Under propped or over propped??? please help

[sharps45]

I started a new thread with new info on my boat. I have a Chris Craft 23 foot deck boat, Volvo Penta SX, 5.7l, 260 hp, 1:66 gear ratio. If I could ask two questions-
1. All the info I can find on this boat shows the 5.7 with 1:43 gears, and the 1:66 with the 5.0 l. I don't know the history of the boat, but what difference will the ratios make in prop choices? Why different ratios at all?
The Michigan prop calculator doesn't list this engine and gear in their options, so how do I figure the correct prop?
2. Did some testing with the props I have 6200 ft elevation (lowest available for 200 miles)
15in X 17 pitch 3 blade alum 33 mph gps 4200 rpm WOT pretty good out of hole
14 1/2in X 19 pitch 3 blade michigan alum 32 mph gps 3750 WOT nohole at all and less top end too

Would going to a 4 blade help with the hole and top end? I understand the 19 pitch being slower out of the hole, but top end should have been better?

 

[tommays]

Re: Under propped or over propped??? please help

You current numbers show the 15 X 17 to a BIT over proped as you should be getting a WOT in the 4600+ RPM range


The 6200 ft is going to reduce your 260 HP about 210 more or less and thats if you have changed the carb jets to run at that attitude

It will be much to rich if it was setup for sea level

I dont know what Volvo does BUT merc has different gear ratios in the drives for high elevation to make up for the powerloss



Tommays

 

[athiker]

Re: Under propped or over propped??? please help

How old is your engine? When was the last tune up?....plugs, wires, etc?

I would think you should get a bit higher max mph as well with that set-up, but like Tommays wrote the 1:66 drive ratio is no doubt to help compensate for the higher elevation where you will lose hp.

A 4 blade 16p is a possibility I guess, but that strikes me as an awfully low pitch for a 5.7L on a 23 footer. Often you lose a little WOT rpm going to a 4 blade and as mentioned, you aren't hitting your recommended upper rpm range now w/ a 3 blade 17p so going to an 18p 4 blade doesn't seem to be the ticket. I wouldn't necessarily expect a better top speed though w/ that 16p 4 blade, but maybe you could improve your holeshot if that's something you'd like.

On a my 2000 21 footer w/ a 5.7L EFI I ran a 4 blade 20p (and a 21p 3 blade) that seemed to be the sweet spot. I tried a 4 blade 18p and it would hardly go and over revved to boot. But I'm less than 1000' above sea level w/ a 1:47 drive. Hopefully some high elevation folks will weigh in.

 

[sharps45]

Re: Under propped or over propped??? please help

Thanks for the quick replies. The motor is only a year old, and I thought it had EFI. The boat itself is a 2000, and I thought all of them were EFI also. This lake at 6200 feet is the low lake in my area, with the others available close by at 7000 or higher. The 15X17 gives me the hole shot I need, and with the small size of the local water I don't need the high end speed. But when I go to Lake Powell in a couple of weeks I'll need both. It's at 3600 feet. The guy I got the boat from ran the 19 pitch there with no problem (he said). Any Powell boaters out there with any advice?
Maybe someone from Tahoe or similar will weigh in.
thanks again

 

[steelespike]

Re: Under propped or over propped??? please help

Not an expert but It looks to me you were pretty close with the 17.
If you were only making 4,200 with the 17 your going to turn less with the 19 and of course the hole shot will be less.I'm guessing your wot reccomended rpm is about 4,000 to 5,000 rpm.
4 blade may help hole shot but top end will be less.Generally you would go to 4 blades in an extreme case.
I guess the elevation makes it tough on the engine.I think 15pitch would put you at about 4,600 with better hole shot possibly a little more top end.
There are some design subtleties that the others can discuss.Generally speaking a ss prop will have more bite than aluminum.sometime requiring a reduction in pitch to maintain rpm but usually gain speed and efficientcy.

 

[athiker]

Re: Under propped or over propped??? please help

Here's another current thread that may give you some reference:

http://forums.iboats.com/showthread.php?t=201569