Will 1/2" on prop pitch make much difference?

colhel

colhel

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The recommended prop size on our Nimbus 3003 is 18x18 4 blade Presently we have 18x17.5 4 blade. We reach max rpm but don't reach max speed.
Edit, we get 19 knots at WOT, but from info gathered on the net 22 knots should be possible though I'm more interested in cruise speed
 
colhel said:
The recommended prop size on our Nimbus 3003 is 18x18 4 blade Presently we have 18x17.5 4 blade. We reach max rpm but don't reach max speed.
Edit, we get 19 knots at WOT, but from info gathered on the net 22 knots should be possible though I'm more interested in cruise speed
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I would guess your top speed with the recommended prop would be at most 18/17.5. I.e. 19.5 - while assuming engine can still reach max rpm and that there is no increase in prop slip.
 
westernman said:
I would guess your top speed with the recommended prop would be at most 18/17.5. I.e. 19.5 - while assuming engine can still reach max rpm and that there is no increase in prop slip.
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You've got that spot on for an 18x17.5.
But if I were to go 18x18 would I get 22 knots at WOT and would my cruise increase from at present 15 ish to 18 ish?
 
Half an inch pitch should give roughly 100rpm less at wot. If you’re getting quoted max revs now you might drop below but not by much. Slightly higher top speed but slightly less acceleration. Is it really worth the not insubstantial investment?
 
If you can’t reach WOT you overload a marine diesel.This is bad very bad in terms of longevity re EGTs .
Petrol and outboard guys have a completely different set of rules ignore any posts from owners of such .

Be happy it reaches WOT and leave it there .You haven’t mentioned WOT , which rings alarm bells in my mind .

They overload across the whole range if they can’t reach rated manufacturers WOT .So the PO has been very thoughtful taking pitch out .

Ideally the holy grail is 3% above WOT rpm to accommodate seasonal growth excess cruising stores and other normal decades weight gains .

Is my Boat Over-Propped or are my Engines Over-Loaded?
 
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FWIW in my own experimentation with props ( bear with me here) C3 - C4 - B5 - C4 - B4 - B3 what I have found is top speed WOT was never affected by more than a knt or two, nor was max displacement speed rpm. What did change was acceleration out of the hole and performance / weight ratios. Cruise revs did change though. Yes they are about 400rpm higher now with B3 compared to B5 but with that came a rather pleasant less loading on the engines for that speed that made them run cooler, quieter and just as economical, which surprised me, but shouldnt have.
I started out thinking that a coarser prop set wouldgive me greater top end at the sacrifice for mid range and hole shot but have found this is simply not true. At least not for my boat, but I would imagine it holds true to varying degrees for most.
As it turned out the graph just had the same beginning and end points, just the curve between was different.
So saying I would not be in a hurry to change your props at all. Any difference would likely be marginal. The only caveat to this is my C3's were wildly under propped. I could hit 20knts like I was in a speed boat but I'd also hit WOT at speeds well below the expected 28 - 30knt performance expectation provided by the other props. I still cant get my head around why all the above is such, but I have thrown a helluva lot of money in experimentation to come to these conclusions.
Lastly I did not have the luxury of trying different style props within a dia/pitch range. Cupping etc etc. I dont know what these would do for you. Not all 17.5 pitches would be created equally in that regard, but it would be a very expensive test unless you could lean on others who have your specific boat engine combo to collaborate with regarding their performance / prop specs
 
You can get the pitch of an existing prop adjusted. Within reason.
I did this on my prop as I could not get close to max RPM at WOT.
After the adjustment, with a clean bottom I could get very close.

In my case, I told the prop guy the RPM I could reach, the RPM I should be able to reach, the engine model and displacement.
 
colhel said:
Thanks @BruceK , and yes @westernman I'll be looking at getting it cupped. The difficulty is because I'm reaching max rpm it's hard to know how much to add on, if I was down on rpm it would be much easier to work out.
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cupping and changing the pitch is not the same thing - not sure if thats clear or not from the above.

Just go and see hamble props and ask them They seem to know what they are talking about.
 
Try Teignbridge Propellors in Devon if you have no joy elsewhere or want another expert opinion. One of the best out there for design, manufacture and modifying props.
 
colhel said:
The recommended prop size on our Nimbus 3003 is 18x18 4 blade Presently we have 18x17.5 4 blade. We reach max rpm but don't reach max speed.
Edit, we get 19 knots at WOT, but from info gathered on the net 22 knots should be possible though I'm more interested in cruise speed
Click to expand...
what did you do in the end? Did you manage to improve anything
 
Funnily enough I was just looking through the old records and thinking about how to proceed. It turns out the previous owner had an 18x18.5 .
Still undecided as when the season progresses and the fouling builds up, it might be better with a finer pitch?

Edit: rather conveniently I found the cutlass bearing size too. Was that you on the Facebook group?
 
Col
1/2 inch of ‘prop’ adjustment will make very little difference in my experience of various Nimbus I have owned ( 2) and helmed in the 27/32 ft range
No 3003’s by the way!
If your reaching max RPM I would leave well alone for the moment
Like you said weed/ growth etc
Which make eng
What HP do you have by the way?
Plus
Maybe the ‘Fun Box’ is overloaded?
Extra weight an all that?
😂
 
colhel said:
Funnily enough I was just looking through the old records and thinking about how to proceed. It turns out the previous owner had an 18x18.5 .
Still undecided as when the season progresses and the fouling builds up, it might be better with a finer pitch?

Edit: rather conveniently I found the cutlass bearing size too. Was that you on the Facebook group?
Click to expand...
yes it was me ! i got the boat lifted on a 'lift & hold' over the weekend so i wanted parts for everything i might need to do. Now its out i can see the bearing is fine anyway. I had a brief run the other weekend until the rope cutter broke which heralded the lift out the water. I was getting 18knots flatout & cruising seemed to be about 15knots with 1/2 tank of diesel and only me on board, i dont remember rpms so when its back in the water i will be checking im getting full rpms. (i did have fenders down & my speedo was a paddle wheel not gps so...:unsure:take the readings with a pinch of salt) Ive got a feeling the turbo might be tired but i havent got round to that yet.
 
adhdan said:
yes it was me ! i got the boat lifted on a 'lift & hold' over the weekend so i wanted parts for everything i might need to do. Now its out i can see the bearing is fine anyway. I had a brief run the other weekend until the rope cutter broke which heralded the lift out the water. I was getting 18knots flatout & cruising seemed to be about 15knots with 1/2 tank of diesel and only me on board, i dont remember rpms so when its back in the water i will be checking im getting full rpms. (i did have fenders down & my speedo was a paddle wheel not gps so...:unsure:take the readings with a pinch of salt) Ive got a feeling the turbo might be tired but i havent got round to that yet.
Click to expand...
I would check with GPS speed and knowing the rpm is important to check engine performance.

kawasake said:
Col
1/2 inch of ‘prop’ adjustment will make very little difference in my experience of various Nimbus I have owned ( 2) and helmed in the 27/32 ft range
No 3003’s by the way!
If your reaching max RPM I would leave well alone for the moment
Like you said weed/ growth etc
Which make eng
What HP do you have by the way?
Plus
Maybe the ‘Fun Box’ is overloaded?
Extra weight an all that?
😂
Click to expand...

If I wasn't reaching max rpm it would be much easier to calculate, but it just feels a bit too revy at cruising as though it lacks bite. Could just be my imagination of course 😏 😁
 
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