My boat is now overprop

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cmedsailor

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After replacing my gear box with a different one (previous ratio 2.35:1, current ratio 2.05:1) and buying a new propeller I have ended up now being overprop (even though I bought a smaller prop). My MD2030D can now achieve maximum rpm 3050 instead of previous 3500 (3200-3600 recommended by Volvo). I now run the engine at 2200 cruising instead of the previous 2500 and get same speed and same consumption. I believe that’s reasonable though because now the prop loads the engine more and even though the engine rotates at currently lower rpm the load is the same as the “ex higher”. Even mathematically (OK that’s simplified) the shaft rotates at same speed (around 1070 rpm) since previous engine rpm of 2500 = current engine rpm of 2200 (2500/2.35X2.05=2200 approx).
Question: Am I harming my engine much?
Yes, I know the story with not having enough left rpm in case needed and like driving your car uphill on 5th gear but am I causing any damage? Is 3050 out of 3500 too much overprop? If I run the engine hard (2900+) it smells a little bit and don’t know if any black smokes will eventually arise because I never had the courage to leave it a full rpm for more than a minute (neither in the past at 3400-3500). There must be some black smoke though, but not much visible, because the area where the exhaust exits the boat is now a little bit dirty.
What do you think? Leave it like this? Decrease the diameter or the pitch by 1 inch (I think it’s more wise to have a 15X9 prop rather than a 16X8)? My main concern is any damage to the engine.
Thanks
 
cmedsailor said:
After replacing my gear box with a different one (previous ratio 2.35:1, current ratio 2.05:1) and buying a new propeller I have ended up now being overprop (even though I bought a smaller prop). My MD2030D can now achieve maximum rpm 3050 instead of previous 3500 (3200-3600 recommended by Volvo). I now run the engine at 2200 cruising instead of the previous 2500 and get same speed and same consumption. I believe that’s reasonable though because now the prop loads the engine more and even though the engine rotates at currently lower rpm the load is the same as the “ex higher”. Even mathematically (OK that’s simplified) the shaft rotates at same speed (around 1070 rpm) since previous engine rpm of 2500 = current engine rpm of 2200 (2500/2.35X2.05=2200 approx).
Question: Am I harming my engine much?
Yes, I know the story with not having enough left rpm in case needed and like driving your car uphill on 5th gear but am I causing any damage? Is 3050 out of 3500 too much overprop? If I run the engine hard (2900+) it smells a little bit and don’t know if any black smokes will eventually arise because I never had the courage to leave it a full rpm for more than a minute (neither in the past at 3400-3500). There must be some black smoke though, but not much visible, because the area where the exhaust exits the boat is now a little bit dirty.
What do you think? Leave it like this? Decrease the diameter or the pitch by 1 inch (I think it’s more wise to have a 15X9 prop rather than a 16X8)? My main concern is any damage to the engine.
Thanks
Click to expand...

Always swing the max dia you can & adjust the pitch to suit the dia.
rule of thumb
add an inch of dia reduce the pitch by one inch & vicky verky
 
Diesel engines need to be run well loaded i am told
So of fuel consumption is ok for same speed then i would have thought that was ok
My brunton is over pitched & fuel consumption at 6 kts went from 1.6 litres per hour ( fixed volvo prop on saildrive)to 2 litres which is quite a lot
However, under propping is worse in a chop or when you need drive in a big sea
 
If your prop works without any issues beyond the rpm, you should stay with the same prop type, design, material and number of blades. You should focus on pitch since the corresponding dia is normally decided by the manufacturer (Volvo or any good brand).

Each inch change in pitch will change engine rpm by 200 (as a rule). Your engine need to reach the recommended rpm at wot range (you aim for the upper limit with normal load of crew and gear etc.) as only then will you know that it is working under acceptable load conditions Struggling at low rpm & full throttle really is not good for the engine - and it will not allow you to enjoy it's capabilities (think about it... it is at the recommended rpm your engine produces max hp - if not there, you won't get max out of the lump).

You may have the prop altered or buy a new. IMO you should stay away from plastic/composite props. Tempting but not worth the money.
 
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In my view, if your engine cannot achieve max power rpm, you have less thrust available should you need it in a minor crisis such as running aground or needing to punch waves and tide.
A boat that will not rev out in flat water is possibly going to be a long way down on thrust.
OTOH making a good cruising speed at medium rpm can be very economical. And quiet.
 
lw395 said:
In my view, if your engine cannot achieve max power rpm, you have less thrust available should you need it in a minor crisis such as running aground or needing to punch waves and tide.
A boat that will not rev out in flat water is possibly going to be a long way down on thrust.
OTOH making a good cruising speed at medium rpm can be very economical. And quiet.
Click to expand...

I tend to disagree with that a little
If the prop is pitched correctly then no problem
However my first brunton was under pitched & whilst i could get 7 knts at 3500 revs in flat water as soon as i hit chop i found speed dropped to 3 kts ( yes i know that this is not a conventional prop so does add certain perameters that confuse the issue)
As i had hit max revs there was no extra power available & the boat slowed
Bruntons changed the prop to a bigger pitch & now i can only get 2900 max revs
However i am getting a lot more push
So i suggest that as long as the engine can rev at optimum torque you should be ok which is a bit like a car engine - max torque is not usually at max revs bit a little lower.
It depends on the power output range of the engine
 
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cmedsailor said:
True but which will make more sense? 16X8 or 15X9?
Click to expand...

both would give similar rpm but the larger dia more blade area to push against the water

16 x 8 reduce the dia by 1" means adding an inch to the pitch so equals the 15 x 9.
to raise the rpm on the 16" prop reduce the pitch

I have a MaxProp so quite easy to alter the pitch. when i re engined from a Perky 4108 to a Nanni 4150 i had to increase the pitch to get just under 3k rpm
 
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The forum is full of advice but the propeller supplier if he is responsible for supplying against the full specification is the one who should be sorting he problem!

Ask them what they will do
 
I have contacted the prop manufacturer and they claim that with my prop, according to their calculations, the engine should have been able to reach max 3200-3250 rpm (which is my requirement).
So if I had to look what limit then the engine to achieve these max rpm what could be the causes?
Exclude:
- Dirt on prop/hull (max 3050 was since the day the boat was back in the water with a clean hull and a shiny propeller)
- Blocked exchaust elbow (I have installed a brand new a couple of weeks ago)
 
The bigger the prop the greater its efficiency, so always swing the biggest you can, but if you decrease the pitch too much the prop will loose bite and increase slip. But their calculations or propeller selection programs (which you can get online) are saying what the computer says, there are too many variables in a boat. Most times the calculations are roughly o.k. but at the end of the day there is a bit of trial and error with propeller selection. You could stay with your current set-up until its hauling out time, but don't labour the engine. Increase the throttle till you hear the revs stop rising and don't go any further. Its very bad for engine but in a yacht its not as bad as a planning motor boat. You really need to reduce the pitch, but may mean going down in diameter as well, to keep a reasonable blade face angle so it does not loose bite. Remember a prop with a 20inch diameter and 9 inch pitch & a prop with a 15inch diameter and 9 inch pitch will both travel the same distance in one revolution. The pitch is not the blade angle.
 
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