CD 30 Powerboat - Strange Doings and WOT RPM question

[Walt Bilofsky]

Recently took my CD 30 powerboat to a very expensive, very good engine guy that I hadn't used before. He said there was some vibration in the boat, and that when they tried to align the engines with the propshafts, they found that the shaft coupling was not properly positioned on the shaft - it was a little bit too far down, so that the shaft was banging into the transmission coupling.

Even more interesting, they found that the shaft coupling was not flat - it was cone shaped with the rim about 6 or 8 thousandths further aft than the center. Although the shafts have been out several times, and the transmissons have been upgraded from MS4s to HS1s, our best guess is that the couplings were that way since the boat was built, since there was no sign of wear, no scalloping, etc. (Wonder if the MS4 couplings were a bit concave to mesh with the convex shaft couplings.)

Anyway, in we went to the yard, where it turned out to be a YA2K problem - Yet Another $2,000 - to face the couplings and balance and true the shafts and props. And now I'm singing "What a Difference Two K Makes." The boat used to vibrate a bit at cruise, and a lot at WOT. Now the vibration at cruise is less, and it's the same at WOT as at 3100 RPM. The change is incredible.

Now the strange part. WOT RPM used to be 3650. Now it's 3975. Neither the yard or I can believe that there was so much friction to be removed from the drive train as to make that much difference. Moreover, the boat is propped with the standard 18x19 size, and the specs on the TAMD41B call for WOT between 3600 and 3900.

So - what do other CD powerboat owners (30, 33, 36) see for WOT RPM? Do you have much more vibration at WOT than at cruise? Does it make sense that there would be that much increase just from engine alignment and shaft truing? And do I now have to repitch the props to get back within spec?



bilofsky@toolworks.com

[Burt Polishook]

Walt,
At the end of the day, was it worth the $2,000? I suspect we all
have some vibration, but how do we can quantify the problem to understand whether the cost of the fix is worth it?
Burt



burtp@home.com

[Walt Bilofsky]

Walt,
At the end of the day, was it worth the $2,000? I suspect we all
have some vibration, but how do we can quantify the problem to understand whether the cost of the fix is worth it?
Burt

Are you asking as a comment on my repair, or because you're wondering about vibration on your boat? Anyway, here's why I'm not sorry I did it:

First, the engine guys said that if the shaft kept banging into the transmission, eventually something would break. That'd be more than $2k and might come at a time on the water that would be inconvenient and possibly dangerous.

Second, if this fix really did remove enough load to up the high end 300 rpm, I'll also see savings in wear and fuel economy.

- Walt



bilofsky@toolworks.com

[Robert Davee]

Hi Walt: When I purchased my CD28 FB Power Hull #124 I was aware of a shaft induced vibration which took out cutlass bearings from time to time. I thought I was smarter then the previous owner and shortly after purchase I pulled the shaft and coupling and propeller and had the whole thing trued up etc. Guess what? When the whole thing was reinstalled the vibration was the same. Then I discovered that the length of shaft - 1 1/2" diameter - from the coupling to the bearing was very close to 6 feet. According to my mechanic this is very close to the maximum recommended unsupported shaft distance allowed. When the shaft was straighted it was within .004 of an inch (considered good). To make a long story short - I added a center bearing as a trial (plain babbit) and the vibration went away. Since, a babbit bearing does not like to operate at that high a shaft speed (gets hot and squeezes out the grease) I adapted a Cooper split self-alligning roller bearing which has been just perfect. I must admit, however that installing the Cooper bearing was a bit difficult. Just a brief explanation - The Cooper bearing (1 1/2" diameter) comes with a rather heavy outer housing which has a spererical bore into which the inner housing with the roller bearing fits. I made a new teak, slit outer housing, the spherical portion of which was made of epoxy using the inner housing as the mold, so to speak. I then epoxyed the teak housing down in the keel area. You may not be able to figure this out. But if you are interested I will photograph it and send it to you.
The important thing is the vibration problem is gone. By the way, I also noted a slightly higher top end but not as much as you.
Boating sure is fun isn't it?

Bob Davee

I have not mentioned the RPM,s at cruise and WOT since the prop pitch and gear ratio on my boat is probably not the same as yours. My prop is 18x16, and the ratio is 1.5:1. I have a new 230 HP Yanmar diesel. I am having another 1 inch of pitch added to the prop now because I seem to have torque to burn. Perhaps I can cruise at a sightly lower engine RPM.




builder@midcoast.com

[J. Merenda]

So - what do other CD powerboat owners (30, 33, 36) see for WOT RPM? Do you have much more vibration at WOT than at cruise? Does it make sense that there would be that much increase just from engine alignment and shaft truing? And do I now have to repitch the props to get back within spec?

My WOT RPM is very dependent upon loading. With less than half tanks I can reach 3850rpm. With a full load of fuel, only about 3600. The boat does have some vibration while on plane at any rpm, but gets worse as rpm increases (enough to make the cockpit hatch cover handles rattle annoyingly). I've been attributing it to unmatched props and am planning to have them matched and balanced on a prop scan system.
I'm suprised at the increase in rpm you mentioned as well. I wouldn't touch the props until you've tested the boat under various loads.

[Walt Bilofsky]

My WOT RPM is very dependent upon loading. With less than half tanks I can reach 3850rpm. With a full load of fuel, only about 3600. The boat does have some vibration while on plane at any rpm, but gets worse as rpm increases (enough to make the cockpit hatch cover handles rattle annoyingly). I've been attributing it to unmatched props and am planning to have them matched and balanced on a prop scan system.
I'm suprised at the increase in rpm you mentioned as well. I wouldn't touch the props until you've tested the boat under various loads.

Joe -

Well, before you have the props removed, you might want to have someone good check to see if you have the same situation as I did with the shaft couplings.

I have never noticed any difference in WOT RPM with loading. Speed, yes, but not RPM. Though I hardly ever ran at WOT because of the vibration, so I might not have noticed. I also see a knot or two difference in cruising speed for the same RPM depending on fuel and water load.

I've been awfully busy, but the first thing on my list is to go out on a calm day and rerun my speed curves. Whatever the resistance in the drive train, a 3000 rpm prop is a 3000 rpm prop, and if the speed curve vs. rpm has changed substantially, the prop's been changed.

The local Volvo dealer notes that the specs for the current version of that engine says WOT 3700-3900, changed from the 3600-3900 for the TAMD41B. Perhaps they were seeing engine wear from engines propped toward the low end. So even if the props were repitched, I may leave them that way, unless fuel consumption goes up.

- Walt



bilofsky@toolworks.com