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Old 01-07-2011, 15:09   #1
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Top Speed Motoring ?

35 foot full heeled cutter. Westerbeke diesel, I believe is 25hp. 1700 top rpm in gear. I am topping out in calm weather about 1.8 knots. Normal? Or do I have a prop problem?

TIA Brad
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Old 01-07-2011, 15:19   #2
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Re: top speed motoring?

Off hand, 1.8 knots does seem a bit puny, and the 1700rpm max does not ring right. A well chosen prop should give you 3000 rpms redline, and about 2400-2700 rpms continuous crusing. If all other things with the engine and transmission are normal, then yes, your prop may be missmatched to the engine.
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Old 01-07-2011, 15:22   #3
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Re: top speed motoring?

1.8? That's a little slow! Surely it should do hull speed, which is what, 7 knots?

Is the engine a 5424? My 31 has one, and does 2400rpm with a 3 bladed prop @ 6.7 knots. However my cruise is 1700rpm, which gets 5.8 knots. Now I've a modified fin keel, but seriously, how much extra drag could your full one have?

So your top rpm is a little low, but 1700 rpm should still move it along. So clearly either you are not exerting much thrust with the prop, or there is a vast amount of drag.

Would it be convenient to get a diver to look?
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Old 01-07-2011, 15:25   #4
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Re: top speed motoring?

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Originally Posted by Astrid View Post
Off hand, 1.8 knots does seem a bit puny, and the 1700rpm max does not ring right. A well chosen prop should give you 3000 rpms redline, and about 2400-2700 rpms continuous crusing. If all other things with the engine and transmission are normal, then yes, your prop may be missmatched to the engine.
The later engines redline at 3K but the earlier Kubotas like the 5424 rev a little lower.
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Old 01-07-2011, 15:36   #5
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Re: top speed motoring?

Check for barnacles. On the hull, but especially the prop.
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Old 01-07-2011, 15:41   #6
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Re: top speed motoring?

Does it go at normal speed when under sail?
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Old 01-07-2011, 15:44   #7
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Re: top speed motoring?

Sounds like the prop is a big chunk of grass.

1700 should be pushing a lot faster and the engine should turn faster..

My 43' with a 4108 cruises at 6 kts. @ 1900 RPM, not max speed, just max fuel economy/speed ratio...
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Old 01-07-2011, 15:48   #8
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Re: top speed motoring?

my formosa 41 goes 5 kts under perkins 4-108 at 2500 rpm----yours should go near 5 under power--if bottom andprop are clean, you dont have proper sized r pitched prop-- find expert and have it evaluated. at 1700 you should be going about 3 kts with all clean and purrty and proper.
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Old 02-07-2011, 00:08   #9
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Re: top speed motoring?

My Farr36 with 29hp volvo at 1700rpm 5 knots, cruises at 2500rpm 6.5knots, 3200-3400rpm tops out at 7.6 knots loaded about 6.5ton.
You need to jump over and have a look at the prop then an engine check.
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Old 02-07-2011, 04:41   #10
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This is the prop (before it was cleaned) on a 30 footer I was delivering. In this state max speed was about 1.5 knots. Once cleaned 5.5 -6 knots easily. Yes I would be checking the prop.
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Old 02-07-2011, 11:31   #11
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Re: Top Speed Motoring ?

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This is the prop (before it was cleaned) on a 30 footer I was delivering. In this state max speed was about 1.5 knots. Once cleaned 5.5 -6 knots easily. Yes I would be checking the prop.
and a prop in that state would be hard for the engine to turn, resulting in low maximum rpm, just like the OP mentioned.
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Old 03-07-2011, 00:02   #12
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Re: Top Speed Motoring ?

Thanks for the replies everyone. I was slowly motoring 20 miles, since I posted. Everyone who suggested prop fouling, go get yourself a cold one on me. Had the boat hauled when I bought it in January. Everything was cleaned. Today I took a plunge in the sound brrrrr. It is a mass of grass and barnacles. I am hiring a diver.

Can anyone recommend a good anti-fouling prop paint?
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Old 03-07-2011, 00:30   #13
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Re: Top Speed Motoring ?

Having worked hauling boats up onto our slipway for antifoul for the past eight years I am able to see a lot of boats year after year.
The best propeller treatment I have seen is a two pack product called "Propspeed".
I don't know what it would be called in your part of the world and it is VERY expensive but some vessels have had amazingly clean props after as long as two years [with regular use].
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Old 03-07-2011, 08:10   #14
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Re: Top Speed Motoring ?

all my divers do is clean my bronze prop well and it doesnt grow stuff so fast when is shiny --oops i used the dread word..LOL...... but is somewhat resistant to growth when shiney and clean--- and used a lot
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Old 03-07-2011, 08:14   #15
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Re: Top Speed Motoring ?

Quote:
Originally Posted by Ironhorse74 View Post
Can anyone recommend a good anti-fouling prop paint?
I haven't found propspeed to be good on my oldish propeller. Don't know what to try this time.

But while the boat is in the water there not much you can do. Try lanoline smeared on while diving it?

I just keep my scraper handy.... but the water here is warm
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