Questions on general inboard boat design...

[IDBoating]

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!

[thtrog]

I searched and found a picture of the wake behind a Yamaha AR230 and it doesn't look very fun. I could only imagine the amount of turbulence you would have after adding additional weight.

170178.jpg

[dom w. forte]

Jetdrives normally dont have enough torque to move the load,they will effect wake shape, they are more difficult in maintaining speeds when loaded, and all data shows they use a lot more fuel.
Picking up your rider is normally more difficult, and they dont track as well when pulling a rider when they cut at the wake.
The more precision in the pull and handling is important to a good rider.
As far as drivabilty in docking or any kind of manuver once you master an inboard nothing comes close to what it will do.
We are show skiers, you have to have a precision boat to do all of what we pull , we can turn , back any direction on a dime, it is all technic, with practice you learn how much you can do.
It all has to do with rudder , throttle, and transmission.
All of what i said apllies to wakeboarding as well.

[chpthril]

I've owned 2 I/O's and 1 inboard as well as worked on dozens of boats of different styles and sizes and from many different manufactures. Based on my experience, a single transom drain is the norm. Having 2 drains on a DD is very common, but I would say self-bailing scuppers/drains on boats is not the normal for a run-a-bout, but maybe normal for an off-shore type fishing boat.

Go grab a wakeboard set or surf run behind a jet or I/O (not surfing though) and you will see why the in-board design still dominates the tournament water-sports scene. Same as why you dont see an I/O running the slalom course.

[Ewok]

Coming from outboard boats to inboard/outboard boats to my current inboard there is a HUGE difference in the wake. Especially the surf wake. The inboard design was preferred in ski boats because the angle of the prop shaft and the relative propulsion pushes the bow down into the water which makes the wake smaller and allows the boat to get on plane faster. In a wakeboard boat, you want to get on plane quickly as well but you don't need to push the bow down to make a small wake. You want to make the bow rise to displace more water to make a bigger wake. When you do this with an inboard/outboard boat with outdrive trim, it does affect the wake. I know there is a little difference in hull shape but the I/O boats had wider wakes and they didn't have the "pop" of an inboard boat wake. It also felt as if you could pull against an I/O when it tries to pull you out of the water vs an inboard which had great traction and pulls you out of the water with much more power.

I know its almost winter up there but next time your out on your jetboat, look at the wake, the jet exhaust is too close to the surface of the water and it would wash out the wake, just like surface drive offshore power boats. IMO, the only viable alternative for an inboard drive system is the new tractor drives such as the zeus from mercruiser. It combines the advantages of I/O and inboard hull and propulsion designs.



As far as water draining overboard, jetski boats are typically used in a very different manner. Most people use them like giant jetskis, turn sharp and have much greater potential to get the interior wet. Most ski boats are built to try to keep the people in the boat dry, as are most other types of boats. Many center console boats also drain overboard because they are meant to take breakers when running the inlets and other harsh weather conditions.

[IDBoating]

Next time your out on your jetboat, look at the wake, the jet exhaust is too close to the surface of the water and it would wash out the wake

Trust me, I've spent LOTS of time studying the wake behind the jet {grin}.

You're correct, on my current jetboat the output of the jet generates some turbulence. Not enough to wash out the wake, though... think of it as some extra froth immediately behind the boat. I've noticed outboards and I/O's do the same thing. They all have this little rooster tail that pops up some distance behind the transom.

However, today's jets and outboards and I/O's also share another characteristic: Their propulsion point is at or behind the transom. That is the big difference - the inboard's prop is forward of the transom.

Imagine a v-drive setup with a jetdrive instead of the propeller shaft, strut, and exposed propeller. The point of thrust is now forward of the transom, just like an inboard. If desired, it could be angled downward like an inboard's prop (indeed, some jets are articulated to make that angle adjustable!).

Once the propulsion point is moved forward like an inboard's, I can't see why there would be any difference in the water behind the boat. In fact, because the jet's stator recaptures the swirl energy, I suspect there could be LESS turbulence and simply more direct, straight "thrust".

[IDBoating]

I searched and found a picture of the wake behind a Yamaha AR230 and it doesn't look very fun. I could only imagine the amount of turbulence you would have after adding additional weight.

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:

[IDBoating]

Jetdrives normally dont have enough torque to move the load,they will effect wake shape, they are more difficult in maintaining speeds when loaded, and all data shows they use a lot more fuel.

Those things are true of the little jetski engines. Sometimes they put one or two of those in a larger hull and call it a jetboat. I'm not talking about those... I'm talking about a real jetdrive and a real engine. If they can use jets on tugboats, they definitely have torque! {grin}

As far as drivabilty in docking or any kind of manuver once you master an inboard nothing comes close to what it will do... it is all technic, with practice you learn how much you can do.

Tony said the same. I'm sure you're both correct. The jet has just spoiled us.

[IDBoating]

Most ski boats are built to try to keep the people in the boat dry, as are most other types of boats.

Yes, but with watersports (slalom, wakeboarding, wakesurfing, etc.) you've got people climbing in and out of the boat all day. Their clothing and equipment are soaking wet, and they drain/drip into the passenger area. No matter how the hull is designed from a water-piercing perspective, you're going to have people carrying lots of water into the boat while engaging in the very activities for which these boats are designed. I would think they'd manage it better than "just let it drain into the bilge". Scupper valves aren't exactly a new invention. {grin}

[Ewok]

Yes, but with watersports (slalom, wakeboarding, wakesurfing, etc.) you've got people climbing in and out of the boat all day. Their clothing and equipment are soaking wet, and they drain/drip into the passenger area. No matter how the hull is designed from a water-piercing perspective, you're going to have people carrying lots of water into the boat while engaging in the very activities for which these boats are designed. I would think they'd manage it better than "just let it drain into the bilge". Scupper valves aren't exactly a new invention. {grin}

Big difference between this and dripping off after a surf session.

[dom w. forte]

It still wouldnt be as fuel effient, and tugs use it not for its torque, but more for being able to change trust angle.
Also look at the size of the housing required to accomadate an equal size prop.

[Ewok]

And I know you can do a similar maneuver with an inboard, its just not designed for this kind of abuse:



This is what he was trying to do:

[IDBoating]

It still wouldnt be as fuel effient, and tugs use it not for its torque, but more for being able to change trust angle. Also look at the size of the housing required to accomadate an equal size prop.

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.

[IDBoating]

Big difference between this and dripping off after a surf session.

Yes, but that's not what I'm talking about.

I think keeping unnecessary water out of the engine compartment is a good thing always and everywhere. Particularly when it's so easy to do. That's why I'm surprised it's not done by everyone as standard practice.

[Ewok]

Yes, but that's not what I'm talking about.

I know, it's a slight exaggeration to your point, but still fun to watch. And the jet boat designers did think about this situation when they designed the topsides, MC did not.

I think keeping unnecessary water out of the engine compartment is a good thing always and everywhere. Particularly when it's so easy to do. That's why I'm surprised it's not done by everyone as standard practice.

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