Just curious?......

Designs for inboard or outboard power

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DSR
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Just curious?......

Post by DSR » Sat Jan 13, 2018 4:09 pm

In a previous post, it was mentioned that full-size jet pumps were a pricey drive choice for boats compared to direct drive (or stern. And V-drive for the sake of discussion) and I was wondering what kind of difference there really is between the them? From what I've seen, jets are actually a pretty cost-effective choice.

For example:

A person has just built a bare hull and already has, say a 350 Chevy on a stand ready to go, minus exhaust and cooling, and start from there with new components?

Thanks,
Dave
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753

283
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Location: Maine

Re: Just curious?......

Post by 283 » Sun Jan 14, 2018 5:01 am

Should we include the cost of upgrading to a 454 to offset the efficiency of a prop vs jet?


(Sorry, I’ve got a twin engine jet boat and never get tired of the efficiency argument) :lol:
Mike

DSR
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Re: Just curious?......

Post by DSR » Sun Jan 14, 2018 9:16 am

LOL!!

Hi Mike

Yeah, my own experience through the years with jet boats (and a few v-drives) have all been powered by big blocks running 500-1000 hp, so we'll just say that fuel economy wasn't a high priority....

But with that being said, in my travels through the net I've stumbled across a few naval propulsion research papers (a couple of the papers for naval military) that had shown that , when properly sized and powered for the hulls they were installed into, jet drives were as efficient as other propulsion systems above 20 knots, and at 35-40 knots and up to 60 knots even, they were actually more efficient than other systems. In those papers it was shown that while the jet was less efficient at converting power to thrust, at the speeds shown, it was offset by a lack of running gear, or appendage drag while moving through the water. From 40 knots up, the efficiency does show dropping back and eventually below other systems as speeds increased, caused by the drag of the jet intake opening in the bottom of the hull gradually increasing. I thought that was pretty interesting.


Researching the powertrain for my own project with the 165 hp Honda turbo engine and jet pump, in the original 842 lb jet ski with a pretty deep V in the hull, it shows fuel economy of 3.1 gals per hour at 35-40 mph cruise speeds and 12 gph at WOT. From what I've understood, basic rule of thumb for marine power was always 1 gph for every 10 hp at WOT, regardless of propulsion system, so these numbers seem pretty good for a jet to me.......

So, where would that leave a person that was open to the various propulsion systems available, to choose a cost-effective system?

Thanks Mike,
Dave
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753

Steven r
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Re: Just curious?......

Post by Steven r » Sun Jan 14, 2018 11:13 am

Hi,
I had a jet boat once, it was like driving on ice! I think they have come a long way since the old days. For performance you need serface drives like power boats.
Steve.

DSR
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Re: Just curious?......

Post by DSR » Sun Jan 14, 2018 1:29 pm

Hi Steve,

Yeah, the surface drives definitely work, but judging by the prices that I have seen, I'm not sure that I would consider them cost-effective for most builds .......Yikes! :shock:

Thanks,
Dave
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753

Steven r
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Re: Just curious?......

Post by Steven r » Sun Jan 14, 2018 3:12 pm

Dave,
Just been thinking, on the price of shafts and props on our boat not cheap!
Steve.

DSR
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Re: Just curious?......

Post by DSR » Sun Jan 14, 2018 5:40 pm

That's what I was curious about Steve. A complete brand new American Turbine AT309 jet pump assembly can be bought for less than 5 grand, which is still a lot of money, but if you purchase a new FNR transmission (and a down-angle output is extra cost), prop shaft, prop, prop strut, a whip strut, shaft coupler, shaft log, turn fin and rudder with rudder post, tiller arm, stuffing box, etc....... What would you have into brand new hardware?
And this doesn't even get into the cooling components (sea pump, circulation pump, water pickup, etc.) that you don't need with a jet.....
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
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283
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Location: Maine

Re: Just curious?......

Post by 283 » Mon Jan 15, 2018 9:45 am

Without actually doing the math, I think a stern drive would be a bit cheaper then a jet drive. Assuming you’re a buying all new. You can buy a replacement alpha drive for less then $1500 on eBay.

I like the mechanical simplicity of a jet drive... no transmission.

And thankfully there are plenty of lightly used pwc around.
Mike

Steven r
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Re: Just curious?......

Post by Steven r » Mon Jan 15, 2018 12:33 pm

Hi,
I think a Monal shaft and a 24x24 super fine prop may cost near the price of a jet unit!
Steve.

DSR
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Re: Just curious?......

Post by DSR » Tue Jan 16, 2018 8:48 pm

Hi Mike,

I have to agree that the mechanical simplicity of the jet drive is pretty attractive. It's not for everyone, but in the right circumstances, I believe it works very well as a cost-effective option.

The donor ski I picked up for my TNT is an '04 with 66 hrs on it (just broken in and the engine and pump still look like brand new inside and out, and runs perfect) that I was lucky enough to pick up at a salvage auction due to an accident that broke the hull. This particular unit is kinda uncommon and pretty highly sought after so it was more money than most skis go for, but I still ended up spending quite a bit less than what a new 4-stroke 10 horse Mercury outboard is going for........

I did see the Alpha replacement drives by Sterdrive Engineering on eBay that it looks like you're referring to, but unless I misread it, the $1500 is just for the stern drive "leg" itself. You would still need to get a transom housing / gimbal assembly (only available from Mercruiser new at $2300) to complete the unit. Then you have to grab a prop, all the hydraulics and still have to deal with the cooling system components to boot, so we're right back in the thick of it doing a stern drive also.

.........huh...It looks like I may have just answered my own question..... :roll:

Thanks, :D
Dave
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753

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Mr Hot Rod
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Re: Just curious?......

Post by Mr Hot Rod » Tue Jan 16, 2018 9:17 pm

The following link might be of interest :
If the hull you're starting with was designed for a centrally mounted inline inboard, you'll have to add a jackshaft extension kit to maintain the original hull's balance.

Glen-L's Inboard Hardware Catalog has very competitive pricing on Velvet Drive and Casale V-Drive transmissions.

See our website's Transmission Page for inline, down-angle, v-drive and performance marine transmission suppliers and pricing.

____________________
Paul Kane
Kane Custom Boats Ltd.
Chelsea, Quebec

Building the Glen-L Hot Rod : http://www.kanecustomboats.com
Glen-L Hot Rod build thread
Kane Custom Boats YouTube channel
Glen-L Boat Video Directory

DSR
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Location: Allegan, Michigan

Re: Just curious?......

Post by DSR » Wed Jan 17, 2018 7:29 pm

Hi Paul,

Thanks for the links. They are definitely of interest and I appreciate it!

I agree 100% regarding keeping a hull balanced as originally designed Unless you've really researched the art of designing a hull and getting all the specific details correct, or get professional help with it (Yeah, a little of 'that" kind of professional help might be good too..... :D ), otherwise the end results could be anywhere from disappointing to dangerously unstable.

I like the jackshaft idea to allow sterndrives to become an alternative drive choice for specific hulls, and the same is true for the jet drives also.

If I remember correctly, a member here on the forum built a Glen-L design (I think it was a Ski King?) and used a Berkeley pump with a jackshaft to a centrally-mounted engine with good success. I don't recall his name, but I believe he was from Wisconsin??? The boat looked great and seemed to run pretty well in the videos I caught.

Thanks again Paul,
Dave
DSR Performance - Home of yet another jet TNT build :D
Codename "Just A Little....."
viewtopic.php?f=23&t=29753

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