Advise on prop for 3.0L

[Cortes100]

I have an 08 Searay 175 with the 3.0L Merc, Alpha 1 drive. Currently running the stock alum 3-blade, 14-1/2 x 19. My WOT is 4800 and top speed around 42mph. With this prop, it actually works quite good. Pulls a couple tubes no problem, and quick on plane. The only thing I really notice is when we have a full load it slows down a bit, obviously the 135hp has something to do with it.
So using the Merc prop selector, here are a couple props that come up for options;
#48-815758A46......High-5...SS....13.25 x 19
#48-825900A48......VenSura, 4 blade .......SS......14 x 19
Same pitch, but SS and number of blades would change.
The merc website doesn't really explain what these props do. I have an idea, but looking for more info on what/how/if these will improve the overall performce. I know test drives are the best, but around here, nobody does that.
Any help is appreciated.

 

[Robbabob]

Re: Advise on prop for 3.0L

Your drop in performance is typical. If you don't mind giving up some top speed, the 4-blade is a good choice. The high-5 may cost more, but will also improve hole-shot and keep a decent top speed.

The Aluminum is flexing under the heavier load. SS will fix this. I assume you are familiar enough with your waters that you have no concern of hitting large rocks or stumps. SS is not forgiving like aluminum.

Good luck!

 

[jayhanig]

Re: Advice on prop for 3.0L

Re: Advice on prop for 3.0L

1) The Spelling Police issues a spanking citation for the misuse of "advice" vs "advise". You ask for advice. You advise others. "Advise" is something you DO; "advice" is something you GIVE or RECEIVE.

That being said, here's what I hope will help you with your prop selection questions (the carrot that goes with the stick):

Stainless vs aluminum: Stainless is recommended for heavier boats and also boats operated in saltwater. More expensive obviously but also sturdier. There is always the danger of damaging or losing a prop so you may want to stick with aluminum. But definitely the stainless is the more desirable (at least to me; I live at the beach).

3 blade vs 4 blade: the more blades, the smoother the operation. Most boats like yours are equipped with 3 blade props as the best balance between price and performance. You will note a fair number of 3 bladed prop offerings vs a very limited selection of 4 bladed props in your range. You have to figure there's a reason for that. But if you're determined to go with a 4 blader, you'll want to decrease the pitch by 1".

Diameter: You can vary bigger or smaller up to half an inch without any apparent effect. Of course if you go bigger, clearance may become an issue.

Pitch vs RPM: They have an inverse relationship in that increasing one factor decreases the other. Figure 1" = 200 RPM. So increasing the pitch from 19" to 20" will decrease the RPM by 200; decreasing the pitch from 19" to 18" will increase the RPM by 200.

The bottom line: I think I'd stick with a 3 bladed prop of the same diameter and pitch. You can find them in both stainless steel ($210-$414) or aluminum ($100-$147).

Hope this helps.

 

[Cortes100]

Re: Advice on prop for 3.0L

Re: Advice on prop for 3.0L

1) The Spelling Police issues a spanking citation for the misuse of "advice" vs "advise". You ask for advice. You advise others. "Advise" is something you DO; "advice" is something you GIVE or RECEIVE. QUOTE]

Ummm, spankings are sometimes ok, but all depends from who. Guys with beards aren't my style.

Anyways..........since my current prop is getting the proper rpm range, why would the 4 or 5 blade lower the WOT. The pitch is the same but in each case the diameter gets smaller.
I'm not too worried about top end, but on the other hand I don't want to give up 5 mph. Also under normal cruising (lets say 3200 rpm) with the 4-5 balde props, are my mph going to go up or down.
I assume on both 4-5 bladers, my holeshot should be a little better.

 

[jayhanig]

Re: Advice on prop for 3.0L

Re: Advice on prop for 3.0L

Anyways..........since my current prop is getting the proper rpm range, why would the 4 or 5 blade lower the WOT. The pitch is the same but in each case the diameter gets smaller.
I'm not too worried about top end, but on the other hand I don't want to give up 5 mph. Also under normal cruising (lets say 3200 rpm) with the 4-5 blade props, are my mph going to go up or down.

If you go from a 3 blade to a 4 blade prop, you're supposed to reduce your pitch by one inch. If you keep the same pitch, it's as if you actually increased the pitch, which then reduces your RPM by 200.

So a 3 bladed prop with a 19" pitch is the equivalent of a 4 bladed prop with an 18" pitch. If you mount a 4 bladed prop with a 19" pitch, it's functionally the equivalent of a 3 bladed prop with a 20" pitch.

 

[steelespike]

Re: Advice on prop for 3.0L

Re: Advice on prop for 3.0L

20 lashes with a limp control cable.
Your performance is just about spot on.
Can't be too specific but typically a 4 blade will improve hole shot,stay on plane with less throttle and handle better on plane and at the dock.Top speed and rpm will be reduced thus the need to drop an inch.Very difficult to pin point any of these changes.
If I had to guess speed drop would be 1 to 3 mph.But load speed may hold up better. The Solas Amita 4 blade has produced good rersults; a search may reveal some posts.
Generally stick with diameters within about 1/4 to 1/2 inch.
As far as the al to ss change; to me too darn expensive and can be unpredictable.
Some of those better acquainted wth ss may have some suggestions.If you boat where hard contact is likely ss repair is expensive.

 

[xxxflhrci]

Re: Advise on prop for 3.0L

I think your performance sounds about right on the money with your current prop. I wouldn't change a thing if it is working for you.....However, if you use a trash lake like I do, you may want to switch to SS....The first couple of summers I had my boat, I had 2 aluminum props. One to run and one to send out to get repaired after hitting floating crap, usually a stick. After doing this routine one too many times, I wised up and bought a SS prop. I've been using it 10+ years and it still looks like brand new.