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Old 29-04-2015, 03:25   #1
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Propeller exchange project: what do I need to know?

Hi everybody,

We are overpropped and want to exchange the prop this summer. We will only have one week on the hard and need everything beforehand, since suppliers here in Tromsø are pretty useless. What do I need, and which unforeseen difficulties will I encounter?

I found two nice prop dimension calculators in an earlier thread, so I will use those to calculate the size and pitch, and compare with the current one (assuming we find imprinted dimensions when diving down in 4 centigrades cold water). This is where I encounter the first problem: I could not find a gear ratio imprinted on the gear box, and I do not know the rated maximum RPMs of the engine. It's a Ford/ Sabb F4 144 GR from 1977 with 54 hp (imprinted on a plaque on the engine). I'm also not sure what it means how many shaft bearings between gearbox and propeller I have (only one cutlass) and what my desired maximum speed should be (I suppose hull speed calculated from water line?).

Now, what do I need to know about the prop to shaft connection? All I know from the previous owner is that the cone on the shaft is 1:10, and the key is 10mm (he thinks). Is that enough information to order a new propeller? Getting a new shaft together with coupling might be an option, but I don't know its length.

The next step will be to get the old prop off. I have no clue what conditions it will be in, but expect pretty bad. Any experience or tips with this? Will removing the nut and using a puller tool suffice?

And last, any special advice for installing the new prop or preparing the surfaces etc?

Any advice on this will be highly appreciated!

Cheers,
Phil
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Old 30-04-2015, 01:49   #2
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Re: Propeller exchange project: what do I need to know?

EDIT: I just found some engine specs in the manual that came with the boat regarding RPM: They write that max hp is reached at 3600 RPM (which sounds quite high?), while max torque is reached at 2250 RPMs. Which one is more relevant in this context?

Right now we don't dare to go over 1600 or so, since then quite some white smoke comes out and it doesn't sound good, either.
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Old 03-05-2015, 18:20   #3
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Re: Propeller exchange project: what do I need to know?

I have no experience with your engine but I assume it is gasoline fueled. 3600 rpm is about right for small gas engines of that time. High speed diesel can run at 3600 rpm, but my experience running high speed diesels fast shows me they don't last as long when run at high speed. If the engine won't run over 1600 rpm, you may have a problem beyond prop size. I assume it will reach higher rpm out of gear? What was the rpm the former owner used for maximum speed? Running at max hp will use more fuel. Maybe much more fuel. My old commercial fishing boat got 2 mpg @ 7 knots and under 1/4 mpg at 14 knots.
Your propeller shaft probably is 1 piece. You need to know the diameter and you should be able to measure that where it comes thru the packing/bearing. It should be in usable shape if the zincs were properly renewed and the there isn't excessive bearing wear. My 1942 vessel has the original shafts, but the bearing contact areas have been renewed. Using a proper puller, pull the prop while everything is wet (and lubricated). When the puller is pulled up very tight, hit the screw end hard with a big hammer. It may take more tightening and hits. Some use heat on the prop hub, but I don't. On big props it is common now to use a small explosive. Also check for stern bearing wear.
I would suggest you contact 1 or more major propeller dealers in Norway or Europe and get their recommendation. Their experience could give you a better prop match.
If you cannot find any info on your gear ratio, mark the front engine pulley and the shaft. With the transmission in gear, manually rotate the engine and count the revolutions of of each. For example, if the engine turns twice and the shaft turns once you have a 2:1. Usually they don't come out with an even number. A more accurate way would be to count the gear teeth on the input and output shafts and divide the small gear into the large one.
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Old 03-05-2015, 19:35   #4
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Re: Propeller exchange project: what do I need to know?

I don't know the engine either but I found this on the web.
http://www.nordsabb.com/files/Sabb/sabb54hk.pdf

It seems to have a fairly flat torque curve so I'd size the prop for full rated rpm or close to it.

According to the spec sheet the engine came with either a Borg Warner 71CR or a PRM 160 reverse gear. Both of these are hydraulic gearboxes.
I'm not sure if they will show an accurate ratio when rating by hand. I'd suggest investing in a cheap hand held tachometer such as this CyberTech Digital Photo Tachometer Non Contact Tach: Automotive Instrument Panel Tachometer Gauges: Amazon.com: Industrial & Scientific and measure to measure the engine and prop rpm while running in gear.

Good luck, and stay warm!
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Old 03-05-2015, 19:46   #5
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Re: Propeller exchange project: what do I need to know?

btw - when ordering a new prop, besides the diameter and pitch, you'll also need to know the length contact length on the shaft. I'd measure the internal diameter for and aft rather than relying on the PO's memory.

There's a drawing of a standard shaft for the engine on he above spec sheet. It may be worth ordering the complete shaft with prop if you have doubts on the trueness of the current shaft, and you can be confident that the drawing matches yours.
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Old 04-05-2015, 05:33   #6
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Re: Propeller exchange project: what do I need to know?

Hi there,

Thanks for your replies. It's a Diesel, not gasoline. Its rpms go all the way up when not in gear, and we can get it further up when in gear, too, but then lots of white smoke comes out.

The file you sent seems like the right engine. Specs also fit with the manual we have. 3600 as rated max rpms seems quite high, but if that's how they designed it... I suppose fitting a prop for 85% of max rated rpms is a good way to conserve some power for special situations, so if I aim for 3000 rpms as cruising speed it should be alright? Or maybe even a bit less to conserve a bit of fuel when cruising?

I used two different calculators to get a prop size estimate, and each has different assumptions and accordingly different recommendations. One uses more data on hull design as the other one. Contacting different prop makers is a good idea, even though I had bad experience with trusting shops earlier, which is why I go great lengths to figure things out myself.

I talked to the previous owner and he remembered right away that the gear is 2:1. He also mentioned that he tried two other props before that and kept the current one because it was still the best choice then. He used calculators, too, but apparently without success. So it seems to be difficult without a bit of trial and error. He also ran the engine at these low rpms.

With the uncertainties regarding the shaft I decided to postpone the prop project until next year. This year I will take the thing apart and measure everything, and if it's possible to get a prop plus maybe shaft within one week after hauling out, I go for it. Otherwise I order for next year. Thanks also for the advice on removing the prop!

Cheers,
Phil
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Old 04-05-2015, 06:50   #7
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Re: Propeller exchange project: what do I need to know?

First, I'd dive and be sure the prop is clean. (do you have a dry suit?)
A badly fouled prop won't tell you anything, it's just possible that once cleaned it may be OK. Plus hull fouling can play a major part. I assume maybe though in those cold waters fouling isn't much of an issue?
Before buying a new prop I'd pull the old one, in the water so you can tell everything about it, taper etc. Might even have it re-worked?
I have a Yanmar, it's also a 3600 RPM engine, as I bought the boat with a clean bottom it was propped to obtain 2600 RPM max, which is overpropped, just not as bad as yours. 3600 RPM Diesels are pretty common, I cruise mine at 2,000 RPM, at 3,000 the vibration and noise go up and fuel economy comes way down
A possible solution for you is an adjustable prop, those you can dial in the correct pitch. They are less efficient blade shapes though, nothings perfect.
I have an Autoprop that is sort of a self adjusting prop with an efficient blade profile, I'm very happy with it, once I got over the cost.
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Old 13-05-2015, 09:40   #8
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Re: Propeller exchange project: what do I need to know?

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Originally Posted by phipseml View Post
...Right now we don't dare to go over 1600 or so, since then quite some white smoke comes out and it doesn't sound good, either.
The white smoke could be unburned diesel, or maybe there is water in the diesel. Use a rubber bulb with a hose connected to it and get a fuel sample from the bottom of the tank, or some other method to get a fuel sample. Do fuel filters show water? It could be something else like overheating, but that would show on the engine temperature gauge. I am having some difficulty thinking about the problem because engine speed should not influence whether the engine is missing or not. Does the white smoke have a strong diesel smell? Does it matter if the engine is hot or cold? Does the engine start easily when the engine has not been warmed up, a cold start?
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Old 13-05-2015, 10:20   #9
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Re: Propeller exchange project: what do I need to know?

You are going to need a propeller puller such as the Walter #2N, The Walter Machine Machine Co., Inc. These are expensive, but if you damage a propeller and have a spare, you can fix it in the water yourself. Some boat yards will use a slide hammer, but I have seen postings about a slide hammer damaging the gearbox. Finding a boatyard with a propeller puller is also an option.
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Old 13-05-2015, 10:34   #10
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Re: Propeller exchange project: what do I need to know?

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You are going to need a propeller puller such as the Walter #2N
That is waaaay more puller than the OP will need and difficult to use underwater, should it come to that. The most versitile puller for general use is the Pro-Pull PPf-113.



The OP should also know that there is almost no reason to ever haul a boat for prop work. Any prop on any pleasure boat can be pulled while the boat is in the water and most hull cleaners are equipped to do this. Of course, maybe finding a hull cleaner in Norway is not an easy thing...
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Old 13-05-2015, 10:35   #11
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Re: Propeller exchange project: what do I need to know?

Your best prop sizing method will be to do a test run to determine max rpm when in the water. If the engine is rated at 3600 rpm you would like it to max out at least 3200 rpm or more according to most people. A lot depends on how much HP you have and need.
Use that information to determine how much to change the pitch on your propeller in consultation with the prop shop.
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Old 13-05-2015, 16:39   #12
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Re: Propeller exchange project: what do I need to know?

Are the propeller and engine original equipment? If so then the propeller should be properly matched to the engine by the marine architect that did the original design. It could be that you are not reaching the RPM that you should because of an engine problem and so you do not need a new propeller. Get the engine running so there is no white smoke.
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Old 13-05-2015, 16:43   #13
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Re: Propeller exchange project: what do I need to know?

If you can find someone to pull the prop while in the water I think it would be a good idea to pull it so that you can find the exact size of everything. As to the pitch I can't help except that there are props that are adjustable and they can be adjusted while in the water. Max Prop was extremely helpful when mine needed adjustment. -- PYI Inc. Max-Prop PSS Shaft Seal Seaview Radar Mounts R&D --

Very expensive though.
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Old 13-05-2015, 21:50   #14
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Re: Propeller exchange project: what do I need to know?

Dave Gerr's propeller handbook is great
Worth ten times the price


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Old 14-05-2015, 04:02   #15
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Re: Propeller exchange project: what do I need to know?

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Dave Gerr's propeller handbook is great
Worth ten times the price
His Boat Mechanical Systems Handbook is also worth ten times the price
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