Jet Boats vs Outboards and IOs

[Sharp Shooter]

Re: Jet Boats vs Outboards and IOs

I see a big ol' air intake, sure it isn't blown?

Now here's some tail:

244 MPH Nitro drag boat - YouTube

A big airscoop on a tunnel ram is what you see. And I like your prop boat video.

Here's the video that settles at least the jet boat vs outboard portion of this conversation. HAHAHA!!!

 

[WIMUSKY]

Re: Jet Boats vs Outboards and IOs

A big airscoop on a tunnel ram is what you see. And I like your prop boat video.

Here's the video that settles at least the jet boat vs outboard portion of this conversation. HAHAHA!!!

Duh, I should have known.......:facepalm:

Vid.....................Good one! :laugh:

 

[Chris1956]

Re: Jet Boats vs Outboards and IOs

I would think the real proof would be found in the unlimited hydroplane racing boats. What kind of drive system do the race winners have?

 

[ssobol]

Re: Jet Boats vs Outboards and IOs

I would think the real proof would be found in the unlimited hydroplane racing boats. What kind of drive system do the race winners have?

Hydroplane boats are a very special case. These are inboards that have surface piercing props. When at speed the prop is half out of the water. At lower speeds with the prop completely submerged it is less efficient.

Recently one of the motorboating magazines (I can't remember which one, I was reading it at an airport news stand) did a test with basically the same boat model with I/O power, outboard power, and jet power. Based on noise, fuel economy, installation, available onboard space, cruising speed, ride, cost, etc., they came to the conclusion that outboards work out the best. I/O only came out ahead for style points (some people don't like the look of having an outboard hanging in the back).

One thing pointed out in the article was that jets boats don't have motor trim. You either have to have tabs, or change the loading to change the boat trim.

 

[WIMUSKY]

Re: Jet Boats vs Outboards and IOs

I would think the real proof would be found in the unlimited hydroplane racing boats. What kind of drive system do the race winners have?

Turbines.......

 

[Brian 26]

Re: Jet Boats vs Outboards and IOs

Gas mileage....some variances-inboard (which for safety are comparable do worse):
3.39 MPG at best range for an Yamaha SX240.
3.78 MPG for Cobolt 240
3.83 MPG for Chapparal 224 at best range 3.83 MPG (and yes...the Chap has less wetted area).
2.25 MPG for Mastercraft X-45
3.09 MPG for Tige RZ4

Great info, thanks for posting, I've never seen those numbers side by side like that.

I hear that power loss comment a lot but don't necessarily believe it either. I like jet boats because you don't have the outdrive to worry about, kids jumping off the swim platform and hitting it, under water collisions etc....

 

[Jetboaters]

Re: Jet Boats vs Outboards and IOs

Obviously a prop guard is not a jet. I never claimed it was. I was simply trying to describe for those who have never seen it. The propguards predate Jet Outboards.

No one has yet described how the efficiency of the modern jet boat has been improved. Intrinsically, it would seem that a jet boat would have a disadvantage in efficiency, since the jet pulls in and accelerates water, which then pushes on the large body of water. I would think the efficiency loss would be in the pulling of the water into the tube, the friction of the tube, and the need to push water with water, rather than directly with the prop blades.

Prop boats waste a ton of energy with water thrown off the prop up and down, and also in initial acceleration when waste a ton of energy literally lifting the bow of a boat (due to the location of the prop). So both types have energy loss issues. Jets accelerate faster because the thrust exits at the transom level not below it - so no energy wasted lifting the bow. Also because the impeller is enclosed, no energy lost throwing water off the prop.

Older performance jet boats have vertical trim on the nozzle(s), the newer models have not incorporated trim, mostly as a cost saving IMO. No reason it can't be done.

 

[thumpar]

Re: Jet Boats vs Outboards and IOs

Around here the only jet boats we see are the old shallow hulls with big blocks and jetavators. The only outboards are either bass boats with large engines, pontoons or old Bayliners. It is probably 85% I/O on our lake. I don't know that I have ever seen one of the newer style jet boats on the water. They are kind of the looked down on in this area.

 

[Jetboaters]

Re: Jet Boats vs Outboards and IOs

Around here the only jet boats we see are the old shallow hulls with big blocks and jetavators. The only outboards are either bass boats with large engines, pontoons or old Bayliners. It is probably 85% I/O on our lake. I don't know that I have ever seen one of the newer style jet boats on the water. They are kind of the looked down on in this area.

There are at least 2 Jetboaters on Lake Coeur d'Alene, but unless you knew that they were jet boats, you might not realize it....

 

[thumpar]

Re: Jet Boats vs Outboards and IOs

We stick to Long Lake. It is only 11 miles from our house. CDA is about 1.5 hours from us since we are pretty far up North.

 

[Sharp Shooter]

Re: Jet Boats vs Outboards and IOs

I would think the real proof would be found in the unlimited hydroplane racing boats. What kind of drive system do the race winners have?

All the modern Unlimited hydroplanes utilize Lycoming T-55 turbine engines ran through a v-drive gear box to a propeller . There was a team recently that I don't know if they're 2014 registered that was running piston power.

 

[Sharp Shooter]

Re: Jet Boats vs Outboards and IOs

Hydroplane boats are a very special case. These are inboards that have surface piercing props. When at speed the prop is half out of the water. At lower speeds with the prop completely submerged it is less efficient.

Recently one of the motorboating magazines (I can't remember which one, I was reading it at an airport news stand) did a test with basically the same boat model with I/O power, outboard power, and jet power. Based on noise, fuel economy, installation, available onboard space, cruising speed, ride, cost, etc., they came to the conclusion that outboards work out the best. I/O only came out ahead for style points (some people don't like the look of having an outboard hanging in the back).

One thing pointed out in the article was that jets boats don't have motor trim. You either have to have tabs, or change the loading to change the boat trim.

Most jet boats do have trim adjustment capability. They basically ad a device called a jet-o-vator to the end of the pump. I've also seen jet boats (and I have pics) where they utilize adjustable cavitation plates which most of you would know by the name of "trim tabs".

It's true that outboards are very efficient (when clamped to an air entrapment hull) and they're great for good fuel economy, reliable and don't take up much space in the boat, BUT they are horsepower "limited" unlike inboards. This is why the worlds fastest boats are not outboards.

 

[Sharp Shooter]

Re: Jet Boats vs Outboards and IOs

Prop boats waste a ton of energy with water thrown off the prop up and down, and also in initial acceleration when waste a ton of energy literally lifting the bow of a boat (due to the location of the prop). So both types have energy loss issues. Jets accelerate faster because the thrust exits at the transom level not below it - so no energy wasted lifting the bow. Also because the impeller is enclosed, no energy lost throwing water off the prop.

Older performance jet boats have vertical trim on the nozzle(s), the newer models have not incorporated trim, mostly as a cost saving IMO. No reason it can't be done.

I don't think you really have a firm grasp of all this. Lol

Propellers come in many shapes and sizes and some even utilize adjustable pitch. Also, saying jet's accelerate faster is a blanket statement that isn't true for all situations. If you attend a drag boat race where the big dogs race you'll realize that the quickest and fastest boats are propeller driven.

I'm a big fan of jet propulsion because I've been around them all of my life, but I've never owned a jet boat and never will.

 

[Frank Acampora]

Re: Jet Boats vs Outboards and IOs

No one here has adequately explained the basic physics principles: Jets are USUALLY (given the same horsepower input), faster in acceleration because---the jet develops its maximum thrust when you hit the throttle, not as the boat accelerates. The impeller is sized to the engine, lets it rev up into its maximum power band quickly, and is also usually a zero slip design.

Jets USUALLY ( again given the same horsepower input), slower top end because thrust at the nozzle is a constant (whether the boat is moving or held at rest) and as the boat accelerates, it is running away from the water jet, thus effectively reducing the reaction and thrust. THUS, given the same horsepower input, a jet drive boat will apparently perform like a boat with lesser inboard or outboard power.

However, look at the supercharged jet skis that will top out at well over 60 MPH--If you throw enough horsepower into the impeller, and match the impeller to the engine, you will get blazing performance

 

[thumpar]

Re: Jet Boats vs Outboards and IOs

No one here has adequately explained the basic physics principles: Jets are USUALLY (given the same horsepower input), faster in acceleration because---the jet develops its maximum thrust when you hit the throttle, not as the boat accelerates. The impeller is sized to the engine, lets it rev up into its maximum power band quickly, and is also usually a zero slip design.

Jets USUALLY ( again given the same horsepower input), slower top end because thrust at the nozzle is a constant (whether the boat is moving or held at rest) and as the boat accelerates, it is running away from the water jet, thus effectively reducing the reaction and thrust. THUS, given the same horsepower input, a jet drive boat will apparently perform like a boat with lesser inboard or outboard power.

However, look at the supercharged jet skis that will top out at well over 60 MPH--If you throw enough horsepower into the impeller, and match the impeller to the engine, you will get blazing performance

Some of that depends on the boat. Take a 26' boat and throw a Bravo 3 behind a 8.1l and you will get up quick. Put a jet behind it and I don't think it would get going so fast. When you get down to the 17' size on a light hull the jet might have an acceleration advantage.

 

[Chris1956]

Re: Jet Boats vs Outboards and IOs

Well, it would seem we have played this thread out, IMHO. Almost like a oil thread, there is no consensus. At least we did get some analysis, which is different than what is usually provided in an oil thread.

 

[haulnazz15]

Re: Jet Boats vs Outboards and IOs

No one here has adequately explained the basic physics principles: Jets are USUALLY (given the same horsepower input), faster in acceleration because---the jet develops its maximum thrust when you hit the throttle, not as the boat accelerates. The impeller is sized to the engine, lets it rev up into its maximum power band quickly, and is also usually a zero slip design.

Jets USUALLY ( again given the same horsepower input), slower top end because thrust at the nozzle is a constant (whether the boat is moving or held at rest) and as the boat accelerates, it is running away from the water jet, thus effectively reducing the reaction and thrust. THUS, given the same horsepower input, a jet drive boat will apparently perform like a boat with lesser inboard or outboard power.

However, look at the supercharged jet skis that will top out at well over 60 MPH--If you throw enough horsepower into the impeller, and match the impeller to the engine, you will get blazing performance

^ Best answer of the thread.

 

[haulnazz15]

Re: Jet Boats vs Outboards and IOs

Gas mileage....some variances-inboard (which for safety are comparable do worse):
3.39 MPG at best range for an Yamaha SX240.
3.78 MPG for Cobolt 240
3.83 MPG for Chapparal 224 at best range 3.83 MPG (and yes...the Chap has less wetted area).
2.25 MPG for Mastercraft X-45
3.09 MPG for Tige RZ4

I just picked similar boat lengths of brands that my friends have. It is correct that Jet boats aren't as economical at no wake speeds....(3.93MPG for the shorter Chapparel 224, and 1.29MPG for the Yamaha).

The problem with the above numbers, is that the boats aren't really that comparable to each other. The Cobalt 240 is going to weigh a good bit more than the Yamaha 240, almost 900lbs more dry hull weight, yet the Cobalt still has significantly better fuel efficiency. The only way to truly illustrate the efficiency differences is to compare boats which offer jet / inboard / i/o packages on the same hull. Not too many of those exist, especially in the runabout class.

 

[RotaryRacer]

Re: Jet Boats vs Outboards and IOs

Boating Magazine just did a comparison of the 18' Glastron that offers all three power options.

The hulls are very similar but do differ slightly to accommodate the particular power plant. But in general this is the most apples to apples to apples comparison possible in the current market.

The results (3rd picture) and other pictures:

Boat Engine Comparison Test: Outboard vs Sterndrive vs Jet Drive | Boating Magazine

 

[haulnazz15]

Re: Jet Boats vs Outboards and IOs

Boating Magazine just did a comparison of the 18' Glastron that offers all three power options.

The hulls are very similar but do differ slightly to accommodate the particular power plant. But in general this is the most apples to apples to apples comparison possible in the current market.

The results (3rd picture) and other pictures:

Boat Engine Comparison Test: Outboard vs Sterndrive vs Jet Drive | Boating Magazine

Excellent reference, RotaryRacer. I'm a bit surprised by the use of the 150HP outboard in the comparison, simply because it's 100HP less than the jet, and 70HP less than the I/O. I would have thought a 200-225HP option would have been great, which would better approximate the difference between I/O and O/B fuel efficiency and weight differences. I'm a bit skeptical of 19s for the I/O to get 0-30mph while heavy, vs ~8s for the jet or O/B, but the rest of it looked pretty good.