Under propped? thoughts?

I know that this has been talked about many times. i have read many great thoughts on the topic but i would like specific comments about my situation.

Boat is a Albin 27FC with the 78 Nissan.

for a cruising speed i am running 1800 at 1/3 throttle which gets me, in no current, about 6knts. I am able to hit the redline at slightly more than half throttle and that gives me 7-7.5knts. I am told by boat owners that the redline is 3K. The Nissan car people (ld28 is early/mid 80's Maxima) are saying 3800. Id rather error on the side of caution and call it 3.

I am not sure what pitch the prop is currently.

As for what i would like to accomplish, I am not necessary trying to gain cruising speed but i would like to have ability to run a little harder to fight currents when necessary. 6knots is OK but when i am fighting a 3knt current it makes for a slow ride and and somewhat limited maneuverability.

From your post, it appears that WOT on your engine is supposed to be 3800 rpm. While cruising, push to WOT (only for a few minutes or you'll get really hot) and check your rpm's. If you hit close to 3800 rpm at WOT then it doesn't matter what speed you reach, you are spinning the right sized and right pitched prop.

If you are not able to hit 3800, then you should either decrease the diameter or pitch. A one inch decrease in pitch will increase the rpm by about 200.

If you hit over 3800 rpm, you should increase the pitch or diameter. Of course, if you are using the boat for skiing, tubing, etc, you may want a faster hole shot and should keep the prop/pitch a little smaller. You'll give up a little bit of top speed though.

the max rpm is somewhat a matter of debate. The marine and industrial application users say 3000. the engine was used in cars and the car guys (they are a bit of a cult following) say it is 3800. Not a huge difference but i had been limiting it to 3000. I have no doubt i could, very easily hit 3800. I am able to reach 3000 at only slightly more than half throttle.

If your at half throttle and at 3k, have you ever ran it all the way on the throttle? I'd say your over pitched at this point. There are several online websites that will take everything into consideration and give you your prefered pitch for your boat. However, I would call a local prop shop, they have the experience and would probably help you. Repitching is a no brainer - sometimes our boats are handed over to a repair facility, a relative or close friend, being properly pitched eliminates the possibility of someone over revving our plants by accident.

I had one of those cars. The 6 cyl Nissan diesel. I'm sure it would have no trouble reaching 3800 but you should find out at what rpm the engine develops it's maximum power. Probably 3800. That's just slightly faster than a 36 Chevrolet.

The high speed idle speed (WOT in neutral as with no load) should be a little above the rpm where max power is developed. If you only get 3400 under no load your max power rpm could be 3000 but I'd guess it will be about 4000rpm. And IMO your max rpm in gear would be ideally 3900. A little under propped is best.

When/if you get the WOT rpm up to speed you'll experience a whole new world of performance. Your engine will make more noise but it will run freely feeling less restrained and probably smoother.

Go on the internet and see if you can discover other applications of that engine. Join BoatDiesel.com and ask around there.

OP go to Seaboard marine and read Tony Athens articles on overload and all related articles on diesel motors then come back and ask questions if you still have any. You need to understand Sea trial-WOT, load and no load-manufactures specifications- what propellers do and how all effects fuel burn and motor life.

You probably could find the engine specs for the car engine but they may or may not be very relevant. Could have different cams and injectors ect.

The Nissan was marketed by Chrysler Marine as I recall but they went out quite awhile ago. I think Mark as in Daddyo here on TF had a 27 Albin and perhaps he had the Nissan engine.

BoatDiesel is still probably your best bet.

Changing props to attempt to get a 25ft LWL to operate at almost plaining 7-7K will require a higher PM to be carried at normal cruise.

If you change the prop pitch to reach 3800 or 3000rpm from where ever it is now (2200?) at 1/2 throttle the cruise RPM will change UP by the same percentage .

So instead of an 1800 cruise it might become 2300 forever .

This IS higher up on the power band so may set you up for underloading and a slobbering engine.

As its a car engine this should not happen , but the fuel consumption will be up, and the engine life shorter from increased piston miles.

REEDS nautical tide tables can keep you out of 3K currents.

IF you attempt full throttle operation with the existing prop, a 10% reduction from whatever RPM you see on the pin is suggested.

No black exhaust smoke down 10% your fine.

About this loading issue talk to engine manufactures or;

http://boatdiesel.com/

Also I have hundreds of hours at 2300 rpm and its joy to have an engine singing along on a light load. I know another member who has just ceased to overprop and he is very happy w the results. I think he's cruising at 2300 also. With my Albin 25 I cruised at 2750rpm (3450rpm engine) and had no issues w noise. Does your Albin 27 have the same style engine cover/compartment as the 25?

I did check out Boatdiesel and even there they have very little about the LD28.

I was able to come up with this brochure about the engine.

Interesting thing is that in its marine form it is rated at 78HP. Which is less than the automotive rating and more than the industrial rating. I have not been able to find anything specific to the marine version.


Either way, I opened it up this weekend and cruised at 2400 which, according to the attached document is in the peak torque curve of the engine. Which was more comfortable as far as speed and my ability to run against the tide and in the slack tide increased my speed to 8 +/- knts. the engine sounded happy and had no issues with heat mitigation, smoke or other issues.

I am able to hit 3800 but that seems to be where the prop is limiting...as i come above the torque curve of the engine and go well over hull speed i am to the point of diminished returns to push much harder than 3000 for any reason.


Thanks for the information everyone.

Sounds like you're right where you should be and want to be re your expressed desires to be able to maintain speed against current.

Any idea what your speed is at 3800rpm?

9knts. That is against a 1knt (approx) tide, 2' waves on the nose and a stern wind of 15knts.

I didn't hold there long, I know that technically i am a SD hull and i think that the planing speed of the hull is somewhere north of 10knts. It is going to take more than 78 HP to get her up and moving. I do hear that some of the 157HP Isuzu equipped boats will jump up and run on top. The details are a bit sketchy as to what it feels like and how capable it is at running there. At 9knts steering is VERY twitchy. For the size of boat the rudder is HUGE.

I would be interested to see a video or talk with someone who has one with the 157HP in it.

I had an Albin 25 w a 34hp Yanmar and could get slightly over 10 knots at WOT. It was the earlier rockered banana bottomed boats. At 8.5 knots visibility over the bow was completely acceptable and for over a year I cruised there .... At 2750rpm. I spent lots of time at that speed and rpm. When we took the boat to Alaska I dropped back to 7.35 knots and I don't recall why.

I suspect your boat would be happy at 3000rpm cruising for many years. But I'd still keep digging for better numbers on your engine.