3 bladed propeller vs 4 bladed propeller

Hi,

I have constructed a vessel which is plying at a speed of 12 knots with a power of 170 hp x 2 nos . The vessel is creating vibrations and we are trying to find ways to reduce vibrations.

If we increase the prop to 4 bladed, do you think it can have a effect.

Also I need reasons why a-bracket flanged bush bolts are getting loose. Can you suggest any solutions.

 

Maybe.

Who designed the boat and what are their qualifications?
What kind of boat is it?
How large?
Displacement?
Pictures?

 

As BlueBell says, not enough info. Realistically, the shaft could be causing the vibration.

 

All else being equal, a 4 blade propeller should have less vibrations than a 3 blade propeller.
However the real cause of your vibration (as Jehardiman notes) could well be mis-alignment of your propeller shaft.
And as Bluebell noted, a lot more info is required.

 

I very much doubt that four will be better, might even be worse.

 

The boat is a catamaran having a round bottom hull a slender hull.We have two engines of 170 hp in both hull.We have a fixed engine girder without any antivibration mounts.
The displacement of boat is around 35 tonnes.

 

Vessel specifications

Length overall -24.36 m
Breadth - 7m
Breadth demi hull - 1.48 m
Depth -1.6 m

Meet Google Drive – One place for all your files https://drive.google.com/file/d/1-4zyMBOC3-Mb93tJCE5NmZsA0ltr8v0C/view?usp=drivesdk

 

Can you post a photo (or two) please?
I clicked on your Google Drive link (do you have photos here?) but it said that I 'needed permission' and that I would receive an email if permission was approved - but no email received yet....
That is good going, to be doing 12 knots (is this maximum or cruising speed?) in a 24 metre cat with 'only' 2 x 170 hp.
What is the reduction ratio on the gearboxes, and size of the propellers?
Are you getting similar amounts of vibration with both engines / shafts?

 

Nope. Starboard side have more vibration compared to port side. please give ur email id i can sent it across.

 

Gearbox ratio is 2.06:1.The propeller rpm at full speed would be around 1800 .Yes the service speed of the vessel is 12 knots

 

Re posting a photo (or two) on here, it is very easy really - just click on 'upload a file' in the bottom right hand corner of the box where you write the post.

If the starboard side has more vibration that could suggest a mis-alignment issue?

You mentioned originally that 'the vessel is creating vibrations' - are you actually getting vibration in the hull structure, or is it just with the engines and propeller shafts?

 

Yes. The vibrations can be felt in deck of the vessel.
I can only find a just image url upload.

 

Really vibrations can be experienced in the the aft side of the vessel where we have a - brackets to support the shaft.

 

All hulls have a 'natural' frequency of vibration - the only way that this natural frequency can be changed is to significantly alter the hull structure (ie change the second moment of area of the hull section). Maybe this frequency is close to the local frequency of vibration caused by the shafts, and hence it is being amplified / felt on deck? I am rather hazy on this subject - we covered it at college almost 40 years ago, but I have never had to use it since.

Re the vibrations in the aft side of the vessel, are these in way of the deck, or the hull bottom shell in way of where the A brackets are through bolted?

Re photos, if you have any 'ordinary' photos in jpg format on your phone or computer, just attach them in the 'upload a file' box below.

Re the A bracket securing bolts becoming loose, any vibrations will only aggravate the situation. Do the bolts have lock nuts and / or washers on them?

 

Yes. A brackets are bolted to hull and it have lock nuts bolted to hull.Regarding natural frequency, do the hull can generate vibrations. Any possible method to reduce vibrations.
The main issue of picture upload is only by directly clicking picture with camera no file upload possible. I have many pictures of boat in my phone