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Old 22 October 2013, 11:39   #1
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Country: USA
Town: Hood River
Boat name: Seal Team 7
Make: Zodiac SRA-750
Length: 7m +
Engine: Evinrude ETEC G2 300
MMSI: WDI 8895
Join Date: Sep 2012
Posts: 264
Willard 730 Diesel Repower Project

Well unfortunately my Navy-abused Cummins 6BT only made 40hrs before needing the head off. When we pulled the head, there was severe scuffing of cylinder walls. Basically, that was the end of my relationship with it. I sold it to someone who will rebuild it and make her like new again.

Lesson here is to be very careful buying these boats as you can't really know the internal condition of these engines.

For me, I'm taking the opportunity to upgrade to more horses and speed in general.

Starting this thread to document the repower.

Here's the old one out of the boat and on a pallette.
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Old 22 October 2013, 11:56   #2
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Country: USA
Town: Hood River
Boat name: Seal Team 7
Make: Zodiac SRA-750
Length: 7m +
Engine: Evinrude ETEC G2 300
MMSI: WDI 8895
Join Date: Sep 2012
Posts: 264
Here are the existing propping numbers from the 220hp 6BT.

Top
2600 rpm | 29.50kts | 1.43 ratio | Slip 10.5% | 17 1/4" prop | 23 pitch

Cruise
2300 rpm | 25.50 kts | 1.43 ratio | 11% slip

At cruise, burning under 6 gal per hour.


I would like to go into the 300hp range which should give a much higher top end and cruise. Some of the Hurricane interceptors use the Yanmar 315hp. They max at 3800rpm. The new Volvos max at 3500rpm. Cummins max RPM is lower. I am planning on sticking with Cummins for a number of reasons, mostly because the boat is already setup for Cummins geometry and engine mounts. I've learned a lot already about Cummins and I do think they are good engines. Also, the parts are reasonable and the cummunity support is excellent.

The problem with say a 330hp Cummins is that max RPM is only 2950. My transmission is 1:1 and my Konrad 520 drive is 1.43:1. At that gear ratio, I can't go up much on the prop pitch or enough to gain top end.

There are two possibilities to change gear ratio that I know of.

#1 Trade out the Konrad leg for another Konrad at 1.21 ratio.
Pros: Fairly simple change
Cons: Torque to existing tranny (Borg Werner 72C) exceeds max rated torque by a lot. Cummins 330 QSB can put out 814lb/ft. BW gear is rated to 450lb/ft, and that is at max rpm only.
Also torque at Konrad exceeds Konrad max of 738lb/ft although Konrad doesn't seem to worry about that.

#2 Change out BW gear for a ZF280-1 gear with .814 ratio which gives final gear ration of .814 x 1.43 or 1.16:1.
Pros: ZF280 gear can take the torque. Expansion ratio reduces torque to Konrad by 20%. Ratio gives fairly optimal numbers using my existing props
Cons: Custom order ratio, adds cost. Not sure of availability.

Using a 1:16 ratio and 330hp QSB my new prop numbers would be

MAX Speed
2950 rpm | 23 pitch | 1.16 gear ration | slip 10% | 49.9 mph
2950 rpm | 22 pitch | 1.16 gear ration | slip 10% | 47.7 mph

Cruise
2600 rpm | 22 pitch | 1.16 gear ration | slip 10% | 42 mph
2300 rpm | 22 pitch | 1.16 gear ration | slip 10% | 37.2 mph
2100 rpm | 22 pitch | 1.16 gear ration | slip 10% | 33 mph
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Old 23 October 2013, 19:08   #3
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Country: USA
Town: Hood River
Boat name: Seal Team 7
Make: Zodiac SRA-750
Length: 7m +
Engine: Evinrude ETEC G2 300
MMSI: WDI 8895
Join Date: Sep 2012
Posts: 264
OK well #1 and #2 options are now out of favor. What a difference a day makes.

Thinking of sticking with the velvet drive 1:1 tranny an possibly getting it beefed up. Finding this a common approach with a lot of the faster boats.

Pitching up in prop and calling it good.

All existing geometry stays the same.
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Old 23 October 2013, 19:45   #4
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Country: USA
Town: Seattle
Boat name: Water Dog
Make: Polaris
Length: 4m +
Engine: Yamaha 60hp
Join Date: Apr 2007
Posts: 1,152
Pretty boat, hope it all works out. I'd love a diesel inboard in this size range.
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Old 23 October 2013, 20:08   #5
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Country: USA
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Make: Zodiac RIB-P
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why don't you stop thinking small and throw a bigger diesel in there with an areneson surface drive and call it good

Jason
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Old 23 October 2013, 22:29   #6
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Country: USA
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Boat name: Seal Team 7
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Posts: 264
It would slash the tubes on the turns

How's yours coming?
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Old 23 October 2013, 22:41   #7
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Country: USA
Town: Hood River
Boat name: Seal Team 7
Make: Zodiac SRA-750
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Join Date: Sep 2012
Posts: 264
Here's the hole.

Sucks to have a boat that is a BIG paperweight, but she'll be right again.

The good thing is that all the geometry should be almost the same by using another Cummins 5.9 block engine.

Take a moment to look at the Gianormous NAVY specified Sea-strainer. That needs to move somewhere else or traded for sane sized one.

6BT is not aftercooled but the QSB is and more hardware on that side of engine.

Exhaust looks huge but it needs to be bigger. Will need a different exhaust port on transom or go with twin ports.
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Old 23 October 2013, 23:36   #8
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Looks like the arneson's fit on the 11m boats,

All of a sudden it seems like things are moving along. The new gas tank is in, and I am getting the transom cap fixed and a handle for the forward hatch fabricated. I'm going to chop up the original diesel tank (gasp!) so I can use the deck top. Then I need to swap trailers and hang the OB. Need to purchase the hydraulic steering system and do some basic electrical. Then it should float and go like a boat

Jason
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Old 24 October 2013, 01:17   #9
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Arneson Surface Drives

Surface drives, less drag and a lot less money in your bank account
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Old 24 October 2013, 10:56   #10
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Country: USA
Town: Hood River
Boat name: Seal Team 7
Make: Zodiac SRA-750
Length: 7m +
Engine: Evinrude ETEC G2 300
MMSI: WDI 8895
Join Date: Sep 2012
Posts: 264
Yeah a lot less.

You'll be excited when those engines are hanging off the transom. That will be a big difference.

On the fuel tank, seems like a shame to chop up. You don't need the vent so why not create a fiberglass plate? Easier to keep painted as a bonus.

I would like to figure out how to install a second kicker tank somewhere. Not a lot of options with the Diesel.

On my tranny, you can get a velvet drive totally beefed up and redone to take a lot more torque. It's out of the boat and on a palette so now is the best time to do it.
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