I think I am experiencing cavitation on my repowered (BV-1505) sailboat. In neutral at high rpm the engine is smooth and quiet. In reverse there is a slight increase in sound but the engine is relatively smooth. In forward gear I feel an increase in vibration and there is a burbling sound. At 2100 rpm and above the vibration and sound increase in intensity and I am concerned I will cause dammage to the boat, prop and drive train. The boat is a Bristol 40 and the prop is located in an aperture. The prop seems to be located futher forward, closer to the keel, than a Bristol and a Hinckley of about the same size which I observed while on the hard. Any ideas would be appreciated. MikeB
I doubt that you will cause any damage to the boat or engine or transmission from your description of your problem. It certainly sounds like it could be prop wash, or cavitation but I will need some more info from you before I can comment. If this is the case, it is much more annoying tran dangerous.
Is it possible for you to send me a photo of the propeller's position in the apeture?
It will also help if you can let me know the transmission ratio and the size of propeller on the boat (diameter and pitch and number of blades) and well as telling me the waterline length of the boat. Another thing that I need to know is how much "tip" clearance there is from the tip of your propeller to the bottom of the hull (just above the prop).
Are you using a flexible coupling?
Are you able to acheive 3,000RPM in forward gear? How does the boat perform under power?
Your answers will help me formulate a reply. Regards, Stanley
I have some photos of the aperture. How can I send them to you?
The transmission ratio is 2 to 1. The prop is 3 blade 14"-13 pitch MP-3 RH. It has very wide blades. The LWL is 27'6". There apears to be about 1" of clearance at the tip from the pictures. I did not measure while out of water. I am not using a flexible coupling. I believe I can attain 3000 rpm in fwd but will verify tomorrow. At 2000 rpm I can move at just under 6kts. When I increase speed to 2200 and above there is a significant increase in the noise. When I look at the shaft when underway at 2000 and above the shaft seems not to be vibrating. Hope this will help. Let me know how I can send the photos.
Your prop appears correct. You should be able to send photos through this forum, dont use the quick reply, use the advanced one. If this still gives problems, send the photos to me at stan@betamarinenc.com
Everything appears correct. You should reach a maximum boat speed of 6.7Knots with this set up which is also close to your hull speed.
You may be able to fit the flexible coupling that Beta Marine supply with the engine and this will move the propeller towards the rudder by 1-1/4". The photos indicate that there is enough room but please check and turn the shaft by hand with the rudder over to one side and make sure that you dont have an interferance issue before running the engine. This should lessen the noise from the water being forced past the hull by the prop without affecting performance.
What you have is a case of a bit too much power and the compromise that had to be made with the propeller sizing to allow the prop to fit the apeture.
Please let me know how you get on. Regards, Stanley
Thank you for your analysis. I will look into getting the shaft coupling. One further question. If the prop is moved futher aft, the shaft will be exposed for about 3 inches. Will this cause other problems?
I am not sure of your shaft diameter and this is not something that I want to guess at. The general rule of thumb for full safety is 1 to 1.5 times the diameter as clearance between the end of the support (cutless bearing) and the front of the propeller hub. For the sake of a test, (I assume that you do have the R&D flexible coupling as we supply one with every small engine/transmission) it will do no harm. A test is not a long run, but a test that is long enough to see if this will help.
There are a lot of propellers that break this rule. For example, there are some folding propellers that have a 4" long hub before the blades and this has the same effect as lengthening the shaft. If your shaft is in good condition, I believe that you will come to no harm with this test. If your shaft diameter is 1-1/8", I would not worry at all as your tip speed is not going to be high enough to cause any problem.
One more thing. I finally got to the boat and ran the engine. Here are the results. I was able to get up to 3000rpm in neutral and reverse but was only able to get up t 2600rpm in forward at the dock. I assume the same would happen while under way.
You may find that you can get a few more RPM when underway to bring your revs up to 2800. If this is the case, I suggest that you have your prop re-pitched when you haul out at the end of the year and lose 1" of pitch. This should have the effect of bringing your RPM up about 200 and will help to reduce your prop wash effect.
That makes a lot of sense I was beginning to think the prop was over pitched when I found out that I could attain 3000rpm in revarse but not in fwd. Thank you for your time you have beena great help.
