New guy, need prop pitch advice

[Bobby S]

Just got a 1996 175 w/ 115 2 stroke.

Engine runs out real nice. But 2 blades are bent and one slightly cracked.

I want to go to a 4 blade prop. I'm looking for fuel efficiency and easy cruising and good control idling around. Hole shot performance isn't a concern.

I understand the 4 blade props plane easier and have those characteristics.

After beating up the internet and searching the threads in here, I need some advice.

I'm not sure of the proper size and pitch for my needs. Any help is greatly appreciated.

 

[pakama]

Try reading this: https://www.mercurymarine.com/en/us/propellers/selector/#/step-one

 

[Bobby S]

Try reading this: https://www.mercurymarine.com/en/us/propellers/selector/#/step-one

Yeah, I did that before my post.

The engine year only goes back to 2000.

I have a 97'. And even if I choose 2000, there are several sizes and pitches for the 4 blade options in SS and aluminum.

Any other advice? Thanks in advance.

 

[Bobby S]

Okay. I've researched it down to the solas titan 4. (SS 4 blade prop)

Only remaining question: What pitch?

Options are 13, 15, 17, 19, 21

I'm thinking 13 and 15 will be weak, or use higher rpms, getting up on plane.


Can anyone recommend 17, 19 or 21?

I want easy planing and best fuel economy. Top end speed isn't a concern.

 

[pakama]

If you are happy with the performance of the 3 blade you have now contact a prop vendor and they will tell you what size 4 blade you need...if you want to change the current performance they will probably tell you to decrease the pitch a little more. The pitch for overall performance and efficiency should match the WOT specs of you engine so that it is not over (or under) loaded. Every unit of pitch roughly equals a 200 rpm difference from the recommended pitch for you engine. IIRC

 

[blaster]

What size propeller do you have now? What RPM's are you seeing when you run full throttle? I don't believe a four blade will save you any fuel.

 

[beeker]

C7heck out prop thread dated 7/30/16 should help,
But you need to know max r.p.m.

 

[randmhall]

Do you have a good local prop vendor in your area? If so, go see them. We have a local shop that only sells, services props. They were most helpful a few years ago on our old boat. My old Wellcraft Cuddy was heavy and underpowered, and struggled to pull me up on a ski.
They recommended a couple of props, and then let me try them. They ended up swapping props for me about 5 times before I found what i liked best. The 4 blade I settled on helped the hole shot tremendously over the stock 3 blade aluminum.

Yes, I could have saved 50 bucks buying online - but their advice, time, and willingness to let me exchange until I was happy, is priceless.

 

[Leardriver]

Four blade props have more blade surface in contact with the water.
Less slippage, more drag. Think of it as four wheel drive for your boat.

If hole shot/ski starts aren't important, stick with 3 blade.

 

[Bobby S]

Thanks for the responses. I ended up going with a 4 blade 17p. I'll be installing later today and in the water tomorrow. Hopefully, I'll be with in RPM limits @ WOT with the 17p.

 

[blaster]

Thanks for the responses. I ended up going with a 4 blade 17p. I'll be installing later today and in the water tomorrow. Hopefully, I'll be with in RPM limits @ WOT with the 17p.

It would be very helpful to other boaters with the same power to post more specific information. For example, I have a Power tech ELE4 15"X17" four blade propeller which runs 48 mph at 4950 RPM's. It sometimes replaces a Vengeance 14"x19" which runs 50.5 mph at 4950 RPM's.

By posting complete information it makes the thread much more helpful to other people.

 

[Bobby S]

Got out on the water today and it looks like I picked the correct pitch. The limit for this Mercury 115 @ WOT is 4800-5200 rpms.

I peaked @ 5100 rpms going 36 mph (phone GPS and speedometer are within 1 mph) with three souls onboard, a full cooler and a portapotty(don't ask).


Low speed maneuverability was outstanding and little to no cavitation experienced when accelerating from deadstop to WOT.

Now..... this engine has a very noticeable surge in rpm's when cylinders 3 and 4 kick in. Approximately a 1000 rpm jump around 1800-2000 rpms. Did it on old bent up prop as well. It will surprise anyone not sitting down against a backrest.

So the proper rpm range is correct for cylinders 3 and 4 to fire up, but its a pretty violent transition.

I understand this surge is somewhat common for these 2+2 engines, but a 400 rpm surge seems to be the target, from what I've read.


Any advice on reducing some of this surge, or is this just something to live with?

Carbs are synced, but I don't know about linkage adjustments yet.