Newb question on props

Any help/comments appreciated! Posted this over at the Grand Banks site but thought I would ask here as well. Hi to all, too Took delivery of my new-to-me 1987 Grand Banks 42' yesterday. Many years sailing and center console/outboard experience but this is my first twin engine inboard trawler. It has the FL SP135's w/Borg Warner Velvet Drive gears. Beautifully well maintained with almost no issues. One issue though is a very slight vibration from the port prop/shaft. Surveyor feels strongly that it is a prop issue, not the shaft and no immediate need to pull the boat and fix but it should be done in the not too distant future prior to any cruising. The installed props are 4 blade which over prop the engines. I believe the engines are rated at 2600 RPM's and the boat gets 2300 or so. I have what I assume to be the original 3 blade props in the lazerette.

My question is whether or not I should have the 3 blades tuned, maybe re-hubbed just because they are being serviced, and have them installed instead of keeping the 4 blades on. I know that people overprop for various reasons but at this point I am only interested in having props that are correct for the boat, don't over stress the engines and push me at a reasonable speed and maneuverability. Question is should I flip them since I don't have a real reason or enough experience to know whether 4 blades are the best for me? I do not have the numbers off the props. I can get them off the 3 blades but will have to search to see if I have the purchase record for the 4 blades in the piles of maintenance records. Any comments or help greatly appreciated. It is nice to finally be an owner!

The limited research I have done on GBs says you need to look at fuel tanks and engine shaft alignment. If you are going on the hard anyway, have a competent yard check out both powertrains and all components.

Thanks. All of that has been done prior to purchase. The prop issue is new and most likely the result of running aground during delivery. Not too bad, backed off fairly easily, but...the vibe wasn't there previously.

Ah, you did not mention that in your original post. Forget whatever I said. Good luck.

firstbase said:

Thanks. All of that has been done prior to purchase. The prop issue is new and most likely the result of running aground during delivery. Not too bad, backed off fairly easily, but...the vibe wasn't there previously.

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Since the vibration is due to grounding, suggest you pull the boat at a yard conversant with and skilled at fixing your possible problems. No advantage to waiting. Prior to grounding was the vessel able to achieve rated RPM with current 4 blades?

firstbase said:

Thanks. All of that has been done prior to purchase. The prop issue is new and most likely the result of running aground during delivery. Not too bad, backed off fairly easily, but...the vibe wasn't there previously.

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Seeing as a quote from your OP says... "Surveyor feels strongly that it is a prop issue, not the shaft and no immediate need to pull the boat and fix but it should be done in the not too distant future prior to any cruising."

First of all - I would not wait very long to get this rectified. Vibrations on shafts for any reason can lead to other problems.

Due to surveyor's input... It appears that the prop alone has been bent out of line by the grounding. If it were my boat I'd either have pro diver pull the offending prop and take it to a certified prop alignment shop for check up and realignment if needed - or - haul-out the boat and do similar with prop as well as to check out the strut, cutlass bearing etc. Without leaving the boat firmly on the hard you may be able to get a good deal from a yard to just pull the boat long enough (i.e. leave it in the slings for hour or so) to remove the prop and take close look at other drive line items. Then, if all but the prop looks OK, immediately re-splash the boat and have a diver reinstall the prop while in water. Good Luck!

Happy Power-Boating Daze! - Art

firstbase said:

One issue though is a very slight vibration from the port prop/shaft. Surveyor feels strongly that it is a prop issue, not the shaft and no immediate need to pull the boat and fix but it should be done in the not too distant future prior to any cruising. The installed props are 4 blade which over prop the engines. I believe the engines are rated at 2600 RPM's and the boat gets 2300 or so. I have what I assume to be the original 3 blade props in the lazerette.

My question is whether or not I should have the 3 blades tuned, maybe re-hubbed just because they are being serviced, and have them installed instead of keeping the 4 blades on. I know that people overprop for various reasons but at this point I am only interested in having props that are correct for the boat, don't over stress the engines and push me at a reasonable speed and maneuverability.

Click to expand...

firstbase said:

Thanks. All of that has been done prior to purchase. The prop issue is new and most likely the result of running aground during delivery. Not too bad, backed off fairly easily, but...the vibe wasn't there previously.

Click to expand...

Might be that 4 blades isn't causing over-propping; could be diameter and pitch instead of the number of blades.

If vibration only started after the grounding, I'd agree that's the likely cause... and could be one or more blades are bent. Prop shop can probably fix it easily enough.

Were it me, I'd probably have both props balanced, and fix the over-propping at the same time. You can often tell the prop guys what your WOT is and what it should be... and they can adjust the props to achieve the RPMs you're looking for. Magic. Within reason, of course, but prop shop guys can usually advise.

-Chris

Almost certianly too much blade area.
Check the three blade props at a prop shop and put them back on the boat.
Sell the four blade props. Two problems probably gone.

Don't you have to be unnecessarily unlucky to touch a prop on something if you're only decorously grounding? Aren't the props' paths above the bottom of the keel by some reasonable distance? Mine are something like 6" above the keel and about 30" oc away.

23-2400 isn't so bad for a FL135 and I agree with Ranger. If you're pretty sure its just a prop issue, have the prop(s) pulled and at the same time ask the prop shop to adjust the pitch. They can advise you as to what can be done and if/how much it will help. As far as the three blade props go, you don't really know if they'll be correct either and they might have problems that a prop shop would need to address, too.

Ken

You have two problems: a vibrating prop/shaft and both installed props have too much pitch to reach rated rpms. But you need to know the diameter and pitch of both the 3 blade and the 4 blade props to know which one to base your fix on.

Get that info and talk to a good prop shop about which set should give you the best rpms. Then have them true up that set, adjust the pitch if necessary and install. While that work is being done, check the vibrating shaft for alignment (but not on the hard), trueness, cutless bearing wear, etc.

As a guess, I would expect the three blade set is more likely to give you what you want. Increasing three hundred rpm is probably too much for the 4 blade set, as a rule of thumb is an inch for each hundred rpm and a 3" change is too much.

And finally, I bet there are lots and lots of trawlers out there with 300 rpm over pitch. It should work fine as long as you keep the rpms down- 2,000 rpm or less.

David

I have a spare set too. As I also had no idea what the spares would do when on the shafts, when I had a prop bending incident I took the opportunity to install the second set while the first were being trued. Then I knew for sure the spares were in perfect condition.

It might be worth grabbing a mask and flashlight and getting the numbers off the 4 blade props first, that way you'll at least know what your options are.

No vibration - grounding - vibration now ... is a no brainer. Lots of things can happen, even in a gentle grounding. Struts can crack or shift, shafts or blades bend, etc. Much more force than happens in high rpm shifting is put on the thrust bearings, too. Insurance should cover it. Prolonged running could lead to stern bearing, seal or transmission bearing wear.
I carry different spare props, too. But close enough to keep operating in remote places.

You mentioned "re-hubbing " the 3 blade props. Do the props have rubber hubs? If they do, they probably are outboard props and not suitable for use on your boat.

I'd be inclined to take your four blade props to a prop shop and talk it over with them. They can re-pitch the props if needed.

sunchaser said:

Since the vibration is due to grounding, suggest you pull the boat at a yard conversant with and skilled at fixing your possible problems. No advantage to waiting. Prior to grounding was the vessel able to achieve rated RPM with current 4 blades?

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Thanks. I don't see any advantage to waiting either. I think he was calming me down a little! I'm not going anywhere right now so will look at doing something soon. Prior to grounding (not absolutely sure that is what did this really) the 4 blade props got it to 2200 rpm's not the 2500 that the Lehman 135's are rated. Overpropped but the boat has not and will not be driven hard. 1650 RPM's seemed to be just fine. Previous owner said thats where he stayed day in and day out.

Art said:

Seeing as a quote from your OP says... "Surveyor feels strongly that it is a prop issue, not the shaft and no immediate need to pull the boat and fix but it should be done in the not too distant future prior to any cruising."

First of all - I would not wait very long to get this rectified. Vibrations on shafts for any reason can lead to other problems.

Due to surveyor's input... It appears that the prop alone has been bent out of line by the grounding. If it were my boat I'd either have pro diver pull the offending prop and take it to a certified prop alignment shop for check up and realignment if needed - or - haul-out the boat and do similar with prop as well as to check out the strut, cutlass bearing etc. Without leaving the boat firmly on the hard you may be able to get a good deal from a yard to just pull the boat long enough (i.e. leave it in the slings for hour or so) to remove the prop and take close look at other drive line items. Then, if all but the prop looks OK, immediately re-splash the boat and have a diver reinstall the prop while in water. Good Luck!

Happy Power-Boating Daze! - Art

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Thanks Art. Seems to me that if I am doing one prop I should probably have the other done as well. Or is that somewhat overkill? I'm a symmetry type of guy

DHeckrotte said:

Don't you have to be unnecessarily unlucky to touch a prop on something if you're only decorously grounding? Aren't the props' paths above the bottom of the keel by some reasonable distance? Mine are something like 6" above the keel and about 30" oc away.

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I was on the boat when it happened it wasn't a major league grounding. I said in a previous post that the vibe wasn't there before but truth be told it is so slight that it may have been there and I just missed it. The grounding was on sand and not that big a deal. The props are protected by the chines and I just can't get myself to believe we grounded hard enough to bury the hull to the level need to get the props that low....don't know. being that it is a new to me boat I will most likely have it hauled and everything checked to be sure all is well. More money but peace of mind....

firstbase said:

Thanks Art. Seems to me that if I am doing one prop I should probably have the other done as well. Or is that somewhat overkill? I'm a symmetry type of guy

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When I had one that I knew had to be straightened, I took them both to Osborne and when I picked them up I was told the second one had been in worse condition than the first.

Take them both in.

Benthic2 said:

It might be worth grabbing a mask and flashlight and getting the numbers off the 4 blade props first, that way you'll at least know what your options are.

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Thanks, going to have a diver clean the bottom next week and will have him get the numbers for me.

HopCar said:

You mentioned "re-hubbing " the 3 blade props. Do the props have rubber hubs? If they do, they probably are outboard props and not suitable for use on your boat.

I'd be inclined to take your four blade props to a prop shop and talk it over with them. They can re-pitch the props if needed.

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Thanks, that was my ignorance showing. I assumed....coming from my outboard days!!

firstbase said:

Thanks Art. Seems to me that if I am doing one prop I should probably have the other done as well. Or is that somewhat overkill? I'm a symmetry type of guy

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Nothing wrong with simultaneously making luv to your twin "screws"! Would be good as a new owner... then you're starting with both screws well tuned from the get go.


Also, while pulling props on the hard you can check to see if the "grounding" disturbed anything else. I guess you'll haul instead of diver to do both props... that seems logical.

Art said:

Nothing wrong with simultaneously making luv to your twin "screws"! Would be good as a new owner... then you're starting with both screws well tuned from the get go.


Also, while pulling props on the hard you can check to see if the "grounding" disturbed anything else. I guess you'll haul instead of diver to do both props... that seems logical.

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I think that would be wise. As much as I would like to save some $$$ like anyone else it would be better to pull it and have everything checked just in case. Can have both pulled, tuned, and make sure that it is a prop issue only. Other choice is to guess, have one prop pulled and looked at. If it fixes the issue then great but... I still don't know if everything else is ok.

Take them to a prop scan shop. They will computer test them, free, and tell you what's up. They will also do a great job adjusting them and advising on how much pitch you can remove.

Doing this out of the water is probably best as everything else should be inspected after a grounding. Dont waste money on a diver reading the numbers.

I'd get the spare 3 blades checked (get the dia. and pitch) and do what we call a short haul (cheaper down where I come from) and put the newly checked 3 blades on. Inspect shafts and struts and put it back in the water and test run it. If vibration is gone and RPM are correct your done. If not, you can give the numbers that you get from the test run of the 3 blades to the prop shop and they can adjust the 4 blades to run properly. At the speeds that we go in our trawlers, blade choice doesn't do a lot as far as economy are cruising rpm verses MPH.

bayview said:

Dont waste money on a diver reading the numbers.

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I was suggesting he get the numbers himself, not hire someone to do it.

ddalme wrote;
"I'd get the spare 3 blades checked (get the dia. and pitch) and do what we call a short haul (cheaper down where I come from) and put the newly checked 3 blades on.'

Probably the original. Perfect for the boat.