Prop Size MD21b

jamesyboyz

jamesyboyz

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Hi guys a bit of advice needed,

I’m running two Volvo MD21B engines in a 30foot 10 ton boat.

I am struggling with very high revs to get the thing moving not to mention the fuel I’m burning.

The props are 13.5x10 recently cleaned along with the bottom.

Speed and revs with 1:93 gearbox

5kts @ 2500
6kts @ 2800
8kts @ 3200
9.5 kits @ 4200 (and overheats)

Do I need bigger props as I’ve looked online and seen that these are about right? I am desperate for better fuel economy and more speed at lower revs.
 
4200 is over reving a md21which is a fairly old engine now and would indicate that the props could be bigger however The horses on these engines were always a bit undernourished though sold as 75hp
I would suggest that you take the likelihood that these units will not have much life left in them before spending more than maintenance costs on them
Measuring fuel consumption should give an idea of actual HP is produced
They were commonly used in such 30ish foot semi displacement boats that needed more like 2 x 100hp to plane
 
If the engine is over revving the props can't be correct and increasing pitch would help. You need to be careful that you don't overload the engine by over propping though.
 
jamesyboyz said:
......... My thinking was to increase the pitch to bring the revs down and save fuel?
Click to expand...

Engines does not move the boat, the propeller does ..... so irrespective of the RPM you run at, it is the thrust that the propeller generates which moves the boat ... to generate the thrust, the propeller gets loaded and the engine governor responds to the load..

So to move your boat at 8 knot, the propeller have a certain load (resistance to push through water), the engine governor give the fuel needed based on the load on the propeller ... changing the propeller (size or pitch) does not alter the load it will take to move the boat through the water at 8 knots ... The only thing you will achieve is spending money to run the engine at lower rev, but at same load ... ie take out same HP and thus burn same amount of diesel...

In respect to overheat (common issue on the MD21's)... Heat main generated in the engine is a result of the combustion... heat therefore is a result of Fuel / Air volume in combustion chamber ... so by bringing down RPM, but maintaining the Fuel/Air mixture in the combustion chamber, you are reducing the volume of coolant circulating in the engine (coolant pump also run slower) ... this results in same overheating at lower rev...

If overheating is an issue, it indicates that the cooling system need some TLC ... if you want higher speed, then resolve this first them be prepared for a higher fuel burn as you will take out more HP of the engine(s) as load increases as water resistance does...
 
At the risk of being shot down in flames, those engines in that size boat are either too small or the boat is a displacement boat.

What boat is it? And have you ever exceeded those speeds. i.e. is there a real problem or just a failed expectation.
 
BruceK said:
At the risk of being shot down in flames, those engines in that size boat are either too small or the boat is a displacement boat.

What boat is it? And have you ever exceeded those speeds. i.e. is there a real problem or just a failed expectation.
Click to expand...

Hi thanks for the reply. the boat is a Princess 30DS semi displacement. It has 13 knots shown on a journey on the Garmin from previous journey prior to me buying this. (ive only had it a couple of months).
 
A quick look about shows these came with a variety of engines from ~60hp to ~160hp presumably to reach different markets, river and coastal. I'd be very surprised if 2 x ~60hp engines will push her any faster than she is going currently and that 13knts was either with current or downstream. I'll wait to be corrected though. On the plus side I'm finding going at displacement speeds more and more attractive of late and no bad thing.
 
Divemaster1 said:
Engines does not move the boat, the propeller does ..... so irrespective of the RPM you run at, it is the thrust that the propeller generates which moves the boat ... to generate the thrust, the propeller gets loaded and the engine governor responds to the load..

So to move your boat at 8 knot, the propeller have a certain load (resistance to push through water), the engine governor give the fuel needed based on the load on the propeller ... changing the propeller (size or pitch) does not alter the load it will take to move the boat through the water at 8 knots ... The only thing you will achieve is spending money to run the engine at lower rev, but at same load ... ie take out same HP and thus burn same amount of diesel...

In respect to overheat (common issue on the MD21's)... Heat main generated in the engine is a result of the combustion... heat therefore is a result of Fuel / Air volume in combustion chamber ... so by bringing down RPM, but maintaining the Fuel/Air mixture in the combustion chamber, you are reducing the volume of coolant circulating in the engine (coolant pump also run slower) ... this results in same overheating at lower rev...

If overheating is an issue, it indicates that the cooling system need some TLC ... if you want higher speed, then resolve this first them be prepared for a higher fuel burn as you will take out more HP of the engine(s) as load increases as water resistance does...
Click to expand...

Great advice. Thank you for taking time to help
 
Hi
Something doesn’t add up here. Going by your numbers somewhere in the middle. Presuming 2800 rpm, 10” pitch and 1.93 ratio...at 7.2 mph....you’re at 48% prop slip. This is amongst the worst I’ve ever seen. Is your coupling good ? Is the prop slipping on its shaft or key way/ taper ? Something is terribly wrong, unless I’ve missed something.
 
QBhoy said:
Hi
Something doesn’t add up here. Going by your numbers somewhere in the middle. Presuming 2800 rpm, 10” pitch and 1.93 ratio...at 7.2 mph....you’re at 48% prop slip. This is amongst the worst I’ve ever seen. Is your coupling good ? Is the prop slipping on its shaft or key way/ taper ? Something is terribly wrong, unless I’ve missed something.
Click to expand...

You've got your speedboat head on. Those are expected figures for a displacement / sailboat pushing the speeds he is climbing out the hole.
 
BruceK said:
You've got your speedboat head on. Those are expected figures for a displacement / sailboat pushing the speeds he is climbing out the hole.
Click to expand...

Not sure BruceK. Physics are physics. I took the mid range figures mentioned to get what should be a fairly efficient calculation. Prop slip is prop slip. And nearly 50% is just far too much really.
 
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