Prop selection for economy

I have been using several prop sizing programs found on several different sites on the internet and none seem to address TWO (2) questions I have.

First the details: I have a propellor on my Gulfstar 36 (20,000 lbs) that is an 18/18 and it is driven by a Perkins 4-108 (47hp at 4000 and 37hp at 3000) through a 2.57:1 Velvet Drive gearbox. I cannot achieve anywhere near hull speed and cant even achieve 7 Knots which is what I think the economical cruising speed under power would be. When I get it up to 6.5 it begins to overheat. (I have checked water flow and replaced the raw water impeller. The fresh water is suffient in amount and does not vary in level over long periods of time..50 hours or more). There is NO black smoke and the engine runs smoothly. I just completed a 35 day, 1400 mile trip with it running at 5.5 knots at 200 degrees F and averaging 0.671 GPH.

The questions I have are as follows:

1) according to several of the prop sizing programs I could increase the diameter of the prop to as much as 23x20 and reduce the RPM to as little as 2000 and then have a slower running engine but still have the HP to push it to 7 knots. BUT....none of these programs let me enter the prop specs I now have just to check the program and to determine if my performance is any where near what the prop should be giving me. (there could be something else that I should be looking at) I think I would prefer a less radical change, perhaps to a 20x18 and reduce RPM to 2500. (2500 rpm, 996 prop rpm, 35HP about 75% of full power and 7 knots) Wheather reducing the RPM would result in a cooler engine or not is also a question since it is still doing the same amount of work, just as a slower pace. One program suggests that the optimum diameter is 20" for this combination but it doesnt even suggest the pitch and doesnt say how it comes to that conclusion.

2.) None of the programs seem to make a distinction between sizing for economy and sizing for speed. And most seem to be designed for speed in that their language suggests they are refering to larger power boats and sometimes have an input for hull shape. I am only interested in displacement speeds. To further complicate the problem the Gulfstar 36 uses the same hull for the MS as for the Trawler so its not really a sailing hull.

I am fully confused now and would like some input on this.

 

What is the available room for the propeller?

 

I am not sure, but I think I could go as large as a 23" prop. But I will have to dive down and take a look as I have never seen the bottom of my boat.

 

This is the first thing you have to be sure of. And it would be necessary to accurately measure it on the dry, not diving.

 

"When I get it up to 6.5K it begins to overheat."

There are dozens of reasons why a boat motor will overheat, so I would recomend a compression test , injector check and then timing checked.

Also look for heat elsewhere in the system , hot drive bearing or hot transmission?

The power numbers you quote are advertising numbers , about 25 hp is all that Perkins will do. About 1 1/2 galons per hour of fuel burn.

Your numbers ,6 gph is normal , and about 12 or 13 hp is all you're using.

Before looking for a "crusing" prop look to the condition of the engine & drive train.

You might start by diving and cleaning the prop to as near new as you can.

FAST FRED

 

My manual states 47HP at 4000 (pleasure) and 37hp at 3000 (commercial). Thats quite a bit more than you are saying???

The prop and bottom were cleaned one day before leaving on a 1400 mile journey and it was data from that trip that produced the points of referance I mentioned.

The water pump has a NEW impeller and is pumping more than enough water (so the exchanger is not plugged), the temp guage is NEW, The exhaust is cool, and the engine is cool enough to lay your hand on it for2 seconds or so before you yank it back. The timing can only be judged from the fact that it starts immediately and the engine does NOT smoke at all. Could still be a problem there, but not a big one.

 

I would definately figure out the overheating issue before looking at new props. And you definately don't want to spin a bigger prop if you are already overheating. My only other question is whether or not you can make full WOT rpm with the current set up.

It seems like you are pretty good at the diagnostics and have a pretty clear understanding of how the cooling system works, so I can only humbly offer a few more thoughts.

Sounds like the raw water side is working pretty well, so lets consider the freshwater side.

Heat Exchanger- Slide out the tube bundles, give them the HCl dip or take to a radiator shop for cleaning. They will build up a scale on the freshwater side that impedes the flow of heat.

Thermostat- usually pretty rugged, but they can stick and not allow enough fresh water throught the heat exchanger.

Fresh water pump - less likely



You are dead on about a larger prop doing the same work at a lower rpm. Its very unhealthy for the engine to do this, however, and is a pretty lengthy conversation to boot. There will be no fuel saving, but you will stress the bearing and cranks, etc, etc, etc.

Here's a little something I typed up for another post, and it makes sense to me, hope you find it helpful...


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Alot of interesting points in this thread. This stuff is really quite complicated, and there are no good resources where you can read the definative answers.
I find more and more that alot of the the yard guys and marine diesel guys - the guys that get paid alot more money to be marine experts - don't really get it either. Some are just god awful. It sure does seem less science and more "art" by the time you factor in trim and prop tuning, and some have suggested above that you just need to try the prop you are interested in, rather than speculate.

Just ask around at the next boat show you go to about the difference between nibral and bronze props. You will get as many different answers as times you ask.

I don't claim any special knowledge, but I sure have been trying for the last year and a half.

Anyway, I think Larry is correct about the fuel savings with a larger prop. (and by larger we mean increase pitch, +/- diameter) There probably are none, but you do stress out your engine.

As I understand it, this is because the "throttle" is not regulating the fuel flow to the engine. There is a middle man who does that, and you can only make recommendations to him with the "throttle" control. The middle man is so important, that we call him the "Governor". ( If you are from around here, you say "Guvna". )

You set the "throttle" to suggest to the governor that you would like to cruise at 1500 rpm's, and he will add fuel when climbing a wave or lay off the fuel when surfing down the face. As sea conditions, winds, whatever change, the governor will adjust fuel to maintain that rpm. So the throttle might really be called the "rpm suggestion lever". Kind of like how the "gas" pedal on a carbureted engine is really an "intake air velocity lever".

So if you put larger props on, you should be able to push the boat to a given speed at a lower rpm, but the governor will be adding more fuel to achieve that same rpm compared to the smaller props. If you consider that it takes a certain amount of horsepower to push the boat to any given speed, no matter how large or small the props, you can expect the same horsepower output from the engine if using the larger props, even if at a lower rpm. You can thank the governor for that.

How this extra fuel at a lower rpm is so damaging to the engine is a great topic for another day.

Anyway, thats my current understanding, and it hasn't changed for many months. I hope it makes some sense to you.
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Mike

 

jimisbell,

As per a quick estimative, asuming a Lwl of 30' (I cannot find the data anywhere!), and using your 37 HP at 3000 rpm, and 20000 pounds displacement, I get Diameter: 19", Pitch: 13 " for a 7.1 knots speed (Around 1.3 S/L ratio) and a 3 blades propeller. With this, the 557 pounds per shaft horsepower available, equal the pounds per shaft horsepower required, so the thing seems balanced (Power to weight ratio). If the waterline is 33', you should reach 7.45 kn with the same.

Alternative: Diameter 18", Pitch 16"

Using 47Hp at 4000 rpm, you should reach 7.75 kn with a 17" x 11" prop.

May your propeller have an excess of pitch, having 18"?

You can contact other Gulfstar 36 owners for advice. You may try http://gulfstar36.com/ and http://www.gulfstaroc.org/ or http://www.gulfstarownersclub.com/ (Which Gulfstar 36 is yours?)

 

Your numbers are very close to mine. I agree the 18x18 has too much pitch.

The WL is 31feet, weight 18000, beam WL 11 feet, Very close to what you assumed.

 

With this data and 37 HP at 3000 rpm, diameter becomes 19" and pitch 14" for 7.45 kn (1.34 S/L) or, as alternative, 18" x 16.5"
This 1.5" difference in pitch with your 18"x18", will probably take out something like 675 rpm from the engine, so dropping from 3000 to 2325.
To find out the economical speed you should find out the minimum point in the engine's specific consumption curve.
In my opinion, you should calculate the propeller for the 90% of the rated power, this is 42 HP. Asuming 3500 rpm for this, we get diameter 17" and pitch 12" and a reachable speed of about 7.8 knots.
Lowering too much the rpm, let's say to 2500 (around 30 HP and the same torque as for the 37 HP and 3000 rpm, this is 63 lb/ft) you could reach 7 kn with a 20"x16", but your engine would probably develope cylinders damage in the long term.
(All the above are rough numbers)

 

I do not know the magic of propellor design so I may be tottaly wrong on this, BUT...wouldnt decreasing the pitch from 18" to 16.5" cause an INCREASE in RPM to get the same speed not a DECREASE. I am just looking at this as though the water were a solid and the screw pitch increases thus more forward motion for each revolution. Since we are aiming for as little slippage as possible, then we are aiming for "water being a solid" and decreasing the pitch would cause an INCREASE in RPM just to keep the same speed???? No???

 

What I'm saying is that having a pitch of 18" instead of the theoretical. 16.5" should be causing the mentioned reduction in r.p.m. (Precisely because pitch is higher than necessary)

 

Too much pitch will increase the amount of Hp absorbed by the prop , but do nothing to help the speed.
This is because the efficency falls so rapildly from the overpitch.

A boat like yours should be set up to run about 6.5L at nomal cruise , and should burn about 3/4 gal per hour doing that , 1800 is a good long term RPM.

The advertising trash is for pulling skiers up, 10 seconds if that.
Engines may carry as many as 5 ratings from 24/7/365 to pleasure boat use of about 100 hours a year.

With the proper gear box (about 2-1) and 17 or 18 inch diameter and 12 or 13 pitch the 1800 LRC should be a pleasure for your ears and wallet, and NEVER overheat.

Attempt 40 hp ONLY at great peril for any cruise.

This website deals with these engines and looking about (or a phone call) may give you a more realistic HP/Rpm picture.

http://www.tadiesels.com/perkins

If std tap water was ever used in this engine G is correct that the heat exchanger should be looked at. You may need to run a 2 part flush if the scale and deposits from hard southern water are there.
AFTER its clean 35% antifreez (as corrosion protection) and the rest DISTILLED water ONLY !!! will keep the engine side serviceable.NEVER tap water.

Good luck,

FAST FRED

 

It is interesting that while the Perkins Shop Manual shows 47 HP at 4000 RPM, this engine rebuilding site shows 51 HP at 4000 RPM. The numbers just keep going up and these are not people selling the engine!!!

But I doubt that I will run it over 3500 RPM anyway because I dont even like the sound of it at 3500. My last engine was a Perkins 6-354 which I cruised at 1400 RPM. It sounded real nice there. I realise that the 4-108 was not designed for the slower speed of the 6-354.

You mention 1800 RPM as a good speed to run the 4-108. At that speed with the 2.57:1 reduction in my transmission the prop would only be turning at 700 RPM. Am I right in thinking that at the lower prop speed I sould use a steeper pitch??

 

prop selection

Jimisbell.....A couple of things to consider (and forgetting for the moment about economy and speed) you have a fairly large boat 36 ft and 18000...20000 lbs ,and a little perkins 4-108 to push it ,engine temp 200 degrees,which is way to high .It would appear the engine is overloaded,therefore explaining the overheat.Can the engine reach rated speed,with the boat fully loaded,passengers gear,etc.etc,if not the engine is definately overloaded ,and will result in a very short life indeed,the only answer to this is to take pitch out of the prop.....cheers