Some help pitching a prop please.

[GILow]

Hello Cruisers,

I am troubleshooting some performance problems with our engine. We have a 96 HP Nissan ED33, in good condition, 2100 hours, recently overhauled injectors, but I can not get it over 2000 rpm, at which point we are doing around 6 knots. The boat is clean though the prop is slightly fouled.

I have done the calculations and everything tells me that the current 20 x 12 prop is way too big, but I am worried that I am missing something subtle as the person who set the boat up knew his stuff, and even went to the trouble of including a SPARE 20 x 12 prop.

Critical figures are:

Engine - see attached torque/HP curves from Diecon, the people who marinised what was basically a small truck engine.

Boat - LWL approx. 37", beam at WL, 12", hull depth 3", displacement 15 -17 tons.

Gearbox - Borg Warner Velvet drive 1.5:1

Typically these boats get by very well with 75 HP and should certainly go faster than 6 knots with the kind of HP I have available.

As an odd aside, the rated maximum rpm of that engine is 3500 rpm according to everyone except Diecon. I understand that there is a difference between maximum rpm and maximum SUSTAINED rpm, but as you will see from the torque curve, they seem to make the differentiation based on much lower figures. Perhaps they modified something in the engine to make it more suited to marine use, but either way, I should be getting more revs.

What do you think?

Matt

[GILow]

Hmmm.... might help if I attach that torque curve hey?

[Albro359]

Fouled prop will get you every time
This is a very difficult issue to resolve without knowing more about the design of the boat ..e.g fin keel vs long keel
15-17 tons is a LOT for a 37' boat
Looking at the graph it looks like the revs max out at 3200...your fuel consumption at those revs will be horrific...
Marine mods generally only have to do with cooling...the motor is the same....

[GILow]

FWIW, 3/4 keel design, and the boat is 42 feet overall, 37 is the actual waterline.

True, the Swanson is a heavy boat though.

As for the mods, yes, I would have thought it was simply a case of the exhaust manifold and the heat exchanger, but I am curious that they rate the engine to a maximum of 3200 rpm, when everyone else rates it at 3500 or 3600 rpm.

Matt

[djmarchand]

The basic criterion for prop pitch is that the engine just reaches (or maybe 100 rpm more) the max rated rpm- 3,200 presumably, in gear and at wide open throttle.

75 HP should easily push even your heavy hull to its displacement speed of about 8 its.

David

[Jim Cate]

G'Day Matt,

I'm not particularly knowledgeable about propping, but what jumps out to me is the 1.5:1 reduction in your gearbox. In heavy displacement hulls it is common to have much higher reductions, ie 2-3:1 or so. A large, slow turning prop is generally indicated for hulls like yours.

Cheers,

Jim

[scoobert]

just to throw this out there, have you checked your fuel system?
sounds like it may be restricted, perhaps a clogged filter? bad fuel pump?

[Cheechako]

Prop Pitch: You want the engine to max out at maybe 15% less than maximum rated rpm. You need a clean prop to tell where you are at. Will the engine go over 2000 rpm in neutral? if not you have an engine problem not prop pitch issue.
Consider if the engine has a commercial rating (lower HP , Lower max RPM) If so, pitch your prop to run the engine at that lower rpm and your engine will last longer.

[GILow]

Originally Posted by scoobert

just to throw this out there, have you checked your fuel system?
sounds like it may be restricted, perhaps a clogged filter? bad fuel pump?

Checked...? No.
Completely replaced at vast expense... yes.

Oh if I had only CHECKED in time. S

[GILow]

Originally Posted by Jim Cate

G'Day Matt,

I'm not particularly knowledgeable about propping, but what jumps out to me is the 1.5:1 reduction in your gearbox. In heavy displacement hulls it is common to have much higher reductions, ie 2-3:1 or so. A large, slow turning prop is generally indicated for hulls like yours.

Cheers,

Jim

Yes, it is a strange gearbox arrangement. I would have expected a lower ratio too. About 2:1 would be about right according to the prop calculators, but no small job changing gearbox ratios and also, for some reason, probably to do with the number of water ski boats in Aus, it is very easy to get a s/h 1.5:1 box, but the other ratios are rare as hen's teeth.

[GILow]

Originally Posted by Cheechako

Prop Pitch: You want the engine to max out at maybe 15% less than maximum rated rpm. You need a clean prop to tell where you are at. Will the engine go over 2000 rpm in neutral? if not you have an engine problem not prop pitch issue.
Consider if the engine has a commercial rating (lower HP , Lower max RPM) If so, pitch your prop to run the engine at that lower rpm and your engine will last longer.

Yep, easily wants to go over 2000 in neutral. Sounds quite happy about it too. This comes back to the point that the engine apparently had a top rpm of 3500 -3600 rpm, and it is as if it has been de-rated twice over.

The performance curves give an intermittent marine use curve which is apparently derated by 10%, and then a further derated curve of 25% for continuous use.

I am quite happy to go with that rated lower figure, just can't work out why he has done it this way. Seems such a basic mistake. Still, within reason we seem happy with the concept that the engine should get to its max rpm +/- 10% depending on your philosophy on these things, so replacing the current 20 x 12 with something more like a 19 x 8 seems to be indicated by the three prop calculators I tried.

That's seems a VERY big difference though...


Matt

[transmitterdan]

I agree that the 1.5:1 transmission ratio is a bit out of whack for this heavy boat. The prop is too big with too high pitch and trying to turn it too fast.

The BoatDiesel prop calculator says for this engine/transmission configuration and a 20" prop the pitch should be a whopping 1.7".

It also says that for your existing prop (20X12) the ideal transmission ratio would be about 2.5:1 which sounds right.

It recommends for the 1.5:1 transmission that the optimal prop is 15" X 7.5". It seems likely your engine and transmission could turn that prop at the rated RPM. And it ought to be significantly less money than those big 20X12 props.

[GILow]

Originally Posted by transmitterdan

I agree that the 1.5:1 transmission ratio is a bit out of whack for this heavy boat. The prop is too big with too high pitch and trying to turn it too fast.

The BoatDiesel prop calculator says for this engine/transmission configuration and a 20" prop the pitch should be a whopping 1.7".

It also says that for your existing prop (20X12) the ideal transmission ratio would be about 2.5:1 which sounds right.

It recommends for the 1.5:1 transmission that the optimal prop is 15" X 7.5". It seems likely your engine and transmission could turn that prop at the rated RPM. And it ought to be significantly less money than those big 20X12 props.

Thank you Transmitterdan, those are interesting numbers. Your prop size comes out a fraction smaller than what I devived using vicprop and one other bit of software (can't remember the name right now). They came up with a three blade 15.8 x 7.6

Would you mind telling me what figures you used to get the prop size you are recommending? I might be missing something.

Matt

[RaymondR]

I upgraded my engine installation from a Yanmar 3GM30, 27HP to a Kubota V2203, 50hp. It revs to 2,800 rpm.

The Yanmar had a box with a 2.36:1 ratio and was spinning a 15 1/2" x 11" prop. It would rev out when the prop was clean but stall down when the prop became fouled. The Yanmar revs to 3400.

I did not want to have to change the prop so put a box with a ratio of 1.52:1 box on and the V2203 revs out and in fact I now suspect it may be slightly under-propped which is OK as it provides a fouling allowance.

Whilst the prop you are spinning is quiet a lot bigger in diameter it appears to have a lot less pitch and you have quiet a bit more HP.

The point of this is that those calculations tend to look a bit iffy to me in comparison to what I am observing on the installation I have..

One of the give-aways on an over-propped engine is that they will blow lots of black smoke if you try to rev out the engine.

Diecon have been around for a while and I have not heard any complaints regarding their proficiency in marine matters. What do they say regarding the situation?

Has the boat been re-engined and what was in it before?

[scoobert]

Originally Posted by GILow

Thank you Transmitterdan, those are interesting numbers. Your prop size comes out a fraction smaller than what I devived using vicprop and one other bit of software (can't remember the name right now). They came up with a three blade 15.8 x 7.6

Would you mind telling me what figures you used to get the prop size you are recommending? I might be missing something.

Matt

unless i am mistaken, and i may be, they can change the pitch of a prop you already own.