Prop Drive, and then Jet Drive.

mikey.himself

Petty Officer 2nd Class
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Sep 11, 2007
Messages
130
So, I got to thinking. Why don't they use jet-drive systems more than they use Prop drive systems in the I/O applications? Is it because their not as reliable? I should think in this day and age they can make a heck of a solid system. And also, they are a much better system as far as propulsion.
 

RotaryRacer

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Jul 18, 2004
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1,361
Re: Prop Drive, and then Jet Drive.

Efficiency.

Reverse?

Off Throttle Steering?
 

mudmagnet63

Petty Officer 1st Class
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Sep 28, 2007
Messages
231
Re: Prop Drive, and then Jet Drive.

As a former Jet Boat owner there are some drawbacks I have seen. Shallow water, especialy sandy bottom or thick vegitation sucks right up then jams the impeller. at low RPM. The boat I owned did not have the handling capabilites of a prop. Preformance wise it took a lot of HP and a lot of fuel. I'm sure they have improved since then, but just my opinion.
 

mikey.himself

Petty Officer 2nd Class
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Sep 11, 2007
Messages
130
Re: Prop Drive, and then Jet Drive.

No, all opinions appreciated. I just want to hear everyones thoughts. The only problem in the past i heard of was sucking things up , and obviously pump wearing out. I just find it so odd that they aren't used more frequently.
 

45Auto

Commander
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May 31, 2002
Messages
2,842
Re: Prop Drive, and then Jet Drive.

What makes you think they're a much better system? A jet drive is generally much less efficient than a prop. They are good for shallow water and where you're looking for a simple system. A jet drive is more efficient than a conventional prop above 60 MPH or so but really sucks (literally ...) below that!

There's a reason all the serious hot-rod race boats use semi-submerged props - they're the most efficient at high speeds. A conventional prop is more than twice as efficient as a jet drive below about 40 mph, which I would guess is where about 99% of the boats out there spend their life.

Chart below came out of a Fluids textbook, sorry I don't remember the author.
 

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mikey.himself

Petty Officer 2nd Class
Joined
Sep 11, 2007
Messages
130
Re: Prop Drive, and then Jet Drive.

I don't know, I just thought they were pretty good units. A seadoo, for example, while dramatically smaller, is a great example.... you hit the throttle and that thing is leaping out of the water, you turn and its turned before ya know what happened. Those seadoo boats also use them...i dont notice control at low speeds very poor, no worse than a prop. but I just thought they might be something more people would want to see on their practical boat. What companys produce them? And what is the cost compared to a prop drive?
 

RotaryRacer

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1,361
Re: Prop Drive, and then Jet Drive.

Efficiency has been covered by others.

They are much less efficient in most applications.

Reverse: any experience I have had with jet drives in reverse has been less than positive. Try steering in reverse with a jet drive some time.

Off throttle steering: without thrust a jet drive needs to have some significant fins or otherwise to effect the direction of the boat. Take your Sea-Doo example. Try to steer after you release the throttle. It only responds when you have thrust. I am sure most jet boats are better than they were many years ago now though.

I am not saying there isn't a market for jet drives. I just don't see them becoming as popular as prop systems for the reasons stated.

Yamaha and Sea-Doo seem do be doing pretty well selling some pretty large boats with jet drives. Most of the larger ones need two engines to keep up with a similar sized single engined I/O or OB boat.
 

drewmitch44

Lieutenant Commander
Joined
Jun 26, 2005
Messages
1,749
Re: Prop Drive, and then Jet Drive.

For the reason that a jet reduces your HP. There are some lakes that allow a 85 hp limit. But you can use a 150 if its a jet. And the 85's are blowing the 150 jets away.
 

drewmitch44

Lieutenant Commander
Joined
Jun 26, 2005
Messages
1,749
Re: Prop Drive, and then Jet Drive.

gas costs too much to use a 150 when you can use a 85 with the same effect. In my opinion its that plain and simple!
 

tashasdaddy

Honorary Moderator Emeritus
Joined
Nov 11, 2005
Messages
51,019
Re: Prop Drive, and then Jet Drive.

i have 3 PWC, with out thrust you have no steering, reverse is a redirection of the forward thrust, and totally sucks. there is a big difference in and 600 lb, pwc, with 110 hp than a 1500 lb boat, 5 people aboard, with the same set up. also a sea doo is a sea don't. on a boat with a jet you loose +- 40 % of HP in the conversion to jet, than propped motor.

jets have their place and use, in rocky rivers , and hard to get places, but if you have ever worked on one, they are a real PITA.
 
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