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  #1 (permalink)  
Old 10-09-2017, 03:58 PM
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Join Date: Oct 2017
Location: Benfleet, UK
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Default Df300 prop woes

Thank you for accepting me into this superb forum!

I recently purchased a beneteau antares 8.80 with the df300 fitted, my first outboard boat!

When I picked it up I was very disappointed with the performance, Iíd struggle to get 25kts at wot, it would only rev to 5500rpm and fuel burn was terrible!

I have taken advice but cannot get a definitive answer to my question of what is the correct prop for this setup. The engine was mounted too low, this has been put up about an inch and itís correct now. Now down to the prop, I took off the original 15 1/4 x 19 and replaced it with a 16 x 17. The revs rose to 5650 and the speed to 33kts, was pretty pleased with this but have been advised that these engines need to rev so we have changed it to a 15 1/2 x 17. Instantly the revs have gone up to 6200 which I think is correct based upon suzuki information but the speed has dropped to 30kts.

This confused me a little, I understand the diameter being reduced has made it lose a bit of speed but Iíd have thought the extra rpm would have overcome this.

So my question is what should I use, what would be the best prop for general cruising, would the 16 diameter prop be more economical at 22kts or would it be labouring? Is the engine going to be better on fuel revving a bit higher but under less load with the 15 1/2 prop.

Should I be trying a different prop?

Any advice is more than welcome and thanks for a fantastic forum!

Regards

Stuart
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Old 10-15-2017, 11:51 AM
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Location: lake oswego oregon
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do you have a Suzuki dealer/ mechanic to work with?
here is a page that gives reccomended sizes
http://www.suzukimarine.com/~/media/...llers_PDF.ashx

the other question is number of blades on the prop
from the chart the 16ís are 3 blade props and the 15ís are 4 blades
the 3-15 1/4-19 looks to be designed for the 150/175 HP models

From the boat specs 30 knots is the top speed for this model
I donít know what you achieve trying to push it beyond this point
Engine specs indicate operating range of 5,700 to 6,300 rpm. If you get to the middle of that range you should be on very good shape

the folks that probably know the most about the question you ask is the boat manufacturer themselves. you might reach out to them to see exactly what prop they discovered works best


The Antares 8.80 has a choice of single engine or twin engine power: (300 HP or 2x 150HP max) for a maximum speed of 30 knots and a speed of 20 knots cruising. It has a bow thruster to make manoeuvring easier and handling safer.
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Old 10-15-2017, 02:52 PM
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Location: Benfleet, UK
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Thank you for the reply, the only props Iíve tested are 3 blades. You would be shocked that we cannot get a specific recommendation from beneteau via foxís yacht sales. The knowledge database is sadly lacking greatly. Problem is the 15 1/2 revs too much, I got it to 29 kts max speed today but at that speed itís fuel consumption was over 100 litres an hour. By changing to the next size, the 16x17 it does not rev enough! I will swap the props back over and try again tomorrow and get some better data. Iím ideally looking for the best economy at cruise speeds, not necessarily interested in the top speed.

Will report back tomorrow
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Old 10-16-2017, 02:04 PM
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be careful of the prop you use..
from the specs this engine is not designed for a 15 inch 3 blade prop
the 16 / 17 is the least pitch for the 16 inch prop
then next size is 16 - 18 1/2
if you are going to test a 15 inch get a 4 blade
and it would be nice if you can get your hands on a 16- 18 1/2p or 16-20p just to compare performance results

the other question has to do with boat and engine trim
both effect rpm and burn rate
when you get up to speed you may need to adjust both for optimum performance
I know from my experience, I can get 200 to 300 increase in rpm from just trimming the engine slightly at WOT
and you can get additional rpm from the boat trim tabs besides from using them to level the boat when running at WOT.
one last thing.. year of the engine and how many hours on it?
service records?
good luck and hope this info to be helpful

the following is a guide from Suzuki

pitch
Pitch is used to describe the theoretical distance that a
propeller “cork screws” through the water in inches of
forward travel. In theory, an 18” pitch propeller would
move ahead 18” with each complete revolution of a
propeller. There is always some percentage of inefficiency
as water conditions, boat weight and propeller style are
introduced into the performance equation. The lower the
“slip factor,” the more efficiently the propeller will perform
and the faster the speed at a given engine RPM. Lower
pitches accelerate faster, but have a lower top end speed.
Higher pitches have a slower acceleration, but may attain
higher top speeds. An inch of pitch generally is equal to
150 – 200 engine RPM at wide open throttle (WOT). If
pitch is increased by an inch, RPM will decrease by 150
– 200 RPM at WOT. If pitch is decreased by an inch, RPM
will increase by 150 – 200 RPM at WOT. Always operate
your boat/engine combination within the recommended
RPM range guidelines as stated in your operator’s manual.
Operating outside the recommended range can cause
severe damage to your engine and may void the engine
warranty

Last edited by artdf175; 10-16-2017 at 02:43 PM. Reason: add info
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Old 10-16-2017, 03:58 PM
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Location: Benfleet, UK
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Thanks for some great info here.

Suzuki actually recommend the 16x17 for the boat I have found out. We done some proper testing today in good conditions and with this prop we could only get 5600 fully trimmed which is outside of the operating range, but the fuel economy was atleast 20% better with the 16 than the high revving 15 1/2. Top speed was similar but more consistent with and against the tide.

Ken reeves (propgod)has done some in-depth calculations and has thinks that a 16x16 would get us to within the correct operating range and get us up to around 5800-5900 rpm. This prop is not available from suzuki but it is from others.

I cannot believe how difficult it is to select the correct prop and that it hasnít been extensively tested for this boat as there are a lot of them out there in this configuration.

I will upload my test results as they are quite interesting

Regards

Stuart
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Old 10-16-2017, 03:59 PM
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Location: Benfleet, UK
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Itís a 2015 engine with 150 hours, just been serviced and has full service history, no errors on service downloads
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Old 10-16-2017, 10:18 PM
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Location: lake oswego oregon
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you have approached this correctly
check out Solas props

my original prop was damaged beyond repair and the prop shop recommended one made by these folks
good luck and post back your results
Art
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  #8 (permalink)  
Old 10-19-2017, 04:32 AM
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You are correct that it can be difficult to select the best prop for your boat. Not only pitch and diameter are important but there are lots of other things that you have to consider. Prop A from manufacturer A can be a lot different with prop B from manufacturer B. Even when they have the same pitch and diameter.

Special curved trailing edges enable the prop to maintain performance at higher trim levels and in tight corners. Cupped boat props allow most boats to achieve a higher top-end speed or at least the same speed at a lower engine RPM. They also promote more efficient fuel consumption.

The use of a aluminum or stainless steel prop can make a huge difference as well.

Interesting to read for all boat owners who wants to know a little about props:
prop-school-part-1-introduction
prop-school-part-2-terminology
prop-school-part-3-blade-rake
prop-school-part-4-blade-cup
prop-school-part-5-blade-efficiency
prop-school-part-6-slip
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