My Boat Videos - Gotta Love the 351

Jumping backwards a bit (didn't check in for a while), Thanks Ried and Pete! So it sounds like I've got the green light to ramble on & on & on & on...! Wait, I think I feel a three pager coming on!
Just kidding.
It was interesting what you were mentioning above about the flex factor of the NiBrAl. You usually see a lot of comparing between aluminum and stainless and folks don't mention the NiBrAl a lot. It's like "Oh that's what goes on an inboard" and that's about it. This has been a cool thread with a lot of neat info.

I had always noticed the huge differnce in the typical inboard props pitch (as compared to sterndrives and outboards). Didn't know for a long time that most inboards were close to 1:1 ratio. I think I just assumed it (in my ignorance) since the were direct drive and the pitches were so low. Most outdrives ratios are in the 1.5:1 or more (the old Volvos for V8's being largely 1.61:1 and then for 4 cylinder engines, as much as 2:1 and I think even higher). Noticed above the mention of 1.23:1 inboards.

Ried; you're right, the main wild variable is the slip factor. It's like that's what you're really calculating if you have known actual speeds, and then theoretical speed, rpm's and gear ratio. It's fun to try to predict the numbers. As mentioned, cup does some things too. Most of my rambling has been from memory from reading, experimenting, and talking to the guys at the Donzi web site. I gotta dig out my old Mercruiser "All you need to know about props" book. It is very cool and full of great info.

Wow; that's all I could come up with. I must have web writer's block.   

TRBenj wrote: 8122pbrainard wrote: Luciano, Tim (TRBeng) was doing a big summer test on props. Tim, Did you ever finish the prop testing and compile the results? The slip results should be very interesting because it is the big vaiable!! Prop testing is finished and I have a bunch of numbers. I havent crunched them yet, but have a pretty good feel for what I would recommend on all the 1.23 direct drive boats. There isnt a whole lot that would help all the 1:1 guys, as I was dealing with a totally different set of props. I will be putting together the full writeup and having Keith post it. Its on the to-do list.

Is it the same thing you've always recommended for the 1:23 boats? Any surprises?

Luciano, Tim (TRBeng) was doing a big summer test on props. Tim, Did you ever finish the prop testing and compile the results? The slip results should be very interesting because it is the big vaiable!!

Pete, I thought your comment was classic and I know you and Craig are pals. I think indirectly I was probably justifying my own longwindedness (is that a word?) and looking for company in Craig, since I'll give anyone a run when it comes to babbling on.

I fortunately or unfortunately share Craig's interest in the prop calc's but they continuously stymie me and mostly in the area of slip. I just keep playing with the numbers and along the way you learn prop sizes/results, engine power and speed as a function of rpms, etc., all the stuff a gear head or speed freak likes to know.

First off, Craig, you can NEVER write too much, so don't apologize. They just don't have to read it, or can skip thru it if they're not interested. But I know a few of us anyway are certainly interested.

As far as stainless on inboards, I've got one of them (12x13 OJ) running right now on my blue Mustang and I think it's fine, but I can't tell that it's any faster than the same size OJ I also ran/run in Nibral. I couldn't see the SS's being that big of an improvement mainly because I feel Nibral is such a good material, and while not as strong as SS, it surely gets cast to as fine and sharp an edge as any SS, which is a far cry from alum, which is never cast with a sharp edge on the leading or trailing side. Eric Johnson at OJ told me the SS wouldn't flex as much as nibral and theoretically could be faster than nibral with all things equal. That stainless sho is purty though.

Joe, on the question of damaging the SS's, they will certainly fold a blade just like the others, but maybe not as easily. So I don't know that you'd have escaped unscathed after solid contact.      

with the aluminums, i had a guy that his boat took forever to get up, he wanted prices on rebuilding his engines. the strange thing was he was getting 200 more RPM's at w.o.t, He was a friend so i suggested some new props first, it cured the problem, the boat had alot of hours and what was happening was the blades were flexing straight and not allowing the boat to pop up, the aluminums do wear as in this case

I don't know about Correct Craft, but Malibu and MC was using SS props in the late 90's. IIRC, they were made by CVC. They offered improved performance at the time, but the Acme and OJ CNC props are even better. Two big drawbacks with SS was some got stress cracks and shed blades, and if you hit something, you did far more damage to the drive train.

Craig, Our host's server is running out of memory!!!

Might be jumping ahead here, but found some good info. in my February 2006 Boating World on props; a neat piece from the Q&A section with a bunch of prop info.
It says (and I knew some of this) that the rule of thumb is that by adding an inch of pitch, you'll decrease rpm's by 200 to 300 (and of course the opposite would be true - subtract an inch to gain 200-300 rpm's).

Also says (something I didn't know) that the tendency, by going from aluminum to stainless, is that you will also experience some (amount not stated) reduction in rpms's.

And finally, that, as a rule, stainless props give better bow lift on heavier boats, resulting in better performance. And I think, just my observation, when people, speaking about props, say "performance" they're talking mostly about speed (unless the say something like "all around" performance).

So, applying all of this to my scenerio (my Cobalt - (same should apply closely to a 16' Donzi and loosely to an 18' Donzi)) it seems that if I start with what I am observing now (4400 rpms at WOT hitting 54mph with the 14x23 cupped aluminum prop. and my 1.61:1 gear ratio (GR) for the Volvo 280), I should be able to predict what the 14x21 cupped stainless will do (with an assumption about HOPEFULLY improved slippage thrown in). I plugged in the numbers and, since I know the speed with the 23 alum. I can calculate the percentage of slippage it's experiencing. I did that and have estimated that it is experiencing about 10.23% slip (if I'm doing this right). Theoretical speed, considering the 4400 WOT rpm's, 23" ptich, and 1.61:1 GR would be 59.52 but she's only actually running about 54 mph. 954/59.52=1.1023). Assuming I'd get the same slippage with the 21" stainless - it worked out to a theoretical speed (not adjusting for slip) of 60.52 mph.

Apply the same 10.32% slip factor and I got an calculated speed of 54.90 mph with the stainless.

This was taking into account that the 2" reduced pitch takes the rpms (up 400 to 600) from 4400 to 5000 (on the high side I know; I was being a little optimistic here)and then also the "change to stainless" effect, I estimated would drop the rpm's say 100 (?) to 4900. I felt that these seemed like reasonable possibilites based on all this jiberish!

So 54.0 mph to 54.9 mph? Hmmmm? Not a whole lot better.

BUT, if the slippage is decreased, and I'm guessing that that will be the case, based on the fact that a big reason stainless gives improved performance is that it doesn't flex as much as aluminum, I can calculate various reductions in slippage like so:

At 2% less slip than the alum., theoret. speed = 60.52 mph / slip (1.0823) = 55.92 mph.

At 3% less slip than the alum., theoret. speed = 60.52 mph / slip (1.0723) = 56.44 mph.

(Skip 4% less) (it came to 56.97 mph)

At 5% less slip than the alum., theoret. speed = 60.52 mph / slip (1.0523) = 57.51 mph.

Hey, I'd be happy with that because that's how fast she went (actualy 1/2 mph faster) with the old engine and the big ole' 27" pitch prop!

The up side to it all is that the 21" would be able to hold slower speeds better than the 27" ever could making it possible to pull wake boarding kids (as well as occassional 45 yr. old wakeboarders that need to go on a diet and take better care of their backs).

The old 14x27" was not so good for that (as you can imagine). I once told a prop shop repair guy that I used the boat for skiing (with that 14x27 prop)and he said something like "That's like trying to pull a Winnebago with a Volkswagon!!!!!!!!!!!!!!

So what does all this aluminum vs stainless prop junk have to do with a bunch of guys that all (98%) have NiBrAl (or bronze?) props under their boats?

I'm not sure! I just started typing and couldn't stop!

Reid,
Sounds like your expanding your knowledge of props into the realm of stern drives. Interesting stuff, aint it?

I didn't even pop over to the Donzi site to check, but I'll bet you were talking to Randy ("Big Grizzly" on the Donzi site). Am I right? I'd say he's the guru over there.
He's very knowledgeable about the props and I'm most certain I read (probably over a year ago) the stuff about him working with Solas. Pretty neat info. Randy has at least two "way cool" Donzi's, a (I think late 60's or early 70's) Inca Gold 16' Donzi with a Grumble Green stripe and interior, as well as an old 18' Corsican which is very sweet. I believe the 16 has a Mercruiser outdrive which was less common than the Volvos in that era, but because of the "better" gear ratio, it's pretty quick (well over 60 mph as I recall - maybe even 65 and that's screaming in 16' Donzi). His Corsican has a Volvo drive I believe and is probably the boat he did a lot of the Solas prop testing in.
I was able to find a new Michigan Wheel stainless 14x21 cupped prop for my Cobalt (same as the Donzi 16 and the boat in the video links at the beginning of this thread) but I've never tried it yet (2 yrs now) because my Volvo 280 drive is one of the ones that didn't have the threaded hole in the tip of the prop shaft. I haven't gotten into that project. The guys on the Donzi site assured me that it's no big deal but I'm a little reluctant to rush into it. I'll do it some time! The current prop runs the boat well in the mean time. It is a 14x23 aluminum Michigan cupped prop. It turns the engine at around 4400 rpms and the boat is hitting low to mid 50's (like 54). Lately it's even been a little slower. Haven't changed plugs, etc in a while. And I always use 87 octane. Just those two things might improve matters a bit (at least back to 54 mph). My goal with the 21 stainless is to hopefully get the rpms up to 4600-4800 range (which is recommended for my 351 Ford), in hopes that it might increase speed a tad. So many factors. It would at first seem that the increase in rpms might slow the boat down, but my hope is that the stainless will experience less slippage and therefore raise the speed a bit with the increased rpm's. Won't know til I try! Something to look forward to! You can go through the little prop pitch vs rpms vs Gear Ratio = theoretical speed (allowing adjustments to the theoretical pitch change due to cupping) and then, based on how fast the boat does end up going on the GPS or a good gage (vs theoretical speed), calculate the amount of slip. It seems the aluminum props I've tried are always in the high teens (percentage wise) for slippage and the guys with stainless often get less slip. I won't be getting much above high 50's (I don't think) any way you look at it because my HP is at 290 (and I believe that's rounded up). I have the old engine (needs rebuilt) that was rated at 290 and it was definitely stronger, but my current engine is a newer PCM crate motor disguised as the original Homlan Moody 351/290 hp (for the proper look). In the small print, of my paper work for this newer crate motor, I noticed it said 285hp in really small print so that's why I say it's rounded up.
The old engine would turn a massive Michigan Wheel 14X27" aluminum prop at 4600-4700 rpms and do a solid 57mph! That's pretty quick for any aluminum prop. And in the 16' Cobalt it was a blast. Still exciting but not quite as much. Hard to lose any top end after a bunch of work! But, being newer, it's been very dependable and it's really plenty fast.
Yeah, it's fun to fiddle with this stuff. As far as what's more fun (a deep V that slices through big ole' waves easily while the ole' speedo is dancing around the mid 50's, or turning on a dime so fast you can break your ribs (right Pete?) in a nice inboard on a smooth day?), --- that's a tough one for me. Almost like the chicken or the egg thing. As I slowly (very slowly) get my old inboard together, I'm looking forward with anticipation to having the same tough decisions to make!   
Actually this particular old inboard isn't going to be able to do the fast turns (apparently due to it's design), but if I get it back to where it's suppose to be, it will be able to hit the mid 50's in smooth water! Pretty good for a 17' inboard built in 1964! I can't wait!

ReidP; Biggest "thrill" of my summer? Driving that cool Donzi back down the lake in the early morning (Saturday) during our visit. Think we were coming back from chasing down the guys taking a boat to breakfast. All I know is it was fast!

john

Reid, Interesting about the pin settings. My brain is more tuned to a 16 and you're right the 16 behaves well with the pin on the low setting. I haven't heard from anyone that the almost total opposite occurs with the 18 with regard to the pin setting and how the boat behaves, but that's interesting. I think part of it may be that, as you've said, you've been mostly an inboard guy. The extreme deep V of the 16 and 18 Donzi does tend to shock people at first in hard turns who have been in other boats a lot. I'd say probably "especially" inboards because their degree of deadrise is so slight (many with nearly flat bottoms).

That is how these boats handle. I think it's fun, but you definitely gotta get used to it. Of course the big plus to that extreme V is that you can blast into big waves at high speed and fly without fear of being severly pounded upon landing. In fact, that's the other "surprise" with the handling of these boats. When you do that every "new" passenger grabs a rail and grits their teeth...
Then the boat lands with amazing smoothness and your passenger says "What happened? Why doesn't my spine hurt?"   

The down side is that while they have enough HP to take off quick, the extreme V makes it hard to maintain nice slow skiing speeds for things like tricks and wakeboards. The wake is big at those speeds, but hard to maintain.

Plenty of speed for barefooting!

I barefoot ski with my Cobalt at nearly wide open, of course the mid 50's speed gets reduced some by the drag (so I'm probably barefooting at high 40's - but I weigh (I think) 225-230? (I've lost track ). The porpoising can be a little bit of a problem. My wife doesn't like driving it when it does that and, getting up to speed to step off a slalom ski to barefoot, it does tend to hop some.

Thanks for the feed back Craig. In regards to the trim and manual placement on the highest pin, this boat surprisingly doesn't porpoise even at mid throttle. The prop will catch air in slow hard turns. The prop by the way is an original style, large blade aluminum. I don't even know where to start to find a stainless perf type. We only picked up the boat in October, replaced the dead engine with a stout alum head 302, and here we are. At higher speeds with a low pin setting the boat will grab the starboard chine and jerk suddengly in that direction and lean, but won't do this with a higher/higheest pin setting. Also, at the highest pin setting you get no cavitation/ventilation whatsoever. This boat by the way, came with a 289 Interceptor. The 16' with trim, conversely will porpoise anytime at low to mid-throttle unless the drive is almost all the way in. A completely differently set of manners from the 18. Headed out of town tomorrow for a few days of business and away from computer. Thanks again and we'll catch up later. Sorry everyone else for the boring non-CC babble.     

Yep, my Volvo drive has a plate that says Holman Moody Marine where the plate would normally be the model number plate that many will remember (270, 280, etc.) being used probably into the late 80's.
I've got way too many pics. I gotta get around to checking into the photobucket thing to post some pics.

Forgot to mention, reidp, you're right; there were Mercruisers with Fords too in a few of the Donzi's I've seen; 302's for sure. (mostly at a (I guess, "the") Donzi website - not the official Donzi company one). Those made a fast little set up because they had a gear ratio of 1.5:1 vs typical Volvo of 1.61:1. Makes a pretty good difference!

It is funny; when you think Mercruiser, you're usually thinking Chevy, but it wasn't always that way. Oh, there were also some Jet drive options (with classic Donzi's)and even Chrysler marine engine options. The Donzi 16 was even offered with outboard power - called a Baby Donzi. They're neat little boats and quite rare I believe.

Preach on, Brother X: I could read this kind of valuable info for hours. But rest easy, as I easily out-ramble you I'm afraid. Thanks Pete for egging him on. I'd like to see some more up-close boat and engine picts if you have them.

Riley, lots of Ford powered I/Os up thru late 70s and probably a dominant number. MerCruiser even used Fords for quite a spell along side the GMs. Still a handfull of Volvo and OMC Fords up thru mid-90s. Hard to find an early orig Donzi with anything but.
EDIT: Disregard most as Craig beat me to it above.

Holman Moody actually marketed the Volvo unit with their own name on it, HM that is.