Are jet boats that bad on gas?

[Quick01GT]

I went to a marine supply place that sold newer boats, the guy told me my 73' sleekcraft is way too outdated and will get horrible gas consumption on the lake, I have not taken it out since I got it and it has twin 30 gallon tanks, do you think this is enough capacity for a day out on the lake? im not gonna race around, just cruise. It's a 23' executive by the way, is the guy right? are jet boats that primative? you guys know more about this so let me know if I bought something that sucks more fuel than my escalade with a 6.2L

 

[Lyle29464]

Re: Are jet boats that bad on gas?

The early 70's hulls were a bit heavy. The jet drives were not very efficient. They did not handle well at a slow speed.

 

[roscoe]

Re: Are jet boats that bad on gas?

23' executive..... with what powerplant ? nevermind, it is a guzzler.


outboards equipped with a jet drive lose 30% of their power.

I imagine inboards are similar.

Boats as a whole are terribly inefficient, then knock off another 30%, and you have a guzzler. $30-$60 per hour.

 

[26aftcab454]

Re: Are jet boats that bad on gas?

one of my lake buds has a 19ft 70's Jet Boat with a "stock" 455 Buick . We run about 12 miles round trip to the sandbar .
He cruises 30-35mph and says he burns 6 gallons of gas.
Tell us what motor, carb(s) & jet drive you have. Photos tell alot.
There are many here on this forum that can tell you alot more than I can.

Your boat sounds bigger & heavier.

 

[Scott Danforth]

Re: Are jet boats that bad on gas?

you bought a boat, yes, it will suck more fuel than your escalade. my 2002 Searay could only get 3 miles to the gallon on a perfect day. at WOT, it was more like gallons per mile. Jet boats are less efficient than props. so best guess is about 1-1.5 miles to gallon on reasonable cruise, and a lot less with the holeshot and WOT

 

[Quick01GT]

Re: Are jet boats that bad on gas?

My boat has a stock 455 olds with a 650 edelbrock and rebuilt heads with performance marine transom headers, as long as I get a full day of use I would be a happy camper

 

[H20Rat]

Re: Are jet boats that bad on gas?

outboards equipped with a jet drive lose 30% of their power.

Outboard jets are one of the worst designs ever for a jet, centrifugal flow only, other jets are mixed/axial flow. With that said though, those old jets had all kinds of factors against them to turn them into gas sucking pigs... heavy hulls, jet pumps that need some tune ups, big v8 engines, and owners that want to drive them fast.

Modern jets (yamaha, etc...) get mileage that is good or in some cases better than a comparable I/O.

 

[TomB985]

Re: Are jet boats that bad on gas?

I completely disagree, smoking crater. Too lazy to look it up right now, but go to www.boattest.com and check out the fuel efficiency of Yamaha or SeaDoo jet boats. Then compare with a stern drive or outboard competitor.

It's not even close. I nearly bought a jet boat, but after some research even the new ones just have too much of an efficiency loss.

 

[26aftcab454]

Re: Are jet boats that bad on gas?

My boat has a stock 455 olds with a 650 edelbrock and rebuilt heads with performance marine transom headers, as long as I get a full day of use I would be a happy camper

I think how hard you "mash the gas" will make a huge differance. Go out & have fun!

 

[PCUK]

Re: Are jet boats that bad on gas?

Generally speaking jet drives are more efficient above about 30 knots and then are near equal to props. Below that they are less efficient. I would test it at various different speeds on and off the plane to find the most economical speed. Obviously the usual caveat applies that the boat must be either planing or running at displacement speed. Anything in between will just waste fuel.

 

[Frank Acampora]

Re: Are jet boats that bad on gas?

The formula for thrust is mass times velocity squared. Thus a small mass moving at high velocity generates a large thrust-- greater than that developed by a large mass moving at lower speed.

It is a common misconception that because it is a "Jet" and because jet aircraft are efficient and fast, that a jet drive boat will be also. However, we are comparing apples and oranges. An aircraft jet engine develops thrust with a high velocity flame and is only very efficient at high altitudes and 90-95% of its RPM.

A boat jet drive, on the other hand uses a relatively low velocity massive column of water squirted out the back to develop thrust. These drives develop maximum thrust around the hole shot. As the boat speeds up, it "runs away" from the column of water and thrust drops off until drag on the hull equals thrust. Then the boat is topped out at speed. You can try this effect with your simple garden hose nozzle.

Because of the low speed thrust characteristics, jet drives were popular years ago on SK type hulls used for water skiing, especially in California. However, for top speed, they will never--given equal horsepower--match a prop driven hull.

Since the jet drive does lose thrust, to match speed you must use more horsepower and thus fuel. A 90 horsepower jet drive will only deliver the speed potential of about a 70 horsepower engine while using the same amount of fuel as a 90. THUS: for a given horsepower, they are less fuel efficient than prop drive engines. So, yes, you could say they are "bad" on gas.

 

[gtochris]

Re: Are jet boats that bad on gas?

We had a '96 Sea Ray F16 with the Merc Sport Jet 90Hp, not only did it "feel" like 25hp, but it was HORRIBLE on fuel! I do 2x the MPG it seems with my stingray as both have similar size tanks. Even though the Stingray is said to be 45 more HP and 2x the weight, it is WAY more powerful too!

I completely disagree, smoking crater. Too lazy to look it up right now, but go to www.boattest.com and check out the fuel efficiency of Yamaha or SeaDoo jet boats. Then compare with a stern drive or outboard competitor.

It's not even close. I nearly bought a jet boat, but after some research even the new ones just have too much of an efficiency loss.

I love how new Jet boat manufacturers love to push the "MPG at 3,000 rpm's" as if that is some kind of benchmark. a prop can be on plane where as a Jet is doing 5MPH at 3,000 RPM's....

 

[drrpm]

Re: Are jet boats that bad on gas?

Boats are inefficient compared to cars. Jet boats and especially older jet boats are less efficient than prop drive. However, most of us are boating for recreation, not for work so any fuel use is inefficient. While gas prices are a daily irritant, they are a small portion of a vehicles total cost. The question to answer is whether the fun you have on the boat is worth the cost. There's a famous quote by J. P. Morgan when asked about the cost of maintaining his yacht which was "If you have to ask you can't afford it"

 

[ssobol]

Re: Are jet boats that bad on gas?

.....As the boat speeds up, it "runs away" from the column of water and thrust drops off until drag on the hull equals thrust. Then the boat is topped out at speed. You can try this effect with your simple garden hose nozzle.....

Same thing happens with a prop. With a prop the column of water is not as constrained (focused by the nozzle). With a prop there can also be some loss of thrust due to cavitation and vortices off the tips of the prop. There is a speed (rpm) limit on any given prop where efficiency starts dropping fast due to these effects. A jet drive impeller should be able to operate at a higher rpm before these effects occur.

There should be some efficiency gained with a jet drive at higher speeds by the ram effect of the intake providing a solid water column to the impeller. A well designed jet will also limit the tip vortices. Cavitation can be a problem if the impeller expels the water faster than it can come in (intake choking) or if there is disrupted intake flow (occurs when the boat is turning).

 

[Frank Acampora]

Re: Are jet boats that bad on gas?

You are not quite correct on your prop anaysis. A well designed prop has blades shaped in cross section like an airplane wing. Negative pressure developed on the forward side and positive pressure on the rear help the prop to "screw" itself Forward through the water thus moving the hull forward. Just as an airplane propeller (airscrew to the British--for a reason) screws itself forward through the air. The column of water that you see is the "slip" of the prop. Now mind you-- I am not talking about special applications such as surface running props or specialty racing blades. Just your everyday work prop versus everyday jet drive. I won't argue that props also lose efficiency at higher RPM because that is true, but the boat does not move away from the prop. The net effect is still that for the everyday boater, jet drives use more fuel for a given hull and speed.

 

[rallyart]

Re: Are jet boats that bad on gas?

...so let me know if I bought something that sucks more fuel than my escalade with a 6.2L

A 4hp on an inflatable will get worse mileage than your Escalade. The Escalade might be a boat but it certainly is not as bad as a real one.

 

[NSBCraig]

Re: Are jet boats that bad on gas?

Sounds like a fun boat. Just take it out and enjoy.

 

['78 Crusader]

Re: Are jet boats that bad on gas?

Generally speaking, the 70's jet drives do consume more fuel than a comparable prop drive of the same vintage. With that said, they were also a lot more fun and could do things no prop drive could do. Bottom line when people bought the jet drives it wasn't for miles per gallon but more along the lines of smiles per gallon.

Agreed!

Sounds like a fun boat. Just take it out and enjoy.

X2 Agreed!!

I have a 1978 21' Crusader jet boat with a 540 CID BBF motor and though the fuel mileage is terrible, the smiles are plentiful and the sound of that Big Block Ford.......well lets just say it very pleasant to the ear.