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Old 09-10-2016, 06:11 PM   #1
db2
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Californian 42LRC Prop Size/Over Prop

I need to replace the props on my Californian 42LRC and I would like to correct what I think is an over prop situation in the process. When I bought the boat the starboard prop was damaged by electrolysis. The Electrogard system has now been bypassed and conventional zincs are in place.
The boat has fresh Perkins T6.354 engines. The Velvet Drive transmissions are 1:1. The shaft size is 1 3/8". The props are 4 blade 24D x 19P.
My Perkins workshop manual says that these engines should reach 2250 rpm. At full throttle I get about 1900 rpm. I don't think the turbos even kick in at this low of a rpm.
Can any members with similar set ups share their prop size with me? Are 4 blade and 3 blade props similar in performance when it comes to selecting the proper diameter and pitch? Which should I change, the diameter or the pitch? Any help will be appreciated.

take care all,

Dan
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Old 09-10-2016, 08:25 PM   #2
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Take a look for the engine RPM stamped on your high pressure fuel pump for the correct RPM on your installation. That'll give you an accurate target to shoot for.

Check your tachs' accuracies with an inexpensive digital photo tach. They're available on Amazon for under $20.

Yes, there's a difference between the performance of similar dia/pitch of 3- and 4-blade props.

Maybe this prop calculator will help.

https://www.vicprop.com/calculator.htm
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Old 09-10-2016, 08:40 PM   #3
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That's very low pitch.

You's be better off w a three blade probably w less dia and more pitch. You're eating up most of your power trying to spin up those big props through the drag of the huge blade area. Best prop very roughly would be something like pitch = .8 of dia.

Look in the other prop thread from yesterday. There was someone posting that had access to Vic Prop or some formula for prop selection. Or go to a prop shop

Actually your problem is the gear ratio ... 1-1 is direct drive. I'm surpprized you got up to 1900rpm. You need a much lower gear ratio ... 2.57-1 or deeper like 3-1.

Are you sure your gear is direct drive?
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Old 09-11-2016, 12:10 PM   #4
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This Kit Build sheet supplied with my 42 LRC lists the prop size for the Perkins as 4 Blade 23 x 21 x 1 3/8.
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Old 09-11-2016, 07:29 PM   #5
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Thanks for the help. The collective knowledge here is fantastic. I checked my high pressure pumps and they are both stamped 2750. Do these turbo Perkins really spin that fast? If so, no wonder my boat will only do 8kts. The PO may have over propped the boat. It was a Seattle boat named Porgy and owned by a gentleman named Yapel. It made many trips from Seattle to Cabo. If it will only spin 1900 rpm with the 24 x 19s it is very over propped.
I could not find a tag on the transmissions, so I may be mistaken about the 1:1 ratio, I have one too many boats.
TG says that the build called for 4 Blade 23 x 21 x 1 3/8 props. This would be 1" less diameter, but 2" more pitch. TG, does your Californian have T6.354 Perkins engines and how many rpm do they rev at what boat speed? I hear that the 42LRC can do 12 kts.
Thanks for the help boys, I really want to get my prop choice right the first time as you may guess. I will consult with local prop shops tomorrow, but there is no substitute for real data.

Dan
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Old 09-11-2016, 09:57 PM   #6
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Originally Posted by db2 View Post
If it will only spin 1900 rpm with the 24 x 19s it is very over propped.
I could not find a tag on the transmissions, so I may be mistaken about the 1:1 ratio, I have one too many boats.

TG says that the build called for 4 Blade 23 x 21 x 1 3/8 props. This would be 1" less diameter, but 2" more pitch.

I hear that the 42LRC can do 12 kts.
You didn't say what year, but assuming your T6.354's are not range 4 and are T6.354M. The manual says Max RPM 2400 and continuous cruise speed is 2250. Running maximum engine speed should not exceed one hour.

The non turbo'd 6.354M has a Max RPM 2800 and continuous cruise speed is 2400.

Something really wrong with your prop setup, I don't know what happened there. Most of the turbo'd boats I've seen had 1 1/2" or larger shafts. Maybe they used a higher grade of steel in these. I have 1 3/8" shafts on my non turbo'd 6.354M's 130 HP engines. Were the engines originals or previous owner upgrades?

Your transmission won't be 1:1. If they're turbo'd engines and BW Velvet Drives, should be model 72C and ratio 1:1.94 or greater.
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Old 09-11-2016, 10:10 PM   #7
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Originally Posted by db2 View Post
Thanks for the help. The collective knowledge here is fantastic. I checked my high pressure pumps and they are both stamped 2750. Do these turbo Perkins really spin that fast? If so, no wonder my boat will only do 8kts. The PO may have over propped the boat. It was a Seattle boat named Porgy and owned by a gentleman named Yapel. It made many trips from Seattle to Cabo. If it will only spin 1900 rpm with the 24 x 19s it is very over propped.
I could not find a tag on the transmissions, so I may be mistaken about the 1:1 ratio, I have one too many boats.
TG says that the build called for 4 Blade 23 x 21 x 1 3/8 props. This would be 1" less diameter, but 2" more pitch. TG, does your Californian have T6.354 Perkins engines and how many rpm do they rev at what boat speed? I hear that the 42LRC can do 12 kts.
Thanks for the help boys, I really want to get my prop choice right the first time as you may guess. I will consult with local prop shops tomorrow, but there is no substitute for real data.

Dan
My 79' 42 came with the 3208's but the Build Kit lists the prop sizes for the Perkins and the Cats. See Attached. The first line is the prop size for the Cat 3208's and Near the bottom it lists the prop size for the 185hp Perkins.
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File Type: pdf 42LRC-Kit.pdf (637.1 KB, 19 views)
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Old 09-12-2016, 11:52 AM   #8
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Edelweiss, thank you for the info on rpm and transmissions. The boat was built in 1973, so I believe the engines are the early models. I will plan for 2250 rpm. I don't know what the PO did, but indications are that he too lost a prop to electrolysis. The is a spare 24x 19 on the boat, so I think he changed prop size. What size are the props on your non turbo boat and are you happy with them?
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Old 09-12-2016, 01:07 PM   #9
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You'll get a much better prop prediction working with a major company. Even if you don't buy their props. Using an online calculator with no current performance info is just a shot in the dark. They may produce props that reach the rpm desired, but probably won't be the best speed or economy.
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Old 09-12-2016, 11:21 PM   #10
db2
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Pacific Propeller in San Diego was very helpful. It appears that my 24 x 19 props may be correct. I was encouraged to perform a WOT test at the dock and the engines only hit 1900 rpm. It looks like I need to learn to adjust the governor on the HP pumps. Can anybody tell me how?

thanks to all for help,

Dan
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Old 09-12-2016, 11:25 PM   #11
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Edelweiss, thank you for the info on rpm and transmissions. The boat was built in 1973, so I believe the engines are the early models. I will plan for 2250 rpm.
Db2

This is the CAV pump manual. If it doesn't copy through properly Dan, just PM your email and I'll send it that way.
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File Type: pdf Lucas CAV DPA injection pump instruction book.pdf (3.70 MB, 11 views)
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