MD21b, and barnacles on prop.

[pandos]

Headed off yesterday, and beat with reefed main and foresail, into what must have been more than f6 with huge swell running, right on the nose. (at times seemed like an episode of Deadliest Catch.

Eventually with about 10 kts to go to our planned overnight spot I decided to relent and to motor on-wards.

Started engine, it seemed sluggish and would rev right off the scale, (4200 ish) until it was put in gear, it then only would manage 1900, it was rough and sounded terrible with a horrible vibration and we were only able to make 1.2 ks over ground (it was into wind and waves and tide).

Fearing the worst and in the interest of safety I turned tail and sailed back to where I had left from, arriving back at 12.30 am ( of course wind died soon after I turned tail)

This morning my son, put on a wet-suit and scraped barnacles from the prop apparently it was covered in them, this made an immediate difference and we had a huge improvement and most likely the engine is back to how it has been for years, it now revs to a max of about 2200 and trots along at 1800 which is what I would generally run it at.

Question, should the engine be able to rev up higher when under load ? and if so what do I need to look at ? it does not emit bad smoke or sound bad so I am confused. If it was an injector I figured i would get clouds of smoke, if worn out engine likewise I expect blue smoke, no oil being burnt.

 

[TQA]

Do you have a torque curve or max torque rpm figure for your engine? Your engine is at it's most fuel efficient running at max torque. So if you prop the engine to run a little bit above that you get good fuel consumption with a little left over for a headwind. But get a bit of fouling on your bottom or barnacles on the prop and your speed will drop off dramatically.

Do you reach hull speed in flat water with a clean bottom and prop?

 

[pandos]

So if you prop the engine to run a little bit above that you get good fuel consumption with a little left over for a headwind. But get a bit of fouling on your bottom or barnacles on the prop and your speed will drop off dramatically.

Do you reach hull speed in flat water with a clean bottom and prop?

I understand those points, my issue is not whether there is any usefulness in the prop rotating quicker, I am trying to identify at what point my engine under load should stop reving, and what would be a likely cause of a drop off in engine power.

 

[Tranona]

Difficult to say without knowing the maximum permitted revs set by the governor. 4200 sounds right without load, but you need to check with the specification for the engine. Achieving only 2200 under load suggests it is way over propped. With the correct size prop you would expect to achieve around 4000 rpm and cruise at 3000. It may well be that your boat is vastly overpowered and it has been deliberately overpropped.

 

[Steve_N]

The pleasure version of the MD21B should be governed to 4500, 'other installations' to 3000.

To give you some figures to compare to we are 9.42m LWL and about 10 tons displacement and get the following speeds swinging a 17x11 3 blader.
5.6 knots@ 1500rpm
6.1 knots @ 1800rpm
6.75 knots @ 2200rpm
7 knots @ 2400rpm

It will rev to about 3000 under load but with no worthwhile improvement in speed.
We tend to tootle along at 1800 which gives both a nice relaxing engine note and a lovely sounding bow wave and also doesn't immerse any of the transom.

 

[pandos]

The pleasure version of the MD21B should be governed to 4500, 'other installations' to 3000.

To give you some figures to compare to we are 9.42m LWL and about 10 tons displacement and get the following speeds swinging a 17x11 3 blader.
5.6 knots@ 1500rpm
6.1 knots @ 1800rpm
6.75 knots @ 2200rpm
7 knots @ 2400rpm

It will rev to about 3000 under load but with no worthwhile improvement in speed.
We tend to tootle along at 1800 which gives both a nice relaxing engine note and a lovely sounding bow wave and also doesn't immerse any of the transom.

Yes those are the kind of figures that I remember and expect to achieve, but cannot get revs under-load up past 2200, 1800rpm was definitely the sweet spot as regards note, speed and economy.

My problem is not over propping or anything like that, as no changes have been made to the combination.

Tony.

 

[jamesyboyz]

Those props appear to be rather big. I am running 2xMD21B on 13.5 x 10 pitch. I am revving at 3200 to get 8kts and 4200 to get 9.5kts.

Fuel consumption is high so I am wondering if I should enlarge the props to be somewhere near yours? I need as much speed as I can get though