Elec. Diesels - GREAT READ - Power vs. Prop Curve!

Boatingfool

Boatingfool

New Member
Feb 13, 2008
389
Annapolis, MD
Boat Info
560 Sedan Bridge
Engines
V10 1050hp Mans
Must read for all Electronic Diesel Engine Owners!

There was a post about smokey 480 Cummins Electronic Diesels when getting up on plan a while back. I posted that my boat, especially now that its warmer now and I have all my gear, etc. weight aboard, takes a noticeable 30-seconds or so to get up on plane.

Everything checks out 100% on the electronic MAN engines, and once on plane the boat torpedoes between 18knots and 36knots.

I know nothing is wrong, but its always made me wonder WTF was going on when I get up on plane...

I stumbled across this today:

"Special Considerations for Boats with Electronic Diesel Engines"

http://www.hydrocompinc.com/knowledg...hECEngines.pdf

Awesome reading! This makes total sense as smaller displacement diesels (which are lighter and easier for manufacturers to install, and leave more room for other amenities such as bedrooms) produce the same horsepower that larger displacement (ie. more low end torque generally) diesels made years ago. Explains why it takes so long to get my 70,000+ lbs up out of the water!

This probably also explains why boats equipped with the Cummins 8.3L 540 and 600hp engines aren't much quicker than their 480/500 alternatives at similar RPM (they just can't be propped much larger, so the extra speed only comes from the higher rpm).
 
How do you trailer that monster boat??? I just had to ask.....
 
That makes sense. It would be interesting if you could find the torque curves for your MANs. I don't know about variable pitch propellers; I'd hate hanging complex machinery in salt water, but the two speed gear boxes would work. Marine gears are just single speed automatic transmissions. It's not new science to put an extra gear set in there with control hydraulics.

Trailering is pointless on the East Coast. Everywhere one would want to go with a big boat is accessible by running her on her own bottom. Can't think of a single place I'd want to take my boat where I'd need a trailer. A bigger boat with longer legs, but not a trailer.

Best regards,
Frank
 
I don't know... You can rag on my Cummins engines but I've never had an issue with getting up on plane when they are performing correctly. You can feel the turbos kick in around 1500 RPM but it gets over the hump just fine and quickly with no black smoke. The only time I've had trouble getting up on plane is 1. dirty bottom/gear and 2. had a leak in the crossover tube from the aftercooler to the head... I'd be suspicious if your boat is taking a long time to get up on plane...
 
After reading that link, it would seem apparent that if it is taking too long to get on plane that there is a mismatch in the 'system'...props, trans or maybe even the motor itself....but non the less, a mismatch....

I'd have to agree with Gary...

Four Suns said:
....I'd be suspicious if your boat is taking a long time to get up on plane...
Click to expand...
 
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I would go look at some published performance curves of the boat... (boattest.com?). I bet Sea Ray even has some... My boat goes from 0 to 26 knots in about 12-15 seconds which matches the performance curves that are out there.
 
Here's some dated info and specs for your boat:

http://www.seamag.com/boattests/DM_article.asp?id=2846

Why do you think your boat weighs 70,000 pounds? Has the yard weighed it? If it weighs that much more than the thing did new, maybe you have some "extra weight" somewhere you don't want?

They also have WOT posted at 34 knots... Are your props right?

I'm sure there are performance curves out there for your boat and engines... I just find it hard to believe that the 560 DB was designed to take 30-45 seconds to get up on plane with black smoke coming out the back end...
 
That article was very interesting, but the writer doesn't make his point very clearly. The simple answer is, you can put a smaller engine in a boat with the same horsepower of a larger engine, but it will have to run at significantly higher RPM to achieve that horsepower (HP = torque X RPM). Then, the power isn't there when you need it - getting up on plane.

It's funny that cars don't need multi-gear tranmissions, but they have them. Boats could benefit greatly from them, but generally don't have them. A constanst speed transmission may be the answer (or whatever they're called). They're used in buses and probably other applications as well.
 
Boatingfool,

Interesting. My boat, which is basically a newer version of yours is rated at 51,500 pounds dry and was weighted last season at about 65,000 lbs fully loaded (without crew). Your boat is rated at 50,000 but that's probably with the standard engines. I don't think 70,000 is an unreasonable guess, but you should have her weighed the next time you haul out.

My boat was delivered with 28x27 props that seemed to have a hard time getting the boat up on plane. They allowed the correct WOT (after some tweaking) but slipped a lot as the boat was getting up. SR gave me new 30x33 props that solved the problem.
It still takes longer to get up on plane when the boat is fully loaded, but without the cavitation and slippage of the smaller props.
 
By the way, the transition from hull speed to plane is quite smooth on my boat. The RPM's increase as I slowly increase the throttles. I do not see any of the hesitation that the article talks about.
 
Sea Gull said:
My boat was delivered with 28x27 props that seemed to have a hard time getting the boat up on plane. They allowed the correct WOT (after some tweaking) but slipped a lot as the boat was getting up. SR gave me new 30x33 props that solved the problem.
It still takes longer to get up on plane when the boat is fully loaded, but without the cavitation and slippage of the smaller props.
Click to expand...

Given that I am the "little brother" to your boats, not sure how relevant this is but...There was a prop switch done on my boat after it had been delivered, I do not know the details as I was not the owner at that time, but before I bought the boat, Sea Ray cust service told me that they had determined that the MAN engines needed a different prop and that replacement had been done on my boat (including the spares).

I do not have that kind of delay getting up on plane, and it is very smooth. I also have 2 small childeren and my wife brings EVERYTHING to the boat, so mine might weight 80k (at least it feels like it when hauling the stuff in!)

Don't know if it makes a difference, but I thought I would chime in...

Ryan
 
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Right on Ryan. This is exactly my experience.

My boat was delivered with 28x27 props and did not achieve rated WOT. The dealer was paying big bucks to haul/ re-work props and test, when we finally agreed that 28x26.25 was the right prop. After all that, SR chimed in and said that they determined with MAN that the boat needed 30x33 props instead. The new props were put on and lo and behold the boat gets to rated WOT AND caviation is eliminated!

I don't know enough about propeller geometry to know why this worked. Going up in diameter and pitch would seem counterintuitive to me, but it worked. Maybe there is less cupping or other shape variation that contributes to this phenomenon?
 
Same number of blades, Sea Gull, on the 28x27 and 30x33 wheels?
 
Jeff....that falls into the :smt017category. I don't understand how moving up in pitch AND diameter decreased your time to plane and maintained your WOT RPMs....acutally had no effect on your WOT RPMs.

What about your SOG at different RPMs...don't tell me they went down......
 
I'd say cup, but the rule of thumb is 1 to 2 inches of pitch for cupped blades. But this is a diameter increase, too. Might be different blade area and a change in disc, too. Hard to say.

Best regards,
Frank
 
Great input (and the technical reading I love!) on all your boats :grin:

When the boat was last brought up they said 71,000lbs. which I thought was high (as Sea Ray says 50,000 for the base boat) but she had 3/4 fuel, water, all my stuff, the upgraded V10's, many factory options, etc.).

The props on the boat are 4-blade 31"x37" Nibral Cupped. I do not think there is any cavitation or slippage because the RPM needles hold rock steady as they slowly increase.

Also, if you put the throttles 1/2 way up (which originally didn't seem like much, but I learded that 1/2 way on the throttles is more like 3/4+ on the engines) there will be a lot of black smoke. The boat smokes but doesn't make any noises and it ultra smooth.

I learned that if I bring up the throttles slowly (1/16th at a time) she'll still take 30-seconds to get on plane but she'll do it with no smoke and is still ultra smooth.
 
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