think boat is over propped, need advice on which direction to go

[turbocorrado]

1997 wellcraft eclipse 2000s 4.3 v6
boat is currently topping out at 4000rpm WOT 36-37mph - current prop is 14.5 x 21

according to wellcraft the boat is supposed to go 48mph at 4800rpm with a 13.75 x21 prop
im not sure what gear ratio the outdrive is, but im assuming 1.81

according to mercurys prop selector page a 18.26 pitch prop will bring the boat to 4600rpm and 44mph

should i drop to 13.75 or stay at 14.5

 

[jimmbo]

Your gear ratio should be on the upper gear case on the MerCruiser label. Until you have that info the prop calculator is of little use. Wellcrafts numbers are based on an empty new boat. Add a few hundred pounds of gear and a few people and it will be slower. If you do have the correct gear ratio, I don't know if 3/4 dia will raise it 800 rpm. 18.26, what does that mean?

With 1.81:1 ratio
21 in pitch @ 4000 rpm and 10% slip calculates to 40 mph
21 in pitch @ 4800 rpm and 10% slip calculates to 48 mph

Maybe your engine needs a tune up

 

[turbocorrado]

Your gear ratio should be on the upper gear case on the MerCruiser label. Until you have that info the prop calculator is of little use. Wellcrafts numbers are based on an empty new boat. Add a few hundred pounds of gear and a few people and it will be slower. If you do have the correct gear ratio, I don't know if 3/4 dia will raise it 800 rpm. 18.26, what does that mean?

With 1.81:1 ratio
21 in pitch @ 4000 rpm and 10% slip calculates to 40 mph
21 in pitch @ 4800 rpm and 10% slip calculates to 48 mph

Maybe your engine needs a tune up

mercruiser label is gone, boat has been sunk twice, so im sure it has some added weight to it which i think is alot of my issue. motor is new. just trying to get to about 4400 rpm or so

also the 18.26 is the pitch that the mercury prop website came up with, apparently it doesnt just stick to 18,19, etc. so i was looking at a 19 pitch

 

[jimmbo]

You want to prop it so it runs in the upper half of the rpm range. I am assuming it is a carbed model and based on what you said the Wellcraft test said, I would guess the WOT range 4400 - 4800. Generally speaking, you gain/lose 200 rpm for every inch of pitch change. So to get to 4800 you will need to drop down to 17 in. However if there is a diameter change too, an 18 or 19 might be correct. You are talking about 3 blade props?

 

[Scott Danforth]

Is this a new issue or has this always been the case?

on the 4.3's with ballance shafts, they run smooth on 5 cylinders if you have a dead plug. that will bring you down on power.

Had a buddy loose a plug, and drop an exhaust shutter, boat ran great, however down on power. marina mechanic told him the motor was over propped. after a few days of testing props, I told him something was causing his lack of power/RPM and asked him to run a power ballance test. Spark plug #3 wasnt firing.

changed plugs and motor gained most of his power back. found the exhaust shutter when we did the bellows a month later.

 

[turbocorrado]

Is this a new issue or has this always been the case?

on the 4.3's with ballance shafts, they run smooth on 5 cylinders if you have a dead plug. that will bring you down on power.

Had a buddy loose a plug, and drop an exhaust shutter, boat ran great, however down on power. marina mechanic told him the motor was over propped. after a few days of testing props, I told him something was causing his lack of power/RPM and asked him to run a power ballance test. Spark plug #3 wasnt firing.

changed plugs and motor gained most of his power back. found the exhaust shutter when we did the bellows a month later.

first season it would run 4000rpm but the rear main seal was dumping, so i rebuilt it 2 winters ago. last season it would only run to 3600rpm after i rebuilt the motor, found cyl 3 plug gapped wrong over the winter and that got me to 4000 rpm this season. the most ive ever gotten out of the boat was 4000rpm, even after a rebuild. boat was sunk twice so im sure there is some added weight but im 800 rpm and 10mph off of factory specs

 

[steelespike]

It appears you would need to drop 4" to a 17 to make up 800 rpm.
My limited info indicates a 1.81 is a 190 hp. A 17" at 4800 would be about 37 mph but with full rpm.
Don 't assume a fresh rebuild is setup right. check the timing is set right and advancing as it should.
Throttle set up right for wot.
Does it respond to up trim?
Is the bottom clean and smooth?
If it sank twice the foam under the floor may' have absorbed water,
adding 200 or 300 lbs right where you don't need it.

 

[turbocorrado]

It appears you would need to drop 4" to a 17 to make up 800 rpm.
My limited info indicates a 1.81 is a 190 hp. A 17" at 4800 would be about 37 mph but with full rpm.
Don 't assume a fresh rebuild is setup right. check the timing is set right and advancing as it should.
Throttle set up right for wot.
Does it respond to up trim?
Is the bottom clean and smooth?
If it sank twice the foam under the floor may' have absorbed water,
adding 200 or 300 lbs right where you don't need it.

well i found out the prop on it is a 14.5 x 19 when i went to the boat today which changes somethings, i was 100% sure i put a 21 pitch on there. im gonna have to do more tinkering.
i need to triple check timing.
throttle is opening all the way
it does respond to trim
bottom has alot of old bottom paint on it

 

[jimmbo]

A 19 @ 4000 rpm with 10% slip calculates out to about 36 mph

 

[turbocorrado]

A 19 @ 4000 rpm with 10% slip calculates out to about 36 mph

yeah it appears its right where it should be, just gotta figure out why its not revving out all the way,

 

[Watermann]

Are you sure your tach is correct? Has it been replaced since the multiple sinking events?

 

[turbocorrado]

Are you sure your tach is correct? Has it been replaced since the multiple sinking events?

yeah i replaced it when i bought the boat 2 seasons ago