RPMs too high?

[-Spades-]

Searay 17 ft bowrider with a 1999 Mercury 115 2-stroke outboard,

A sticker on the engine shows the RPM limits to be 4750-5250, im getting 5500+ RPM with speedo showing 42mph, it would actually climb higher if not for the rev limiter. Im thinking the prop is underpitched? The stamping on my prop says Quicksilver 48 16988 18P, its in great condition with no dings at all. If i trim it down it will slow down to 5000 but the speed drops significantly, which tells me its not trimmed up too much to cause the overrev. I would like the RPMs to peak right in the middle of the engines Max range at 5000, how much of an RPM reduction can i expect out of each degree of prop increase? such as a 19P or 20P prop?

 

[Alumarine]

Are you sure your tach is reading properly?
What does the tach read at idle?

 

[-Spades-]

No im not sure about its accuracy. It reads around 1200 @ idle, the sticker says it should be at 850 but if i bring it down that low it chugs and dies. This was its first time out since purchase. I do have a wire wound pickup type RPM gauge that i can verify against.

 

[Alumarine]

I'd check it against another tach.
There might be a selector switch on the back for different pole settings.
Try clicking it slowly through all the positions a few times.
Sometimes that gets them working properly.

 

[Scott06]

Verify your tach is accurate first.

Rule of thumb is 1" pitch change (+/-) will increase or decrase rpm by 200 ish if you are staying with the same prop type. Depending on the prop type cupping etc if you change the type of prop like 3 to 4 blade, basic aluminum to SS with a lot of cup as well it gets more compliated

 

[-Spades-]

Ok on my next outing i will have my other RPM gauge with me, as well as a GPS to get accurate numbers. Does an18P prop seem too low for my 17ft boat or am I too quick at blaming the prop?

 

[mr 88]

Ok on my next outing i will have my other RPM gauge with me, as well as a GPS to get accurate numbers. Does an18P prop seem too low for my 17ft boat or am I too quick at blaming the prop?

You are getting ahead of yourself. IMHO the odds are you have the correct prop. Follow the advice of the replies you got. If that was the wrong prop and it hits 42 I would think with a 21" prop or so would put you at 50 +mph and I do not see your setup hitting that speed without some modifications.

 

[Texasmark]

You can go to the Crestliner www and look up my boat: 2002 1750 Fish Hawk, dual console, hull is aluminum but it's a heavy aluminum...not a tin can open skiff with a 9.9 type alum boat.

I run a 2002 115 2 stroke and a SS Laser II, early edition, 3 blade 20P with me, full tank of gas and gear runs right at 5250 limit on a 85* day, and max out at about 48. A SS 4 blade 19P Solace I just put on will pull it down to 5000 to 5100 on a cool day (I was out last Friday) but will do 45-46 . I didn't look up your prop but if SS with cupping and rake and all the goodies of SS props, you put on a couple of folks and more gear and your 18 may be just right. If stock, general purpose aluminum without the goodies, then yes you might need an inch or two more pitch.

My limiter works as the Laser will put it into the limit and I can tell it's limiting by RPMs and throttle position....that would take some explaining what I am doing and what I am experiencing but I know I am in the limit and the module is working. Your limiter must not be working or somebody unplugged or removed it. According to Faztbullet, the piston skirts will be damaged with higher RPMs on this engine.

For your numbers with an 18P and 5500 gives an 11% slip for 42 and depending on your boat and load, those are reasonable numbers at WOT trimmed to the "sweet spot" on a warm day.
I believe your 5500/42 numbers so your tach and speedo seem to be accurate if your boat is anything like mine.....but boats "are" different and the Crestliner hull, for a non "pad" hull with it's reverse chine, planes easily and has little boat in the water at WOT making for prop slip numbers in the 5% give or take range with my normal load....me!

So, sir, my suggestion is that you leave your prop alone and fix your RPM limiter.

 

[-Spades-]

You can go to the Crestliner www and look up my boat: 2002 1750 Fish Hawk, dual console, hull is aluminum but it's a heavy aluminum...not a tin can open skiff with a 9.9 type alum boat.

I run a 2002 115 2 stroke and a SS Laser II, early edition, 3 blade 20P with me, full tank of gas and gear runs right at 5250 limit on a 85* day, and max out at about 48. A SS 4 blade 19P Solace I just put on will pull it down to 5000 to 5100 on a cool day (I was out last Friday) but will do 45-46 . I didn't look up your prop but if SS with cupping and rake and all the goodies of SS props, you put on a couple of folks and more gear and your 18 may be just right. If stock, general purpose aluminum without the goodies, then yes you might need an inch or two more pitch.

My limiter works as the Laser will put it into the limit and I can tell it's limiting by RPMs and throttle position....that would take some explaining what I am doing and what I am experiencing but I know I am in the limit and the module is working. Your limiter must not be working or somebody unplugged or removed it. According to Faztbullet, the piston skirts will be damaged with higher RPMs on this engine.

For your numbers with an 18P and 5500 gives an 11% slip for 42 and depending on your boat and load, those are reasonable numbers at WOT trimmed to the "sweet spot" on a warm day.
I believe your 5500/42 numbers so your tach and speedo seem to be accurate if your boat is anything like mine.....but boats "are" different and the Crestliner hull, for a non "pad" hull with it's reverse chine, planes easily and has little boat in the water at WOT making for prop slip numbers in the 5% give or take range with my normal load....me!

So, sir, my suggestion is that you leave your prop alone and fix your RPM limiter.

Actually my RPM limiter works, i feel that it would climb even higher than 5500 if not for the limiter, i can hear & feel it working.

 

[-Spades-]

If that was the wrong prop and it hits 42 I would think with a 21" prop or so would put you at 50 +mph and I do not see your setup hitting that speed without some modifications.

Well, im not really trying to get more speed out of it as 42 seems to be the correct Max speed for this boat spec. I would like the prop to bring down my peak RPM and maintain the 42 mph... id be happy with that

 

[TyeeMan]

Something to keep in mind when you're propping your boat. When you prop your boat you should have in the boat (people, beer, gas, equipment, etc) what you would normally have in the boat when you are going on an outing. Find out what WOT RPM is with all that in the boat and adjust prop pitch accordingly to get in the 5000-5200 rpm range.
If you are going on an outing and the boat is light ( one person, no beer, a little gas, etc) you may likely over rev so you will to throttle back a little.

 

[rallyart]

The prop pitch is effectively the same as the gear ratio on your car. If you go up in pitch you travel further with each rev so your top speed goes up if you have the power to move the boat at that speed. As you go up in pitch you also can lose some acceleration as you don't have that same torque multiplier effect. Some shops will let you test out several props as long as you are buying one of them. Then you will know what all the effects are of changing the pitch.
Your boat probably gets reasonable economy below 35 mph so most of the time you are likely to cruise at part throttle. Also at any particular speed your are only using so much power to move your hull through the water at that speed. That means that if you want to go 30 mph, for example, you will need a particular amount of generated horsepower to do it. Part throttle with one propellor or a different part throttle with a different propellor will bot be making the same HP to go that speed and the economy will be almost identical. If you want to go faster try a higher pitch prop, Perhaps that is a 20" instead of your 18" but you will come out of the hole slower. The easiest solution is to just throttle back when you get to the speed you wish to travel at.

 

[Sea Rider]

If that's a second hand combo, previous owner could have prop it right for his needs, higher loads, etc which you are not duplicating. Check tach rpm readings against other. How loaded, number of souls do you plan going boating with ?

Happy Boating​

 

[Captain Caveman]

Exactly!

I have two props for my boat. It's good to have a spare on the rocky lakes that I boat. More importantly, the stock prop is perfectly matched for the boat & motor and maxes out top-end speed right at the top of my RPM band. My other prop steps down so that I can pop water skiers / tubes out of the water....or just get on plane better when fishing with a bunch of big guys in the boat. With my second prop, my engine would over-rev if not for the rev limiter kicking in. I just have to be cognizant of my RPMs when boating without much weight in the boat.

It sounds like the previous owner merely went down in pitch and left that prop on your boat. Some prop shops will let you test out a prop before buying. Ask around.

 

[jhande]

Sorry for stepping in and asking a question on someone else's thread. But I didn't want to start a new one for similar question so...

When and if I get my boat in the water should I concern myself with prop and tilt/pitch? As far as I know everything has been the same since 1978. I'm not looking for full throttle water skiing or anything, just putting to the fishing hole. After all, she's old like me LOL.

 

[Sea Rider]

Counting now a days with better boating information, better prop selection than before it's a must tach current motor and check if runs inside its min-max wot rpm range factory stated. It's a day/night fun expience boating with a motor correctly proped for given load to run towards its max wot rpm range, once there can go skiing, speeding or throttle at the speed you feel comfy boating with. If the old motor runs strong go for a prop maximization. To accomplish that, the combo must necessarily count with a tach in order to play safely with prop pitches.

Happy Boating


​

 

[jhande]

Thank you!

 

[Irvin648]

But - Most modern car engines cooling systems are often pretty over-dimensioned, arent they? Shouldnt it be able to "get rid off" the heat of the engine when pushed to the limit without trouble? Or is that just something you expect?

 

[jhande]

But - Most modern car engines cooling systems are often pretty over-dimensioned, arent they? Shouldnt it be able to "get rid off" the heat of the engine when pushed to the limit without trouble? Or is that just something you expect?

Well... if you push it to the limits for an extended length of time your car will start to run too hot.

It would be more a kin to a race car engine. At a certain RPM level the motor is making max horse power. But yet it still drives fine at slower (lower RPM's) speeds.

 

[Sea Rider]

Can't posibly compare a car to an outboard motor. A car is cooled by means of a radiator and the green krypto stuff, the OB on pure fresh or salt water which will achieve a spot on cooling provided that their entire water passages are immaculate clean. In a near future when all OB's use the green stuff for cooling issues can be compared to a car in terms of spot on refirgeration...

Happy Boating​