Help with prop size- Marine Trader 36/Lehmans

All,

I recently purchased a 1984 Marine Trader 36 with twin 90 HP Lehmans. It currently has 22x22 4-blade props, which is way overpropped. She will only turn up 1800 and I currently cruise at 7.4 kts at 1450. My understanding is that she should turn up to 2600. Does anyone with a similar vessel and power package have any suggestions? The prop guy says I should probably have 22x13/14" props on there. Any advice is appreciated!

Are you certain that your props are free of marine growth? Barnacles and other plants and critters can kill your rpm. Do your engines reach desired rpm with transmissions in neutral?

Giggitoni said:

Are you certain that your props are free of marine growth? Barnacles and other plants and critters can kill your rpm. Do your engines reach desired rpm with transmissions in neutral?

Click to expand...

Props are clean and run up to full RPM under no load. They just have too much pitch. I am wondering if I should try to go to 13 or 14" pitch- and where I could buy these props.

boatdiesel.com prop calculator?

I had a great experience with North Harbor Propeller. I purchased the Hung Shen HTP-3 heavy thrust propeller for my displacement trawler (16x10) I see that the HTP-3 comes in your size (22") and ranges in pitch from 14 to 20. They helped me determine the correct pitch based on various parameters: current prop size/pitch, RPM at WOT, hull type, displacement, etc. The prop performs great!

North Harbor Propeller Sales and Repair-Anacortes Washington

A quite good calculator is at the following link

www.vicprop.com/calculator.htm

See a reputable prop shop and change to 3 blade props.

Are your transmissions working properly?

My understanding is that she should turn up to 2600.

True if you wish to cruise at or near 2600RPM.

Sounds like a knowledgable owner installed a cruising prop on your boat , and the results are good.

Why would you want to re prop to gain 300 or 500 RPM at cruise at the same speed and burn more fuel?

Simply install a RED line at 1650 and do not cruise over that RPM.

If you are a "belt and suspenders" guy install an EGT gauge (about $120) and cruise efficiently as few other trawlers do.

FF, while I get what you are saying, you are just grabbing at random numbers. How do you know 1650 is a safe RPM for the way he is propped? And what if he does get an EGT gauge? What is considered a safe range of temperature??? In the end you need prop and power curves to safely operate in that manner. And also someone knowledgeable in those engines to help you determine a safe EGT.

My local prop guy suggested 22x 13 or 14" props. The gent I bought my boat from had rarely used the boat since restoring and splashing her and the existing 22x22's were available to him so he threw them on. I am currently cruising at 1450 rpm and can only max out at 1800, when the engines were apparently designed to cruise at a higher RPM and should max out at 2600. So- I think I may be overloading my engines with the current setup. I was just wondering if anyone had personal experience with propping a boat with similar setup to mine before I go throw 3k at new props. Again, it's a 1984 Marine Trader 36 Double Cabin with T/90 hp Lehmans.

As David suggested, I would recommend joining www.boatdiesel.com and explore your parameters with those folks. The cost of admission would be well worth it! Good luck.

Bama,
Yes it would be hard to find two used props the right size. Call Tacoma propeller and others for deals and other economy routes. Deep Blue also has good prices on props but they positively do not exchange. Glad you have 90s instead of 120s.

Something else for you to think about is CG. Looks like you've got way too much weight forward. Just in your avatar picture she looks bow down.

propeller pitch

Another thing you may want to think of is transmission gearing. My MT 40 sedan with a single Ford Lehman has a 26d x 19pitch 3 blade prop driven by a 2.65 to one transmission. With the price of new props around $1,500. you may be able to have your transmissions regeared for less money. I purchased a spare transmission off e-bay for $350.
Contact Michigan wheel to help with determining what would be best.

How do you know 1650 is a safe RPM for the way he is propped?

Easy , he gets 1800 RPM and a glance at any prop graph shows loads drop very rapidly with lower RPM.

The usual safe HP is 10% less than the engine can pull ,
and HP required drops more than 10% on any prop graph with a 10% RPM reduction...

As there is no mention of blowing a fog of black smoke at his cruise of 1450 or even at 1800 , Id say the previous owner got it just right!

I wouldn't change a thing.Except to add a red line.

And an EGT gauge as it will tell you NOW if you have pushed just a bit too hard yet before the black smoke starts to tell you with a bit of practice.

I used to run up to about 650 - 675oF max which was my cruise with confidence that i wasn't overloding the engine.

I would check the engines with a phototach before I did anything. Even if the tachs are close changes should be made with the most accurate information possible.