Proppy, Prop Prop Prop! SO CONFUSING!

[comstox]

My 1984 -75HP Merc had a prop marked: 12 3/4 R 21 MWC (Mercury Water Carft?) well i broke that one when I hit a rock last year, and I replaced it with a prop that FIT, (?) but wasn't the right prop. It was too big and I could never achieve full RPM for my boat. So, I can either buy an original 2 3/4 R 21, but I'd like to try a prop a local vendor has which is a Vortex 13 1/4 x 17. - WHAT should I expect?????
Basically, going from a 21 pitch to a 17 pitch. Keep in mind that my 21 pitch 12 3/4 prop @ WOT, DID bring me to peak MFG RPM.
Thank ye, oh wise prop masters!

 

[Faztbullet]

21 pitch is pretty steep for a 75Hp unless its on a super light hull..The 17P sounds a whole lot closer to being right.

 

[jimmbo]

MWC-Married with Children, but in this case, Michigan Wheel Company/corp.. The new Prop is cupped and the MWC may not have been. If the new prop does have a better grip on the water, it could account for the RPM drop

Your motor is the 66.6 cubic inch 4 cylinder? WOT RPM range is 5000 - 5500. Gear Ratio 2.3:1

What RPM were you getting, with the old prop and with the new one?

 

[comstox]

21 pitch is pretty steep for a 75Hp unless its on a super light hull.

It is a pretty light boat. 1968 Sidewinder LowPro. WIth original 12 3/4 R 21 MWC prop, I could go 45 MPH! (Confirmed by GPS on Mavionics App,) but the bigger, wrong prop I can only get about 38 - 39 MPH. I would give you the specs on THAT prop, but it has NO number on it whatsoever.
And I can't remember what MAX WOT RPM I was getting with the original or the wrong sided prop.

 

[comstox]

21 pitch is pretty steep for a 75Hp unless its on a super light hull.

It is a pretty light boat. 1968 Sidewinder LowPro. WIth original 12 3/4 R 21 MWC prop, I could go 45 MPH! (Confirmed by GPS on Navionics App,) but the bigger, wrong prop I can only get about 38 - 39 MPH. I would give you the specs on THAT prop, but it has NO number on it whatsoever.
And I can't remember what MAX WOT RPM I was getting with the original or the wrong sided prop.

 

[ahicks]

I say try it! The 17" might be fun as a ski prop, but you'll need to be careful to see what max revs are with it so you don't over rev the engine.

 

[comstox]

I say try it! The 17" might be fun as a ski prop, but you'll need to be careful to see what max revs are with it so you don't over rev the engine.

Do you think I'll loose any top speed, or get less MPG while cruising just above plane?

 

[ahicks]

Props are all about compromise. If you prop for max speed or minimum rpm at cruise, you're going to have a dog when accelerating from a stop. If you prop for max performance pulling toys/skiers, you are for sure going to loose potential top end and will be turning higher rpms at cruise.

You can also try for a compromise prop, that isn't the best at wide open, or at pulling, but does a reasonable job at both. Your boat, suit yourself!

 

[WesNewell]

Going down 4" in pitch will bring your rpm up ~800 rpm. if you were getting max rec. rpm with the 21P, you can assume you will over rev your engine by 800 rpm at wot. Which is not good.

 

[Sea Rider]

Going down 4" in pitch will bring your rpm up ~800 rpm..

I'll bet you that going 4 pitches down won't dial + 800 wot rpm, will dial way, way, way more. The stupid rule of thumb casted on stone that states +,- 200 wot revs with one +,- pitch can't be applied for all combos. That math only works well on heavy combos as this one and OP's has a light boat with a large motor.



Try a 4 less pitch prop from your current installed one on your motor and tell us it it was fun blowing the motor out..

Happy Boating

 

[comstox]

Okay, I have been having a trigger issue and haven't been able to test my current MAX RPMs yet, but I found my outboard's manual and it say that my max RPM is 5500. My new trigger is on it's way and it looks like it's going to be a GREAT boating weekend! I'll fill you guys in with my findings on Monday!
BTW, thanks for the great feedback and important info you guys share!!!

 

[comstox]

Update! - Well... I was having some no spark issues I just took care of and was SO EXCITED to get back out and get new MAX RPM data to share with you guys but.... when I got out on the water yesterday.... My tach wasn't working!!!! Oh my boat! I WILL fix that tach and get back to you prop-gurus.

 

[comstox]

Alrighty boys and girls! (This has been quite a ride,) one issue after another... But I have replaced stator and trigger, replaced rectifier to restore my tach's function, and I'm READY TO GO with new prop testing!!! I'll get back to you guys in the next few days with my findings.

 

[QBhoy]

Keen to hear how you get on. A 21” seems mega for a 75 merc on any boat. I had a really fast fletcher hull at 16ft with a merc 90 (same gear case I think) and with a 19” stainless. It was just into the peak rpm range and at 46/47 mph gps at a push. That’s about as extreme as I’m aware of for such a set up. I tried a 21” on that boat and it ruined it.

 

[comstox]

Keen to hear how you get on. A 21” seems mega for a 75 merc on any boat. I had a really fast fletcher hull at 16ft with a merc 90 (same gear case I think) and with a 19” stainless. It was just into the peak rpm range and at 46/47 mph gps at a push. That’s about as extreme as I’m aware of for such a set up. I tried a 21” on that boat and it ruined it.

QBhoy, When you say it "ruined it." Do you mean it ruined the sweet setup of "just into peak rpm range," or ruined, as in you broke something on the outboard?

I do plan on getting out this weekend for some test runs. I'll let you all know how it goes!

 

[jimmbo]

Keen to hear how you get on. A 21” seems mega for a 75 merc on any boat. I had a really fast fletcher hull at 16ft with a merc 90 (same gear case I think) and with a 19” stainless. It was just into the peak rpm range and at 46/47 mph gps at a push. That’s about as extreme as I’m aware of for such a set up. I tried a 21” on that boat and it ruined it.

An Apples to Oranges comparison

66.6 cubic inches for the 75hp,
99.8 cubic inches for the 90hp
Rpm range for the 75, 5000 - 5500, peak HP at about 5400 rpm
Rpm range for the 90, 4500 - 5000, peak HP at about 4750 rpm
Gear ratio for the 75, 2.3:1
Gear ratio for the 90, 2:1

 

[comstox]

An Apples to Oranges comparison

66.6 cubic inches for the 75hp,
99.8 cubic inches for the 90hp
Rpm range for the 75, 5000 - 5500, peak HP at about 5400 rpm
Rpm range for the 90, 4500 - 5000, peak HP at about 4750 rpm
Gear ratio for the 75, 2.3:1
Gear ratio for the 90, 2:1

Good to know Jimmbo!

 

[Sea Rider]

This thread is full of misleading and confusing tech speculation, each boat/motor combo is unique and should be propped right to run at least middle to max wot rpm range as loaded, if with a near fixed load much easier to dial any of both conditions. Will depend entirely which condition suits best the OP's needs. Let him post wot runs end results with both props...

Happy Boating

 

[QBhoy]

QBhoy, When you say it "ruined it." Do you mean it ruined the sweet setup of "just into peak rpm range," or ruined, as in you broke something on the outboard?

I do plan on getting out this weekend for some test runs. I'll let you all know how it goes!

Hi there. I meant it ruined the all round performance of the boat. A few people sitting in the back or trying get on plane was sometimes a struggle

 

[QBhoy]

An Apples to Oranges comparison

66.6 cubic inches for the 75hp,
99.8 cubic inches for the 90hp
Rpm range for the 75, 5000 - 5500, peak HP at about 5400 rpm
Rpm range for the 90, 4500 - 5000, peak HP at about 4750 rpm
Gear ratio for the 75, 2.3:1
Gear ratio for the 90, 2:1

Jimbo

mmm. Not so sure either of us are spot on here.
Here is my thinking. But having not looked it up, please excuse me if I’m wrong.

The 90hp I had was from around 1990. It was the 3 cylinder model and I’m sure the same base engine as the 75hp. Pretty sure it’s the same gear ratio too. It certainly wasn’t having the same specs as you say above in terms of rpm range at least.

Where I may be wrong is that the posters engine is from 1984. Maybe that’s a different case altogether? The 90 of thre same age was probably a straight 6. Maybe that’s where you are thinking and perhaps rightly so. But I was comparing my 1990 ish era 3 cylinder.