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Author Topic:   Prop for 13'5 SuperSport 40HP?
dnoha posted 07-22-2002 02:06 PM ET (US)   Profile for dnoha   Send Email to dnoha  
Hi. I’m new here.

And new to Boston Whaler’s, and new to outboard motoring for that matter. Our family has a long history of small boat sailing (Sunfish, Hobie, etc.). But we don’t know squat about powerboats. Yet.

What we do know is just enough to start out right. Meaning that when we did buy a boat (last week), we made sure it was a Whaler. We live on a lake and wanted a small one (easy to pull out of the lake without going to the ramp, easy to tow, shallow draft). We’re just going to be tooling around the lake and letting the kids ski/wakeboard, etc. So we bought a used ’86 Super Sport (13’-4”). It came with a ’98 40hp Mercury (2cycle/3cylinder, 1.88/1.0) that was hardly used. Everything in very nice condition.

My first question is about the prop. The one we have now is labeled “Quicksilver” and is a three blade aluminum job. It has some stuff stamped on it, the last bit was p15 which I take to mean the pitch. I tried using the prop calculator on this site and on Mercury’s, but neither was particularly helpful. The Merc site kept telling me I was overpowered for their calculator and should seek professional help. (I’ve had people suggest I “seek professional help” before, but that’s another story.)

I don’t have a tach (should I get one?) and I just “feel” like I’m over-revving (specs say 5000-5500 for max RPM) the motor when I run WOT so I don’t. (Well, just that once.) According to our GPS we’re making 32mph at just under WOT where things don’t sound suspicious. Being a bit paranoid about over revving it, I’m keeping things around 25mph or so or less.

I know enough to realize that prop selection depends on loads of variables, including what we want from one, quick power from start for pulling skiers or top end speed or whatever. But since I’m going to let the kids and others drive this thing, what I really want is a prop that won’t invite the motor to wreck itself with overly high RPM’s.

Any words of wisdom to share with me?

Have enjoyed reading this forum over the last week or so.


dnoha posted 07-22-2002 03:24 PM ET (US)     Profile for dnoha  Send Email to dnoha     
My calculations result in 4658rpm at 32mph assuming 10% slippage. So that's not over-revved, but sure sounds like it to me.

But then what do I have to compare it with? Like I said, I'm new to powerboating. I do have an antique tractor (1949 Farmall Cub), and max rpm on it is 1800. Maybe that's why 5000rpm sounds too fast ...

5000rpm would be around 18% slippage. Any chance of that on this setup? Or am I just paranoid?

jimh posted 07-23-2002 09:09 AM ET (US)     Profile for jimh  Send Email to jimh     
A 40-HP Merc with a 15-inch pitch prop sounds about right. I think your lower unit ratio is 1.83.

The only way to tell what is going on is to get a tachometer. It is a modest investment ($65) and it will give you much return.

With a tachometer you will be able to see just where you are running the engine. You'll also be able to notice problems. For example, if the engine suddenly won't turn to its usual crankcase speeds, you'll see it right away.

The tachometer is also useful when other people operate your boat. You can tell them to "run it at 4000 RPM" for example, and they'll be able to put the boat right into its groove.

By the way, a prop's "slip" is really not exactly "slippage" as in something being loose, but has to do more with the generation of thrust from the blades. A prop can be "slipping" when it becomes loose on its hub and fails to rotate at the same speed as the prop shaft. (A tachometer will also let you discover this phenomenon, too.)

The 13-SS with a 40-HP is a great little boat. You have done well to get yourself one. If properly maintained you'll be watching your grandchildren use it, maybe even with that same engine! (I have a 1976 Merc on my Whaler and it starts and runs like new.)

dnoha posted 07-23-2002 12:02 PM ET (US)     Profile for dnoha  Send Email to dnoha     

Thanks very much for your thorough response.

You’re right on the ratio of 1.83, don’t know where I pulled the 1.88 from. And your advice on getting a tach sounds well reasoned to me. I’ll do so.

The more we use our little Whaler, the more we feel good about having picked the right boat for our needs. I share your opinion that it will be in great shape when the grandkids arrive. I just hope my two beautiful daughters don’t surprise me with grandkids too early…

JohnAz posted 07-24-2002 12:55 AM ET (US)     Profile for JohnAz  Send Email to JohnAz     
Once you get a tachometer, and know what your engine is doing,,,you should get a second prop to water ski with[low pitch}, and the higher pitch for more speed,,my 13 ft, with a 40 OMC 2 cyl. does 36 mph gps,,,with a ski prop it does 31 mph
my1st13sport posted 07-26-2002 08:58 PM ET (US)     Profile for my1st13sport  Send Email to my1st13sport     
does your 40hp Merc have a Quicksilver remote??
The reason i ask is that I also have a '98 3cyl 40hp merc and was wondering about the color coding of the wires coming out of the remote for a tach\accessory. If you are going to hook up a tach i would like to know which wires\colors you use to get the tach working on a 3cyl merc.
Thanks in advance,Ron
dnoha posted 07-31-2002 01:21 PM ET (US)     Profile for dnoha  Send Email to dnoha     

I'm new to this too. I'm assuming from your question that the thingy that houses the throttle lever / ignition key hole / fast idle / choke is the "remote"?

If so, yes it says "Quicksilver" on it. When I find out from the local Merc dealer about getting a tach put on it, I'll post back what I've learned. Might be a week or so ...

my1st13sport posted 08-04-2002 09:20 AM ET (US)     Profile for my1st13sport  Send Email to my1st13sport     
yes,quicksilver is the remote package that usually comes with a merc, since merc owns quicksilver.Yes that is the throttle,thanks in advance for any info,Ron
LiquidDiet posted 08-05-2002 09:24 PM ET (US)     Profile for LiquidDiet  Send Email to LiquidDiet     
a 13' Sport with a good 40 hp OMC will run a good 40 mph with a light load. If you are only getting 32 with a light load something is wrong. I always ran a 11.75" dia. by 17" pitch with the OMC
jstachowiak posted 08-06-2002 08:13 AM ET (US)     Profile for jstachowiak  Send Email to jstachowiak     
My 15 GLS with a Johnson 40 HP was overreving (acted like it was missing right at 5800 RPM's) and I took it to my mechanic and asked him about it. He took the prop I had it "reworked" and I never had a problem after that. And max RPM was now 5200. Unfortunatly I do not know what he had it "reworked" to. The tach will tell you if it is near max RPM.
GlennGlenn posted 08-12-2002 06:23 PM ET (US)     Profile for GlennGlenn  Send Email to GlennGlenn     
"a 13' Sport with a good 40 hp OMC will run a good 40 mph with a light load. If you are only getting 32 with a light load something is wrong. I always ran a 11.75" dia. by 17" pitch with the OMC"

I have an 87 13 SS, 50 HP Johnson TNT and with a light load, 6 gallons of gas and one male at 165 LBS run near 50 on GPS fully trimmed out; a good 40 HP will run it over 40 MPH on GPS with the trim out; Check the prop and the trim because thats the combination where I get the most gains. running a 11 3/4 X 18 pitch; I can even get a 20 pitch to run but it bogs out of the hole . RPMs with the 18 are at 5,200 and max on motor is 5500.

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