twin prop help/puzzle

[cav scout]

the problem i have is WOT(should be 4500). running now around 3000 to 3100 with 15x15 twin counter rotating aluminum props maxing out around 28mph on the gps. my boat is a 1991 regal 290 commodore with twin 5.0L mercs alpha ones that seem to run fine. i have been looking at smaller pitch props but it doesnt make sense and i am wondering if there is something else i should look at. comments / ideas ?

 

[QC]

Re: twin prop help/puzzle

Yeah, doesn't make sense at first glance. So when you say twin props you mean the both "individual" props on the two Alphas?

Assuming that . . . do you have any history on the boat? Have you ever seen either of them turn into the 4000+ range. I always recommend verifying the tach(s) first. Also, what condition is the bottom? What gear ratio? We should do a reality check with the 15" props, 28 MPH and the RPM. Even 28 MPH takes some serious horsepower, so I am not convinced the tachs are right at this point.

BTW, welcome aboard!!

 

[QC]

Re: twin prop help/puzzle

1991 5.0s had either a 1.5 or 1.65 ratio. 230 and 200 respectively. Either way, unless you are getting some awesome slip numbers (you aren't) the tachs are off. In the case of the 230 with a 1.5 ratio, you'd have to have a 1.4% slip which just isn't happening, and the 1.65 ratio puts slip into negative territory (impossible) . . . Tach first, bottom condition, then another test while finding the fastest speed (GPS) you can go and good accurate RPM.

 

[cav scout]

Re: twin prop help/puzzle

first thanks alot !!
i have had the boat for about a year..and she has always performed the same.
the bottom is smooth with bottom paint.
the highest rpm underway around 3100.
dont know the gear ratio. is there a way to look it up via serial number ?
what would be the best way to test the tach ? some thing from the autoparts store ?
do you think changing the dia. of the props from 15 to 16 make any difference ?

 

[QC]

Re: twin prop help/puzzle

dont know the gear ratio. is there a way to look it up via serial number ?
what would be the best way to test the tach ? some thing from the autoparts store ?
do you think changing the dia. of the props from 15 to 16 make any difference ?

Are there any horsepower decals? Yes, S/Ns will help if they are still stock . . . We only really need the gear ratio for reality checks.

First thing is to look behind the tachs for a little switch to set them for different number of cylinders, usually 4, 6 or 8. The tool you need is sometimes called a phototach or a shop tach, see what they have available for rent or loan, I've never checked. There are fairly cheap ones available these days where you put a piece of reflective tape on the crank pulley and then aim at it with a "gun" that records the RPM.

Do not change the props or even consider size changes until the tachs are right. Good data first, then make decisions. Things may be just right as is. Don't worry about diameter even then, pitch is the number to concentrate on and if RPM is low, we go down, not up . . . Right now we don't know.

 

[Randybeall]

Re: twin prop help/puzzle

the problem i have is WOT(should be 4500). running now around 3000 to 3100 with 15x15 twin counter rotating aluminum props maxing out around 28mph on the gps. my boat is a 1991 regal 290 commodore with twin 5.0L mercs alpha ones that seem to run fine. i have been looking at smaller pitch props but it doesnt make sense and i am wondering if there is something else i should look at. comments / ideas ?

Have you had a professional tune up of the engines. If not, check the throttles and make sure that when the throttle lever is in full throttle position the throttle butterflies on both engines are exactly wide open. Then check engine timing, factory spec only. Lots of guys will say some other amount but the manufacturer wanted the boat to run as well as possible and last a reasonable amount of time. Third have you replaced your fuel filters? A slight flow restriction can kill top end power when you need maximum fuel available.
I agree with all the tach info questions and need for gps speed accuracy, just make sure you are getting all the horses that you have available.