Trim Tabs and Rough water Operation

I'll measure tomorrow when daylight...but you can see in the pics I already posted that I have what looks like 4 -5 inches of ram travel and the tabs are extended maybe 8 inches below bottom of hull... my rams are mounted almost mid point on the tabs from front to back. If just mounted an inch to far to the rear it will reduce travel significantly more in angle. If to far to rear your ram is extending outward more and downward less.
 
Captain PartyMarty said:
I had look I can raise the engines by 1 more hole. If I wanted to go more than that I would have to take out two bolts and run with a 4 bolt installation instead of the 6 I have now. Would raising it by 3/4" really make much of a difference?
Click to expand...

The question about getting the motor height right is not about the mounting bolts...it's about where the anti-ventilation plate is relative to the water surface when you're trimmed neutrally while running. The anti-ventilation plates should be skimming above the water surface...obviously getting splashed but above the solid water surface. You need to get somebody to look over the transom when running and take pics to see how it looks.

Our boat has a single outboard. The motor was way too deep from the factory. We went up 1 hole and it improved overall performance. Went up another hole (2 above factory mount) and performance was better except we "blew out" in sharp turns. So for our boat/prop combo 1 up was the sweet spot, so we lowered back to 1-up and all is well.

When I say overall performance, we noticed a few things: the boat responded to changes of trim better. the boat was more efficient at cruise. steering feel improved.
 
Need some better pics. Side view with engine down in pic

Also better pic of the trim tabs. I want to see the bottom of the boat and close up of underneath them and how they are mounted etc.
 
Last edited:
If you want to push your trim tabs deeper into the water, you can put shims between the cylinder and the tab. Most tab manufacturers can supply them or you can build your own. Just make sure that the tabs will still retract to a position at least flush with the hull bottom. You can also build extensions onto your tabs. It helped my boat.
I really don't think that the trim tabs are your problem although they certainly can help to compensate for the real problems. Most of our fishing boats are stern heavy and yours may be severe with big fuel tanks at the stern. Motor height is important and I've seen lots of boats with "conservative " motor heights ( too low).
For stern heavy fishing and cruising boats where top speed is not really important, I really like 4 blade props. A properly sized 4 blade will improve hole shot, load carrying ability, cruise efficiency, slow speed (rough water) performance and do it all at lower rpm.
Good luck with your boat. A few changes can sometimes make it "a whole different boat" and result in more enjoyable cruising.
 
Old Fisher said:
If you want to push your trim tabs deeper into the water, you can put shims between the cylinder and the tab. Most tab manufacturers can supply them or you can build your own. Just make sure that the tabs will still retract to a position at least flush with the hull bottom. You can also build extensions onto your tabs. It helped my boat.
I really don't think that the trim tabs are your problem although they certainly can help to compensate for the real problems. Most of our fishing boats are stern heavy and yours may be severe with big fuel tanks at the stern. Motor height is important and I've seen lots of boats with "conservative " motor heights ( too low).
For stern heavy fishing and cruising boats where top speed is not really important, I really like 4 blade props. A properly sized 4 blade will improve hole shot, load carrying ability, cruise efficiency, slow speed (rough water) performance and do it all at lower rpm.
Good luck with your boat. A few changes can sometimes make it "a whole different boat" and result in more enjoyable cruising.
Click to expand...

I think hes got engine height and prop issues myself.
 
I have a boat with several hundred lbs of lead in the bow. With the engine as high as it can go, along with a 4 blade. That boat constantly moves heavy loads. I believe the engine cavitation plate on that machine sits about 5 inches above the original bottom v. The pod is back 33 inches. Pod is raked so what ends up happening is the carving the hull makes through the water, with the bow trimmed to it's best riding position, the pod ends up sitting ontop of the carving, and so does the cavitation plate on the outboard. This is the easiest way to pod a boat. Sometimes you need more flotation in the stern, which is a whole different topic all together. Looks to me like his pod sits a few inches higher then the original bottom V, and is not raked. Unless it's just the picture and I cant tell.

Also you dont need 6 bolts holding the engines on. So dont worry if you have to take it up and only use 4
 
I would look at using a wedge between the motor and the pod to increase negative trim before carrying more weight around all the time if the tabs can't push the bow down on their own.
 
I took more measurements of my tabs...ram is mounted 6.5 inches from the transom (centre mark) tab is 11 inches deep total. Ram travels 3 inches from retracted to fully extended. Bottom edge of tab is 6 inches below the hull when fully extended downward. (using a level and tape from the bottom of the tab to hull)
 
So I measured the motor height today to do this I leveled the POD then I leveled the engine and clamped a 2.25 square tube to the bottom of the cavitation plate. I ended up measuring about 2.75in between the bottom of cavitation plate and the original hull lined up with the center of the engine.

I think I Remember from the Pods Builder that the for every 12" of setback the engine should go up 1 in. I measured the setback along that tube from the back of the transom to the front of the motor leg. and got 44in. Using the offset rule I get a engine height of 3.67in based on that it would seem that Ship Happens is right and the motors are too low.

I also researched a bit about offset vs height and seems that 10:1 is also pretty common this would indicate that 4.4in would be the right height.

I am guessing that moving up 2 holes is in the cards to see what happens.

Also as you can see my POD has no rake upwards but the pod is offset from the bottom of the hull by about 3"

Profisher thanks for measuring all that I only get about 3in drop under the hull with a 2.5" stroke. Originally my boat had those same actuators/tab setup and so would have had the same extension. Maybe that's why I feel I had more Bow control with the Diesel Setup,

IMG_0981.jpg
 
I always end up moving an engine up. I most the time end up and inch and a half per foot of setback. You can always go down. 4 blades shine here. You will get less cavitation at lower speeds which enables more movement from a smaller pitch.

Nice looking setup. Little fine tuning and she will be singing


I really like the idea of shimming the actuators on the trim tab planes vs extensions. Very modular and super simple. Its genius. If need be
 
I got this response from Bennett today.

Sounds like an additional option which might help, I think I will order these.


"
Thank you for the photos. The installation looks proper. It looks to be your outboard bracket and the twin outboards are pushing your center of gravity further off the stern causing the trim tabs to be less affective than expected.

Your situation requires more trim tab lift capability, but you have limited mounting area to work with on your vessel to improve your situation. For this reason, we offer a product that is called 3” Drop Fin Add-ons (DF12, $32.64 USD set), which is an addition that can be added to a currently installed 12” Chord (fore-to-aft) trim tab to increase its performance by channeling the water flow under the tab and holding it there to produce the maximum lift capable of a given size trim tab. They are 3” fins that angle 90 degrees down on the sides, there by not allowing the water flow to “escape” and keeping it under the tab for the full length. You attach them to your current trim tabs by drilling holes (16 TOT, 8 per trim plane) and using stainless steel machine screws & locknuts (not included). You can Special Order them from any of your local marine dealers, catalog &/or Internet retailers – area marine distributor. Please use our “WHERE TO BUY” webpage (https://bennetttrimtabs.com/find-a-dealer/) to locate someone in your area that will be able to help you with your Special Order."
 
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