Prop selection 20' MA 175 Mariner

[Chaser]

I have a couple of questions regarding the propeller on my Master Angler 20. I'm not a prop guy, know nothing about them, so pardon my ignorance. I'm just trying to get some info.

The boat is a MA 20 with a re-built 175 HP Mariner 2 stroke. I put the boat together two seasons ago, and its run great every step of the way. Its got excellent hole shot, and good top end speed, maxing out at about 38 knots with one person and minimal fuel. Cruises at 25 knots at about 4000 RPM using 10 GPH.

As I mentioned I know nothing about props, and I don't even know what is on there, since it came with the motor when I bought it re-built.

The only problem I have is the boat doesn't want to stay on plane with RPM's lower than about 3800-4k. At that RPM I'm doing about 22-25 knots, but any lower R's and it will slowly fall off plane and slow down. I'd like to be able to cruise a little slower for fuel economy. Like I said everything else about the boat/motor works great, and I'm very pleased.

Is it worth even considering a new prop, will I get any benefit in fuel economy? I don't mind sacrificing top end speed but would like to keep the acceleration I have.

[Chaser]

Just found the prop dimensions. 14.5 x 19 pitch. I guess I should also mention at WOT I'm getting to 5800 RPM.

[CHANCE1234]

Your planing speed has a lot to do with how much your 175 weighs. Theses boats are designed for engines in the 300lb range. Any more than that and your min planing speedily increase. Trim tabs and a 4 bladed stern lifting prop will help. With one person and minimal fuel I would think your top end should be a little better than that. Go to you tube and search for "20 Seacraft minimum planning speed". That will give you an idea of what my 20 with a 2 stroke 115 yamaha does with a 15 pitch prop, 3/4 fuel and me on board. 3000 rpm, 13-14 knt min planing speed.

[Blue_Heron]

Originally Posted by CHANCE1234

Your planing speed has a lot to do with how much your 175 weighs. Theses boats are designed for engines in the 300lb range...

X2.

Minimum planing speed has more to do with the boat's weight and center of gravity than it does with your prop, but correct propping will definitely help. Try taking your largest friend for a boat ride and asking him/her to sit on the forward casting deck while you reduce throttle and trim the motor down. You'll probably find the minimum planing speed much improved.

Based on WOT engine RPM, your boat sounds like it's propped ok, but I would have expected more speed and less rpm with that combo. So the prop may be long on pitch and short on blade area, which might hurt it's performance at lower speeds. A Mirage 17 is a pretty good choice for all around performance and they can be had pretty cheap on ebay if you're patient. When I put one on my 20 with a 200 Merc, it improved low speed and cruise performance without losing a lot in top end.
Dave

[gofastsandman]

Originally Posted by Blue_Heron

X2.

Minimum planing speed has more to do with the boat's weight and center of gravity than it does with your prop, but correct propping will definitely help. Try taking your largest friend for a boat ride and asking him/her to sit on the forward casting deck while you reduce throttle and trim the motor down. You'll probably find the minimum planing speed much improved.

Based on WOT engine RPM, your boat sounds like it's propped ok, but I would have expected more speed and less rpm with that combo. So the prop may be long on pitch and short on blade area, which might hurt it's performance at lower speeds. A Mirage 17 is a pretty good choice for all around performance and they can be had pretty cheap on ebay if you're patient. When I put one on my 20 with a 200 Merc, it improved low speed and cruise performance without losing a lot in top end.
Dave

I think I`m up to 6 or 7 props now.

I think you will be blown away when you find the right one for your boat.
Every boat is set up differently.

[FishStretcher]

The prop is like the transmission and tires and air ride suspension all in one. It makes a HUGE difference in how the boat works and to a degree, the fuel economy.

See if you can borrow props or rent or buy the cheaper aluminum ones to start. I have a Solas Amita 4 blade stern lifter. I went down in pitch and up in diameter. And up in speed and up in acceleration. All for ~$110. The prop I started with is my backup prop.

I bet a little stern lift on a 4 blade, maybe down in pitch a bit, would work wonders.

[Bushwacker]

I agree with the 4B prop suggestions for more stern lift; some 3 blade props with lots of blade area, like the Merc Mirage and the BRP Rebel also have good stern lift that will help you hang on plane at low speed. There are a couple more things I'd check before you get too deep into changing props:

1. Make sure your tach is accurate. The digital tachs on the new motors that tie into the engine computer to get rpm are dead nuts accurate, but some analog tachs are notoriously inaccurate and they often read too low. Although not likely, your speed could be low because your motor is actually over revving and you need a prop with more pitch!

2. Make sure your motor height is correct. The AV (Anti Ventilation) plate should be above the water when you're up on plane and fully trimmed out, which generally means it's at least an inch or so above the keel. Some props like the Stiletto like to run even higher than that.

[Fr. Frank]

You've gotten some good advice here. One more thing I'd like to add: Even when you've think you've found "THE" prop, don't be afraid to keep testing and experimenting.

I ran a 17" Mirage on my V6 200 hp Mercury on the back of my 20' Seafari, and was amazed at how much better it was than the stainless 19"P Michigan I got with the motor. I tried a bunch of props before finding the Mirage. I went from 5500 rpms at 38 mph with the Michigan to 5750 rpms at 43 mph with the Mirage.

But then one day the owner of the local Mercury dealership offer to let me try his 4 blade 15x17" PowerTech he was running on his 19' Aquasport cuddy with a 200 efi Merc. That was even more amazing! I ran 5600 rpms at 45 mph, but minimum plane went from 18 mph down to 13 mph, and time to plane went from 7-8 seconds down to 5 seconds.

So I called PowerTech, and spoke with an engineer, and sent him photos of my hull, along with actual weight and balance data for normal usage, nose-cone design, etcetera; and they made me a custom one-off 15.25" diameter x 16.5" pitch 3 blade with a very aggressive rake progression on the trailing 1/3 of the blade and heavy cupping. It cost me a little over $700.

My new top speed was 53 mph at 6050 rpms with 10 gallons of fuel and one 200 lb driver, at 40F into a 7-8 mph headwind with a light chop. Time to plane was still 5 seconds, or just under, and minimum planing speed was now 14-15 mph.

I could have just stopped experimenting with the Mirage. But the Michigan I started with was "in the zone" for rpms, speed, and all that stuff. I just knew I could do better. I gained 15 mph in top speed, reduced my minimum planing speed by 4 mph, and reduced my time to plane by 2 seconds or more. [And the incidence of rake progression and cupping on the new prop threw a 25' high rooster tail at 50 mph :-) ]

Okay, it did cost me over $700. Worth every penny.

[Chaser]

Thanks for the feedback guys. Based on what I've read here and elsewhere, I figured I was close given the RPM range and WOT RPM I've gotten. Maybe I'll try a 4 blade with similar characteristics. I wish there was a prop shop around that I could borrow different props and try them out, instead of having to buy them!

I don't know if the tach is accurate or not. Its a new analog unit, but I don't know if its possible to calibrate it. How do I check the actual RPM without the tach? Is there a way to put a timing light on it? Maybe I should read more in my manual...

I think the motor height is pretty good. I just guessed when I mounted it (based on info I'd gotten here, thanks!) after raising the transom to 25". Running on full plane the AV plate is just above the water coming off the transom.

I know the boat is heavy too, and it sounds like this has a lot to do with my symptoms. Replacing the cockpit sole would help a lot as I know its wet. But this is more work than I want to do right now. The boat has the larger 70 gal fuel tank as well, and sometimes we pack it totally full. If it turns out I'm going to keep the boat another few years, the cockpit sole is a repair I would like to make, and would probably make a big difference in performance. I would move the fuel tank forward as much as possible at the same time, maybe even install a smaller tank.

Anyway thanks for the help guys. Great forum!

[Bushwacker]

Originally Posted by Chaser

. . . I don't know if the tach is accurate or not. Its a new analog unit, but I don't know if its possible to calibrate it. How do I check the actual RPM without the tach? Is there a way to put a timing light on it? Maybe I should read more in my manual...

Sounds like your motor height is about right. It's amazing how many dealers mount them too low!

I've had new factory tachs that were off by 300 rpm, so just because it's new doesn't mean it's accurate. I'd see a dealer about checking your tach. There are universal strobe tachs that work somewhat like a timing light, where you turn a dial that changes the strobe light frequency until a timing mark stands still, then just read the frequency/rpm from the dial. Some dealers also have a dyno they can attach to the prop shaft and run the motor up to max rpm using a known good tach while using a flushing attachment. Need to check it over the whole rpm range, as some tachs are ok at low speed and only off at high speed.

One crude check is to plot MPH (from GPS) vs RPM. It should be pretty linear once on plane. If the plot isn't a straight line, that's a clue that the tach may be off.