Modern planing hull/top speed trim tab requirement. A question for hull designers

[BMcF]

Interesting conversation!

Trim Tabs on a well designed hull allow the hull to run at its optimum running attitude over a broader range of speed, sea conditions and weight distribution.

Far from band-aides, they allow the good characteristics that a designer has developed to work in a larger envelope than a "static" design.

For example if the boat can plane at lower speed and still keep its bow down cutting an oncoming chop it is essentially running on its lines and the good characteristics of its hull design are working.

Calling Trim tabs a crutch for bad design (if it is a good design to begin with) is like faulting an aircraft designer for including flaps for low airspeed performance.

Tom McGow
Bennett Marine

Very well summarized, Tom! I heartily concurr. For almost 30 years, we've used trim tabs to make good designs perform better and bad designs perform "good enough". It's a rare vessel we work with that does not have trim tabs or interceptors on the transom..or end up with them eventually. SWATHs and hydrofoils being about the only exception.

 

[philSweet]

I can't speak to the design of the boat, but I might be able to shine a little light on the mentality on an engineer


I think what we have here is an engineer who is accustomed to having an extra 25% runout of any adjustment beyond what should be needed. Good thing if you're designing a packaging machine, but hydro doesn't really degrade over time the way machines do (I haven't seen gravity break yet). There's no reason to provide more trim capability than that required at WOT with full tanks. I'd be inclined to put a limiter in to remove excess travel. As long as you don't suffer handling problems, and particularly porpoising, then I guess they got it about right. I looked at the photos of your boat on the web- does yours setup like the ones in the adverts? Unless it doesn't perform as advertised, I'd guess any excess travel has been removed to prevent poor performance at different load and power conditions.

 

[Joakim]

For example the NA optimizes the rather heavy -7.5 tons for a 11.50m- boat for the cruising speed (normally 0 trim and 0 angle flaps) to get a good mileage and a sweet good behavior, as it's the situation the boat spends most time.
At top speed as the planing surface is reduced and goes aft, the boat has maybe a tendency to sink (after all it has more than 1000 kg of engines at the stern, a common problem with sterndrives) and to have too much angle of "pitch", making it very sensitive to waves.

Does boats behaving like this really exist? If a good planing trim is achieved at cruising speed, typically trim is too low at top speed. Only exception I know to this is possible porpoising. Mostly trim tabs and low stern drive trim are needed while getting to plane or at (low) cruising speed.

 

[FAST FRED]

In France we call that "le complexe de la petite bite", the complex of the small dick that the guy need to compensate with big cars, useless jewels, big watches and these monstrosities...Or the nouveau riche complex, the bling bling society.

In the USA this type would simply purchase a Harley Davidson bike.

A 1913 design , but it can be heard 3 miles away , surely more important than a Platinum Rolex,

 

[gonzo]

Or zee expenseevee wine

 

[William George]

Interesting comments. You can't all be right! Some observations.

'Tunnels' says its common sense, and that's true if you've been around boats a long time as I have. But do you run your boat at maximum negative sterndrive and tab trim to optimise WOT and still not achieve a good pitch angle for maximum speed? If so there's really no value in these trim devices, because if they don't achieve a relatively flat pitch at WOT with maximum negative trims then they'll be very little use to adjust for sea conditions.

'Tabman' again is right with 'to optimise running over a broader range' but again that is no good if it doesn't actually achieve a correct pitch even at WOT. Your comment about critising aircraft design is wrong as that's exactly what flaps are for, to improve low speed performance(like a boat) and your comment overlooks the fact that planes don't usually use flaps to correct wing profile at WOT maximum speed for a light load.

'Ilan Voyager's' comments about the Cancun with twin Ford V8's is not surprising as that is a boat/installation from 20+ years ago or more. My comment are about a boat designed very recently by a well known designer with an International boat builder.

I'm not sure what 'philSweet' is getting at but I would state its obvious that you need no more trim than that required at WOT. The question would be 1)has it got enough at WOT and 2) Is it achieved with a satisfactory balance of organic inbuilt hull trim as opposed to external devices.

'Joakins' comments are were I came in.

Shouldn't a modern design with crudentials achieve a better organic trim, especially with the design actually locating the heavy tanks/batteries and adjusting ballast to aft, when the design could accomodate a more forward positioning?

 

[FMS]

A boat that requires drives to be fully tucked in at WOT and interceptors down at WOT is unusual. It is assumed the transom angle is a standard 12 degree angle.

Can you describe in more detail what happens if the drives are not fully tucked in and if the interceptors are not mostly down approaching WOT?

Can you also specify the drive and interceptor position at optimal cruising speed?

 

[Mr Efficiency]

Worth trying bigger, wider tabs as an experiment, doesn't necessarily create more drag either.

 

[tunnels]

Worth trying bigger, wider tabs as an experiment, doesn't necessarily create more drag either.

How can more surface area in the water not create more drag ? what about water friction ??:idea:

 

[FMS]

How can more surface area in the water not create more drag ? what about water friction ??:idea:

Compare drag created by moving a larger tab at a smaller angle (less depth aft) to a smaller tab at a larger angle (more depth aft)

This is a separate discussion as it was specified this boat has "QL interceptors"
A study comparing drag of tabs to Interceptors was published by
http://maritimedynamics.com/interceptor.pdf

 

[tunnels]

Compare drag created by moving a larger tab at a smaller angle (less depth aft) to a smaller tab at a larger angle (more depth aft)

This is a separate discussion as it was specified this boat has "QL interceptors"
A study comparing drag of tabs to Interceptors was published by
http://maritimedynamics.com/interceptor.pdf

What is the better trim tab and wide one each side but going almost completely across the boat ?? or a long but narrower one that exstends back from the hull each side? both having the same surface area

 

[Mr Efficiency]

What is the better trim tab and wide one across the boat ?? or a long but narrower one each side?

The usual answer, "it depends" Certainly wider is more effective in generating lift per unit of area, if they remain in the water at all material times, but I guess if they aren't flat plates, that too could vary. If the specification is interceptor, then surely one that occupies a greater wide of the hull could be the ticket.

 

[Ilan Voyager]

"Shouldn't a modern design with crudentials achieve a better organic trim, especially with the design actually locating the heavy tanks/batteries and adjusting ballast to aft, when the design could accomodate a more forward positioning?"
Yes, but you won't get the "amount" of accommodations that customers of today are used to for the same length. You can even have planning boats without tabs, interceptors and another devices.
That exists in several navies; small patrol boats.
I've uploaded several pics of a very successful one of the French Navy, a bit too big, but a good example of the results obtained when a boat is designed for efficiency and not fashion. Designed by RAIDCO, a design office I know well.
The boat is very heavily built (Lloyds A100) as shows its displacement of 41 metric tons full loaded. As you can see, there is nothing fancy but just optimization born from years of experience at sea.
The boat is able to navigate with comfort at sea state 4, 350 days a year, or 1200 hours (about 10 times the annual use of a yacht...)
The VCSM is highly apreciated and has been sold to several navies. It would make the base for a nice yacht.
Look at the trim full speed at 28-29 knots...and the easiness of going on the deck. I'm maybe too old but I find it beautiful.

 

[bertho]

Ilan,
agree but , to be fair, should give to camarc design www.camarc.com/ ( " sir"alastair cameron ) the palms .. they are the designer for raidco , JM M ...... as you know !
all the best
bertho

 

[Ilan Voyager]

Ilan,
agree but , to be fair, should give to camarc design www.camarc.com/ ( " sir"alastair cameron ) the palms .. they are the designer for raidco , JM M ...... as you know !
all the best
bertho

Bertho, You're right. (Raidco engineers have their part also, it's a collaboration like in all the working boats, where there is no place for hypertrophied egos) I love their boats!!! If I was rich, I would have a yacht designed by Camarc on the basis of a patrol boat.
Thanks for the link, I'm having great pleasure in detailing it.