Vibration & Misalignment

[Downeaster]

There are a couple of current threads that touch on this subject so I'll toss in a couple of quick and dirty stats or remedies that are often overlooked or just unknown.

Prop wash against the hull isn't just a blade-induced pressure wave causing a knock on the hull but it is also a once-per-(blade) revolution pressure on the shaft and bearings. There is a standard wheel size to a given aperture and it calls for 17% of the wheel diameter for blade tip clearance to the hull (Gerr). For instance, a 20" diameter wheel would have to have a minimum, blade tip-to-hull distance of 3.4". How does yours measure up?

Another issue is soft engine mounts. If you elect to go with these, you want to soft mount the transmission-to-shaft coupling too (Downeaster). If you you have soft engine mounts and a solid transmission-to-shaft coupling, what is going to absorb the engine's movement? I've heard good things about ACE engine mounts (Pascoe et al) and they are scheduled for my next build.

I haven't seen the video mentioned in other threads but here are a few quick tips that are often overlooked:

• Of the pair, the thin nut should butt up against the propeller hub.
• Dry fit your propeller without the keystock in the keyway and mark the shaft with a pencil - then reinstall the keystock and make sure that the propeller hub reaches the pencil mark.
• Your final shaft alignment check has to be done with the boat in the water (preferably for a few days after launch), not on the hard.
• According to the National Fisherman (Feb 2011), you're aiming for shaft-transmission alignment of 0.004" or less.

 

[Sailorgp]

I've read the same thing about the thin nut being against the prop hub...but it's rarely seen in the real world. Also, I believe solid coupling run out is .001 per inch of diameter in the coupling i.e. a 4" couping should be within .004", a 5" coupling .005 etc. Soft couplings can be twice that spec.

 

[tunafishhkg]

Ace is a good mount especially for large 12cyl engines but do look at the Barry Controls mount that besides having supreme resonance dampening characteristics, has the advantage of allowing you to see the loading on each mount in order to balance the loads out between all the mounts. As far as soft mounting the coupling, IMO soft mounting the coupling while sensible in theory, creates another weak link that does not justify the ends. The shaft can take the little tweaks when accelerating or loading and kind of self centers do to rotational forces. That is of course unless your coupling has to be closer to stuffing box than recommend, then the option is to soft mount by evolution drive or other drive savers everything has its place though you can add the soft mount if you like LOL. Seen the flexible drive savers fail with age/salt damage and with little impact that would not have caused damage if it was the standard method. I personally would never install my shaft without having the two couplings faced, Shaft checked for run out, and propped lapped. I agree with letting the boat sit in the water before final adjustment and Keith got zero zero run out on my boat.

 

[jojobee]

The Aquadrive system eliminates the need for stiff, hard mounts and for careful engine alignment to the propeller shaft. A lot of the high end builder use this system. Instead, the propeller shaft is aligned to a thrust bearing which absorbs all the propeller thrust. The prop pushes the boat directly. It no longer pushes the engine. CV joints and tubular shafts transmit engine power to the thrust-bearing and propeller shaft while allowing engine movements in every direction. CV joints allow misallignment(sp). Soft engine-mounts isolate nearly all vibration.

 

[tunafishhkg]

Aquadrive must have a rubber mount or gambled bearing on the load side also as how do you make up for the inevitable wear in the cutlass bearing especially for the low maint people who do not change there cutlass for yrs at a time? Evolution drives controls this by design since their unit goes all the way to prop. Only my opinion of course but I tend to never go just by the literature since all manufacturers spend big money to look fancy on paper as that’s the only way to get people interested. Everything is a trade off on boats and have their place pro and con LOL

 

[jojobee]

Tuna,
you still have a shaft to a coupling, but it is very short now. Change the cutlass like you normally would. The load side does have a rubber mount. This is a simple system.

 

[tunafishhkg]

Agree maint is key just that I have seen so many boaters who change out thier cutlass every 5ys or longer. Then again those who do go the extra mile and install a syst like this, are anal with maint to make sure there is no vibration. Of course I might be foolish to do mine every 2yrs even if only in water for 3-4M especially if I am harpooning LOL.

 

[captainlarry84]

17% Rule

Prop wash against the hull isn't just a blade-induced pressure wave causing a knock on the hull but it is also a once-per-(blade) revolution pressure on the shaft and bearings. There is a standard wheel size to a given aperture and it calls for 17% of the wheel diameter for blade tip clearance to the hull (Gerr). For instance, a 20" diameter wheel would have to have a minimum, blade tip-to-hull distance of 3.4". How does yours measure up?

I think in today Downeast keeled boats you will have a hard time with this rule of 17% tip clearance. That rule was true years back before they went high HP. With proper wheel cupping most keel boats are sporting anywhere from 2 - 3 of tip clearance. If the tip clearance is to close the water flying off the bladed hits the bottom of the boat, which starts a rubble and makes air which then starts to burn the tips of the propeller. Nibral is much less affected by close tip clearance than bronze.

In order to swing the larger wheels and smooth things out the wheel must be cupped. The cupping helps to hold the water on the blade and the water that escapes and gets thrown off the wheel is pitch in a direction away from the bottom.

Some photos of tip clearances

 

[tunafishhkg]

Good stuff Larry Interesting! Will cupping prevent hull from wear if to close? With 450HP on a 31 BHM with 2-1tranny, a efficient wheel diam is a concern as I already have the trans. Also since it will be a stick boat, how will cupping effect noise of wheel. Will go with wet tube but can only drop it down so much and rec shaft angle should be around 8degree's I have read and 15 degree's max for engine angle per Cummins spec. Hate to have to get another trans cost wise in order to reduce engine angle.

 

[captainlarry84]

I know the vessel well. With a 450 Cummins & 2:0 things will get tight. The right ratio would have most likely been a 1.77 gear. We need to know a few more things before we can move forward and hone in on your needs:

• What is the distance from the bottom of the boat to skeg?
• What is your WOT RPM of the motor?
• What is what you feel is the current largest diameter wheel that will fit?

 

[tunafishhkg]

Just brain storming/pre planning since out of the country bored till April but its a 6CTA 450HP @2600. 2" shaft and the wet tube is not installed yet. Already have the trans but if its really that crucial, then would get another trans since I really hate going backwards on a project or worse, regretting my actions later just to save a few bucks that in the big picture, is not that much. Would have liked to be able to use my 26X24 nibril prop from my Duffy and get a new DQX for the duffy but do not think I could get the clearance on the 31 BHM. Wonder if a higher shaft speed would make more noise in the water?

 

[captainlarry84]

This is what I come up with for your BHM. However I would consult with experts in the field before move forward, as a mistake can be very costly.

The BHM can take a very large wheel, however the largest fit may no be the best for what you want to do.

She is only a 31 footer so I would look to max my diameter out at 24. This will give you plenty of tip clearance and it will be an easy wheel to feed water to after flowing round that fat keel.

I would not change the shaft angle at all. However I would push the wet tube back about 2 or maybe 3 to get the wheel a little back from the keel. Next I would then fair and mold the wet tube in to give a very smooth and aero dynamitic surface for the water to glide over. Wet Tubes are the only way to go for max performance.

Your current 450 Cummins @ 2600 RPMs & a 2:0 gear gives a shaft speed of 1300 RPMs Based on that shaft speed & load I would compute your propeller to be a 26 X 24 which is what you would like to turn. However that is a lot of wheel to feed water to on such a small boat.

With a 1:77 gear your shaft speed would go up to only 1468 RPMs which is a 13% increase in shaft speed. This would in turn reduce your wheel size by approx 3 of pitch or diameter. With the 1.77 your propeller would be a 24 X 23 or possibly a 24 X 24. Which would be the perfect wheel for your boat. It would be quiet and there would be less load on the shaft & smoother shifting.

Years back we did a 31 BHM with a 375 CAT & 2:0 gear the boat turned I believe a 24 x 23 propeller. The owner wanted more and he jump up to a 2:5 gear with the 26 wheel with a pitch somewhere near 24. He gained nothing lost speed and had to add 200 pounds of weigh in the stern to get her to run trim.

 

[traditions]

31 BHM with a 260 Cummins 2 1/2 to 1 gear,lobster boat 26x28 4 blade ,just barely over turns when boat is empty.Absolutly no vibration from wheel wash,and it has a cage on it,which is good for at least 100 rpm.I think the 26 diameter is a good size for this boat.The wheel gets good water,the keel is pretty narrow.A good way to check is when the boats out of the water,stand in front of the bow and see if you can see all 4 blades of the prop.The hub is not that critical for balance.On my sons 31 the skeg is not much wider than the hub of the wheel,and the boat is pretty deep aft.I have noticed that most BHM's run flat.The only one I know of that lifts the bow has a cruiser house on it and we had to put wedges on the stern to bring the bow down so you could see.Most all of the lobster boats are rigged quite well forward.

 

[traditions]

Heres a picture runnung at about 1800,I tried to edit on the previos post,but couldnt figure it out

 

[tunafishhkg]

Thank you Larry, your time and info, your help is appreciated. Looks like I will have to really consider my budget on the drivetrain for this project as to the cost vs efficiency since I already have a 600hr since new 1.97-1 trans. To get a new trans will add a lot to the cost. I do have a 5 blade 24X23 I could use but it is bronze that on with 450HP, gets close to the margins strength wise. I am basically doing this because I had a lot of spare parts to take advantage of the ability, equipment, and deal I got on this hull. I did not start out to build this BHM as a stick boat since My Duffy is already set up for harpooning that was so enjoyable but after the last few yrs of slow harpoon fishing, I added spray rails, lifting rails, and creature comforts on the Duffy making it heavy and maybe noisy in the water compared to what it was originally built to do. This coming yr we will see how that plays out on the Duffy but I always try to keep a back door especially on the fun things in life and the BHM should do nicely.

 

[captainlarry84]

If you go with what you have the worst that will happen is that you will overturn your 24 X 23. No big deal just some lost speed. Also keep in mind that since you will be overturning the wheel with the 2:0 gear the motor will have a light load, so at full RPMs your fuel burn & load will be well under spec, so you can run her higher on the RPMs to make up some speed and additional load.
Also keep in mind that your bronze 24 X 23 is plenty strong for your application. It is stronger that a 24 X 23 four bladed wheel because you are sharing the load with more blades. If the wheel has a good ring us it. Plus you can re pitch that wheel by 2" if needed.
Lastly if the do the complete job with the 2:0 gear 2” shaft & 450 Cummins and you are not happy, the only change is a simple gear swap out. No other changes are needed.
On one of our first repowers on a 31 footer we when with a 1:77 gear. We sea tested the boat and we were only turning a 22 X 18 wheel. We hauled and went two a 2:0 gear & jumped up to a 23 X 21. All in one day. Total cost for the change with a ZF220A gear came to $2500.00 plus the old gear, not so bad.

 

[tunafishhkg]

I do not mind getting a new wheel for the 2-1 trans since I have a 26X24 nibril on my duffy and a new DQX might have better speed plus give me a spare. Seems you think it would be better to repitch my 5 blade 24X23? that would work for me cost wise.
Thanks again

 

[captainlarry84]

I prefer a four bladed wheel on all Keeled boats for earlier stated reasons. You have two very good starter wheels to get the job done. Yes I would re pitch a bronze wheel before touching that diamond of a Nibral.

In your case however I think the bronze wheel may work better with the 24 diameter. It will most likely be quieter, smoother and easier to feed water two.

450 HP is a lot of power for a 31 BHM. It would be nice to get all of the HP used but it may not be the best all a round.

What is in your control and cost nothing is what you decide on doing with the length of the wet tube that will also be a factor in getting a smooth running vessel. The fairing and molding in of the tube will be key.

 

[tunafishhkg]

The wet tube part is easy and have seen many M style fairing on DE though was told by a respected experienced builder that a clean rudder was as much or more important speed wise on a Duffy at least. How long do you recommend to set back the tube from say the vertical end of the keel as a reference? I will repitch the 5 blade and see how it goes since I own it anyhow but if I have to buy a wheel, would rather get one that would improve the Duffy since my Duffy is also 450HP though on the volvo, it is under pitched and I like it that way for the reasons you stated earlier.I use it for long canyon runs and a DQX might improve my numbers but maybe not enough to be worth the cost.
thanks

 

[captainlarry84]

We never spoke much about the rudder. But yes I concur a thin flat S/S runner is key to getting rid of the water after the propeller work is done. On my rudder I always mount 2 of the flatter 6 zincs on the rudder. One zinc is place up very high and out of the flow of heavy water traffic. This one always lasts the full season. The second zinc goes on the rudder dead center where there is a hole in the rudder for shaft removal. That zinc is directly in line with the nibral wheel for excellent protection. However due to water flow it erodes quicker.
My rudder is bronze as apposed to s/s which is a better match when you have a nibral wheel. Today most boats come with only S/S as it is much cheaper to make. However you do need to make sure that you a well zinced with that much S/S behind a bronze or nibral wheel.

The wet tube how far back.good question. I picture of your current aperture area would be helpful. On a guess, I would want the end of the shaft to be the length of the propeller hub plus 2 away. Meaning if the hub is 6 long then the end of the shaft should be 8 away from the front of the rudder. This will give you swing room for removal plus enough distance away from the rudder to have smooth water flow.

I would re-pitch nothing until sea test. Your Nibral can go from a 26 X 24 to a 26 X 25 max, if more load is need then maybe a little cup. Your 24 X 23 Bronze can go up to a 24 x 25 max. A 1 increase in pitch with a 2:0 gear on a 2600 RPM motor will reduce RPMs by approx. 175 RPMs per inch increase.

Lastly the less wheel bending the better.. On most of the Downeast repowers with every boat being a little different we always like to run the first season in bronze. If need we bend & cup a little until we get the sofe bronze wheel to where we want to be. The we order a factory casted from birth Nibral wheel which comes in is original box with matching serial numbers depicting such.

Going to a DQX on your Duffy most likely will give you little. If your get a larger DQX you will indeed pick up speed, which in turn will increase load & fuel burn. I would only jump to the DQX if you need a 26 X 26 or larger. If not then rework the 26 x 24. Also keep in mind that a DQX is harder to turn because of great blade area.