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Old 28 October 2012, 22:40   #1
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Country: USA
Town: San Clemente
Boat name: Scotty D
Make: Cabo 35' Express
Length: 10m +
Engine: 3208 Cat Diesels
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Otech 7M RHIB with Cummins 6bt

Picked this up on thursday and making good progress cleaning her up.

2002 Otech 7M RHIB center console. Similar to the Willard 7M Navy.
2010 Wing tubes.
289 hours on a center mounted 2006 Cummins 6BT 210hp.
Jackshaft to a Bravo 2X diesel 1.50 ratio sterndrive.
17.75X 21 pitch prop pushes her to 30.7 knots per Garmin GPS. Cruise is 23-27 knots with current prop.
3.6 nmpg cruise economy.
36 gallon fuel tank.
Trick freshwater flush system and fuel strip pump.
Garmin GPS and sounder
Standard Horizon VHF
Newmar battery charger
Whelen yelp/siren/PA (non-op)
5600 lbs.

Anyone know of a good used galvanized or aluminum dual axle trailer for sale (7,500 GVWR minimum)?
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Old 28 October 2012, 23:49   #2
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Looking good. Pretty quick work. What kind of work have you had to do to get it operational? What kind of rpm's were you turning to get that kind of speed at WOT (excellent performance for that set up, by the way) ? Are you sure that the prop is a 21 and the gear ratio is 1.5?
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Old 29 October 2012, 10:49   #3
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Boat name: T/T Something Bigger
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Engine: Yanmar 6LP 315hp
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Is there a transmission or does yours shift thru the Outdrive? I added a ZF Hurth 1:1 transmission and a vulcan torflex dampener. Mine is scary quiet with no vibration. Looks pretty
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Old 02 November 2012, 14:03   #4
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Quote:
Originally Posted by ncp View Post
Looking good. Pretty quick work. What kind of work have you had to do to get it operational? What kind of rpm's were you turning to get that kind of speed at WOT (excellent performance for that set up, by the way) ? Are you sure that the prop is a 21 and the gear ratio is 1.5?
Lube, fluids, checked fuel tank for crap, added fuel, motor paint, unstuck the turbo, purge air from fuel system, add batteries, chase some wiring ghosts, patch one chamber and splash. Shift is a little gummy but with fluid change and use, hopefully it works itself out other wise will rebuild clutch cones in the drive.

Motor runs like new which given it only has 289 hours would be my expectation. Very tight, no smoke.

No tranny straight jack shaft to bravo 2 1.50 ratio (i thought, will check again). Prop is 17.75 x 21 pitch (mercury part # 4818616A40 21P). Better fit would most likely be a 23 pitch since I am over WOT by 10% ish.....
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Old 02 November 2012, 18:13   #5
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Quote:
Originally Posted by ShadBurke View Post
17.75X 21 pitch prop pushes her to 30.7 knots per Garmin GPS. Cruise is 23-27 knots with current prop.
Actual speed from testing?

Quote:
Originally Posted by ncp View Post
Are you sure that the prop is a 21 and the gear ratio is 1.5?
The reason that I asked is because based on the above, I did some rough calculations and thought you might have negative prop slip numbers...which, obviously, isn't possible.

Quote:
Originally Posted by ShadBurke View Post
Prop is 17.75 x 21 pitch (mercury part # 4818616A40 21P). Better fit would most likely be a 23 pitch since I am over WOT by 10% ish.....
But, when you say you're 10% over rated rpm's, it makes much more sense.

Rated WOT RPM for Cummins 6BT210 2600 rpm
10% over equals 2860 rpm

Prop slip, based on 2860 rpm, 21 inch pitch prop, 1.5 : 1 gearing, 30.7 knots/35.33mph, is 6.82% slip.

Fuel economy, at WOT, based on the above and 11.9 gallons per hour (2600 rpm rating...Cummins doesn't provide data for being over rated WOT rpm), is 2.58 nmpg/2.97mpg

Low prop slip....sounds like it's running well. I'd love to hear how it runs with a 23 in pitch prop. 35.33 mph is alread great. I've heard of Willards getting up to 38-39 mph with the right prop and a clean bottom (I'm running my Willard with a 6BT210, a 17 1/4 x 23, and make 36.00 mph at ~2700 rpm; I have bottom paint on though instead of a clean gel coat...I sometime wonder how it would go with clean bottom and a 24 inch pitch)

I wonder if the extra 6 inches of beam on the Otech makes planing easier? (versus a Willard at 9'0 beam). Though top speed, I'm guessing, wouldn't be affected because the wetted surface is the same? (don't know!) ePerformance-wise, I wonder how the different lifting strake design works.

Keep the pictures coming!
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Old 02 November 2012, 19:17   #6
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I would expect single digit slip numbers given no bottom paint, cg, drive depth and prop diameter. Keep in mind, knots reported are SOG, so there are some variables that were uncontrolled. Over the next two weeks, I shall see what comes of further break in.

Lobster are quiet thick right now over at the island, most likely running tuesday to the west end for some so we shall see.
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Old 02 November 2012, 19:52   #7
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Quote:
Originally Posted by ShadBurke
I would expect single digit slip numbers given no bottom paint, cg, drive depth and prop diameter. Keep in mind, knots reported are SOG, so there are some variables that were uncontrolled. Over the next two weeks, I shall see what comes of further break in.
I was down at 8% when I was running an 18 1/2 x 21. My slip is just over 10% now with the smaller diameter and higher pitch (which makes sense)...but much happier with performance.


Quote:
Originally Posted by ShadBurke
Lobster are quiet thick right now over at the island, most likely running tuesday to the west end for some so we shall see.
I was over there two weeks ago ( West End)...maybe we were in the wrong place, but I ended up freediving Cherry, 4th, and Isthmus later the same night (used a tank at West End) and did much better!

Have fun. Looking forward to a report!
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Old 02 November 2012, 20:21   #8
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Have you tried any of those Hill Marine Big Cow 4 blade ss props? 18.25 x 22??

I was thinking of calling them since they are local and seeing how they perform
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Old 02 November 2012, 23:42   #9
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Nope. Haven't tried them.

I started with a Merc Aluminum 19 in pitch (don't know diameter), then tried a Powertech BRA321 (18 1/2 x 21, 3 blade), and settled on a Powertech BRS323 (17 1/4 x 23, 3 blade).

I talking to Konrad, the "prop expert" suggested a 16 1/2 x 22/23 Powertech. I emailed them later when I got the 17 1/4 x 23 and told them that I'm still making rpm's, so I wasn't going to bother cutting it down! I'm guessing that theNavy uses the 19 in itch for having the max 18 people loaded up.
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Old 10 November 2012, 00:06   #10
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Did you take the Otech to Cat the other night and if so, how did it go?
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