5000 RPM w/21 pitch prop...too high???

[chiguy454]

So first off, hi everybody....long time guest, first time poster.
To make a short story long.....I've been boating for several years (jet boating that is) and got a steal, at least I think it was, on a 1985 Chaparral 200 XLC 260 Merc/Alpha 1($400 with a very nice Shorlandr trailer). The boat had a cracked block and was left out for a year, but only had 600hrs and is very solid. So, being the guy that I am, I built a 383 vortec SBC for marine application. HP is unknown, but probably (conservatively) around 325??? I have been breaking in the engine, and just finally opened it up the other day. I reached 60mph gps but watched the rpm's climb to 5000rpm at wot. Now coming from an automotive background, for me, that seems pretty high.
To get to the point here, I would like some prop help. I have read many posts, got opinions from other boaters, and talked to the local dealer, but still get mixed ideas. The current prop I have on the boat, came with it, is a SS 15.25x21. I have no problem getting up on plane, seem to have plenty of pull, however, I would like to see my wot rpm drop to around 4600 or so. I'm a little confused, Aluminum or SS? Diameter, pitch? I don't necessarily need to go any faster, but would like to keep a good hole-shot and keep the rpm's down a little at wot.
Thanks

 

[steelespike]

Re: 5000 RPM w/21 pitch prop...too high???

It would seem that a change to a 23" pitch would put you at about 4600.
For a more predictable result you should stick with the same brand/model ss prop.
Usually as pitch goes up diameter goes down.
The experts may be along with more specific suggestions.
As I understand it the Alpha 1 is only rated up to 300 hp.

 

[JoshOnt]

Re: 5000 RPM w/21 pitch prop...too high???

I am with steelespike on this one, I would say the same as him but remember the number of blades as well. I have found same model and brand props before with different amounts of blades.

 

[jestor68]

Re: 5000 RPM w/21 pitch prop...too high???

If want to keep your good hole shot, you really don't have to do anything but control the rpm with that handle in your right(I assume) hand. ( Cheaper alternative)

If you insist on going 60+ mph, you'll need to step up the next higher pitch available to pull the rpm down to whatever is suggested in the literature that came with the aftermarket engine.

You should under stand that increasing pitch will affect the acceleration (slower), while probably increasing your top speed.

5000 rpm with a modern( 4 bolt main) 5.7L is not unreasonable. There are plenty of Mercruiser motors that are rated for 5000+ rpm.

I have a 502 that I routinely run over 5500 rpm(92.6 mph) for short bursts, although I limit it to 5000 rpm using the throttle the majority of the time.

 

[craze1cars]

Re: 5000 RPM w/21 pitch prop...too high???

Are your RPMS too high for what? The motor? The outdrive? You think you're past your peak power curve?

Most 383 Vortec builds in cars make their peak HP closer to 6,000 rpm when put on the dyno. Of course most of them are built to make betwee 400 and 475 hp. 383 ci Vortec is a pretty meaningless info without knowing what kind of cam/bore/stroke/lift and other such details about what you built. If you built a low rpm stump-puller torque machine, you might be revving too high. But if you built it like most people build 383s for their cars, you're revving way too low....

The cool thing about boats is that they essentially have built-in dynos. Just gotta buy a variety of similar props and see which one gives your boat max top speed in equal conditions. That prop will tell you what RPM your motor makes peak HP at.

I do see possible exploded Alpha drives in your future. But the fact that it's in a relatively light boat will help it live if you keep RPMS under control and keep up with maintenance.

Since you say you don't really want to go any faster but want to slow your WOT down and maintain good holeshot, I'd suggest a 4 blade 23 pitch. 4 blades will maintain strong holeshot at the expense of top speed a bit, and the extra pitch should lower your rpms a bit. I was always very impressed with Solas HR Titan props for getting bang for the buck. Had one on a 350 Vortec (truck engine...torque cam) in a 20 ft Stingray that I sold recently. I always guestimated it at near 300 hp. It pushed a 23 pitch 4 blade Solas prop no problem at all with a 1.61 ratio Volvo drive at about 4800 rpm, to about 58 mph. Ripped outta the hole too. Yours should be stronger than that, though my Stingray probably had a faster and lighter hull than your older Chapparal so that makes a difference too.

OH....stainless is pretty much mandatory for your power output and application.

 

[chiguy454]

Re: 5000 RPM w/21 pitch prop...too high???

Wow, thanks for all the responses! I left out a lot of info because I didn't want to bore yo u guys with a 2 page story about me and my boat. So, yes, the engine may be a little too much for the alpha 1, but reason for the 383 was that I already had most of the parts to build that combo.
Here is a the scoop on the engine: '88 4 bolt block with stock bore, 400 cra nk, KB 12cc d-dish, vortec heads, aluminum intake, and a roller cam work ss roller rockers. It is a fairily mild build for a stroker, it has a roller cam designed for a 1600-2600 rpm torqe curve and a 1500-4500 rpm power range (lift i/e .460/.460 and dur @.050 i/e 195/210 with 112 lobe center.
As for the rpm's, I just kind of figured since the cam is rated around 2500 rpm for the max powerband, that I should try to limit the engine to that range. Seems getting max performance at the lowest engine revs an keeping it within the power band would give me the best results.
The Chaparral is 2800lbs dry, and had a 260 merc from the factory and obviously a a 1.50 ratio outdrive.
Based on the responses, I should probably be leaning toward a 4 blade prop, as we do a lot of skiing and tubing, but also like to go fast from time to time. Thanks for the ideas, an I think I am getting a better understanding of prop selections.

 

[chiguy454]

Re: 5000 RPM w/21 pitch prop...too high???

Sorry, I meant 4500 max rpm's in the above...not 2500

 

[craze1cars]

Re: 5000 RPM w/21 pitch prop...too high???

Yeah that build sounds like basically it's all about low RPM torque, which should be great for a boat. I think it's a very reasonable guess that your peak power will come in somewhere near 4,500, and by 5,000 that motor is falling on its face and is running nowhere near its potential.

Definitely more pitch is in order, and I'm quite certain it will VERY easily push a 23, if not even a 25. Trial and error will be the only way to determine for sure.

Got a prop shop nearby that will give you a few used loaners for test-drive and shopping purposes? There is no better way to find the perfect prop...