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New Cummins/Mercruiser Project

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Old 11-25-2010, 12:26 AM
  #201
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I believe your green engines were brought back to 300hp when they sold the boat? If the propellers you are running today are doing well (not much slip) then when you add 360more Hp 50% more you will not add much more speed. You will only be able to add a few inches more pitch but will be limited by the 1.15 gear ratios.

If today you're running propellers that give you a lot of slip then with the extra Hp you can go up in Dia and in pitch reduce your slip which adds speed and the extra pitch will add some speed and you might see a big difference.

My remarks regarding dia are based an assumption. Say that boat has 16" props now and you figure you are running with only 10% slippage at full rpm. If you go to 18" dia you may only get up to 8% slippage and although your speed may increase slightly it is not going to be in leaps and bounds.

With the extra power you might be able to run 6 blades or something and really get the efficiency up there but still can't run very fast with that final drive ratio.

The ASD-8's, I choose them because they work & don't break - would not take them off for nothing!
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Old 11-25-2010, 08:44 AM
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Don't forget these are 3400 RPM engines not 2800 RPM.

The boat ran pretty good with the 300's in it so with the extra HP and RPM I am expecting a corresponding increase in top end. I should probably have tried to get a special overdrive ratio from Twin Disc for the marine gears but had some concern with the boat being able to get on plane without having to rely on vent tubes and the like.
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Old 11-25-2010, 09:07 AM
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You are right no more 2,800 rpm days - LOL!

Me I would have went with the OD's and a 2 speed, best of both worlds. I'll say this if you are not using vent tubes you are probably not using the bulk of the Hp the engines can make.

I posted on here somewhere a dyno chart of a 380hp B the exact engines your boat had in it. We were having problems with propellers - no surprise there!

I kept feeling we have plenty of power left or the engines were having problems in the boat itself for some reason. We took boost readings at a range of rpm's while in the boat running and some exhaust temps for haha's. Pulled one engine out threw it on the dyno and the chart shows 381Hp no problems and it could make 380 down to about 2,600 and up to 2850. We then ran the engine at 2,800 and watched to only get I think 28psi boost and the engine only makes around 305-315Hp.

We were not loosing but not USING 20% of the Hp we had available. Again, getting on plane was the issue, if we had a true 2 speed back then that is what would have gotten the extra hp back, we could have taken a prop that we could not get on plane with and used all the available Hp.

On the other side, take what we learned on the Malestrom and from Buzzi and really OD the crap out of it, use small propellers and go fast as hell. The delema was and still is then I can't stay on plane below 50 mph because the propellers don't make enough thrust to carry the boat at slower speeds.

The user has to decide what they want driveability or top speed - the two speed gives some of each!
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Old 11-25-2010, 02:48 PM
  #204
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Quote:
Originally Posted by THERUSH View Post
Don't forget these are 3400 RPM engines not 2800 RPM.

The boat ran pretty good with the 300's in it so with the extra HP and RPM I am expecting a corresponding increase in top end. I should probably have tried to get a special overdrive ratio from Twin Disc for the marine gears but had some concern with the boat being able to get on plane without having to rely on vent tubes and the like.
What is the setup on this boat?

350 hp .. 2,800 rpm Cummins???

What is the current reduction ratio, boat speed and propeller info?

How does it run?

Boat size, make and weight?
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Old 11-25-2010, 10:42 PM
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The boat is a 40' Super Hawaii American.

Had 300HP @ 2800 RPM Cummins 6BTA's, TD 1.15 Reduction gears, inline ASD-8's and 18 x28.5 4-blade Rollas.

Putting in 480HP @ 3400 RPM Cummins QSB's same reduction and drives.

Boat weighs in about 12,000 lbs and would run mid 50's with the 300's.
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Old 11-26-2010, 12:19 AM
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Quote:
Originally Posted by THERUSH View Post
The boat is a 40' Super Hawaii American.

Had 300HP @ 2800 RPM Cummins 6BTA's, TD 1.15 Reduction gears, inline ASD-8's and 18 x28.5 4-blade Rollas.

Putting in 480HP @ 3400 RPM Cummins QSB's same reduction and drives.

Boat weighs in about 12,000 lbs and would run mid 50's with the 300's.
You will be happier with a different reduction ratio than What you have choosen. Also a different propeller will also improve the boats performance also from bottom end to top end.
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Old 11-26-2010, 08:07 PM
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Quote:
Originally Posted by Rik View Post
You will be happier with a different reduction ratio than What you have choosen. Also a different propeller will also improve the boats performance also from bottom end to top end.
Any Suggestions?
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Old 11-29-2010, 09:33 PM
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Herings would be the best choice, especially for the diesel application. I usually like going with a pair of 6 blades for diesels.
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Old 11-29-2010, 10:28 PM
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bar3 - what diesel boats have you done?
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Old 12-05-2010, 11:44 AM
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Joe ,I,m still busy with my diesel project ,and would like to post a few pics here ,to show you guys what has been done..but damn I,m registered member only ... can anybody post pics here for me?
link : http://www.seriousoffshore.com/forum...?t=9651&page=5

Last edited by stirling; 12-05-2010 at 12:40 PM. Reason: link to pics
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