Should I try a different propeller on my sailboat outboard?

[OrangeCrush]

I posted a thread years ago about choosing the right propeller pitch for my sailboat. I've got a 2002 15hp Mercury 2 stroke on my Pearson 26. Based on the advice I got here, I went with the smallest-pitch propeller I could find, which was 9.25 x 6.50. It sounded like that was the safest bet to avoid straining the engine, but I struggled to fully understand all the parameters at the time.

Here is the old thread:
http://www.cruisersforum.com/forums/...buy-39688.html

The last poster, hamburking, disagreed with everyone else, but this happened two years after I bought the low-pitch propeller. His rationale was that since my 15 hp motor is already larger than it has to be, it has more than enough torque to propel the boat at lower rpms, and that with a greater pitch I would get the same thrust with less fuel, noise, vibration, engine wear, etc.

I've put hundreds of hours and thousands of miles on the 9.25 x 6.50 propeller without any probles. Then again, maybe hamburking is right it's still not ideal. His argument makes more sense to me the more I think about it. The motor sounds a little unhappy above 75% up to WOT, with very little extra speed as throttle increases. Maybe it's over revving? I've also got pretty poor stopping power in reverse, but I always figured it was just normal. Since I'm almost always running it below 75% throttle anyway, maybe I've got torque to spare and a steeper pitch would improve my power and efficiency.

I know that the "If it ain't broke" school of boat maintenance may chime in here, but I'm happy to spend $85 to try a propeller with greater pitch and see if it improves things. But how steep should I go? The three choices that I see listed as parts for my motor are 9.00 x 9.00, 9.25 x 7.00, and 9.25 x 6.50 (the one I already have). Hamburking recommended 9.00 x 9.00, but maybe then I would end up straining the motor like I was trying to avoid in the first place. Should I then try a smaller change and go with the 9.25 x 7.00? Is there any way to know if it's straining or over-revving other than sound and feel? I know there is a formula for pitch, but I don't know how to use it and I don't think it factors in horsepower, which seems like would make a difference.

Anyway here is some other potentially relevant info:
Waterline length: 21.7
Displacement: 6000 lbs
Outboard Model number: 1015211ZD
I'm not sure what the rpm range should be on this motor.

Anybody able to offer confident guidance on this? Any help is greatly appreciated.

Jack

[a64pilot]

You need to know both the operating range of the engine and it's RPM, you need a tach.
You should prop any engine so that at full throttle it is running in the middle of it's max RPM range.
You can over prop an engine without damage as long as you never run it a high throttle settings, but it's no recommended and lugging the engine at higher throttle will over heat it and damage it.
In short I'd leave well enough alone.
Probably what was said in the other thread, but I didn't read that one

[OrangeCrush]

Thanks for the reply.

Well the original question wasn't whether to change it or leave it alone, since I didn't know what propeller I was replacing in the first place. Now I'm just questioning whether I made the right choice back then. It seems like a pretty influential decision; maybe it would be worth getting a cheap tach and measuring the rpms at WOT, if that would give me a clearer answer.

[Jim Cate]

Another approach is via an aftermarket prop vendor such as Solas. They offer a greater variety of props, including 4 blade models, and can offer sound advice about choosing, even for sailboat applications.

I've never used one on a displacement hull, but was able to markedly improve the performance of a Suzuki 15 on a heavy inflatable by mounting a four blade "cupped profile" prop. It would be worth a call...

Cheers,

Jim

[onestepcsy37]

From my own experiences I've found the so-called "sailboat" props are more suitable for pushing barges than boats, I had an 8hp on a 5000lb boat and found when I traded the sailor prop for the "standard" powerboat prop I picked up speed and fuel economy with no real loss in low end torque. Same experience with a 6hp on a 4000lb boat.

With your 15hp on a 6000lb boat you should have no need at all for a sailor prop. You're probably overpowered already.

[SkiprJohn]

I'd stay with the 6.50 and run your engine at what sounds like the right rpm. Check the engine specs to see what best rpm for best fuel economy is and run your engine at that rpm if you can find a tach for it. Just remember that if you're trying to push your boat faster than 5 or 5.5 knots you're wasting fuel.
You could probably make it go as high as 6.5 knots but your burning more fuel to do it.

[a64pilot]

Originally Posted by OrangeCrush

Thanks for the reply.

Well the original question wasn't whether to change it or leave it alone, since I didn't know what propeller I was replacing in the first place. Now I'm just questioning whether I made the right choice back then. It seems like a pretty influential decision; maybe it would be worth getting a cheap tach and measuring the rpms at WOT, if that would give me a clearer answer.

A tach will give you your answer, but as Jim pointed out calling a prop manufacturer may well get you where you need to be just as easy.
Used to be any marine mechanic had a tach as most boats back in the day didn't, don't know if this is still true, maybe you can rent / borrow a tach?
Under propping an engine as long as you don't over rev the engine will cause no harm, over propping one can hurt it, that's why I suggested leave it alone

[OrangeCrush]

I'll definitely give Solas a call and see what I can learn from them. Thanks for that suggestion.

I was looking into buying a photo tachometer (at least about $100 or so, it seems) when a little voice in my head called out, "There should be an app for that." Sure enough, there is an iphone app called Strobe Tachometer that uses an adjustable strobe light to determine the RPMs of a spinning object. Pretty clever.

So a few questions remain.

1) Can I test the outboard's RPMs at WOT in a tank or at the dock, or do I need to get the boat up to full speed? I can't seem to work out this concept in my head. It seems like you would need the boat to be moving at normal speed to test the RPMs, but then again I'm guessing mechanics do everything in a tank.

2) Where can I look up the desired RPM range for my motor? I have a Seloc manual but I'm not finding it in there. I don't have the owner's manual with me right now. Is it searchable online somewhere by model/serial number?

3) For the sake of argument, if I did try a new prop and it did have too much pitch for my boat/motor, would I be able to hear and feel it straining (as I imagine) or might it be too subtle to notice?

I'll still call Solas about this and see what they say, but trying to work out some of this stuff on my own first.

Thanks again!

[a64pilot]

1. No, you need the boat up to speed, but a sailboat may be so slow that it doesn't make a whole lot of difference. A prop turning through a moving fluid has what is called induced flow, all that means is that the effective pitch of the prop is reduced by the amount of the forward motion of the boat. Look at a fast planing boats prop, the reason they have such a huge amount of pitch is because of the speed of the boat. A fast boat will often ventilate it's prop to unload the engine until it gets up to speed.
Your ear cannot tell if an engine is bogging down or not, a dealer for your motor can tell you optimum WOT RPM, but I'll be they or a prop manufacturer can recommend a prop that is close enough.

On edit, take the cowling off an try your iphone on the flywheel, never tired the iphone thing myself

[sy_gilana]

I would stick with what you have.
The higher pitched props are more raked, they have a curve on the blades.
The "work" prop is more flattened and gives you better astern power.
The blade area is also normally larger.
The thing is you must not compare speed on a calm day with prop x or prop y. You must look at pushing the 6000Lb boat into a stiff breeze with a smart chop and still make headway. Under those conditions the standard prop, which is designed for planing craft will probably pump a lot of air.
Get a spring balance and tie the boat to it and measure the bollard pull, there lies the secret.

[perchance]

Solas offers a 4 blade high thrust prop in your hp range designed to push small heavy boats like sailboats etc.

[onestepcsy37]

Think he already has too much thrust. With that displacement/hp I would be looking for a faster prop - 8" pitch comes to mind. I think he could achieve hull speed at a much lower rpm which would lower fuel consumption and increase engine life.

[Opie91]

As already stated several times you really need to put a tach on that motor, otherwise it is just guess work.

You do not need to spend $100 on a photo tach check this one out:

Digital Laser Photo Tachometer Non Contact RPM Tach Meter Motor Speed Gauge USA | eBay

An alternative would be a decent automotive multi meter. Most have a spark plug sensor for rpm. Check around the dock someone has to have one.

Modern Mercury motors have a sticker with the max rpm located on the midsection. (see pic)

Spend the time and check out the max RPM it takes no time at all.