Prop question

[ErieCan]

I have a Lund Rebel 1650 with a Mercury Command Thrust 60hp. The boat came with a 13.75 diam, 15 pitch 3 blade. With that prop I'd get max 33 mph at about 5700-5800 RPM (according to the analog gauge). 

 

I needed a spare and wanted to try a 4 blade (I fish Lake Erie and wanted a rough water prop). The dealer said that Merc 4 blade Spitfire props are designed in a way that you shouldn't have to go down in pitch. So he sold me a 13.4 diam, 15 pitch spitfire. 

 

I took it out today and although I really liked the hole shot, control, and the fact that it really lifts the stern up on plane, I was maxing out at about 5300 RPM and barely hitting 32 mph. 

 

The only other option in the Spitfire for my set up (according to the Merc prop selector site) is a 13.8 diam, 13 pitch. Increased diameter, but two pitch down. 

 

Do you think this prop will solve the RPM issue? Will it drop my top speed too much? 

[Way2slow]

Speedometer speed is normally useless, way too inaccurate to really tell much about the true speed.  

It's pretty common to loose a couple MPH when going from the three to a four blade prop because of the extra drag the fourth blade creates, especially in and aluminum prop because of how thick they have to make the blades.  Plus your current prop could have some damage that's causing extra slip.  If you had an accurate gps speed, you could go into one of the prop calculators and see what percent of slip you have with one versus the other.  The less the slip, normally the better the fuel economy.   

I'm thinking a 13" pitch is probably a bit too small.  That in a three blade is what's on my 24' pontoon that weighs 4200 pounds and only has a two  stroke 90.  For almost a 1,000 pounds of bodies, and other gear, I've just ordered a four blade 11" pitch to try.  However, going by your numbers, they support going one size down.  

If max speed is what you are looking for, you need to consider SS props.  However, no matter what prop you use, you are probably talking about 2 to 3 miles per hour difference, at about $150 per mph.    

[desmobob]

I think the frustrating thing about the Spitfire props is that when you pick a different pitch, it also has a different diameter.  That makes it a little trickier to guess which one you need.

 

I ended up with two Spitfires for my boat; one is ideal for when I'm solo and the other is better for when the boat is loaded (I have a little Bass Tracker Pro170/40HP Merc four-stroke).  Either will work fine in each situation though.

 

A prop right in the middle of the two I have would be just right but it's not an option in the Spitfire range.  Great props though... an interesting design and solid value.

[gim]

I recently went from a 4-blade with a 21 pitch down to a 3 blade with a 19 pitch based on advice of the prop shop on my Merc 75 hp 4-stroke.  The prop specialist was positive I'd get higher speed and better overall performance so I tried it.  It turned out to be completely wrong.  Not only did it take way too long to get on plane, my speed was slower too.  I kept this 3 blade prop as a back up but I immediately went back to my 4 blade prop in 21 pitch.

 

I have not tried the 4 blade mercury spit fire SS prop yet because the cost makes me cringe.  The thought of damaging a $500 prop on some gravel or a rock has extremely high risk and the water is down this spring.  Based on the advice in another thread, I think I will eventually give this prop a try but for now I'm a little burned out experimenting with different props.

[desmobob]

1 minute ago, gimruis said:

 

 

I have not tried the 4 blade mercury spit fire SS prop yet because the cost makes me cringe. 

 

Mine are both the inexpensive aluminum versions.  I don't know if a stainless steel prop would offer me any big advantages on my little 40HP outboard.  And the Spitfire design in SS isn't offered in a size to work on my engine.  The stainless steel Trophy Sport that is sized for my boat/engine looks very similar to the Spitfire design.

[WRB]

Try raising the engine 1/2”.

Tom

[ErieCan]

I found something interesting on another forum. A guy had a 60 Command Thrust like me and tried 4 blade props on a 16' aluminum console. He was getting too much lift in the stern. Complained he was blowing out with the four blade props. 

 

It was found that because its a command thrust, a motor meant to push more weight like pontoon boats, and the props therefore are larger diameter, that it's too much thrust for a smaller boats which are light in the stern (like mine). Most of my weight is in the mid to bow area. Me and a buddy at the console and two trolling motor batteries near the bow. 

 

The spitfire may not even be an option for my setup. A standard gear case would probably do fine with it.  

[WRB]

Move the batteries to the stern.

Weight distribution is critical for performance. 13 1/2” 4 blade prop is standard size. The blade is like a lower gear ration, less speed more power. Nearly the same speed at lower rpm indicates a bow heavy wet hull.  

Tom

[desmobob]

4 hours ago, ErieCan said:

 

It was found that because its a command thrust, a motor meant to push more weight like pontoon boats, and the props therefore are larger diameter, that it's too much thrust for a smaller boats which are light in the stern (like mine).

Mercury's Command Thrust engines generally have greater displacement and a lower gear ratio.  For example, my 40HP four-stroke is three cylinders totaling 747cc. with a 2.00:1 gear case.  The 40HP Command Thrust version is a four-cylinder 995cc. with a 2.33:1 gear case.  These are two very different engines needing different propellors.  

[E-rude dude]

You can get it in half pitch sizes now to really tune in performance.

 

https://www.boatpropellerwarehouse.com/category/8128/spitfire-x7

7 hours ago, ErieCan said:

I found something interesting on another forum. A guy had a 60 Command Thrust like me and tried 4 blade props on a 16' aluminum console. He was getting too much lift in the stern. Complained he was blowing out with the four blade props. 

 

It was found that because its a command thrust, a motor meant to push more weight like pontoon boats, and the props therefore are larger diameter, that it's too much thrust for a smaller boats which are light in the stern (like mine). Most of my weight is in the mid to bow area. Me and a buddy at the console and two trolling motor batteries near the bow. 

 

The spitfire may not even be an option for my setup. A standard gear case would probably do fine with it.  

They are putting 115 command thrust on bass boats turning 23-24 pitch props. You just have to get the right one. It takes trials. 

[HenryPF]

You have a 60hp motor - no way you have enough HP to benefit from SS as you don't have the power to flex aluminum. SS is usually for 150hp+. The lunatics out there scream and shout how good SS is, but who wants to spend $500 on a SS prop to get a whole 2mph more and then you gotta screw with lifting/lowering the motor? Throw another guy in the boat and everything goes outta whack.

 

Anyhoo - 15 to 13pitch from should drop you 5mph and raise your rpms by 400. Unless you need instant hole shot and less top end, you were better off with the 3 blade.

 

Example - my merc pro xs 115, 44mph top end, 3 blade 14 x 15 pitch aluminum at 5800 rpms. To 13.4 x 15 4 blade, 40mph top end, better hole shot, stainless steel. Giant waste of money

[Russ E]

8 hours ago, HenryPF said:

You have a 60hp motor - no way you have enough HP to benefit from SS as you don't have the power to flex aluminum. SS is usually for 150hp+. The lunatics out there scream and shout how good SS is, but who wants to spend $500 on a SS prop to get a whole 2mph more and then you gotta screw with lifting/lowering the motor? Throw another guy in the boat and everything goes outta whack.

 

Anyhoo - 15 to 13pitch from should drop you 5mph and raise your rpms by 400. Unless you need instant hole shot and less top end, you were better off with the 3 blade.

 

Example - my merc pro xs 115, 44mph top end, 3 blade 14 x 15 pitch aluminum at 5800 rpms. To 13.4 x 15 4 blade, 40mph top end, better hole shot, stainless steel. Giant waste of money

Curious what kind of boat you have that a 115 pro xs will only run 40 to 44 mph.

I have a 2017 115   pro xs with a command thrust lower unit. It is turning a 23 pitch tempest  prop at 6000 rpm and 49-50 mph. With a very quick hole shot on my Ranger z175 fiberglass boat.

It came with a 24 pitch prop. Hole shot was a little slow. Top end was the same but wot was 5600 rpm

[E-rude dude]

5 hours ago, HenryPF said:

You have a 60hp motor - no way you have enough HP to benefit from SS as you don't have the power to flex aluminum. SS is usually for 150hp+. The lunatics out there scream and shout how good SS is, but who wants to spend $500 on a SS prop to get a whole 2mph more and then you gotta screw with lifting/lowering the motor? Throw another guy in the boat and everything goes outta whack.

 

Anyhoo - 15 to 13pitch from should drop you 5mph and raise your rpms by 400. Unless you need instant hole shot and less top end, you were better off with the 3 blade.

 

Example - my merc pro xs 115, 44mph top end, 3 blade 14 x 15 pitch aluminum at 5800 rpms. To 13.4 x 15 4 blade, 40mph top end, better hole shot, stainless steel. Giant waste of money

Lift your motor one hole then report back ?