Spetz

Hey guys,
I need to get a prop that works for my boat/engine but I haven't the slightest of clues about them, and was hoping someone can help me out

Ok, here are the stats:
Boat is a 930mm deep V hull, weighing at 2.2kg
Motor is a OS 15 CV-R with a marine head. It's claimed output is 1.2hp and I would say in real life terms it's probably inbetween a Aqua .18 motor and a OS 18 CVR

I need a prop is 4mm shaft, which I've found. I know the standard prop is 41mm (I measured it) and it doesn't rev too high with it and does not move fast
I put a 48mm prop and it didn't rev at all
And a 39mm prop which just revved with no result
I have no idea about pitch

The props I found have pitch stated in mm
For example, a graupner prop states: Diameter 36mm, Pitch 50mm
What does this mean?
And roughly, what do I need?

Any help is greatly appreciated, please do help me!

Spetz

http://www.offshoreelectrics.com/products.php?cat=60

These are the list of props I am looking at btw

Sean Bowf

I think I would try the grp-2318.34.5 prop. Diameter is 34.5mm and pitch is 48mm. This would be real close to a 1435. If the boat turns this one real well, I would try the next size up (grp-2318.36).

Sean

Spetz

What does the pitch mean in mm?

And, are you sure it's not too small? As I tried a 39ish mm prop (from Kyosho, unknown pitch) and all it did was rev out

Can I calculate pitch somehow by looking at the prop?

Sean Bowf

Pitch is often used in reference to "pitch inch" of a prop. The pitch inch is the diameter multiplied by the pitch.

The props lists a pitch of 48mm...so that would be 1.89 inch for us americans...I am pretty sure this is the pitch inch. Divide that by the diameter (1.36 inch) and you get the pitch of the prop blades, which is 1.3897 (1.4sh).

I am used to octura props...which uses the pitch and diameter for their part numbers. So an X435, is actually a prop that has a 35mm diameter and a 1.4 (x4) pitch.

Sean

Spetz

And how much of an effect does pitch have?

ie, considering a say 38mm prop just made the engine almost free rev, would this mean that a smaller prop (35mm like you mentioned) with more pitch make the boat go better?

Dan S

Spetz,

take for example an Octura 440/3 and 440/2, they both have the same diameter and the same pitch and remember guys I'm comparing stock props no mods.

lets use one of my boats as an example running a Novarossi 21 LS with a 3280 pipe on 35% nitro, it has no problems turning a 440/2 but cannot turn a 440/3 in its stock form, the difference between the 2 props, the extra blade.

Too keep the explanation simple, think of the pitch as the distance the object (boat) would travel per full rotation, so if a prop has 1.5 inches of pitch the boat should travel 1.5 inches for each full revolution of the prop.

Again, without talking about modification, if your boat can't spin a 40mm (diameter) prop with lets say 1.5inche of pitch you could get a prop of the same size with less pitch or get the next smaller size with the same pitch.

Look at a full size boat, using 3 different propeller all same diameter but with a different pitch, for example your boat came with a 14x21 prop (on a 115HP engine), this means 14inches diameter and 21inche of pitch.

With that prop your getting a Max speed of 57mph at 5900rpm's, now you put on an other prop (same dia) but with 19 pitch your results would be a gain in torque (fast out of the hole) and a loss of top speed and your rpm just went up, so lets say you want to it 60mph and keep your rpm at 5450, you could go with a different prop,
just a bit smaller like 13 3/4 and lets say with a pitch of 22 and now you have the numbers you were looking for (lets assume you tried the 23 already and that was too much prop).

hope I didn't cloud it too much for you.
and if the 38mm prop you tried had the same pitch has your 40mm and you were over revving or cavitating, you need to readjust your strut depth.

Dan.

MiniMental

in simple terms, pitch is the distance the propeller would move in one revolution... (although not as simple as this in reality as you have to add apparent slip, real slip.etc )

Regards
Aidan

Spetz

I see
That kind of made sense

The strut depth is not adjustable. but it's pretty much at the lowest point of the hull. I can turn the strut down (change the thrust angle) to lower it a bit more I guess?

TCHedOff

Dont change your strut angle unless your looking to force the nose of the boat down and lift the transom more - the strut angle is for adjusting how a boat planes (rides on the surface) not for addressing prop issues.

TCHedOff

Dont change your strut angle unless your looking to force the nose of the boat down and lift the transom more - the strut angle is for adjusting how a boat planes (rides on the surface) not for addressing prop issues.

If you have the $$, you can buy hardware that aloows for you to adjust the depth of the prop (give you more tune-ability).... generally deeper is better.

Spetz

Well it's a surface drive system
And I don't want to spend $$$ on it (it's not a great boat or anything)

It doesn't plane well though, the nose is too high up so I think I might need to adjust the strut angle anyway and hopefully it'll also help with the prop issues
Though somehow I don't think it's gonna be that easy