Prop for '72 18

[A2VeeDub]

I am new to the forum so I guess I will introduce myself first. My name is Bryan. I live in Pasadena MD. I spend most of my boating time in the Chesapeake Bay, particularly the Magothy and Patapsco Rivers. My first interest was watercooled VW's (hence the screen name). I build custom turbo engines and such (Not much comparison to the domestic V8's in boats).

Anyway, my father aquired a '72 18 2+3 about 6 years ago. It had been knocked off the trailer in a car accident. The trailer was twisted. The boat had skid across the asphalt. It was cracked behind the rub rail on the passenger side from the point of impact and most of the chines were worn off on one side of the boat. It was sad to see, but good for us because we didn't have the money to buy a "nice" Donzi. It didn't have a motor so he got it for about $2k if I remember. We fully restored the hull and painted it back to the original color scheme. That was our first endeavor into major glass work, and I now have a lot of respect for those guys (There is an expert in the boatyard in our community that walked us through most of it.)

I have since bought the boat from my father, because he bought an '87 22. I think it is the "Special Edition" or something like that. This past winter we redid the half rotted transom and put an alpha drive on with a 1.5 gear ratio. I know you purists would curse me, but the drive was brand new and free. The engine is a half fresh water cooled, aluminum heads, 9.5-1, forged internals, 350ci small block chevy. Guestimate around 320-340 hp. It also has a 100 shot of nitrous. I am currently running a 3 blade 23 pitch mirage prop. Trimmed up the boat did 62 mph (gps) off the bottle at 4400 rpms. On the bottle it hit 5500 rpms at an even 70 mph. Not a bad rpm increase for the 100 shot. Problem is, if you run the numbers, prop slip goes from 7% off the bottle to around 15% on it.

So on to my question. Does this sound common? or should I be looking for a different prop to lessen the slip at higher speeds? The pitch seems right on and if I can get the slip down, I can always up the nitrous dose a little (125-150 shot) to get the same rpm for a few more mph.

Sorry for the long intro. Here are some pics. By the way I have been brousing the forums for a while and some of the info from here is priceless and you guys really know your stuff. Some of the guys picking out years by small details and hull numbers reminds me of the obsessive VW guys I know. Its great.

[BUIZILLA]

hello... and welcome.... IMO your about 50-60 hp shy of the 320 expectation if your only turning 4400 with a 23... a 270hp will turn about 4800 or so with a 23 Mirage and a 1.5 drive and run about 60-62... hope this info helps.

:shades:

[A2VeeDub]

I thought it seemed that the hp was less than expected. Doesn't matter that much. I was really wondering about the increased slip at the top end. The motor is strong. I can easily add a little more nitrous to get the same rpm with less slip. Numbers say I should get 74 mph at 5500 with the same slip I get at 62 mph. If I can get enough power to get closer to 6000 rpm I should approach 80mph. (Not likely with the current power) My goal for now is 75mph.

[JimG]

Wow! Boat looks great!

[Lenny]

Slip numbers going UP at the top end is odd. They should be going the other way. If you were at 7% at 4400 I would have expected 4-5% at 5K+.

Have you tried/borrowed a different prop from someone for comparison?

[A2VeeDub]

I haven't tried anything else yet. Any suggestions on types of props to start with? I think I can borrow a 22 pitch cleaver. A lot of people I know are running a lot heavier pitch, so prop selection is limited.

[mrfixxall]

Slip numbers going UP at the top end is odd. They should be going the other way. If you were at 7% at 4400 I would have expected 4-5% at 5K+.

Have you tried/borrowed a different prop from someone for comparison?


Lenny nos i uncontroled hp its 100 hp now! s i wouldnt doubt hehas tht much prop slip...i would be doing 60mph in my 72 monte carlo and drop it down to 2nd gear punch it and hit the nos switch and my tires would go up in smoke with a 100 shot..P.s. i took it off my boat in fear of braking my drive + i wanted to put my hatch back on with out cutting any hole in it..

[DonziBuoy]

The boat looks gorgeous, would like to see close up of your gauges and engine.

[A2VeeDub]

same X dimension. Wouldn't have minded raising it some, but already cut down the flame arrestor to clear the hatch now. Plus risers are barely rubbing the hatch as is and I couldn't bring myself to cut that hatch.

[A2VeeDub]

Pictures are the wife's department. I'll try to get some more. Probably be a few days because I keep the boat at my dads house.

[Just Say N20]

I had a 125 shot on a 1979 Donzi 18 2+3 that had a "tired" 260 hp Mercruiser in it, also running an Alpha drive. My rpm numbers sound like yours. I had a cleaver prop that was the original prop, and then a round ear prop.

The cleaver had a LOT more slip when I hit the N2O than the round ear prop.

I never held it for more than 7 seconds, and in 45 hours of run time, only used 3, 10 lbs bottles. Never had any engine or outdrive issues.

It is an interesting dilemma because you want a prop you can run without the N2O that lets the engine be where it wants to be (4,400+ @ WOT), but you really don't want to get much above 5,500 rpms (my opinion) with the Alpha drive. I would think 6,000 rpms would be flirting with disaster. They aren't that strong of a drive for big power bursts and high rpms.

Your speeds sound pretty good, but I realize fast is rarely fast enough.

[A2VeeDub]

My goal was to get to about 75ish. If I could reduce the slip some I could do it with 5500 rpms. I realize I might have to step up to the 125 shot to do it. I am very happy with the current cruise speeds. I can run 45mph at less than 3500rpms. Running in the Chesapeake I don't see to many days where I can use the nitrous anyway. I might use one 10lb bottle in 1-2 seasons.

[A2VeeDub]

Just Say N2O. What rpm's and speeds did you get on and off the bottle with the two props?

[Just Say N20]

Just Say N2O. What rpm's and speeds did you get on and off the bottle with the two props?

It was 15 years ago, and NO GPS back then. The boat had a Teleflex speedo that felt pretty accurate. This Donzi was my 7th boat, so I had a pretty good feel for speeds.

Cleaver prop; Speedo 58 mph @ 4,600 rpms, with N2O 62 mph @ 5,400 rpms

Round ear prop; Speedo 57 mph @ 4,400 rpms, w/N2O 65 mph @ 5,200 rpms.

[A2VeeDub]

Borrowed a 23 pitch cleaver. Went 64.4 (gps) at 4650rpm. That gives me a very low slip number (<5%).

Is it common to get slip that low in an 18 or do you think maybe my tach is off?

Something was wrong with the nitrous. It cut in and out on me so I didn't get numbers on it. I'll have to look into it more. Could be the fuel pressure cut out, or the wide open throttle switch. Bottle pressure was good.

Prop handles completely different. The boat turns a lot harder without sliding or leaning so much. It also feels like it has brakes when you back off of plane.