Rookie needs some advice

[wufwee]

Hi, I just got my first Donzi. It's a 1990 Sweet 16 w/4.3 OMC. I'm not sure what prop to run as the one that came with this boat has been repaired and the #'s got ground off. I'm looking for as much top end that I can get. Any advice? I wonder what came on it from the factory? Thanks, Russ

[younger]

When I first got my 16 it had a 4.3 OMC HO. It is a 90 as well. If I recall it ran a three blade 21" pitch and would hit 53mph. Hope this helps!

[wufwee]

Thanks Younger, I would be happy with 53mph, for now, that's fast enough. Russ

[wufwee]

I checked with the former owner about the prop pitch. He told me he thought it was a 21. It has four blades. Will it make a big change for me to go from four to three blades? I was also wondering about the difference between aluminum and stainless for performance purposes. I do want as much as I can get with what I have, and I'm not worried about the extra $ for stainless if it is a top end increase involved. BTW, this boat is fresh water only (not sure if that matters or not). Thanks for your help, Russ

[Dr. David Fleming]

Since you are a self proclamed rookie - props are a very complicated subject and you will never fully understand them. Here are a few issues.

Pitch - this is the basic measurement of the screw factor in the prop, if it were in a solid material and it made one revolution how far would it screw forward in one turn. A 22inch pitch would travel 22 inches. a 33 inch pitch would travel 33 inches. Now we all would like to have the 33 inch pitch so that every time our engine turned one revolution of the prop we would be going farther and faster. There are limits, however, and most of us do not have the engine power to drive the 33" prop. You must have the right pitch for your boat and this may be several pitches and props. Your boat may act one way with one pitch and another way with another pitch. If you pitch it for speed it may not carry any passengers but yourself. IF you fill your boat with passengers and do not change to a low pitch prop you might not even get any speed at all. So you have to choose the right prop for the right boat for the right situation.

Blades - props come in two, three, four and five blades and sometimes more. Some US Navy submarines have ? seven or nine? blades. A single blade prop is the most efficient, but is not practical because of the imbalance of one blade so the two blade is the fastest with the least drag. In some racing situations it is absolutly necessary to run the two. Most modern boats use the three or four blade because of many complications in modern props and hull designs. Merc BRAVO III uses two propellors, one in front of the other each rotating in opposite directions which may seem totally illogical but is very efficient - good gas milage! Some propellors are mounted in front of the lower unit and these are called "tractor" props because they pull the lower unit. Conventional props are "pusher" props and are somewhat fouled by the water disruption of the gear case in front of them. Generally, "tractor" props run faster, but don't turn well because the gear case is in unsolid water behind the prop.

Metal - Props have been made in the past mostly of bronze becasue it is extremely repairable and is strong and corrosion resistant. Bronze props went into second place with the fine stainless steel alloys and designs of modern propellors. The steel can be made into thinner blades and these are like "ginsu knives" very nice to run on your boat. Aluminum props are the the modern "average" props - cheep to make, easy to repair and the new aluminum alloys of modern propellor design are great especially if you have a stainless prop stolen - big bucks.

Repair - props are like knives - You pay for quality - a "standard" prop is made and pitched for ordinary folks - you can get a stainless or aluminum "standard" prop - it has thick blades and if you hit something - like backing your boat into a police car - it won't bend too much. Props can be prepared for racing in a special way - this is called "lab finish" - labratory finshing - this means that a racing propellor expert has taken the time to prepare every blade to its optimum shape and made it very thin and sharp. Usually these props have a special serial number and a record of what they are all about. These "labed" props prepared for racing are used by many boaters for pleasure use and provide 3-4 mph more speed than the same "standard" prop. You have to be sure you have the right prop for your boat before you have one "lab finished" or you could be very disapointed about owning three or four of the wrong props just to decorate you boat house. "Standard" tainless props are about $600 the same "lab finished" prop is $900 ish. Its a great final touch to the perfect boat.

DAmage - Thats right props do get damaged - look at the blade edges are they knicked bent or show marks - this is bad - you want to have the prop looking perfect. You can hammer or file down the damage to get by, but every thing you do will not make it right - it has to go to the shop for repair. Most boaters take the prop off when convenient - its just a thing to do. Taking off the prop is not difficult - wedge a block of wood between the drive unit and the blade. Use a big socket wrench and spin the nut off - it is a special locking nut with a nylon insert to make it tight. Also there is special stainless steel lock washer which has arms to fold over the nut so it will not come loose. These must be bent out of the way when installing and removing the nut. You should be careful about replacing all of the parts just as they are suposed to on the drive unit. A special wrench called a "torque wrench" can be used to tighten the nut, but many people know how tight just by the feel of the wrench.

The Hub - The center of the driving hub on most Merc props is a special designed with a form of clutch to allow the prop to slip on the hub if you hit an object at high speed - this prevents breaking the drive gears and shafts but won't necessarily protect the blades from damage. There are several types of "clutch inserts" used in Merc props and the real race props have a solid hub.

Discussion - Some props have special blades, why? because they have special jobs. If you see a prop with one side of the blade perfectly flat like a "meat cleaver" this prop is designed to be operating half way in the air its called a "cleaver." If the back edge of the blade is turned over slightly this is called "cup" and the rear edge of the blade is bent over to cause it to grip the water better. This can make the prop act slightly higher or lower in pitch and can be used to fine tune the speed of the prop. The center of the Mercury props forms a tube - this is unique to Merc because they developed "thru hub exhaust" back in the 1970's. This was an advertizing gimick where the exhaust pipe of the engine fed into the center of the prop for a quiet exhaust on outboard motors. When Merc developed the stern drive V-8 engine they kept the design even though most boats use a transom exhaust. Some boats have underwater exhaust pipe diverter systems that allow the driver to flip a switch and send the exhaust into the prop. However, on the really fast Merc engines they do not connect this to the engine as it screws up the exhaust function in some way and so on these engines there is a broken connection to keep the exhaust from the prop.

Basics - This should get you started on props - it just gets more complicated from here. Merc probably makes six or seven props that would work on your boat - find a Power Boating magazine issue where they test a bunch of props on one boat and read up on how some props work in one situation and some props in another.

Donzi Registry - You are fortunate to be checking this site. Keep asking the members questions and keep learning - the people on this site have boats like yours and can tell you what is new and unique. Information you cannot get any where else in the world, even the Donzi factory.

[wufwee]

Thanks, Dr D. Great info. Russ

[ezervos]

Thanks Dr. D, that was the most cogent discussion of prop physics I have ever read. Your carbeurator screed was also quite informative. I know it takes a long time to put them together, please know that many of us uinitiated read and appreciate them!

[The Hedgehog]

My father has a 1990 16 and had a similar setup.

Younger is on the money with the #'s

You will go faster with the right 3 blade. No need for a 4 blade on that setup. It has a conservative X dimension (prop runs deep in the water) and comes out of the hole great with a 3 blade.

[z33donzi]

WOW Dr. D that was great what a book of knowledge :popcorn: i have to say this is a great site with great people most boaters are Its a group of folks that i am proud to be a part of thanks again Dr. D and to every one else

[wufwee]

You said that right z33donzi, I'm new on here, and absolutely amazed at all the helpful advice.