Unable to reach cruising speed- prop sizing questions

[Kindle]

Cheechako,

The prop was originally replaced because we could only get around 2500 rpm wide open throttle.

[SimonV]

I have the same engine but have a kiwi prop that has been deliberately over pitched.
My theoretical hull speed is 7.3 but I cruise at 6 at 2300 revs if I push the revs to the point it will rev no more it is 7 at 3400 if I give it more throttle I get no more revs but I start to blow soot. at 2300 revs and 6knots I use 2lt an hour

[Astral Blue]

Keith and Deborah... I am in the process of repowering my boat and have chosen the Perkins 4.108 as well. We have different sized boats but similar gear ratios and prop sizes. I have done a considerable amount of research into this engine's capabilities before making the purchase. I also used David Geer's book and it did underscore the calculations I made elsewhere.

For what it is worth, I think the speeds you are attaining are consistent with the engine's output with respect to your boat's displacement, LWL, gear ratio, etc.

In performing calculations to determine whether the engine is appropriate for my hull and my cruising goals for my own repower, I made extensive use of the power required calculator and the prop calculator from boatdiesel.com. This calculator is known to be right on the money with it accuracy; and the results have been underscored with David Geer's formulas. I ran several calculations using the data you provided and have posted the results here. They are consistent with the results you are currently getting. The only thing I am noticing is that your prop seems overpitched. I ran the calculations for cruising at 2500 RPM and performed separate calculations for 14, 15, 16, and 17 inch props. I also performed a power required calculation, which gives power ratings needed for each type of engine. The Perkins 4.108 is considered a "Pleasure" engine.

Hope this helps...

[Astral Blue]

Just for kicks and giggles, I performed the same calculations using 3000 RPM as the cruising speed. By the way, it is completely safe to run the Perkins 4.108 at 3000 RPM for extended periods under load. It is often run at 3600 RPM for many gensets that use this engine.

[Cheechako]

Originally Posted by Kindle

Cheechako,

The prop was originally replaced because we could only get around 2500 rpm wide open throttle.

Hmmm,.... not sure they can repitch over 20%. It would be nice to see you get to 3200-3500 max. I would likely buy that 17" prop with less pitch shooting for that rpm zone i suppose. You wont be overworking the Perkins in a 35 ft boat....

[goboatingnow]

For what it is worth, I think the speeds you are attaining are consistent with the engine's output with respect to your boat's displacement, LWL, gear ratio, etc.

right on the money. given the weight and lwl I would say what you are getting is about right. Note while the 4000 rpm is given the unloaded rated max my understanding from the perkins manuals I have is the marine version is 3500 rpm one hour use and 3000rpm max continuous. The engine would fail if ran at 4000 for any length of time. I don't know where you are getting the 3800 from. Torque curves would suggest a 1900 to 2200 cruising range.

Dave

[Astral Blue]

Originally Posted by goboatingnow

right on the money. given the weight and lwl I would say what you are getting is about right. Note while the 4000 rpm is given the unloaded rated max my understanding from the perkins manuals I have is the marine version is 3500 rpm one hour use and 3000rpm max continuous. The engine would fail if ran at 4000 for any length of time. I don't know where you are getting the 3800 from. Torque curves would suggest a 1900 to 2200 cruising range.

Dave

Glad you shared that info. I always wondered what the maximum RPM is at continuous load. I have the Perkins 4.108 workshop manual and didn't come across any information with respect to what it is rated to take continuously. I know the builds used on (non-marine) gensets operate at 3600 RPM's continuously, but the marine application is different by many respects.

My 4.108 is being installed as I write this; and I'm sure I'll have tons of questions building up to the sea trials. So it's great to know there are so many people here who have real world experience with this engine to share.

[goboatingnow]

Originally Posted by Astral Blue

Glad you shared that info. I always wondered what the maximum RPM is at continuous load. I have the Perkins 4.108 workshop manual and didn't come across any information with respect to what it is rated to take continuously. I know the builds used on (non-marine) gensets operate at 3600 RPM's continuously, but the marine application is different by many respects.

My 4.108 is being installed as I write this; and I'm sure I'll have tons of questions building up to the sea trials. So it's great to know there are so many people here who have real world experience with this engine to share.

great engine, it can always be kept going. Thats its trick. Noisy mind u
Dave

[japarker11]

How about chasing down the gearbox? For that era of Oyster, the Perkins was probably coupled to a Hurth transmission. I found ratios from 1.88 to 2.14. Also, you could be getting slippage at your higher power settings. As you apparently have a tach, you could use a hand held for the tranny output shaft.

Check out an UK website, Castle Marine, for a prop calculator. Using your numbers, it came up with a 16x12.

[hellosailor]

If you hear hooves think horses not zebras.

However there are some assumptions here to be eliminated.

Is the problem new to the rebuilt engine and if so was it properly rebuilt, i.e. full compression, correct injectors?

Clogged exhaust elbow or anything else that might be strangling engine?

Tachometer checked, speeds accurate vs a portable one reading flywheel rpms?

Before you go nuts on the prop, just make sure everything else is not just assumed but certain.

[Kindle]

Thank you to everyone for your advice and time-very appreciated.

After reading the posts and speaking with the prop shop again, before we look at replacing the propeller, we are simply going to ramp up our rpms a bit and watch the temp. It seems this may be enough. We will post our findings after we have had a chance to take the boat out again. We have a short two week shake down cruise coming up next month and this should give us even more time to feel out where our cruising rpms will be and give us a better handle on the performance.

Thanks again!

Hellosailor, your questions were good-and yes we looked at all of the things you listed.

[savoir]

Just a small side issue. I don't get your preference for high revving. That can really cost some buck$. My diesel is a Yanmar and its diesel consumption doubles from 2300 to 3400 rpm. I'm guessing a Perkins would be similar.

All the high revving does with a heavy displacement boat such as yours is dig a deeper hole in the water and blow a little (maybe a lot) smoke. I suggest aiming to set your propeller for a cruising revs of 2200 and never rev above 2500.

Here is the fuel chart for the nearest Yanmar to yours.

http://www.yanmarmarine.com/theme/ya.../PDF/4JH5E.pdf

[Astral Blue]

Originally Posted by savoir

Just a small side issue. I don't get your preference for high revving. That can really cost some buck$. My diesel is a Yanmar and its diesel consumption doubles from 2300 to 3400 rpm. I'm guessing a Perkins would be similar.

All the high revving does with a heavy displacement boat such as yours is dig a deeper hole in the water and blow a little (maybe a lot) smoke. I suggest aiming to set your propeller for a cruising revs of 2200 and never rev above 2500.

Here is the fuel chart for the nearest Yanmar to yours.

http://www.yanmarmarine.com/theme/ya.../PDF/4JH5E.pdf

Very good point here. In a matter of increasing the speed under load by 1000 RPM's, the Perkins 4.108 can go from 1.2 GPH to 3.1 GPH.

[savoir]

More than double ! Slurrrrp ! One thing is for sure too - diesel isn't getting cheaper anytime soon.

I'm afraid I have never owned an adjustable pitch prop so I know nothing about that aspect. Possibly a good thing.