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View Full Version : Sterndrive props vs Inboard props



Ghost
08-02-2009, 04:17 PM
Maybe these are stupid questions and assumptions, but here goes anyhow.

In all cases, I'm referring to medium to high performance (40-60mph) boats from say 1960 to the present, but most of the inboards that fit that description are pre 1975 at the latest, so maybe 1960-1975 makes more sense. The early classic era lets say.

In general, I'm just trying for more perspective and understanding on why sterndive props and inboard props look like such different animals.

Assumption: the props used on inboard boats of this genre seem to have substantially less pitch than their sterndrive brethren. Maybe mid to upper teens. The props used on sterndrive boats have props typically from upper teens to upper twenties, and use more gear reduction in the drive.

If the above assumption is wrong, maybe most of the discussion goes by the wayside.

But I'm trying to understand why the props look so different. Certainly with sterndrives, it is clear that the hubs are much larger for obvious reasons. But beyond that, a couple other things seem very different.

Why the difference in pitch, if the boats are basically similar, why use more gear reduction in a sterndrive with a higher pitch prop? One would think the optimum diameter and pitch would be about the same for similar hull shapes and somewhat comparable speeds.

Blades on sterndrive props, on average, look to me like they cover more area. Maybe this is an illusion because the hub is larger, but I swear the blades look like they cover more area usually. If yes, why?

Anybody ever make a stainless inboard prop? I figure probably not often if at all, just because most inboards stay in the water, but curious if it's been done, to get more performance with thinner blades. Did early racers use any stainless props on inboards?

Obvious differences in I/O vs inboard, which I'm unclear about whether they explain some of the differences, include fixed shaft angle versus trimmed outdrive, and the smaller hub, and the materials (Ni-bral and such versus aluminum or stainless). Diameters may be different too, which I could imagine being tied to hub size in part if they are.

've seen some early I/O props that "look like inboard props", and one presumes they figured out better prop designs for the sterndrives later on. If so, did inboard props change too, and I'm just unaware, or did inboard props kinda stay where they were, and if so, why?

All thoughts, ideas, speculation welcome. (Knowledge and experience are even better.)

Just Say N20
08-02-2009, 05:36 PM
I'm sure that the fact that inboard props are NEVER running parallel to the direction of travel, and hopefully I/O props are pretty much always running parallel to the direction of travel has something to do with it.

MOP
08-02-2009, 06:01 PM
Just a thought, inboards that are about our size class most run 1:1 ratio that would account for the lesser pitch. I have also though about shaft angle and how it lifts the stern, maybe they have to make the prop in a way that reduces stern lift. The nice thing about inboards is the reliability they seem to run for ever, over the years I rebuilt or re powered quite a few boats rarely was any tranny work done. A Borg seems to be pretty bullet proof in pleasure boat.

DonziBuoy
08-02-2009, 08:38 PM
My 16 has a Michigan bronze 21 pitch inboard prop, meaning the hub is much smaller and the elephant ear blades seem to push a lot of water. The boat has a worked over 350 crate, and if I punch it, I can stand the boat on end. She's planing at 2200 and at 3000 I am flying. HOwever, after 3200 the engine (outdrive?) makes a straining sound, and I am not going much faster. I have thought that I am catching air and maybe cavitating. But I have seen so many outboard race engines with a small hub and obviously, it works for them. Any thoughts?

BigGrizzly
08-03-2009, 09:32 AM
Ghost, They do make SS inboard props. MOP is correct in his statements od gear ratios. Next in the old days all the props were the inboard style you refer to. The reason is technology was old and a guess at what would work. Props have advanced tremendously in the past 25 years, with the most in the past 15 years. The attack angle has a tremendous amount to do with prop design, especially with cupping and the type of cup. In an inboard the cup can cause plowing more then stern lift. the flow off the bottom of the boat to the prop is another issue. all this is evident in the speed difference in a V drive Donzi Vs an out drive one such as a 19 hornet. The out drive is always faster with the same power. Also ride is different. the v drive runs wetter too. I could go on for hours on this, but I will save it.

Just Say N20
08-03-2009, 10:34 AM
MOP. You've got to change your avitar! That thing is too weird. Creeps me out. :eek:

gcarter
08-03-2009, 12:02 PM
Inboard props are GENERALLY square, the diameter and pitch are in the same ballpark because the diameter of the inboard prop is less restricted in comparison to an outdrive prop. If you want a 30" pitch prop on a Bravo, you're still limited to a 15 1/2" diameter and the prop must look quite a bit different because the blade area is so restricted by space available in the outdrive, that a lot of rake must be added to make up for it, i.e., to regain the blade area lost in comparison to the inboard prop.
Is that sentence clear????:confused:
I think we all understand the reason for the large hubs, they allow better hydrodynamics than a small hub. Of course that large hub further restricts the available blade area.
Many of you have probably noticed that many racing outdrives have smaller diameter prop shaft bullets and smaller diameter hubs.

BigGrizzly
08-03-2009, 02:40 PM
Actually there are a few 16 diameter props for the bravo. The short drives limit the prop size. the 3 inch shorty will actually only be good to 15 1/8 yhe 2 inch 15 5/8 . The konrad will turn 16 and the TR dtiv does a 22.

Ghost
08-03-2009, 03:58 PM
Ghost, They do make SS inboard props. MOP is correct in his statements od gear ratios. Next in the old days all the props were the inboard style you refer to. The reason is technology was old and a guess at what would work. Props have advanced tremendously in the past 25 years, with the most in the past 15 years. The attack angle has a tremendous amount to do with prop design, especially with cupping and the type of cup. In an inboard the cup can cause plowing more then stern lift. the flow off the bottom of the boat to the prop is another issue. all this is evident in the speed difference in a V drive Donzi Vs an out drive one such as a 19 hornet. The out drive is always faster with the same power. Also ride is different. the v drive runs wetter too. I could go on for hours on this, but I will save it.


Inboard props are GENERALLY square, the diameter and pitch are in the same ballpark because the diameter of the inboard prop is less restricted in comparison to an outdrive prop. If you want a 30" pitch prop on a Bravo, you're still limited to a 15 1/2" diameter and the prop must look quite a bit different because the blade area is so restricted by space available in the outdrive, that a lot of rake must be added to make up for it, i.e., to regain the blade area lost in comparison to the inboard prop.



Thanks, this stuff is interesting.

I see the point about the size restriction on sterndrive props. One thing I don't get is how the choice was made to build sterndrives with reduction ratios in the 1.5:1 to 1.8:1 range, pretty serious reductions, where inboard boats like mine are much milder gear reductions, not far from 1:1. Naturally, the props must fit the reduction ratios. Somehow, I figure someone concluded that somewhere about a 1.6:1 reduction was a good thing for optimizing the sterndrive. What I don't get is why that would be. Is that one ultimately driven by prop diameter restrictions?

If it is, I probably have similar prop diameter restrictions. Maybe I could handle a 15 diameter. Maybe. But in theory, my boat could have a reduction more like a sterndrive, and take a prop with far more pitch than the 15,16,17 inchers I could hang on it now.

I wonder if that would be better, swapping the gears to more reduction ratio and upping the pitch? (Oops, I said "better". I guess, we haven't even touched upon the whole top-end-speed versus holeshot versus range of good performance. But for now, maybe top end speed is a good thing to consider. Constrains the variables in the discussion anyhow.)

gcarter
08-03-2009, 05:33 PM
Mike, unfortunately, your boat was set up for just how it is. And geometrically, it would be hard to change.
Maybe you could use an in-line reduction gear of 2:1 and increase the pitch significantly. It would make for a more efficient boat. But you probably don't have room to do this w/o moving your engines further forward and higher. You see where that goes!:shocking:
You can also increase increase the diameter slightly, but you have to maintain an edge clearance of at least 20% of the prop diameter.
Good luck.

Ghost
08-03-2009, 07:05 PM
Mike, unfortunately, your boat was set up for just how it is. And geometrically, it would be hard to change.
Maybe you could use an in-line reduction gear of 2:1 and increase the pitch significantly. It would make for a more efficient boat. But you probably don't have room to do this w/o moving your engines further forward and higher. You see where that goes!:shocking:
You can also increase increase the diameter slightly, but you have to maintain an edge clearance of at least 20% of the prop diameter.
Good luck.

Thanks, while I'm probably not interested in blowing the cash on it (yet), I think I could get replacement gears for my v-drive and change no geometry at all, but get a 1.77:1 reduction instead of my current 1.15:1 ratio. Probably can't go up too much in prop diameter, maybe from 14 to 15. But still, just knowing it might be a real upgrade option is very INTERESTING. Appreciate the thought.

If anyone has any notions of what this might do for performance, feel free to speculate wildly... ;)

gcarter
08-03-2009, 07:15 PM
Thanks, while I'm probably not interested in blowing the cash on it (yet), I think I could get replacement gears for my v-drive and change no geometry at all, but get a 1.77:1 reduction instead of my current 1.15:1 ratio. Probably can't go up too much in prop diameter, maybe from 14 to 15. But still, just knowing it might be a real upgrade option is very INTERESTING. Appreciate the thought.

If anyone has any notions of what this might do for performance, feel free to speculate wildly... ;)
Ahhh...V-drive! :banghead:
I should have known.
There're some nice integrated tranny/reduction gear/and V-drive in one tidy package.

MOP
08-03-2009, 07:16 PM
Just route your V drive to an Arneson, get rid of all the under water restrictions.

zelatore
08-03-2009, 07:41 PM
Don't forget, you can't get the blade tips too close to the hull or you'll get some nasty vibrations.

Personally, I think you should just pop for some of the nifty ZF 2-speed gears. I've always wanted to try them.

Come on, you know you can afford it.

gcarter
08-03-2009, 08:18 PM
I think Mop's idea is good.
And here's a different slant on it. I mean if you want to really change things......
Surface props, outboard rudder, could be lots of advantages;

http://www.donzi.net/forums/attachment.php?attachmentid=48884&stc=1&d=1249348669

Ghost
08-03-2009, 08:36 PM
Sweeeet. Now some suffiently wacky stuff is getting kicked around.

Of course, would any of that stuff even compete with spinning the engines around, pulling the v-drives, and slapping a couple of outdrives on?

I have to say I am intrigued what top end mph delta would result from a 1.77 reduction gearset and an aggressive set of stainless props, relative to the best I could do with my original setup, propped appropriately.

Hmmmmmm....

BigGrizzly
08-04-2009, 06:24 PM
Your going to get into something really strange. Remember some drive do not like big diameter props while others thrive on then. Also Like Zel sait too close to the hull gets big time vibrations. it also gets big time cavitation. This is why some props come out after the stern anf the rudder is ona stand off.

silverghost
03-24-2011, 01:33 PM
To revive this old inboard & V-Drive propeller thread a bit.

There are som FANTASTIC new Inboard & V-Drive High-Tech propeller designs available today ~~~For very reasonable money $$$$ !

I have a 24 foot 1988 American Skier Eagle Inboard ski-boat.
This was a limited production, High Priced Euro-style, ski-boat.
Possibly the biggest performance ski boat ever built ?
It was built with a money is no object mentality !
It looks like a smaller Fino.
The Rolls~Royce of Ski-boats.

This boat was buit by Kenny Elkind a famous offshore racer, & young friend of "Donzi" Don Aronow.
Kenny also built the famous Monza race boats , along with American Skier & many other boat brands.
Don Aronow according to Brownie & Michael Aronow crashed a young Kenny Elkind's jet-boat on an infamous ill-fated test drive. You may have read, or heard
about this infamous incident ?
What a Great Don Aronow & Kenny Elkind story !


Inboard Prop Test Results.

BOAT: 24 foot American Skier Eagle Inboard Ski-Boat
Weight 3500 lbs.
Beam 8 feet !

Boat Engine Specifications~~~
FWC Indmar Chevy 4 bold main 454 HO straight conventional inboard engine installation .
400+ horsepower ?
850 Carb & marine performance cam.
Water-cooled Marine
Header-Style 4" exhaust manifolds to 4" under deck & under water exhaust until the boat gets up on plane.
Direct drive 1-1
NO reduction gear ratio !
13* prop shaft angle
1" Aquamet 22 SS shaft
Warner Velvet Drive 72C BAM beefed-up marine gear transmission.

PROPELLERS TESTED ~~~

It was originally set-up strictly as a waterski-towboat .
14X12 NiBrAl 3 blade Michigan Dyna-Jet ski-tow prop.
A very OLD prop design !
Top speed 55 MPH GPS !
Tons of pulling & towing torque.
Lots of wasted energy & horsepower.
Great low wake for water-sking.

Since I do not water-ski
so much anymore & wanted more top speed with more fuel economy I swapped my old ski-tow prop for a
13 X 16 heavily cupped Stainless Steel Central Valley Propeller race/performance unit and the boat now does 66 MPH GPS !
The stern also dug-in much less.
Much less wasted power & energy !

I then swapped to a Stainless Steel
OJ Force 4 blade heavily cupped 13 X16 Stainless Steel performance prop.
This prop also has a heavily aft angle raked blade angle which pushes the tips of the blades aft & much closer to the rudder. Approx 1/2" away !

This boat now does 68 MPH GPS and has much less vibration with the 4 blade Stainless Steel OJ prop.
It is very smooth running; with a fantastic hole shot !
The stern does not dig-in as much.as the three blade SS CVP or Michigan Dyna-Jet NiBrAl props.
It now also turns on a dime & rides like it is on rails !
The boat planes much better ; and the hull rides higher & dead level !
It now handles like a different speedboat !
It is much faster & is much more responsive in handling !

My Conclusions~~~

My ski-boat inboard hull has come alive !

AND~~~ I think I have much more top-end speed in this hull to go as I am red-lining my RPMs with more secondary in my carb left unused~~~
5000 RPM !
I did not want tp press my luck !

I NEED MORE PROP PITCH !

There are many more modern High-Tech Performance propeller choices for inboard & V-Drive performance applications today !
Not like 30-40 years ago when the Donzi Inboard V-Drives were first built !

Most Inboard & V-Drive Donzi owners have been using props that were designed in the 1930s-1940s
by old time racers like Garr Wood; and little has changed on these old prop designs since WW II ! ! !
They push a lot of excess & un-necessary water and use a fantastic amount of wasted horsepower ~~~
But your boat's speed using these 50-60 year old prop designs is very limited !

Some very high Tech inboard props can be bought today for -$350-$450 .
A lot of Bang-for Your -Buck $$$ in my opinion !

There are also some fantastic High-Tech CNC Milled 3 & 4 blade NiBrAl props out there today too ! $450-&600
OJ, ACME, Mchigan & others offer these !
I hear great things about them ; but have not tried a CNC milled prop~~~
At least not ~~~YET !

Inboard boat racers have used Stainless Steel props for decades~~~
They used to be very expensive !
Many were custom one-off built designs.
Not So EXPNSIVE Today !

Someone with a V-Drive Donzi inboard should experiment a bit with these newer High -Tech prop offerings !
I KNOW you will gain more speed & handling performance than with the 30-40 year old out-dated design Antique bronze & NiBrAl propellers you have all been running on Donzi Inboard V-Drives for decades !

You all saw what Grizz & his Special Volvo Solas props did for the old 270-280 Volvo~Pennta outdrives !
They brought these old Volvo drives to life !

These newer performance props will bring your old Donzi inboard V-Drive to a new High-Speed performance level with much more Life & handling !

Try a new modern High -Tech inboard & V-Drive prop~~~
You will LOVE the results !

Why are you still running with a 60 year old propeller design on your Donzi V-Drive ? !

I believe Grizz would be asking us this same question today ~~~~
He was THE Donzi Propeller Go-To Guy !

We are all missing Randy more every day !

R I.P. Grizz !