Getting up to motoring speed

[Ed 87Cat]

I am working on a problem with motoring speed that I am looking for some guidance from the group.
I have been sailing my 87 Cat 30 for 16 seasons now. It has the XP25 (23 hp) diesel engine. The boat seems to sail normally, but motoring is becoming a bit of a chore.
It has been a long time, but I am sure I use to be able to motor at 6.2 knot (GPS) when the motor was revved up to a comfortable rpm, maybe 2400 to 2800 rpm. These days, I am struggling with making 5.8 knots at 3000 rpm. Tow a dinghy (hard bottom with 90 lbs motor on the back) and speed drops to 4.7 knots. If I add some sail and a bit of wind, I have no problem towing the dinghy at 6 knots.
I am trying to figure out the loss of speed.
The motor seems to run OK: no white, black or blue smoke out the back.
I don't think the tranny is slipping, because powering up quickly doesn't show odd revving.
The bottom is not slippery slick, but I would not have thought it would have that much effect.
I am looking for ideas and ways to verify and would appreciate any help.
Ed

 

[Ralph Johnstone]

Hi Ed and welcome to the site.

I'm sure you've already checked out ground speed vs. water speed vs. GPS and all that other good crap to arrive at your conclusions. I also know you've checked the condition of your prop.

The only thing that comes to mind is the accuracy of your tachometer. Maybe invest in a cheap optical tachometer to be sure you are getting 3000 RPM on the engine. Then check your transmission output (slipping ?) to ensure its RPM = to the engine speed divided by the gear ratio. If that checks out, then you have some significant resistance outside of your drive.

 

[Anonymous 121765]

ED,

Ralph brought up some good points but, I'm still missing something here.

I have a C30 running a 3M20 Universal. I'm in salt water so, my bottom & prop need stay clean as a little bit of growth can make a big difference in performance speed, especially on the blades going in/out my fast tide channels. A crapped up bottom also adds resistance to speed & performance.

I didn't see anything on prop cleanliness but, from your detail that the bottom is not slick clean, makes me wonder just how clean it is.

I use a scheduled scuba clean service to ensure my bottom & prop are at their best. Sure, it may cost alittle more but if you think about it, you didn't buy a sailboat to SAVE $$$ now did you.

To sum it up, it doesn't take much growth to foul your prop/hull & cut down on performance.

CR

 

[Benny17441]

You are looking for a single source but perhaps it is the cumulative result of different ones. Over the years we tend to add weight to the boat and install things on deck that might increase drag. The engine may have some extra wear on it plus perhaps a new larger alternator. Our mind also play tricks by recalling the indicated speeds but not necessarily the actual prevalent conditions at each reading years apart. It would matter if you have a 1/2 knot current working against you on the bow or working with you by pushing on the stern. For a reliable GPS speed reading it is necessary to make two runs in opposite directions under the same conditions and use the average of the two readings. Readings made in a single direction and years apart cannot be compared. I would start anew and calculate motoring speed by making a number of two way runs at same indicated RPM and seeing what your boat realistically does now. You can clean the bottom and look for other ways to reduce drag and perhaps you can make some gains but mechanical replacements and or improvements would likely not be cost effective. In my book any speed around 5.5 knots under power is quite acceptable.

 

[Stu Jackson]

When was your last bottom job?

I'm running a 1986 M25 (not the XP) and just finished a six week cruise from SF to Vancouver Island. No speed issues at all, had a bottom paint job last year.

Rarely does your engine slowly die. And I've become intimate with M25 series engines for the past 18 years.

Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too
http://www.c34.org/wiki/index.php?title=Diesel_Engine

Good luck.

 

[SFS]

You mention the hull and its condition, but have you also inspected the running gear and prop? I agree with Benny, it may be an accumulation of factors, but you should eliminate the easily checkable ones. Verify that the prop is not damaged or fouled (with marine life or an entanglement of some sort), and that the shaft and cutlass bearing are ok. Checking tach accuracy wouldn't hurt, as mentioned above. And if you are relying on memory for your statistics, double check your logbooks to confirm your recollections.

 

[Ed 87Cat]

Thank you everybody for the quick responses. They are all very good ideas that I need to look into.
Most likely there is an impact from a number of locations.
I would agree the tach may not be accurate, but my speed tests were based on a notch back from maximum throttle, on Lake Huron on a calm day (no waves, no current).
I will have to look at the bottom and see if I can improve.
Has anybody seen problems with power loss due to packing or cutlass bearing. I was going to replace those, although the cutlass bearing has no play in it? Anything I should look for?
The prop is clean, but can these wear?
Ed

 

[Ralph Johnstone]

Has anybody seen problems with power loss due to packing or cutlass bearing. I was going to replace those, although the cutlass bearing has no play in it? Anything I should look for?

The fact remains, that in spite of possible increased friction at these points, you're still able to get the engine to 3000 RPM and the prop to 3000/gear ratio. Something else is holding you back.

 

[Benny17441]

I have not seen much prop wear in sailboats. Do not underestimate a bottom job. It can easily add 1/2 knot of speed to a boat. Even a little amount of growth on the hull can create a significant drag. Regular bottom cleanings help but nothing like a fresh coat of paint.

 

[centerline]

You mention the hull and its condition, but have you also inspected the running gear and prop? I agree with Benny, it may be an accumulation of factors, but you should eliminate the easily checkable ones. Verify that the prop is not damaged or fouled (with marine life or an entanglement of some sort), and that the shaft and cutlass bearing are ok. Checking tach accuracy wouldn't hurt, as mentioned above. And if you are relying on memory for your statistics, double check your logbooks to confirm your recollections.

I also agree..... something does not add up here.
what kind of prop do you have? (fixed, folding, 2 or 3 blade).... maybe its fouled with line or maybe the key way is sheared and although there is enough friction holding the prop so it turns at low power, it may be allowing the prop to slip enough when pulling hard, which will reduce the performance....
if the trans was slipping you would notice it in severe heat build up in a short while of running at max throttle.
the trans can get hot to the touch when running at full power for extended periods, but it should never get so hot you cant hold your hand on it for 15-20 seconds.

a fouled bottom is more noticeable at the higher speed than it is when going slow, so it could very well be a combination of a couple factors causing the problem.

 

[Ed 87Cat]

Lots of ideas!!
So let me throw this back at the group and tell me if I am making wrong assumptions.
The bottom may be part of the issue, but in Lake Huron the water is super clean. We don't get any kinds of growth, only a bit of algea. I clean the spots where the cradle pads are a couple times in the summer to make sure they are clean. There is no growths on the prop also. Also note that my sailing speed does not appear to be impacted. In the end though, I agree with the group that it should be done.

I looked at transmission slippage this weekend. It was a nice calm day. Gunning the throttle, the RPM's stayed steady. If the tranny was slipping, wouldn't there be some variation as it tried to grip? There was no overheating. Just to be sure it wasn't the throw range of the lever on the binnacle, I shifted it into forward from the lever on the tranny itself. I also did my yearly oil change and the fluid was nice and red with no burnt smell. That said, if I can my hands on a stroboscope, I will try to measure the revolutions of the engine and drive shaft.

I am still thinking power loss, because trailing a dinghy takes another knot off my speed. That seems excessive for me.

My next thought is that the driveshaft may be developing a wobble, possibly because the alignment is not correct. My motoring speed has been decreasing slowly over a number of years. I will replace the packing, but will look at the cutlass bearing and see if there is any wear, particularly at the front. I plan also to see if the shaft is true and no wearing.

If anybody can suggest a test, this weekend coming is my last chance for in-water testing.

Thank you, all
Ed

 

[centerline]

Gunning the throttle, the RPM's stayed steady. If the tranny was slipping, wouldn't there be some variation as it tried to grip?

I am still thinking power loss, because trailing a dinghy takes another knot off my speed. That seems excessive for me.

My next thought is that the driveshaft may be developing a wobble,


If anybody can suggest a test, this weekend coming is my last chance for in-water testing. Ed

gunning the throttle wont tell much as far as trans slippage, due to the prop slippage.... but after 30 minutes of running at WOT, and where the trans operates smooth, yet builds a lot of heat, it is most likely due to slippage.....

if the engine maintains its rpm, its not power loss..... power loss will drag the engine rpm down, so if you can maintain rpm, it means the engine has enough fuel and air to develop the power necessary to keep the prop turning/pushing against the water at the force you have set with the throttle...

the wobble you may feel could STILL be a failed key way in the propellor, which would explain everything else... I dont know what type of shaft coupler you have, but you might want to check for a failure in that area.

and engines have a drive coupler between them and the trans, and depending on how its constructed, it could fail with the same symptoms you are experiencing...until it wears enough until it doesnt go anymore at all.

 

[dziedzicmj]

Your packing cannot slow you down that much. If it did, you would have bigger problems (and you would have noticed that).
A simple option for a strobe tachometer is an app for your smartphone (there are numerous available).

 

[AXEL]

What have you been using for anti-fauling on your prop? I had a problem several years ago similar to your (OP). At the time I was using the same ablative bottom paint that I used on the bottom (with a barrier coat). Over the years the leading edge of the prop loaded up. One season while the boat was hauled I stripped the prop down and got all my speed and power back. I now use Lanocote (wax) on the prop, cheap, easy, effective!