Glen-L.com

So, you are saying that you get approx 10kts @ 1000rpm, 20kts @ 2000rpm, and 30kts @ 3000, correct?
And, you are presuming you should get 40kts @ 4000rpm (but your 40kts isn't happening), correct?
And, you are getting 4500rpm WOT.

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God Bless our Troops; especially our Snipers

From memory, yes.

Speedo (GPS) pegs out at 32 knots, despite engine giving an extra 1500 rpm past 3000.

I was about to order a down angle ZF63 1.2:1 tranny with a mercruiser 8.2 MPI for my 24'6" tahoe.. should i be getting the velvet 1:1 instead??

Cheers

That could be a topic all its own. The subject of whether a 1.23 (by adding prop pitch) is the same as a 1:1.
I say no, it is not the same; others will say there is no difference.
Back in the day when the plans were drawn for Tahoe, Monaco, Riviera the standard small block Chevy was 260hp. BBC was about 330hp. The tranny of choice back then was the Borg-Warner VelvetDrive in direct drive ratio. 71c handled the small blocks and the 72c series handled the big blocks.
With modern engines the SBC is no longer 260hp, it is now making the power that the old BBC used to. The fuel injected BBC is now in the 400hp class.
The original design of these boats ran well with the SBC 260hp, 1:1 gear, 13x13 class prop on a 1" shaft. (This just a generalization of prop size here.)
If you drop in bigger HP the prop size would naturally go up in pitch or diameter, (with tranny & shaft being constant).
Lets assume you upped from the old SBC 260hp to the MPI SBC 340hp. Your prop is not the old 13" class but rather a 14x14 class size.
Now assume you upped tranny gear ratio from 1:1 to a 1.23:1. You no longer at 14x14 but you go to a 14x16 4blade heavy cup or similar. Now your at the upper limit of a 1" shaft. Probably go to 1 1/8" shaft for prop size availability & safety.
Now assume you upped the power to a Big Block MPI. You would up the size of the prop again accordingly. Now you would likely be looking for a 15 diameter. 15 diameter is a oddity and I dont know why. Nobody makes a 15 dia 4-blade.
I believe you would be better off with a 72c 1:1 and a 1 1/8" shaft. The 1:1 gear would allow lesser pitch/diameter than the 1.23 which would lead you to a more standard strut, and better prop selection.
dont forget that as prop size goes up, the size of strut goes up along with size of rudder, length of shaft also.
My vote goes for the 1:1 72 series velvet.

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God Bless our Troops; especially our Snipers

Wow.. thanx much for this detailed info.... Since its another subject all together, I'm gonna send you a pm with another question since u seem you know inboards!!

Thanx again!

Shaft rpm also goes in the equation. In the '50s and early '60s engine turned in the 3000 rpm range. With engine rpm near 5-6000, prop tip speed can enter in.

Bill

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Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Speaking of shaft RPM and equations ...

To compare optimum diameter for 1:1 and 1.23:1 ratios for increasing power levels, click on the link below. It will download an excel spreadsheet which uses the DIAMETER-HP-RPM Formula from Dave Gerr's Propeller Handbook to find propeller diameter from HP and RPM.

http://www.chelseacoachworks.com/boats/ ... 23vs1.xlsx (excel spreadsheet)

Here's a link to a web page if you can't download the spreadsheet :

http://www.chelseacoachworks.com/boats/ ... 123vs1.htm (web page)

The Diameter-HP-RPM Formula is also included in our Propeller Calculator (click on the Diameter-HP-RPM tab at the bottom of the spreadsheet).

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Paul Kane Chelsea, PQ

Building the Glen-L Hot Rod : http://www.boats.chelseacoachworks.com

Paul,
I couldnt get the link to work, but, I can say that I've been thru all of that book and he sizes diameter per the size of the boat. Reduction gear comes from a formula of how fast the installed HP can make the boat go.
In reality the smaller the HP the larger the prop, and vice-versa.
Anyway, after going thru all the formulas, my own boat worked out to a 16" dia wheel per his formulas, and a 14" 4-blade equals the blade area of a 16 3-blade. I have run less dia and still run fine. David Gerrs numbers are quite conservative.
So, any boat smaller than mine would be perfectly happy with a 14" prop. Builders and owners can experiment with 3 blades vs 4-blades..... cup vs no cup.......rake vs no rake......14" dia is more than adequate for a runabout sized hull.
When you start installing higher HP without adding more diameter, then you contain all that extra HP is via pitch & cup.
A riviera/Tahoe with a 8.2 MPI reduction geared that was held to 14" dia prop would run out of prop size options as far as pitch is concerned.

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God Bless our Troops; especially our Snipers

So is the 8.2 not a very good idea then??

There is a Riviera already running with a 5.7L MPI with 1.23 gear box. As I understand it, he had to get custom cupping to get enough prop on the 1" shaft. He is topping 60mph. It is my understanding that a Riviera gets loose at that kind of power. I got a ride in it. Felt loose to me........It is your boat, power it up any way you like. That is the beauty of building your own project.

If you were to go up to 8.2l MPI with 1:1 box, you perhaps might turn a similar prop as his. Then again, you may be looking at a totally custom prop. A 1 1/8" shaft could easily size a prop for that though.

_________________
God Bless our Troops; especially our Snipers

Ah no the 8.2 isn't going in the riviera... Im putting that in the 24'6" tahoe. The Riviera will receive a 5.7

I definitely don't want a loose boat... what did the guy do to fix the problem?

Loose - ubetcha. Over 50 and you feel the lightness of the boat in a hurry. There was more than one person who mentioned how light the boat felt I higher speeds despite have 4-5 full size men in it for added weight. The Tahoe and Riviera are very similar is weight.

With the added torque using the reduction gear I had to go large on the prop to hold my 5.7L. I suspect you are going to have to deal with the same issues with a 8.2L and no reduction. You are going to have to experiment to hold the engine RPM's down to a manageable wide open throttle.

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Dave

My Riviera build - the Midnight Cry Project

Video of Midnight in Action
Blog

Few things in the world measure up to the thrill and satisfaction of boating in a boat that you built.

I havent place the order yet for the motors... What do u guys recommend me to order for each boat? It needs to be a Mercruiser tho. What tranny to go with each? I want to plug n play with least headache.

I think this would be a good topic for a new thread.
Please open one.

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God Bless our Troops; especially our Snipers

300 hp should put you at about 50 mph. After that you're on your own.

Bill

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Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build