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MV Wanderlust · 28-06-2017, 05:42

Good Morning,

Our 42' Chris Craft's optimal RPM is about 2300. When we run it up to cruising RPM and then extend the trim tabs, we level out and pick up anywhere from 3.5 to 5 mph on speed. All of that is normal and expected.

So here's the situation and question: Not long ago, we were running about 10 mph, maybe around 1400 or 1500 RPM. The bow was up a bit. As an experiment, I extended the trim tabs. The bow came down a little and we picked up an additional 2 mph. I was surprised since I don't think we were going fast enough to be on plane. Our Chris weighs 36,000 lbs and has 660 hp. We were not on plane at that speed, right? Yet the trim tabs had an obviously positive effect. Is this normal? Does everyone experience this? In normal seas, is there a point where trim tabs aren't going to help?

ranger58sb · 28-06-2017, 05:55

Quote:

Originally Posted by JohnEasley

s this normal? Does everyone experience this?

Normal, but actual impact depends on boat. And every boat is different. You're experimenting the right way.

-Chris

St. Elsewhere · 28-06-2017, 06:24

Trim tabs have greater authority at higher speeds. As you have discovered they also have some effect at lower speeds. The best practice is to adjust the tabs at the desired RPM's to find the tab position that yields the best speed through the water.

tuffr2 · 29-06-2017, 04:40

Question - does the RPM increase a small amount when the speed increases using the trim tabs.

I am used to outdrive and outboard trim. When I trim the engine the RPM's will also increase.

MV Wanderlust · 29-06-2017, 05:11

[QUOTE=tuffr2;2423353]Question - does the RPM increase a small amount when the speed increases using the trim tabs.QUOTE]

The RPMs stayed steady on our boat. Only the boat's angle of attack and speed changed.

rhubstuff · 29-06-2017, 09:05

With your trims tabs up and the bow in the air, your waterline length is decreased. When you force your bow down using trim tabs, your waterline increases, thus increasing hull speed.

MV Wanderlust · 29-06-2017, 09:39

Quote:

Originally Posted by rhubstuff

With your trims tabs up and the bow in the air, your waterline length is decreased. When you force your bow down using trim tabs, your waterline increases, thus increasing hull speed.

Makes sense.

Tabman · 29-06-2017, 14:33

It's quite normal, particularly if the Tabs are properly sized for the boat, we have seen this sort of thing on many boats, and the speed you were going is just about where it happens.

Since you have straight shafts (no power trim) the propellers angle of attack is governed by the hulls attitude. Not only did your increase the boat's waterline length as you brought the bow down a bit, you also brought there angle of the prop closer to being parallel with the water flow. This is more efficient, reducing slip and increasing thrust. Thus you go a bit faster.

Tom McGow
Bennett Marine

tuffr2 · 29-06-2017, 17:21

That does make sense...now what if the boat has outboard power and trim tabs. Not sure how those two will act together and not sure why even put trim tabs on a boat that you trim with the outboard tilt angle.

Tabman · 29-06-2017, 17:38

Adjusting the boat’s attitude with the prop is not as efficient. It's function is to produce thrust, when you angle it to control attitude the thrust is at an angle and not pushing the boat straight forward and as mentioned you induce more slip.

There ia as section about this in this article I wrote about using Trim Tabs a while back.

Tom
Bennett Marine

If you are new to using Trim Tabs you may find this helpful in learning how to use them to achieve the best results. All boats react differently to Trim Tabs and the best way to find out what works best for yours is to experiment, and remember, use short bursts of the controls and let the boat settle down between corrections. Using them will soon become second nature.

Also some boats that suffer from "wandering" at low speeds (particularly I/Os) will benefit from having the Tabs fully down at no wake speeds. They sort of act like feathers on an arrow and make steering a little easier.
Tom McGow
Bennett Marine

Getting and Staying Trimmed

All boats assume different fore to aft attitudes at different throttle settings and vary in sensitivity to lateral weight distribution.

A boat’s optimum running attitude is determined by the operator. While some people may define optimum running attitude as the highest possible speed for a given amount of engine RPMs, others desire the best possible fuel economy, yet others may be trimming the boat to get just the right mix of speed and wake (such as for waterskiing.)

Optimum running attitude is when the boat is running to the operator’s satisfaction for the given operating conditions. There are as many optimum running attitudes as there are boats and boat owners

A good way to determine a boats optimum running angle is to run the boat lightly loaded at full speed in calm water. During this test observe the boat’s bow in relation to the horizon. Most boats run at or near their optimum attitude under these conditions. This should give you a feel for the appearance of the wake and bow spray when running at an efficient attitude. Note that not all boats will achieve their optimum running attitude under these conditions. Some boats will benefit from extra lift even when running at their maximum throttle settings. If you feel the boat will benefit from added bow down trim when running at speed start with the trim tabs fully up and deflect the trim tabs in short bursts. Be alert to changes in the boats handling, as you bring the bow down. Observe any changes in RPMs and/or speed. Adjust power trim if applicable.

Indications of Running Untrimmed
When a boat is running untrimmed the bow spray will exit the sides of the boat far aft. The stern wave (wake) is high and curling like a breaker on the beach. The rooster tail is high and close to the stern. The engine is laboring and the ride tends to be less smooth.

Indications of Running Trimmed
The bow spray moves forward and is flung not as far from the boat. The wake diminishes in height, as the rooster tail flattens out and moves away from the boat. The engine is operating under less load as evidenced by the tachometer and speed as well as sounding “less strained”.

One Step at a Time
The key to obtaining optimal results from trim tabs is to operate them in short “bursts” and let the boat react before making another adjustment. The amount of time between corrections is influenced by the size of the trim tabs and the boat’s speed. This will help avoid overtrimming or ending up with one tab too far down when correcting lateral trim. You will quickly become acquainted with a boat’s particular traits.

Take Off
Properly sized trim tabs can significantly reduce the time needed to get up on plane. They also allow a boat to keep its bow down and stay on plane at lower speeds.
As the throttle is advanced the stern of the boat begins to squat, lifting the bow. As the boat accelerates, push the bow down position of the helm control in short bursts. The boat reacts by the stern lifting, the bow coming down, speed increasing, and reduced engine laboring. If you over do it and deflect the tabs too far the boat will end up overtrimmed. When over trimmed, the steering becomes “over sensitive” and wants to pull off course to port or starboard. If this occurs, operate the control “bow up" until the desired attitude is established.

Getting the Most from Power Trim
Adjust the trim tabs to achieve the desired running attitude. Then use the power trim to position the propeller thrust parallel to the water flow. If necessary, re-adjust the trim tabs to fine tune the attitude. By observing the boat’s speed and engine RPMs the best combination of trim tabs and power trim will be apparent. Trim tab angle indicators and a power trim angle indicator are particularly useful in duplicating effective settings.

Trimming to Sea Conditions
When running into a head sea you want to trim the bow down so the sharp forward sections of the boat do their work cleaving the waves. This provides the most comfortable ride and minimizes stress on the boat (and passengers). In a following sea the tabs should be fully retracted for maximum steering response.

Correction of a List
The normal control setup for trim tabs operates in relation to the desired changes in trim and not the actual movement of the tabs. Therefore, do not think about what the tabs are doing, but rather on the control and what you want the boat to do. As above, make the corrections in bursts and allow the boat to settle to the new settings. You may find it easier to correct the boat’s fore and aft attitude before you correct the side to side trim.

Correction of Porpoising
Operate the tabs in very short bursts of about half a second. Continue until porpoising subsides. The objective is to have only a very slight amount of tab deflection, just the amount needed to cure the up and down motion of the bow.

MV Wanderlust · 30-06-2017, 04:46

Quote:

Originally Posted by Tabman

Since you have straight shafts (no power trim) the propellers angle of attack is governed by the hulls attitude. Not only did your increase the boat's waterline length as you brought the bow down a bit, you also brought there angle of the prop closer to being parallel with the water flow. This is more efficient, reducing slip and increasing thrust. Thus you go a bit faster...

If you are new to using Trim Tabs you may find this helpful in learning how to use them to achieve the best results. All boats react differently to Trim Tabs and the best way to find out what works best for yours is to experiment, and remember, use short bursts of the controls and let the boat settle down between corrections. Using them will soon become second nature.

Wow, such a wealth of information! Thanks, Tom. That helps a lot. The clear explanation of the dynamics makes it very understandable.

Ken Z · 30-06-2017, 07:11

Your right in running different rpm with the angle of your trim tabs. Every boat and sea condition are different. A situation when i wouldnt think to trim to is a large following sea where a bow down angle would maybe create a more wetter and unpleasant ride.