What prop to use?

[donzi92]

:cussball: :shark: I have a 1992 23.5 foot donzi with a 454 king cobra on it what is the best prop for my boat? Right now it has a stainless 21 pithc three blade and it takes forever to plane out and not enough rpm while at wide open throttle. Can anyone help?
Thanks Chad

[boldts]

Hi Chad,

First, what type of boat do you own? (model) Reason I ask, is because if your measuring a 22 Donzi Classic at 23.5', your including the drive in your measurement. Also, if your having trouble getting the boat on plane and are low on the wide open RPMs, the boat must be much heavier than a Classic.

Do you have trim tabs? Are you using them in full down when attempting to get on plane?

Low RPMs at wide open could be a couple things. Are you running with your drive trimmed out or completely tucked in because of the planing problems? If you have tabs, are they completely in the up position when running wide open?

Last, the prop falls into the equation. Assuming the engine is in top tune and running like new, First indication from what you have stated, is that the prop pitch needs to be lowered. Your running a 21", try a 19" and see if she performs better for you. Can you tell us what your top RPM is? Also, can you give us a speed at wide open? Basic rule is for each 1" of pitch, you gain or lose 200 RPMs. I'm not familiar with the OMC prop models, but as a sponsor of the Midwest Donzi Club, Turbo Propellers I believe is now offering props for the OMC drives.

Just need a little more info. from you to give you help in where to start. After that, prop testing 101 is your next step. Good luck!

[donzi92]

Scott,

Thanks for the response the boat is a Z24 so yes it is a very heavy boat. I do have trim tabs that I use and it helps to plane out but should still be better. Even with the lower unit trimmed up while wide open I still do not get enough RPM still about 500 short of recommended red line. Also while selecting a new prop should I consider a four blade. Sometimes during heavy cornering the prop begins to cavitate. And the boat loses enough momentum that it comes of plane. Also what is a good web site to purchase props?
Thanks again, Chad

[RedDog]

Is this the boat? http://www.donzi.net/photos/osaffell531.jpg

Black Widow 24. Very nice! If not, see the model list link at the top of this page and help us zero in on what you have. 4 blade would help with cavitation but then an equal pitch as a 3 blade will mean yet less RPM. Are you sure the 454 is tuned / running to potential? The King Corbra has a 1.43:1 gear ratio. I ran a 22" 3 blade on my old 18 Classic with a good running 454 and King Cobra. Maybe you need to try a 19" 3 blade.

[donzi92]

Yes that is the boat! (except turquiose in color) The motor is brand new. unfortunitlly I justbought the boat last year and on my third cruise on it the engine ran tight on me, so it has a brand new motor. Is there a difference between a regular Z24 and a black widow?
Regards,Chad

[donzi92]

That being the boat, What is it worth in perfect shape and an e-z loader trailer? I have my first baby on the way and would think about selling it.

[RedDog]

...Is there a difference between a regular Z24 and a black widow?


I don't think there ever was a boat designated as a "Z24"

[boldts]

Chad,

Very nice Donzi. Great family ruff water boat. You see many more red ones than turquiose, but really, you don't see that many Black Widows period. If you'd like to sell the boat, I'd do a search on the boat in some of the "boats for sale" websites and get an idea of what they are being sold for across the country. I may have a gentleman interested in it as I sometimes receive e-mails from folks looking to buy a Donzi. I'd be happy to send you his contact information if you wish.

As far as your performance, like I said, I'm not that familiar with the OMC products, but with a 1.43:1 drive gear, I would think the 21" pitch prop you have would be right in the neighborhood for your engine. Most of the guys with 1.50:1 gears in the drive are running a 25" or more prop pitch. This is on a Classic though so obviously, there is a big weight difference between your Donzi and theirs. But, that is where your drive gear should be helping your boats performance.

The cavitation you are experiencing again might be a driver issue. Do you trim the drive in before starting a tight turn? This is the recomended procedure for deep-v boats. It brings the strakes back into contact with the water and helps the boat turn. It also brings the prop back into the water so that it does not slip. (cavitate while making the turn) Note: For those of you running a ventilated hull, DO NOT trim your drive in when making a sharp turn. It's a known cause for boat spin outs.

For where to buy or which prop to try, again, the internet is a consumers best friend. First, it allows you to find the best price once you determine which prop you want to buy. Most shops will allow you to try a prop and as long as it's not damaged, return it if it's not what your looking for. If you find a couple prop shops up in your area, give them a call and ask them what they would suggest for your boat. These folks sell props for a living and if they are not reputable, they won't be in business very long. Throttle-Up is used by many here on the boards, but they are in FL. Still might be a very informative phone call for you to find out information and then search closer to you for what they suggest you try. Another wealth of information would be the fine folks at Turbo Propellers in Indianapolis, IN (1-800-922-9955) They are a sponsor of our annual gathering in KY and a little closer to your location. Hope this helps.