Prop ???'s for Inline 6

[Oshkosh1]

So I was reading the factory manual (from 1973) last night...which is the year of the motor in my 1985, 19' Wellcraft.

Just reading about the prop recomendations in the appendix...and it stated that a 17-19' boat should turn between a 14.5 and 15.5 inch/21-23 pitch Aluminum prop.

I'm running a 13.5/17P SS and get good performance(top out at 42 or so with 3 adults).

Seems like the manual is suggesting quite a big wheel for this motor. I wouldn't think this I6 can push more than a 19P without problems.

Now...I DO have an Alpha one.

Did the older drives(1973) have a much different ratio?

Any input/expeirence/prop selections from other I6 owners?

 

[Georgesalmon]

Re: Prop ???'s for Inline 6

I have an I6 and use a 4 blade 15" prop. What matters is the boat. My boat is heavy. You need to post a bunch more info before the guys who know about props can help you. Answer the questions in the sticky about this forum.

 

[Bob_VT]

Re: Prop ???'s for Inline 6

That Wellcraft is a good size boat and you are performing right. You can verify it with a tachometer but those inlines like to rev at about their max. Going larger will not yield you more speed but it will also hurt your holeshot.

Here is a post where I tested my old inline 115 http://forums.iboats.com/showthread.php?t=177235

 

[Bondo]

Re: Prop ???'s for Inline 6

That Wellcraft is a good size boat and you are performing right. You can verify it with a tachometer but those inlines like to rev at about their max. Going larger will not yield you more speed but it will also hurt your holeshot.

Here is a post where I tested my old inline 115 http://forums.iboats.com/showthread.php?t=177235

Ayuh,.... He's got an I-6, I/O Bob....

I'd think the Op is runnin' an Outboard prop, at 13.5" diameter...

I/O props run 'bout 14" to 16" diameter...

 

[Oshkosh1]

Re: Prop ???'s for Inline 6

Interesting...

I'm running the prop that it came with(the 13.5, with a spare Aluminum of the same size) assuming it was for an I/O.

If it a'int broke...don't fix it in this case?

 

[QC]

Re: Prop ???'s for Inline 6

Funny, but nobody understood your question . . . Heck Bob doesn't even know how many cycles you're running and Bond-o wants to talk Diameter. Maybe I'll throw in a Doelfin recommendation

So to your question . . . I also thought it was an odd recommendation from the manual. The MCM 165 originally used a 1.65 gear ratio from what I can find. You don't say what you have now. I am guessing a 1.5. A 21 at 4000 RPM and 15% slip would be right at 41 MPH with the 1.65, so maybe that would be OK. Is your speed GPS? RPM? I think those I6s were rated pretty low WOT range.

 

[steelespike]

Re: Prop ???'s for Inline 6

A quick calculation it seems the motor is over revving at 4700 at 42 mph, Its rated at 4300.
We need your wide open throttle rpm and gps speed. to know for sure whats going on.
The but will have better overall performance and mileage with the right prop.
You could run as it is but stay away from wot.

 

[Oshkosh1]

Re: Prop ???'s for Inline 6

My Tach hasn't worked since I bought the boat...just "earballing" it(I know, I know), and I only ran WOT a couple of times...and then just to "see what it could do" for a very short period, all the while ensuring there was no cavitation or overspeed signs.

Speed was Via GPS and the motor didn't seem to be in any distress...I was guessing about 45-4700 so not far off. Other than those couple of times, I "cruise" @ 3/4 throttle and maintain 28-30 MPH. May have been a bit above 4300...but defineately nowhere near "grenade" speed.

Because this is a "Frankenboat"(a 1973 250CID/1985 Alpha-one) I'm not certain what the ratio is. My GUESS is that it's the drive from the original 3.0 litre 4cyl which was OEM. Apparently this motor was rebuilt and retro-fitted.

I've got a new Faria tach sitting on the bench...I will install it "post haste" and when I get hard numbers I'll report my data.

 

[QC]

Re: Prop ???'s for Inline 6

I'm not certain what the ratio is. My GUESS is that it's the drive from the original 3.0 litre 4cyl which was OEM.

This would make my calcs worse. I think you have a 1.5:1, but a 2:1 (stock 3.0 liter) would suggest that a 21 or more would be in play.

I've got a new Faria tach sitting on the bench...I will install it "post haste" and when I get hard numbers I'll report my data.

Hard numbers will require verification of the gear ratio. Barring over the engine and counting prop turns is the "right" way when you don't know Good luck!

 

[Oshkosh1]

Re: Prop ???'s for Inline 6

Hard numbers will require verification of the gear ratio. Barring over the engine and counting prop turns is the "right" way when you don't know Good luck!

I'm awaiting some parts and when I get the drive back on I'll do it.

 

[Oshkosh1]

Re: Prop ???'s for Inline 6

I have no idea why I thought I was running a 17P...It's actually a 19P SS Quicksilver. I also have a 19P Aluminum as a spare.

Has anyone seen my Geritol??:facepalm:

 

[QC]

Re: Prop ???'s for Inline 6

Jeez, you suck. I never get anything like that wrong . . . OK, back to the calculations. Doh!!

 

[steelespike]

Re: Prop ???'s for Inline 6

If I remember right a 1.65 gear ratio and with the 19" prop, guessing 10% slip puts you at 4280 rpm.
It would be much better to have a working tach.
your motor may be a little tired or out of tune maybe the throttle doesn't open all the way.
Guessing again if the motor could turn a 21 up to 4280 at 10% slip speed would be about 46.
The way it stands now the motor couldn't do it.

 

[QC]

Re: Prop ???'s for Inline 6

We think it is a 2:1 at this point. The drive probably came from an old 3.0 setup. Yet to confirm.

 

[steelespike]

Re: Prop ???'s for Inline 6

While I noted the alpha one mention I didn't occur to it was from a lesser motor.I don't think that info came till post #8.
Oh well I guess he'll get us the actual count at some point.

 

[Oshkosh1]

Re: Prop ???'s for Inline 6

I've got the sterndrive and prop #'s

Stern is A317841 B
Prop is QS 48 13980 AS 19P 140C7

I've got parts "in hand" and I'm checking the alignment tomorrow or tues with a local tech then I'll have the drive in by tues or wed and start counting to confirm.

Also...
A local is selling a 21P "High-Five" prop for $150...says(and looks from the pics)it's "like new"

He stated it came off of a 4.3L.

Opinions?

 

[QC]

Re: Prop ???'s for Inline 6

Let's confirm some numbers first and go from there. Defintiely need the tach going as well. Those High 5s are good for hole shot for sure. Not so much for top speed.