My volvo 3.0 is under-propped, need new prop advice

[nathandarby67]

I need help selecting a new prop. I have a 2006 Four Winns Horizon 180 with the 3.0L Volvo Penta. Has a VP SX-M outdrive with a 2.18 ratio. Current prop is 3-blade aluminum 14.3 x 21. Engine redline is listed at 4600 in the manual. I can not even get an exact full throttle speed as I am screaming right up past 5000 rpm's with the revs still slowly climbing. This is with the boat medium-loaded with 3-4 people plus a little gear (typical load for me). The closest I have come is 42 mph at 5200 rpm. I could probably get 200-300 more rpm's out of it if I let it run on out and dialed the trim in just right, but I would prefer to keep my con rods and pistons on the inside of the engine block.

About me: We never do any towing. No skiiers, tubes, etc. Just used for cruising, so hole-shot is not very important to me with the new prop. We do often take 30-40 mile trips up the river from here, so cruising speed and fuel economy are very important to me. I am thinking I could increase my pitch a fair bit and get more speed at a given rpm with resulting improvements in fuel economy, but I don't want to overshoot and overprop the boat. If we ever do want to pull anything, I could easily swap back to the current prop for the day. So basically I am looking for the best "cruising/efficiency" prop for my setup. Want to stay with aluminum also.

THANKS GUYS!

 

[Tail_Gunner]

Re: My volvo 3.0 is under-propped, need new prop advice

http://www.go-fast.com/Prop_Slip_Calculator.htm

As a reference your number's look sound at 11% slip which is very good for a I/O. Using the std 200rpm drop for every 1" of pitch a 23 would be in order...But that does sound quite large for a 3.0 in a 18 foot boat...Solas has some good alum props in that catagorey...Just stay away from a alum prop advertised with cupping at that high of a pitch..

Id call I-Boats and see if they would allow you to exchange if a 23 is to large..Again it is uncommon for a 18' boat using a 23 w/3.0 more like a 21 but if your tach is right that should be the number.

Or a 22p 4blade alum would enhance your cruising speeds and lower planing speed.

 

[nathandarby67]

Re: My volvo 3.0 is under-propped, need new prop advice

http://www.go-fast.com/Prop_Slip_Calculator.htm

As a reference your number's look sound at 11% slip which is very good for a I/O. Using the std 200rpm drop for every 1" of pitch a 23 would be in order...But that does sound quite large for a 3.0 in a 18 foot boat...Solas has some good alum props in that catagorey...Just stay away from a alum prop advertised with cupping at that high of a pitch..

Id call I-Boats and see if they would allow you to exchange if a 23 is to large..Again it is uncommon for a 18' boat using a 23 w/3.0 more like a 21 but if your tach is right that should be the number.

Or a 22p 4blade alum would enhance your cruising speeds and lower planing speed.

Thanks for the quick reply. From reading other threads I was thinking a 22 or 23 was not typical for a 3.0L. Maybe I was blessed with a "hot" engine, who knows. A few questions:

1) What is the concern with a cupped aluminum prop with a high pitch?

2) Easiest (i.e. cheapest) way to verify true rpm?

3) What differences would be seen between a 23p 3-blade and 22p 4-blade?

 

[Tail_Gunner]

Re: My volvo 3.0 is under-propped, need new prop advice

Thanks for the quick reply. From reading other threads I was thinking a 22 or 23 was not typical for a 3.0L. Maybe I was blessed with a "hot" engine, who knows. A few questions:

1) What is the concern with a cupped aluminum prop with a high pitch?

2) Easiest (i.e. cheapest) way to verify true rpm?

3) What differences would be seen between a 23p 3-blade and 22p 4-blade?

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1.Again that's a lot of pitch so adding cupping such a Turning point could really change things.

2. I know thats a tough one...Put a set of ears on the motor...run the engine in your driveway...get hold of seperate tach and check it.

3 Four blade create Lift...That is they both push and lift resulting in lower planing speed and better econ..The cost is generally 1-3 mph loss at WOT..Adding a 4th blade generally cost's you about 100 rpm.

 

[nlain]

Re: My volvo 3.0 is under-propped, need new prop advice

To give an example to your question #1, the issue is not the pitch it is the cupping at that high a pitch, example: my boat, 2004 SeaRay 185 Sport, 4.3L 190 hp, 1.81:1 ratio, came with a 14X23 prop that ran 5100 rpm and 49 mph gps, I bought a Truning Point 14 1/4X23 prop, it killed my wot rpm and speed, 4400 rpm and 41mph gps, due to the cupping, I had to drop to a 14 1/4X21 Turning Point prop to get back where I was.

 

[nathandarby67]

Re: My volvo 3.0 is under-propped, need new prop advice

-----------------------------------------------------------------------3 Four blade create Lift...That is they both push and lift resulting in lower planing speed and better econ..The cost is generally 1-3 mph loss at WOT..Adding a 4th blade generally cost's you about 100 rpm.

So far it sounds like 4-blade would be more suited to what I am using the boat for. I spend very little, if any, time at max speed, so having a couple mph shaved off my max would be ok it I got better economy in the mid range.

So, to further this dummy's (me) prop education:

1) So am I correct in saying that for a given mid-range engine rpm, say 3500, the four blade would run a little faster than the three blade, thus giving the better economy? Or is the better economy only seen at the new ultra-low planing speeds attainable with a four blade that would not be possible with a 3-blade?

2) In looking at some of the props available, some are listed at 14 1/4 and some at 14 1/5. Is there going to be any difference there?

 

[steelespike]

Re: My volvo 3.0 is under-propped, need new prop advice

Not meaning to butt in; the 4 blade will get it on plane at less power and speed.But depending on the setup the most efficient practical cruising speed could be signifigantly higher than planing speed. Careful testing or a flow meter are about the only way to tell what is best.Generally prop diameter is simply a function of the design and not taken into consideration
except in the extreme.

 

[TilliamWe]

Re: My volvo 3.0 is under-propped, need new prop advice

nathan, with a 2.18 ratio!!!! a 23 pitch prop seems very reasonable. Mercruiser liked to put 2.0 drive on those 17 and 18' bowriders with the 3.0 and they used 21 and 23 pitch props. Was that boat originally sold at a high altitude location?

Also, I know Tail Gunner ran the proslip calculator on it, but please confirm your tachometer's readings, before you go too far, okaay?

 

[Tail_Gunner]

Re: My volvo 3.0 is under-propped, need new prop advice

nathan, with a 2.18 ratio!!!! a 23 pitch prop seems very reasonable. Mercruiser liked to put 2.0 drive on those 17 and 18' bowriders with the 3.0 and they used 21 and 23 pitch props. Was that boat originally sold at a high altitude location?

Also, I know Tail Gunner ran the proslip calculator on it, but please confirm your tachometer's readings, before you go too far, okaay?

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