Prop Size

[mvermeer]

Hello,

I have a 1986 Monk 36 with a Lehman 135 engine and a Newage 401 2.5:1 transmission. The propeller is a 24:22, and she averages 7.5 knots at 2000rpm, 9 WOT at 2600 (which I only do for diagnostics lol).

I am wondering if people could share similar data. I am considering increasing the propeller pitch but of course do not want to overload the engine. I am not sure if the propeller is original, as it seems to be in excellent shape for a boat this age.

 

[rslifkin]

Hello,

I have a 1986 Monk 36 with a Lehman 135 engine and a Newage 401 2.5:1 transmission. The propeller is a 24:22, and she averages 7.5 knots at 2000rpm, 9 WOT at 2600 (which I only do for diagnostics lol).

I am wondering if people could share similar data. I am considering increasing the propeller pitch but of course do not want to overload the engine. I am not sure if the propeller is original, as it seems to be in excellent shape for a boat this age.

If it tops out at 2600 and that's rated WOT for your engine then any pitch increase would leave you overpropped. Check your max RPM in neutral to confirm what RPM the engine is governed to (likely somewhat more than 2600) to confirm that it's not reaching max governed RPM at WOT in gear.

What's making you think that you should increase prop pitch?

 

[High Wire]

Your pitch is right. Even a little on the high side. Most non turbo monks have Perkins 135 hp, are propped 24x20 or 24x21. I would not mess with it except bring your cruise speed down to 6.9 kts if you’re looking for better mileage.

 

[mvermeer]

If it tops out at 2600 and that's rated WOT for your engine then any pitch increase would leave you overpropped. Check your max RPM in neutral to confirm what RPM the engine is governed to (likely somewhat more than 2600) to confirm that it's not reaching max governed RPM at WOT in gear.

What's making you think that you should increase prop pitch?

The need for speed lol JK…I’ve spoken with other owners of similar vessels who have comparable arrangements and speak of lower rpm and higher SOG. Am just curious if I’m under wheeled.

 

[Fish53]

A propeller doesn't necessarily have to be pitched to obtain an engines max rpm. Depending on how you use your boat being pitched to just a bit over torque peak is usually most efficient. Just remember not to overload your engine trying to get higher rpm. A manufacturers engine graph will help chosing pitch and diameter.

 

[Comodave]

The need for speed lol JK…I’ve spoken with other owners of similar vessels who have comparable arrangements and speak of lower rpm and higher SOG. Am just curious if I’m under wheeled.

If you are concerned about SOG then buy a planing hull boat. Otherwise just accept that it is what it is and be happy. I have a planing hull and would love to trade fuel burn rates with you…

 

[mvermeer]

If you are concerned about SOG then buy a planing hull boat. Otherwise just accept that it is what it is and be happy. I have a planing hull and would love to trade fuel burn rates with you…

Oh I’m just used to 8-9 knots from my commercial fishing days. It’s just a thing in my head lol now I’m on tractor tugs that do 11-14 knots! So 7 just seems…slow lol! But, considering it’s called “pleasure boating” and “cruising”, I suppose I should just relax and enjoy the scenery.

 

[Jeff F]

The need for speed lol JK…I’ve spoken with other owners of similar vessels who have comparable arrangements and speak of lower rpm and higher SOG. Am just curious if I’m under wheeled.

You could bump it up to 2200 and cruise at 8 knots, assuming the motor and cooling system is good. No need to jump to prop changes.

Sent from my moto g play (2021) using Trawler Forum mobile app

 

[Nomad Willy]

Some like to prop by running a given # of rpm in hundreds down from max.

I ran my boat mostly 700 rpm down from max at wot. My Mitsu engine made it’s max power at 3000rpm. I ran basically 700rpm down .. 2300.

Those that use this method can compare w other boaters power system easily.
I fudged a bit as I usually propped for 50-100rpm over wot max. That way my engine sounds better w a little less load while cruising. Kinda like singing along happily rather than working hard.

 

[kapnd]

Exhaust gas temperature numbers are valuable in fine tuning a propeller dimensions. An overpitched prop will cause higher EGT #s across the board, and will often show a distinct jump at the point where the manufacturers published torque curve is exceeded.

 

[catalinajack]

A Lehman 135 that delivers 2,600 RPM at WOT has a propeller that is "properly-sized". The torque and horsepower peaks between 1,600 and 1,800 RPM ands falls off from 1,800. Same for a FL 120.

Most Lehman owners run their boats at around 1,650 RPM, some a little more, some less. We owned a DeFever 44 with twin 120s. At 1,650 RPM we travelled at 7.4 knots. Pushing up to 1,800 gave us eight knots and higher fuel consumption. Why bother, more fuel, more noise, marginal speed increase. At 1,650, on the upper helm, the only sound we could hear was a soothing purr of the engines. The only time we EVER ran at more than 1,700 was to overcome about one mile of VERY swift current passing by the brewery to get to the Old Port in Montreal. There we ran at 1,850 and the engines were screaming, comparatively. I was not comfortable running at that speed.

 

[RichHurst]

Have you posted on Monk Owners site? My Monk 36 had pretty much the same numbers as you state. The newer Monks with their 220 Cummins will leave you in the dust racing to the fuel dock.

 

[mickand]

If you consider your waterline length to be 34ft, then your theoretical hull speed is around 6.75kts. Any speed above that and your just pushing water.
considerable fuel savings can be had by traveling under hull speed, but you seem to be a 7+kt speed demon?

 

[OldDan1943]

Hello,

I have a 1986 Monk 36 with a Lehman 135 engine and a Newage 401 2.5:1 transmission. The propeller is a 24:22, and she averages 7.5 knots at 2000rpm, 9 WOT at 2600 (which I only do for diagnostics lol).

I am wondering if people could share similar data. I am considering increasing the propeller pitch but of course do not want to overload the engine. I am not sure if the propeller is original, as it seems to be in excellent shape for a boat this age.

Most good prop shops have the data for the original prop. Then, you talk to them about your desired goals.

 

[Nomad Willy]

There’s another way to the best cruise speed. Load

If you can find or otherwise determine at what rpm you burn 55 to 65% of your fuel burn at the rated rpm. That would indicate you’re at a 60% load. 60% is an excellent place to be for a good balance of engine temps, load and power output. Example; the FL engine burns 6gph at max rpm/load. If you burn 3 gph your running at a 50% load. 4.5 gph would indicate a 75% load and so on. You may want to do some soul searching re load and speed. If you’re a speed freak you must pay. And if you’re a relaxed cruiser or more or less allergic to diesel engine noise you may whan to tune for a tune for a 50% load.

Re the prop go more or less heavy on pitch and use diameter to make adjustments. You may get a bit more speed but flexibility re load/speed will be lower. In other words A boat w a bit larger prop and less pitch will be more tolerant of added weight in/on the boat. A boat w a heavier load will suffer performance sooner w an increased load.

But rec trawlers are mostly all SD hulled boats and that design feature means they were designed to run at least but mostly above hull speed. And they were designed quite awhile back when fuel was cheaper. So now the saying “run what you brung” applies to a great deal. Fact: there are not as many FD boats as one may like on the market so many rec trawlers are run as a FD hulled boat (where they actually SD boat). and even more are run at SD speeds above what the NA had in mind (a guess or assumption).

I am not a NA so some of what I say may need to be taken a bit lightly.
Also one or more of my recommendations likely will not be the most economical course to what’s desired.

 

[rslifkin]

If you consider your waterline length to be 34ft, then your theoretical hull speed is around 6.75kts. Any speed above that and your just pushing water.
considerable fuel savings can be had by traveling under hull speed, but you seem to be a 7+kt speed demon?

I'm getting a hull speed of 7.8 kts for a 34 foot waterline (1.34 x sqrt of waterline length). 6.75 is probably about right for a cruising speed though, that last knot approaching hull speed requires significantly more push.

 

[Lepke]

When I was a commercial fisherman, I ran at 7 knots unless the fishing was great and I had money to burn. At the time diesel was 35¢. Salmon paid $3/lb. and tuna $2000/ton.

If you want to go faster economically, get a longer boat. It's not cheap or easy to overcome hull speed.