Need a new prop. Pitch question.

[STIguy]

Boat is a 17 foot Crestliner aluminum "bass boat" with a 1995 75HP Merc on it.

I do not have a tach so I can't say for certain what my WOT RPM's are.... but I have been told that these motors have a rev limiter on them so if it over revs it will cut out. I have run it at WOT several times and never had that happen so I guess it's within a safe limit.

The prop on it looks to be the original that came with the motor. Part # is: 48 77348 A40 21P

One of the fins on it are bent and another has a pretty deep gash on it so I am looking for a new prop.

The current prop being a 21p seems to work fine (even damaged as it is). The boat easily gets up on plane within seconds, cruises at a nice speed and the top speed seems pretty darn good. (sorry no GPS)

- Does reducing pitch ALWAYS reduce top speed?

- What about the diameter? The current prop is 12 3/4 inch. If the prop I get is 13 or 13 1/4 how does the diameter affect performance?

I am NOT looking for a better holeshot as it does fine now with the 21P and I don't plan on towing anything/anyone. If I went with a 19P would it kill my top end? I am looking for top end speed and cruising fuel economy... without killing the holeshot of course.

Are stainless props really that much better than others? Enough to justify their price?

Any advice on what pitch to go with or what brands to look for/stay away from is greatly appreciated!

Thanks in advance.

 

[steelespike]

Re: Need a new prop. Pitch question.

You should be able to get the prop repaired.You shouldn't run the damaged prop it could cause premature problems with the gearcase.Run the repaired prop and get the max rpm and speed.Generally prop diameter is a product of the prop design and is not considered in prop selection except to be sure it is intended for your application.Don't make any prop,changes without knowing your present prop size and max rpm and speed.A tinytach is reasonable and easy to install.perhaps you could borrow a gps for the speed.
If a prop change is indicated you can keep the repaired prop as a spare.
Generally a stainless prop will outperform aluminum.Some think they are worth it others not.The Hustler Turning point is said to give close to stainless performance for the cost of aluminum.
Changing props without your specs could result in premature engine failure at worst and poor performance and mileage at best.

 

[STIguy]

Re: Need a new prop. Pitch question.

I think I'm going to stick with a 21p and go stainless.

I am looking at these two props:

1. SOLAS RUBEX NS3 Stainless Steel Propeller (Prop: 9431-130-21 Hub: RBX-102)

2. Stiletto Advantage with Guardian Hub Stainless Steel Propeller (Prop: 811321 Hub: DE-501)

Which one should I go with?

 

[steelespike]

Re: Need a new prop. Pitch question.

Stainless prop selection is outside of my expertise.
a 21 sure seems like a lot of prop for your setup.
I couldn't find a Merc 75 test but a 90 on a 18.5 ft aluminum uses a 17.
evinrude 75 uses a 17 on boats similar to yours. I think the gear ratios are similar. I'm a little surprised you won't spend $40 for a tach but $300 for a prop that may be wrong.Too much prop will cause lugging,poor performance and mileage.Coking, premature repair problems.

 

[Randybeall]

Re: Need a new prop. Pitch question.

I would be with steelspike on this one. With the 21 pitch you will most likely never reach max rpm. That seems good but it costs efficency, read that fuel milage. The engine will perform best in all areas if it has the correct prop. Iboat has an exchange program which you should check out. What that offers you is the chance to try out a 19" pitch stainless and see what it does for you. That and a tach to tell the true rpm of the engine will let you know if you are doing the right thing. Remember the stainless prop will load the engine more so going down in pitch is to your advantage. You may find a little more top speed even though you go down in prop size. With the exchange program you can send that prop back for another which better suits your needs if needed and you don't damage the prop.

 

[CaptAhab]

Re: Need a new prop. Pitch question.

I have basically the same problem as you.. the thing is aluminum prop blades are not as stiff and cannot be cupped and raked like they can be in stainless.. which gives you more bow lift and you can run the motor at a higher setting without blowout.That can give you big gains in top end, if you run light and alone.
Otherwise if you don't want to get messed up about it a stainless 21 would probably do pretty well. You would be able to run it more semi-surface then the aluminum prop and eke out a few mph... just my opinion..

 

[STIguy]

Re: Need a new prop. Pitch question.

Thanks for the replies folks!

I went ahead and ordered the 21p Stiletto stainless.

My reasoning is this:

1. The motor came with a 21p and runs great IMO with it.

2. The boat is very lightweight. It's a 16' 8" aluminum with 2 plastic 6.6 gallon removeable fuel tanks which weigh almost nothing. I almost always go out by myself, occasionally with a fishing buddy and very rarely with 3 people in the boat. So I run light most of the time.

3. The max RPM's for this motor are low. The owners manual says 4750-5250 RPM. I believe thats pretty easy to achieve with a 21p on a 75 HP, light boat.

4. The motor can and most likey should be raised a bit. I have no ventilation problems at full throttle or when turning... even when trimmed way out. Plus, the stainless should "bite" better than my current prop so raising the motor a hole or two shouldn't be a problem and will raise the RPM's a bit if indeed I am below the recomended RPM.

Yes I do need a tach to be certain I am running in the recommended RPM range and I plan on getting one to be sure.

I do have a hard time believing that a $40 tach will be accurate though.... Can anyone verify the accuracy of these TinyTachs?

I hope my gut is right on this one... otherwise there will be a 21p stainless Stiletto Advantage for sale shortly :redface:

 

[steelespike]

Re: Need a new prop. Pitch question.

There have been a lot of tinytachs applied here I only recall one problem
and I believe the problem was user related.Correct tach properly installed(simple installation) a+ results.If you get a tiny tach it will work on any similar motor without regard to stator poles.

 

[STIguy]

Re: Need a new prop. Pitch question.

Well in case anyone was wondering... The boat does 43.2 MPH (GPS) at about 5000 RPM (TinyTach). With just me and half a load of fuel.

So thats pretty good... I guess with another person in the boat and some fishing gear it might drop out of the recommended range (4750-5250) I plan on raising the motor a hole or 2 How much RPM will that get me?

 

[Silvertip]

Re: Need a new prop. Pitch question.

You probably have an incorrect prop. My 17 foot Alumacraft Navigator has a 75 HP Merc which has an upper rpm limit of 5250 RPM. A stock 19P Merc aluminum prop allows that boat to run at 5200 RPM with two aboard, full live well and a rather heavy load of equipment. With a 21P on your boat you are very likely lugging the engine unless you typically run with a very light load. To answer your question "does a prop with less pitch always slow you down" the answer is no, and in fact it can speed you up. That holds true however only if the current prop has way too much pitch -- so much pitch that the engine can't reach anywhere near its wide open throttle rpm. Spending money on props when you have no idea what the engine rpm and accurate speed are is a waste in my view. Just because the rev limiter never kicked in tells you only that the engine never over reved. It does not tell you that the engine is being lugged (or under-reving.)