Prop Change

So we refurbished our spare prop which was in excellent condition. The boat was pulled and we are swapping out the prop as the one that was on there had some damage, etc.

The original prop was a 22" 16 pitch. The prop I want to put on is a 24" 17 pitch.

I know the 1 degree of pitch is about 100 RPMS. I wanted to get a little better speed out of the boat as I was only hitting 6 knots or so at 1675 RPMS. I am concerned that I may be over propping too much going to the new prop.

I am running a FL120 and a 2.10:1 Velvet Drive. The boat weighs around 19,000 lbs and 35.6' by 13.6'

Anyone have any experience with this? I am on the hard until Monday morning. Maybe I should try it and if it loads the motor too hard buy a new prop. I know the true test is to run it up to the rated 2500RPMS but I would not ever run the boat at that speed. Everything I have read is that they like 1600 to 1700 all day long. I have also read that it was not uncommon to over prop to get more speed knowing you won't ever get to the max rated RPM. I just don't want to hurt anything either.

Thanks for your help.
Jeff

Jeff, opinions will run the gamut but as long as you have proper clearance I don't see a problem over propping a bit. I'd do it on my boat if it wasn't so fun to get up on plane.

Re work your current prop and have a diver put it back on prior to selling the boat as that's the only time it's likely to run full rated RPM.

Increasing D by 2" and pitch by 1" is a pretty big change. I'd advise thinking twice about your goals. At 100 RPM increments up to 2500 what is your boat speed and temperature with smaller prop? How about WL length? Without good data guessing is all we can do to advise you.

I strongly concur with Sunchaser. Your engine should be able to reach your diesel's manufacturer specified WOT RPM. Overpropping has been discussed extensively on this forum and you'll be able to find some very good information on this topic. I've done considerable research on it and share the wide-held opinion to avoid over propping.

Over propping will kill your engine faster than almost anything.

My boats manufacturer is famous for over propping their boats. Every single of my model boat came out of the factory marginally propped empty, and over propped for real world use.

The generally accepted test is to make sure your engine will make full rated rpm when fully loaded, but I believe you know that.

When you indicated that people intentionally over prop a bit to increase speed, that can be true but more with my style boat that I believe yours.

Here's why....

With my style boat, IF you are going to ONLY cruise at or below hull speed then over propping a bit, as my boat came from the factory is not a real engine killer.

That's because the engines at hull speed have very little load applied to them, and at the slightly lower RPM the over propped condition represents there is still enough reserve power in the engines to support the load. Thats because my boat has 660 horsepower. Its designed to run at semi displacement speeds, and hull speed represents very little load in comparison. In short even slightly over propped my boats engines are not overloaded at hull speed.

With your style boat and power plant size you appear to be optimized for hull speed operation. Adding more load into that equation means you are over loading your engine. This will equate to premature death.

I agree w Sunchaser and ksanders to a significant degree.

If Jeff's boat were a twin (240hp total) overpropping would far more desireable than w 120hp. The situations where Jeff would be tempted to apply more power would happen much more often increasing the hazard of overloading greatly.

So I'd go w Vashon Trawler for Jeff but if it were a twin I'd agree more w ksanders.

And Jeff overpropping won't increase speed. Over propping reduces power (often considerably depending on how much overpropped) and hence speed. However at low speeds you may increase speed at a certain rpm. From your OP that may be your goal. Are you just trying to get to a certain speed at an rpm that is smoothest w your engine? That can be done and safely IF one can resist the temptation to increase power to get to an anchorage before dark or get through a rapid that's slowed you down to 3 knots.

And again I agree w Tom that 2" more dia is a lot of increase.

So I just read several threads on over propping, etc. the one thing I got from that was confusion! . Well not completely.

I am going to run this prop the 10 miles back to the slip and see how it runs. I will run up to WOT and see what I can achieve. I think I would not want to run the boat at WOT as a regular thing and really like the 1675 area as it feels about right for my boat.

I don't want to be very over propped at all. The surveyor listed the original prop as a 26" with a 16 pitch. If you can't eyeball the difference between a 22" prop and a 26" prop you may want to change professions. Grrrr!,,

I will most likely have to purchase a new prop. I just don't have that money right now. I may have to cruise at a much lower RPM for awhile until I can get a new one. I know many will pass judgement but you can only do what you can do and in paying cash and fixing the other things that I did I am close to empty for a few months. All good though, I do care and want it to be right. And as usual, I really like all of the information on this site.

Jeff

You can probably get the one you have cut down and repitched for much less than a new prop. Got a good prop shop in your area?

twistedtree said:

You can probably get the one you have cut down and repitched for much less than a new prop. Got a good prop shop in your area?

Click to expand...

Yes and they can take off from the TE, LE, both or the blade tips.
Tom is a good prop man. Don't remember his last name or the name of his business but he's on the right at the bottom of the hill going to Anacortes past the casino and before the WI turn off. Recommended.

Overpropping on a trawler, and running at or under hull speed is not going to kill the engine. Provided, of course, that it is not grossly overpropped. Run the boat at hull speed, check for black smoke or dark water spilling from tailpipe. Either is not good. Now push engine up in rpm and notice where black smoke or dark water is noticed. That is your "smoke limit", don't go there. Below the smoke limit, engine should be happy.

This topic comes up all the time. Engine co's plot prop load curves, but most of our boats are powered to well above hull speed. The boat power curve looks nothing like the displacement hull curve. Well, its the same up to the hull speed, but then has a sharp upward kink in it where HS is reached.

So propping for rated rpm on a boat that can exceed hull speed puts the engine in a very lightly loaded state when operated at or below hull speed. Since knowledgeable trawler operators don't try to push hull speed, why prop it for that region past the "kink"?

Just know where the smoke limit is, and stay under it. If a NA (naturally aspirated, or non-turbo) engine is chugging along at 1500ish at hull speed and not making smoke, it is happy.

JBear, let us know how the prop works out.

Many sail boats that by their nature run at or below hull speed are over propped and survive it. There is no absolute truth here but a good deal of mechanical wisdom suggests a motor should be able to run wide open throttle full load clean bottom at or preferably 50-100 rpm above rated RPM. Some more stringent engine manufactures like my JD would not pass sea trail and give me a extended warranty if even slightly overloaded. There is also the concept that overloaded motors are overloaded at all RPM'S not just at upper end. There are good articles about all this at boatdiesel.com and Seaboard marine.com under Tony's tips. The advantages, if any of overloading are small if any at all and no disadvantage of propping to manufacturers specs.

As much as I don't like to admit it there are two advantages to over propping.

1. Less noise for a given speed well below hull speed.

2. Less fuel burned at a given speed as long as the speed is low enough. A very small amount. About a knot below HS is best. Or perhaps HS w a SD overpowered hull like a 36' w twin lehmans or equivalent.

A trawler that is not over propped will run at idle speed in the harbor slow enough to please almost everyone. If your trawler runs too fast at idle you're almost certainly over propped.

Eys,
Most all over propped trawlers are only overloaded at the upper end IMO. I think even highly over propped trawlers would not be overloaded if kept at least one knot under hull speed. I can't relate to the concept of a boat that is overloaded at any or all speeds. I'd guess no one on this forum is that over propped. That would be overloaded at an idle.

I think you were referring to Olympic Propeller.

That is who did my spare, now on, prop. He did exactly what i asked and did a great job. Problem was me though! I will test the prop and then see what he has to say. I work on his phone system so I will be out there on Thursday to do a couple of things and hit him up then.

I am going to make sure I hit 2300 to 2400 rpm with the prop I settle on. I will still probably run at 1600 to 1700 most of the time though.

Jeff

Just trying to think this through. If a trawler is proceeding through the water at one K below hull speed it is doing so because the prop is pulling or pushing it via its rotation. The key element must be the energy transmitted from the motor to the prop which does the work. The amount of fuel burnt must be proportional to the amt of work done not so much the RPM of the motor. There is evidence that large diameter slow turning props are more efficient, but I don't think we are talking about that here what we have here is mainly a pitch and loading issue. Putting a motor up to a higher rpm does not mean a lot more fuel burnt unless the load is also increased. The over propped motor just increases the load at lower rpms and since the load is there so is the fuel burn. The motor that is not over propped will produce the same HP at a slightly higher rpm, and producing the same HP means burning the same fuel for the same boat speed. As I see it the only time prop loading is important for speed is if you want max speed and HP from your motor then under propping would fall short in transmitting HP from motor to prop and resulting in less speed. I have also read where some boats actually go faster by removing lugging usually in planning boats that have trouble getting over the hump. There is also the issue of lube oil distribution. If traveling at low speed and lower rpm due to over propping affects lubrication it can not be good. Most Diesel motors are more efficient at distributing lube at mid and higher rpm not at the low range. So I still am thinking there is no significant benefit of overloading a motor prop combo.

Jeff,
I was referring to Tom Ashbrook.
Northwest Propeller.
360 466 4206/360 630 7867
PO Box693/LaConner WA 98257
Tom's been banging props for many years. He's fair and very easy to deal with.

eys,
I like your post. Especially the part about lube oil distribution. I'm kinda vague on that could you expand on it some?

Manyboats diesel motors as you probably know are dependent on lube oil for two major reasons. One is coating of metal parts to make slippery and deter wear. The other is cooling and some motors are more dependent than others on this. The heat build up in some critical areas is not always reflected or dependent on none oil coolant. One of the reasons a tech might use a heat gun to find hot spots. A simple rubber band entering the oil system from an oil filter change can clog a oil cooling channel and cause compete catastrophic failure.The distribution of oil while not the same in all motors is dependent on an internal pathway with a pumping system. Something I just remembered. When I was back east and tearing apart a Yanmar 6LY 2A the chief instructor made a point of stating that that motor was highly dependent on oil flow for upper end cooling and it was more important than the coolant. It is not a long stretch to consider that the weak spots in oil distribution would be at start up and the upper and lower ends of rpm and of course thicker oil running cold would also hinder good distribution. I would think on an engine designed to run most of the time at 60-80% load when used in a trawler at lower loads a little extra rpm to produce the same HP would benefit oil distribution. While I can't prove what I think with flow facts, I suspect consultation with engine manufacturers might confirm the logic used. Its not to dissimilar to a very simplistic blood flow system, the major difference being the sophisticated feed back and variable controls in the human machine. The oil pump in the motor is probably dependent entirely on RPM unless there is something I am not aware of and of course there are lots of things in that category. So my conclusion is if you run your motor slow why not prop to spec or under prop and get a little extra rpm for the same load. My personal boat is intentionally under propped and at WOT will show no more than 80% load. I cruise the boat at coolant spec temps at 40-60% load with clean out at end of day. What I try to do is use the motors at or near their peak torque and let them spool free with modest load. The exhaustive sea trails preformed by JD on my 6068 common rail engines had no problem with this philosophy and granted a 5 year warranty, and that would not have happened even with slight over propping.

Max RPM at WOT is great for sport fish and little gas powered IO with car engines..

Volvo and many other diesel eng mfg usually >overprop< today for very good reasons.

I would install your replacement and after the boat is warmed up, say 30 min of modest operation would go to WOT for 30 seconds and note the RPM seen.

Yes there SHOULD be black smoke.

Then I would pull back 10% or 300RPM and see if the speed and new RPM is where you want to cruise.

If it takes even more than a 10% or 300RPM reduction you are home free in terms of not overloading the engine.

Enjoy the pleasures of lower RPM cruising with longer engine life , lower noise and less fuel burn!!