Best prop for 91 Z24 with 7.4L to hit 60MPH?

[wrasse]

Just picked up a 1991 Z24 with 7.4 (I think it has 330HP?) and king cobra outdrive. It has a 20 pitch SS prop at the moment and I can get it to 55 MPH.

Would change out to 21 pitch help top end? Would be great if I can break 60MPH without spending too much dough.

[joel3078]

I have basically the same boat - 1990 24' black widow. Your gonna need more horsepower and a bigger pitch prop to break 60. Oh to do 70, figure about 500hp. Guess about 1-2mph speed increase with each pitch increase. Evenually you run out of horsepower with bigger prop and then you will actually loose top speed. Your top speed rpms on a stock king cobra 454 motor should be around 48-50K by the way. For now, try a 3 blade 14.5 x 21 or 22 prop and play with your motor trim and trim tabs to get the best top speed. A 23 pitch prop will be too big. Got any pics of what a Z24 is by the way? If it has the round hatch and round cuddy windows, it's a black widow. Does it look like the pic attatched?

[wrasse]

Hi. Thanks for your input. It looks like that one but the top hump part is white with red stripe. No pictures right now but will get some later.

My speedo reads 55 MPH when trimmed out. I'd love to go over 60 MPH true speed. What'll be the easiest way to get there you think? engine mods?

[joel3078]

Your boat is a black widow. It should have that name on 3 places. The stern back by the Donzi name with flag/griffin, on the dashboard, and on inside gunnel area - sides of back seat. If you have snap in carpet, the front piece was custom embroidered also with the name and a special widow spider graphic.

Engine mods to go 60 are all outside stuff. Gotta burn more fuel/air so do the exhaust, intake manifold, and maybe bigger carb. I see ads for used stainless marine exhaust on this site that would work on your motor. The 7.4L chevy engine in king cobra came in 2 editions. 330hp and 365hp. The 365hp is the high output verision. Talk to some local marine engine guys close to you for more details/advice. Speed costs money - I would guess 2 grand outta get ya to 60mph. Also, use a portable GPS to measure your speed. The dahsboard speedometer lies - usually you are really going slower that what the speedo shows.

[wrasse]

Hmm, the boat doesn't have a single place that says black widow. Title lists it as Donzi Z24

Also found a demo 22 pitch 3 blade prop and tried it today and it was a total slug. Couldn't push it more than 4000 RPM fully trimmed and actually lost top speed by 10 MPH from 55 to 45MPH. Will go back to original 20 pitch or try a 21 pitch if I can find one.

Also, I notice that after I ran it (smooth waters), the cuddy carpet is completely wet. Just the bottom areas, the sides and top are fine. Any ideas?

[wrasse]

With the 20 I think I was around 4700-4800 RPM.

Any ideas on the wet carpet issue? Hoping it isn't anything too serious.

[wrasse]

I'm not sure, the spline is 14.5 I think.

So there is a drain in the cuddy on the bottom somewhere? Where does this water come from in the first place? Wouldn't the auto bilge pump take care of it? Maybe the bilge pump isn't working right? Sorry, i'm a complete newb,.

[Donzigo]

The fuel cavity between the front and back of the boat has a small PVC pipe running through the bottom of it. The fuel cavity is sealed. My guess is that you have something (like carpet pieces) in that PVC pipe. Take a hose and shoot water into that pipe and it should drain in the engine room. Then, your water problems shold be done. If there is water after that, look for a hole/crack in your hull or a drain hose that isn't connected.

[joel3078]

Get us some pics of the boat. Sounds like a black widow without all the widow stuff was just called a z24. Hmmmm, learn something new everyday.
The floor drains in my boat are on the bottom corners of the back seat - were the seat bottom meets the floor. There are 2 of them that drain into the bilge area. Take your carpet out (it's snap in type?) and let it dry out in the sun. To unplug the drains, run one of those plastic barbed sink cleaner rods for clearing out hair from bathroom drain back and forth in your floor drains. Mine plug up sand, grass clippings, small leaves, etc. After you clear it out, shoot a garden hose down each drain to make sure she is good to go.
www.flexisnake.com is an option or go to walmart, ace hardware ,etc. and get a white plastic snake that has little barb hooks. About 2' long x 1" wide. Both of these items are less than 5 bucks.
Now that your floor is draining, why you getting water in the cabin in the first place? And let's be clear. You getting water in the cuddy cabin part or the main driving/riding area. I thought you were talking about the main driving/riding area. If your talking about the cuddy area, mine has a simple rubber/brass foldover transom plug that goes in the floor. Located front and center just as you step down into the cuddy area. Sorry no pics of that.

[wrasse]

It looks just like yours without the black widow markings. I'll post some pictures when I return from my trip.

The water is in the cuddy area. I noticed the drain hole in the bottom under the carpet just as you enter the cuddy area. This area and the area that flips up just in front gets wet on the very bottom.
This drain hole did not have a plug as you described. I filled the hole with water using a hose and it didn't seem to drain down to the the rear bilge even with the bow tilted up. Why am I getting water there in the first place? Is it front flow from the rear bilge area? Just the very bottom gets wet in the cuddy area. No leaks from the cuddy seen when washing the thing.

Any ideas on how that cuddy drain area drains? Is there a front bilge pump that's maybe not working?

[joel3078]

I have that fold over drain plug in mine. It's in the floor just as you enter the cuddy area. Don't know what it is for or how water would come up from that hole in the floor. I have never removed the plug. Put a plug in yours, take it for a spin and see if the carpet stays dry. Next time I take my boat out I'll look into it more and see what the heck it's for. I always thought that it was a floor drain that would go back to the bilge. NOTE: The round cuddy and hatch windows leak water if they are open at all. Also, those cool little round screens in the side port holes are easy to loose. They do a 1/4 turn and pop right out. Again, put a transom plug in the carpet hole and see if she stays dry.

[wrasse]

Maybe this is what's happening? http://www.offshoreonly.com/forums/showthread.php?t=98365&highlight=transom+plug

Maybe I should try pluggin the thing. Is there just one of these drain holes in the cuddy?

[wrasse]

I think this is likely it since both of you have plugs here and I don't. I wonder what this forward drain area is for anyways? Does it help with planing (get the bow down by getting water to the front to load up when stopped then drain back when the bow goes up?)

Great help guys, I'll post pics of my this boat when I get back, it's definately called a Z24 but without the black widow graphics and the black bulge area where it has a red stripe instead.



I have that fold over drain plug in mine. It's in the floor just as you enter the cuddy area. Don't know what it is for or how water would come up from that hole in the floor. I have never removed the plug. Put a plug in yours, take it for a spin and see if the carpet stays dry. Next time I take my boat out I'll look into it more and see what the heck it's for. I always thought that it was a floor drain that would go back to the bilge. NOTE: The round cuddy and hatch windows leak water if they are open at all. Also, those cool little round screens in the side port holes are easy to loose. They do a 1/4 turn and pop right out. Again, put a transom plug in the carpet hole and see if she stays dry.

[joel3078]

I think it's just a drain for the cuddy area in case water get in. Does nothing performance wise for the boat, hull, speed, etc. I'll ask another guy that has this same type of boat if I don't get mine out in the next couple days.

[FISHIN SUCKS]

I don't recall seeing a plug in my drain hole but then again I have never had any water in my carpet their either. I am going up to the lake today and will check it out to let you know what I see. Be back Sunday night.

My suggestion is to try a 14 1/4" x 21 pitch prop for your speed. I tried a 14 3/4" x 21 Lightning, was very impressed with the whole shot, but went from 5000 rpms to 4400:garfield: Motor has had work done to it as well, 454 + .030:yes:

Supposedly the Blackwidows were only in red and in black color schemes. However, I saw a teal one on ebay last fall, so who knows:confused: I know, Donzigo probably knows:yes: . That cat has impressed me everytime, I need to buy him a few :beer: and learn more.

[joel3078]

Tom and the rest of you. I ran into 2 more boats that I swore were black widows. There are not! They are Z-24. One was yellow/white and the other was teal/white. They all have the same shapped hull, deck, round hatch and round cuddy windows. From what I can tell, the black widow was only done in red/white with black trim strips and had black widow labels on the back hull, seat side areas, dashboard, and snap in carpet. The look alikes were simply called Z-24. Windshields seem to come in 2 versions. A 1 piece plastic molded with no frame, and a silver metal framed version that has pieces of glass or plexi.

As far as the cuddy drain plug goes, it's just that. Keeps bilge water from coming up into cabin. Or if Cabin gets wet, allows it to drain to bilge. If you store boat on lift, trailer, etc, keep it bow/nose high so all the water drains out the rear/transom plug.