DuoProp sizing question on KAD42

[banflu1]

Hi everyone

So I’m looking at a boat (abroad) fitted with a single KAD42 and in a video from the seller I can see the boat making 15 knots at 3,200 rpm.
I felt like this is rather slow, knowing that the cost should make around 32 knots max.

I asked the seller about this and he says he doesn’t see it as an issue, and that it makes 26 knots max. Apparently it revs to 4,100 rpm max!

I think it’s rather obvious that it is underpropped; however he says that they had put new props on it 4 years ago when they bought it, and he won’t be replacing them again since he feels like it is performing well. Unfortunately he can’t remember what size they are. They look like J series though.

So if it is doing 26 Knots at 4,100 (max) with undersized props, will it make 30 knots at 3,800 with a ‘next step up’ prop set?
I know you “lose” around 300 rpm with every size up, but how does that affect the top speed?

I’m just trying to see whether this is a simple prop issue or whether there might be a bigger underlying problem, before spending money to go see the boat abroad.

Thanks

 

[MapisM]

I would neither spend money nor waste time to go see the boat regardless, 'cause silly owners are the real boat killers.
God only knows what else he did wrong and/or neglected.

 

[Portofino]

Overloading marine diesels, kills them .
Two ways often together but can be exclusive.
1- over propping ie too large usually pitch in duoprops but can be the diameter and pitch in others .
2- fouling of the stern gear props , usually associated with the hull .The hull to a lesser extend compared to the props .

Duo props set ups - your specific example are particularly susceptible to loss of top end speed from dirty props the 3200 from the alleged max .

They are easy to clean up , both in the water snorkel or lifting the leg and reaching over .Or diver or lift with survey if a bigger vessel .
Demonstration of a sick engine / power plant(s) in a mobo and fobbing off the inability to achieve WOT is a huge red flag .

The single biggest golden rule of the many buying “:rules “ with any mobo purchase is it should reach the manufacturers wide open throttle rpm , hold it for at least 10-15 mins without any drama .
Drama being overheats , alarms , change in colour of exhaust fumes etc etc .

You can get ( gets expensive ) engineers to calibrate the tachometer , laser the thermostat house , laser the sump / oil filter , laser the charge air coolers to verify the cooling systems are within specs .You would probably do this with the larger twin set that the engines cost £100 k + .

The principles apply to single KADs as well .


Having said all this old diesels esp marine do age with time loose a % of Hp , valve do not seat as well as 20 y ago , the fuel pumps get out of time a fraction. Injector spray patterns only go in one direction over the years etc etc .
Weight tends to increase with crushing stores and clobber .That nice glass like shine on the bottom disappeared with layers of antifoul.
So in time they can loose manufacturers WOT due to the above .

Manufactures and builders know this so in fact on test and sign iff slightly under prop them .
They say something like 3 % over WOT rpm should be a achieved to semi balance the above a few years down the line .

My 2003 , MAN 2876 le 401 s are rated 700 Hp @ 2200 rpm .
I can get 2270 rpm at max load when it’s just been freshly anti fouled .
I only do it once / year and nether bother until next time .
Rarely exceed 1950 rpm , tend to sit it all day at 1760 rpm which achieves 28/ 29 knots @76- 80:% load depending on fluids .carried .

The motors are nursed along , not over loaded .


One of the first indicators of chronic overloading ( over propped / fouled running gear ) is what you see no Hp or a drop off .
Not enough Hp to spin the props up assuming they were once correctly sized .This is because the excess EGTs - exhaust gas temps mushroom the exhaust valves.They work but loose that crucial 100 % gas fit and compression is lost = power loss .
Its not a binary bang fail it’s insidious- happens very gradually without owners noticing until they can‘t reach WOT with a clean boat , clean sterngear and no excess weight .Too late then the damage is done .

 

[julians]

What boat is it?

You might be able to find the official performance figures and with what prop for that boat and engine and see how far away it is from those - if at all

 

[Mr Googler]

When moving to J series prop you are supposed to go up a size

i.e…if oe was B5 then you go J6. Quite possible they replaced B5 with J5?

on my previous boat they did exactly that however, I liked the fact 20+ year old engines where less stressed.

New…on B5 props my boat was good for 38 knots (that’s advertising WOT so done light as a feather). On the J5 props it would max out at 34-35 knots @4k revs. Full fuel…loads of crap onboard etc…

it would cruise at 25 knots 3300 ish revs so 14 knots at 3200 does sound low but as already said…could be fouled, dirty hull etc compounding the issue. The hole shot on those J series prop is excellent.