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Old 30-08-2006, 10:53   #16
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Jentine, I am warried that you might be right. I hope not. That is another reason why I don't want to invest serious money in a prop. I am gonna get a used one for under $100 that has a lower diameter and see what happens. If I don't get the RPMs up then I know what to problem is. I need a bigger engine. But i've talked to other atomic 4 owners with larger vessels and the prop seems to be too big in my case. At least I will install a switch to shut off the alternator (125A Amptech) so that if we are motoring into weather and current I can keep the load off. Also I'll check what the compression is. Last time I checked (3 years ago) it was in the 90's. The acceptible range is 90-115PSI so I am on the lower end which is not good.
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Old 30-08-2006, 11:27   #17
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"That was the original engine. This boat was configured originally for racing. " That explains a lot. Racing boats usually do not have engines--even when they have engines. (Huh?) What I mean is, they have "docking engines" designed to bring them into the docks or take them home on windless days. The A4 would have been the lightest engine they could buy at the time, half the weight of a diesel and half again the weight of a 35hp. diesel.
I don't think you'll be able to reprop it and get acceptable performance, no matter what you do. Again...IIRC David Gerr has a consulting business (and a good reputation) and you might try to shortcut the process by contacting him for a professional opinion from someone who's not selling props.
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Old 30-08-2006, 13:12   #18
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I switched out a power boat 3 blade prop for a 3 blade sail prop. I now get more speed and more rpms. The two are the same pitch and diameter, but the power prop really did not work properly. It was likely the cause of the transmission that died when I had the boat purchase surveyed. It was terrible at low rpms. and never got much speed either. It sailed slower as well.

You want to reach max rpms for the best power and peformance. Size the prop until you get there and you should have the proper prop. It should be a factor of the engine and the transmission.
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Old 08-09-2006, 22:06   #19
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update

I ended up getting cheap 3 blade fixed sailboat prop on ebay. It was worked on before. Someone re-pitched it from 10 to 9 to 8. I took it to a local prop shop and it looks like with some heavy machining and some luck I'll have a 15/9 3 blade fixed prop (bronze) next week. It was interesting to see how they measure the pitch. It actually varied from 8.1 to 9.9 on each blade. There were some crevices and one of the blades was warped. the guy was very knowledgable and after some persuading he agreed to give it a shot in restoring it.
Petar
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Old 09-09-2006, 07:02   #20
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This is probably a power-boat prop:

I have 2 of them, the one on this picture is the spare.

Looks like it came of a tug-boat.

http://www.cruisersforum.com/gallery...p?i=2384&c=500

CSY 33 sailboat with a Perkins 4-108 rated at 51 HP @ 4000 RPM.
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Old 09-09-2006, 19:58   #21
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Yup, I had one of those too a RH 16 / 11. I sold it on eBay. Those fat blades were pretty bad on my Perkins 35 @ 3000 rpm. Youn really don't want one of those for more than getting out of the slip.
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Old 09-09-2006, 20:28   #22
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Yup, I had one of those too a RH 16 / 11. I sold it on eBay. Those fat blades were pretty bad on my Perkins 35 @ 3000 rpm. Youn really don't want one of those for more than getting out of the slip.
Aye, it works pretty good my boat:

Slightly bigger engine, and as ya know: A diesel needs the resistance.

I get 4.8 knots @ 1800 RPM.
5.3 @ 2100
5.6 @ 2400
6.2 @ 3000.

Max RPM under way is about 3900, not sure what the speed is then, but foam is flying and speedy power boats are left behind...
Close to hull speed anyway, 7.2 perhaps.

Lots of drag under sail, but I need the power for the New River, the current, the bridges, the crazy traffic, etc.

Otherwise I would have gone to a feathering or folding prop for another .5 knots under sail, and $3,000.00 out of the checking account.

For the time being, the big fat prop is most efficient under power, and that is where I spend more time than under sail...
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Old 29-09-2006, 16:00   #23
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update on the prop

The prop on ebay I found was a 3 blade sailboat fixed prop. One blade was bent about 1/2" and the pitch varied from 10 to about 8. It was a 15" diameter. At the end of the day I found a machinist to fix it up and now I have a 15-9 fixed three blade prop. We got in the water about a week ago. fully loaded I was motoring over 2000 rpms. There are no noticable vibrations, althogh I still want to allign the engine better. You can never be happy with those allignments. At around 500rpms I feel some vibrations. Nevertheless it is a tremendeous improvement in speed and rpms while cruising as well as reversing. I didn't max out the throttle yet because I want to break it in slowly. This engine has never run over 1500rpms or so under load.
here are the numbers

prop rpms at full throttle speed
prior 18/10 folding 2blade 1200 5-5.5
now 15/9 fixed three blade over 2000 6+

reverse is a breeze, and being a cruiser I don't mind the reduced sailing perofrmance. I think this old atomic 4 with reduction drive will be able to push our loaded C&C38 down the ICW.

Petar
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Old 29-09-2006, 21:02   #24
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I was there. I had the right porp on my sail boat but it was a power boat prop and the change when I removed it was night and day. I gained speed sailing and at low rpm it was more stable. At high rpms it was even faster.

There is a right prop for every boat and it needs to be the right one. Normally you can go to any goofd prop shop and they can go by the numbers and nail it. It's engine and transmission and nothing else.
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