What Pitch Propeller to Buy ?

[OrangeCrush]

First of all, let me say that this website has been the greatest resource I could have imagined since I bought my first sailboat a year and a half ago. Everybody is so incredibly helpful and I really like the tone of the forum. Thanks!

So, I have another important question. I need to replace the propeller on my Pearson 26's outboard motor, a 2002 Mercury 15hp two-stroke (model number 1015211ZD). The boat is around 6,000 displacement.

They make three different propellers. 9.25 x 7.00, 9.00 x 9.00, and 9.25 x 6.50.
I don't know what the pitch is of the propeller I am replacing. I could inspect it but I don't know what to look for and I am pretty sure it is corroded beyond recognition of any numbers that may or may not exist, etc.

The on the phone at the vendor suggested the 6.50 pitch when I told him it was a sailboat. But he also mentioned that using the wrong pitch can strain the engine either by pushing too much water or revving too high. Is there a way of calculating or finding out the right pitch for my outboard?

Thanks again, Jack

[Ex-Calif]

Jack - Check out this thread. Gord posted in the past quite a collection of prop calculators.

http://www.cruisersforum.com/forums/...-info-269.html

[djmarchand]

Go with the 6.5 pitch. Your Pearson 26 won't go much faster than 6 kts. That outboard motor is designed to work with planing hulls that will go 15 kts and need the coarse 9 pitch. Your slower speed hull will work fine with the flat pitch prop.

David

[klem]

You will want to check using a prop calculator but the 6.5 pitch is probably correct. Pitch refers to the distance that the prop moves through the water each revolution with no slippage. Since sailboats motor quite slowly compared to a planing hull, they require much less pitch. Too much pitch will mean that the load is put on the engine at lower rpms which prevents you from revving up since the engine doesn't produce the necessary power at low rpm. Too little pitch will simply limit your top speed which tends not to be a problem for sailboats.

[OrangeCrush]

Thanks, I am starting to understand. I am not sure how to use any of those calculators though. How do I look up engine RPM, gear ratio or propeller slip for my outboard?

Should I just go with the 6.5? Or should I look that stuff up to check? Thanks

[Paul Kotzebue]

Wide open throttle for a 2002 Mercury 15 hp 2 stroke is between 5000 and 6000 rpm with 2:1 gear ratio. You'll likely find that 6.50 inch pitch is more than ideal, but is probably OK since you may never actually use WOT.

[OrangeCrush]

What do you mean by more than ideal? That I would be better off with even smaller than 6.5?

On the prop calculators, the only number I didn't have was prop slip.

As for wide open throttle, I usually open it up all the way and back it off just a hair until it sounds like it's in a nice groove. Is that a good way to do it?

Thanks!

[Paul Kotzebue]

Assuming apparent slip is 45% (just a guess), and your desired speed is 6 knots, 5.30 inch pitch would be ideal.

[OrangeCrush]

Understood. If I go with the 6.5, will I be doing any damage to the motor by running it at or near FOT?

[Paul Kotzebue]

Probably not. However, if you increase throttle and the boat doesn't go any faster, you should back off a bit.

[OrangeCrush]

Good advice, thanks!

[hamburking]

I'm going to go way out on a limb and disagree with all the previous posts...I would recommend the 9 x 9 .

Your 15hp outboard has way more power than you need for a Pearson 26...you could push it with a 9hp motor. So, since you've got tons of power, your should get the most aggressive prop available. This will allow you to make full use of the low end power of your outboard....in english, this means low rpm, nice and quiet, with lots of thrust. And you would burn alot less fuel, and your engine would have less wear.

IMHO, if you get the 9x6, you will end up revving high and going slow. You'll use more fuel, make more noise.

My pearson 30 has only 18hp (inboard diesel).

[OrangeCrush]

Well it's been a few years now since I bought that propeller (9.25 x 6.50). It's been working pretty well for me so far, with several thousand nautical miles under its belt (assuming propellers have belts). Don't have much to compare it to, but fuel efficiency is good.

Are you saying that if my outboard were a 9hp, I would want a flatter propeller so that it could run at higher RPMs, but since my outboard has more torque at low RPMs, I could get the same thrust with less fuel?

This makes sense to me, and it's hard to remember my exact thinking at the time, but I seem to remember the actual speed the boat (and motor) would be moving through the water was a big factor as well.

If you're feeling very sure about your reasoning, maybe it's worth me experimenting with a different pitch prop, but it seemed like I got the best one possible, and I certainly haven't had any issues with it....

[hamburking]

Originally Posted by OrangeCrush

Are you saying that if my outboard were a 9hp, I would want a flatter propeller so that it could run at higher RPMs, but since my outboard has more torque at low RPMs, I could get the same thrust with less fuel?

Thats exactly what I'm saying. The smaller engine would need to run at higher rpm to achieve the same power output. But running a prop slower is always prefered (on a displacement hull) because it increases prop efficiency...or reduces "slip". And running slower is also nice and quiet, reduces engine wear, etc.

I've been researching this topic because I have a repowered boat, a pearson 30, and although the engine never strains at all, I can't go very fast even at full throttle. The newer volvo diesel just doesn't to the rpm of the old atomic 4, so even with a bigger prop I now have, I never achieve anything near hull speed. I think I need to address the reduction gear. First step...figure out what I've got!

Anyway, if your boat is running well, then you made a good choice. If you happen across a used 9x9 for cheap, it might be worth trying it out, just to satisfy your (ie my) curiousity.

My previous boat was a paceship 29 with an atomic 4. The standard 2 blade had been replaced with a 3 blade, high pitch prop. I got almost max speed as soon as I put it in gear, and had to keep the idle speed high so as not to stall it. And it would not rev up hardly any more. It was not a good situation. Quite the opposite to my current situation.

Anyway, props are not an exact science. With efficiencies of around 50%, there is lots of room for improvement.

In general, the slower the engine, the bigger the prop, and the higher the efficiency. The new electric engines can run very slow with nice big props and achieve much higher efficiency than internal combustion engine setups. They do this because of the limited battery power available for most electrics. With gasoline engines, the opposite is true....gasoline holds a tremendous amount of power, and its easy to bring gallons and gallons with you. At least that was the thinking back when gas was cheap.