Prop Talk: What should I do?

[stormchaser]

From a previous thread...I'm overpropped by 150-200rpm. And due to all the weight of our liveaboard crap, she won't cleanly plane off anymore. Engines are running fine otherwise. When virtually empty, with no water and 1/;2 fuel she planed off beautifully and topped out at 17.9knots. Now she semi-planes and tops out at 15.5. I've decided I want to correct the situation with being over propped. But the question is..which way to go?

1. Have existing props reworked to knock off 2" of pitch (maybe 1"?) and have tuned to the highest level of acuracy possible.
2. New/Used 4-blade
3. New/Used 5-blade
4. New/Used 3-blade

My goals are: proper sizing/loading to allow proper WOT RPM, hopefully restore ability to plane off cleanly, hopefully regain lost speed, and NOT totally kill fuel efficincy at 9-10 knots.

Thoughts?

 

[REBrueckner]

Are you going for economy or perfection? You can't make a decision without some objective.
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Make you decision based on one or the other..... or your own criteria. Or look on Sams and see if they have pair of props available at a reasonable price that you like.

 

[Brian Degulis]

I would reduce pitch by 1" that will get you the lost RPM or close to it. If you go 2" it's probably going to be to much and speed and fuel usage will suffer. Your better off working with the props you have. If you buy used or new you still have the expensive of getting them checked and adjusted. If you want to go to the highest acuracy possible your talking prop scan which is not cheap probably around $800 per prop for the highest level.


Brian

 

[stormchaser]

My objectives, in order of importance are:

1. Proper loading and RPM at WOT
2. Try to regain lost speed and clean planing
3. Efficiency
4. Smoothness and vibration reduction (not really a problem now but a properly reworked prop should have less as a bonus).

Yeah, I know propscan isn't cheap, but if I'm going to have the screws reworked, I'd rather do it once and do it right, to the highest level possible.

Honestly if money were no object, I'd have a whole new drivetrain, modern electronic diesels in the 500hp range. I suspect that would REDUCE fuel consumption quite a bit AND increase speed a bit. I'd love to cruise at 17-18 on her but know that isn't realistic with these old 871TIs no matter what I do to the props. hahaha...

 

[SKYCHENEY]

Go out with normal loading and get speed numbers at all rpm ranges. Then take that info to the prop shop and they will be able to calculate how much pitch to take out. I only took out 1" the first time as I was skeptical of their calculations. As it turned out, I needed to take another 1.5" off to get where I needed to be. Had I gone with their recommendations, I would have only done this once.

You will also need the approximate weight of the boat, the horsepower, the gear reduction ratio, etc for them to plug into the prop calculator program. A good shop will use the computer data coupled with their knowledge and past experience to come up with a good estimate of what you need.

 

[REBrueckner]

150 RPM short at the top end when you usually run at 10 knots is not even worth discussing...you can't load the best at 10 knots and at also at WOT...
.
If for some reason you want best loading and throttle at the top end, as you post, take an inch or inch and a half as Brian posted.

Anything you do will be a "waste of money" based on what you have told us:

My goals are: proper sizing/loading to allow proper WOT RPM, hopefully restore ability to plane off cleanly, hopefully regain lost speed, and NOT totally kill fuel efficincy at 9-10 knots.

but NOT spending money has never had anything to do with owning a boat!!!!! So do what makes you happy...

 

[Blaine Thorpe]

Here's a formula that will give you an estimate of what you need for pitch. This only works if you stay with the same style of propeller. Again, it is just an estimate.

Present prop pitch x present WOT rpm / desired (rated) rpm = estimated prop pitch

Blaine

 

[SeaEric]

Go to Black Dog on Kent Island. They will get you squared away.
http://www.blackdogprops.com/

 

[Brian Degulis]

Take into acount that he really doesn't know the condition of his props right now. He said ther's no vibration so they probably aren't way out but they can be out quite a bit and not cause vibration. What I'm getting to is he's probably going to gain some RPM and thrust just be getting the props near perfect. That's why I'm sugessting just an inch off the pitch. If he takes 2" and it's to much he just wasted all the money he spent on the prop scan. It will all need to be done again if the pitch has to be changed again. I'm betting that the propscan and taking an inch out is going to get the RPM back to 2350 and him planing as good as he's going to.

Brian

 

[SKYCHENEY]

Or, they may be near perfect right now and he needs to have 2" taken out. That is why I said that you need to take the props and the performance info to the prop shop. Once they scan the props, they will be able to tell. Don't just guess or it's likely to be money down the drain.

 

[saltshaker]

You could also try and loose some weight. Tough when you live aboard but you'd be surprised how much weight is added by all the junk that doesn't need to be on the boat. Move what you can into storage and or your dock box. Then determine what needs to be done to your wheels. Have you ever reached 2300RPMs since you've owned her? If the wheels haven't been checked in a while they may very well be in need of a tune up. What wheels are you running? I know a lot of 53's have 28x28 4 blades.

 

[ron6785]

I',,m not sure with all the crap we liveaboards put on the boat it has anything to do with your props. 871's tend to a clean plane only at WOT anyway. As you mentioned when you bought the boat it was only capable of two knots more than now and it was empty.

 

[Brian Degulis]

Or, they may be near perfect right now and he needs to have 2" taken out. That is why I said that you need to take the props and the performance info to the prop shop. Once they scan the props, they will be able to tell. Don't just guess or it's likely to be money down the drain.

Yeah that makes sense I guess the prop shop can read them and figure out how much he's losing now if they're out.

Brian