Questions on general inboard boat design...

[IDBoating]

I'm not getting into this little argument, but I probably will. Breaking out the popcorn.

That's nothing compared to the "jet vs. prop efficiency" argument. Just you wait... {grin}

[da.bell]

The issue you have with a scupper valve would be that we add ballast to our boats. That pushes the boat further into the water which creates additional presure onto the flap. That flap can only take so much presure before it fails and allows water going back into the boat. IMO, I wouldn't put my money on a scupper valve to keep water out of the bilge. It's just another point of failure that can/will eventually happen especially with a tow boat.

[IDBoating]

The issue you have with a scupper valve would be that we add ballast to our boats. That pushes the boat further into the water which creates additional presure onto the flap. That flap can only take so much presure before it fails and allows water going back into the boat. IMO, I wouldn't put my money on a scupper valve to keep water out of the bilge. It's just another point of failure that can/will eventually happen especially with a tow boat.

Aren't most scuppers mounted on the transom, above the on-plane waterline? There would be no pressure when underway. But I get your point. (BTW, there are flapless scuppers - mine uses a sort of ping pong ball, so it cannot fail under pressure.)

Dedicated slalom boats aren't ballasted, though, so this increased pressure would not occur. Do they use floor drains?

[jwanck11]

Why do you think towboat manufacturers don't use jet drives WA?

[IDBoating]

Why do you think towboat manufacturers don't use jet drives WA?

That's why I'm asking the question - to find out.

It could be as simple as "tradition". "Always been done this way." Perhaps any experiments that were performed were before people had or understood mixed-mode jetdrives. Perhaps they mounted them on the transom in the traditional way, instead of using a v-drive setup to push the jet forward and under the hull. Maybe folks believe some old wives' tales about jetdrives (hey, I'm the first to admit that jetski engines do not make good boat engines!).

Technology marches on. I'm just wondering if there's a good reason no one does it besides "We haven't tried it lately" or "I've heard they're not good for towboats". Has anyone actually tried it with a modern jetdrive on a modern hull? What happened?

[Ewok]

That's why I'm asking the question - to find out.

It could be as simple as "tradition". "Always been done this way." Perhaps any experiments that were performed were before people had or understood mixed-mode jetdrives. Perhaps they mounted them on the transom in the traditional way, instead of using a v-drive setup to push the jet forward and under the hull. Maybe folks believe some old wives' tales about jetdrives (hey, I'm the first to admit that jetski engines do not make good boat engines!).

Technology marches on. I'm just wondering if there's a good reason no one does it besides "We haven't tried it lately" or "I've heard they're not good for towboats". Has anyone actually tried it with a modern jetdrive on a modern hull? What happened?

Ok, I'm back in. To answer that rehtorical question, you know Yamaha makes a "wake" boat, did you look and test-drive one prior to getting the inboard? I did not test-drive one because I was under the impression they threw a crappy wake. If you did, what were your impressions?

To adress the other points on propulsion, I know a ducted fan is more efficient than an open blade design but I think direct drives are favored because there is less drivetrain loss, and v-drives are favored to get the traction of direct drive and more weight in the back for a better wake.

[IDBoating]

You know Yamaha makes a "wake" boat, did you look and test-drive one prior to getting the inboard? I did not test-drive one because I was under the impression they threw a crappy wake. If you did, what were your impressions?

Jetboat makers market "wakeboats" but they are a joke. It's pure marketing, trying to ride on the coattails of boardsport popularity.

Example: BRP made a version of my boat (same hull, same deck, same everything) they called a "wakeboat" by adding a tower and throwing a 600 pound fatsac in the ski locker (literally - they were shipped with the fatsac still in its plastic bag). Since I have the non-wake version, I was in a unique position to run the experiment. So I did!

I replicated their setup by adding my own (better) tower. That helped immensely. But when I added weight in the ski locker the wake actually got SMALLER. This was because the CoG of the boat was aft of the locker, so adding weight ahead of it had the effect of raising the stern out of the water, exactly the opposite of what is desired. This was confirmed by some buyers of the "wake" version; when they filled their OEM fatsac the wake got smaller. A classic example of marketing hype over engineering.

However, that does not mean a jetdrive cannot be used on a true towboat. It just means that an existing jetboat cannot necessarily be turned into a wakeboat by adding a tower and a quickie fatsac. That does not render the basic concept invalid.

To adress the other points on propulsion, I know a ducted fan is more efficient than an open blade design but I think direct drives are favored because there is less drivetrain loss, and v-drives are favored to get the traction of direct drive and more weight in the back for a better wake.

Now keep the engine and v-drive. Dump the transmission and all of its losses. Connect a jetdrive instead of the propshaft, strut, and propeller. The weight is still in the rear, plus you get lower drivetrain loss, higher efficiency, less turbulent water (thanks to reduced swirl), etc.

The big question is whether there is any relationship between wake/wave quality and the type of motive unit moving the water beneath the boat. I suspect there isn't, if both types are located ahead of the transom and under the hull.

[91Terminator]

Not to start an argument, but you wouldn't need as large an impeller because jets are actually more efficient than open props. This is primarily because the jet can recover two forms of energy that are lost in an open prop environment.

First, an open prop thrusts water in two directions: Axially (in a straight line) and radially (in a circle spreading out from the prop). Centrifugal force causes a large amount of the water to be "spun away" from the prop, sort of like an expanding doughnut. This radial thrust is entirely wasted energy because it does not help to push the hull forward.

In a jetdrive, the impeller also accelerates water radially. However, the housing around the impeller can be designed to recapture and redirect this radial thrust so that it contributes to the axial thrust rather than being wasted. Water pumps that do this are called "mixed mode" pumps and the better jetdrives are also known as "mixed mode" (because they take advantage of both types of thrust coming off the impeller.) Open props have no way to recover radial energy, so it is lost.

Second, all spinning propellers (and impellers) impart swirl to the water. Like radial thrust, energy spent swirling water does nothing to move the hull forward so it is wasted energy. However, a jetdrive has a stator behind the impeller. The stator's vanes are tilted opposite those of the impeller. Why? Because that deswirls the water coming off the impeller - and uses it to move the hull forward.

Think of a nut on a bolt. If you turn the nut, it moves the nut linearly. But you can also move the nut linearly by turning the bolt. Likewise, if you put a spinning prop in still water the prop will want to move. But the prop will also want to move if you put it in spinning water. That's what a jetdrive does; it puts the "stationary prop" (stator vanes) in a column of spinning water (the swirl coming off the impeller). The spinning water moves the stator forward, which transfers the energy to the hull. In other words, the swirl energy is recaptured and transferred into linear motion.

Both of these characteristics are unique to jetdrives, and make them more efficient than open props. Because of that, a smaller jetdrive can often be used in place of a larger open prop.

Why do all the test show Tige(Inboards) are a ton more fuel efficient??? There is nothing fuel efficient about a jet ski or jet boat they have always sucked fuel and always will. They are nothing but high rev-ing, high RPMS motors, and there is no way around horrible fuel efficiency and high rev-ing motors, im sorry but this has been proven to many times. i cant tell you how many people hate the fuel efficiency of their jet boats. We sold a 06 22Ve to a guy that had just bought a AR230. He hated it after he had drove our boat. he will tell you time after time how much easier it is to maneuver the Tige. Also he came in like the first week of owning the tige and wanted us to check the fuel sender cuz he said it didn't move at all all weekend, he used to go through 2 tanks a weekend, he was dead serious about us checking it and we said dont worry keep using it it will eventually move. He was so amazed at the fuel economy he gets with his tige. Facts are facts. Check the tests.

[AdaminMN]

[QUOTE=Ewok;451216]And I know you can do a similar maneuver with an inboard, its just not designed for this kind of abuse:



I know that guy, he told me to buy a Nautique.

[IDBoating]

There is nothing fuel efficient about a jet ski or jet boat they have always sucked fuel and always will. They are nothing but high rev-ing, high RPMS motors, and there is no way around horrible fuel efficiency and high rev-ing motors, im sorry but this has been proven to many times.

I totally agree, which is why I dismissed the jetski engines from this discussion. That's not what I'm talking about. Indeed, many of the "real" jetdrives are powered by diesel engines. I doubt anyone would claim those are "high revving motors".

There is a difference between the jetdrive and the engine powering it. I completely agree with you that jetski engines are inappropriate for a boat. I'm talking about real marine engines coupled to real jetdrives. For example, a PCM engine powering a jetdrive through a v-drive gearset. That is entirely possible. It's just a different "lower end" on the drivetrain.

i cant tell you how many people hate the fuel efficiency of their jet boats.

On the other hand, I love the efficiency of mine - but that's because it's not powered by jetski engines. It has a real, marine class, Mercury 2.5L V6 EFI engine. I burn 4-5 gallons per hour when boarding or skiing, which is right in line with what other "big block" engines burn.

For comparison, I also have two jetskis powered by traditional jetski engines. THOSE things suck gas. The manual for one of them even says, in black and white, that at WOT a full tank will last 30 minutes (!!!).

In this thread I'm talking about real engines coupled to real jetdrives, not jetski engines and their little screaming impellers.

[dogbert]

Wow, that looks awful. My jetboat doesn't generate anything even close to that much turbulence.

It's not a great shot (I was trying to catch my then 5YO son, not the turbulence), but here's an animated GIF that shows the water behind our jetboat:

Of course you have less turbulence, you're only going about 5 Mph. Try running 22-24 (which is the speed at which most accomplished wakeboarders run) and you'll see a big difference. It's not just the propulsion system...it's the hull design, too. Most I/O and jet boats do not have tracking fins or a rudder. So, if you're a wakeboarder or skier, it's pretty easy to drag the back of the boat around. I used to pull my I/o 1-2' either way every time I cut on my ski. I can't do that with my Tige.

Once you get used to driving an inboard, it's really no big deal...even in reverse.

[evil0ne]

OK a few things that have probably already been said:

1) Drain - even with a dripless packing you are bound to get a few drops of water in the bilge with an inboard so you're going to have water in the bilge.

2) Maneuverability - They both suck. Jets don't back up for sh&t either. If they have two motors it helps but that's twice the headache. And as everyone says if you work the throttle you can move the boat around pretty well.

3) Towing - there's the wake, there's the planing speed and lastly trackability. Those are the three biggest differences between an inboard and a jet drive runabout.

[zad0030]

We're new to inboards - our new 24Ve is our first. At the moment we own two boats so we're naturally doing comparisons and some questions have arisen.

1) Our jetboat has a dedicated floor drain that is plumbed to a dedicated scupper valve. Water that enters the passenger area (dripping clothing, boards, etc.) never gets to the engine compartment/bilge because it's routed straight to the lake. As a result, my jetboat's engine compartment remains dry all season long (and I do mean DRY). It appears that most inboards, though, just expect such water to drain into the bilge where it must be actively pumped out. What is the reasoning here? What would be wrong with a "sealed" floor and a floor drain?

2) Is there an inherent reason that wakeboats must be traditional inboards? I don't think the propulsion method plays a huge role in the shape of the wake or wave; I believe the hull is the dominant factor. Does it matter that the hull is being pushed forward by an exposed prop instead of an impeller? I'm asking because everyone jokes about "inboards only back up to the right". A jetdrive fixes that, as well as dramatically simplifies the entire drivetrain (no transmission at all, ultrafine control over F/R behavior, etc.). I'm not talking about using jetski engines - there are professional grade jetdrives used in everything from fishing boats to tugboats to cruise ships, powered by everything from electric motors to enormous diesels. The technology exists and is well proven. Is there some reason it can't be used on a wakeboat?

Thanks!

Well in order to understand you will have to look back at where wake boats started.

Wake boats started as ski boats obviously. Ski boats came out in a time where outboard motors where just about all there was. And this meant a slow hole shot and a long drag to get the boat on plane. Inboards boasted a quick hole shot.
Inboards are towboats because they provide a quick hole shot, minimal bow rise, tracking, and constant speed.

Ok jetboats are terrible. Like someone said, any halfway decent slalom skier will drag the boat around with them. They don't track straight, they dont hold speed well, and they throw a mean wash.
Have you ever seen a jetboat from shore? The rooster tail is moving at about 45 but the boat is only moving 30.

The technology is not well proven in a tournament boat market. Never, will I see a jetboat pull a 3 event tournament. The sport is built around the inboard boat.

[dom w. forte]

I totally agree, which is why I dismissed the jetski engines from this discussion. That's not what I'm talking about. Indeed, many of the "real" jetdrives are powered by diesel engines. I doubt anyone would claim those are "high revving motors".

There is a difference between the jetdrive and the engine powering it. I completely agree with you that jetski engines are inappropriate for a boat. I'm talking about real marine engines coupled to real jetdrives. For example, a PCM engine powering a jetdrive through a v-drive gearset. That is entirely possible. It's just a different "lower end" on the drivetrain.



On the other hand, I love the efficiency of mine - but that's because it's not powered by jetski engines. It has a real, marine class, Mercury 2.5L V6 EFI engine. I burn 4-5 gallons per hour when boarding or skiing, which is right in line with what other "big block" engines burn.

For comparison, I also have two jetskis powered by traditional jetski engines. THOSE things suck gas. The manual for one of them even says, in black and white, that at WOT a full tank will last 30 minutes (!!!).

In this thread I'm talking about real engines coupled to real jetdrives, not jetski engines and their little screaming impellers.

How do you compare your mercury v6 to a 5.7 v8 when you dont come close to the weight ratio of the boats, and you just proved that the 5.7 with the inboard is more effient, cause im pulling over 4000lbs and burn from 3.5 to 5 gals per hour dependent on rider.

[dom w. forte]

How does your jet propulsion reverse?