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Recently bought my 1st whaler, love the boat, 40 hp may be a bit small but that's what it had. Having trouble getting it on plane with any kind of load in the boat and/or quickly. Once planed I am getting approx. 28 mph @4800 rpm. Any suggestions or experience on what is the best prop pitch for this set-up?

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I had a similar issue with my 16' and 60hp Evinrude power. Tough to plane with any load, maxed out at 28mph, similar RPM. Turned out the throttle cables were messed up. Now I can get around 35+. If your prop checks out, there could be an issue somewhere other than the prop.

As for prop pitch, it depends on the gear ratio of your motor. What is the make model and year of your motor? They aren't all the same.

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You may want to explain exactly which 15 foot boat you have, what 40 HP outboard it has on it, what the outboard's WOT rang is as well as its gear ratio. You should also tell us what propeller is on it now, size, brand and model.

With that basic information we should be able to offer some suggestions.

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I rented one here in California before I bought mine. It had a 40HP Honda on it. It ran about 29 MPH with 2 adults and a toddler on board (340lbs) plus a couple gallons of gas.

It planed off ok, and ran decent, just not fast. As far as I know, the hull I was in has been in the water for the better part of 15+ years (I rented the same boat 13 years ago and it was worn out then!), so it may be a bit waterlogged.

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Should have given more info, it is a 1986 custom (side consol) 15'3", motor is a 1994, 4 cylinder, dual carb merc 2 stroke, don't know what the max RPM is as I don't have the manual for the motor (guess I should get one). Pitch on existing 3 blade prop is 17p. I am thinking about going to a 13 or 14 P. I have another 16' fiberglass at a northern lake that I run a 50 hp 4 stroke yamaha on with a 13 P prop and it moves the boat well (boat is rated for up to 80 hp), I can a pull out a 120 water skier no problem. Or load the boat to the gunnels and get it on plane (2 people & 400 lbs of gear) no problem. I will check throttle cables (never thought of that) The previous owner had this boat from new for his kids, he may have adjusted throttle cable to reduce max speed.
Thanks for the suggestions!!!

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OK, you have a classic 15 foot Whaler with a 1994 Mercury 40 on it.

If I am not mistaken, I believe that motor uses a 2:1 gear ratio and has a 5000-5500 RPM WOT range.

If you are only hitting 4800 RPM with a 17" pitch propeller, you are overpropped a bit.

The trouble is that 28 MPH (which is typical of 40 HP on a classic 15) is only possible with a 17" pitch propeller if the slip is over 27 percent, which is extremely high. I could only explain that if the propeller were badly chewed up or had been modified.

I want to know what 17" pitch propeller is on there now. What brand? What material is it made of? How many blades? What part number? What diameter?

Though I probably do not need to ask, I'll ask anyway, how high on the transom is your motor mounted? Wait, wait! don't tell me, it is mounted as low as it can go.

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Prop part No. is 48 73144 A40 17P, I believe 10 in. Diameter (no numbers on prop indicate this) based on looking the part no. online. Other nos. on the prop 13 91 2 and 2533.
Motor is mounted so anti-cavitation plate is pretty much level with bottom of boat, yep mounted as low as it will go. There is no whale fin on the motor, also thinking of installing one if changing prop size & pitch does not help getting out of the hole enough.
From some reading reducing pitch size has an approx. effect of increasing RPM 200 rpm per a a drop of one pitch size. Thus if I went to a 13 or 14P I would increase max rpm to about 5600 or 5400 respectively (currently max RPM under load is about 4800 RPM)
What effect does diameter size have. The 13P prop is available in a 103/8 in. or 10 1/2 in. Diameter.
I appreciate the help.
Cheers
* forgot to mention it appears to be a aluminum Merc. prop,

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Diameter is left up to the engineers.

We go by the pitch and the manufacturer.
I don't know of any manufacturer that sells a different diameter with the same pitch for the selected motor.

Our choices may be as an example: 13p, 15p, 17p.... We don't have a choice in diameter as they already have that figured out when they calculate the pitch...
If you change manufacturers, their diameter(s) may be different but we still only have a choice in the pitch of the prop from that manufacturer...

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Yes, the Mercury 48-73144 A40 17P is a 10" x 17" Aluminum BlackMax propeller. I am perplexed as to why it is slipping so badly.

At any rate, reducing pitch will gain RPM. I would not use less than 14" of pitch, and if you want the Mercury BlackMax model, use the 10-1/4" x 14", part # 48-73138) NOT the 10-3/8" x 14", part # 48-816706 (Large Blade Area) propeller.

I recommend you raise the motor on the transom two holes and install a stainless steel propeller if you want to really improve the boat's performance. Either the 10-3/8" x 14" Mercury Vengeance, part # 48-855860 or the 10-1/2" x 14" Stiletto Triad 3.5 (or Turbo Hot Shot 3.5)

Everything about your boat's performance will improve with a stainless steel propeller from top speed to fuel economy.

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So, reducing pitch will give me higher RPM and better out of the hole power, lower top speed. Excuse my ignorance but what will raising the motor a couple of holes do? From everything I've read the anticaitation plate should be level with the keel of the boat. Does the hull design of the whaler affect this principal?
I wouldn't say there is slippage, it just doesn't have the umph to take the boat onto plane that I have experience with other other boats. I have put new carb kits in, checked the fuel pump diaphrams (they're OK), new plugs, checked timing, checked compression, all is good. Is there possibly something I'm missing? Is the engine too small for the boat?
I'll put on a smaller pitch prop and see what happens.
Is ther anyone out there in Whaler land that has a 40HP on the 15'3' (1986)
Thanks for the info & suggestions

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The advice you've been reading to put the AV plate at the keel line is very out of date. You'll have better luck with the AV plate around an inch above the keel. I'd start the investigation by moving the motor up a couple of holes. It won't cost you anything except maybe some caulking and then you'll be able to see where you stand with performance and RPM. I'm guessing you'll pick up a bit of speed at the top end, maybe get up to around 31 MPH.

Next, change out the prop - for the best accelleration, you want the WOT RPM to be right at the max of 5500. Think of lower pitch like using a lower gear in your car. You're running a 17 pitch, so you want to drop 10% in pitch to pick up 10% in RPM - that would be roughly a 15-pitch. But the modern SS props have a lot more cup than your aluminum, so that effectively adds pitch, so Tom's recommendation of a 14-pitch is right on. The Stilettos are very nice and priced right - should be able to get one, with hub kit for your motor, for somewhere around $225.

Your 40 is definitely toward the lower end of the horsepower range for that boat - it's never going to be all that quick out of the hole. But if you follow the recommendations you have received, you should be able to improve the hole shot and general driveabilility significantly. And it's by no means a given that you'll have a lower top speed with the lower pitch prop - it may actually improve the top speed by getting the RPMs to where the motor can generate more power.

Tim

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As Tim and Tom stated above, raise your engine so that your Cav plate is about an inch above the keel. With today's modern props, raising the Cav plate an inch isnt a problem.

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No, reducing pitch will NOT necessary reduce the top speed, in fact it ought to increase it in this situation, especially if a decent model of propeller is chosen.

No, you do NOT want the AV plate even with the bottom of the hull, that is too low. Again, raise the motor on the transom to get it above the bottom of the hull.

Propeller slip is a calculated value based on the pitch of the propeller, RPM and boat speed through the water. There is nothing "wrong" with slip; it is a value we calculate to see if the particular model of propeller is behaving consistently with how it runs on others boats. Every model of propeller will run a slip in a normal range of values. Once the slip exceeds this expected range, eye brows are raised.

Porpoising and poor acceleration are both symptoms of an outboard mounted too low on the transom.

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Thanks for the education. I will raise the motor up 2 notches and will try out the 14P prop Tom has recommended. I will go with an aluminum (even though I would like to use stainless) as where I run the boat there are a lot of reefs.
Thanks for the help and will let you know how things wrok out once the changes are made.