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?

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

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.

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

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.

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

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 can’t get to 2400 rpm and are producing black smoke in exhaust, you are over propped.

What makes you think anything is wrong with the prop that is on the boat now?

prop

Pics of the prop showed chips and was very pitted.

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.

jeff brown said:

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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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

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.

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 .

Fish53 said:

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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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

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/

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.

Easting said:

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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Guessing would be a mistake, any reputable propeller shop that deals with more than just outboards would be happy to make a recommendation.

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.

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.

psneeld said:

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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Do you know what diameter will fit this boat with the necessary tip space? Three blades is generally better than four depending on a number of factors.

Sorry, I wasnt clear.

I did the calculation for my boat to see how easy and accurate the calculator is.

The calculator is only good as far as it goes and dependent on accurate input, but it's a good starting point.

My 34 marine trader had 2:1 gear and used a 24"×16p prop

IMO you should work with a good prop shop. They know how to get the correct prop for optimum performance and will even let you try different props if their first recommendation doesn't work.

Call Michigan Wheel.

You need,
Good hull clearence.
Full rated rpm at WOT. (2500 for your FL I believe)
This gives the propper emgine/prop loading.
Then you should insure there is a good dia./pitch relationship that will insure you don’t have too much dia and too little pitch. Or too much pitch and too little dia. Most trawlers that go astray here have too much diameter and too much blade area and too little pitch. A good pitch, dia and blade area combination will vary little from the perfect combination.
Michigan Wheel has the experience to be able to tell you what you need to know over the phone.

As I recall you have a Michigan machine pitch prop on your Willard, how's that working? I'm putting one on my new boat, an 18x14.

prop

Thanks everyone for the information!
I had the bottom stripped,barrier coated 2 coats of paint. Bow thruster and new windlass installed, Hull painted with awlgrip new stripes. I asked them to check the prop size for me because I saw chips in the pictures they sent me(I'm 1200 miles away). It didn't happen before it was put back in the water 2 weeks ago. I like my toys dialed in right, so doing all the testing with a junk prop won't really give me a perfect baseline anyway. I'm told there's hundreds of these boats out there that's why I asked for help . I figured I would look for one while there was no rush and put it on the next haul out. Then send the original out to rebuild.

You didn't buy a high performance machine so "dialing in right" while certainly your perogative is a bit overkill for an 8 knot or less boat.

Certainly no rush towards some mythical perfect solution unless you are going to keep the drag coefficients, boat weight, engine performance, and cruising rpms to an absolute either.

As to the" junk" prop on there....it might be the perfect one.... as it goes.

jeff brown said:

Thanks everyone for the information!
I had the bottom stripped,barrier coated 2 coats of paint. Bow thruster and new windlass installed, Hull painted with awlgrip new stripes. I asked them to check the prop size for me because I saw chips in the pictures they sent me(I'm 1200 miles away). It didn't happen before it was put back in the water 2 weeks ago. I like my toys dialed in right, so doing all the testing with a junk prop won't really give me a perfect baseline anyway. I'm told there's hundreds of these boats out there that's why I asked for help . I figured I would look for one while there was no rush and put it on the next haul out. Then send the original out to rebuild.

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I might do just the opposite. Run the boat as is with expected load / weight. If you could have the boat weighed it would be ideal.
1. Be sure that the engine can reach full rated RPM in nutreal (no load)
2. Run the boat at many different RPM including wide open throttle
3. Record RPM and associated speed at several different RPMs on a spread sheet. (Be sure to include expected cruise RPM and speed and wide open throttle RPM and speed)
Give that information to the prop shop along with the old prop and let them run with it. A good prop shop like Michigan can offer great advice with the above data.