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maybe that hard racing paint on the bottom is as good as they say, plus I have learnt alot from this and I plan to increase the prop pitch from 8 to 10, I am told that will slow down the prop by 150 rpm and will allow the Yanmar engine to go more slowly to achieve the same hull results.
In June I plan on taking the same trip so I will be able to compare results. The dock master and yard foreman here has just said I must have had more current with me than I realized. I was trying to be as neutral as possible. Now the only thing I need to a water maker instead of carrying all this extra weight.
bahama-here I come
John Vigor wrote:Mike, firstly, the credit for that saying goes to Boyd, not me.
Secondly, if you're really getting 29 miles to the gallon I am truly amazed. Just be thankful and don't touch a thing.
On a Gulf delivery this past summer of a PS44 w/Yanmar 60hp set to 3/4 recommended top rpm. The fuel consumption was slightly less than 1.2 gph. There was no wind and the waters quite smooth for four straight days and except for an occasional eddy current no other factors were noted.
Years ago on fishing boats I always used .5 gph/10hp Which gives a reasonable safety factor. Running above 3/4 recommended rpms under load causes a steep non-linear rise in fuel consumption, at least that has been my observation on both agricultural and marine applications.
Didereaux- San Leon, TX
last owner of CD-25 #183 "Spring Gail"
"I do not attempt to make leopards change their spots...after I have skinned them, they are free to grow 'em back or not, as they see fit!" Didereaux 2007
Mike C wrote:maybe that hard racing paint on the bottom is as good as they say, plus I have learnt alot from this and I plan to increase the prop pitch from 8 to 10, I am told that will slow down the prop by 150 rpm and will allow the Yanmar engine to go more slowly to achieve the same hull results.
Mike, I thought the main reason you fiddled with the pitch was if you weren't able to reach the top rpm that the engine was designed to operate at (engine working to hard) or if you were reaching top engine rpm too easily (engine not working hard enough). I'm not sure it should be done just as a fuel saving technique. I think "slowing" the rpm down means loading the engine more. As I understand it, that extra rpm is a reserve in case you need it in extreme conditions.
I repowered and I was able to get within 200 rpm of top engine rpm on her initial test run. Our yard mechanic who is considered one of the best in our area told me I don't have to worry about that differential but a slight pitch change remains an option. I guess its no big deal to have done. But as someone else noted, the engine will probably run with a bit less friction as she breaks in and perhaps the rpm gap will close some. In any event, I don't feel compelled to do anything about it right now.
I'd love to hear what John has to say on this subject.
Dick, This is probably not the John (V) you were hoping to hear from, but I'll keep the seat warm until he arrives.
I have to agree with your statement concerning prop pitch and how it affects performance. I think that what you say and what what Mike C said go hand in hand.
Too agressive a pitch and the engine starts lugging and cannot attain top RPMs and deliver top hull speed.
Too little pitch and the prop (sometimes called screw) grabs too little water and the screw travel in one RPM is much less than desirable for top efficiency. The prop is literally "spinning it's wheel(s)". and not moving along with fuel efficiency in mind
The hull should, ideally, reach hull speed at or a little more than two thirds of top RPMs. An engine that tops off at 3,600 RPMs should deliver hull speed at 2600-2800 RPMs. This is cruising speed, not to be confused with top speed. I believe that this also applies to heavier than air vehicles, ie: planes, choppers, etc.
I think that the higher, more agressive the pitch, the less prop slippage and vice versa, the less pitch, the more slippage. A boat with a prop that has the correct prop pitch might be considered analogous to a trained runner. Under most conditions, the runner can lope along for long distances with little or no problems.
When the runner lengthens his stride to an unatural length, (too much pitch per RPM) he loses efficiency, slows down and starts to lag behind the pack. Consequentially, when the runner shortens his stride to a degree less than ideal, (too little pitch), he can be running full bore and not going anywhere compared to the effort he is expending.
Therefore, proper pitch, as with proper stride, must be matched to the boat and runner to reach maximum efficiency. The more deviation from the norm results in a proportional reduction in whatever form of efficiency being sought after.
I was thinking of JV because he has been so involved with these type of discussions but I also feel I can take your words to the bank. Whenever I post about something I don't know a lot about (which is most of the time), I'm always relieved when the real authorities don't cut my idea to pieces. Now I can relax and enjoy the game today.
Unfortunately the way to knowledge for guys like me is often when something goes wrong and it needs to get fixed. What knowledge I now have has been very expensive! But its all fun.
The Charger's have handled us twice very easily but I'm thinking their respective playoff records may carry the day for NE. Enjoy!
Well, OJ and Dick are correct, of course, but we're really working in the dark here since we don't even know if we're dealing with a two-blade or three-blade propeller.
Boyd was right when he suspected something was amiss with Mike C's figures. I feel the same. I don't believe he's getting 29 miles to the gallon. There has been some miscalculation somewhere, one that has led Mike to believe he's getting poor fuel consumption figures when, in fact, they're unbelievably high.
If we assume that we have a three blade propeller, here, and an engine that delivers about 13 horsepower to the shaft (I'm subtracting 2 hp for the alternator, pumps, friction, etc) then we can enter the tables to find the correct propeller diameter. The two things the tables want to know are shaft horsepower and maximum shaft rpm.
With a maximum engine speed of 3,600 and a reduction gear of 2.62, we get a shaft speed of 1,374. For these parameters the tables give us a propeller diameter of 13 inches.
This is good news, since it's what Mike already has. Diameter is the most important single factor in deciding the correct propeller because even small changes in diameter have much more effect on power absorption than large changes in pitch or blade area.
As for the pitch, we can now turn to a nifty chart in Skene's Elements of Yacht Design, which is entered with the percentage of propeller slip and the speed of the boat in miles per hour.
Skene's estimates the percentage of slip for auxiliary cruisers such as Mike's at between 46 and 55. Let's assume it is 50 percent. To make 7 miles an hour (6 knots) at 1,374 rpm, the chart calls for a pitch of a little over 10 inches.
So Mike has the right diameter (although I don't know if he has enough clearance between the prop and the hull, because a 12-inch prop is usually the maximum for a CD27) but he may be a little underpitched. I believe he's thinking of pitching up to 10 inches, so he's probably on the right track -- if he has a three-bladed propeller.
But, as we all know, this theoretical pitch/diameter calculation is just a starting point, a ballpark figure to get roughly the right prop on the boat for proper performance trials.
Individual propellers can give different results, depending on their shape and area. The whole object of the prop trials is to see if the boat will reach maximum hull speed (6 knots in this case) with an engine speed of about 90 percent of its advertised maximum rpm.
If your engine reaches maximum rpm while giving much less than maximum boat speed, your propeller needs more diameter or more pitch. If the engine lugs badly at maximum boat speed, that is, if the engine doesn't get up to near maximum revs at 6 knots, either the propeller diameter or the pitch needs to be reduced.
This, of course, is what OJ and Dick have already said, and usually you have to try a couple of different props before you find the ideal for your boat.
The temptation to over-pitch a prop, so that it gives you greatly improved fuel consumption in calm water should be fiercely resisted. It means that the engine will lug. It will never be able to reach maximum prm, and as rpm is a factor of power, you'll never be able to get maximum power from the engine when the sea is rough and the wind against you. It's like trying to cycle up a steep hill in the highest gear.
A better way to get good overall fuel consumption is to run your boat at three-quarters of hull speed or less, keeping that last 25 percent in reserve for those rare occasions when speed counts more than poor mileage.
As usual I've learned something from one of your posts.
By the way, I went from a 12x2 to a 13x3 blade on my 25D to accomodate the increased 13.5 hp of the Beta. There is plenty of clearance. Same engine in a CD 27 calls for a 12 inch. They did my boat and a CD27 at the same time last winter. As I remember he said it was because the D is 2000lbs lighter so they had to make the engine work a little harder. I'm not sure if that makes any sense to you. Another major difference from the 1 GM was the reduction ratios. They are a Beta dealer but most of the prop selection was done in consultation with Michigan Wheel and their computers.
One things for sure, my three blade is going to be locked tight this summer for maximum speed!
Goodmorning,
Just wanted to add my 2 cents. My repowered CD27 with a 2GM20 Yanmar (1998) has a 12x14 Michigan Prop. I believe a 13" diameter would not have adequate clearance. The 14 pitch was suggested by Orr's Island Boat Works in Maine and his sources. (now retired)
My experience with performance. The engine will reach about 3600 rpms (easily to 3500 and then slowly to about 3600 with no smoking) I criuse somewhere between 2000 - 2500rpms. I move fine at around 2100 (about 5 knots - calculate this to be 54% prop efficiency) but feel that I should work the engine part of the time to keep it burning clean. As I increase rpms I loose efficiency.
I have done calculations on mpg over a wide range of speeds. If I stay at about 80% of theoretical hull speed I get about 18 mpg.
On a 1 yr cruise from Lake Ontario to Maine to Key West - Bahamas and then home we traveled about 5200 nm and powered about 4000 of them. We also voyaged out the St Lawrence around Nova Scotia and back home through NYC and powered about 2000 nm. This engine and prop combo have worked fine for us. I am actually slightly overpowered but there have been a number of times when we used every rpm we could get.
One comment on locking the prop. With my 14" pitch I calculate that at 2 knots of boat speed the prop is turning at 174 rpm (@100% efficiency so its actually less) which gives a prop tip speed of 6.2 mph. With a pitch of 10 the numbers are 283 rpm and 8.7 mph. I still question whether these speeds would generate the drag which is a factor at high rpms. I have observed a decrease in boatspeed when I locked the prop (at 1-4 knots boatspeed) (as stated in another post I usually sail with it locked for the peace and quiet and to minimize wear)
I believe my calculations are correct but please have patience with me if someone finds a problem.
Spring is just around the corner,
Loren
SV Whippoorwill
Hi Mike,
I did some calculations. The flaw with the figures which resulted in a 6 knot boatspeed at 2387 engine rpms was that there was no correction for prop efficiency.
My method for your boat (8" pitch and max engine rpm 3600 and a 2.62 trans. ratio)
6076ft = 1nm
6076/60min=101ft/min (to achieve 1 knot)
101ft*12=1212in/min (to achieve 1 knot)
1212"/8"=152rpm (total in /8 in =rpm to achieve 1 knot)
3600rpm/2.62=1374 rpm (max engine rpm/trans ratio = max prop rpm)
1374rpm/152rpm=9.04knots (max prop rpm/rpm per knot of boatspeed with 100% efficiency)
6/9.04=.66= 66% efficieny necessary to cruise at 6 knots
After reading the posts in this thread I was inspired to do calculations for my prop efficiency. I guess it should have been obvious earlier but I found that as I start to approach hull speed my prop efficiency decreases. The 2GM20 can push the boat to over 6.5 knots (prop efficiency 41%) but I'm sure my mpg would be terrible. Some aother efficiencies: 54% @ 4.5 knots and 50% @ 5.5 knots.
Concerning mpg achieved, it can be calculated from the charts but actual observations from my own boat is the most useful for me. Running at max tourque is not where I have seen the best mpg. Traveling at boat speeds that probably aren't the best for my engine do result in the highest mpg. While powering for many miles on the ICW I have used gps miles (realizing that current can be a factor) over a about 200 nm at a time divided by gallons used. Its amazing how similar results were while motoring at a givien engine rpm. Increasing from 2100rpm to 2800rpm resulted in a significant decreased in mph.
And finally - not sure why a person who I believe is working on theory would chose to question what I have observed in tenths of knots on my boat as a result of locking the prop many times.
Thanks to all who motivate us to think - It's tough when the outdoor temp is predicted to dip to -6F as our weatherman did tonight.
tartansailor wrote:Great post John, I need to get a copy.
BTW did you hear the one about the guy that said he felt a speed reduction with a locked prop?
Dick
Dick,
yeah, why would you question someone who is sharing their experience (considerable experience BTW). is it just because they dont agree with you? and then to make fun of them? (tsk-tsk)
