Prop info?

[starsnstripers]

250ci to 292ci L6s est. 35hp increase. The boat was running a 19p prop so if i install the 292 and it over revs should i go to a 21P? The two 19p props that came with the boat have #s4873146A40. How do i determine the Dia. by the #s and whats the largest Dia. prop you can use on a prealpha drive?

 

[walleyehed]

Re: Prop info?

Are you wanting to stay with the same style prop you have now, or are you wanting to move up in technology if you have to change??
Also, as a general rule, given the same boat at the engine size we're talking, it takes 17HP to increase 1mph..so looking for a 2mph gain, I think I'd try the prop you have first. Are you exceeding the max rpm with the smaller engine?

 

[starsnstripers]

Re: Prop info?

Not sure thats what i got the boat with i havent run it yet. So you think a 35hp increase wont be enough to change prop to 21P? I was more worried about an rpm increase with theextra hp and same prop.

 

[walleyehed]

Re: Prop info?

That 35HP on the same drive May not add a great amount of difference. It also is dependant on what RPM the initial engine would turn the 19.
I just don't think you'll see a 400RPM increase with the 292.
Whats the 250 rated at for HP?

 

[achris]

Re: Prop info?

That 35HP on the same drive May not add a great amount of difference. It also is dependant on what RPM the initial engine would turn the 19.
I just don't think you'll see a 400RPM increase with the 292.
Whats the 250 rated at for HP?

He's got an old 165 with a 1.65:1 drive ratio. He's looking at putting a 292 in with the same drive... I have to agree with you on the amount of speed he'll pick up. As the man said 'Stay with the 19" until you know what's happening'.

The 19" props you have are 14" diameter. Merc usually never bother with diameter spec. They make the props to suit the drive they are designed for, and they been doing it a long time!!! Merc props are the most efficient on the market, and don't let the Solas guy tell you any different. I done lots of prop tests when I had my shop.... The biggest prop you can put on a MC-1 drive is about 16" diameter.

Something else to think about. As the pitch of the prop goes up, so does the efficiency

Chris...........
Was that only 6 days .... Gee how time flies.....

 

[MikDee]

Re: Prop info?

From experience with Mercruisers, Yes I like Merc props, better then comparible Michigan wheels anyway, there may be better brands out there, but I think they're the best bang for the buck. I had a Merc. 14X19" Alum., on my old 18'5" Wellcraft Airslot with my rebuilt 165hp 6cyl that would do about 45. I've used, that prop, and the 13 3/4X21" on a few of my different boats as well, and liked them both.

 

[starsnstripers]

Re: Prop info?

Yeah, Walleye the 250ci is 160-165hp vs 292 200hp. AND, for you Chris "very funny" time flies huh? you only came over here because i complained so you could rub it in. I think it's gonna be like Don said a tril thing. Are you running a 21p Chris? The same prop may just be ok, I was just curious if a HP increase would matter. Thanks for the input people. BTW> And the Quickness too. <Chris>

 

[achris]

Re: Prop info?

Ho hummmm... another 6 day reply

When I bought the boat (1986) it had a 165 engine, 1.65:1 drive and was running a 17" prop. It would only pull just on 3900rpm. I changed to a 15" prop and the whole boat felt a lot better. Engine WOT was 4300 rpm and speed was 30 knots (34.5 mph).

1994 - installed 4.3LX (180 propshaft hp) with 1.84:1 drive. Ran a 21" prop and was doing 40 knots (46mph) 4800 rpm. Changed to a recupped (to 21") ali 4 blade for the smoothness. Pulled 38 knots at 4950rpm. More blades, less efficiency. I then changed to a 21" 4 blade 'Offshore Series' prop until..

2006 - installed 4.3MPI (220 propshaft hp) with a 1.62:1 drive (in 1996? merc changed the lower unit ratio slightly to reduce a noise problem, hence the slightly different overall ratios now). I'm currently running a 17" prop and pulling the rev limiter in at 5200 and doing.... wait for it.... 40 knots!!!

Have a look at this post in 'Non-repair Outboard engine' and read the full story....

http://forums.iboats.com/showpost.php?p=1563486&postcount=12

Cheers,

Chris.......

 

[starsnstripers]

Re: Prop info?

Yeah yeah! 4.3 jibber jabber.haha! Good story "i read it" you had me confused for a minute though, geesh this> " means inches or quotes. It doesn't mean prop pitch this> P means prop pitch!!! I was like how the he** did he fit a 21" inch prop on an outdrive? Dont do that to me, I'll need a heat exchanger for my brain I can see i'll have to do a trial and era with the big fat hp increase.

 

[achris]

Re: Prop info?

ok... sorry to confuse you.... inches (") is inches of pitch.... That just happens to be what the 'P' means..... I usually never refer to the diameter of the prop. For the particular applications that we use here the pitch is the critical measurement. It has a far greater influence of performance than the diameter.

The only real place for significant diameter changes is the ski world of inboard engines (shafts not legs).

Cheers......

 

[starsnstripers]

Re: Prop info?

Auh, no problem Chris i figured that when you said 14" prop is usually standard. I was wondering because when i looked at it i have maybe an inch clearance dia. wise. darn i was hoping to sqeeze a 21 on there. haha "kidding". Ok it's boring here i'm going back where the fun is ODs.

 

[walleyehed]

Re: Prop info?

"The only real place for significant diameter changes is the ski world of inboard engines (shafts not legs)."

achris, this is where things are really changing...Diameter is of the utmost importance for max performance on any rig that can adjust the running height of the drive. Diameter is begining to play a huge roll in differing designs now as well, and in the case of an I/O, it's coming back into play there.
I received a BUNCH of props from Merc last summer to test...they are breaking the old school rules left and right.
I would agree not to get hung-up on diameter, but pay enough attention to the design to understand why 1 17" may be 15"D and another only 14"D. Blade design will be different and so will performance, and it may not be in the direction one would expect.

 

[achris]

Re: Prop info?

"The only real place for significant diameter changes is the ski world of inboard engines (shafts not legs)."

achris, this is where things are really changing...Diameter is of the utmost importance for max performance on any rig that can adjust the running height of the drive. Diameter is begining to play a huge roll in differing designs now as well, and in the case of an I/O, it's coming back into play there.
I received a BUNCH of props from Merc last summer to test...they are breaking the old school rules left and right.
I would agree not to get hung-up on diameter, but pay enough attention to the design to understand why 1 17" may be 15"D and another only 14"D. Blade design will be different and so will performance, and it may not be in the direction one would expect.

I totally agree with everything you've said.... I am also aware of the significance diameter makes, especially with the higher torque engines. I have experimented with that myself, having played with Merc's Mirage props, some 4 blades and most of their 3 blades. At one stage I had 7 different props for my own boat . I have sine found the ones I like and 'e-bayed' the rest.

In the case of sns he has an MC-1 drive and is limited in the maximum diameter he can use, that's the only reason I said, for him, that diameter was basically irrelevant.

As an aside, you seem to be very knowledgeable on this subject... Do you have a prop shop or boat shop? Or do you specialize in the shaft driven ski boats? Just curious....

Chris...........

 

[walleyehed]

Re: Prop info?

I'm a consultant for a major prop manufacturer, and yes, I test props from all manufacturers for hire....just over 400 last year alone-not on 1 boat..I have about 6 test boats of different makes and models.
And I do sell props...just not on this forum...

 

[starsnstripers]

Re: Prop info?

Well i'm no prop pro but i can see there's not much room for diameter on my drive. I'll stick with the basics I think i can get what i need on my old stove bolt to please me by a pitch change if needed.thanks though