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kamapmj
07-17-2002, 08:23 AM
Looking for some expert advice on finding the perfect prop to bring the RPM's down from 5500 to ? due to the over-revving since the installation of the new outdrive with different gearing. All tech info is listed below. Should be noted that the drives were swapped due to shifting problem under warrenty some years ago.

Thanks for the input.

Donzi Specifications
1992 Donzi Classic 18

Engine:
351 OMC HO (High Output) King Cobra is a Spitfire 5.8 L 285 HP Ford
WOT (Wide Open Throttle) recommended RPM is 4600 - 5000
(Standard options were 5.7 L 260 HP OMC Cobra L.E. & 350 King Cobra Chevrolets)

Carburetor:
4V (Four venturi), adjustable idle circuit, fixed main fuel jets, electric choke

Outdrive:
Gear ration of 1.64:1 “JV” (This is the ratio for the 3.2 litre OMC Cobra Diesel!)
The original (Replaced) was 1.50:1 “AMH”
The non HO 5.8 L 351 (240 HP) is 1.41:1 at 4000 - 4400 RPM WOT recommended

Propeller:
OMC Raker 14 ΒΌ X 21

Performance:
WOT RPM = , too high

Solution:

boldts
07-17-2002, 10:17 AM
kam,
No expert here, but anyone who owns a performance hull as been in search of the best performance at one point or another. Try this website and plug your numbers in to the calculator. Prop Help (http://www.boatpropellers.com/propcalc.html) That 1.64:1 gear ratio is going to require a serious pitch increase. Something in the 24 to 25" range. I'm not familiar with the OMC props, but diameter and cupping the prop also have major factors in how many RPMs an engine will turn. Adding cup to a prop is the same as adding an inch of pitch to the prop. Cup usually takes a little speed away, but also helps the prop keep a grip while the boat is in a turn. It appears that a 15" dia. will also be needed. How-ever, if you use a 15" dia., you won't need as much pitch in the prop. It all works hand in hand.
Best advice would be to contact a prop shop and ask them for their recommendations. They may need the information you supplied here plus the boats weight. Another factor in the equation is what you want from the prop. Good all around use? Top speed? Sking? Right now, number 1 on your list is to get the RPMs in the recommended engine range! Those folks work with these figures every day and know what is available from the prop companies for your application. They should be able to give you a starting place and then it's a matter of testing the prop on your boat. Each drive company obviously makes their own props for their drives, but does that mean it is the best prop to run on your boat? Not always! wink
Good luck and let us know how you make out. There's always someone in the same position as you, looking to maximise their performance also.

Craig
07-17-2002, 11:02 AM
Not to contradict, but cupping increases speed.

boldts
07-17-2002, 11:10 AM
Craig,
Thanks, I stand corrected! I've only heard the negative of that aspect, but I guess it makes sense if it's like adding pitch. :)

tailwind
07-17-2002, 11:45 AM
Certainly no expert here, but do have recent experience with a similar setup.

I have a 1991 18 with the OMC King Cobra (1.5 gears), and the 350 Spitfire (Chevy) setup.
Approximately 270 plus hp. The Donzi came with an OMC Viper 14 1/4 x 21 prop which had
been filed or severley modified, even to remove some of the cup area. GPS speed, trimmed out with this prop was 57 mph @ 4800 rpm.

Recently replaced with the "current" OMC Viper (new Dim) 14 3/4 x 21. Current GPS is 62.5
@ 4200 rpm. According to boat prop calcualtors, on the web, this is not possible, at this rpm!
In order to be running this speed, my rpms should be closer to 5000. Therefore, the next
opportunity, I am going to check my (VDO) tach calibration to see whats going on.

Realizing your gear ratio of 1.64, you should be able to turn a 23 pitch, or even a 25. I would
be interested to see how close your tach reads, as like mine could be off at wop.

Good luck, keep us posted
Randy

Craig
07-17-2002, 12:03 PM
Boldts,
Yeah, I had to keep thinking, am I thinking backwards, but like you said if its "like" adding pitch... Have had two aluminum props that reved high and ran slow. Cupped both and it lowered rpm's and increased speed. What are possible negative effects? I've often wondered why every prop isn't just produced "cupped" once someone recognized the benefits. Lower RPM, better top end and better on take off.

Barry Phillips
07-17-2002, 12:03 PM
Im running a 350chevy in an 18 with a Bravo 1 drive, 1.5 gears turning a 23" Vengeance SS 3 blade this combination works well with only 250hp I can run mid to high 60s at 46 to 48 RPM. I tried a 21" Mirage Plus and lost 3mph.

SO-SLO

mattyboy
07-17-2002, 08:51 PM
definetly not an expert but I have the basic same prop, a ski prop, the name raker comes from the rake or lift, opposite of cup, that gives an extreme amount of lift, at speed the prop heads for the surface, in airated water then over revs,
I bet your boat plans quickly,
think you need a 24or 25 with less rake

Matt

tailwind
07-17-2002, 09:01 PM
Speed/ shaft hp/ observed rpm/ are all vital parts of this equation!

When taken one by one, overall expectations will arise!

Although, defintive GPS measurements will suffice, Tach calibration will indeed tell the tail!

Regards,
Randy

HyperDonzi
07-17-2002, 09:01 PM
on some prop calculators, they ask for prop slip. how do you know what to put in here?
im trying to find the gearing of the drive.

MOP
07-17-2002, 09:20 PM
This may help a little, you will have to guessimate a little. Then go and borrow props!
http://www.go-fast.com/Prop_Slip_Calculator.htm

HyperDonzi
07-17-2002, 10:37 PM
how do i knwo how much im slippin?

boldts
07-17-2002, 11:10 PM
Hyper,
Simply put in all the other numbers except the slip percentage and it will calculate it for you. You need to know your drive ratio, WOT RPMs, Pitch of prop (Remember to add 1" if the prop is cupped) and finally your WOT speed (GPS speed prefered). Usually the goal is to keep the slippage around 10 to 15%. Less of course is better and boat speed increases. Here's how my boat worked out.

RPMs = 4200
Drive = 1.36
Pitch = 25
GPS Speed = 67.1

Slip percentage worked out to 9%.
My boat needs less pitch to increase RPMs and the best pitch appears to be 23". RPMs are still only 4600, but speed goes up to 68.

HyperDonzi
07-17-2002, 11:18 PM
68, are you planning on thaking that to alum this summer? theres only one problem with entering all of it except the slip, i dont know the gear ratio.
it is an alpha from 1990 if that helps.

boldts
07-17-2002, 11:34 PM
Hyper,
Wanted to get up there this past weekend. Just wasn't able to finish things up around the house. But yes, I want to get up there and meet you.

As far as the drive ratio, you can use the calculator to get close on that also. Just place 10% slip into the formula and leave the drive blank. It might not be right on, but it will be close. I believe the Alpha's came with a 1.5:1, but I'm not an expert on Mercruiser drives and gears used with different engines. The V6 may have used a higher number gear to allow the engine to turn a larger pitched or diameter prop. The Mercruiser website may be able to help you there if they still offer a V6 engine. Chances are they used the same gear ratio when your boat was built as they are today. Just a thought.

HyperDonzi
07-18-2002, 12:59 AM
thanks scott, i work at the restaurant there on the weekends. theres a 2 weekends ill be off and one of the days i will be here (the others in erie) ill get a date up when i know.

HyperDonzi
07-18-2002, 01:00 AM
thanks scott, i work at the restaurant there on the weekends. theres a 2 weekends (8am-8pm sat and sun) ill be off and one of the days i will be here (the others in erie) ill get a date up when i know.

boldts
07-18-2002, 01:12 AM
Sooo, you work until 8PM, that leaves a good hour to take you out in the Donzi if you'd like. Best bet for me to get up to Alum will be on a Sat. Send me an e-mail at: scott@mwdonziclub.com That way I can let you know when I'm sure I'll be there. Next weekend looks like a trip to MI.

HyperDonzi
07-18-2002, 01:17 AM
sent and check you private messages, sent one before you put up your email. sorry kamapmj for using this thread as a conversation box :)