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-   -   Motoring RPM (https://www.sailnet.com/forums/pacific-seacraft/330728-motoring-rpm.html)

Jonah71 07-25-2019 10:03 AM

Motoring RPM
 
1988 PSC 34 Yanmar 3hm35F 2 blade prop 17X10 RH
I understand that this engine is designed to run at a peak continuous rpm of 3200. My throttle pegs at 2400. It seems I am leaving a lot of power unused and therefore losing potential speed motoring. Is the solution a throttle adjustment? What role does the propeller play in this? I have sailed the boat since I purchased it 4 years ago typically motoring at 2000-2200 rpm because this seemed appropriate given that flat out is 2400.

Thoughts?

Damon Gannon 07-25-2019 10:39 AM

Re: Motoring RPM
 
Does the boat reach hull speed? If it does, then it won't go any faster, regardless of increasing engine RPM. Most likely explanation is that you are "over-propped" (either the pitch or diameter of the prop is too great), so the engine is unable to get up to full RPMs. If it is over-propped, you should consider an appropriately-sized prop because you are increasing the wear on the engine and transmission. Another simple explanation may be your fuel filters. Have you changed them? How does the engine run at idle? Does it run rough or seem like it is being starved of fuel?

Jonah71 07-25-2019 10:49 AM

Re: Motoring RPM
 
Thanks for the input. I will need to do some test runs with the throttle maxed out at 2400 to see if I reach the rate hull speed of 6.9 kts. At 2200 I am certainly not getting anywhere near that speed - typically +/- 5.5 kts

The engine seems to run great with no apparent issues. Frequent fuel filter changes and new fuel pumps (both lift pump and primary fuel pump) in the last 2 years.

PhilCarlson 07-25-2019 10:54 AM

Re: Motoring RPM
 
If you google 3hm35F RPM there is quite a lot of discussion out there:

https://www.sailnet.com/forums/pacif...35f-psc34.html
https://forums.sailboatowners.com/in...-3hm30f.19055/

The Max RPM rating on that engine is 3400 for 1 hour or 3200 continious. Cruising speed should be around 70-80% of the continious rating. Have you verified your tac reading?

Does the engine top out at 2400 when it's not in gear? If so you're probably looking at a throttle adjustment. I don't know if this engine is governed.

Does it feel boggy or blow black smoke at the top end? Both would suggest resistance in the system which could point to over-prop if in gear, or restricted exhaust if not in gear. There are other possible culprits.

Jonah71 07-25-2019 11:08 AM

Re: Motoring RPM
 
Thanks for the input. Two questions: When you say "cruising speed" is the assumption that you are motoring at or near full hull speed (flat water no head winds etc)? Second, if an engine is rated for 3200 rpm continuous is this intended to be the rpm under running load? It would seem odd to me to rate an engine for 3200 rpm but have typical max rpm under running conditions much lower at 2400.

I will definitely test the man rpm in neutral next time I am on the boat to see if I can reach the 3200 or even the 3400 amp limit. I understand that many owners are using different props from mine. I am curious if anyone else has the same 2 blade set up that I do. Does anyone know if there was a "stock" propeller set up?

PhilCarlson 07-25-2019 11:21 AM

Re: Motoring RPM
 
The engine RPM ratings are based on what the manufacturere says the the engine can do without damaging itself, and yes that is loaded. 'Cruising RPM' has more to do with optimal fuel consumption. You'll need to find the sweet spot between RPM, acceptable cruising speed, and fuel consumption.

You can use this calculator to get a theoretical best prop size for your boat/driveline: https://www.vicprop.com/displacement_size.php It's not gospel, but it is useful.

You would want to make hull speed, or very close to it, at your cruising RPM in calm conditions. In this context, all boat speed measurements are through the water, not speed over ground.

SloopJonB 07-25-2019 11:33 AM

Re: Motoring RPM
 
Yanmar says a 17 X 11 two blade is correct so over propping would not seem to be the issue.

Jonah71 07-25-2019 11:45 AM

Re: Motoring RPM
 
I ran that prop size calculation on the link you sent using 3200 for the max rpm (max continuous rating) and for a 2 blade prop the recommended size came out 16.8 dia. 9.4 pitch. Pretty spot on for my existing prop. Given the info compiled here it seems I should feel comfortable running with my throttle maxed out at 2400 rather than 80% of that or 2000.

PhilCarlson 07-25-2019 12:06 PM

Re: Motoring RPM
 
Quote:

Originally Posted by Jonah71 (Post 2051616948)
Given the info compiled here it seems I should feel comfortable running with my throttle maxed out at 2400 rather than 80% of that or 2000.

Yes, the engine can handle it. 2400 right in the middle of 70-80% of max rated RPM. Are you sure your tachometer is giving you a true reading?

olson34 07-25-2019 12:34 PM

Re: Motoring RPM
 
Quote:

Originally Posted by Jonah71 (Post 2051616914)
1988 PSC 34 Yanmar 3hm35F 2 blade prop 17X10 RH
I understand that this engine is designed to run at a peak continuous rpm of 3200. My throttle pegs at 2400. It seems I am leaving a lot of power unused and therefore losing potential speed motoring. Is the solution a throttle adjustment? What role does the propeller play in this? I have sailed the boat since I purchased it 4 years ago typically motoring at 2000-2200 rpm because this seemed appropriate given that flat out is 2400.

Thoughts?

What is your motoring speed at 2400 rpm? I would imagine that your boat would motor at at least 6.5 kts.


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