Yet AGAIN..more prop info. needed

[TOPSY II]

Help!!!: I bought an '86 Minx (Hull #59) late last summer, which had sat up for about a year and a half to maybe two years. She has a total of 384 hours. I had it checked out by my local mechanic here in Atlanta, and we corrected a number of nagging issues, water circulating pump, new fuel lines, replaced noisy gimbal bearing, new water pump kit, new water pump impeller etc. replaced all gear oil and lubed..boat now runs at around 150 degrees. Compression is good.

It has a bored (30 over) 350 Mag (Carborated),with Crower Cam which apparently was done in late 1995 from the limited paper work that came with.

It has a "no name" 14 1/4 x 21 stainless steel prop, and at WOT (4700) I'm only getting about 57 mph ( hand held GPS).

I've been reading threads, going on line to the various prop venues, but now I'm REALLY confused.

I'd like better overall performance, she seems "sluggish" coming out of the hole, mid range is "ok", but top end from all I've read should be closer to the mid 60"s ( is that realistic?)

Question: Should I increase pitch to say around 23", stay with a 3 blade, or consider a 4 blade, cupped or not? I KNOW there are some experts out there..any and all advise will be greatly appreciated.

TOPSY II:confused: :confused:

[Woodsy]

Topsy....

First thing to check is your tachometer reading properly. It sounds like you are not too far off with the 21P prop. I am assuming your minx has a Mercruiser Alpha drive with a 1.5:1 gear ratio.

Check out this website: http://www.rbbi.com/folders/prop/propcalc.htm

If you punch your data into the boxes, it comes up with a prop slip ratio of .085 or 8.5%. Thats pretty darn good.

I don't think any sort of prop change will get you the 8mph you need to get to 65mph. You can probably get to 60 by playing with different props. My guess is your motor is actually down on HP, probably only making about 250-260HP.

Woodsy Von Outboard

[TOPSY II]

Woodsy: Thanks for the input...is 4700 RPM on the low end ( if it's reading correctly)? and if the tach is the problem, which it could be since it appears to be the original, would you recommend Lavorsi,Gaffrig etc. as a replacement. And secondly, how do I test it for accuracy?

I've been kicking around the idea of installing new gauges and a GPS speedo just to dress her up..(All Ladies like to dress up!!).

BTW: Yes it's an Alpha I, :yes:

THANKS!



Topsy....

First thing to check is your tachometer reading properly. It sounds like you are not too far off with the 21P prop. I am assuming your minx has a Mercruiser Alpha drive with a 1.5:1 gear ratio.

Check out this website: http://www.rbbi.com/folders/prop/propcalc.htm

If you punch your data into the boxes, it comes up with a prop slip ratio of .085 or 8.5%. Thats pretty darn good.

I don't think any sort of prop change will get you the 8mph you need to get to 65mph. You can probably get to 60 by playing with different props. My guess is your motor is actually down on HP, probably only making about 250-260HP.

Woodsy Von Outboard

[Woodsy]

If the motor only pulls the prop to 4700, then its just about right, assuming of course the tach is accurate. You could put in another tach or use a shop tach from a marina. There is no way of gauging if the rpms are low or not because your motor is no longer "stock". Most lower HP (non-magnum) Mercruiser engines are not rated to spin over 5000rpm. You are within 300 of that so your pretty close. You could try having your current prop labbed, that will give you an extra 200 rpm or so, and maybe 2-3 more mph. But that might put your motor on the ragged edge, considering how many hours it has. Labbing will thin and true the blades of your propeller. it makes the prop much more susceptible to damage.

Woodsy Von Outboard

[TOPSY II]

Thanks...I'll get a shop tach and check it out this weekend. If all that checks out, maybe I'll just leave well enough alone, and in a year or so, repower with a Magnum or 6.2L I love the boat, and plan on keeping her around for a long time.( they're not of them around!)

Appreciate your input.

TOPSYII



If the motor only pulls the prop to 4700, then its just about right, assuming of course the tach is accurate. You could put in another tach or use a shop tach from a marina. There is no way of gauging if the rpms are low or not because your motor is no longer "stock". Most lower HP (non-magnum) Mercruiser engines are not rated to spin over 5000rpm. You are within 300 of that so your pretty close. You could try having your current prop labbed, that will give you an extra 200 rpm or so, and maybe 2-3 more mph. But that might put your motor on the ragged edge, considering how many hours it has. Labbing will thin and true the blades of your propeller. it makes the prop much more susceptible to damage.

Woodsy Von Outboard

[Bad-Tat]

Topsy,
Take the Grizz up on his offer to come up to his place and do some talking and testing. He has many, many hours doing prop tests.

[BRIAN73]

Topsy,

My friend has the Testterossa Minx. We have been doing alot of mods to the engine and prop testing.
He is currently spinning a 23 stilleto at 5050rpm.

Your figure of 4700rpm Wot seems to be very close to being correct.

I would suggest running a 21 pitch tempest or a 21 pitch laser 2 with a lab job.

In my oppion the only way to get close to 65mph mark would be to run close to 400hp and be able to really spin a good 23 pitch wheel.


Brian