at 2000rpm, and keep the props turning at 1000rpm or whatever. you feel that as "no effect on speed", but you could be burning 5% more fuel

As you say, it varies according to the boat. Some boats might burn less fuel by having the arse lifted up a bit. There are no universal rules and you need to experiment, if you have fuel flow or % load data. Also it might be a perfectly good trade off as MapisM says to have more fuel burn but a nicer trim angle

If dropping the tabs creates more drag, then the engines squirt more fuel in to achieve same RPM, I get that bit, but if there's more drag, surely the props slip more, so the boat goes slower? If that's correct (and that's a big "if"!), then if I set RPM to 2,000, then play around with trim to get max speed, haven't I also found max prop efficiency and therefore min load?

 

[MapisM]

if there's more drag, surely the props slip more, so the boat goes slower? If that's correct (and that's a big "if"!)

I'm not saying this isn't correct in principle, but it's not so relevant.
It takes a helluva drag difference to significantly increase the prop slip (e.g. a badly fouled hull).
If for instance your props slip is 5%, assuming it goes to 5.5% (which is already a big jump!), at 20kts you would get a speed reduction of less than 0.1 kt.

 

[jfm]

I'm not saying this isn't correct in principle, but it's not so relevant.
It takes a helluva drag difference to significantly increase the prop slip (e.g. a badly fouled hull).
If for instance your props slip is 5%, assuming it goes to 5.5% (which is already a big jump!), at 20kts you would get a speed reduction of less than 0.1 kt.

All agreed. When you're doing "20kts" the SOG is flickering 19.9-20-20.1 all the time, and so you will never the see the extra prop slip

Gotta laugh at MapisM though: "Assuming your props slip is 5%". There speaks a man who hangs out with uber high performance race boats a bit too much and for whom "fast" merely starts at around 60knots

 

[MapisM]

LOL, must admit that I'm more used to debates on prop slip related to performance boats.
Is 5% so much off the mark, for a planning flybridge on shafts?
If anyone can post its prop pitch, the gear ratio and any speed/rpm combination, I could find out.

 

[epervier]

Us semi-d owners are a desperate bunch, aren't we?

Everyone else talks of their top speed to the nearest 5 knots. But we know it down to one decimal place and we've all got evidence in terms of the gps read out.

Like this casual shot:

which, of course, is only here to show the large number of AIS units in the Solent. But what's that on the right of the screen? 15.2, y'know. And I wasn't really trying. And don't imagine I couldn't tell you exactly the rate and direction of the tide flow... if I wanted - but you might have to waterboard me before I let on.

Oh, and here's a picture of one of the trim tabs on my Aquastar:

Just a couple of years back, I was using tabs to try and stop chine walk in the other boat at 34Kns!!!! what a sad old bar steward I've become

Really tearing up the water in this one 6.4kns vis carp radar needed could hardly see the bow of the boat!!

 

[BlueHeaven]

Many thanks for the helpful comments. I will follow up the link to saltwater-marine. I can’t actually see my trim tabs when going along due to the bathing platform and general turbulence so I am encouraged to try and fit some sort of indicator. Otherwise I worry that I’ll end up with one up and one down and not know.