Mercruiser 3.0 - another 4 blade prop question

[FFDave]

Hey everyone,

Ive been searching and getting ideas about props.
I have an 18ft open bow glasstream with the 3.0 mercruiser.
The prop on it is a 17p and seems to be great. No complaints.
Nice hole shot and good top end.
I believe im right around 42 mph with me and a buddy and 46-4800rpms
Im not concerned about the top end speed at all, or the rpms, because im in the safe rpm range for the motor, and top end speed doesnt really do anything for me.
Wakeboarding and tubing is what we will be doing plenty of this summer. The 17p does fine for both, BUT
Ive heard that a 4blade will increase the hole shot, steering torque, and smoothness of the ride. My thought process on this is - if i want to have more people in the boat this summer while wakeboarding or tubing, the 4blade will help with the added weight...right?
And if so, is staying with the 17p but moving to a 4 blade be ok? or do i need to switch my pitch with a 4 blade.


Thanks!

Dave

 

[hwsiii]

Re: Mercruiser 3.0 - another 4 blade prop question

Dave, You might try reading this for a better understanding of motor height, changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

Buy a Solas Amita in the same pitch here at Iboats.



H

 

[FFDave]

Re: Mercruiser 3.0 - another 4 blade prop question

H,
Thanks for the article, it was well written.
I guess the only questions i have that i didnt see in the article, was about steering torque and overall ride quality improvments moving from a 3 blade to a 4 blade.
Unless i read over it.

Or is that a bunch of crap.

Dave

 

[fossill]

Re: Mercruiser 3.0 - another 4 blade prop question

I changed to an aluminum 4 blade made by Merc. First thing I noticed was it got on plane a lot quicker and made the boat run way smoother vibration wise. I went from a 23 to a 20 pitch to stay in the rpm range, but I was running at the bottom of the rpm range to start. Top speed stayed the same, but probably because I wasn't at the upper rpm limit with the 3 blade 23 pitch. My guess is that you would need to go to a 16 inch pitch with the same prop type/ manufacturer as your current prop to keep the same performance. My boat is an 18ft bowrider with a 4.3L

 

[FFDave]

Re: Mercruiser 3.0 - another 4 blade prop question

I dont know if i want to go down in pitch any more than 17.
I dont go WOT a whole lot.
But, when switching to a 4 blade, you loose rpms right. SO if i have a 17 and am at 4600-4800rpms, switching to a 4 blade will bring me down a little bit, but i'll still be in the safe range.
My question about this is..
If i have 5 people on the boat, or 4 people and towing one person..will the 4 blade at the same pitch im running now, lagg the engine? since im getting that 46-4800 rpms with only a couple people on the boat with my current 3 blade 17p?
Sorry if im sounding stupid, this is my first year boating.

Dave

 

[fossill]

Re: Mercruiser 3.0 - another 4 blade prop question

Yup.
You want to be as close to the upper rpm limit as possible. Lugging the engine puts more stress on it then revving freely. Plus if you are already lugging somewhat goiing straight, you'' really drag the engine rpm down pulling through a turn.

 

[hwsiii]

Re: Mercruiser 3.0 - another 4 blade prop question

You will only lose 200 RPM, and the boat will plane at lower speeds, and it will carry the extra people much better than the prop you have now.

If you will get me your exact RPM and speed I can fine tune the pitch. My numbers are not any better than the information I get from you.




H

 

[tawood]

Re: Mercruiser 3.0 - another 4 blade prop question

I dont know if i want to go down in pitch any more than 17.....

Why not? If that is the prop you need, then you should.

 

[campion505i]

Re: Mercruiser 3.0 - another 4 blade prop question

I've recently dealt with the same issue in reverse (looking for more top end, sacrificing hole shot)....everything I've read indicates that 4 blades are not recommended on 4 cylinder engines...too much prop/ not enough power.

 

[tawood]

Re: Mercruiser 3.0 - another 4 blade prop question

For "all around" performance they are not the best, but to suite a specific need....

My boat has a 3.0L 4 banger stock with a 3 blade 21P prop....last year I was planing an outing with several of my friends, and I was worried my 4 cylinder wouldn't be up to the task of pulling a skiier with 5 adults still in the boat. I was also worried because I knew this same weekend my wife would be going out with her friends in the boat, and just the thought of trying to explain "over rev" to her, (because I geared the boat down for pulling), made me cringe....she is not a mechanical person by any means.
The 4 blade, 19P prop was the answer...gave the boat a fantastic hole shot, but didn't over rev at WOT. Downside was it dropped top speed a bit.

Funny thing was it turned out I didn't need it: my boat pulled skiiers just fine with 5 in the boat on the original prop. I keep the 4 blade in the boat as a spare, and also use it in early spring for trolling (slower idle speed too).

 

[FFDave]

Re: Mercruiser 3.0 - another 4 blade prop question

Thanks for the help on this old post.
I actually screwed up my prop today at the lake on a freak accident involving me and and some rocks at the launch - who put those there? and since when do rocks chew props up?
Im ready to order a new prop asap, i am now worried about ordering the 4 blade i origanally planned on. Ive been happy with the 3 blade 17p prop and i even put on the 19p i had and was happy with that too.
I'd be pleased with the 4blade if it made for a smoother ride and got on plane faster..which is the entire idea of the 4 blade. But i dont want to over prop, like i read from a response above.

hmmmm..
Dave