Prop Question

MO_CPA · 04-16-16, 10:03 PM

Good day gents,

So I have a Tracker 175 TXW with a 60HP Mercury 4 stroke. I had a 3-blade 10.375 x 14P prop on it and it was a DOG coming out of the hull when I had two guys in it and a normal fishing load of gear. When I say dog I mean 10+ seconds to plane.

Since this wasn't quite good enough for me (thinking it was somewhat of a safety hazzard) I switched to Mercury's new Spitfire aluminum 4-blade in a 10.3 x 13P. I tell you what, it's like I added an extra 50hp back there...the hull shot was improved by at least 50% and probably more (4-5 seconds with two guys and gear). The boat just handles all around better, turns better, trims better, reverses better, and etc.

Here's where the quandary is, I now reach the top of my manufacturer's rpm range much quicker than I did before (6,000 rpm). I hadn't lost much if any top-end performance but I get to the top before I have laid the hammer all the way down. If I do lay it down, the rpms will get all the way to 6200 rpm give or take.

So, should I be concerned or should I just be mindful of my tach and not over rev it? I'm concerned if I pop up to a 14 pitch 4 blade I will lose my new-found hull shot and performance improvements that I am so in love with. This boat is not a speed demon by any means, so should I just be happy with what I have?

Thanks for the input.

MO

mikechell · 04-18-16, 07:39 AM

Propping has always been a balance of hole shot and top end.
Less pitch (or diameter) on the prop: get faster revs and better hole shot, but you lose top end speed.
More pitch: you get more top end speed at the expense of the hole shot.
The main variable you need to be careful of is the rev limiter. (you cannot actually "over-rev" a modern day engine).
If you are getting to max RPM too quickly and hitting the rev limiter, you have too little prop. You can (rarely) cause damage if you're continually bouncing off the rev limiter.
If you aren't actually getting to the rev limiter, then you are propped perfectly ... best hole shot and best top end.

mikechell · 04-18-16, 09:33 AM

When my guys are setting up a new boat, I always have them prop a boat to HIT the rev limiter at top speed.
That boat will NEVER be as light as it is right then. The customer will immediately fill the fuel tank to the top (63 pounds per 10 gallons), put in a few items (gear, life vests, coolers maybe) and one to several people. All these additional weights will bring the RPMs lower and lower at WOT.
It sounds like your original prop was set for "proper" RPM with one person and no extra weight onboard.

MO_CPA · 04-18-16, 12:46 PM

Thanks for the info Mikechell.

I never hit the rev limiter, even trimmed out and hammered down. I did jump over the manufacturer's recommended RPM's per my tach by 200rpm. I pulled it back quickly in fear that I would damage something.

Another thing to think of, it was relatively cool yesterday and very low humidity. I heard that the motor will run less RPM's in hot sticky weather, which is normally what I fish in.

The other thing that might be changing my RPMs is the fact that I can trim much more with the four blade than I could with the three blade without porpoising.

I think I will stay where I am at. The boat is running well, just very different. It will take some getting used to.

Captmikestarrett · 04-19-16, 11:43 AM

This is good info.. It makes me think of something my dad explained to me while on an Air Force C130. The turbo prop and how it feathers the blades to achieve lift off and then air speed. The blades rotate to adjust pitch.. So some time ago a Mercury tech put together a prop that does just that.

http://www.saltwatersportsman.com/me...-new-smartprop

They got mixed reviews at first but as the design got better the prop got better.

Capt Mike

mikechell · 04-20-16, 01:09 PM

Funny you should bring that up, Captain. The first aircraft I worked on in the Marine Corps was the C-130. You are correct about that AC ... the engines are maintained at about 90% full speed and the props are angled to get whatever performance is desired. Take off required about 110% from the engines (very easy to do, short term, with a jet engine) and max pitch on the props. Or they used JATO ... which was a fantastic kick in the pants on the bird, very cool to watch from the ground.

Mercury's variable pitch prop is no longer made. It was great for heavy boats, not so much for the average "joe".

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04-16-16, 10:03 PM	#1
MO_CPA BassFishin.Com Active Member Join Date: Jun 2011 Location: Ozark, Missouri Posts: 361	Prop Question Good day gents, So I have a Tracker 175 TXW with a 60HP Mercury 4 stroke. I had a 3-blade 10.375 x 14P prop on it and it was a DOG coming out of the hull when I had two guys in it and a normal fishing load of gear. When I say dog I mean 10+ seconds to plane. Since this wasn't quite good enough for me (thinking it was somewhat of a safety hazzard) I switched to Mercury's new Spitfire aluminum 4-blade in a 10.3 x 13P. I tell you what, it's like I added an extra 50hp back there...the hull shot was improved by at least 50% and probably more (4-5 seconds with two guys and gear). The boat just handles all around better, turns better, trims better, reverses better, and etc. Here's where the quandary is, I now reach the top of my manufacturer's rpm range much quicker than I did before (6,000 rpm). I hadn't lost much if any top-end performance but I get to the top before I have laid the hammer all the way down. If I do lay it down, the rpms will get all the way to 6200 rpm give or take. So, should I be concerned or should I just be mindful of my tach and not over rev it? I'm concerned if I pop up to a 14 pitch 4 blade I will lose my new-found hull shot and performance improvements that I am so in love with. This boat is not a speed demon by any means, so should I just be happy with what I have? Thanks for the input. MO __________________ Whatever you allow is what will continue.

04-18-16, 12:46 PM	#4
MO_CPA BassFishin.Com Active Member Join Date: Jun 2011 Location: Ozark, Missouri Posts: 361	Thanks for the info Mikechell. I never hit the rev limiter, even trimmed out and hammered down. I did jump over the manufacturer's recommended RPM's per my tach by 200rpm. I pulled it back quickly in fear that I would damage something. Another thing to think of, it was relatively cool yesterday and very low humidity. I heard that the motor will run less RPM's in hot sticky weather, which is normally what I fish in. The other thing that might be changing my RPMs is the fact that I can trim much more with the four blade than I could with the three blade without porpoising. I think I will stay where I am at. The boat is running well, just very different. It will take some getting used to. __________________ Whatever you allow is what will continue.

04-19-16, 11:43 AM	#5
Captmikestarrett BassFishin.Com Super Veteran Join Date: May 2006 Location: Accokeek MD Posts: 3,315	This is good info.. It makes me think of something my dad explained to me while on an Air Force C130. The turbo prop and how it feathers the blades to achieve lift off and then air speed. The blades rotate to adjust pitch.. So some time ago a Mercury tech put together a prop that does just that. http://www.saltwatersportsman.com/me...-new-smartprop They got mixed reviews at first but as the design got better the prop got better. Capt Mike __________________ Capt Mike Starrett light tackle guide Potomac River http://www.indianheadcharters.com

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04-18-16, 07:39 AM	#2
mikechell BassFishin.Com Active Member Join Date: Apr 2011 Posts: 464	Propping has always been a balance of hole shot and top end. Less pitch (or diameter) on the prop: get faster revs and better hole shot, but you lose top end speed. More pitch: you get more top end speed at the expense of the hole shot. The main variable you need to be careful of is the rev limiter. (you cannot actually "over-rev" a modern day engine). If you are getting to max RPM too quickly and hitting the rev limiter, you have too little prop. You can (rarely) cause damage if you're continually bouncing off the rev limiter. If you aren't actually getting to the rev limiter, then you are propped perfectly ... best hole shot and best top end.

04-18-16, 09:33 AM	#3
mikechell BassFishin.Com Active Member Join Date: Apr 2011 Posts: 464	When my guys are setting up a new boat, I always have them prop a boat to HIT the rev limiter at top speed. That boat will NEVER be as light as it is right then. The customer will immediately fill the fuel tank to the top (63 pounds per 10 gallons), put in a few items (gear, life vests, coolers maybe) and one to several people. All these additional weights will bring the RPMs lower and lower at WOT. It sounds like your original prop was set for "proper" RPM with one person and no extra weight onboard.

04-20-16, 01:09 PM	#6
mikechell BassFishin.Com Active Member Join Date: Apr 2011 Posts: 464	Funny you should bring that up, Captain. The first aircraft I worked on in the Marine Corps was the C-130. You are correct about that AC ... the engines are maintained at about 90% full speed and the props are angled to get whatever performance is desired. Take off required about 110% from the engines (very easy to do, short term, with a jet engine) and max pitch on the props. Or they used JATO ... which was a fantastic kick in the pants on the bird, very cool to watch from the ground. Mercury's variable pitch prop is no longer made. It was great for heavy boats, not so much for the average "joe".

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