Friends:
I forgot to mention that the velvet drives turned out to be 2.57:1. Everything is OK, and all the angst regarding 2:1 was for nothing. I noticed while at the Anchorage Marina in Lindenhurst, NY that a good many of the cruisers had 4 blades on instead of 3. Of course, these boats were much newer than ours. The Viking 35 that the motors came from had a 4 bladed prop. How long (hours of cruising or similar)would it take in gas savings to pay for new (or used if available) 4 bladed props instead of the 3 bladed ones? Or would the proformance be significantly different enough to justify the switch regardless of the gas savings? I do not remember how big my current 3 bladed props are. I may have to shove someone overboard and tell 'em to take a look as long as they are down there.
Jerry
With your increase in power but similar torque to the 427, I would stay with the stock prop specified for the 38 Commander, which is a 23x25 3-blade, or as Paul has experienced, a 23x23 4-blade.
The 4-blade will get you onto a plane with less throttle, you'll have a more efficient cruise, and less top speed. As for fuel milage making up the difference between a 3 and a 4 blade, my head gets tired just thinking about that one. Not sure you'll ever really see it, maybe in my spare time?
Tom:
I understand everything you pointed out, except that the top speed will be lower. Why is that? Does the extra blade make the prop cavitate out sooner? If so, why not change the pitch of the 4 blades? What about the vibration effect. Or is the whole (or sum) of the differences just not that big to worry about? (Kind of like a sentence of 99 years compared to life for a prisoner.) I certianly lean toward not buying new gear and doing my own R and D!
Brainless(and clueless) in VA.
Jerry
3-blade for speed, 4-blade for efficient cruising speed
March 13 2008, 6:34 AM
Thats the way it is, tried and tried again thousands of times. The 4 blade has real good performance out of the hole and throughout the lower to moderate speed ranges. The 3 blade it traditionally faster, and many of the unlimited hydroplanes use a two blade.
My theory on top speed issues is the fact that although the 4 blad has better "bite", it also has more surface area to create drag and friction. It's a compromise. Since most of us in these days of high fuel cost don't run our boats at top speed much or at all, the 4 blade props make a lot of sense and enable boats to cover longer ranges with less fuel and less vibration, at lower rpm too. The ultimate difference in top speed isn't much, maybe one or two mph at the most, depending on the boat and power.
Tom:
Given that the 4th blade adds more area, and if I change to a 4 blade in the future, does the diameter of the 4 blade need to be as large as the 3 blade is now? (I keep remembering that the road to hell is paved with compromises, or something like that!)
4-blade photos ( WARNING - You must be 18 or older to view these images )
March 13 2008, 8:11 AM
Hi Jerry,
I did a lot of research on my 4-blade installation, and at the very very last minute I did a dumb thing, I had them pitched at 24" instead of 23".
Normally if you have a 23x25 standard prop for the 38 Express, the wisdom of the day is to keep diameter the same and drop by 2" of pitch. I only dropped one inch, and as a result I have an over-propped boat, not by much, but it should have been a 23x23 in a Dyna Quad.
Here are photos of the installation. These props made quite a difference in lower speed cruising performance, and they would be even better if they were 23x23.
Here's what my high speed wake looks like. Its still fast, but I can pick up a little more speed with the repitch some day. Ah well, I'm not losing any sleep over it. With the massive low end torque of a 427, the big dogs are still happy. Based on your motors, I think the 23x23 would still be a good option for you. They're tough to find in a 1-3/8" shaft size, mine were custom, but that just means they were not an on the shelf item. They had to be specially bored for the 1-3/8" shaft, and that took them about 2-1/2 minutes extra time and no additional cash.
Paul:
Thanks for the data sets. The differences between the two engines are very small. My scientific training reminds me that there are a multitude of factors involved in producing the sets and that is why I suggest the differences are very small. Wind, current, tide, boat bottom surface condition, variables associated with the props used, condition of the engines, bushing frictions, course piloted, etc. can effect outcomes. But this is data that is useful to us cruising types. We must not let this info get in the hands of our wives! My wife is smart enough to know what it means. Maybe I could paint an orange stripe on the Summer Palace or a big Red Cross and tell her we need the dollars for the gas for emergency trips. Yeah! That's the ticket. I wonder how much an oil well costs? (That is one with oil in it, and not just the well.)4 knots is toooo fasst for me.
Jerry
With a title like that.................................
March 14 2008, 5:38 AM
...........all it would take, is one twig to snap and you could start the Mother of all flame wars, ha ha.
Since I own GM and Ford motors in boats, both of which do an admirable job, I really can't get too hyped by a Ford ~v~ Chevy comment. They both represent fine American iron, and they both do what they're supposed to do.
I recently topped off my tanks, at a cost of $400. Ouch! However, that $400 is going to put a lot of smiles on a lot of faces the way we use our boat. It is a way of sharing an experience with people that sure trumps dropping $400 in a restaurant for a few people. Yeah I know, when you add the refreshments and snacks, the cost goes up, but you know what I mean.
Being on the water in a classic Commander, nothing quite like it! Your 38 not only represents the initial design unvieled in 1964 (yours being a 1964 boat, itself) it is such a good looking icon, get ready to have people wave and give you a steady stream of compliments once you get her commissioned and cleaned up. That is also one of the real kicks of Commander boating, people can't help but appreciate a boat that looks this good.
Paul:
You are so right! From TV commercials (cost of X,Y, or Z,$400. Seeing the smiles, priceless.) There is nothing better than sharing our boat experiences with others. Dave Mehl recently refered to boaters as a group and the core philosophy of our willingness to help others. Here in the DC, Annapolis area are a multitude of programs to give kids, handicapped individuals, elderly, etc. opportunities to go out on the Bay. This forum is another example of sharing. How many $80 per hour mechanics would be willing to give away the secrets that are revealed here?
There are many competitions that have made America great. Fords vs. Chevys. Cowboys vs. Redskins. Army vs. Navy. Sails vs. Power, beef vs. chicken to name a few. We are all better off with FRIENDLY competitions. Let's hope we can continue to instill this lesson in the next crowd to follow us.
Jerry
I have been lurking here on your forum since seeing a note in Classic Yacht Magazine. The volume of specialty information you have here on the Commander boats is mind boggling. We are doing our homework about a future Commander purchase, and this forum is worth its weight in gold to us.
