RPM at WOT too low? (Mercruiser 3,0 TKS)

[Ben Nevis]

Hello,

Below is the basic info:
1) Performance issue you are trying to correct: understand if RPM at WOT is appropriate (maybe, correct throttle position under question).
2) Current prop manufacturer, model, aluminum or stainless as a minimum; 3) Current prop diameter and pitch (required): Mercury (stock), 21 pitch, diameter 14.25, aluminum, part code 832832A45P21, in almost new condition.
4) Wide open throttle RPM and speed with an average load (very helpful): 4100, speed approx. 53 km/h, load 370 kg.
5) Engine/drive make, model, year, and HP: Mercruiser 3,0 TKS (carb), 2007, 135 hp
6) Boat make model, year, length and weight: Larson 180 Sport 2007 (2008 model year), length 5.33, weight 1072 kg (2363 lbs), max load capacity 578 kg (1275 lbs).
During test runs under WOT with total load of about 370 kgs reached abt 53 km/h. Water was nearly still, when started to plane rose the stern slightly. Was quite surprised with the results, however started to select a replacing prop until one experienced Mercruiser local dealer calculated with Mercruiser software and told me that actually boat should reach specification WOT of 4400-4800 rpm with 21 pitch prop and 400 kg load, speed should be approx. 64 km/h. He expressed the idea that throttle position might not be aligned with acceleration handle position or there might be some other technical problem.
Compression checked when buying the boat and it was perfect.
What is experienced people’s opinion, 4100 rpm max for the boat is generally OK or suspicious and needs a deeper look? Need to clearly understand that to fix the problem if it exists and stay using current prop or make a justified decision which prop size/type/material to move to.
Plan to use the boat for rides with 4 to 6 people at cruising mode (not maximum speed), sometimes for watersports.
Currently can not visually check throttle position since the boat is in a garage under snow.
Thank you!

 

[Bifflefan]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Per the info you posted, and assuming that all of the adjustments in the linkage are correct, and the engine is in good working order (i.e. has good spark plugs and wires and such).
I would say that you could use a 19p prop. Don't worry about the diameter to much, just make sure it will clear the steering trim tab if you have one.

On the other hand, if WOT is the only issue, (i.e. hole shot is good and mileage is good). I would leave it alone. Someday you are going to unload the boat of all the gear and persons and see what she can do. That's the day you will need the 21p prop.

I would suggest getting a 19p to pull a skier or a tube and use it only when doing that.
Remember that MAX RPM is just that, the maximum. You are close enough to it and that engine is not an old 2 stroke that loads up if you don't keep it running at high RPM's.

With a 19p and very low weight in the boat you run the risk of over revving the engine and causing some damage. That is worse than running it below the max rpm.

 

[Don S]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

You need to find out what the problem is. Along with the throttle linkage check, Does your drive have a 1.98 gear ratio? Make sure the tach is correct. Verify with a known accurate shop tach.
Running an engine overpropped and lugging is a good way to burn valves and cause excessive wear.
An inch of prop pitch will only give you a 100 -200 rpm change.

 

[fishrdan]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

There could be things not allowing the engine to perform 100% as the Mercruiser dealership alluded to, carb opening all the way, bad fuel, improper timing, etc... When was the last time the engine was tuned-up, it needs to be running right before selecting replacement props. Are you operating at high altitude?

The engine should operate withing the WOT RPM range "under a normal load". During your testing WOT was only 4100, so you are pretty far off. I'd be shooting to get all the way to 4800RPM, so the engine has some room to loose RPM and still stay around 4400RPM when the boat is loaded down.

4-6 people, how much weight is that? How many people were in the boat during testing?

I'm thinking a 18P 4 blade prop (like an Alpha4 or a Solas equivelant) for an all around heavy load and watersport prop. But it could easily over rev the engine with a light load...

With a 19p and very low weight in the boat you run the risk of over revving the engine and causing some damage. That is worse than running it below the max rpm.

If the engine can't reach it's WOT RPM range, it's lugging the engine, that's bad. I would rather overshoot WOT RPM then be under since the operator can control how fast the engine is going. If inexperienced people are going to be driving the boat and not paying attention to engine speed, then yeah, make sure the prop won't allow the engine to spin over 4800RPM.

 

[steelespike]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

I think the prop is a good match;but you need to figure out why its not closer to wot rpm, about 4800.
Is the motor in excellent tune,good compression?
Is the bottom clean and smooth?
Is the throttle opening all the way?
To make up 700 rpm would require a 3 or 4" reduction in pitch Putting you at 17 or 18" pitch.
Close to a size for heavy duty water sports with a load aboard.

 

[Don S]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Are you using the trim? If you are just running the trim all the way down, then the boat is plowing and it will overload the engine and keep you from top rpm and speed. Don't look at the trim gauge if that is what you are going buy, trim up until the engine starts over revving a little, and the speed starts dropping, then you know you are trimmed to high. Trim down some until you get the best rpm and speed. If the boat is porpoising, you may need to come down wit the trim just a bit more till it stops. Now you are properly trimmed.

NOW. report what your WOT rpm is and speed.

 

[jestor68]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Buy a 19 pitch prop and run tests with in the spring. It should not over speed with the 19 pitch; even fully timed out.

It is not unusual to have to change prop pitch(down) after a few years, as the motor has lost some of it's original compression(power). Since lost compression cannot be regained(short of over haul), the only choice is to drop down a step in pitch to keep things going.

The Mercury Marine web site has tests of the 3.0L on like 18 ft boats. They were using 18 pitch props to turn 4800 rpm. Only a couple of 17 footers were running 21 pitch; being several hundred pounds lighter than your boat.

 

[Ben Nevis]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Thanks for your ideas!

The motor has 1.98 ratio, running at normal altitude. It had ran for 20 hours when i bought it last autumn (condition of the boat confirms that, motor was checked by the specialist and he said it's in perfect condition, don't remember exact compression but was same as for new motor). Condition of plugs and wires not checked.
I did use the trim after starting to plane and seemed to do it properly (was assisted by an experienced boater). The result with 4 people plus some cargo such as petrol (total weight approx 370 kg) was around 53 km/h at 4100 rpm. 6 people with cargo would be around 570 kg.
As for hole shot, it was medium easy (generally have not so much to compare with, but expected it to be somewhat easier).

I well understand that WOT is a maximum rpm and generally do not plan to run in this mode, but with such maximum speed/rpm, i believe my cruising speed/rpm is suffering as well (did not measure exact speed/rpm when cruising).
As for tach - yes, definitely need to check it.

I am ready to buy a new prop, but if i find out that there is a problem with the motor (such as linkage) which i do fix then this prop (was thinking of 4 blade 18 ss) may become inappropriate and give too high rpm.

Looking at some boattest results, i see comparable boats with same (or even higher pitch) prop performing much better, such as:
Bayliner?185 Bowrider (2009-)?2009? Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com
Tahoe?Q4 SF (2004- )?2004? Reviews,performance,compare,price,warranty, specs,Reports,Specifications Layout, video | BoatTEST.com

That's why i decided to ask if motor seems to run properly or rather no.

 

[dan02gt]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

By looking a all this it's really hard to say what's going on. Have you ran the boat with just you in it to what kind of numbers you get? It may just be that you are over-proped for the load you're trying to carry.

 

[Ben Nevis]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

No, i haven't run it on my own yet. Will try in May only. When i run on my own, what should be rpm and speed to say that motor is OK?

 

[jestor68]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

This is pretty simple. The boat MUST BE propped to operate within it's recommended rpm range of 4400-4800 rpm with whatever is the "normal" load onboard.

The accepted practice is to prop it to run at or near the upper limit(4800) with just one or two people in the boat, so that it will still be able to muster the proper rpm when carrying a heavier load.

Your motor is telling you what it needs.

 

[steelespike]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

You certainly need to confirm the tach,but a prop calculator says at 4100,1.98 ratio 21" prop your speed indicate about 20% slip.Reasonable at that reduced rpm.Is your speed by gps?
The 21" pitch is pretty typical for good speed a reasonable all around performance.
I have seen faster smaller 3.0 boats with different pitch and speeds hovering in the 50 mph range.

 

[Ben Nevis]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

The speed is not by GPS, but by onboard speedometer. Sorry, i did not catch the meaning of "at 4100,1.98 ratio 21" prop your speed indicate about 20% slip.Reasonable at that reduced rpm". Did you mean that speed could be by 20% higher but actually it fits lower rpm?
If speed and rpm is proven, does there seem to be a problem with motor?

 

[trendsetter240]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

You need to identify the problem before you start prop shopping. Could be any of the above suggestions or a combination of them.

That motor, on the boat described should be able to turn the stock 21p mercury prop.

I was out yesterday with a friend who has a Bayliner 192, 3.0 TKS, Alpha I Gen 2 outdrive 2.0:1 and the same stock merc 21p prop. The boat weighs 2500lbs and with two people, fuel and fishing gear it can spin 4600RPMs trimmed out.

So a nearly identical setup and 4600RPM can be achieved by trimming out.

Here's a short video clip from yesterday. Bayliner 192 Fast Cruise - YouTube

 

[jestor68]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Based on your current performance, the Mercury prop selector says a 4 blade 18 or a 3 blade 19.

FYI; the 18 ft Larson tested by Mercury with the 3.0 was using a 15.5 X 15 to turn full rpm. That model was only a couple hundred pounds heavier than your boat.

 

[trendsetter240]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Based on your current performance, the Mercury prop selector says a 4 blade 18 or a 3 blade 19.

FYI; the 18 ft Larson tested by Mercury with the 3.0 was using a 15.5 X 15 to turn full rpm. That model was only a couple hundred pounds heavier than your boat.

This may be true but the current performance is in question. There could be timing / carb adjustments needed. He might not be trimming the out drive properly. etc.

Calculating using boat weight, 1000lbs of people and gear and 20 gallons of gas the same calculator recommends a 20-21 pitch prop.

 

[steelespike]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

The speed is not by GPS, but by onboard speedometer. Sorry, i did not catch the meaning of "at 4100,1.98 ratio 21" prop your speed indicate about 20% slip.Reasonable at that reduced rpm". Did you mean that speed could be by 20% higher but actually it fits lower rpm?
If speed and rpm is proven, does there seem to be a problem with motor?

The prop calculator indicates that your figures indicate 20% slip.Slip is how we tell how efficient your prop is working.
For instance a 12" pitch in a solid would move forward 12" in one revolution.But water isn't a solid so there is some slip.
Slip on your boat at wot 4800rpm would be about 10% a little less or little more.As you slow the boat the slip tends to go up so
at 4100 slip of about 20% could be reasonable.A light fast boat would tend to have lower slip numbers heavy hard to plane boats higher slip. It does appear that your motor may actually be at about 4100 thus the relatively low speed with reasonable slip.
You need to confirm the tach is accurate and if so see why the boat won't reach normal wot rpm.

 

[Ben Nevis]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Thanks for your valuable recommendations.
Could you pls describe in more detail (or give a link) how i should check the real rpm. Above is mentioned about "accurate shop tach". How it should be linked to the motor? Shall i buy this device or it's usually rented at a service shop?

 

[steelespike]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

If you have a regular marina they may let you borrow or rent a shop tach.
Maybe a tech would make a quick run with you for a nominal fee.
I'm sure the hook up is an easy procedure but on a 2007 that likely has electronic ignition
I don't even know if it has a true distributor or electronic advance. The shop tach may even have a
plug in.You also need a gps for an accurate speed check.Some cells have a gps app.

 

[Ben Nevis]

Re: RPM at WOT too low? (Mercruiser 3,0 TKS)

Thanks very much!