Prop Advice (all questions answered)

[njmedic3306]

So here is what my boat is...(owned about a month)
21 foot 1982 Chris Craft Cuddie
5.7 Goodwrench Crate (about 350hp) Alpha Outdrive
Prop Vortex MI/992003 15x7 3 blade

It runs out great, gets up on plan in what seems like seconds. However I am not super impressed with these results 5000rpm 40mph (gps)

There are a few very small pits on the prop, could this be hindering performance?

Any suggestions on props? I am not rich so I am thinking budget, however she was just winterized so I have all winter to look on ebay/craigslist.


Thanks

 

[njmedic3306]

Prop Advice (all questions answered)

1) Performance issue you are trying to correct.
Not enough top end

2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Aluminun Vortex x MI 992003 (on the prop)

3) Current prop diameter and pitch (required).
15x17

4) Wide open throttle RPM and speed with an average load (very helpful)
Just me and 20 gallons fuel 5000RPM at only 40mph GPS

5) Engine/drive make, model, year, and HP
5.7 GM Crate motor, 400hp not sure of year but Alpha I outdrive

6) Boat make model, year, length and weight
1982 Chris Craft Scropion Cuddy 210 (21 feet)

 

[steelespike]

Re: Prop Advice (all questions answered)

I'm not a pro with the 5.7 But in my opinion you would normally run a 21 or 23" prop
at about 4800-5000.A 17 would be off the scale.Don't know your ratio but at 400 hp you should be able to twist a pretty serious prop with a 21 ft boat.
Is the bottom clean and smooth?Does the throttle open all the way at the carb?
Is the timing set right and advancingg as it should?Just because its a new install don't assume all is done right.

 

[njmedic3306]

Re: Prop Advice (all questions answered)

So do you suggest I bump up to an 15x21? Would that give me more top end. I am not too worried about loosing too much off the line as it planes in a few seconds now as it is.

 

[steelespike]

Re: Prop Advice (all questions answered)

I believe your motor is rated for 4400 to 4,800 rpm.
Not knowing the year, difficult to pin ot down.
I think your motor should over reving 5,500 or more rpm with a 17" prop.
Your at 5000 If you go to a 21" prop your rpm will drop "ABOUT" 800 rpm
(200 rpm per inch) that would put you at about 4,200 Below the rating. of 4400 to 4800.Are you sure its 400hp? If its not a marinized motor the cam may be wrong
and may may be difficult to prop for.Also The outdrive gear ratio may be wrong
further complicating prop saelection.

 

[Robbabob]

Re: Prop Advice (all questions answered)

Much of what everyone has stated is spot on.

We have the '95 CC Concept Cuddy, with 5.0 EFI that turns out 230 HP. Our WOT hits 4800ish and 52ish mph with a SS 14.25 X 21+

Although I am not familiar with your hull, I know you would achieve more speed by adding pitch. Having an aluminum prop might be flexing under such a heavy load from the 400HP power-plant. If you are buying a new prop anyway, might as well go with stainless.

Good luck and be sure to let us know how it goes!

 

[njmedic3306]

Re: Prop Advice (all questions answered)

I understand I will loose 200rpm for every inch of pitch I add but what effects will I get if I move from a 15" to say a 14.25 inch diameter??

One more silly question, how do I know if I need a LH or RH prop?

 

[steelespike]

Re: Prop Advice (all questions answered)

As you add pitch diameter generally goes down.Diameter is generally not considered
as a direct affect on rpm.A reduction of 3/4" "may" marginally increase rpm.
Most props are righthand rotation standing behind top of prop rotates to the right
and blade angle would push water toward you.Most props would be marked R or C(clockwise) for right,L or CC (counter clockwise)for left.

 

[QC]

Re: Prop Advice (all questions answered)

Going to assume right hand rotation at this point. Couple of questions though . .

1) Do you know your gear ratio? I am using 1.5:1 for now, but you note Alpha 1. They weren't out until later than '82, so could be a drive change although, it would still most likely be 1.5

2) What are you doing with trim at WOT?

3) Are you absolutely positive the tach is right?

4) The 400 bhp thing and "crate" has me worried this isn't a marine engine. Can you tell us more about it? Who built it? Where'd you get specs? Be aware there are very few marine small blocks that make 300 bhp for a variety of reasons. Exceptions? Yes. But doubtful at this point.

I think she should run faster as I'm getting 25% slip with a 1.5 ratio. Important to nail this down before you buy something. A jump from 17 to 21 is serious . . .

 

[njmedic3306]

Re: Prop Advice (all questions answered)

1. I am not sure what ratio it is and I don't think I will find out since its rapped for the winter. I thought the boat came with an alpha drive but since I dont know what years they were produced I cant speak. I could swear all the other similar Chris Crafts Scorpoins had alpha outdrives.

2. At WOT I tried it trimmed in all the way and trimmed it out a bit and I didnt get much difference in speed. The hull seemed low in the water trimmed in but I didnt gain much speed by lifting it.

3. I am not sure the tach is accurate so I guess I should test it.

4. Actually I don't know much about the motor except the following.
a. Says GM Goodwrench on the valve cover.
b. Has what appears to be a Edelbrock dominator 4 barrel carberator.
c. Except for having the marine risers and manifolds I dont know how
"marine setup" it is.
Here is a pictures of it running 2 years ago....So maybe you can help identify the outdrive. I have had a few boats but this one is the first without an outboard so I took a few risks but I got it cheap.

http://www.youtube.com/watch?v=hk324Q-XMs0

http://www.youtube.com/watch?v=yMfh7_w1cZE&playnext=1&videos=ePHoZtL7v14&feature=mfu_in_order

 

[QC]

Re: Prop Advice (all questions answered)

In the pics it's "pre" Alpha. Still probably 1.5:1. Yeah I think you need to play with weight balance and trim to get the bow up. That will cut down on slip and she'll be faster. Idle sounds a little rough for a marine cam and Good wrench seems like a clue as well. The cam is the concern. Not much you can do about it now, but I still don't believe much more than 300 bhp with a carb.

If the tach is right, more trim will make RPM worse, so definitely will need more pitch. I'd:

1) Verify tach

2) move some weight aft and see what happens

3) if RPM rises more you may in fact need a 21 inch pitch, but I'd check that before jumping unless they'll let you return it

 

[fossill]

Re: 15x17 on a 5.7 Mercruiser sound right?

Re: 15x17 on a 5.7 Mercruiser sound right?

Merc High 5 in a 21 pitch or 23 pitch. Pricy new but lots of deals on ebay for used ones. Best prop out there in my opinion. And if you want to sell it again, they go quick.

 

[captkevin]

Re: 15x17 on a 5.7 Mercruiser sound right?

Re: 15x17 on a 5.7 Mercruiser sound right?

3 blade stainless 19 pitch should be a good all around prop. Give you more top end & maintain performance overall.

 

[njmedic3306]

Re: Prop Advice (all questions answered)

WOW well thanks for that. I had no idea it was not an alpha. Now when I am looking for a new prop will I sill look for one that fits an Alpha 1?

 

[QC]

Re: Prop Advice (all questions answered)

OK, I merged these two threads to eliminate any confusion, although that will add some as well.

While High Fives are good, a 23 inch High five is a way different recommendation than a 3 blade 19, and we have that three minutes apart . . .

Let's try and zero in on this and we all need to recognize that different props perform differently on different hulls, and for different purposes. While a High Five is great for hole shot and midrange performance, a three blade is usually better for top speed. Four blades seem to be good compromises, but again it is very difficult to predict success without a lot of testing . . .

 

[Robbabob]

Re: Prop Advice (all questions answered)

Everything mentioned still sounds like there is way too much HP for the prop that is installed. High possibility the boat isn't trimmed enough... which is good, in this case, to prevent the over-rev.

Testing is the only way to know for sure. Bet you're glad to hear that! Ask around at the dealers or marinas and see who might be willing to work with you this testing/selection process. You may find the SS 3-blade 15X21 works best.

Good Luck and let us know!

 

[njmedic3306]

Re: Prop Advice (all questions answered)

Ya know now that you mention it when I tried to trim out (bring bow up) it would only go for a second or 2 before the RPM seemed to take off (above 5000rpm). Of course I didnt do that for a second or 2. I thought the prop was coming up too close to the surface but maybe now that its discussed it was because I am so badly under propped.

Thanks I will keep an eye out for spring.

 

[Robbabob]

Re: Prop Advice (all questions answered)

Ya know now that you mention it when I tried to trim out (bring bow up) it would only go for a second or 2 before the RPM seemed to take off (above 5000rpm). Of course I didnt do that for a second or 2. I thought the prop was coming up too close to the surface but maybe now that its discussed it was because I am so badly under propped.

Thanks I will keep an eye out for spring.

Test in the Spring at about half-throttle, once on plane. Bump the trim/tilt a little at a time. You will feel her free up and only add a littel rpm. Keep going up until your bow begins to bounce a lot, then back it down a bump or two.

Once you find the sweet spot, you can start increasing rpm and adjusting the trim accrodingly. I suspect you'll achieve about 45 mph while staying below 4800 rpm (maybe 3/4 throttle). As you add pitch, speed will increase while staying at or blow 4800 rpm.

Though opinions vary, SS is probably your best prop, but some stick with Aluminum when they know they'll be around too many stumps or rocks. It may depend on how well you know your waters