Trim tabs don’t alter pitch of boat

Getting back to this discussion, I think I’ve decided on the following, pending response from you guys. There are pics of my boat’s transom in a previous page of this thread.

1. The existing trim tabs are 12” cord length x 20.5” width, and extend out only to the edge of the transom. I am thinking of adding a 12” cord length x 26” or 28” width extension tab to it, not overlapping the existing one, but using angle SS or aluminum supports on each side and the middle. Each angle piece would be bolted in three places on both the existing and extended tab. The angle pieces on each end would be 3” deep to act as drop fins on each side. I may need to bend the outer edge of the extension tab to conform to the side chine on the transom.

2. The “V” of the keel between the prop tunnels is 12.5” long on each side (that is, each leg of the “V” is 12.5”. I propose to add a pair of extensions that extend this “V” at least 12”. In this case, I would overlap the bottom of the keel by at least 12” and attach the extension using hefty screws or lag bolts (I have to verify the thickness of the hull here). I would support the upper sides of the extensions with SS support rods attached to the transom.

Thoughts? Questions? Am I crazy?
 
Found a cool app on the iPhone store. Clinometer. Measures bow to stern pitch; side to side angle. Wont replace your inner ear it it’s kind of cool to put phone on dash for adjusting your trim tabs.
 
AllanS said:
Getting back to this discussion, I think I’ve decided on the following, pending response from you guys. There are pics of my boat’s transom in a previous page of this thread.

1. The existing trim tabs are 12” cord length x 20.5” width, and extend out only to the edge of the transom. I am thinking of adding a 12” cord length x 26” or 28” width extension tab to it, not overlapping the existing one, but using angle SS or aluminum supports on each side and the middle. Each angle piece would be bolted in three places on both the existing and extended tab. The angle pieces on each end would be 3” deep to act as drop fins on each side. I may need to bend the outer edge of the extension tab to conform to the side chine on the transom.

2. The “V” of the keel between the prop tunnels is 12.5” long on each side (that is, each leg of the “V” is 12.5”. I propose to add a pair of extensions that extend this “V” at least 12”. In this case, I would overlap the bottom of the keel by at least 12” and attach the extension using hefty screws or lag bolts (I have to verify the thickness of the hull here). I would support the upper sides of the extensions with SS support rods attached to the transom.

Thoughts? Questions? Am I crazy?
Click to expand...
B02CAAEF-18D0-462F-A919-A520AEDAA8BD.jpeg
 
Hoping for some feedback from Dave and Mike. Thoughts, guys?
 
>>I’ll have more specific questions for Mike and Dave as I consider my options and decide on what mods to make. Do either of you mind a PM discussion?

No, I'm fine with a PM.
 
Where can I find the PM email addresses? I don’t see them under the members section.
 
My apologies for the length, better refill your coffee cup before this one:

The effect that trim tabs have, is simply changing the shape of the hull's bottom, thus, causing a change of support. Most people have a simple understanding about how they work, and usually, that's sufficient to get a desired result, but in this case, there's more one needs to understand.

In the field of fluid dynamics, there are two types of elements. There are drag elements, and lift elements. A common farm windmill uses flat pieces of sheet metal or wood slatting, which will be deflected to one side upon the appearance of wind. This is called a 'drag' element. The converse of a drag element, is a LIFT element, which is what's found on the blades of a modern wind turbine, or an aircraft wing. It is not a flat shape, it is a rounded shape... an airfoil... and when looking at the cross section, the airfoil has TWO curves... the upper, and lower curve... air travelling beneath the lower curve travels a shorter distance than the air passing above, and the result, is a differential of velocity above and below, and it is this differential speed which causes differential pressure, thus LIFT. A DRAG device generates force strictly by deflection, and the back side of the surface is nothing more than a turbulent zone. In aircraft terms, the back side of the blade is in a 'stall'... generating no lift.

The amount of physical force that EITHER will produce, is a function of mass and velocity... the amount of fluid passing over and under an element, determines how much deflection it will present. More mass, or more velocity, equates to greater force.

Let's put this in perspective of non-compressible fluids... like... water... for obvious reasons, water will support more lift than air, because of it's greater DENSITY. Now, one could say that water is not compressible, but in the case of naval engineering, you CAN'T say that... because the water passing under the hull of a PLANING vessel... is well-entrained with air bubbles.

A side description: If you wanna sink a boat, remove all the water from beneath it in the middle, and it will split in half under it's own weight. That's the idea of a torpedo or depth-charge. Yeah, the shock is high, but taking away support is what does the deed.

When splashing over waves, a hull's support isn't pure... and my prior note about the prop tunnels not providing substantial LIFT, is relevant to that issue. Velocity through the tunnels is high, but it's not providing much hull support.

Now, this is a big boat, and it wasn't intended to be an interceptor. There's a substantial amount of surface, but the transition from displacement mode, to planing will not be very dramatic. A planing hull is referred to as 'generating lift'... but it's not doing it like an airfoil... it works like a drag element... it's skidding across the surface. What's happening, though, as a planing hull increases speed, is that the deflection force of the water being forced along it's CHINES, are causing higher surface pressure, thus reducing wetted surface and concurrent drag. With your hull, those 8-lung kitties are at full-snot, the hull is probably at it's highest elevation possibility, for no reason other than there's just no higher place to climb.

With exception of trim tabs that can be retracted to a point HIGHER than the running surface of the hull, a tab does NOT increase wetted surface... it merely changes the angle of the surface it projects... and being adjustable, it allows the operator to make adjustments to the hull's attitude, as long as the speed through water is high enough for the reaction force of water against the tab at that angle, is sufficient to act against the boat's center-of-gravity.

(Sorry Pirate Lady... I'm doing my best to avoid the math here! :-D )

In a high-performance application, a trim tab's existance at ZERO ANGLE (meaning, it's flush with the bottom of the hull) doesn't generate any lift... provided the boat is travelling at a high planing speed, and has a neutral pitch (look up roll, pitch, and yaw). What the tabs WILL do, at a neutral pitch, is provide surface that, as the nose rises, the back of the tabs appear as an EXTENSION of the transom, so a rise of the nose becomes translated to high forces on the back of the tab, and effectively, the hull becomes that-much-longer, so the center-of-gravity effectively moves FORWARD. Take a look at my little runabout... those tabs basically NEVER run down below neutral unless I'm teaching a 5 year old to ski... but when I'm running above 55mph, having those tabs sticking aft means that when the nose comes UP, the tab's back edge tends to keep it from continuing it's climb upward. Not relevant (in velocity terms) to Allan's issue, but if you were at the helm of my runabout at 65, you'd recognize just how nice it is for that to happen when the wind blast wants to make aircraft out'a ya... but it's important to understand how that physical extension affects running attitude... because of what it does to the apparent relationship of CENTER OF GRAVITY. Running angle of ANY vessel is a combination of static balance, and the myriad of forces acting upon it... hydrodynamic deflection being one, thrust ANGLE being another (tunnels for the props having significant impact on BOTH here).

What Allan is looking for here, is a more level RUNNING attitude at a generally slow plane.

In order for trim tabs to have a substantial impact, they'll need to have lots of surface AND have to be far back from the center of gravity. A trim tab, extended deep, is nothing more than a remote-controlled sea anchor... most of it's useful work will occur within a range of zero to 15 degrees of angle.

Adding more surface WILL help. Getting it farther aft WILL help... but I think the biggest challenge, will be getting enough surface, far enough back, to effectively take over command of it's center of gravity.

The keel extension is effectively increasing the length of the hull, and in the case of exceeding displacement speed, provides a certain amount of supporting lift. It would be better if that extension reached all the way from keel to gunwale edge, but that puts it right through center of the prop wash. If it were me, and I REALLY WANTED it... that is exactly what I would do... I'd make it out of stainless, and put a teardrop-edge on the front of that edge, partially for strength, but also to help distribute the load of spinning propwash. I would put some strong stainless angle on the top back edge, and run supports to the transom, probably adjustable via large turnbuckles so that they could be adjusted, or disassembled for service, as I'd expect to be removing fouling flotsam every so often.

If you were REALLY REALLY dedicated to it, making basically two big trim tabs that span from keel to gunwale on large actuators...

But just adding the additional running surface would be the first-and-foremost concern.

And I'll note- I'm NOT a naval engineer... I'm just a hands-on guy and a very sick polymath that spends too much time tinkering with all sorts of boats, and my favorite is older Sea Rays. I suspect the engineers who laid out the lines of your hull had a planing profile in mind that had only a certain amount of planing lift, and that the angle you're seeing might be intentional for seakeeping reasons. Big cruisers are heavy. Planing a hull high means higher hull pressures and reaction forces, which requires more materials thus, a heavier hull, which means more power required, and more fuel consumed, thus, shorter range or more fuel weight necessary... to beat the snowball, they had to draw a limit somewhere.
 
Thanks for the shout out!
I actually understand your post.
Some points, tell me if I am wrong…
1. SeaRays have a natural tendency to run very bow high.
2. On 2 268 I had I replaced the tabs with ones for a 340. Boats were very ass heavy boats and the tabs improved on plane performance amazing to the point I could throttle down to 2300 rpm and remain on plane.
3. My 250 requires tabs to get on plane fast, with tabs I can get up quick without opening the 4 barrels and keep side balance even (8.6’ beam), so unless I want to run balls out it requires some degree of tabs.
4. Can’t stay on plane below 3k rpm without tabs down.
5. May replace tabs with the biggins so I can throttle back in Chesapeake chop and remain on plane.
All this make sense?
 
Dave, thanks for that extensive explanation of hydrodynamic forces in play regarding the boat hull. I understood them as well; you have a gift for explaining complex forces in an understandable manner. I was comfortable with your discussion of drag and lift, as I like aviation, although not a pilot.
So there was a lot of theory in the physics of how a boat moves through the water. I wish I could shrink my boat to a small scale replica so that I could experiment with potential modifications that would increase stern lift under a variety of boat speeds. As a scientist myself, there’s nothing like empirical data to tell you what is really happening. I’ve never been one for modeling the data to predict what is happening.

I tend to believe you about what is really needed, namely an extension that spans the entire stern, almost like a “wing” to provide a long running hull and the lift that provides. That’s a drastic alteration for me in a single go. Again, as a scientist, I like to test modifications a step at a time to see if I’m on the right track. Also, these modifications would be under a large swim platform that make adjustments impossible once the boat is in the water, so I’m thinking this is a year-by-year adjustment.

That said, do you think my proposed modifications (expansion of current trim tabs + adding extensions to the keel) would provide a testable solution for the coming year? Anything you would do differently? How much of an extension should the keel extension be, 12”, 18”?

BTW, I see Pirate Lady talking about RPM’s above 3000…my maximum rpm of the diesel Cats (3208 Turbo) is about 2700-2800, generating a speed of about 15 knots.
 
AllanS said:
Dave, thanks for that extensive explanation of hydrodynamic forces in play regarding the boat hull. I understood them as well; you have a gift for explaining complex forces in an understandable manner. I was comfortable with your discussion of drag and lift, as I like aviation, although not a pilot.
So there was a lot of theory in the physics of how a boat moves through the water. I wish I could shrink my boat to a small scale replica so that I could experiment with potential modifications that would increase stern lift under a variety of boat speeds. As a scientist myself, there’s nothing like empirical data to tell you what is really happening. I’ve never been one for modeling the data to predict what is happening.

I tend to believe you about what is really needed, namely an extension that spans the entire stern, almost like a “wing” to provide a long running hull and the lift that provides. That’s a drastic alteration for me in a single go. Again, as a scientist, I like to test modifications a step at a time to see if I’m on the right track. Also, these modifications would be under a large swim platform that make adjustments impossible once the boat is in the water, so I’m thinking this is a year-by-year adjustment.

That said, do you think my proposed modifications (expansion of current trim tabs + adding extensions to the keel) would provide a testable solution for the coming year? Anything you would do differently? How much of an extension should the keel extension be, 12”, 18”?

BTW, I see Pirate Lady talking about RPM’s above 3000…my maximum rpm of the diesel Cats (3208 Turbo) is about 2700-2800, generating a speed of about 15 knots.
Click to expand...
Allan,
I got single 7.4 gas, you have dual diesels. We can’t compare. Total different.
 
Well, you can't compare an apple to a pineapple, but one can compare the physics of each, to the circumstances they both operate within.

PL's 7.4 gas in a 25ft deepish V, and your twin diesels in a 44 Aftie both live in the same realm, and live under the same laws of physics.

Let's start with the very two basics: There's really only TWO things we can rely upon: Gravity and Triangles... Cats not so much, but Gravity and Triangles.

PL- your statement of 'natural tendancy to run bow high'... well, in comparison to many manufacturers, one COULD make a statement like that, however, it's not likely so. My personal experience with Sea Ray hulls, is mostly from 1969, to 1984ish, from 17 to 26 feet... but in that same time period, I've worked with Cruisers Inc, Wellcraft, Century, Donzi, Thompson, Tom Sawyer, Glastron, Reinell, Boston Whaler, Grady-White, Mako, Fabuglas, and many, many others... and I've worked with wooden runabouts (my Dad had a beautiful '54 Century Resorter 19 inboard)... and a dozen homemade round-nose and pickle-fork hydroplanes (some much safer than others)... and one thing I've learned, is that there's a whole lotta factors that make a flavor seem 'bow high' or 'stern heavy'.

Case in point, the Sea-Ray hulls prior to 1972 all seem to have a little bit of 'rocker' in the last foot or so of hull, while those after, were basically square, or slightly hooked. Other companies did similar, and the reasoning I've found most universally, was that manufacturers made the slight change in response to the 'fuel crunch' of that era... so that running less horsepower would still allow planing. One could jump to the conclusion that this was the same effect as adding trim tabs, but that wouldn't be entirely accurate... a planing hull needs a certain amount of thrust in order to climb. Less than sufficient, and you'll stay in a displacement mode, just digging a big hole in the water. It's not until you can get enough velocity for water pressure acting on the surface to break the transom clear, and start generating more surface pressure, than the weight of the boat... then you get lift... and with lift, becomes less wetted surface, less friction. The paradigm is that, by reducing horsepower, one can reduce the WEIGHT of propulsion equipment, thus, requiring less surface, and getting slower planing. In THEORY this all works, but we're all damned to live in a world of reality, not theory... running a lighter powerplant IS a great thing... and doing it with less fuel (and a smaller tank) DOES make for more available performance, but with 8 people and three coolers, 20 gallons of fresh water and a half-full porta-potty, four sets of waterskis... the idea of shedding 400lbs by going from big to small block... or dropping 250 by knocking a pair of cylinders off that small block, is simply NOT a significant change, save for just ONE fact: The center of gravity. Passengers and gear aren't so concentrated rearward.

Now, the comparison between the two boats... PL's 26foot is a very straightforward V hull. It has several chines to climb up on. The forward hull angle is probably significantly steeper than the transom (technical term... 'forward deadrise' vs 'deadrise at transom'. Off top of my head, my '70 17'er has probably twice the deadrise, if not more, than the aft end. Look at the little thumbnail, that's what... 10 to 15 degrees up on each side? Up front, it's 25 degrees. This has somewhat of an effect on how planing occurs at varying velocities, not because of the V angle, but because of how the chines are laid out on that vee... the line they run determines how they generate lift. On my 22'er (a '77), the change is more significant, but there's a lot less taper, and of course, there's no rocker to the back end. Comparing the two hulls, the latter SEEMS like it runs 'bow high', but pictures of the two side-by-side, the 22, and the 17, both running at 45mph with retracted tabs and drives trimmed for that speed, at minimum throttle , have the same running attitude. At 14mph though, throw all that away, they're relying on the prop thrust angle to keep 'em barely-on-plane. Given insufficient thrust, ANY planing hull will run 'bow high', and given sufficient thrust, it'll flatten out. Given AMPLE thrust, it will try to climb to the next higher level-of-plane (higher chines) until either you run out of chine, run or run out of thrust... and the latter is not just a function of hydrodynamic drag... it's also AERODYNAMIC. My 17' runabout has a low profile, but at 60+mph, there's much more wind-drag, than water-drag. On my dad's 25' Wellcraft Suncruiser, 27mph is all the 5.7 will do, and when there's a headwind, you know it...

Back to Allan's boat:

I don't see why you couldn't MODEL TEST modifications. If you have it hauled out, you could MEASURE the hull dimensions out well enough to make a scale model that's some workable size, then calculate what the scale WEIGHT should be (by doing a displacement test), and then BALANCE the weight in that displacement test, to determine center of gravity... and then put it in a flowing tank to see what the hull WANTS to do.

Would it be quick and easy? Depends on your skills. I'd think that a 4x8 sheet of pink foam (Foamular 250), with a hot knife, some glue, and a palm sander, you could get pretty close... or if you've got the right friends, a tabletop CNC router, and cut sections, glue 'em, then spray a light coat of epoxy on it, you could model the thing to amazing precision. If you're handy with woodworking, cut thin strips of wood, glue 'em together, and sand the exterior to shape, and hit 'em with some marine varnish.

Displacement test is easy: Look at YOUR boat, figure out where the waterline IS when it's sitting at slip. Duplicate the waterline on your model, and note how high it sits when you float it... then start adding weight, until you have the model hull form sitting SAME as the real thing. How much weight, and WHERE you put it , will prove ITSELF.

Once you have it modeled, you'll need a test tank... if you got any friends at schools that have a hydraulics department, they'll not only have one, but they'll probably have a student who needs a study project, and would be willing to help out, simply for the opportunity to do a practical analysis paper on it.

What I'd do, is set it up as noted, and drag it up to scale speed, and see if it exhibits the same planing attitude as the real thing. If it doesn't, put propshafts, propellers, rudders, and electric motors on it, and power it up. This will prove how much prop thrust is affecting running attitude.

THEN start adding elements in whatever way you can think of... and see what works. This will be the perfect 'acid test' before you go throwing money into screw-holes in the hull.
 
Not to pee in your Cheerios, but that almost sounds like more work than throwing a set of big trim tabs on his boat and see what happens. I mean, I love the idea of proving out a theory beforehand, but dang!
 
It might be, Nate- it certainly would be on my runabout, or my 22... or any of my others... they're all on trailers, dollies, or racks, and I have a marine forklift that'll easily put 'em wherever I choose... and my Dad's Well is in the Dri-Stak, so easy there...

But a 44'er isn't as easy to haul out... ;)

Now... building a model might be tough for some, but for others, it's easy. For a 44'er, just go 1" = 1ft... and you have a 44" model... Styrofoam is downright easy, and if he got really carried away, he could make a full running RC model of it, just to play with after all the experiments are done...

Oh, and one other important note with RE to PL's engine difference:

Big boats have their peculiarities...planing or not, moving lots of boat, means moving lots of water... lots of thrust. There's a basic bit of marine propulsion reality that slaps us in the face once we start striving for speed and efficiency:

Marine thrust is not linear, nor hard-coupled to horsepower numbers. You can build an engine for more horsepower, but that doesn't mean a boat will go faster. You can build more torque, but it won't assure that a boat will go faster. Why? Because the drive unit, regardless of wether it's a propeller, or a ducted prop, or a 'jet', or a paddle-wheel, there is a point where the SPEED at which the unit can move, will limit the velocity of whatever it can move.

Notice I didn't mention VOLUME...

Forward thrust is about moving a volume of water from the front, towards the rear. The DESIRED reaction is that moving water in that direction, results in thrust to force the vessel in the opposite reaction. The ultimate hope, is that in shoving 20 tons of water aft, you'll be shoving 20 tons of boat forward, right?

Let's say you want more thrust... so you go to a larger diameter prop, and put more torque into it... thus, moving more water, generating more reaction.

Spin that prop faster, to move more water, and you get more thrust... however, big props require lots of torque, and the large diameter of the blades mean that the tip speed of the prop is going very fast, even if the shaft RPM is slow. Put a reduction gear on it, use a big torquey engine, and you get that thrust.

What you cannot get, though, is forward speed.

Charlie Strang pointed it out to Karl Kiekhaefer... It doesn't matter how much horsepower you put on an engine... once you've reached the maximum speed that you can SPIN a prop, even with no load, you've identified the ultimate limit of speed. Pick whatever diameter and pitch you want, when you hit that max limit, you're done. In real-life practice, your ultimate speed is that maximum prop RPM or LESS, and it's determined by the amount of prop slip, and engine loading (hydrostatic drag + aerodynamic drag). IN a big cruiser, asking a 3208 to swing a 30" prop to 4000rpm just isn't happening. The torque curve for a 3208 fueled to 435bhp peaks at 1030ft-lbs at 2000rpm, and drops to 816ft-lb at 2800rpm.

I've got a Grumman step-van that came to me with a Cummins 4BT. Rated for something like 115hp, it was a dead-solid reliable engine, torquey and fuel-efficient, noisy as heck, and generated NO waste heat to warm up the aluminum breadbox in a -15F night. With the truck's gearing, and that engine's fuel feed governed to about 2250rpm, that truck would not go one ounce over 54mph. Didn't matter how much torque it had there, it would simply not spin any faster... and I didn't have another gear to throw into it's TH425. I made that gal midwestern-interestate-highway-worthy by putting in a '76 Buick 455 in it's place. Even in smog-years trim, the BBB had every bit as much torque and more, weighed about a third LESS than the oil-breather, and would spin all day at 4300rpm. Not surprisingly, 54mph to 75mph, that breadbox's aerodynamic drag is amazing... but surprisingly, the Buick still manages about 12mpg, and now I don't freeze on the way to/from a show).

Getting Allan's 44 to plane flat, means increasing planing surface aft (to combat the center of gravity) and getting a bit more velocity going (because surface and velocity are the two components of lift) and angle is a function of balance (getting more lift BEHIND the CG).
 
Yeah, I agree with Nate…while I am an RC modeler (planes), scratch-building a scale boat is beyond my skills and time available. You’ve given me lots to think about tho, Dave. Seems more and more I read from your explanation is to create more hull aft to move the CG. Isn’t that what I propose to do?
 
Yes, it is.

I wouldn't put it past ANYONE's skills to scratch-build one... but if you already have it hauled-out... have at it... but if you DO, I'd suggest take good pictures and measurements for future use.... as long as you have those, you can always hand the measurements and photos off to a neighborhood kid with a cheap 3d printer to make one... a guy can do a whole lot, in a short time, with a CAD program and a $250 3D plastic printer.
 
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