Prop size?

[piperca]

Can someone tell me what props I will need when running sterndrives (single props) on a 28' boat that has a max loaded weight of 9500lbs, twin 150hp diesels, 4000rpm with 1.61:1 gear ratio? What would the approximate cruise/max speed be?

[Jango]

16 x 13 Props @ 26 MPH Max. Cruise about 18 MPH @ 3000 rpm

[piperca]

Thanks!

That's maximum load; I don't see me running that way all the time. I can see me dropping about 1000lbs when I'm not fishing. Will dropping the weight make much of a difference in speed? When not loaded, should I switch props to ensure efficiency and to avoid overloading the engine?

[jonr]

Most people size a prop based on being able to reach max HP output and max speed. But you don't have to do that - say in your case, a higher pitch prop that won't let you exceed 3600 rpm will give you better economy and longer engine life.

[Jango]

I agree with jonr. With 8500# gross @ 4000 rpm Prop needs to be 16 x 14 Speed increases to 28 mph.
Adding 1000# should Drop rpm,s to abt 3750 and max speed to 25 -26 mph

[piperca]

I don't think they make a 16x14 prop ... how about 16x15 ... or can a prop shop tweak a 13 or 15 pitch to a 14?

[Jango]

If you're fairly confident in having 300HP, go to the 15 Pitch otherwize use the 13 Pitch. Bronze or S.S. props can be repitched to 14 P., but I have not had much luck with the pitch staying as modified.
With 300 HP your max rpm will decrease to around 3700, even at 8500# gross. Shoudn't be a problem with Diesels.

[piperca]

It looks like I need to start out with the 13s and see how she performs. I can always invest in a set of 15s in the future, if need be. The engines are rated at 150hp, but that could mean 147 or so ... I'm sure it's not over!

[piperca]

Well, I have a slight change. Looks like I'm going to be running Volvo DP drives with 2.30:1 gear ratio.

What props am I looking at now and what kind of performance am I looking at?

[Karl2]

Based on the data in your post (28' and 9,500 lbs) you are looking at 35 to 37 mph WOT swinging A7 or A8 props.

If you can provide deadrise at the transom + mid ships and chine beam at the transom this could be dailed in with better precision.

I'm assuming these are AD31P - You should prop for 4,000 rpm. Prop for 3,600 and you MAY get some small cruise efficience improvement but you WILL reduce service life of the engines.

Karl2

[piperca]

Thanks, Karl.

The boat has 18 degrees deadrise, but I'm not sure of the other two measurements. The boat has a narrow beam of 8'0", if that helps.

[piperca]

Karl,

I purchased an outdrive (Volvo DP-E) that has a 2.32:1 gear ratio. The drive came with several sets of A8 props ... five, to be exact. My mechanic thinks these props are too large for my application and thinks that A5s might be a better place to start. What do you think and what is your reasoning? I looked back and compared the speed difference between using a single prop and duoprop drive and there appears to be quite a large gap there. Why would the DP give me such a performance increase?

[Tantalus]

Anyone have the formulas needed when working out what prop is best?

[Çemberci]

sizes

16 * 15 inc % 81 DAR 3 Bladed 24.7 knot

15 * 15 inc % 88 DAR 4 Bladed 25 knot

Oktay Çemberci

[gonzo]

Before calculating anything, you need to look at what the engine manufacturer recommends as top RPM range. That is the target. Anything else is a wild guess.

[Tantalus]

My engine should rev ideally 2100, but I can only get it up to about 1600. I'm told this is because she's over- propped and if I go for a smaller / different pitch, I can rev higher, go faster, for about the same fuel consumption? If this is the case, then I have nothing to loose by changing the prop. However, I don't want to just do it because some ol' fella says so.