Author Topic: Prop for a Banka  (Read 788 times)

Offline Keithg1

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Prop for a Banka
« on: July 08, 2018, 12:55:39 PM »
Greetings, I have a 56 ft Pump-boat with a Mitsubishi 4DR5 diesel motor and the hull is pretty lightweight being a wood-fibreglass composite design. Can anybody advise the best prop-size for her? She currently runs an 18" 3 blader but I feel that there is more speed to be had.

Offline JoeLP

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Re: Prop for a Banka
« Reply #1 on: July 08, 2018, 10:35:08 PM »
Welcome to the site Keithg1.  Please take time to introduce yourself to the group when you can get around to it.
There may be someone on here that can help you with your question.  It's in an arena where I lack the knowledge to help on so many levels.  I do enjoy nice big boats to get out on the water(been on more than my fair share of boats in that size range on the great lakes when I lived in Michigan).  That said, I never owned one nor got into the maintenance and workings of such boats.  It does sound like you have a nice boat to get out on and enjoy.
« Last Edit: July 09, 2018, 06:07:02 PM by JoeLP »
In the land of the blind, the one eyed man is king.

Offline Peter

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Re: Prop for a Banka
« Reply #2 on: July 09, 2018, 04:47:45 PM »
Greetings, I have a 56 ft Pump-boat with a Mitsubishi 4DR5 diesel motor and the hull is pretty lightweight being a wood-fibreglass composite design. Can anybody advise the best prop-size for her? She currently runs an 18" 3 blader but I feel that there is more speed to be had.
.

The 4DR5 engine would have come from a Mitsubishi jeep or light truck originally. This engine is basically a "grunt" engine, "built for comfort, not for speed, as we used to say.  :) :) " but would have developed around 80 HP at 3,500 revs, when new.

Bottom line, as it's a land vehicle engine, there will be no recommended/best/optimum matched prop size for marine use. Everything would be a trade off.

Hopefully it has been properly prepared for the marine environment, or it probably won't last very long. Replacement engines in our area (Bataan) are around PhP 70,000 to 100,000. Cheaper up in Zambales.

Here is what I would do in your situation.

Looking on the internet, there are free service manual downloads, so it would be possible to work out what the gearbox output drive shaft RPM should be, at 3,500 engine RPM.

I would then look at the pumpboat gearbox (is one fitted, or is it a direct drive?) output shaft with a rev counter strobe, or get my friendly mechanic to do it, and see what we're getting under running load with an 18" prop. If it is near the value of the original gearbox spec, then any increase in prop size may not give any appreciable increase in speed, and could cause mechanical problems down the line. Greater stress on the output shaft/couplings/gearbox etc. etc.

If I still wanted to go with a bigger prop, I would try to "borrow" a bigger one, 20" or 22" say, and see what the difference (under load) in shaft revs are, with the engine running at 3,500 RPM, and select accordingly.

Of course, that is what I would do, others may disagree.   :(

Peter

« Last Edit: July 09, 2018, 05:20:01 PM by Peter »
Noli nothis permittere te terere.
Virtus autem corruptibilis est,
summa virtute prorsus corrumpitur,

Offline lost_in_samoa

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Re: Prop for a Banka
« Reply #3 on: July 11, 2018, 09:38:08 AM »
Mitsubishi 4DR5 diesel motor


Does this help ?

4D5-Step III manual pdf file

4D56 engine specs in a blog post

Note - links to the manuals at the bottom.


Offline Peter

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Re: Prop for a Banka
« Reply #4 on: July 11, 2018, 05:51:47 PM »
Does this help ?

4D5-Step III manual pdf file

4D56 engine specs in a blog post

Note - links to the manuals at the bottom.


lost_in_samoa

On the right track to get the OP thinking. There are posts on other sites which go into building a banka with different engine types, including the 4DR5. A quick search by him should bring up posts and contacts, to address his question.

I haven't the time today to go deeper into the subject, as it's the World Cup football 2nd. semi-final tonight.

For what it's worth, however, the 4D5 and 4DR5 engine specs are different, so the manuals for them may have differences as well.

The 4DR5 is a larger engine of 2,659 cc, 59 kW (80 HP) at 3700 rpm, while the 4D55 is 2,346 cc, 48 kW (around 65 HP) at 4,200 rpm.
(The 4DR6 is a 74 kw (100 HP) direct injection version of the 4DR5.)

The unknown factor is the gearbox type, if fitted, to enable the drive shaft revs to be monitored and the info to be used to go forward.

Hope this helps.

Peter

« Last Edit: July 11, 2018, 06:04:48 PM by Peter »
Noli nothis permittere te terere.
Virtus autem corruptibilis est,
summa virtute prorsus corrumpitur,

Offline lost_in_samoa

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Re: Prop for a Banka
« Reply #5 on: July 11, 2018, 06:59:34 PM »
On the right track to get the OP thinking.

Took me about 5 minutes to find those.  Just trying to be neighborly.

 


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