Help for prop replacement

[billy67]

Hi all bought a 24' boat witha mercury verado 250hp L6 , boat was previously owned by another guy who didn't really care about performance numbers and after giving it a good run it seems that figures are way off, the prop used for this boat is a mirage plus 15.5x17 and w.o.t only creeps up to 5400 @ 32 knots gps speed, was thinking of buying an Enertia 15.3 x 14 prop to see if it can at least get it up to recommended specs of 5800-6400 rpm , any suggestions will be greatly appreciated, spark plugs and ignition coils have been checked and are ok.
Thanks..Bill.

 

[ChooChooSnakeMan]

What kind of hull is this, the hull design has a big impact on what type of prop works best. A high performance 24' hull that gets up on a pad would need a way different prop than a 24' pontoon boat for example. Let us know what boat this is and I'm sure you would get more help. As a general rule all other variables being kept the same (weight, engine height on the transom, etc.) you should see about a 200 rpm increase for every 1" drop in pitch with the same brand / model prop. Generally speaking at least on higher performance hulls you want to be close to the WOT upper limit when running light so I think your target should be 6200 rpm or so. That way you won't go over and bump the rev limiter on a cooler day. Your hull must be pretty heavy for a 250 Verado to only pull 5400 with a 17Pitch prop. You might not what to read this but are you sure your engine is healthy and putting out full power? At 200 rpm per inch you would need to drop 4" in pitch or to a 13 pitch which seems ridiculous unless your hull is very heavy, doesn't run on a pad, or is a pontoon. I hope this at least gives you something to start with. Post what hull you have and I'm sure you will get some good input, hopefully from someone running your exact boat / motor combo.

 

[billy67]

What kind of hull is this, the hull design has a big impact on what type of prop works best. A high performance 24' hull that gets up on a pad would need a way different prop than a 24' pontoon boat for example. Let us know what boat this is and I'm sure you would get more help. As a general rule all other variables being kept the same (weight, engine height on the transom, etc.) you should see about a 200 rpm increase for every 1" drop in pitch with the same brand / model prop. Generally speaking at least on higher performance hulls you want to be close to the WOT upper limit when running light so I think your target should be 6200 rpm or so. That way you won't go over and bump the rev limiter on a cooler day. Your hull must be pretty heavy for a 250 Verado to only pull 5400 with a 17Pitch prop. You might not what to read this but are you sure your engine is healthy and putting out full power? At 200 rpm per inch you would need to drop 4" in pitch or to a 13 pitch which seems ridiculous unless your hull is very heavy, doesn't run on a pad, or is a pontoon. I hope this at least gives you something to start with. Post what hull you have and I'm sure you will get some good input, hopefully from someone running your exact boat / motor combo.

 

[billy67]

Hi it's a local build im way out here in the Mediterranean and it's quite on the heavy side originally it was built with inboard but got converted with an outboard extension.

 

[dingbat]

Hi it's a local build im way out here in the Mediterranean and it's quite on the heavy side originally it was built with inboard but got converted with an outboard extension.

Your 32 kt. speed is excellent for a boat that size with only 250hp but something doesn’t sound right with your prop situation.

How much does the boat weigh?
What much horse power is the hull rated for?

 

[Scott06]

What is the gear ratio on the engine. Assuming it is in perfect working order seems like the engine may have too tall a gear ratio for the application

 

[dingbat]

Running a prop calculator on your reported performance results in 23% slip.
Should be running 1/2 that figure, but would drop your WOT RPM even lower.

Possible scenarios:
Tachometer is wrong - has it been checked?
Motor not setup properly - A common problem with conversions
Damaged or under powered motor - inline 6 motor

Earlier questions still open.......
5400 rpm
17 pitch
1.85 ratio
36 MPH

 

[jimmbo]

So it originally had an Inboard, and was converted to an Outboard.
Concerns:
1)As mentioned above, Engine Height, also factoring any Setback from the Transom.
2) Tachometer Accuracy.
3) Engine Condition. Have you done a Compression Test? If yes, what were the Numbers?
4) Are you operating the Power Trim properly? Trimming in too far will drop the WOT Rpm by several hundred rpm, even over a thousand.
5) Switching from a Mirage to an Enertia is not quite as easy as saying 200 rpm change per inch. Within the same Prop Lineage, that Rule will hold true, but a 17" Enertia may well have a different Rpm and/or Speed at WOT, than the Mirage. I know the Boat itself will feel and handle differently as well.

 

[Scott Danforth]

Being an Verado, is the CAC leaking and taking out the supercharger?

in addition to the questions above, have you had the boat weighed?