Trim tabs don’t alter pitch of boat

Revisiting this topic, now that my boat is out of the water. What metal and how thick should the extensions to the trim tabs be? Stainless? 1/8”?
 
1/8" stainless will be sufficent, but you'll need to add 90 degree bends down on the sides, and a 90 degree UP on the aft end to maintain rigidity on the tab under load.
 
Thanks, DaveKamp. Would 1/8” aluminum work as well? Much more affordable, and will be painted with anti foul paint anyway. Agreed on the edge bending.
 
You could use aluminum, but you will find that it will fatigue and crack along either the side bends, or the back bend, because it is most susceptible to fatigue along the grain lines. Extending the tab length imparts lots more pressure on the ram's load-point than increasing the tab width, so the edges will see much higher stress.

The problem you're faced with, Allan, is that the hull, while a planing design, does not generate lots of lift in the rear, simply because the propellers are recessed into tunnels. Hydrodynamically, the tunnels 'steal' the lift that would normally be generated by this flat surface, thus, the hull naturally 'squats'. This same character occurred in the Penn-Yan 'tunnel-drive' design. IF the hull were to move fast enough to flatten out, your propellers would be surfacing... and it's possible to actually RUN it that way, you'd need different wheels and a whole lot more power swinging'em to get to that point.

I spent all night thinking about it, and the only PRACTICAL way I see that might help, is a somewhat radical thought... Would be to remove the hydraulic tabs, and make a flat extension like GFC's, but instead of just the keel, take it all the way up to the chine, and pass it THROUGH the direct wash of the props. I'd make it so that it had about 4 degrees of downward cant, on adjustable (threaded) struts, and the leading edge (directly in the prop wash) folded over, so there was a 'soft' edge on the lead. It would need angle welded to it through center and at the edge for stiffening... That would provide SOME respite from the loss of lift. Unfortunately, it would generate a little loss of thrust, and it would be susceptible to fouling with whatever flotsam the wheels didn't shred...

The reason I came to this conclusion, is because the running angle you're at, is the result of the center of gravity being supported more forward, rather than in the area of the tunnels. Hydraulic trim tabs are dandy for hulls that are consistently supported, but the drive tunnels aren't lift-generating once the initial climb has been made... they're basically just plumbing.

Another possibility I pondered which you could TRY... is to form an extension with a curve in it, that would direct thrust downward, to help provide the flatter running angle, but what you'll actually be doing, is trying to change the boat's planing attitude by simply using propulsion thrust to LIFT it, rather than using the hull's shape to support it's center of gravity at your chosen angle. This means it'll be using fuel to stand upright, rather than go forward... and when you have a pair of 3208's roaring away, you'd probably prefer that fuel go to better use covering distance than providing a better view and digging a very temporary hole in the lake....

After all, 3208s are VERY efficient at turning diesel fuel into noise and smoke...
 
Wow, Dave, I’m really impressed that you spent so much time thinking about and responding to my issue. Seems you know a lot about hull design, certainly much more than me!

So, am I stuck with a hull design that will never plane well unless I do some drastic stern redesign like what you suggest? I truly wondered whether the huge propeller tunnels were a good or bad thing. The boat does begin to plane if I’m at WOT, but that’s not how I typically travel.

I guess wouldn’t mind not planing at lower speeds, if I knew that my fuel efficiency and speed were not “suffering” from the stern being low. I like puttering along at 8-10 kts, and even at that speed the stern is low, and a huge wake is being generated. It just doesn’t seem right…I can’t believe the SR engineers got it that wrong. It would be nice to know if other SR owners that have my hull design have similar issues.

While I am pretty handy making/fixing things, I think a stern alteration that you describe would be out of my league. BTW, what is a “GFC”? Fiberglass?

What I may do this winter is to widen and lengthen the existing trim tabs as much as possible, and see if that makes a significant difference to the trim for the 2022 season. That much I can do myself. It may be for naught, but then I would know if the more drastic measures you describe are necessary. Do you think adding stationary SS extensions to the bottom of the stern at the keel would help? That would be relatively easy to add as well.
 
AllanS said:
Wow, Dave, I’m really impressed that you spent so much time thinking about and responding to my issue. Seems you know a lot about hull design, certainly much more than me!

So, am I stuck with a hull design that will never plane well unless I do some drastic stern redesign like what you suggest? I truly wondered whether the huge propeller tunnels were a good or bad thing. The boat does begin to plane if I’m at WOT, but that’s not how I typically travel.

I guess wouldn’t mind not planing at lower speeds, if I knew that my fuel efficiency and speed were not “suffering” from the stern being low. I like puttering along at 8-10 kts, and even at that speed the stern is low, and a huge wake is being generated. It just doesn’t seem right…I can’t believe the SR engineers got it that wrong. It would be nice to know if other SR owners that have my hull design have similar issues.

While I am pretty handy making/fixing things, I think a stern alteration that you describe would be out of my league. BTW, what is a “GFC”? Fiberglass?

What I may do this winter is to widen and lengthen the existing trim tabs as much as possible, and see if that makes a significant difference to the trim for the 2022 season. That much I can do myself. It may be for naught, but then I would know if the more drastic measures you describe are necessary. Do you think adding stationary SS extensions to the bottom of the stern at the keel would help? That would be relatively easy to add as well.
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GFC is a member. “GoFirstClass”
 
Allen, I do have a set of 12 x 30 drop fin trim tabs from Bennett in my garage. I was going to add them on my 340 Dancer but I sold it first.

Let me know if you are interested.
 
Steve S said:
Allen, I do have a set of 12 x 30 drop fin trim tabs from Bennett in my garage. I was going to add them on my 340 Dancer but I sold it first.

Let me know if you are interested.
Click to expand...

Can you send me a photo or two, Steve? I might be able to adapt them.
 
AllanS said:
Sorry, another newbie question. While at cruising speed, say 12-15 kts, I expected that activating the trim tabs would lower the bow, but no matter how I adjust them, they seem to have no affect. I did test the trim tabs this spring while on the hard, and then do activate when the helm switch is used. PO said “not to expect much bow down or planing out from this boat”. Something doesn’t sound right to me…thoughts?
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12-15 knots is kind of on the low side to see much trim tab effect. You are just barely out of the hole. If they are functioning, larger tabs would improve things as others have mentioned.
 
Allan, sorry to not reply sooner. I've been busier than a 1-legged man in a cat kickin' contest.

The design of those tab extensions was the work of the guy who owns the boatyard that recommissioned my boat after it was decomissioned before trucking it across the country from the Detroit area to Portland, OR. He is a nautical engineer with his doctor's degree (IIRC) in some wild azz marine specialty.

He designed a much longer (fore to aft) SWP and it worked well but it created a new problem: The center roostertail came up beneath the SWP and created a drag effect on the boat. He put his head together with another nautical engineer and they came up with the idea of extending the running surface to push that roostertail further aft. It worked fine so they designed the trim tab extensions to go along with it.

The tab extensions and the center hull extension were both made of 1/8" stainless. In about 9 years of use before I sold the boat, the extensions worked great. They were tested in everything from running at low speeds in calm waters to running at planing speeds in some 8' to 10' waves. (don't believe the size of those waves? Go to youtube and search for "Umatilla Days video 1". Watch that, then #2, #3 and#4).

If I can answer any further questions don't hesitate to ask.
 
AllanS said:
Thanks, DaveKamp. Would 1/8” aluminum work as well? Much more affordable, and will be painted with anti foul paint anyway. Agreed on the edge bending.
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Aluminum will not work. Its far too soft, and will corrode in salt water and then break. You need to use stainless steel. You should contact Bennett Marine and get their thoughts and advice.
 
I did talk with Bennett, and they had no ideas or solutions. No help at all.
 
Yeah, they won't... it isn't in their best interest to do so.

1/4" aluminum is really hefty... it is rigid enough that side and end bends aren't necessary to maintain rigidity. Corrosion is a problem in ALL water, but there's lots of variables. The little black high-speed tabs on the runabout in my avatar are factory Bennett, and aluminum... and they do NOT have side bends, only along the back edge, so the grain-line going fore-aft will not result in a fracture.

The think one has to keep in mind, is what kind of load the tabs are REALLY under. Water pressure is an evenly-distributed load across the surface. The leading edge is securely supported by the hinge at the hull, and the load is supported at the trailing edge, or somewhere midpoint from hinge to trailing, by the actuator. IF the load on the tab's flat surface is high enough for flex to occur between the hinge and actuator, or from the actuator to the trailing edge), then fatigue will occur.

Stainless will tolerate the deflection through a whole lot more cycles, and to a lot greater distance, before it will start to work-harden and fatigue.

If it wasn't such a big boat, I would be willing to bet more on aluminum, particularly if one were to rivet on a pair of angle-iron braces to the fore-aft edges, and bend the rear edge up (with grain going fore-aft).

In this case, we really have two problems- One, it's a really big beast, that has enough horsepower to push it up a bit, but it has two tunnels in the bottom where hydrodynamic lift can not efficiently match what's happening up front.

If there were a pair of Merlin V12's down there, or better yet, a 1MW gas turbine coupled to both props, you'd certainly get more speed, and more lift, but that spot in the tunnels would STILL not bring up the transom. You could go Full-Bond, and stick a third prime-mover in the middle, to a waterjet in that low keel, and a set of flaps to cover the tunnels... none of these are really cost-effective or sensible.

Now, I don't know the REST of the lines of the hull, but I'm willing to bet that the naval engineer who laid the hull lines was well-aware of this, and they were probably very deliberate in it... they may have done it specifically for seakeeping (keep the bow high when running tall swells?).

The reason for putting the wheels in tunnels, is 1) to protect the wheels and 2) provide a shallower draft. It DOES work nice in that respect, running tunnels have helped many large cruisers get houseboat-like drafts They even do this with river towboats (albeit, the 'tunnels' are really just a big wide-open flat space aft of the rest of the hull). The layout allows the propshaft angles to be more in-line with the waterline... but don't pour on coals when the water is shallow, you will suck yourself down against a sandbar simply because you've pushed all the water out from beneath your hull.

Adding an extension to the 'keel areal' that exists would probably help a bit, but without having a whole mess of numbers, or SOME empirical data from testing, it'd be hard to say how much it'll take, or how much benefit you'd get... but one way, would be to build it way MORE than you'd think, and then trim it off a bit-at-a-time until you've reached a happy compromise.
 
Gofirstclass said:
Allan, sorry to not reply sooner. I've been busier than a 1-legged man in a cat kickin' contest.

The design of those tab extensions was the work of the guy who owns the boatyard that recommissioned my boat after it was decomissioned before trucking it across the country from the Detroit area to Portland, OR. He is a nautical engineer with his doctor's degree (IIRC) in some wild azz marine specialty.

He designed a much longer (fore to aft) SWP and it worked well but it created a new problem: The center roostertail came up beneath the SWP and created a drag effect on the boat. He put his head together with another nautical engineer and they came up with the idea of extending the running surface to push that roostertail further aft. It worked fine so they designed the trim tab extensions to go along with it.

The tab extensions and the center hull extension were both made of 1/8" stainless. In about 9 years of use before I sold the boat, the extensions worked great. They were tested in everything from running at low speeds in calm waters to running at planing speeds in some 8' to 10' waves. (don't believe the size of those waves? Go to youtube and search for "Umatilla Days video 1". Watch that, then #2, #3 and#4).

If I can answer any further questions don't hesitate to ask.
Click to expand...
Enjoyed those videos, that’s crazy, loved the progression from 1-4
 
Thanks for the multiple responses, lots to cover here.

Mike, I too loved your videos…#4 reminded me of my wife and I headed to Block Island last October, where 8-10 footers made her turn green and the furniture was rearranging itself on the aft deck. I turned back and sought shelter behind Fisher Island. Waves over the bow were scary!

Dave, I appreciate your clarity on SS vs aluminum. If I go with aluminum, it will be 1/4”, not 1/8”.

Regarding alterations to my current trim tabs and hull, I understand that no one can give a firm solution on what to do. I DO appreciate everyone’s observations; it does help with the thought process.

That said, what would be most helpful is to find other 1989 440 AC owners to ask the what their experiences are. Does anyone know how to search this forum based on boat model?

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?
 
Mitch, I appreciate your point, I just didn’t want to drag this out online for those that weren’t interested.

I have not had the boat weighed, in fact, I don’t know if the marina I’m in can do so. The factory specs say 23,000 lbs. I’m sure it’s much more than that equipped. In fact, this boat has a 120 gallon water tank under the stern bed…recognizing that water is heavy, I tried to keep that tank only 1/3rd full to lessen the stern weight.
 
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