Prop Sixe

[Boatafloat]

Hi All,
to carry on with the prop conundrum, I think my boat has the wrong prop fitted it is a semi displacement cruiser 28 ft weighs 4.5 ton has a Iveco 8141 130 hp in board vee drive with Hurth 2.1 gearbox max revs 3800 but only acheives 3200 revs and gives 8 knts. I know the hull is clean but not had the boat out of the water to check prop size yet.
What are your thoughts and does any one have anything similar to compare size of prop with and speeds as I would like to get it up to 12 Knts if possible.

thanks for any help

 

[Spare Thyme]

Hi and welcome
You need to determine what the rated rpm is, possible that the out of gear 3800 ( sometimes revered as the high idle) is correct and the 3200 in gear is the rated rpm. All marine diesels should have the 2 numbers on the plate. Over propping a marine diesel is bad for engine life, as is under propping.

Boatdiesel.com is a good source of info including propping info, however I could not find the 8141 listed, can you have another look at the plate and see if the 2 numbers are listed

 

[Boatafloat]

Hi Spare Thyme
The engine spec is 4200 rpm but run at 3800 on marine engine.
If that makes sense

 

[Spare Thyme]

Is it the SRM13?

 

[Spare Thyme]

If this is the one, level 2 membership of boatdiesel will give you access to the prop calculator for your set up

 

[Boatafloat]

Hi Spare Thyme
yes thats the engine I will have a look on boatdiesel for the prop caluclator

thanks for your time

 

[QBhoy]

Hi
I’d maybe be looking at the other things before ending at the prop...unless you know it was changed at one point for some reason.
Things like intercooler cleaning. Air filters and turbo health etc. Leaking injectors another one...if you are certain the hull is clean.
All the above will drop you a few hundred rpm.
Was it reaching your desired rpm previously ? Most boats won’t be perfectly propped and usually the engines that are rated at a single max rpm are only making a statement on its max rpm. It’s usually a limit in this case. The engines that are rated with an rpm range are suggesting you should be in this range though.
You could spend a fortune with trial and error with props if you are only a couple hundred or so off it. Then there is the inevitable risk of a drop in performance, cruising ability and not to mention fuel consumption, should you go down in pitch.