Re-powering a Duffy 35

Im in the middle of restoring a 1987 Duffy and Duffy 35.Im re powering this vessel with a Cummins 450 hp.Im at the point where Im wondering what gear ratio and prop size to use.I would like to get as much as I can out of this little motor but dont want any issues later on down the road.What would you run for ratio on box and prop size?I dont know what this boat weighs,Ive spoken to Bill @ atlantic boat works and he doesnt know becuase of the age of boat.Can anyone here help me with this puzzle?

 

Dear Drunkin clown,

It will be useful to know at least the length , the beam at waterline level and the draught.
An important information also is the immersion of the propeller shaft.
You can extract all these measurements - if you are still in dry dock - from the
waterline painting ; the end line of the underwater coating.

 

Well like I said Its a 35 Duffy,beam is 12.6 feet and it draws around 38"The existing prop is a 26x24 nibral 4 blade.I really dont how deep the shaft sits in water.Thanks

 

Propeller's Data Estimation

Dear,

I supposed that your engine is a Cummins NTA855 M 450 hp at 1800 rpm.
See the corresponding attached file contains the Engine's Leaflet.
I also supposed that you will install a 4-blade propeller.

If all the above are correct then see on the second attached file the information you asked.

I calculated the possible combinations for your case with different reduction ratios.
The estimated speed for this Power of 450 hp is 20 kts.
You have to choose the correct New Propeller's Dia looking carefully the clearances around the New Propeller's Dia to follow the next sizes:

1. Distance of the Blades' Tip to the keel not less than 10% of Propeller's Dia.
2. Distance of the Blades' Tip to the Upper Hull not less than 17% of Propeller's Dia.
3. Distance of the Blades' Tip to the End of Hull's Stern not less than 20% of Propeller's Dia.

I hope that you find the answers on what you asked.
If the engine is different type please give back the necessary information regarding her nominal speed on which it produces the 450 hp regarding to modify the calculations.

 

These guys might have a clue what it needs http://www.atlanticboat.com/duffy35.php

 

Ive called Atlantic boat works the other day and spoke to a fella named Bill,he told me that the molds have changed ownership twice since Duffy and Duffy sold out.He said that info was available in 94.He also told me to give him serial number and the distance from steering bulkhead to centre of transome.Maybe he can help.I would like to see at least 25-27kts out of this hull and motor.The engine is the 8.3 llitre 450 Diamond series.450 hp at 2400 rpm.Does anyone know the weight of these hulls with the lobster cabin,300 gals fuel.All hauling gear was removed aswell as rotten timbers.Would love to post a pic but they are too big for this forum or site

 

Please see the attached file
Is this your engine?
Note that if the referred engine on the attached file is that you said the rated speed is 2600 rpm and not 2400 rpm.
So please confirm the correct data.

 

And..........

 

and....... see the attached file

 

Thanks Adler! appreciate that!

 

Well yesterday was a good day for me.A friend of mine who is a Cummins dealer here brought down a Cummins engineer who is visiting the island to see the boat.We sat off on the boat for about 45mins talking about the boat and engine.I just have to give him some proper measurements and he said he will work everything out for me.Hope my wallet is up to task!He said the only issue he see with the install is exhaust.I may have to run an inline check valve.He said Cummins also makes risers that should be perfect for this application.Has anyone seen these Cummins risers before?Should I think about making my own?

 

If you do install a riser have the riser portion insulated versus water cooled. Eventually all risers will corrode through and if it is uphill from the engine it can allow water into the turbo and exhaust manifold.

 

I had a riser made for my old boat and it was insulated.There was a raw water feed to it from engine which helped cool it too.The pipe welded on for the water was on the downhill portion of the riser so there was little chance of water going back into engine.I really wish I could figure out how to downsize my pics so I can post some!Pics speak a thousand words...Clown

 

Another option from a riser would be a silencer that seperates water from gas in the exhaust.

These are Vertical Cylinder units with a drain in the bottom. The gas and water enter the unit from the elbow, the water drains out the bottom immediately, and the gas now cooled and similar to its original volume exits traditionally out the transom dry and quieter.

This might actually be cheaper than having a riser made with the added benefit of a quieter boat. There is no danger of water getting up to the elbow because it drains immediately through a drain in the bottom of the silencer. For your size engine something like a 2.5" drain.

Do you know Travis Lewis??

Steve

 

I know a couple of Peters and a Terry.What does Travis do?