Advanced Prop Calculator on Boatdiesel Forum

[BillD]

Boatdiesel forum has a new advanced prop calculator.

For those of you who a boatdiesel members "give it a whirl".

Peter Compton, who owns/administers boatdiesel forum, is looking for input on the new calculator.

The calculator is more detailed than the old one on input data and output data.

FWIW, Bill D

 

[Powderpro]

Thanks for the heads up, I played around with it a bit. It's kind of cool to be able to pick your engine and transmission and custom input your hull factor.

As I've said before, I think if you are accurate with the info you put into the calculator, it is pretty accurate.

When I put my 38 Wesmac info in, the calculator is real good at predicting my top speed. One thing a guy needs to remember when using this calculator is it assumes you want 100% engine load at WOT. For my use, 92-93% is ideal at WOT. Some guys may want to see 96-97% load at WOT. Because the calculator assumes this 100% load, my experience has been that it over-props a little bit. The Calculator says my Wesmac should have 35" of pitch, I ended up with 30" pitch which gave me 88% load. I think 33" of pitch would have gotten me to 100% engine load. So like I said, the calculator almost nailed perfectly my top speed, but over-propped me a little bit. Also, and this isn't very important, but the fuel burn that the calculator says is also a little high. It said 29 gph at WOT, but in reality I think Deere says WOT is about 24.5 gph.

 

[BillD]

I'll be under propping the 370 (3000 rated) to 3200 to "de-tune" the available hp.

The new prop calculator illustrates exactly what Tony helped me calculate when decided on the 370/ZF6A 1.54:1 package

 

[Powderpro]

I'm assuming the boatdiesel calculator assumes a non-skewed prop. My Wesmac had a skewed 4 blade (DQX), and I bet the calculator is assuming just the standard 4 blade when it recommends 35" pitch. So maybe the calculator was spot on for my Wesmac if I wanted 100% load at WOT with the standard 4 blade prop. I've heard you should reduce pitch by at least 1" when going from the standard to the skewed prop (DQX). What size of prop and what kind of speed are you expecting from your boat?

 

[BillD]

I'm assuming the boatdiesel calculator assumes a non-skewed prop. My Wesmac had a skewed 4 blade (DQX), and I bet the calculator is assuming just the standard 4 blade when it recommends 35" pitch. So maybe the calculator was spot on for my Wesmac if I wanted 100% load at WOT with the standard 4 blade prop. I've heard you should reduce pitch by at least 1" when going from the standard to the skewed prop (DQX). What size of prop and what kind of speed are you expecting from your boat?

I have a spare set of DynaQuads for the Blackfin 20X22

I'll try one of these props...should be "close". Using the data from this prop I may go to a DQX. The DQXs on the Blackfin now are 20X20.55

I'm figuring 19 knots @ 2200 rpms

 

[bestfish]

What do you use for a hull factor?

 

[BillD]

Interesting question.

I bought the 25 T Jason as a "long sea trial test" to figure out if I wanted to move from a twin planing sportfish to a single keel DE. I ran the snot out of the Volvo (122 hp) that was in it so I had some real sea trial data for the calculator. 1.91 gear, 18X14 four blade, "cleanish" bottom, boat heay with 6 water barrels filled.

I kept changing the factor numbers using the known WOT speed and actual WOT of the Volvo until the calculator came up with 15 knots.

Ended up a factor of 3

 

[Powderpro]

What do you use for a hull factor?

For my Wesmac, planing hull with a factor of 5 is what I used and results in the most accurate numbers. It very slightly under estimated my top speed and slightly over estimated the prop pitch I needed. No calculator is ever going to be perfect, because there are just too many variables to capture. But it's really amazing if you punch in all the correct numbers, how close the calculator is.

 

[osprey47]

So other folks experience with the calculator is proving to be close to actual. I ran a BHM 36 through and am getting top speed of 14kts (Yanmar 6lya-ste w/ twindisc m5050sc and 4 blade) when the known top speed is 20kts. I played with hull factors but no real impact. Posed the question on boat diesel comments and will wait to see what kind of response i get. That said the calculator is an impressive iteration.

 

[bestfish]

I ended up with a factor of 6.4 after trial and error of known inputs on my previous engine. Then I added 1500lbs to my guesstimate total weight for the new one. It seemed pretty accurate but obviously it's a planning tool not an exact science. I ended up with 18500# (36' Harris cuttyhunk 6v92 dd,1.5:1 Allison, 29x25 no cup or a 24x29) gave me a cruise of 22 and top end 28. If nothing else makes me feel a little better about adding all that weight, I'm scared the boat will be a turd

 

[FPTMarineDiesel]

Used the new system based off of real data taken today on a sea trial

Hello all,

I used the new calculator based off of the sea trial on a repower today that we did. The calculator works well with exception to fuel consumption, but I am sure that is just an old general calculation. The calculator shows 3 gph higher than what we actually are burning at rated speed and that really is a large dispersion. But the rest of it was spot on.

Here are the specs if you want to try yourself:

Select planning-> Avg. for coefficient-> Known weight from scales today - 26000 lbs -> FPT-Iveco-> C13 825-> 641 bhp-> 2400 rpm WOT-> ZF 360A -> 2.192:1-> 4 blade -> 32" diameter selected.

It came out with a 32" x 34" wheel which is exactly what we used and a top speed of 23.2 knots which we achieved 23.5 knots so very close indeed! The hull was a 40' RP that did not have any lifting rails, with some lifting rails I expect the boat to gain at least 1 knot of speed... I will keep you posted as we plan on doing this very soon. The boat threw water from in front of the wheel house so badly at WOT that the water was spraying over the rails onto the deck the whole time.

Peter