props

[Way2slow]

One more thing I should remind you of.  You are moving into summertime and as the water gets warmer the slip is going to be greater, so the rpm is going up a little and the speed is going down so remember that as you start troubleshooting.  Given a choice between 200 rpm below max and 100-200rpm over max, I always go for the over max rpm.  

[gimruis]

9 hours ago, WRB said:

What boat do you?

 

Its a Ranger RT178 with a 75 hp Merc 4-stroke.

[Way2slow]

Since I've not run Black motors since the late 60's I've been doing a little research and have come to the conclusion, as for what prop you need, I don't have a clue.  I had all these preconceived ideas on what I would probably start with, but after doing some research on Merc's, I'm totally wet behind the ears and would only lead you down the wrong path.  

The first kicker is the gear ratio of your lower unit being 2:0? something, I've never dealt with one that tall.  I'm used to 2:7 and 2:8.  

Then looking at their props, they only have a couple that would even begin to work with your motor, and neither of those looked like something I would want to run.  

I was thinking you would probably need something like a 17" pitch but with that gear ratio, that's going to be a lot, and looking at Merc's, that would be over 14" diameter, and I don't think that would work.  

So, before I lead you down a blind hole, I'm gonna have to say I don't have a clue on what to recommend for something to start with.  Just don't have the experience with anything close to what you have.  

Four Stroke motor; I'm a two stroke guy

Tin boat, I'm a fiberglass, riser hull guy

2:0 lower unit, I've run 1:1 turning close to 10,000 rpm and 2:8, not even close to 2:0 

[gimruis]

Thanks for honesty @Way2slow.

 

The diameter is 13.25 and the likely pitch is 17.  I've been able to gather that so far.

[Functional]

If I'm not mistaken....the 75 mercs are just detuned 125hp motors. If you are not in warranty you could address the ECU and get a lot more than what a prop will offer you. I recall reading about it a lot looking for prop and motor height position for my boat. If you do a little searching you can find out more on it and decide if it's for you. If I recall, it wasnt all that hard. 

[Way2slow]

I was thinking around 17" pitch and 13.5 would be close, but that gear ratio is going to make it fairly sensitive to weight and how well it gets out of the hole.  I have a feeling if you unload it, taking out a lot of the weight, you should see a rather significant increase in top speed.  If you have a lot of weight in the bow, that's a speed killer because it has to be able to get the bow up or the hull will be running too wet and creating tons of drag.

[gimruis]

7 minutes ago, Way2slow said:

If you have a lot of weight in the bow

 

I don't have much weight up there.  Just the bow mount.  My tackle and rods/reels are mostly mid ship and my live well (which is almost always completely empty), fuel tank, and batteries are all near the stern.

[Susky River Rat]

As much as I joke about not having a prop the same issues arise in the jet world. With impellers and anything else you can think of to try to gain top speed.

[Capt Ed]

You need accurate rpm and top speed numbers.  Don't use the instruments on your boat.  

 

If you don't already have it, Mercury's Vessel View Mobile is a great addition.  It can measure rpm's accurately, speed, fuel flow, temperature, and more.  https://www.mercurymarine.com/us/en/product/vesselview-mobile#:~:text=VesselView Mobile® wirelessly connects,details into your Mercury engine.

 

Or, take it to your Mercury dealer and have them run a diagnostic test.  You'll be able to see rpm's in increments including wide open throttle.  

 

With good numbers, then you can talk accurately with an experienced prop tech.  

[Way2slow]

A tachometer and the GPS in his sonar (assuming he has that since most do these days) is more than accurate enough.  If by chance he is depending on a water pressure mounted speedometer, hell he might be running 40 and just don't know it.  Diagnostics usually tells what percent of time it has operated in different rpm bands, not the exact rpm it's turning at WOT running down the lake.

 

Like I say, start with the facts first.  Get a good RPM reading and GPS speed reading with it trimmed out perfectly.  While doing this, pay attention to where the spray is coming out of the side of the boat.  Run these numbers on a prop slip calculator and see what you get.

Also, pay attention to the sound of the motor while trimming it out.  Make sure you can over trim it.  You should be able to hear the motor change sound and a lot of times you can feel it in the steering wheel because it will start cavitating the prop.  If you can't over trim the motor, that's the rabbit you need to chase first.  Getting the spray coming out behind the driver's seat is the next one if it's not doing that.

[TnRiver46]

Old salty boat mechanic I know said the best way to prop a boat is put it on and go. Guy knows his stuff, said he grew up propping boats at a salt water marina . 
 

used to be a prop swap day for bass boaters around here. Everyone show up to the ramp and try out different props to see which ran better 

 

it’s a no BS method 

[Tennessee Boy]

1 hour ago, Way2slow said:

If by chance he is depending on a water pressure mounted speedometer, hell he might be running 40 and just don't know it.

I did the break in for my current boat before I installed my electronics.  The water pressure speedometer said my top speeds was 47 mph.  When I installed my fish finder the GPS said my top speed was just over 40 mph.  Now the water pressure speed tops out at about 10 mph if it reads anything at all.  

 

Personally,  I only change props when I break one.

 

17 minutes ago, TnRiver46 said:

used to be a prop swap day for bass boaters around here.

It was around there that Paul Allison hit something and bent his prop.  The bent prop was faster and the cupped propeller was born. 

[gimruis]

24 minutes ago, TnRiver46 said:

used to be a prop swap day for bass boaters around here. Everyone show up to the ramp and try out different props to see which ran better 

 

it’s a no BS method 

 

I'm tempted to mark this as the solution and I'd be there in a heartbeat if this actually happened near me.  But being nothing but a pipe dream (at least here), its not a realistic solution.

[TnRiver46]

@gimruis, I bet you would have better luck with a vexilar transducer swap day 😂 

[Choporoz]

This is like listening to the Hydromotive prop guy, on BTL podcast.  I have no clue what anyone is talking about, but it is enthralling all the same

[Way2slow]

Too bad it's not a Go-Fast boat with an OMC motor.  I've got all kinds of custom tuned three and four blade SS props for those, but they only go down to 24" pitch.  My first 70+ mph bass boat was in the early 80's, a 15' ProCraft with a 200 on it.  I had race boats that ran faster but not a fishing boat.  Bought the boat with an 85 that needed rebuilt, (it was only rated for 85) had a modified 200 sitting there ready to go but doing nothing so said, "Why Not".  Don't remember having one that would not run in the high 70's, mid 80's and one that ran in the low 90's since then.  Even my Javelin Renegade 20DC ran 82 with two people and gear with one of my modified motors on it.  Stepped out of that three years ago into a 22mph 24' pontoon boat, talk about an attitude adjustment.  

Used to be, there were not that many choices to make on props, and it was easy to find someone that had a similar setup and swap props just to see how they performed.  Now days that doesn't seem like a very viable option, tons of different boat designs, motor designs and prop designs.  Then add the fact that people just don't seem to be as obliging as the used to be.  I used to always say there's an AH in every crowd. Now I say you are lucky to find someone that is not one.

 

Back to the prop situation.  Start with getting the known good, basic information together, and I'm pretty sure we can walk you through diagnosing the problem.  I'm a firm believer in not just throwing parts and money at a problem.  Start from a known good point, and work into the unknown a step at the time and you will be surprised how easy it can be.  Now, applying a cure might not be so easy but diagnosing the problem should be.

[DaubsNU1]

Hey @gimruis for what it's worth: My brother has a 2020 Lund Alaskan 1800 with 90hp and the same prop...and makes 35-36 MPH.

 

My Lund specs out 17'10", boat weight 1,144lbs (i'm guessing without motor == add 360 lbs for the 90 HP). My boat came with four blade Mercury Spitfire 13x 17P Propeller 48-8M8026590. It will make 38-39 MPH.

 

If I plug in all those numbers to the Mercury propeller selector web site...I get the following results...

 

 

I might pick up the Vengeance some day...for now, I am very happy with the 4 blade...hole shot is great, handles well. And I'm still faster than my Brother ; )

 

 

 

[Capt Ed]

21 hours ago, Way2slow said:

A tachometer and the GPS in his sonar (assuming he has that since most do these days) is more than accurate enough. 

It's not uncommon for the factory tachometer supplied with Ranger aluminum boats to be 200-400 rpm's low as reported on other bass boat forums.  My own Ranger VS 1782 tach read 400 rpm's low.  

 

It's going to be hard to squeeze a few more mph from a 75hp engine, and if the tach is off, the guessing game finding the right one grows harder.     

[Way2slow]

I generally used a mechanical contact tac to check the mounted one and rarely ran into one the was more than a few RPM off (back when they made things in the USA).  I guess I've been out of this stuff too long and the Dumb**s for trying to help since I guess the Chinese don't know how to make accurate tac's, since that's where everything seems to come from these days.  

So, ya'll have a good day, since I don't seem to know what I'm talking about, I will keep my 2cents worth to myself.  

[Woody B]

FWIW my dash mounted tach matches the readout in Mercury Vessel View.

 

Once again,  make sure you're getting full throttle, and see where the spray comes from when on plane and trimmed.   Getting the spray back a couple feet, to the rear of my seat (where Keith said it should be) made a BIG difference in my boat.  Not just speed, but it takes rougher water a bunch better too.   

[gimruis]

3 hours ago, Way2slow said:

So, ya'll have a good day, since I don't seem to know what I'm talking about, I will keep my 2cents worth to myself.  

I started this thread looking for advice on props. You have provided a lot of valuable information. I’ll be honest, some of it is probably too deep for me. But it’s still valuable.

 

I definitely wasn’t prepared for some of these responses.

[Deephaven]

I have 4 props for my tiller.  The quickest, nor the fastest are the ones I like.  Bow lift and mid range response are the most important to me.  Fluctuations in setup are almost 10mph between them.  The most fun and best performing, in particular in rough water is the one with the most capable acceleration when already on plane.

[airshot]

On 5/21/2024 at 4:35 AM, Way2slow said:

One more thing I should remind you of.  You are moving into summertime and as the water gets warmer the slip is going to be greater, so the rpm is going up a little and the speed is going down so remember that as you start troubleshooting.  Given a choice between 200 rpm below max and 100-200rpm over max, I always go for the over max rpm.  

Interesting thought here,  I boat on Lake Erie and its tributaries, all fresh water here, so no information about salt water performance.  On my boat the RPM does run a little higher in the summer heat, but the boat speed increases as well.  On a warm summer day with no wind/ current my GPS will top out at 31.5 mph consistantly.  But early spring or late fall when temps are 50 or below, the rpm drops slightly, Maby 100 at best but so does the speed, can't get over 29/30 mph.  Not sure why it does but your post caught my attention, so I had to respond. 

[gimruis]

On 5/21/2024 at 12:28 PM, Way2slow said:

Get a good RPM reading and GPS speed reading with it trimmed out perfectly.  While doing this, pay attention to where the spray is coming out of the side of the boat. 

 

I had a chance to do this yesterday.  Water was flat and WOT.  RPM is 4500.  I had a passenger with me to even the weight out evenly.  Spray is good coming out just behind where I am sitting while driving.  My top GPS speed was 33.1 to 33.2 mph.

 

I've also had the chance to think about this a little more and maybe it just isn't worth tinkering around with props given the nice hole shot, control, and overall performance I'm already getting.  If anything I might try a SS 4-blade prop at some point in the same pitch.

[TnRiver46]

@gimruis, that’s low for top end RPM, was the motor trimmed all the way down ?