Outdrives

[Jim Bow]

Looked at the Boston Whaler jet boat. Very cute, but ...
the seller had only owned it for a couple of months, very nice guy, but I think it’s the small boat version of a real estate flipper. My son is gonna do a test drive for fun.

Next question: should he stay away from inboard-outboards? Does the industry even make them anymore?

[Curtism]

Can't speak to your " next question " but good move shying away from the whaler. Donzi built a little 13' bow rider jet boat with those OMC jet drives years ago. (Sort of ashamed to say I lofted that homely thing.) It didn't sell well and the ones they sold went on craigslist a couple years later. The powerplants were problematic.

[oldsub86]

Inboard outboards - MerCruiser and Volvo are fine
Stay away from any OMC as they have been out of business for a long time
Also Yamaha - only made for a year or 2 and parts are unobtanium

[Bob (oh, THAT Bob)]

Looked at the Boston Whaler jet boat. Very cute, but ...
the seller had only owned it for a couple of months, very nice guy, but I think it’s the small boat version of a real estate flipper. My son is gonna do a test drive for fun.

Next question: should he stay away from inboard-outboards? Does the industry even make them anymore?

I don't know if you can bolt a prop stern-drive on in place of the jet, on that same engine. On a jet, the engine runs a pump, sucking in water and blowing it out the stern. On an I/O stern-drive, the engine sends power aft to a transmission (either in the hull or the outdrive), and power runs horizontal aft, then through some crazy gear or U-joint (I don't know) that allows the outdrive to tilt, then vertically down to the prop gearcase, another right angle drive. IIRC, each right angle gearset costs something on the order of 5% of the energy due to friction.

The trend has been away from I/O stern drives for a long time. Modern boats either have an inboard engine with a straight shaft aft to a prop underneath the boat, or increasingly massive outboards, and in some cases multiples of them.

My view is don't take the boat if there is any possibility of having to put money into it. It's not very versatile nor saleable. The only plus is that the engine is a 4 stroke (yes?) but probably does not have a closed cooling system (no?) so has had salt water run through it; Even with flushing with fresh water after every run (and that is a big question), you could still have problems with internal corrosion resulting in things like head gasket problems, and then the cost of fixing exceeds the value of the boat.

Useless boats have high disposal costs. Ask Sleek about that.

http://forum.woodenboat.com/showthre...ite-conversion

[BrianW]

It's pretty simple, having owned and operated all three choices... stay the f*** away form outdrives.

Only two acceptable forms of engine driven propulsion... Straight Inboard, or an Outboard.

Anything else involves too many holes below the waterline.

[Reynard38]

Seems that some of the only new boats that favor stern drives are the larger go fast boats. They typically run massive forced induction V8’s making as much as 1350hp.
With the availability of Outboards making 450hp most other boats avoid the complexity of an I/O.

[John Meachen]

It's pretty simple, having owned and operated all three choices... stay the f*** away form outdrives.

Only two acceptable forms of engine driven propulsion... Straight Inboard, or an Outboard.

Anything else involves too many holes below the waterline.

An honest and comprehensive answer that needs no further comment.

[Art Haberland]

I can remember back to the 80s when I/Os were all the rage. Every summer at least one boat sunk at her slip due to the big seal around the outdrive letting go

[Breakaway]

Sterndrives are more complex than inboards, but they offer much better speed and efficiency for the same power than a shaft strut and rudder, due to the ability to trim and because of less drag. And steerable thrust gives one much more control over maneuvering versus a fixed prop and rudder. Hit something or run aground? The drive kicks up instead of the rudder post holing the boat.
.
Versus an outboard, a sterndrive offers a " clean" transom which is an aesthetic some prefer, and also a full width swim platform, which is great if you spend a lot of time swimming or hanging out at a cove or sandbar.

They are heavier than an outboard for the same power and take up room in the boat where stowage, fishwells etc might otherwise reside.

Where they fall short is in two areas. One, corrosion gets them in saltwater. In freshwater, boaters enjoy longtime reliable ownership. Not that people don't use them in the brine, but I personally would advise against it.

Second, they are less tolerant of deferred maintenance than other propulsion choices. An example is what Art wrote: change the bellows annually or it may sink in the slip. Keep up with the anodes. Change the gearcase lubricant. Service the gimbal bearing...plus all the maintenance required of an inboard engine.

Kevin


Sent from my iPhone using Tapatalk Pro

[scoutabout]

I’d say @Breakaway is on the money here. We’ve owned and used both stern drives and outboards by turns for decades and all comments ring true, although I will comment that all thru hull seals are easily examined annually for deterioration. Having a stern drive sink due to bellows failure is 100% avoidable. I know, I’ve sunk one...
Currently we’re running a 4 stroke Yamaha 100hp outboard on the fish n ski. Just a jewel of a power plant. Quiet, smooth, ultra reliable and fuel efficient (“efficiency” being relative of course but 2.5 gal/hr at a 20 knot cruise works for me.)
Back in the bad ol days of stink and smoke and pouring oil overboard that was the reality of two-stroke outboard ownership I couldn’t wait to get into a stern drive boat. Now, since the rise of the 4-stroke outboard I’m not sure if I’d ever go back.

One other plug for outboards is ease of maintenance and winterizing. Almost all outboards will drain all water when vertical vs stern drive motors that need various ports opened in the engine block and related plumbing to achieve same. For us in the land of hard water half the year, this is helpful. Also, I very much enjoy standing beside​ the Yamaha to service it vs contorting myself into all sorts of unpleasant positions to work under the sun pad on the Mercruiser...

[Bob (oh, THAT Bob)]

I’d say @Breakaway is on the money here. We’ve owned and used both stern drives and outboards by turns for decades and all comments ring true, although I will comment that all thru hull seals are easily examined annually for deterioration. Having a stern drive sink due to bellows failure is 100% avoidable. I know, I’ve sunk one...
Currently we’re running a 4 stroke Yamaha 100hp outboard on the fish n ski. Just a jewel of a power plant. Quiet, smooth, ultra reliable and fuel efficient (“efficiency” being relative of course but 2.5 gal/hr at a 20 knot cruise works for me.)
Back in the bad ol days of stink and smoke and pouring oil overboard that was the reality of two-stroke outboard ownership I couldn’t wait to get into a stern drive boat. Now, since the rise of the 4-stroke outboard I’m not sure if I’d ever go back.

One other plug for outboards is ease of maintenance and winterizing. Almost all outboards will drain all water when vertical vs stern drive motors that need various ports opened in the engine block and related plumbing to achieve same. For us in the land of hard water half the year, this is helpful. Also, I very much enjoy standing beside​ the Yamaha to service it vs contorting myself into all sorts of unpleasant positions to work under the sun pad on the Mercruiser...

Agree with your reasoning for the rise of large outboards over stern drives. However, while a stern drive takes up hull space, the weight is further forward and lower which is nice. Those huge outboards in multiples are an awful lot of weight hanging high on the stern (unless on a short motor platform), but they are designed for it.

Breakaway: Stern drive lower drag than propshaft and rudder? I would think not, fat lower unit versus sleek rudder and prop strut, unless considering that with vectored thrust, the lower unit can be much smaller than a rudder, so perhaps.

I had not realized until now, the threat of sinkage with a stern drive. As someone else reminded me, this is also an issue with Volvo SailDrive (an inboard engine driving an outboard-style lower unit poking straight down just behind the keel), which I already avoided like the plague because of bad reviews online due to waiting in port months for service parts, and I would also imagine, water intrusion into the lower unit that is always in the water. Potential sinking drives another nail into that coffin. If I ever get a keelboat, it'll be a conventional inboard diesel driving a propshaft, but bonus points for electric drive, or reversible electric motor between transmission and prop (with trans in neutral when running electric), or electric motor with small generator as hybrid drive in addition to batteries and solar cells.

[Breakaway]

Breakaway: Stern drive lower drag than propshaft and rudder? I would think not, fat lower unit versus sleek rudder and prop strut, unless considering that with vectored thrust, the lower unit can be much smaller than a rudder, so perhaps.

The gearcase is streamlined, plus it tilts up which further reduces drag and keeps the thrust vector almost all forward rather than at +/1- 12-degrees.

The inboards shaft, strut and rudder are way more draggy.

There are references for this--Dave Gerr's Nature of Boats is one that comes to mind. But, all one need do is run comparably-sized, comparably-powered shaft inboard and equivalent sterndrive and it becomes readily apparent. The fuel burn is lower, the speed is higher.

[skuthorp]

I have a long shaft drive I built from a thrown out brush cutter for my Mac canoe. Noisy, and fumy till I directed the exhaust underwater via a flexible tube. Mounts on a hand made rowlock based on a gondoliers fitting with a rope retainer.

[Bob (oh, THAT Bob)]

The gearcase is streamlined, plus it tilts up which further reduces drag and keeps the thrust vector almost all forward rather than at +/1- 12-degrees.

The inboards shaft, strut and rudder are way more draggy.

There are references for this--Dave Gerr's Nature of Boats is one that comes to mind. But, all one need do is run comparably-sized, comparably-powered shaft inboard and equivalent sterndrive and it becomes readily apparent. The fuel burn is lower, the speed is higher.

I believe you, and most especially the part about trim. You could put on trim tabs (at extra drag) to raise the stern, but not raise the bow; For that you need tilt.

However all the premium water ski boats now, I see are inboards, at least I thought. That has the advantage (versus outboard) of putting the engine weight further forward for easier (sooner) planing, and a clear stern so the ski line from the (post thinghy, can't recall the proper name) doesn't get fouled on the motor casing. I don't know if wakeboard boats use an I/O or outboard for tilt to make a larger wake or not.

[Reynard38]

Low drag, no gasket to deteriorate, inboard motors. Cool video.

[Nicholas Carey]

All I know about Whalers is that they're foam-cored glass boats. If was going to get it, I'd want to weigh the hill and see the delta between the spec'd weight and the actual weight. How much water has it taken up?

There is, IIRC, a certain history of Whalers and waterlogged foam cores.

[Nicholas Carey]

Low drag, no gasket to deteriorate, inboard motors. Cool video.

One could always put a longtail on it"



There used to be a guy here in Seattle who had a Vietnamese longtail.

Think big engine on a pintle mount attached to a very long prop shaft hanging out over the transom. The Ur-surface piercing prop. When the water gets thin, you just lift the prop so it's [mostly] out of the water. Might sport a rooster tail of mud when it gets really thin.

This was a sampan with a surrey top, solid teak, maybe 30-35 feet LOA. Powered by an old Chrysler flathead 6 (300 cu in?) ripped out of an old car. Still had the 3-speed tyranny with the clutch and gas pedal on the floor.

He could carry maybe 20-some people. Topped out a bit north of 30 mph or so.

It were a great party sled.

[scoutabout]

Low drag, no gasket to deteriorate, inboard motors. Cool video.

That’s freakin incredible - So two props half out of the water for half a revolution equals the thrust of one that’s submerged all the time?

[Bob (oh, THAT Bob)]

All I know about Whalers is that they're foam-cored glass boats. If was going to get it, I'd want to weigh the hill and see the delta between the spec'd weight and the actual weight. How much water has it taken up?

There is, IIRC, a certain history of Whalers and waterlogged foam cores.

The foam is closed cell. If the boat doesn't sit low, the water logging can't be deep. But I think it could affect the surface of the foam over time, water does get into the void via the center helm and such, specifically the wiring and control cable paths down through the floor and aft. I was around some older ones in years past and wondered that same thing and if over the winter, not only the access hatches left open, but perhaps a vacuum hooked up for long periods to improve evaporation rates, either by drawing air through, or generating a mild vacuum* would help correct if needed.

* You have to be careful there. On a certain keelboat, the vent was plugged for the head waste tank integral to the bow, and when it was sucked out, the hull side pulled in enough to delaminate.

[rbgarr]

My understanding is that I/Os, followed by jet drives and then jet skis were initially designed for the trailerable, freshwater market, which significantly outnumbers the saltwater market. As mentioned, fresh water doesn't corrode the engines or drive units anywhere near as much as in saltwater use. The engines that mate to the drive units are also cheaper in general. I would never purchase the potential problems of a used I/O or any of that type unless it was always a trailered boat and I lived on fresh water. I don't.

[AlanMc]

That’s freakin incredible - So two props half out of the water for half a revolution equals the thrust of one that’s submerged all the time?

not really. the prop design for that application is totally different. here's a basic intro https://www.experts.com/articles/sur...-by-paul-kamen

[scoutabout]

not really. the prop design for that application is totally different. here's a basic intro https://www.experts.com/articles/sur...-by-paul-kamen

Thanks Alan - will check that out.

[Reynard38]

not really. the prop design for that application is totally different. here's a basic intro https://www.experts.com/articles/sur...-by-paul-kamen

Alan, thanks. Great article. I always wondered how those worked.
I may install a jack plate on my Superboat at some point. The motor has a low water pickup already. With a cleaver style prop I could lift the motor underway. Previous owner and I talked about this. He thought it’d raise the top end over 80mph

[AlanMc]

i learned all about them from a world class RC boat racer i know. i think the fastest boats they race use the chopper type props. he hand tunes all of them for different performance features.

[Breakaway]

However all the premium water ski boats now, I see are inboards, at least I thought. That has the advantage (versus outboard) of putting the engine weight further forward for easier (sooner) planing, and a clear stern so the ski line from the (post thinghy, can't recall the proper name) doesn't get fouled on the motor casing. I don't know if wakeboard boats use an I/O or outboard for tilt to make a larger wake or not.

Ski boats and wakeboats are specialized boats. For skiing--at an advanced or competive level--you want a dead flat wake and cornering and a clear transom, as you say. They are used in session of an hour or so and than put away. Efficiency is not high on the design priority list.

Wakeboats ared esigned to ru best at about 12-17 mph, carrying a lot of water ballast ( 2 tons for a 22 foot boat say) plus 8-10 crew. This to create maximum wake size. Again, efficiency not a design priority. Furthermore, for wakesurfing ( not wakeboarding) takes place 8-10 feet behind the transom. This makes ouboards and sterndrives less safe, as a rider falling might fall into the exposed prop. With an inboard, the boat is also tucked under the hull, forward of the transom.

Volvo Penta developed the Forward Drive about five years ago specifically to address this issue and take some market share from inboard/ watersports boatbuilders.( which they have done)

Screen Shot 2020-08-14 at 11.45.26 AM.jpg

Finally, as with many boating groups, watersports enthusiasts have a culture, or subculture, of running inboards. "That's what we've always done so that's what we do." To the devoted and hardcore, its only a "real" towboat if it is inboard powered.

Kevin

[Breakaway]

I may install a jack plate on my Superboat at some point. The motor has a low water pickup already. With a cleaver style prop I could lift the motor underway. Previous owner and I talked about this. He thought it’d raise the top end over 80mph

Make sure you swap out your prop hub for an HD solid hub if you do that. The Flo Torq hub rubber wears out quick in surface-piercing applications.

Originally Posted by Reynard38

Low drag, no gasket to deteriorate, inboard motors. Cool video.

Fast and efficient. Trimmable. Very specialist.

Try docking with surface drives and it makes picking up a mooring with a sailboat in wind against current look like child's play.


Kevin

[bluedog225]

One could always put a longtail on it"



There used to be a guy here in Seattle who had a Vietnamese longtail.

Think big engine on a pintle mount attached to a very long prop shaft hanging out over the transom. The Ur-surface piercing prop. When the water gets thin, you just lift the prop so it's [mostly] out of the water. Might sport a rooster tail of mud when it gets really thin.

This was a sampan with a surrey top, solid teak, maybe 30-35 feet LOA. Powered by an old Chrysler flathead 6 (300 cu in?) ripped out of an old car. Still had the 3-speed tyranny with the clutch and gas pedal on the floor.

He could carry maybe 20-some people. Topped out a bit north of 30 mph or so.

It were a great party sled.

I was looking at these this morning. I can’t figure why they mount them so high up. Must be hell on stability.

[Bob (oh, THAT Bob)]

not really. the prop design for that application is totally different. here's a basic intro https://www.experts.com/articles/sur...-by-paul-kamen

Thanks for that. One would think that the constant impact of the blades from air into water would be heck on durability, but I guess not.

[Bob (oh, THAT Bob)]

Screen Shot 2020-08-14 at 11.45.26 AM.jpg


Kevin

Thanks for the text info. Regarding the design above, clever for the application, but should be inefficient, with the pusher prop blowing aft on the lower unit. An advantage of any "puller" fan such as the conventional aft mounted prop, is that it pulls flow in from the sides and around the lower unit, though this may be less pronounced on a prop moving at speed through a liquid; Puller (air) fans on automobile radiators, generating a lower pressure zone in a fan shroud larger than the fan itself, are notably more efficient than pusher fans, the latter of which generates a torus (donut) of air flow blasting straight back.

[Breakaway]

but should be inefficient,

Again, efficiency is not the highest priority for boats aboard which the operators intentionally ballast the boat to the max and run at high angles of inclination at barely hump speed( 10-12 knots is about optimal for wakesurfing) in order to generate the biggest wake possible.

And, compared to what? A conventional pusher prop sterndrive? Or, versus the inboard powerplants it was designed to compete against?


That said, its ( Volvo Penta) been doing tractor props in recreational marine applications for going on 15 years. Here is there IPS system, which when compared to straight inboards powered with the same engines delivers markedly better speed, efficiency and handling. But that's efficiency versus shaft strut and rudder.

Screen Shot 2020-08-14 at 5.07.52 PM.jpg

[Breakaway]

Thanks for that. One would think that the constant impact of the blades from air into water would be heck on durability, but I guess not.

It does happen. In fact, the boat in that video is being run by a friend of mine. He did break blades off both props during a run from NY to Bermuda. There is a video of that somewhere out there... a blade comes flying over the cockpit.

Here's a still pic. For perspective they are 19" x 44" props.

Screen Shot 2020-08-14 at 5.15.41 PM.jpg

Kevin

EDIT: Another image. That's Tyson on the right.

Screen Shot 2020-08-14 at 5.21.09 PM.jpg

K

[Jimmy W]

Another surface drive.