$ blade prop pitch changes

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jgonnerman
skipper
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Posts: 66
Joined: Sun Oct 10, 2010 5:04 pm

$ blade prop pitch changes

Post by jgonnerman »

I installed a pair of the 4 bladed props about 8 years ago. They are 16" diameter and 13" pitch. My boat is an early build with the 75 hp engines sets.I have about 1350 hours so the engines should still be close to rated output. They reached rated rpm in the first years after the 4 bladed props were installed. In the 2 years its seems the engine rpm no longer reaches design rpm and planning is no longer achievable. '

The major weight gain which occurred about the time the engine rpm was no longer achievable was a second set of 6V golf cart batteries to the house side. I would guess about 200#+. I am sure other weight has accumulated not to mention fuel and water quantities when planning speeds were achieved. Generally boat was full of fuel and water headed out bound and sea conditions usually would make planning speeds uncomfortable. Return usually lighter boat and down wind and swell would permit planning speeds so ability to plane fully loaded was usually not attempted.

My logs which I now regret not being more complete don't reveal quite when this performance changed. On my longer coastwise cruises I usually tow a 14" RIB and limit speed to 9-10 knots out of concern for damage to the RIB. When I first owned the boat I occasionally towed the RIB at 13-14 (still had the 3 bladed props).I read the dinghy manual and realized the pitfalls of that practice.

That said what has been the experience with the 4 bladed props and the action on pitch changes if any?

Thank you
John Gonnerman
San Diego Ca
"Westend Express"
2002 34 Hull 20
AlanH
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Re: $ blade prop pitch changes

Post by AlanH »

John,

I really don’t have much to offer regarding four blade props. We have a 2006 with 75hp engines and three blade props. We were having RPM troubles a couple years ago and Stuart Yacht had a little pitch taken off. I think we are 16 x 13 but I don’t have the papers with me. Four us the real problems surface when the bottom or the props have any growth.

We just had the bottom done and the props done with propspeed. We set off with full fuel, full water, plenty of beer and soda and two adults. We have a 9’ rib in Davit’s and a 10hp motor. At 3400 RPM (Yanmar says I can cruise all day long at 3400) in zero current we are running at 14.3 knots. Before the bottom work and right after the diver came we were doing 12.5. The bottom was not in bad shape but it really made a difference with fresh paint.

Alan
Alan Hendry
Catbo
2006 34' Hull #81
75hp engines
Expatriate
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Re: $ blade prop pitch changes

Post by Expatriate »

John,

We have the 75 HP engines with 4 blade props. I consulted a prop specialist soon after we bought the boat.
He said the pitch was about right. Darn it, I can't remember exactly what the pitch is right now, but I think it is 16x14.

Weight aft does make a difference. When you say another set of 6V batteries, does that mean you now have six 6V batteries in your house bank? We replaced the house batteries last year with four Lifeline GPL-6CT 6V batteries. They have a name plate capacity of 600 amp-hours and weigh 300 lbs. We keep the dinghy on davits and the outboard on the rail behind the "back porch" seat along with a spare 2.5 HP engine. All other weight is near the center of the boat, under the floor or under the seats of the main table.

When we leave for the Bahamas, we have a lot of food, spare parts, tools, and full fuel and water. No way we can plane until the aft tank is about half empty (half full if you are an optimist). When it is about time to take on fuel, we run wide open throttle for a while just because diesels need it. Most of the time we cruise at seven knots at about 2000 RPM.

It worries me to run at such a low RPM, but so far the engines do not seem to have suffered. When we sailed we wanted to run our new Yanmar 4JH5E at the recommended 80% of full power which occurred at 3000 RPM. That put us at 2400 RPM when we had already reached hull speed at 2000 RPM. That extra 400 RPM was just burning fuel. So, many hours were spent at an undesirably lower RPM, but did not seem to harm the engine. After 1800 hours it was fine. Still bugs me though.

Tim
deising
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Re: $ blade prop pitch changes

Post by deising »

Tim, this is off the original topic, but in response to your concern about running our diesels at lower power settings...

We have put over 2,000 additional hours on our 75HP engines since purchasing the boat (with 1500 hours). Most of that has been at 2200-2400 RPM, but I like to throttle up to 3200 for 10 minutes or so every time I run them. BTW, if you find yourself in speed restricted areas, you can run one engine at idle and the other at a high power setting. Your speed will not be too high, but that one engine is getting a hard workout.

While we have had more than our share of problems with our port engine, the starboard engine has been wonderful the entire time. I think if running at lower speeds was a problem, we would have seen it. People talk about running at high RPM, but is more about running at proper temperatures under a suitable load. Running your engine in neutral at 3200 RPM, for example, puts less load on the engine than running in gear at 2,000 RPM.

Since you are not running them frequently at low RPM in neutral to charge batteries (as some sailboaters do), I would not worry one bit about your operating regimen.
Duane Ising
m/v Diva Di
Punta Gorda, FL
2006 PDQ MV 34 - hull 91, 75HP, 3-blade
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