I/O vs. jet(s)

About inboard or outboard motors.

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Denon Osterman
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I/O vs. jet(s)

Post by Denon Osterman » Tue Aug 28, 2007 4:43 pm

So lets say I'm building the desperado, and i wanna go fast...

a 5.7 liter 315 hp inboard (GM vortec engine with bravo one outdrive) with out drive weighs almost a thousand pounds... :shock:

the merc jetdrive 240 hp weighs in at a scant 340 something, WITH the pump

the 5.7 litre 315 hp with, say, an AT pump is like 900.

so my question is, is the extra 65 hp(and the speed advantage of an outdrive, as opposed to a jet) worth the 600 pounds?

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Dave Grason
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Post by Dave Grason » Tue Aug 28, 2007 9:00 pm

I'm no expert but I'd guess the answer is no ....at least in THIS case.

With the Bravo drive, you'll have roughly 5.7 horsepower per pound. With the jet you'll have 4.75 per pound. So the Bravo will have more but barely one more pony per pound. So I think it's going to come down to efficiency. The jet will have some slippage but the I/O will have a lot more internal friction that eats up horsepower. It's a tough call.

Maybe you should introduce some other deciding factors. The jet won't have any worries about ski ropes tangling around the prop. The kids won't get their feet chopped off. You can power the boat right up onto the trailer with the jet. You can skip across some really skinny water with the jet. Rooster tails are fun, too.
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PapaDon
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Post by PapaDon » Wed Aug 29, 2007 9:40 am

Dave, please don't tell people about powering the boat onto the trailer. It is NEVER a good idea. When you're doing that you're digging a hole in the lakebed that people fall into and in which trailers get stuck. At the same time you're building a 'hump' behind it making for even less water getting to your trailer. PLUS, if someone's in the water helping to guide it onto the trailer....well you get the picture. It's never a good idea...that's what the winch is for.
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Caber-Feidh
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Post by Caber-Feidh » Sat Sep 01, 2007 10:41 am

I, for one, always use the engine to get onto the trailer. Anyone that doesn't like it has never tried to pull a big-block powered boat onto a bunk trailer.

Winching is easy with rollers, but if you value a true hull you don't have rollers.

Allot of comp boat trailers (every one I know of) doesn't even have a winch! Just a stainless turnbuckle and a clevis to hold the boat.

Last of all, Dave is talking about a jet drive, not a prop. Jets, by their very nature, are a surface drive. They cause little disturbance to the bottom, even in shallow conditions... and someone in the water has little to worry about with a jet pump! I would take a complete Darwin Award candidate to get hurt.

Denon Osterman
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Post by Denon Osterman » Sat Sep 01, 2007 2:39 pm

I'm thinking the jet is a better idea...but im not sure about maintence. i know on my paretns boat(mariah), theres no maintence to speak of with the outdrive except when it gets winterized...im pretty sure ive heard that jet pumps need to be greased EVERYTIME you take them out???

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kens
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Post by kens » Sat Sep 01, 2007 3:59 pm

Which jetdrive is that @ 340lbs? That seems really light for a engine + drive. Can you post a link to that engine, I would be interested in that too.

Denon Osterman
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Post by Denon Osterman » Sat Sep 01, 2007 8:16 pm

http://www.mercurymarine.com/engines/je ... 240efi.php

the ONLY thing that worries me is the performance of the pump...and the price/availability

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kens
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Post by kens » Sun Sep 02, 2007 4:24 am

Now I see, looks like a outboard powerhead mated to the pump. Novel idea. That might be faster than the Bravo1 drive you mentioned. The outdrive is a lot of drag in the water at high speed.

Rick Heaney
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i/o versus jet.

Post by Rick Heaney » Wed Sep 05, 2007 10:55 pm

Go for the jet drive......

Just make sure that you take notice of how the water will get to the jet.

There must be no skegs etc in front of the intake (you need good water at the intake and skegs can create aeration) and the deadrise needs to be at least 7 degrees and a constant deadrise for a reasonable length fwd of the transom otherwise you can end up skating across the water when you are turning.

The Mercuryjet is a fantastic unit. It is a 10 inch jet pump made by Doen Jets here in Melbourne, Australia and will be my choice for the next boat I build. You can also get them in a turbo diesel engine from Mercury and Turbo Diesel Yanmar engine which is based on the 220hp BMW diesel.

Excellent units, all of them.

As far as maintenance, GO FOR THE JET...... Basic greasing on a regular basis and a good wash through after salt water imertion is about all you'll need to worry about.

The US Navy and Marines have twin high speed diesel/jet powered 40ft RIB's which will do 40knots and pull up in one boat length. They will also "U-turn" at full speed. Both the Navy and Marines need maximum performance and maximum reliability.

Did I mention GO FOR THE JET 8) 8) 8)

Hope this helps

Regards,

Rick Heaney
Shipwright
rickh@seabrookemarine.com
www.seabrookemarine.com
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Denon Osterman
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Post by Denon Osterman » Thu Sep 06, 2007 4:37 pm

Rick you seem pretty knowledgeable...do you know how much one of those will cost? thats my only worry...because i can get the engine AND outdrive for 10 grand ish...im betting the merc jet costs quite a bit more... :?

Rick Heaney
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i/o vs jet

Post by Rick Heaney » Thu Sep 06, 2007 9:47 pm

G'day Denon,

Sorry mate, that's something I can't really help with. I did speak with a Mercury guy at the recent Melbourne boat show and from general conversation the inboard V8 MercuryJet seemed to be around the same at a mercruiser sterndrive but it would depend on the end set up. With a jet, you can go from reasonably basic with manual cable reverse bucket control and cable steering (I have both of those on my Seabrooke 18 and they are excellent) up to full ecu control which matches the bucket control with engine revs etc. to give single lever operation with emergency stop etc. (with emergency stop the engine goes to wide open throttle and the reverse bucket slams down - it's like four wheel disc brakes for your boat.)

I can say that, with combination set up right for your boat, the control you will have with a jet at low speed is exceptionally sweet, the acceleration is amazing, the high speed handling is to shout about and I have found the fuel consumption on the Seabrooke 18 very pleasing indeed. Mine is an inboard mounted 4.0 litre V8 direct drive to an 8.5 inch Doen jet and I love it.

Also, as Dave mentioned, there are no exposed blades to chop up skiers and tangle in ropes either.......... that's worth a lot right there.

Hope this helps.

Rick.
Life's better with a wooden speedboat

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