Prop Selection for a Merc 90HP 4-Stroke

[eudubmad82]

I'm having problems finding a prop that I like for a 90HP Merc 4 Stroke that is on a Lund Pro18 counsol. I'm currently running a 14X13 and it doesn't trim out very easily at lower speeds. Wide open my RPM's are 5200 and the max speed is about 36 mph measured on a GPS unit. Any help would be great as I am a new boat owner and I don't have too much direction as we speak.

Thanks,
Mike

 

[Irv964]

Re: Prop Selection for a Merc 90HP 4-Stroke

I'm having problems finding a prop that I like for a 90HP Merc 4 Stroke that is on a Lund Pro18 counsol. I'm currently running a 14X13 and it doesn't trim out very easily at lower speeds. Wide open my RPM's are 5200 and the max speed is about 36 mph measured on a GPS unit. Any help would be great as I am a new boat owner and I don't have too much direction as we speak.

Thanks,
Mike

First of all welcome to iboats, I think you'll find everything right here that your looking for.
Secondly, I have the identical motor but on a bit smaller boat (16.5 Lowe F&S) so this info should help. Be sure to read the links inside some of the posts as they deal with engine height which you will see is also a very important factor! Anyways here is the link and my test results included.....Irv964
http://forums.iboats.com/showthread.php?t=269778

 

[eudubmad82]

Re: Prop Selection for a Merc 90HP 4-Stroke

Thanks for the help. I'm pretty certain I am going to go with something in the 18" pitch range to get into the 6000 RPM range. Thanks again!

 

[Dhadley]

Re: Prop Selection for a Merc 90HP 4-Stroke

If you're at 5200 with a 13" pitch, an 18" pitch will drop the rpms approxamtely 800-1000 assuming you're staying with the same brand & style prop.

 

[eudubmad82]

Re: Prop Selection for a Merc 90HP 4-Stroke

If you're at 5200 with a 13" pitch, an 18" pitch will drop the rpms approxamtely 800-1000 assuming you're staying with the same brand & style prop.

You're correct, I went backwards. I guess I'm going with an 11" pitch... I may also consider a four blade prop since my boat is a little underpowered. Thanks!

 

[Dhadley]

Re: Prop Selection for a Merc 90HP 4-Stroke

I was going to ask if you've played with the X dimension (height) but you say it "doesn't trim out easily" which I assume you mean it starts to vent when trimmed. That means the motor is high enough or even slightly too high for the condition of the prop or the style of the prop.

 

[eudubmad82]

Re: Prop Selection for a Merc 90HP 4-Stroke

I was going to ask if you've played with the X dimension (height) but you say it "doesn't trim out easily" which I assume you mean it starts to vent when trimmed. That means the motor is high enough or even slightly too high for the condition of the prop or the style of the prop.

Correct. I on the trim gauge I can only go to about an eigth trim before this happens at full throttle. I am thinking of trying a hydrofoil as well as a skeg sleeve. When I bought the boat the skeg has a sizeable nick in it on the edge the parelles the prop. The holeshot is pretty bad on this boat as well so I'm hoping the hydrofoil will help. Any thoughts on these two applications?

 

[Dhadley]

Re: Prop Selection for a Merc 90HP 4-Stroke

The fins are a tool. They allow changes in the set up to gain efficiency. Not all fins are created equal just as all 15" props are not created equal. The Skeg Guard is a simple repair for a damamged skeg or a protector of a good one.

Can we assume the prop you have now is aluminum? Can we further assume it's nicked and worn?

 

[steelespike]

Re: Prop Selection for a Merc 90HP 4-Stroke

While your top speed seems pretty good it seems that a 13" would get a little more rpm. I don't have a clue what your gear ratio is but an 11" pitch seems extreme to me ,closer to what you would find on a pontoon setup. The slow hole shot seems suspect as well.You might try a 13" Hustler turning point they can deliver stainless performance at aluminum prices.Should allow more trim and better hole shot and top speed.You might check for full throttle as
well.

 

[eudubmad82]

Re: Prop Selection for a Merc 90HP 4-Stroke

The fins are a tool. They allow changes in the set up to gain efficiency. Not all fins are created equal just as all 15" props are not created equal. The Skeg Guard is a simple repair for a damamged skeg or a protector of a good one.

Can we assume the prop you have now is aluminum? Can we further assume it's nicked and worn?

It's in pretty good shape void of any nicks or bends. The finish is wearing off a little bit but it is structurally sound. It is, however, aluminum.

 

[eudubmad82]

Re: Prop Selection for a Merc 90HP 4-Stroke

While your top speed seems pretty good it seems that a 13" would get a little more rpm. I don't have a clue what your gear ratio is but an 11" pitch seems extreme to me ,closer to what you would find on a pontoon setup. The slow hole shot seems suspect as well.You might try a 13" Hustler turning point they can deliver stainless performance at aluminum prices.Should allow more trim and better hole shot and top speed.You might check for full throttle as
well.

So stick with the same dia. and pitch? Just change the manufacturer? It is a Blackmax now...

 

[Irv964]

Re: Prop Selection for a Merc 90HP 4-Stroke

So stick with the same dia. and pitch? Just change the manufacturer? It is a Blackmax now...

I don't think he means stick with the same pitch but I could be wrong? In my review above you will see what I thought about the Black Max prop.
Not telling you the Turning Point prop is "THE" prop to get, as you will also see in my review not all props will work the same on different boats.
Also don't forget you have an 18 footer which I am sure is rated for at least a 150 or 175 motor? Usually you don't want to go any less than 25% from the manufacturer's recommended max HP. Also verify your engine height, your cav plate should be at least level with the bottom of the boat or higher.
Double check your set up and then go from there. Good Luck........Irv964