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Old 08-04-2012, 13:39   #1
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Unable to reach cruising speed- prop sizing questions

Hi, we are looking for advice on prop sizing (I know there are several threads on the subject but we just can't seem to make our numbers work out right-so if you have a suggestion we will listen). Below are our boats specs:

Boat: 35 oyster mariner
Lwl: 29
Displacement: 16,000+
Engine: Perkins 410-8 50 hp
Transmission: 2:1
Hull speed: 7.2
Shaft rpm at 95%: 1805 rpm
Unloaded max rpm for engine is 4000

At the moment we have a 16x10 prop. At wide open throttle in gear we reach 3400 rpms with a clean bottom.

At cruising rpms of 2200 we are only getting 4.7 kts, and at 2500 rpm we are only getting 5.2 kts (no current or head wind). We wish to achieve higher cruising speed while keeping these cruising rpms without overloading the engine.

The previous prop was stamped 17x10.5. Our speed was much more acceptable, but were not able to reach high enough rpms leading us to believe that we were overpropped.

To add more confusion, we used Nigel Calders/Dave Gerr's calculations on propeller sizing and came up with the result of a 14x8 propeller?!? Wouldn't this lead to even less speed?

So, how can we achieve a cruising speed of at least 6 kts while maintaining a cruising rpm of 2200-2500 and still be able to get 3800 rpm's at wide open throttle?

Thank you for your suggestion,
Keith and Deborah
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Old 08-04-2012, 13:55   #2
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Re: Unable to reach cruising speed- prop sizing questions

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Originally Posted by Kindle View Post
Hi, we are looking for advice on prop sizing (I know there are several threads on the subject but we just can't seem to make our numbers work out right-so if you have a suggestion we will listen). Below are our boats specs:

Boat: 35 oyster mariner
Lwl: 29
Displacement: 16,000+
Engine: Perkins 410-8 50 hp
Transmission: 2:1
Hull speed: 7.2
Shaft rpm at 95%: 1805 rpm
Unloaded max rpm for engine is 4000

At the moment we have a 16x10 prop. At wide open throttle in gear we reach 3400 rpms with a clean bottom.

At cruising rpms of 2200 we are only getting 4.7 kts, and at 2500 rpm we are only getting 5.2 kts (no current or head wind). We wish to achieve higher cruising speed while keeping these cruising rpms without overloading the engine.

The previous prop was stamped 17x10.5. Our speed was much more acceptable, but were not able to reach high enough rpms leading us to believe that we were overpropped.

To add more confusion, we used Nigel Calders/Dave Gerr's calculations on propeller sizing and came up with the result of a 14x8 propeller?!? Wouldn't this lead to even less speed?

So, how can we achieve a cruising speed of at least 6 kts while maintaining a cruising rpm of 2200-2500 and still be able to get 3800 rpm's at wide open throttle?

Thank you for your suggestion,
Keith and Deborah
You aren't reaching your desired 3800rpm (sounds a bit high to me..) with this prop. Reducing pitch, or diameter, to increase your max rpm, will only reduce your cruising speed even further.

I would look somewhere else than the prop:

Is the engine healthy? boat overloaded? Maybe your goals aren't achievable with 50HP?
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Old 08-04-2012, 13:55   #3
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Re: Unable to reach cruising speed- prop sizing questions

Here in CA, we go to Wilmington Propellor (Google them). You must have something similar in FL. We installed a new Yanmar engine and the install manual had propellor sizing guidelines, so I was pretty clear on what we needed. Went to Wilmington Propellor and didn't tell them what the manual said......just gave them the information they requested and darned if they didn't come up with the same size. This stuff is important....go with the pros.
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Old 08-04-2012, 13:56   #4
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Re: Unable to reach cruising speed- prop sizing questions

well...... I dont have your answer but a prop shop should. If the boat will take a 17" I would stay with the larger diameter. Surprised there is such a substantial differnce between those two props though. A 4-108 pushed my 44 footer at hull speed at less than 2800 rpm. (max prop). Do you still have the 17", maybe have it depitched a bit? 4000 rpm sounds awful high for that engine. I dont think mine ever went over 3400. You've got plenty of HP for your boat, so dont fret over not reaching near max rpm.... unless you like to run up there! The sweet spot on mine was 2500-2600 rpm.
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Old 08-04-2012, 14:18   #5
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Re: Unable to reach cruising speed- prop sizing questions

We received the 4000 rpm unloaded value from the Perkins manual. With a proper prop we would expect to be able to reach close to this rpm at wide open throttle. We do not want to run at this! We were told that cruising rpms should be between 2200-2500 rpm. Maybe this is where an adjustment can be made.

We do still have the 17 prop. If we de-pitch it, would we expect the rpms to remain the same with an increase in speed?

By the way, the current prop is what was recommended by a reputable prop shop. The engine has just been rebuilt and is running efficiently and is suitable for the size boat.
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Old 08-04-2012, 14:34   #6
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Re: Unable to reach cruising speed- prop sizing questions

If you depitch it, the rpms wont stay the same. Yeah....I guess I've not had good luck with formulas or prop shops either... they get you in the ballpark usually... unless they have data on identical boats. Something about this just doesnt add up but I cant put my finger on it. dont know your waterline length but you ought to be getting 7+knots max right?
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Old 08-04-2012, 15:00   #7
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Re: Unable to reach cruising speed- prop sizing questions

The disparity between your LWL and total length might mean that theoretical hull speed can only be reached when heeling (that's why they built boats with overhangs--not for seaworthyness but to cheat a racing rule. An overhung boat has greater waterline when heeled than on an even keel.) Not saying that's the whole issue, just that your hull speed on an even keel might be part of it.
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Old 08-04-2012, 15:09   #8
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Re: Unable to reach cruising speed- prop sizing questions

What speed can you get under sail?
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Old 08-04-2012, 15:20   #9
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Re: Unable to reach cruising speed- prop sizing questions

Cheekchako,

Yes, you are right we should be able to reach 7 kts, waterline length is 29 ft.
It's frustrating, we know we should be able to reach 7 kts, but just don't know which variable to adjust. As you said, "something just doesn't add up".

Keith left for a moment, so please bear with me as he understands this much better than me.

1) if we depitch the 17x10.5 prop to 8 inches, would we expect the speed to decrease, but reduce the load on the engine, therefore being able to run at a higher rpm(gaining back some speed)?

2) Is it better to run a smaller prop with more pitch, or a larger prop and reduce the pitch? As it stands now the engine numbers seem good with the 16x11, but can't reach 7 Knts. The 17x10.5 got us the speed, but we could't get the rpms high enough.


Thanks
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Old 08-04-2012, 15:25   #10
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Re: Unable to reach cruising speed- prop sizing questions

We seem to maintain an easy 6 knots on the wind and 7 reaching. We just got new sails and havn't really had much time to see how they affect her speed.
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Old 08-04-2012, 15:55   #11
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Re: Unable to reach cruising speed- prop sizing questions

There's a discussion here on the Perkins 4-108 power curve.

My John Deere is overpropped and I can only get 2300 out of 2500rpm with everything totally clean. I mentioned this to my sales rep and he suggested I just keep my engine speed below 2250!

I found out halfway through my install that I needed a 4" exhaust. There's a mention in the above link that the Perkins needs a big exhaust to get it's full power.

It's interesting to read actual descriptions of how "happy" Perkins owners run their engines.
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Old 08-04-2012, 17:02   #12
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Re: Unable to reach cruising speed- prop sizing questions

You might be asking more than the laws of physics can provide as far as keeping your RPM's down and the speed up. The faster an engine turns, the more horsepower it generates. Of course the more horsepower it generates, the faster it goes.

At the very least with a clean bottom and a healthy engine, your engine must be able to reach it maximum RPM, otherwise you are effectively lugging your engine which means you will not be able to periodically burn the carbon out of the combustion chamber. This will shorten the life of your engine.
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Old 08-04-2012, 17:17   #13
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Re: Unable to reach cruising speed- prop sizing questions

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You might be asking more than the laws of physics can provide as far as keeping your RPM's down and the speed up. The faster an engine turns, the more horsepower it generates. Of course the more horsepower it generates, the faster it goes.

At the very least with a clean bottom and a healthy engine, your engine must be able to reach it maximum RPM, otherwise you are effectively lugging your engine which means you will not be able to periodically burn the carbon out of the combustion chamber. This will shorten the life of your engine.
A larger prop, or a higher pitched prop requires more torque to turn at a given RPM.

David M has it right, in ICE more RPM = more HP. I think you are going the wrong direction with the prop.
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Old 08-04-2012, 17:18   #14
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Re: Unable to reach cruising speed- prop sizing questions

Making your pitch finer is akin to changing down to a slightly lower gear in your car. The engine will rev a little faster, and produce more power. If you go back to the prop shop and tell them what is happening, they should be able to adjust your pitch accordingly , but it is an art, not a science, so may take more than one go.
Your maximum engine revs should be a fraction less than what it would rev to in neutral.
Your engine is plenty big enough.
Regards,
Richard.
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Old 08-04-2012, 18:15   #15
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Re: Unable to reach cruising speed- prop sizing questions

1) if we depitch the 17x10.5 prop to 8 inches, would we expect the speed to decrease, but reduce the load on the engine, therefore being able to run at a higher rpm(gaining back some speed)?
Going to 8 seems like a pretty big jump. The rpm at a given hull speed would increase,
Unless you dont have enough HP available (you should have plenty) You should be able to reach hull speed with a wide range of pitches.In theorythe engine rpm should max out at 10% or so less than highest recommended engine rpm. In reality that's not always necessary in my opinion.
2) Is it better to run a smaller prop with more pitch, or a larger prop and reduce the pitch? As it stands now the engine numbers seem good with the 16x11, but can't reach 7 Knts. The 17x10.5 got us the speed, but we could't get the rpms high enough. Tradition says to run the biggest diameter....then pitch accordingly. I guess I'm confused why you changed the prop? what rpm were you able to make with the old one... wide open?
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