2013, the year of the jet drive?

[Part-time]

Re: 2013, the year of the jet drive?

I'm not the one who needs to reread.... you just said the same thing I did.... at a given throttle ("unless I throttle up") load does not increase fuel gph but DOES decrease mph so thus both use more fuel to carry more weight. as you said, no exceptions

Forget the speed... I'm not talking mph or mpg. We all know both boats will slow down with increased loads.
If you set the throttle at 1/2 on a jet, it will give you the same rpm's loaded or empty (the load on the engine remains the same)
With a prop you will start off with less throttle and have to give it more as the load increases just to maintain the same RPM's

 

[smokeonthewater]

Re: 2013, the year of the jet drive?

no you just let the rpm's fall and continue at a slower speed.... who cares what the rpm's do.... BOTH will need to burn more fuel to carry more weight at the same speed

 

[QC]

Re: 2013, the year of the jet drive?

I guess the question, and it's why I posted regarding the same, is what do we do with that info? I know it is a pump, but I would argue that inlet pressure will change RPM slightly. i.e. the water is forced in harder as speed increases, so a faster hull will get more RPM for the same throttle setting. I am not posting to be argumentative. Are we missing something?

We all know both boats will slow down with increased loads.

From my experience here I would not bet on that

 

[smclear]

Re: 2013, the year of the jet drive?

Here's a link to some fun jets. Definitely out of my league.


Top 20 Fastest Super Yachts and Luxury Yachts of the world - Boat International

 

[sickwilly]

Re: 2013, the year of the jet drive?

I am a used boat buyer, so how do these boat stack up used?

Here is a quick comparison of 2010 versions, using Nada (I know I know Nada is crap -- what else do we have?)

Jets:

yamaha sc 210 base comparison: 32,499 initial, 24,230 average retail, loss 8,269 or 25%

Sea Doo 210 base comparison: 35,469 initial, 23780 average retail, loss 11,689 or 33%

Sterndrives:

Bayliner 217 base comparison: 30,315 initial, 22,840 average retail, loss 7,475 or 24.7%

Chapp 215 base comparison: 44,279 initial, 35,290 average retail, loss 8,989 or 20%

Glastron 215 BR base comparison: 39,791 initial, 31,350 average retail, loss 8,441 or 21%

Stingray 205 lx base comparison: 29,758 initial, 22,200 average retail, loss 7,558 or 25.4%

inboards:

Mastercraft 214V base comparison: 58,220 initial, 42,830 average retail, loss 15,390 or 26%

Malibu response lxi base comparison: 51,800 initial, 38,800 average retail, loss 13,000 or 25.1%


With this quick limited comparison, I would say the yamaha holds up comparatively, but he Sea Doo does not.

But what I want to know, is how fun are they to slalom ski behind because I am one of a dying bread of dinosaur. I still waterski!

 

[OrangeTJ]

Re: 2013, the year of the jet drive?

They'll pull a skier just fine and have excellent hole shot to get you up on the ski, but they're certainly not the "best" slalom ski boats. In my own experience, they don't hold a line as well as either a competition ski boat or an I/O when you've got an aggressive skier cutting at the end of the rope. Mine improved dramatically in this regard when I added cobra steering fins to the nozzles, but it's still not as arrow straight as a dedicated tow boat, to be sure. The wake is pretty flat at slalom ski speeds, though the center is frothier than on a prop boat. If you're hard core/competitive, that might be an issue...for me it is not as I spend very little time in the center of the wake and I don't cut hard enough that it has an effect on my skiing.

 

[QC]

Re: 2013, the year of the jet drive?

Good research there sickwilly, and good relevant info there OranjeTJ. Cool posts

 

[rbh]

Re: 2013, the year of the jet drive?

As I have never had to play around with a jet drive before, what are the issues?
Are they not just a water pump? takes water in then preasurezes it out?
So from a dead stop it would not preform as good as it would at higher speeds?
I can not see where there would be an issue with prop slip as the efficiancy would be almost 100%, I think?

 

[OrangeTJ]

Re: 2013, the year of the jet drive?

As I have never had to play around with a jet drive before, what are the issues?
Are they not just a water pump? takes water in then preasurezes it out?
So from a dead stop it would not preform as good as it would at higher speeds?
I can not see where there would be an issue with prop slip as the efficiancy would be almost 100%, I think?

Yes, they are just a water pump with the impeller directly connected to the engine (i.e. no transmission). If the engine is on, the pump is processing water. I can't speak for how the "go fast" jets perform, but on the recreational boats, like our Yamaha, I'd say the performance is more the opposite of what you've suggested above. From a dead stop, acceleration is very, very strong. At high speed, the pump geometry becomes the limiting factor. Take a look at 0-30 acceleration figures for these boats as compared to equivalently powered sterndrives. Then take a look at top speed. I think what you'll see is that they run 0-30 much faster than most equivalently powered and sized prop boats but have lower top speed than those boats.

 

[rbh]

Re: 2013, the year of the jet drive?

Yes, they are just a water pump with the impeller directly connected to the engine (i.e. no transmission). If the engine is on, the pump is processing water. I can't speak for how the "go fast" jets perform, but on the recreational boats, like our Yamaha, I'd say the performance is more the opposite of what you've suggested above. From a dead stop, acceleration is very, very strong. At high speed, the pump geometry becomes the limiting factor. Take a look at 0-30 acceleration figures for these boats as compared to equivalently powered sterndrives. Then take a look at top speed. I think what you'll see is that they run 0-30 much faster than most equivalently powered and sized prop boats but have lower top speed than those boats.

I would have thought the opposite, as you go faster the water is rammed into the intake allowing the impellor/impellors? to move that much more water with less HP required.

 

[OrangeTJ]

Re: 2013, the year of the jet drive?

I don't know for certain why it's the case, but my best guess is that you start to run up against the flow limits of the pump, since any pump can only flow so much water in a given period of time. With a prop, there is really no limit to how much water can be exposed to the prop in a given period of time. To be clear, the drive is not efficient at low operating speeds (i.e. going slowly), requiring a disproportionately high rpm to make forward progress. 2K rpm gets me about 3 mph, 4K rpm gets me about 6 mph, 6K rpm gets me about 30 mph, 8.1K rpm gets me 47 mph. On the models with the higher horsepower engines, the extra 140 hp equates to 8 or 9 more mph on top. When you open the throttle quickly from a dead stop, though, even on my 220hp version, the engines spin up to 8K rpm nearly instantly and the boat launches pretty hard and gets up on plane very quickly.

 

[haulnazz15]

Re: 2013, the year of the jet drive?

Actually, rbh, I believe the issue with the efficiencies is mostly related to application. You see, the jet pump is optimized for a certain speed/volume of water to flow through. Thus, at speeds under/over this designed speed, you lose effeciency due to the nature of the beast. This is a large reason why adding HP alone doesn't generally help increase top speed in a jet boat application, as the pump can only process a certain volume of water before it max's-out. I would assume most of the gains in recent years with jet drive technology have been in the design (shape) of the pump housings/impellers themselves to try and extend the operating range where the drive is efficient. Jet drives still suffer from prop slip when volume of water isn't enough or is too much (aeration/turbulence at inlet), thus you lose efficiency at the impeller. The boat prop, being outside of a duct/housing, isn't affected by the limitations of the housing, so you primarily only deal with "prop slip".

I will say that anyone who claims a single-engine jet boat (even the new ones) handles at slow speed like a twin inboard or I/O is full of it. Being able to turn a 360 is useful for positioning in tight quarters, but moving forward/backward causes quite a bit of drift and moving in a straight-line takes quite a bit more manipulation with the steering. The lack of a rudder and generally shallower draft causes the boat to wander.

 

[Part-time]

Re: 2013, the year of the jet drive?

1975 sidewinder 18ss. 455 olds (apx 330hp) berk jet.
top speed 61mph on GPS with apx 25 gallons of fuel and 2 guys with a combined weight of 580 lbs.
I sold it to my buddy next door and this is him playing with it.
Where he goes by the end of my dock at 57 sec. there is only 2' deep and rocks and big beams from the old dock that I still hadn't got all out.

 

[rbh]

Re: 2013, the year of the jet drive?

I am probably looking at this aZZy backwards!
When I think jet drive I think the "opposite" of hydro electric style impellors on a generator getting fed by a stream of water coming down a penstock, or a super charger style where water is drawn in and compressed then shot out, kinda like an old school hand cranked egg beater.

 

[mrdancer]

Re: 2013, the year of the jet drive?

Here's my take on the jets:

Jets are more efficient at higher speeds (or, rather, their designed top speed), and less efficient at low speeds. Yes, they are quick on holeshot, but also inefficient (in terms of "productivity/fuel burned"). Think of a high-performance prop with the plugs removed. If you are concerned about efficiency/economy, a jet would make sense where you are running near WOT for much of your trip. If you are trolling, stick with a prop.

Now, to dispel the "shallow water" myth: A boat with a good "pocket" tunnel setup and a good tunnel prop, operated by an experienced user, can run just as shallow as a similar-sized jet boat. Yes, we are talking 2-3 inches of water. However, very few people know how to properly set up a tunnel prop to run those kind of conditions, so the point is probably moot.

I can run through six inches all day with my 25-foot boat with 115hp 4s and SS prop, and through 3-4 inches for a few hundred yards over hard sand. However, it takes me about twelve inches of water for holeshot, so the jets have an advantage over me in that department. Also, my boat handles like a snowmobile, which isn't a big deal because I can generally clear anything that I can't see in time to steer clear - five years of running shallows and still not a ding on the SS prop. Anyway, point is that the jets have an advantage in handling, as well as depth needed to get on plane, but they don't go any shallower.

We did try jets here a while back (outboard w/ jet foot), but the sand wore down the impellers/housing way too quickly (this is very fine sand, suspended in the water). We went back to props to get away from the maintenance. If I needed to go any shallower, I'd have to get an airboat.

 

[emilsr]

Re: 2013, the year of the jet drive?

The lack of a rudder and generally shallower draft causes the boat to wander.

Our friends' twin Yammie jet actually wanders less at idle than our single engine deep v I/O.

 

[OrangeTJ]

Re: 2013, the year of the jet drive?

True enough for the competitive skier, that's why they like inboards better than an I/O. But for the average guy, I don't think it would matter much, especially as you move up to the larger / heavier boats. One thing I will note though, in most cases you'll want a rope that's a bit longer to get you out of the turbulance of the jet.

I agree that it doesn't matter much for the average guy (i.e. - me). It's probably obvious from my other posts, but I absolutly love our Yamaha jet boat. It's a great family boat that suits our needs/wants nearly perfectly.

 

[sickwilly]

Re: 2013, the year of the jet drive?

Here's the thing I've found. I have never heard a Yamaha boat owner that didn't love the boat. Like you they all say it's a nearly perfect family boat that meets all their needs. I think the safety issue of not having a rotating meat cleaver near their family members is a BIG point.

Hey .... if that is all they know that is all they know

Kidding -- I would love the opportunity to play with one some time. I have only seen one on the lake we boat at. I have seen a couple of the sea doo boats, with their concord like downturned nose, but have not been able to try one of those either.

 

[haulnazz15]

Re: 2013, the year of the jet drive?

Our friends' twin Yammie jet actually wanders less at idle than our single engine deep v I/O.

Yes, but the docking/slow-speed I was referring to, and what Part-Time referred to, was a single-engine jet outperforming a twin I/O at the dock. Sorry, but I just don't believe it. Twin-engine jets are a different story, but will still not track as well as a twin I/O, simply due to design limitations. I'm not knocking the Yammies at all, they are sharp looking boats. However, I don't see them as being any better for general boating purposes than an I/O. Everyone talks about the "safety" aspect, but I haven't ever had an issue with the prop being "unsafe", simply because it isn't turning when people are nearby in the water. To add to it, we have a Kawasaki JetSki (2-stroke) that has been flawless for 13 years, except that now it needs a new shaft seal.

 

[H20Rat]

Re: 2013, the year of the jet drive?

Yes, but the docking/slow-speed I was referring to, and what Part-Time referred to, was a single-engine jet outperforming a twin I/O at the dock. Sorry, but I just don't believe it.

Like I said way back on page one, i've owned all types of boats. Next to a twin screw, a single jet is easily the best handling. Can't change your mind, but I'll take some video this spring and post it up here! Like I said, I go into my slip, spin around, and then move the boat straight sideways until I touch the dock. I'd love to see an I/O do that... (in case you are wondering how you get side thrust on a single jet, you hold the wheel to the opposite side you want to go, and then toggle forwards and back to modulate thrust. Not all single jets can do this, it depends on where the reverse thrust exits the bucket. Of course, the drawback is that steering in reverse is also backwards.)