4 Blade vs 3 blade props

T.R. Bauer · Posted: Tue Mar 16, 2021 9:52 am Post subject:

Nice chart - it's pretty much what I expected. I'm pretty sure the most fuel efficient 26-27 foot Seasport ever put on the water is the 2700 pilot with the volvo KAD series diesel and it didn't get 4 mpg. It does/did get 3 mpg through, which is really good for a boat in that class. It is also rather slow, but the cruise is a very tolerable 18-22 mph. I'd love to have one and I'm not sure why there are not offered with the D4 320 hp or a D6 375 hp diesel. It's probably economics - Volvo diesels are very expensive.

Back to props...As somebody that used to have other boats and do water sports, I know props make a difference. In that application I got what I felt was my best performance with a 4 blade. But remember, this is "felt" better. Was it really? I am convinced it was, but it was not that dramatic a change where I felt I was driving a different boat.

Maybe looking at it like this will help.....lol....You can put all different kinds of tooth counts on your circular saw and each one of the them cuts wood just fine. Some tooth counts are better for some applications, just like some props are better than others in specific applications. A three blade works really well though and that is why they are on the back of so many boats.

smckean (Tosca) · Posted: Tue Mar 16, 2021 12:57 pm Post subject:

starcrafttom · Posted: Tue Mar 16, 2021 3:22 pm Post subject:

Well thats the thing only one or two boats have been rigged that way. The place that sells them is a honda / yamaha (?) dealer so I think they want to not show other motors. I talked to the boat dealer ( not my favorite so not mentioning names) and they will order and rig it any way I want but they are really pushing the twin 225 hondas. Which in the motor on my current cdory 27. Its a good but heavy motor. I think one 400 or 350 is a better what to go even with out the counter rotating props. Not all dealer motor choices are made for you benefit.

The dealer has to cater to a lot of hard core fisherman the want to go fast first and mpg be dammed. Well thats not me, no really. I am willing to go slower, not 50 but 35, if it gets me better mpg. A lot of other people will not be the first to buy a new tech. I had to listen to one blow hard last week going on and on about how the new 600hp mercury was untested. Yeah they sell untested motors for 60k each. That motor has been in testing for 10 years.

bobjarrard

Having spent a great deal of time, money and effort in trying to get a half a knot more from a sailboat - I have found that changing lots of little things in the mix is often less expensive, easier and more effective than dropping the big bucks on the big change. Just a few thoughts:
#1 Bottom paint or not and should you fair the hull - speed paints
#2 Twins, their spacing on the transom (in all directions) and counter rotating props
#3 Windage, especially high and facing forward
#4 Weights - location and amount (stay low - stop pitching moments)
#5 Motor tune, fuel choices, and engine control chips
#6 Lubrication choices, frequency of changes, and temp control
#7 Trim devices and need vs "FARKLE SHINE"
#8 Driver Ed - how to run trim and throttle
#9 etc etc etc etc
Bob Jarrard

starcrafttom · Posted: Tue Mar 16, 2021 4:29 pm Post subject:

Number 8 is a big one.

A unnamed father in law of mine was complaining about his new 17 ft out board fishing boat not planning well and being slow. So we went for a ride. Yep it was not planning well and was slower then it should be. He was talking about taking it back to the dealer and maybe opting for a bigger motor. I leaned over and pushed the " trim tilt " button on the throttle. As the boats bow raised up out of the water and the boat gained 10 mph over its bow down plow position my father inlaws eyes lite up. He had the boat for two month and did not know how to do that. And no one at the dealer even mentioned it. Here we are 25 years later and now I own that boat, he gave it to me, and it still runs just fine, if not a little under powered with just a 50hp on it. took my son James fishing just this weekend.

So yes learn to trim a boat for best performance and do not assume everyone else knows how.

crazy4salmon · Posted: Thu Mar 18, 2021 10:58 am Post subject:

Now that you mention it, the original comparison was really 6 blades vs 4 blades Wink Laughing

Sandy, did you mean 8 vs. 6? Guess when we are talking twins, those would be the numbers. Good catch.

starcrafttom · Posted: Thu Mar 18, 2021 2:32 pm Post subject:

https://www.youtube.com/watch?v=j5xoVM0U8Yo

smckean (Tosca) · Posted: Thu Mar 18, 2021 4:09 pm Post subject:

T.R. Bauer · Posted: Thu Mar 18, 2021 10:30 pm Post subject:

I can tell you without question that on the CD 22 cruiser twin 45s will plane a heavy load that a 90 hp single will not. I don't know why and have really never thought about it until now. I just assumed props since the boat in question with the 90 is ever so slightly faster than my boat. But now, I suspect it is because of the lift and added torque of the twin engine configuration. Of course there are a lot of variables in this, but at least it seems reasonable and makes me feel better Smile