"Correct" Trim Adjustment for Bass Boat

Sorry for the long post, but I want to give you all the details.

I have a 17' Champion Bass Boat. It's an old model (1979) that has the cockpit area set toward the rear. It has a V-150 Mercury outboard. The motor's max RPM range is 5000-5500 rpm. It seems like its happiest "cruise" range is 4000-4500.

Porpoising and turning at speed were problems. An SE Sport 300 Hydrofoil helped a lot, especially the turns. Then I installed a Piranha 4-blade prop. With the pitch choices, I could tune the RPM to run exactly 5000, or exactly 5500, using the same trim settings that I used with the old 3-blade prop. I chose the longer pitch, lower RPM.

Just as everyone promised me, I lost about 2 mph top speed. Now 46 instead of 48. Not a big deal. The 4-blade prop also greatly improved my hole shot, lowered my minimum plane speed, improved fuel economy, and lets me trim up higher without porpoising. A LOT higher.

Which raises my questions. I know that the highest you can trim is not necessarily the most efficient setting.

Here's the scenario. On smooth water, run in a straight line and set the throttle to be running in the cruise range (about 4200), or full throttle (5000).

1. If I trim up to where the steering becomes light and easy, there is virtually no rooster tail. Only the rear 1/2 of the hull is touching the water. (This was as good as it would get with a 3-blade prop.)
2. Trim up until the rooster tail is the same height as the motor. At this point, only the rear 1/3 of the hull is touching the water.
3. Trim up higher, and the RPM's increase about 300. Only the rear 1/4 of the hull is touching the water.

With all this trimming, the water speed (measured with GPS) does not change. Only the engine speed changes.

I can continue to trim up until the rooster tail is over 10 feet high. As I do, the boat wants to wander off the straight line, indicating that less than 1/4 of the hull it touching the water. About that point, the engine revs up higher yet and speed starts to drop off.

Now, a big rooster tail with 3/4 of the boat off the water looks impressive. (Just like a boat commercial on TV!) But I'm more interested in finding the most efficient trim. I can't decide.

Is it just before or just after the point where engine speed increases? Is the engine more efficient when the prop has a more solid bite, and the engine is slower but pushing harder? Or is it when the prop loses a bit of bite, and the engine is turning a little faster from less load? Or maybe the prop isn't losing any bite at all, and the engine speed increase is from less water contact on the hull? If the water speed changed, it would be easier to decide, but speed remains the same.

Rougher water causes porpoising, and the adjustment is easy. But in the early morning, when the water is glassy all the way from the ramp to the fishing hole....

Re: "Correct" Trim Adjustment for Bass Boat

When your motor turn more RPM and the speed remains constant it means your prop is sucking air and tiny air bubbles are exploading on the face of your prop. This will wear your prop fast. Also it can be dangerous if you need to turn as if the prop comes out of the water it can cause the boat to flip over and kill Someone.

In general terms you set the speed you want then without touching the throttle trim motor for the best speed. This will be the best MPG. Triming the motor so far out that you have any roster tail means your useing power to push the stern deeper in the water. Of course the bow will be higher and you will have less control of the boat. I would never trim my boat that way.

When I trim my boat I set my speed and adjust the trim for best speed then bring bow down as far as possible without loosing speed. Any porpoising then I trim down until it just stops.

Riding with the bow high means several things to me. It mean if you hit that little stick or log you did not see then there is more impact. Also more likely to to damage any stern transducers or speed sensors. Instead of the boat just sliding right over the object it has more impact. Any wave you do not see will have more effect on the boat than if the bow was lower and helping to cut thru the wave.

Actually I trim slightly different as I have helm Adjustable trim tabs. I trim the boat with the trim tabs then trim the motor for best speed. Best speed and mpg will be where the motor thrust is straight back, not up or down. Any roster tail means I and waisting fuel pushing the stern down instead of pushing the boat foward.

Re: "Correct" Trim Adjustment for Bass Boat

If you're only getting to 5000 RPM at WOT then your killing the motor, you need to target the upper part of the range for a happy long lasting engine.

Re: "Correct" Trim Adjustment for Bass Boat

ondarvr said:

If you're only getting to 5000 RPM at WOT then your killing the motor, you need to target the upper part of the range for a happy long lasting engine.

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My question is about trim adjustment, not engine rpm range.

The specified rpm range, from the official Mercury service manual, is 5000-5500 rpm. 5000 rpm at WOT when trimmed down can NOT "kill" the motor. When I trim up, the rpm's increase to about 5300 when the rooster tail reaches the height of the motor. I can push engine speed to 5500 or higher if I trim too high.

If I go to the next available lower prop pitch, then it will run 5500 when trimmed down. When I trim up, it will rev up to 5800 and higher. That is more likely to "kill" it than running slower.

Re: "Correct" Trim Adjustment for Bass Boat

DHag said:

My question is about trim adjustment, not engine rpm range.

The specified rpm range, from the official Mercury service manual, is 5000-5500 rpm. 5000 rpm at WOT when trimmed down can NOT "kill" the motor. When I trim up, the rpm's increase to about 5300 when the rooster tail reaches the height of the motor. I can push engine speed to 5500 or higher if I trim too high.

If I go to the next available lower prop pitch, then it will run 5500 when trimmed down. When I trim up, it will rev up to 5800 and higher. That is more likely to "kill" it than running slower.

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Actually 5,800 is better for it than 5,000.

5,000 is lugging it and may cause carbon build (coking) up and stuck rings....this can cause broken rings and a total rebuild.

if it only reaches 5,000 at WOT it means its lugging at other RPMs also.

Re: "Correct" Trim Adjustment for Bass Boat

Boatist said:

When your motor turn more RPM and the speed remains constant it means your prop is sucking air and tiny air bubbles are exploading on the face of your prop. ... Best speed and mpg will be where the motor thrust is straight back, not up or down. Any roster tail means I and waisting fuel pushing the stern down instead of pushing the boat foward.

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Thanks, Boatist. This is the information I wanted.

If you see advertisements for bass boats, they love to show the boat at speed with most of the hull off the surface. Makes for great marketing. Also, it's very common to see bass boats running across the lake with a huge rooster tail. Looks impressive ("My tail is bigger than yours!"), but I've always thought that could not be right.

When the water gets a bit rough, the rear position of the cockpit in mine will make it porpoise. Like you said, I just trim it down until the porpoising stops.

When I trim as you described (best speed with minimal tail), then at least 2/3 of the boat is off the water. Again, that's normal for the modified semi-V of a pro-style bass boat. Minimum water contact equals best speed. The rooster tail at that setting is maybe 1 foot high. Can't really call it a rooster tail. It's just prop wash. I also notice that the wake is minimized at that setting.

Taking what you said, then I can understand.... Minimal water contact on the hull with minimal rooster tail has to be the best setting. Building up the tail may reduce water contact even more, but the down force at the stern will increase the friction to match. Thus, no speed change, and probably with more fuel burned.

Thanks for the detailed answer. Just what I needed. Now I'd like to add a trim gauge, so I don't have to look behind to see how its trimmed. With the old 3-blade prop, I could just trim up until it started to bounce. But with the 4-blade prop, I swear I could make this thing stand on end! The only non-visual cue I get is when the engine has its first speed increase. Then it's apparently already too much.

Re: "Correct" Trim Adjustment for Bass Boat

As for the trim adjustment, just watch the speed and RPM, once you stop going any faster you're there. You also need to take into account how it drives a that trim and speed, if its scary then adjust for good handling. These settings will change every time you hit the water, how the boat is loaded, # of people, where they sit, fuel on board, wind, waves, etc, will all vary each time. Much of it is just the feel and how you like it to run.

Re: "Correct" Trim Adjustment for Bass Boat

ondarvr said:

Actually 5,800 is better for it than 5,000.

5,000 is lugging it and may cause carbon build (coking) up and stuck rings....this can cause broken rings and a total rebuild.

if it only reaches 5,000 at WOT it means its lugging at other RPMs also.

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I'll search the forums for more information on this.

Coming from the 4-stroke, automotive world, I want more opinions. I wouldn't dream of running my Firebird's 400 V8 consistently at 300 rpm above red-line. I've got a rev-limiter on it specifically to prevent such abuse.

Same for my 2-stroke motorcycles. A definite red-line is defined for every one of them.

But I also don't see a "red line" specification for 2-stroke outboards. Just that "range." Of course, they're a different "type" of 2-stroke build. Bikes are made to run at varying rpms. Outboards are made to be set at a speed and kept there for long periods.

I just figured if I was in that range, then it was fine. My "4-stroke automotive" mind says to opt for the lower end of the range.

If investigation proves you right, then I'll just replace broken blades with the next lower pitch. I chose the Piranha prop for its replaceable blades. One lake that I frequent is shallow overall (8 feet at its deepest), and no matter how careful you are, you find a rock with a prop blade from time to time. A single ten-dollar blade is a lot cheaper than having a stainless or aluminum prop straightened or replaced!

Re: "Correct" Trim Adjustment for Bass Boat

The big difference is that an auto engine will go over the redline in almost any gear, possibly not in OD though, but what do they say about pulling a trailer in OD don't do it, too much heat build up in the tranny and motor.

Your OB is always in one gear and at WOT with a loaded boat it should be able to hit or slightly exceed that range. It will have a rev limiter built in so you don't rev too high.

Imagine pulling a trailer up hill with your foot to the floor in high gear, that's what your OB sees all day long.


PS. Yes, do a search, lots of good info on the subject.

Re: "Correct" Trim Adjustment for Bass Boat

ondarvr said:

Imagine pulling a trailer up hill with your foot to the floor in high gear, that's what your OB sees all day long.

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Can't resist...

400 cubic inches. 530 ft-lbs of torque. Even with a trailer up a grade, I don't have to leave 5th gear OR push the pedal to the floor.

Thanks for all the input.

Re: "Correct" Trim Adjustment for Bass Boat

DHag said:

Can't resist...

400 cubic inches. 530 ft-lbs of torque. Even with a trailer up a grade, I don't have to leave 5th gear OR push the pedal to the floor.

Thanks for all the input.

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This just means you need a bigger boat.

Re: "Correct" Trim Adjustment for Bass Boat

I disagree..... if the manufacturer intended the boat to run at or above 5500 then they would have rated it for 5500-6000

In this case the rating is 5000-5500.... the intention here is that low on fuel with one person in ideal conditions it should be at no more than 5500 and full of fuel and people towing a skier with a head wind it should be able to reach no less than 5000

This is the safe max rpm range.... If you are topping out at 5000 rpm lightly loaded then you are slightly over propped for a heavy load but if you are never running it heavy or towing anything then you are ok... but JUST ok... you will likely see a slight performance increase and MAYBE even a slight improvement in economy with a little less pitch...

Next thought is... maximize your engine first. If your plugs are dated or your carbs are gummy correct that first. I have seen a 2 stroke outboard that seemed to run great gain 800 rpm and run better with a plug change.... then we gained another 300 by advancing the timing a few degrees (had to run premium tho)

Re: "Correct" Trim Adjustment for Bass Boat

You need to do a search on DH's and other tuners and builders posts on this subject. The higher the RPM (within reason) the longer the engines survive, motors propped at the lower end of the range self destructed far sooner. This was a study done by OMC, that if I remember correctly, DH had something to do with.

That's the operating range, not necessarily the redline or peak RPM, most motors have a rev limiter that kicks in around 6,000 rpm, some slightly lower, but others much higher. This is to keep you from doing damage at the upper end of the range.

Re: "Correct" Trim Adjustment for Bass Boat

Dh ???

Re: "Correct" Trim Adjustment for Bass Boat

I just found this post in the "Engine FAQ" section.

Wide open throttle RPM's

Some quotes--

"When running your engine at WOT, if it is reaching, but not exceeding the recommended rpm range, the current set up is acceptable."

"If your engine is functioning properly, but not reaching recommended rpm, then something is putting too much stress or resistance on the engine."

"Outboards run best when the resistance, or load on the engine is limited to the point, that recommended rpm's can still be achieved at WOT. Any more load than that would "lug" the engine, lowering performance, and decreasing the life of the engine. Not enough load, and you would over rev the engine."

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My motor is "reaching" Mercury's specified RPM range, albeit just barely. With the next lower pitch prop, it just "reaches" the very top of the RPM range. So seems to me, my present setup won't hurt anything. But if I drop the prop pitch one notch, I MAY get better performance.

This would be so much simpler if one of the two prop pitches available made WOT fall within the range, but they don't.

• 21-inch pitch = 5000 rpm exactly.
• 19-inch pitch = 5500 rpm exactly.

Piranha doesn't make 20-inch pitch blades for their 4-blade prop that fits my motor. I asked.

I'm going to check the motor height against the boat's speed and Mercury's installation instructions. Maybe I'll be lucky and I need to raise the motor an inch. If so, problem solved. No... I couldn't be so fortunate. We'll see....

Re: "Correct" Trim Adjustment for Bass Boat

And then I found this one--

Prop selection, by Walleyehed

In selecting a prop, consider a normal or average load that you will generally have on board. Select a prop that puts you "AT", or "AS CLOSE TO" Max recommended WOT RPM.

Some manufacturers give an RPM "Range". It has been found best for engine longevity, to prop for the "TOP" of that range.

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I think the question is answered.

I'm going to check the motor height. If it can be raised within Merc's mounting specs, then I'll do that. That may raise the rpm more into the range. However, figuring Champion and/or the dealer mounted it correctly, I'll replace my blades to the next lower pitch.

Thanks, everybody!

Re: "Correct" Trim Adjustment for Bass Boat

Hey dhag, this recomended WOT RPM issue seems to have a variety of opinions. Just yesterday I was considering creating a new post to try and sort out some of the confusion.

It seems on this forum the majority of people will tell you to prop so that you can be right at the TOP of the recomended range at WOT with an average load.

A smaller group will tell you to prop slightly over the max RPM in the recommend range.

Nobody will tell you to prop for the bottom of the range. And everybody will tell you that if you are under the recomended RPM range it will damage your motor over time.

My conclusion is based upon what I've read here, what I've read elsewhere, my own experience and the advice of some very experienced long time outboard mechanics here in town.

My conclusion: Your motor should be setup so that it can reach the RPM at which the HP was rated as a minimum with an average load. In most cases this means aming for the middle of the recommended range or slightly higher.

For example my motor was rated 70HP at 5500RPM and the recomended range is 5000 to 6000. My current prop and setup allow it to turn at 5700RPM.

my2c do with it what you will

Re: "Correct" Trim Adjustment for Bass Boat

For some reason I can't get the search functions to work, it just routes me back the main forum page. Someone else will need to do search on Dhadley's posts on this subject.

Re: "Correct" Trim Adjustment for Bass Boat

For some reason I can't get the search functions to work, it just routes me back the main forum page. Someone else will need to do search on Dhadley's posts on this subject.

The other thing is most boats don't come set up perfect, they just get pretty close and if this rig was set up at the dealer there's no way of knowing if its even close to being correct.

Some dealers will put almost any brand of motor on a hull when new, its up to the new owner to specify it. The guy in the back room just kind of eyeballs the setup for a good starting point and hands the boat to the customer. If the customer looks into it they can frequently fine tune it to get much better performance.

Re: "Correct" Trim Adjustment for Bass Boat

ondarvr said:

The other thing is most boats don't come set up perfect, they just get pretty close and if this rig was set up at the dealer there's no way of knowing if its even close to being correct.

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So you're saying that boat dealers are just like car dealers. Oh, now THAT's encouraging....

That explains why the pin that sets the lowest possible trim was set in the bottom hole. Trim it all the way down and the engine would flood at low speed from fuel running out of the bowls. Drove me nuts trying to figure that one out.

So.... I'll take the Mercury setup specs, and go over it point by point. Between mounting height and position, tuning, prop, etc.... Who knows? This thing may push 60 mph when I'm done. I'd be quite happy if it would break 50. Best so far was 48 with its old 3-blade stainless prop. (Went to a 4-blade Piranha after I hit a rock in the above-mentioned shallow lake, and bent the prop shaft.)