Need to determine best prop for weird setup

[Connor0612]

1998 Four Winns 180 Horizon RX
1998 4.3GL Volvo Penta 1.51 gear ratio

Hello! I have a boat that’s had its outdrive unit swapped out with one I’m assuming use to be in a v8. As per Google my 4.3gl should have around a 1.80 gear ratio outdrive but I read the tag and it’s a 1.51.

At WOT I’m only getting 35mph at 3800rpm.
Current prop is a quicksilver nemesis 4 blade 14x21 aluminum.

I’ve also tried a 15x17 3 blade Volvo prop with not much difference.

What prop will work best to get more speed and proper rpm at WOT with this setup?

Thanks

 

[Scott Danforth]

Keep dropping pitch until you get the RPM up to 4800

You will most likely need to re-gear or find someone that wants to swap

 

[alldodge]

Initial thoughts
A 4 blade 21 = a 3 blade 19 , currently get 3800 RPM
Try 3 blade 17 = not much difference

The 17 should have at least seen 4000 if not 4200 RPM
Recheck RPM with 17 and let us know

If you still are not getting much difference the guessing the health of the motor is in question or the drive ratio is different then 1.5, as in maybe 1.3 or similar

 

[Connor0612]

Thanks!

 

[Connor0612]

Initial thoughts
A 4 blade 21 = a 3 blade 19 , currently get 3800 RPM
Try 3 blade 17 = not much difference

The 17 should have at least seen 4000 if not 4200 RPM
Recheck RPM with 17 and let us know

If you still are not getting much difference the guessing the health of the motor is in question or the drive ratio is different then 1.5, as in maybe 1.3 or similar

I looked back on a video and its sitting at 4000 / 4100 with the 3 blade 17 only hitting 33mph ish

 

[Scott06]

I looked back on a video and its sitting at 4000 / 4100 with the 3 blade 17 only hitting 33mph ish

You would need a 13 or maybe 15” prop to get the rpms right

When I repowered my boat with a larger engine, I kept the old higher numerically gear ratio drive and went up 2” in pitch and it works ( 1.81 drive vs 1.62).

As mentioned above double check tune up
Of engine. Would have expected 4” to get you closer.

 

[St1ngr4y]

Small boat like that with 4.3 V6 should easily make 45 mph. Seems to me that the drive ratio is not ideal with this combi. You can try to compensate with smaller pitch prop but you will not get 'the most' out of it. If possible try to swap to 1.8 drive and the try a +/-14" dia 23 pitch propeller.

 

[alldodge]

I messed up in my first comment
A 4 blade 21 = a 3 blade 23 , currently get 3800 RPM
Try 3 blade 17 = not much difference

I looked back on a video and its sitting at 4000 / 4100 with the 3 blade 17 only hitting 33mph ish

You getting more RPM but less speed
Might need the prop checked. If blades are bent some the prop might be ventilating

 

[Connor0612]

I messed up in my first comment
A 4 blade 21 = a 3 blade 23 , currently get 3800 RPM
Try 3 blade 17 = not much difference


You getting more RPM but less speed
Might need the prop checked. If blades are bent some the prop might be ventilating

How much effect does the diameter of the prop change the rpm? Think if i swap my 15x17 for a 14x17 or 13.25x17 it will bring my rpm up from 4000 to 4400?

or would i be better off trying a 15x15 next?

thanks

 

[Connor0612]

Small boat like that with 4.3 V6 should easily make 45 mph. Seems to me that the drive ratio is not ideal with this combi. You can try to compensate with smaller pitch prop but you will not get 'the most' out of it. If possible try to swap to 1.8 drive and the try a +/-14" dia 23 pitch propeller.

I know changing gear ratio back to around 1.8 or even 1.65 would be more ideal but its either hard to find or costs way more money than i want to spend. im just focusing on getting my rpms correct so i dont grenade my engine. Ill have to suck up the fact that my boat is suppose to do 46mph but only does 30ish for now

 

[alldodge]

How much effect does the diameter of the prop change the rpm? Think if i swap my 15x17 for a 14x17 or 13.25x17 it will bring my rpm up from 4000 to 4400?

or would i be better off trying a 15x15 next?

thanks

The diameter has an effect but pitch has the largest roll. Many props increase diameter when pitch is reduced

 

[Scott06]

I know changing gear ratio back to around 1.8 or even 1.65 would be more ideal but its either hard to find or costs way more money than i want to spend. im just focusing on getting my rpms correct so i dont grenade my engine. Ill have to suck up the fact that my boat is suppose to do 46mph but only does 30ish for now

Have you verified the tach against a shop tach and the Speedo is that gps or the dash speedo? The factory water pressure Speedos are notoriously inaccurate

 

[Lpgc]

Not trying to take the thread off topic, Mercury outboards always had smaller props than Johnson, obviously with different gearing to suit, but which is most efficient at low and high speeds (small prop spinning faster or big prop spinning slower)?

 

[Scott Danforth]

Blade style determines efficiency

However if you want efficiency in a boat, get a sail or a bunch of oars and talk your buddies into rowing

 

[Lpgc]

Blade style determines efficiency

However if you want efficiency in a boat, get a sail or a bunch of oars and talk your buddies into rowing

What I'm getting at is how should size/weight/design of boat and (say) engine rpm and torque / hp dictate drive gearing and prop size. Merc or OMC gearing / prop wouldn't be best suited to the Titanic lol, it obviously need much bigger props, but why couldn't a smaller OMC or Merc prop be turned a lot faster and still deliver the same push? That's an extreme example and I would expect answers about cavitation etc but the same principles must apply to us boaters. I might expect a big slow driven prop to be better in some conditions than a small fast driven prop and the opposite in other conditions, I suppose my question is what are those conditions?

 

[alldodge]

It's HP to weight thing
If it's a light boat then the amount of surface area of the blade will not matter as much. Just the reverse if the boat is heavy

The motor should be able to turn Max RPM at WOT. This accomplished by as you know prop, and gear ratio.

During manufacture the drive ratio is determined to have a prop close to the middle. Doing this allows the end user to vary pitch up or down determining after they load the boat

Now if props being used are in perfect condition then going up or down will change x-RPM for every inch of pitch change. If props are not withing original manufactures spec (perfect) then that throws another variable into the mix.

Changing from aluminum to stainless also adds a variable

 

[Lpgc]

It's HP to weight thing
If it's a light boat then the amount of surface area of the blade will not matter as much. Just the reverse if the boat is heavy

The motor should be able to turn Max RPM at WOT. This accomplished by as you know prop, and gear ratio.

During manufacture the drive ratio is determined to have a prop close to the middle. Doing this allows the end user to vary pitch up or down determining after they load the boat

Now if props being used are in perfect condition then going up or down will change x-RPM for every inch of pitch change. If props are not withing original manufactures spec (perfect) then that throws another variable into the mix.

Changing from aluminum to stainless also adds a variable

Thanks, I think I get most of that, but why do manufacturers choose (say) a 14.5" diameter prop instead of (say) a 10" or 18" diameter prop and change gearing/pitch for similar(?) results?

I've wondered about this since my dad's boating days when I was a kid and noticed my dad's Evinrude engine prop was much bigger than his friend's Merc engine prop even though they were around the same hp. I'm not biased OMC or Merc and that's not an argument I'd want to get into, I'd be equally happy OMC or Merc, Chevy or Ford as long as it works lol.

The main reason I'm asking now is because I replaced the prop on my boat, I don't have a point of reference for how the old prop would have performed on my boat because the prop and engine were both in bad condition when I bought the boat but I expect the old prop was close to OEM, the new prop is an aftermarket brand with different pitch/diameter but the seller (marina) I bought if from said the numbers won't make much difference https://forums.iboats.com/threads/1...-engine-and-drive.766501/page-14#post-5924760

 

[alldodge]

I've wondered about this since my dad's boating days when I was a kid and noticed my dad's Evinrude engine prop was much bigger than his friend's Merc engine prop even though they were around the same hp.

Three things can go on with the diameter differences
The prop shaft to Vent plate is taller on the Evinrude which allowed for a larger prop then the Merc. Would need the specs on the motors to determine

The gear ratio on the legs could be different

There could be a load out difference, someone wanted more fuel economy so they change pitch/size to reduce RPM (which actually hurts motor over long term), or something else which could be many things

 

[Lpgc]

Three things can go on with the diameter differences
The prop shaft to Vent plate is taller on the Evinrude which allowed for a larger prop then the Merc. Would need the specs on the motors to determine

The gear ratio on the legs could be different

There could be a load out difference, someone wanted more fuel economy so they change pitch/size to reduce RPM (which actually hurts motor over long term), or something else which could be many things

iIrc both motors (Evinrude and Merc) would be around 60/70 hp. But all OMC stuff used bigger props than Merc stuff. I know there's a difference in prop to cavitation plate and gearing but they were design choices by OMC/Merc respectively, my question is about what the trade-offs are between prop sizes for different conditions (assuming they are geared and pitched to push the same amount of water as each other).

Props aside, dad once bought a new Sims Super Vee boat and new Evinrude 60 we fitted on it, it went well with that engine on it. He sold that boat after a year or so and bought a little 15/16ft cabin cruiser and a seond hand Johnson 70hp. The 70 was a 3 cylinder and I liked the way it ran a lot better than the later 60hp engine. Heh when I was around 12 years old I towed my dad (skiing) 12 miles across a bay when he was waiting to go into hospital for a hip replacement, he said it made his hip ache but it was worth it. His friend was in Guinness book of records for skiing across the English channel with only one leg.

 

[Connor0612]

Have you verified the tach against a shop tach and the Speedo is that gps or the dash speedo? The factory water pressure Speedos are notoriously inaccurate

I haven’t verified the tach with a shop tach. The speed I was confirming with gps so I know it’s accurate. I might have a line on another upper Volvo gear housing with a better gear ratio so I’ve temporarily stopped thinking about props until I hear back