RPM range question

[YellowT]

Hello. Bought a boat last summer, a small (25 foot) but comparatively heavy-ish walkaround with a pilothouse, berth and some amenities. It's equipped with a Mercury 150 four stroke. The original prop was a 14.5 dia with a 15 pitch. The boat had problems: not enough grip, sliding in turns, very poor low speed handling, poor fuel consumption, mediocre acceleration. It did, however, hit the 150's rpm range of 5000-5800 at WOT.

On advice, I re-propped the engine and new prop is a 15.75 dia with a 15 pitch. This prop is also cupped; the original one was not. All problems listed above went away: the boat handles very well at all speeds, much grip in turns and no sliding, very good acceleration to plane, good fuel consumption rate. I'd call it a success, except for one factor:

WOT is now at 4880 rpm, or 120 rpm below the range recommended by Mercury. Here are the qualifiers:

-- I understand that larger diameter props at same pitch will lose top rpm.
-- I don't run at WOT. I don't accelerate at WOT. Ever.
-- Out of deference to my wife, this boat, one of two we own, is a winter and "un-sunny" day cruiser. It lives at 25 knots and 3850 rpm, where engine, boat and wife are happiest in terms of comfort and fuel consumption, mostly inshore.
-- Occasionally I'll run it for five or ten minutes at 4500 rpm to sweep out a little carbon.
-- Back in the last century I put myself through college working as a diesel mechanic. I know what a lugging engine is like and I know what smaller engines need to handle loads. This engine isn't lugging and handles the load well, much better, from what i can hear and feel, with the new prop.

So - is 120 rpm off the Mercury recommended range cause for any concern? I wouldn't think so, given the circumstances outlined above. But... due diligence: Am I missing something?

Thanks.

 

[Scott Danforth]

Lugging a motor kills it. Always best to get the motor to the top half of the RPM window. To me, you're 500 - 600 RPM below where the motor should be. However verify your tach first

 

[dingbat]

What boat are you running?

Without knowing the exact boat, I’m guessing you’re about 100 hp short, if not more, for a 25’ walkaround.

My 22’ is rated for 280hp, hung with a 225hp. My one buddies 24’ is rated at 350 hp, hung with 275 HP. Another friend has twin 175HP on his 25’WA

I’m running a 15x15 prop. Cruise at 27 kts, 4400 rpm at 2mpg. WOT is 5850 rpm at 35 kts. Fuel consumption 1.9-2.0 mpg

Absolutely no doubt you’re lugging the motor badly getting up on plane.

In effect, you only have 1 “gear” to cover the entire speed range. If you only drive at highway speed, do you only have third gear?

May do great at highway speed, but how does starting from a complete stop work for you?

Control the speed your comfortable with using the throttle, not the prop

 

[airshot]

In have found that proping closer to the max rpm range with a light load works best, as then the motor won't be lugging when a heavier load is onboard. Your lugging your engine all the time now even though you don't feel it. Boat engines are much harder to detect lugging over highway vehicles. I would shoot for the 5800 rpm at wot and mabey consider a 4 blade prop for better low speed handling, your motor would be much happier for it !!

 

[YellowT]

The boat is a Jeanneau NC 695 "Weekender". Built in France, there are a huge number of them in Europe, Asia and NZ/Australia, where they're known as the Merry Fisher line. Jeanneau rates the boat at 175 HP max; most sold in North America come with a 150 HP Mercury or Yamaha; in Australia a 175 HP Yamaha; and in Europe and Asia 115 and 150 HP Yamahas. "Low" horsepower by North American standards are the norm on these boats; it's one reason why they don't sell well here. On the other hand, outside of NA they have an excellent reputation as a mid tier mass production boat. They don't break and they don't tend to break their outboards.

The boat goes on plane in 5-7 seconds at 2/3rds throttle with the larger prop; with the smaller prop it took 12-15 seconds at full throttle, and near-full throttle was required to keep it on plane. Cruising at 25 knots and 3850 rpm yields 2.2 mpg. With the smaller prop cruising at 25 knots and 4250 rpm yielded 1.5 mpg.

Again the primary reason for re-propping was how the boat handled and sluggish performance in acceleration and fuel consumption, all of which has been addressed satisfactorily. I'm both puzzled and disappointed to learn that I'll have to re-prop again and go back to having at least some of these same problems.

One question: lugging an outboard seems very different and far less obvious than lugging a gasoline or diesel engine in other types vehicles. If you can't tell from acceleration parameters, sound, vibration, fuel consumption and the tach - what do you go by?

Thanks again.

 

[YellowT]

In have found that proping closer to the max rpm range with a light load works best, as then the motor won't be lugging when a heavier load is onboard. Your lugging your engine all the time now even though you don't feel it. Boat engines are much harder to detect lugging over highway vehicles. I would shoot for the 5800 rpm at wot and mabey consider a 4 blade prop for better low speed handling, your motor would be much happier for it !!

I posted my response before I read yours. This answers some additional questions. The four blade prop is an interesting suggestion. Thanks.

 

[jimmbo]

Get the Rpm to at least the Middle of the RPM range, a 10 or 11 inch will be needed. However adding another blade will require an even greater drop in Pitch, and will likely require a Pitch in the Single Digits.

However, as Mentioned, verify the Tach and Setup of the Engine first

 

[YellowT]

An update:

Tach and engine particulars verified.

The propeller was re-pitched from 15 to 13 by a local prop shop that has done this kind of work for 30 years. About a quarter inch was shaved from the diameter and the cup was maintained. WOT on the boat is now just under 5400 rpm and most of the benefits mentioned previously have been preserved without too-noticeable changes. Presumably the warranty, for what that may be worth, is also preserved.

Thanks to everyone for the responses.

 

[QBhoy]

Hi. What age is the engine ? Is the engine set up well for height ?
Can I ask if you have normal gauges that didn’t come with the engine ?

 

[YellowT]

Hi. What age is the engine ? Is the engine set up well for height ?
Can I ask if you have normal gauges that didn’t come with the engine ?

Dec. 2022 manuf date; height is spot-on by the operating position of the cavitation plate; SC1000, an analog fuel gauge, and a NMEA 2000 connection to a Garmin display.

 

[dingbat]

The boat is a Jeanneau NC 695 "Weekender". Built in France, there are a huge number of them in Europe, Asia and NZ/Australia, where they're known as the Merry Fisher line. Jeanneau rates the boat at 175 HP max; most sold in North America come with a 150 HP Mercury or Yamaha; in Australia a 175 HP Yamaha; and in Europe and Asia 115 and 150 HP Yamahas. "Low" horsepower by North American standards are the norm on these boats; it's one reason why they don't sell well here.

Now I see why none of this made any sense.

This is much smaller boat (OAL rated) with a very different hull design than most the 24’ Walkarounds marketed in the US by its competitors.

The boat is light weight with a suprising shallow deadrise and limited fuel capabilites for a boat marketed as a coastal crusier. More in-line with 19'-20’ models (measured at water line) marketed in the US

One question: lugging an outboard seems very different and far less obvious than lugging a gasoline or diesel engine in other types vehicles. If you can't tell from acceleration parameters, sound, vibration, fuel consumption and the tach - what do you go by?

You can tell from all the above once you have a good baseline for comparison.

My test....bury the throttle from a dead stop. Throttle response should be almost instantaneous and the boat should get up on plane quickly with very little complaint. If it struggles getting on plane or "grouns" during the transistions, your lugging it

 

[YellowT]

double post; repliy to dingbat below:

 

[YellowT]

4400 lbs, 18 degree deadrise, 45 gallon main fuel tank (12 gallon aux), midships pilot house for good balance, marketed as an inshore pocket cruiser, a role performed well in the northern GOM and associated bays and sounds. A 9 foot beam, which might explain some things. Overall, a good, special purpose (winter and cruising), second boat to own, inexpensive with consistent fit and finish. Or so I've found.

As said, developed specifically for the European and East Asian markets, where designs and expectations can be different.

That, said, I've been a little puzzled, too.

Throttle response before the prop was modified was instantaneous, and the boat was on plane within a few seconds with no issues I could observe or feel. The only real difference I can see after prop modification is an increase in rpm's and a slight degrading of low speed handling.

For diligence, I reinstalled the much smaller prop the OB originally came with and took the boat for a run. Throttle response was not instantaneous, plane was difficult to achieve and took much more time, and maintaining plane required high rpm's. OB had that low, half-rumbling tone, and vibrations were more intense - two characteristics I've long associated with lugging an engine....

I do appreciate your observations and suggestions.

 

[Scott Danforth]

FWIW, my 24' boat (26 OAL) is 3600# dry, 4400# wet with gear and a 22-23 degree deadrise to compare to @dingbat comment. at a tad over 450hp, the performance is half what I want and twice what the wife wants.

 

[YellowT]

Ya, we bought this - our second boat, our other one is a CC - for the wife. I'll say, however, that I've quickly learned to appreciate being comfortably "indoors" in the winter and weather, and the stability, too. Nice to be able to take this boat from the island into town and do some shopping and hit a restaurant in January or February, rather than take a car.