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Old 08-14-2012, 08:18 AM   #1
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Thumbs up What's your prop size ?

I'm preparing to repitch my prop and am interested in what boats similar to mine are using. My boat is a 1983 Chien Hwa trawler, similar displacement to other Taiwan Tupperware ofthe same vintage, single Ford Lehman 120 hp with Velvet drive 2.1:1 reduction. I currently have a 23/16 LH Bronze prop which gives me 6kt at 1750 rpm, and just under 9kt at 2800 rpm. Haven't pushed the throttle to wot but there's lots left at 2800. I want to get a decent 1750rpm cruising speed and be able to wot at 2600, which would be slightly less pitch than the engine can handle. I really dont plan on going the max rated rpm of 2400 under load but think I should be able to exceed it a bit if needed. I have begun my calculations of speed at different rpms and plan to take the data to the pros for the technical calculations but would like to know what you guys are using as well. Thanks in advance.
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Old 08-14-2012, 08:22 AM   #2
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Originally Posted by AVALON View Post
I'm preparing to repitch my prop and am interested in what boats similar to mine are using. My boat is a 1983 Chien Hwa trawler, similar displacement to other Taiwan Tupperware ofthe same vintage, single Ford Lehman 120 hp with Velvet drive 2.1:1 reduction. I currently have a 23/16 LH Bronze prop which gives me 6kt at 1750 rpm, and just under 9kt at 2800 rpm. Haven't pushed the throttle to wot but there's lots left at 2800. I want to get a decent 1750rpm cruising speed and be able to wot at 2600, which would be slightly less pitch than the engine can handle. I really dont plan on going the max rated rpm of 2400 under load but think I should be able to exceed it a bit if needed. I have begun my calculations of speed at different rpms and plan to take the data to the pros for the technical calculations but would like to know what you guy are using as well. Thanks in advance.
boatdiesel.com has a good prop calculator.

http://boatdiesel.com/
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Old 08-14-2012, 09:18 AM   #3
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I have a Heritage East 40 and my props are 22/17
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Old 08-14-2012, 09:55 AM   #4
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Originally Posted by AVALON View Post
I'm preparing to repitch my prop and am interested in what boats similar to mine are using. My boat is a 1983 Chien Hwa trawler, similar displacement to other Taiwan Tupperware ofthe same vintage, single Ford Lehman 120 hp with Velvet drive 2.1:1 reduction. I currently have a 23/16 LH Bronze prop which gives me 6kt at 1750 rpm, and just under 9kt at 2800 rpm. Haven't pushed the throttle to wot but there's lots left at 2800. I want to get a decent 1750rpm cruising speed and be able to wot at 2600, which would be slightly less pitch than the engine can handle. I really dont plan on going the max rated rpm of 2400 under load but think I should be able to exceed it a bit if needed. I have begun my calculations of speed at different rpms and plan to take the data to the pros for the technical calculations but would like to know what you guys are using as well. Thanks in advance.

The very best thing to do is start with what you know and take it from there.

Take your boat to WOT and record your RPM.

Decide what WOT RPM you want. (many including myself feel that you should be able to make something above your engines redline RPM).

Give that information and your prop to a good prop shop. They will recommend how many inches to add or remove.

Prop calculators are great if you are starting from scratch, but you are not. You have data from your actual hull, loadout and powerplant. Use that data.
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Old 08-14-2012, 01:00 PM   #5
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Our 1973 GB36 (two FL120s) was fitted originally with three-bladed props, 24" in diameter with a 17" and 18" pitch. The reason for the two different pitches is the starboard Velvet Drive transmission has an extra gear in it to rotate the shaft clockwise when viewed from the rear (the port shaft rotates counterclockwise). Fitting the extra gear into the transmission forced a final drive ratio change so one prop turns a bit slower than the other one. The slower prop had one-inch more pitch to compensate.

A previous owner replaced the original three-bladed props with Michigan four-bladed props. They were the same diameter and pitches. A few years ago we had the props checked by one of Seattle's best prop shops and they said that both props had been horribly set up when the boat had been in SFO bay before we bought it.

We supplied them with WOT rpm, engine horsepower, and the other information they wanted and they totally reworked the props. They are now 23" in diameter and each one has a 16" pitch.

They said that four-bladed props typically should be pitched down one inch compared to a three-bladed prop of the same dimensions and powered by the same engine. In their experience they have not found it necessary on GBs like ours to pitch the props differently to compensate for the slightly different final drive ratios.

We were going to change to three-bladed props until we learned that our four-bladed props were still good (we'd been told differently by another prop shop). Three-bladed props are more efficient and they back better (less prop walk) than four-bladed props of the same diameter and pitch. Four bladed props are smoother. But new props would have been close to $2,000 each where having the existing props totally reworked cost about $400 each IIRC.
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Old 08-15-2012, 07:41 AM   #6
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Thanys Larry M, Blue Heron, ksanders, and Marin. And to others, please keep the info coming. By the way, my single 120 Lehman has huge power for a boat my size. Cant imagine the power twins would provide!
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Old 08-15-2012, 08:29 AM   #7
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20 x 16 RH behind a 2:1 gear. 40 Albin with a 120 Lehman.
Goes right to the rated 2500 rpm.
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Old 08-15-2012, 02:04 PM   #8
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By the way, my single 120 Lehman has huge power for a boat my size. Cant imagine the power twins would provide!
I don't think so if your boat has a semi-planing hull. If it has a displacement hull I would agree with you. But if it's like most diesel cruisers it has a semi-planing hull which means that the boat can cruise at speeds above hull speed fairly efficiently. Efficiently through the water, not so much efficiently in terms of the power and fuel needed to do this. But for a lot of people the ability to get somewhere faster, even if it's just a knot or three faster, is worth the extra fuel. Particularly around here (PNW) where one is often bucking a 1 to 3 knot (or more) current as one cruises through the islands.
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Old 08-16-2012, 08:03 AM   #9
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Marin, I understand everything you said. Still, twin 120s is overkill for a 34' 16000lb displacement hull boat unless you're hauling those big cedar logs between the islands, and of course, fighting those BC currents. Good luck with all that. And watch out for the ferries!

Another thing I can't imagine is how sail powered boats get around over there! I guess for them timing is everything.

Still looking for 34' to 36' trawlers with single 120 Lehmans to input their prop size.
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Old 08-21-2012, 04:52 AM   #10
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34' marine trader single 120 prop 21''x16p
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Old 08-21-2012, 05:15 AM   #11
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Marin, I understand everything you said. Still, twin 120s is overkill for a 34' 16000lb displacement hull boat ...
A Grand Banks is not a displacement boat. It is a semi- planing boat and as such can use the additional power to go faster than a displacement boat of the same size.

Also, our GB36 is 30,000 lbs.
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