Otech 7M RHIB with Cummins 6bt

ShadBurke · 29 October 2012, 03:40

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)?

ncp · 29 October 2012, 04:49

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?

johndoe007 · 29 October 2012, 15:49

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

ShadBurke · 02 November 2012, 19:03

Quote:

Originally Posted by ncp

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.....

ncp · 02 November 2012, 23:13

Quote:

Originally Posted by ShadBurke

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

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

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!

ShadBurke · 03 November 2012, 00:17

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.

ncp · 03 November 2012, 00:52

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!

ShadBurke · 03 November 2012, 01:21

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

ncp · 03 November 2012, 04:42

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.

ncp · 10 November 2012, 05:06

Did you take the Otech to Cat the other night and if so, how did it go?

ShadBurke · 12 November 2012, 17:10

We ran the 22' Whaler Guardian over Thursday night to the West End in 65' of water for a wide open limit style crawl on some nice grade lobster. We were back at the dock at 11pm with 14 lobster. Ran solo last night and it was much slower, water had dropped off 3 degrees to 59.8 due to weekend storm and the current wasn't "ideal". Still managed to get 7 but it took a whole lot longer........

So far, shaping up to be a good bug season.

Let me know if you want to run some time.

ncp · 15 November 2012, 19:17

Quote:

Originally Posted by ShadBurke

We ran the 22' Whaler Guardian over Thursday night to the West End in 65' of water for a wide open limit style crawl on some nice grade lobster. We were back at the dock at 11pm with 14 lobster. Ran solo last night and it was much slower, water had dropped off 3 degrees to 59.8 due to weekend storm and the current wasn't "ideal". Still managed to get 7 but it took a whole lot longer........

So far, shaping up to be a good bug season.

Let me know if you want to run some time.

I don't know anything about hooping (other than what I've read)...I always just swam down and grabbed them!!!

I would like to go sometime with someone knowledgeable though. I'll be in Two Harbors Sat-Mon AM this weekend. Hoping to get a dive in (or two).

ShadBurke · 16 November 2012, 03:46

I was over there today, smooth seas. Rain is schedule for the weekend. Let me know if you want to try the hoop deal, send me a PM with your phone number. Since the opener this season, I am now into the triple digits as far as volume goes........ so I kinda know how to do it........

Going to try a 24 pitch 4 blade after Thanksgiving since still over WOT.

Will let you know how the performance works out.

With the 22P, it's starting to be a little bit of a nightmare around the dock, idle is at 6.4 knots...........

ncp · 16 November 2012, 12:48

Quote:

Originally Posted by ShadBurke

I was over there today, smooth seas. Rain is schedule for the weekend. Let me know if you want to try the hoop deal, send me a PM with your phone number. Since the opener this season, I am now into the triple digits as far as volume goes........ so I kinda know how to do it........

Going to try a 24 pitch 4 blade after Thanksgiving since still over WOT.

Will let you know how the performance works out.

With the 22P, it's starting to be a little bit of a nightmare around the dock, idle is at 6.4 knots...........

You've got me beat this year by a long shot. Tanks and free diving just hasn't been as productive for me this year (mostly north SD, some Point Loma/Cat/SBI). We're heading over this weekend regardless; it's been planned for a while. Hoping for a smooth ride on Saturday even with the 30% chance of rain.

I'd be interested to hear how you go with a 24 pitch 4-blade. I've never tried one but everything I've read says top end suffers while allowing you to plane at a lower speed. A friend just got 41 mph out of a Willard (6BT210) with a Powertech BRS324 17 1/4 x 24. And, I hear you about around the dock...on all 3 props I've tried, I'm 5.0-6.2 mph at idle.

ncp · 19 November 2012, 18:09

Shad, the parking guy at Dana on Saturday though that I was you...asked if I "had the new prop yet?"

Let me know how it runs this weekend!

ShadBurke · 19 November 2012, 20:17

That's funny. I was running different props last Wednesday, Thursday and again on Friday. Was stopped by Harbor Patrol, DFG, and Homeland Security. Perfect trifecta on law enforcement........

I did run the 18.5X24P 4 blade Hill Cow and it got the RPM right where it needed to be. However, performance on top end was same as the 22P, so most likely going to "ear" the 22P to get some more on top end. Cruise improved almost 10% from the 22P, 26 knots (at 2100 RPM).

I saw 3.8nmpg on the stock Mercury 21P 3 blade aluminum running 76 nautical miles on Thursday (at 2100 RPM). Pretty impressed with the economy of that Cummins and with the boat weighing in at 5,600lbs, seems to crush anything in it's way. Very smooth and balanced. Hard turns causes a little excess vibration since the exhaust goes thru the drive but it is what it is....for now.....

Saturday we hooped for lobster. Started early and finished late. Managed limits for both of us in 65' water and macks. Back in slip at 3:30am. Seemed like the bite has slowed since we haven't changed our game plan since the season opened.

Maybe weather, maybe who knows..........

johndoe007 · 21 November 2012, 15:38

No fair it's 30 degrees here... Better get used to being boarded... Nothing like these boats to make friends across the front of an m-60.

Sent from my iPhone using Rib.net

ncp · 21 November 2012, 19:15

Quote:

Originally Posted by ShadBurke

I did run the 18.5X24P 4 blade Hill Cow and it got the RPM right where it needed to be. However, performance on top end was same as the 22P, so most likely going to "ear" the 22P to get some more on top end. Cruise improved almost 10% from the 22P, 26 knots (at 2100 RPM).

I saw 3.8nmpg on the stock Mercury 21P 3 blade aluminum running 76 nautical miles on Thursday (at 2100 RPM). Pretty impressed with the economy of that Cummins and with the boat weighing in at 5,600lbs, seems to crush anything in it's way. Very smooth and balanced. Hard turns causes a little excess vibration since the exhaust goes thru the drive but it is what it is....for now.....

Saturday we hooped for lobster. Started early and finished late. Managed limits for both of us in 65' water and macks. Back in slip at 3:30am. Seemed like the bite has slowed since we haven't changed our game plan since the season opened.

Sounds like you have it figured out.

My calculated slip went up when I went from a 18 1/2 x 21 to a 17 1/4 x 23. Performance is better on average accross the rpm range. Though, I had wondered what a 4-blade would have done - I had always heard that it hurts the top end but reduces mid-range slip. Seems like that's exactly what you found. I'm pretty happy with mine...but, if I were to buy a new prop (again), maybe I'd try a 3-blade 18 1/2 x 22or23.

Sounds like your bug season is going pretty well!!!

ShadBurke · 22 November 2012, 15:57

We ran again on Tuesday night for lobster. Tried some shallower spots 35-55' and we had our limits running thru the nets twice. Much better night. We were back at the dock at 9pm. The offer is still open if you want to get in on the hoop net game, we are going to run again on Saturday night. PM me for details.

ncp · 22 November 2012, 18:40

Quote:

Originally Posted by ShadBurke

We ran again on Tuesday night for lobster. Tried some shallower spots 35-55' and we had our limits running thru the nets twice. Much better night. We were back at the dock at 9pm. The offer is still open if you want to get in on the hoop net game, we are going to run again on Saturday night. PM me for details.

I must be doing it wrong by diving for them...

PM Sent