Any harm in oversized trim tabs ?

[stelican]

I've been looking a while for trim tabs for my fairline weekender 21 and just bought a full Lenco kit at a bargain price. The tabs are 18" wide by 12 deep. From what I can see the recommended tab sizes for the boat are either 12x12 or 18x9. I planned to cut the tabs down to 12x12 but wonder if there's any harm or indeed benefits of sticking with oversized?

I believe Fairline fit was 12x9

 

[Montemar]

Thanks, I’ll put what weight I can forward, keep the trim tabs down and live with the fuel bill!

 

[Hot Property]

Nice to see your are curious of what’s going on under the water , the hull form .The floating apartments guys more interested in fridge size , bed headroom ,prawn grills , joysticks …….stop reading now and skip the rest !

Planing

A planing hull is simply one so shaped that a degree of dynamic lift is added to its natural buoyancy during the time when its speed of advance exceeds that rate at which solid water can close in abaft of it .


I think we all first need to agree that there is no one binary point of planing. It is not either on or off. It is a transitional regime, and therefore, any attempt to define a point of planing is somewhat nonsensical.


What actually defines planing using the most widely accepted definition is —-

The fact that if the speed increases the trim angle will decrease. I did say this earlier ^^^ .

If you aren't planing, increasing speed in the sub planing regime results in an increase in trim angle, for a planing hull. Again, for a semi-planing hull that might not happen at all, but for a true planing hull, the speed where it attains planing status is the point where the trim angle decreases as the speed increases.


Think about an airplane wing. To supply a given amount of lift at a given speed you need a specific angle of attack. If the speed increases you have to decrease the angle of attack or you will get an increase in lift. As you approach planing speed the trim angle doesn't decrease and indeed, the lift is increasing since you are supporting an ever increasing portion of the hull with hydrodynamic lift.
But once you are on the plane, even though the hull may be heavy and still have a significant portion of displacement lift, if &the trim angle drops as speed increases, it has attained planing status.

This is why using arbitrary measures like % of weight or amount of lift, are not appropriate, a heavy hull might have a different lifted height when compared to the same hull when lightly loaded. But in both cases, when the trim angle decreases, planing has been realized.

How much of the displacement volumn must be above the surface to be "on plane"? all of it, most of it, half of it, any of it? What % if any ?
Since planing begins to occur when hydraulic forces lift the hull, how much until it is actualy "planing"? Any lift replaces displacement forces with dynamic ones, so some say the begining of planing occures when there is any lift not associated with displacement.

Therefore to some they see no reason for any of the qualifiers at all. Many shapes can plane to one degree or another, no reason to put those conditions on it. I also do not see why 50 percent is the magic number either, if any portion of the weight is lifted out of the water by hydrodynamic lift you will reduce the drag and increse speed.

Some say that planing occures if any of the weight is suppored by the dynamic forces on the hull from the water. A little or a lot of the weight being supported is illrelevant, the fact that the hull is not fully in displacement mode means that lift forces on the hull are partially supporting the weight of the boat.

I think the real problem is the word "planing" itself is obsolete and based on archaic ideas about fluid mechanics. The origin of the word assumes it is even possible to be above or "on" the plane of the surface. We know a lot more about the process and forces involved but are stuck with these obsolete terms.


Getting back to the topic some suggest taking into account the “subjective feeling “ of the persons on board, I believe that anyone who has been on board a planing vessel can witness that it's movements became "stiffer" and more jerky when encountering waves at high speeds than it was during low-speed navigation. A mathematical explanation for this behavior is that the perturbation of hydrostatic lift component is in linear relationship to the vertical speed of the incoming wave disturbance, while the dynamic lift component increases as a square function of the vertical speed of the perturbation. Hence, the vertical accelerations become much more severe in high-speed regime.
It makes me think that perhaps it can lead us to a completely different criterion for the definition (by convention) of the planing, based on the vertical acceleration response of the vessel to incoming waves, or to a single standardized perturbation which could be reproduced in towing tanks. The planing regime would then be indicated during sea trials by the on-board accelerometers, rather than through the GPS readout .

Just some food for the few creative minds on here . .
View attachment 124507

I think that at the end, it is a matter of semantics and not physics or engineering. It would be better to say, for example, "this vessel at a speed of 54 knots has 93% dynamic lift to displacement ratio", rather than argue whether it is planing or not

I think we can all agree that, when looking trim angle it becomes pretty clear where planing starts.

The more you think about it one soon realises this becomes different for every hull, and for every load condition, every thrust angle, and for every offset of the thrust vector from the line of the planing surface, as well as things like prop rake. It's possible to significantly change the onset of planing by modifying those variables.

When all things are considered, it is far more complex than a simple % of lift. For lightly loaded short, wide hulls, the change in trim angle happens much more quickly with increasing speed. For much larger hulls with heavier bottom loading, the curve will likely be more of a gentle hump, but the reality is, when you are past the peak of the trim angle curve the hull is now in the planing regime. Since there a any number of variables that can push the actual speed at which the trim angle starts to decrease - that's not a bad thing to use for a rule of thumb .

Maybe planing is like time: we all know what it is, until we think about it
View attachment 124508

Does this look right? Bow rises slightly when power applied then definite drop of the bow once 20 knots plus

 

[Portofino]

^^^ Yes it’s a good example, the antifoul line is one to look at below 4 degrees .

Maximum amount of hull length too .Not on flat calm but in waves it helps as the finest V is nearest the bow .
You just end up with more lift spread evenly and thus slightly less drag than a hull with its arse in the drink and bow pointing to the moon = better speed and fuel economy.
Also should it cut up in a chop pov it will be more able to keep going faster the flatter it is
The badly trimmed boats will start slamming earlier and the owners will throttle back before you do .

Advantage of a OB , the trimming should be almost perfect because of the thrust vector effect at the expense of a little speed though , maybe ? Its a balancing act twixt OB trim and hull trim .Looks spot on in this pic . Outdrive boats too can balance leg trim and hull trim with flaps if necessary.Ideally as little flap as possible but many are just stern heavy and need flap down to get up to plane and keep the bow low to see out .Like in the pics I posted.

Shaft drive boats have only flaps which let’s face it effectively act as brakes just like on aircraft .Water is a zillion times denser than air so the time , so the smallest amount of flap = drag .

Having said that shaft angle is a science in its own right , a by product of all this .

V drives .- G box is in front of the motor , or the weight of the motor is aft …..a step away from true mid engine but realise the interior vol benefits today’s market wants .

Semi tunnels . A right builders dodge because they destroy residual lift at the stern , the very place it needs adding back .Or turned around add more suction .
Result huge flaps needed both movable + permanent bolt ons . It can’t really run flat without masses of movable flap which creates drag and they struggle over 25 knots or just have to accept massive fuel burns .Planing sure but at a hefty price .

Mount the P brackets as far back as possible lowers the shaft angle , effects the rudder function; sometimes.

Yours is not a floating apartment never intended to be one so the designer has less pressure to deviate from the “ book “
Its all about increments , the sum of the parts / features under water that not many bother to investigate.

Its too late once the cheque is cashed and you find out you have to stand up to see out planing or it slams in modest waves reultng in throttling back in to semi planing territory or you have to helm from “ up stairs “ if a FB only from a vis pov etc .

Try before you buy in the sea states you think you are gonna encounter .

 

[Farmer Piles]

I have a similar sized boat with an outboard. I always start with the engine right down until she is on the plane as this pushes the nose down and gets her planing more quickly. I then blip the trim up with a few dabs of the button until I feel the boat physically loosen in the water and the revs pick up by about 100 rpm, say 4000 - 4100 rpm and the speed by a knot or two. I might give it another dab up. On my Yamaha 115 on my boat that's about 2.5 out of 10 on the trim scale, but doubtless that varies hugely from engine to engine and boat to boat. You should be able to feel when it's right and watch the revs and speed.
Hope this helps a bit and doesn't sound too preachy.