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Old 01-07-2019, 06:41 PM   #1
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what size prop do you recommend?

I bought a 1978 marine trader 34 DC and would like to replace the prop. It has a Ford Lehman 120 hp with a 2.58 gear ratio.
Any suggestions?
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Old 01-07-2019, 06:45 PM   #2
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What is the diameter and pitch of the current prop? Can you reach the rated RPM of the engine when going forward? What isn't the prop doing that you want to change?

Ted
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Old 01-07-2019, 06:51 PM   #3
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prop

I have no idea what is on it. It runs fine and the rpm seems fine. However I have no experience with this boat. I want to put a new prop on and keep the old one when I pull the boat out vs seeing whats on it and then looking for a rebuilt one in a hurry or sending this one out. Its $40 a day for sitting on the hard plus labor in and out . Can't do any work yourself where i'm located.
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Old 01-07-2019, 07:04 PM   #4
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Any divers where you are that clean bottoms and pull props? Most props have diameter and pitch stamped in the side of the hub or on the end of the hub. When I need my prop pulled, I have my diver do it whether on land or in the water.

Ted
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Old 01-07-2019, 07:15 PM   #5
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There's a formula for figuring out prop sizes, depending on boat, weight, hull, etc, etc. Ask some of the prop shops, they have the formula and can figure out what size you need.

May not be perfect so you might need some tweaking.

However, it would be good into for know what you currently have.
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Old 01-07-2019, 07:20 PM   #6
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Click here, Victoria Propellor has a good online prop calculator......
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Old 01-07-2019, 07:36 PM   #7
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prop

I'm told there were hundreds of these boats built. I was thinking someone would have a pretty good idea what was recommended . I'm not sure if it has the original or the correct prop. Thanks
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Old 01-07-2019, 07:50 PM   #8
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Try a real life test.

Your prop would be sized properly if you reach close to maximum (2500) rpm at full throttle. If you reach it at partial throttle, you are under propped. If you canít get to 2400 rpm and are producing black smoke in exhaust, you are over propped.
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Old 01-07-2019, 08:08 PM   #9
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What makes you think anything is wrong with the prop that is on the boat now?
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Old 01-07-2019, 08:13 PM   #10
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prop

Pics of the prop showed chips and was very pitted.
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Old 01-08-2019, 05:56 AM   #11
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For a few bucks you can join Boatdiesel which has a good prop calculator as well as other stuff. Just enter you boats particulars choose the engine and gear combination and it'll give you a size as well as performance specs. The other route is David Gerr's propeller book. I would suggest using multiple sources if don't have similar boats to compare to or previous experience with yours. Weight makes a difference so try to be accurate and realistic.
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Old 01-08-2019, 06:34 AM   #12
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Quote:
Originally Posted by jeff brown View Post
I have no idea what is on it. It runs fine and the rpm seems fine. However I have no experience with this boat. I want to put a new prop on and keep the old one when I pull the boat out vs seeing whats on it and then looking for a rebuilt one in a hurry or sending this one out. Its $40 a day for sitting on the hard plus labor in and out . Can't do any work yourself where i'm located.
IMO, the odds of getting the exact right prop without knowing what you have, are about zero. You can have 2 props with the same diameter and pitch that perform differently because of the shape and surface area of the blades. Also, you might find the owner's manual for the boat that specifies the diameter and pitch, but not the make and model of the propeller. The pitch could also have been changed to optimize the performance. An inch of pitch can change the engine RPM by up to 300 RPM for rhe same hull speed. Then there is the possibility of a transmission problem in the past that may have been resolved by swapping the transmission with a slightly different ratio. As others have mentioned, loading of the boat may have also necessitated a pitch change. In short, there is a good possibility that the diameter, pitch, make, and model may be different than what the boat originally came with.

If you feel the prop is good enough to keep as a spare, why not wait till your next haulout to get the information off the prop.

Ted
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Old 01-08-2019, 06:48 AM   #13
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If you have accurate information such as WLL, weight and draft for a full displacement boat as well as installation considerations and of course engine and reduction ratio it's fairly easy to find the necessary propeller. You need to understand what type of propeller, you don't use a highly skewed and cupped propeller on a tugboat sort of thing.
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Old 01-08-2019, 07:03 AM   #14
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IF the hull and prop are clean make a graph of rpm/boat speed.

Easy to do , maybe an hour in still, no current water.

After you dive or haul you will have the numbers from the existing prop.

Operating the boat for a couple of months will let you know if you could optimize the prop for your cruising style. Some folks want to speed along others do the trawler crawl looking at the fuel burn.

With this in hand you can order the new prop, and keep it as the spare till the following haul out.

I have had good props made from AHOY in New Orleans , with about a 2 month lead time .
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Old 01-08-2019, 07:21 AM   #15
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Quote:
Originally Posted by Fish53 View Post
If you have accurate information such as WLL, weight and draft for a full displacement boat as well as installation considerations and of course engine and reduction ratio it's fairly easy to find the necessary propeller. You need to understand what type of propeller, you don't use a highly skewed and cupped propeller on a tugboat sort of thing.
There is no way the calculator wil tell you the diameter requirement for the specific boat which the OP doesn't seem to know. The diameter is limited by the space between the shaft and the hull, and or the shaft and the skeg if the boat has one. There also needs to be significant clearance between the tips of the blades and the hull or skeg. While you can go with a smaller diameter prop, this will also effect performance. You really need to know the optimal diameter and make sure the current prop has adequate clearance. On an older boat, there are any number of things that could have been changed which may have effected the choice of diameter and pitch.

Ted
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Old 01-08-2019, 08:23 AM   #16
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You cannot just guess. Nor will the pitch be the same on any 2 given boats because of weight and other considerations. Best to get an expert involved. Mistakes can be costly.

Right in your back yard.

https://www.miwheel.com/
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Old 01-08-2019, 08:24 AM   #17
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I thought I addressed those situations with "accurate information" and " installation considerations" however I may not have made that clear. Not a lot of things can be done successfully in engineering without all the information possible. Certainly something like not knowing a vessels operational weight and speed would make it difficult if not impossible to spec the correct propeller. Also I didn't make it clear that I was speaking in general and not about the specific boat.
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Old 01-08-2019, 08:29 AM   #18
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Quote:
Originally Posted by Easting View Post
You cannot just guess. Nor will the pitch be the same on any 2 given boats because of weight and other considerations. Best to get an expert involved. Mistakes can be costly.

Right in your back yard.

https://www.miwheel.com/
Guessing would be a mistake, any reputable propeller shop that deals with more than just outboards would be happy to make a recommendation.
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Old 01-08-2019, 08:32 AM   #19
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I just for the first time used the basic BoatDiesel prop calculator.

With just the basics, that included prop diameter, it recommended the same size as what was on the boat when I bought it. It is 4 bladed but I think they were originally sold with 3 blades.
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Old 01-08-2019, 08:54 AM   #20
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I ran the sketchy information through the Boat Diesel calculator with a few assumptions. A Marine Trader 34 DC's dry weight is 17,000#, I assumed 20,000, 30 ft. WLL, slow semi-displacement, Lehman 120 with a 2.5 to one ratio and got a 26x19 three bladed wheel. This may or may not be correct as I obviously don't have complete information. This type of vessel would normally be provided with lightly skewed blades and no cup such as a Michigan Dyna-Jet. As I have no installation information I can't address things like added rake or if the diameter is usable on this vessel or not, given these and other information we can zero in on what's needed, as propellers this size are produced in one inch increments it necessarily requires some judgment calls as well.
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