Riviera powered by v12 jaguar

Designs for inboard or outboard power

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jenko
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Re: Riviera powered by v12 jaguar

Postby jenko » Tue Dec 13, 2016 11:47 pm

Kens ,first prop 12 x 15 3 blade attained a speed of 48 mph at 5400 engine rpm 4736 shaft rpm wot
changed vdrive gears and prop
second prop 12 x 12 4 blade attained a speed of 41 mph at 6000 engine rpm 5825 shaft rpm wot
4 blade was smoother ,made for tighter turns ,with less steering effort, got up on plane sooner , boat sat flatter ,but did not work well after 4000 engine rpm
felt like I had a slipping clutch , cruising 1/2 throttle at 4000 rpm hit the gas and engine flairs to 6000 rpm then boat catches up, but only reaches 41 mph .(below 4000 rpm the boat really lurches forward with any throttle increase)
have posted a video on you tube https://youtu.be/W6FnRjuT_z0
So could not see if porpoising had gone as I couldn't reach the speed where it happens.
The prop shop says they can fix the slip /cavitation? by adding some cup and altering pitch

Ozzieboat
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Re: Riviera powered by v12 jaguar

Postby Ozzieboat » Wed Dec 14, 2016 5:06 am

Jenko
You may have what you have already hinted, a slipping BW transmission. Have you put a tacho on the output coupling. Those $15 Chinese lazer ones work very well. Also is the selector engaging in the detent. If a slip is confirmed check transmission oil pressure. Boats looking fantastic by the way.

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jenko
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Re: Riviera powered by v12 jaguar

Postby jenko » Wed Dec 14, 2016 5:39 am

Good point Alan shall check trans

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Bill Edmundson
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Re: Riviera powered by v12 jaguar

Postby Bill Edmundson » Wed Dec 14, 2016 7:30 am

Pete

I've been looking at the two curves and looking at shaft RPM. It looks like both props start coming off the 20% curve at about the same 4000 shaft RPM. I wonder if they're starting to get blade tip cavitation? The higher the RPM the more cavitation you get off the blade, thus little increase with more power. If that is the case the prop man probably can tune a lot of it out.

What is your target engine RPM?

I'm not a prop expert by any means! Just a thought.

Bill
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PeterG
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Re: Riviera powered by v12 jaguar

Postby PeterG » Wed Dec 14, 2016 8:20 am

I think Bill is right, there's a very good chance your prop is starting to cavitate at the higher speeds. Tip cavitation would "unload" the prop and feel like your transmission is slipping. That could be due to a more aggressive prop pitch or simply over-revving the prop. And 20% prop slip sounds too high. Typically the older inboards I have been around were in the 10-12% range, but they were running lower HP engines (6 or 8 cyl. vs. your V12 (!)) and 1:1 gear ratios for runabouts and ski boats. Prop slip is an indication of how well your prop is accelerating or thrusting the water. High slip means lots of acceleration but it may not be the most efficient set-up. You can ask a dozen marine mechanics what the best ratio/prop diameter/pitch setup is for your boat and engine combo, and you'll likely get two dozen opinions. You may need to consult a naval architect/boat designer for the best setup for your boat. Do you know the torque curve for your engine? Most marine engines are set up for high torque, developing their max horsepower at lower rpms than a car engine typically does.
There is an excellent book called The Propeller Handbook by David Gerr that covers how to determine prop size, pitch and transmission ratio based on desired performance, boat type and weight, and engine power/torque. Highly recommended, it cuts through the "rules of thumb" that many marine mechanics and boat shops use and helps you to engineer the best setup.
Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

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Bill Edmundson
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Re: Riviera powered by v12 jaguar

Postby Bill Edmundson » Wed Dec 14, 2016 8:56 am

Pete

I know you can do this. But, for others' info.

Shaft RPM vs tip MPH

Mine 13" prop @ WOT 3100 RPM: 120 MPH - 193 KPH

12" prop: 4000 RPM: 143 MPH - 230 KPH
12" prop: 4736 RPM: 169 MPH - 272 KPH
12" prop: 5825 RPM: 208 MPH - 335 KPH

Bill
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jenko
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Re: Riviera powered by v12 jaguar

Postby jenko » Wed Dec 14, 2016 1:34 pm

Have checked trans oil and it is as new with no burnt smell so I don't think it is slipping.
The prop man can't tweak the prop till after Christmas
will just have to wait but he seems to think its not a problem
Bill I am aiming for max 6000 rpm peak torque is 5600 rpm
would like 50 mph

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Bill Edmundson
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Re: Riviera powered by v12 jaguar

Postby Bill Edmundson » Wed Dec 14, 2016 3:55 pm

Pete

Do you know HP (metric/US) and running weight? You should easily hit 50 MPH. What ever HP you turn at 40 MPH will take almost twice the HP to get to 50 MPH.

OOPPS I messed up the speed conversion, fix it later. Got to go. OK now. Corrected!

Bill
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kens
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Re: Riviera powered by v12 jaguar

Postby kens » Wed Dec 14, 2016 4:44 pm

I have doubt that the prop is slipping in the water. Especially if it is not slipping around 30mph.
I feel that the transmission is slipping, even though it may have good fluid in it, the clutches may not be holding.
Can you get a optical tach on the prop shaft coupling?
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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jenko
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Re: Riviera powered by v12 jaguar

Postby jenko » Wed Dec 14, 2016 11:51 pm

Kens ,I have an optical tach, The run before changing props with the 12 x 15 3 blade was fine and engine pulled cleanly to 5000 rpm and 48 mph ,but for peace of mind I shall check shaft rpm v engine rpm next outing
As I can;t get the prop tweaked till after Christmas I might go out with the 3 blade with the 1.03 gearing to see what happens.
should limit engine to 4550 rpm at around 44 mph by my calcs so if it is the trans same thing should happen and at 4000 it will slip ,if its the prop I will get to 44 mph at 4550 rpm

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Re: Riviera powered by v12 jaguar

Postby jenko » Sun Dec 18, 2016 11:29 pm

Did another run on weekend with 12 x 5 3 blade it ran as expected and had optical tach on shaft and verified no slipping transmission .
so back to the 4 blade does not like more than 4000 rpm

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jenko
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Re: Riviera powered by v12 jaguar

Postby jenko » Sat Dec 31, 2016 12:21 am

have done a few more tests and put the data into a spreadsheet that Paul Kane and I have been working on show hp ,torque on graph with speed and revs with crouchs constant of expected shaft hp to attain speed
Attachments
riviera props.jpg

bob smith
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Re: Riviera powered by v12 jaguar

Postby bob smith » Sat Dec 31, 2016 12:30 pm

I've always liked the area where the Torque and HP curves intersect. Looks to me as if 5 grand and a 3 blade 12x15 are going to be hard to beat.
Jay Leno says, “Horsepower sells cars, torque wins races.” While the principles are the same, a boat experiences significantly greater fluid friction and thus torque is much more important to a boats performance.
IMHO, 50mph is more than enough to impress the grandkids and dust off the occasional wannabe boat racer.
The sound alone of that Jag purring is boating nirvana!
BTW are you using a GPS speedometer?
Bob Smith
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jenko
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Re: Riviera powered by v12 jaguar

Postby jenko » Sat Dec 31, 2016 2:13 pm

Yes Bob GPS speedometer.
I will stick with the 4 blade over this summer your right 50 mph is plenty for the boat just trying for best compromise

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kens
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Re: Riviera powered by v12 jaguar

Postby kens » Sat Dec 31, 2016 6:16 pm

I see the graph for the expected data according to crouch's method, however, where is the graph for the ACTUAL data?
I was looking for the line on the graph that shows 5400 WOT rpm @ 48mph with 12x15 cupped. That is actual data and that is where you should start all remaining data.
I noticed one line on the graph that showed 60mph, that won't happen, you can't get there with this engine/boat setup.
you already posted data of 48mph @ 5400rpm, that is within 100rpm of your posted peak torque. you are real real close to top performance right there.
If you got the throttle actually WOT position @ 5400 with 12x15cupped @ 48mph, and if your Jag engine makes peak torque @ 5500, then you are very very close to as fast as you can go. You can fiddle with props and fiddle with gears, and you might see 50mph, but I doubt you can see 54. you can change props and gear sets and swap hole shot for top rpm, but I doubt you will see much more speed.
I read in one of your post that you were not on plane until 2700rpm, and your info indicated 2700rpm is some like 27mph. Well, at 27mph you are definitely on plane. you are up on plane around 18mph regardless of rpm. I got a 25 foot inboard and I start planing around 13mph and fully up about 16. I do long range cruise @ 20mph and I am fully up on plane at that. your boat and mine are both planing hulls, and our weights are going to be close.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:


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