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TROUBLE
06-23-2011, 01:53 PM
I have a question about what my maximum rpm should be. I have a 43 dcmy with 853 naturals and 26X26 four blade wheels. The bottom is fresh and the props are clean. With 3/4 fuel my maximum rpm is 2500. The boat is struggling to get on plane. This seems low to me. What should i look for?

thanks as always
Art

TROUBLE
06-23-2011, 03:14 PM
Correction The Wheels Are 24x24

Finalee
06-23-2011, 03:30 PM
Are you using the trim tabs? I would think the boat would plane fairly easy with tabs in the full down position. Is it acting diferently from the way it did before?

TROUBLE
06-23-2011, 03:47 PM
I am using the tabs, they are working and last year the boat planed quite a bit easier, although its always taken a little while.
Art

REBrueckner
06-23-2011, 03:53 PM
WOT RPM for 53 series recreational engines is 2800 RPM.

300 RPM below that IS a big difference:

I have prop load curves for1969 8V53's: With standard N50 injectors, these slightly lower HP engines were rated 260 SHP @ 2800 RPM; at 2500 RPM they developed only 180 HP. (yes, those figures are accurate!...The RPM curve is very steep. )

Those were rated with a raw water pump, a 12v- 62 amp generator and twin disc marine gear(says so on the power curve chart).

If your engines are mid 1970's, likely they were the 283 HP model (WOT).

TROUBLE
06-23-2011, 04:09 PM
Thanks Rob,
my engines are 1972. Where do i look for the missing 300 rpms? And are the 24x24 wheels correct?

Art

J's Dream
06-23-2011, 04:12 PM
Where is the boat located? Who cleaned the bottom, and how long ago?

If my running gear (east central Florida) isn't cleaned WELL every 4 weeks, you can forget about performance numbers...and almost forget about planing off.

saltshaker
06-23-2011, 04:17 PM
Thanks Rob,
my engines are 1972. Where do i look for the missing 300 rpms? And are the 24x24 wheels correct?

Art

What was your max RPM before? if something has changed drasticly, I wouldn't look to the props just yet, unless they look like they're out of whack. Older props can flex and get out of shape over time. If this has been a gradual decline I would check the wheels. I would start with the basics first. Check your fuel filters. Make sure you are getting full travel on the throttle cables. Check no load RPM. Verify that the props and shafts are clean. A few barnacles can greatly affect performance.

TROUBLE
06-23-2011, 04:42 PM
Boat is on Long Island , NY, launched mid may with a freshly painted bottom, fuel filters have about 10 hours on them. I purchased the boat last year and actually dont remember ever running it at WOT. It did plane a lot easier and ran 13.5 to 14 knots at 2200 rpms. Now I am running 11.5-12 at 2500 and there is nothing left after that.
I appreciate all the input. Please keep it coming.

Art

lumina
06-23-2011, 04:55 PM
Hi All,

Not to derail Arts thread but Rob, would you know the WOT specs for my 1974 8V71 Naturals 350hp (N70 injectors) assuming "standard" props.

DCMY #92
06-23-2011, 06:20 PM
Since you bought the boat - what modifications have you made that added weight and/or what items have you brought aboard? These boats were not overpowered in the first place. It's easy to seriously tax their performance with additional weight.

TROUBLE
06-23-2011, 06:29 PM
I probably took more things off than i put on.

Art

saltshaker
06-23-2011, 06:42 PM
Boat is on Long Island , NY, launched mid may with a freshly painted bottom, fuel filters have about 10 hours on them. I purchased the boat last year and actually dont remember ever running it at WOT. It did plane a lot easier and ran 13.5 to 14 knots at 2200 rpms. Now I am running 11.5-12 at 2500 and there is nothing left after that.
I appreciate all the input. Please keep it coming.

Art

Check your running gear. It doesn't take too much to foul it. Pull the fuel filters and check them. Could be dirty. If you haven't added weight, I would start checking the basics as mentioned before.

REBrueckner
06-23-2011, 07:58 PM
"... would you know the WOT specs for my 1974 8V71 Naturals 350hp (N70 injectors) assuming "standard" props..."

All 71 series natural recreational engines are rated at 2300 RPM...

And that is what my DD book for 1969 says...except back then the engines developed a bit less HP at 325 SHP. And for unknown reasons, that curve is developed with a 12 volt 25 amp generator. (You'd think the higher HP 71 engine would get at least as big a generator as the 8V53....).

Trojan
06-23-2011, 10:05 PM
My 6-71 is rated at 2650 rpm WOT. Per paper work from Johnson & tower.

BILL

spartonboat1
06-24-2011, 12:11 AM
Boat is on Long Island , NY, launched mid may with a freshly painted bottom, fuel filters have about 10 hours on them. I purchased the boat last year and actually dont remember ever running it at WOT. It did plane a lot easier and ran 13.5 to 14 knots at 2200 rpms. Now I am running 11.5-12 at 2500 and there is nothing left after that.
I appreciate all the input. Please keep it coming.

Art

Are you quoting GPS rated "knots" and not mph, and are you quoting a GPS reading, or a knotmeter?

My 43DCFB with 6-71N's rated 310hp will cruise at 12.7-13kts, GPS observed, at 2,150-2,180 rpm, with some variance due to water depth, winds on head or following and seas head on or following. Since the 8v53 requires more rpm to generate higher hp, I am a little surprised that you can obtain 13.5-14kts at 2200rpm on two points a) I cannot obtain those speeds at what I believe to be at a higher hp setting (the 6-71N at 2,150-2,180 rpm), so I question if that is a knot-meter reading, which can vary quite a bit, and b) you are obtaining a higher speed than me, although at a lower hp, which seems questionable.

Don't mean to be a contrarian, but my first question is regarding the source of the 'speed' quoted.

REBrueckner
06-24-2011, 08:05 AM
Trojan: "My 6-71 is rated at 2650 rpm WOT. Per paper work from Johnson & tower.."

Yes, I forgot to mention such modifications to DD factory engines. Other's have posted such differences from J&T in the forums before.

Any other marinizers modify DD in Hatts???

Be interesting to know just what J&T did to DD factory engines that allowed them to rate their engines at a higher RPM. I've only seen a few J&T engines and the one's I've seen have that "J&T" logo prominently stamped....is it on the heat exchanger?? and maybe valve covers?? Is that standard for J&T??

I think J&T were the marinizers of the ill famed 485HP 671TI rated at 2500 RPM and the 6V53T's which boosted hp from about 197 natural HP to 375 turbo horses. Both those models are short lived when run hard.

As I posted, the flip side is that DD series 71 engines are derated, via smaller injectors and lower RPM ratings for "crew" boat and "work" boat engines. I also wonder if they are different in any other way.

rsmith
06-24-2011, 08:58 AM
Trojan: "My 6-71 is rated at 2650 rpm WOT. Per paper work from Johnson & tower.."

Yes, I forgot to mention such modifications to DD factory engines. Other's have posted such differences from J&T in the forums before.

Any other marinizers modify DD in Hatts???

Be interesting to know just what J&T did to DD factory engines that allowed them to rate their engines at a higher RPM. I've only seen a few J&T engines and the one's I've seen have that "J&T" logo prominently stamped....is it on the heat exchanger?? and maybe valve covers?? Is that standard for J&T??

I think J&T were the marinizers of the ill famed 485HP 671TI rated at 2500 RPM and the 6V53T's which boosted hp from about 197 natural HP to 375 turbo horses. Both those models are short lived when run hard.

As I posted, the flip side is that DD series 71 engines are derated, via smaller injectors and lower RPM ratings for "crew" boat and "work" boat engines. I also wonder if they are different in any other way.


Stewart & Stevenson were hopping up DD's long before Art Johnson.

lumina
06-24-2011, 11:40 AM
Hi All,

Thank you Rob.......Art sorry to temporarily derail the thread. FYI we pretty much run at appx 1800rpm most of the time, gives us 12knts. I don't think we've ever gone beyond 2000rpm.

Mike36c
06-24-2011, 12:44 PM
What is coming out of the exhaust at WOT? Smoke, etc? That will help and if you have racors spring for vacuum gauges where the "T" handles go, that will give you an idea of your fuel system health. Racor makes a direct replacement (but not cheap......)

John Maloney
06-24-2011, 08:49 PM
Art,

Check the wheels for barnicles. I hauled mine last week due to loss of RPM's. I had over 1/4 inch of barnicles on the wheels and shafts and even the bottom of the keel. This is the second year in a row that I have had this issue. The barnicle buildup took place in a 3 week window. My boat is in Islip, not far from yours.

John Maloney
Reverie

saltshaker
06-24-2011, 08:59 PM
Art,

Check the wheels for barnicles. I hauled mine last week due to loss of RPM's. I had over 1/4 inch of barnicles on the wheels and shafts and even the bottom of the keel. This is the second year in a row that I have had this issue. The barnicle buildup took place in a 3 week window. My boat is in Islip, not far from yours.

John Maloney
Reverie

Last 2 years have been pretty bad up here. When I hauled my boat in December it was the worse I've ever seen it.

REBrueckner
06-25-2011, 09:19 AM
"When I hauled my boat in December it was the worse I've ever seen it."

I always wonder about bottom growth variations from year to year....They sure ARE noticeable. Never reached any firm conclusions except that variations in growing conditions ashore sure lead to variations in growth, blooms, disease resistance in plants ashore ...so I guess the same happens in salt water....maybe even the amount of rain affects salt water bottom growth.

One thing you can do if you have navigable fresh water around is to take your boat there and relax for a several days...bottom crud will mostly die off...I used to go up the CT River springtime when headed down east and again when returning in the fall and hang out around Essex....just enough fresh water to enable me to have the bottom painted every other year. But barnacles do NOT drop off...I think they die though, but tough to tell.

TROUBLE
06-25-2011, 11:04 AM
Hey John,
I saw your boat at atlantique on wednesday. You looked busy so I didnt stop by.
Art