Autoprop H5 propeller with Hunter 38

[firebird13]

Hi,

Four years ago, I bought a 2009 Hunter 38 equipped with a Bruntons Autoprop H5 propeller.

From the start, I noticed that the engine overheated above 2400/2500 RPM.
Several specialized mechanics looked into the problem without finding a solution.
Finally, a Yanmar dealer removed the engine from the boat, completely overhauled it, cleaned the engine block, replaced the sensor and thermostat, and re-cleaned the heat exchanger.

The problem persists.
The 3JH4E engine reaches a maximum of 2900 RPM with a clean hull and an overhauled propeller.
We are wondering if the propeller itself might be the cause of the problem.
Does anyone else have this type of propeller and are experiencing a similar issue?

 

[jssailem]

Your prop is automatic self-pitching to maximize efficiency and performance. From your symptoms the first question would be Prop Pitch, but not in your case is the prop is functioning to specification.

Next question is the 15" diameter the correct size for your engine? The Hunter is known to come with a fixed prop that in the 17-18" size. The Autoprop accounts for the smaller size through the adjustable pitch. The 15" size is less expensive and will provide a bit higher sailing speed.

If your gearbox ratio is on the high side (e.g., 2.61:1) or the boat is heavily loaded, a slightly larger diameter (16–17") in the same series could give even better low-speed thrust.

All things on our boats is a compromise. You state that the engine has been fixed. You might take the boat out on a calm day and do a WOT test.

Sourced info:
How to Verify/Optimize It Yourself

1. WOT test (flat water, clean bottom, light load): Engine should reach ~2900–3100+ RPM (near rated). If it tops out way lower → slightly over-propped (rare with Autoprop; contact Bruntons for a repitch). If it over-revs easily → under-propped (go larger dia).
2. Cruise numbers: Expect 5.5–6.5 knots at 2200–2600 RPM (~0.6–0.8 gph) and 7+ knots at higher RPM.

 

[firebird13]

I contacted the Bruntons agent to get his opinion on the propeller fitted to my boat. Here is his reply:

"The AP-9561 propeller is an Autoprop H5, 460mm diameter, designed for a Hunter 380 with a Yanmar 3JH4E, 39hp @ 3000 rpm, 2.36:1 ratio, 1.25" shaft, right-hand rotation."
He replied for a Hunter 380 instead of a 38. But I think it should be the same.

Regarding performance and handling, at 2300/2400 rpm, with the hull clean, the boat cruises between 7 and 7.5 knots. Above 2400 rpm, after 20 minutes, it overheats.

It doesn't exceed 2900 rpm, whereas it should reach between 3000 and 3200 rpm.

 

[jssailem]

Regarding performance and handling, at 2300/2400 rpm, with the hull clean, the boat cruises between 7 and 7.5 knots. Above 2400 rpm, after 20 minutes, it overheats.

It doesn't exceed 2900 rpm, whereas it should reach between 3000 and 3200 rpm.

Trying to decode the issue.

• The variable pitch prop would suggest that the faster the prop spins, the more it tries to increase the pitch. This would be based on the centrifugal force being the driver for pitch change. The 15" Prop my be at full pitch when the engine is showing 2400rpm. Are you seeing any noticable sit down in the water by the stern at max speed?

Just a couple of SWAG possible workarounds come to mind.

• The larger prop may allow the engine rpm to reach above 2900 before bogging down and still provide efficient thrust.
• Perhaps you need to increase the size of your heat exchanger.
• Perhaps the engine thermostat is wrong. It is not opening as the engine heats up.

Question: What is the temperature of the water you are in.

 

[MikeHoncho]

Hi,

Four years ago, I bought a 2009 Hunter 38 equipped with a Bruntons Autoprop H5 propeller.

From the start, I noticed that the engine overheated above 2400/2500 RPM.
Several specialized mechanics looked into the problem without finding a solution.
Finally, a Yanmar dealer removed the engine from the boat, completely overhauled it, cleaned the engine block, replaced the sensor and thermostat, and re-cleaned the heat exchanger.

The problem persists.
The 3JH4E engine reaches a maximum of 2900 RPM with a clean hull and an overhauled propeller.
We are wondering if the propeller itself might be the cause of the problem.
Does anyone else have this type of propeller and are experiencing a similar issue?

Yes, and the shop that did all that work without questioning the prop should took you for a ride it appears. I got the same type of response from the US dealer of Autoprops out of Florida. I bought mine used, so I owned the problem, not them, but the information was wrong, way wrong. The engine would not go above 2500 rpm without smoking like crazy but averaged about 7.2 knots. I had it cut down and now it maxes out at 6.5 at 3500 and is right were it should be. No change in pitch just cut down. I believe a total of 1 inch.