Prop selection

matt h · 14 April 2008, 20:35

ok, after a run to teignmouth this weekend to try out the new prop ,findings were as follows.

Yamaha 90 2 stroke, peak rpm 5500.... 60 Litres of fuel and one person,( still could not acheive 40 knts even on 20 litres of fuel left) i did speak to prop developments and they recommended a 13.25 x 17 s/s prop!!! is this right ???

13x19 ally 5300 rpm and 39 kts
13x19 S/s 4700 rpm 33.5 kts

13x17 ally 5500 rpm 33 kts,

really would like to reach 41 kts, IS THIS DO-ABLE???

(cos chris's boat is still faster than mine)

Turbodiesel · 14 April 2008, 21:07

What boat is it?

doggypaddle · 14 April 2008, 21:08

matts boat is an avon searider 5.4m with a heavier than standard console.

Searider · 14 April 2008, 21:24

Only ever managed 39.7 knots in my 5.4 Searider. Outhill console. 2 persons. 100 litres fuel.

90hp 2 stroke Mariner 20" Laser II.

Reckon it might have gone a bit quicker if the engine was lifted an inch or so but sold it before I had an opportunity to try it.

Chris · 14 April 2008, 21:29

Quote:

Originally Posted by matt h

ok, after a run to teignmouth this weekend to try out the new prop ,findings were as follows.

Yamaha 90 2 stroke, peak rpm 5500.... 60 Litres of fuel and one person,( still could not acheive 40 knts even on 20 litres of fuel left) i did speak to prop developments and they recommended a 13.25 x 17 s/s prop!!! is this right ???

13x19 ally 5300 rpm and 39 kts
13x19 S/s 4700 rpm 33.5 kts

13x17 ally 5500 rpm 33 kts,

really would like to reach 41 kts, IS THIS DO-ABLE???

(cos chris's boat is still faster than mine)

They do say that a stainless prop normally needs to be a couple of inches less pitch than the equivalent Ally prop so I would say your next trial should be a 17" stainless prop, I reckon that will get you pretty close to your perfect set up and if it needs tweaking then send it to steel developments to get it perfect.

Interestingly enough great minds must think alike cause I have been doing some investigation into how I can get a couple more knots out of my set up and I spoke to Ralph the Outboard Wizard (in the US) about options.... he suggested that I should try the new BRP Rogue four bladed prop and that I should run the engine as high as poss on the transom.

Good for 45 knots I reckon

ian mcconnell · 14 April 2008, 21:46

Hi matt

where did you get all those props??

go with the steel developments recommendation, 13x17 titan stainless steel, ive seen 40kts @ 5100 revs approx 20ltrs fuel.

im suprised you only got 33knts with 13x17 alloy, i gone back on my standard prop while mines still a bit damaged and im seeing 35knts @ 5500 with plenty of fuel.

is it worth trying a second satnav unit to calibrate your speed

reinier · 14 April 2008, 23:18

When using a stainless steel prop, try to lift the engine as high as possible without causing excessive cavitation or too low water-pressure, it will give you more RPM's and thus more speed.

mike-stgt · 16 April 2008, 19:57

Hi Matt,
with a transmission of 2:1 , or 26:13, normally 1 inch of pitch means 500 1/min more ore less.
Mike

matt h · 16 April 2008, 22:30

thanks for the input guys, really appreciate it, going to Nick chris's prop while he's adjusting something or other!!

More Speed egor!!

mcinfantry · 19 June 2008, 05:02

what shaft length are your motors? 20" or 25"?

Nos4r2 · 19 June 2008, 09:21

20"

reinier · 21 June 2008, 13:47

Quote:

Originally Posted by mike-stgt

Hi Matt,
with a transmission of 2:1 , or 26:13, normally 1 inch of pitch means 500 1/min more ore less.
Mike

1" change in pitch causes only about 200 rpm less or more

TheAmusedDragon · 19 March 2021, 10:26

I know this really old thread, but did you every manage to hit 40Knots, ive just bought an Avon 5.4 mod spec, with a 90 tohatsu but i think its got the wrong prop on it 13.5 x15 prop on it, just wondering what i should change it to?

Searider · 19 March 2021, 16:27

Quote:

Originally Posted by TheAmusedDragon

I know this really old thread, but did you every manage to hit 40Knots, ive just bought an Avon 5.4 mod spec, with a 90 tohatsu but i think its got the wrong prop on it 13.5 x15 prop on it, just wondering what i should change it to?

Start with a 19".
cavitation plate about level with the bottom of the boat - it will be there or thereabouts anyway.

TheAmusedDragon · 22 March 2021, 09:31

Ok great. What brands would you recommend ?
What's your opinion on stainless steel?

Thanks jacob

Searider · 22 March 2021, 10:08

Quote:

Originally Posted by TheAmusedDragon

Ok great. What brands would you recommend ?
What's your opinion on stainless steel?

Thanks jacob

I'd start with a standart Tohatsu 19" aluminium and get a baseline from there.
I started with the 19" aluminium Mercury prop that came with the engine.
Fitted a 20" laser II Stainless steel propeller - as it came up for sale as a secondhand item. This gave the best performance. This was back in 2004 - so evel older than this thread!

TheAmusedDragon · 24 March 2021, 13:12

Quote:

Originally Posted by Searider

I'd start with a standart Tohatsu 19" aluminium and get a baseline from there.
I started with the 19" aluminium Mercury prop that came with the engine.
Fitted a 20" laser II Stainless steel propeller - as it came up for sale as a secondhand item. This gave the best performance. This was back in 2004 - so evel older than this thread!

Ok great , i will have a look out for a 19 ali,

Ive been looking on ebay and found a 19p stainless Laser II would that be alright or a 20pitch would be better?

Many thanks jacob

TheAmusedDragon · 24 March 2021, 13:14

https://www.ebay.co.uk/itm/Yamaha-OU...Cclp%3A2334524

would this work?

Limecc · 24 March 2021, 13:33

Quote:

Originally Posted by TheAmusedDragon

https://www.ebay.co.uk/itm/Yamaha-OU...Cclp%3A2334524

would this work?

I don't think so. Here's what I would do and already did.

Call Steel Developments and see what they recommend.
Maybe it will be a 17p as mentioned here in post#6.

Buy from them, see how it goes and they will adjust it one pitch up or down for free. This eliminates any risk of you buying the wrong expensive prop.

The other thing you can do is to work out your existing prop slip at various rpm and max revs which may have a bearing on the type and choice of prop. You made sure the engine height is optimal didn't you? Cavitation plate level would be the lowest, then try one and two holes up and hopefully still won't ventilate.

Try not to be obsessed with top speed or bragging rights, the boat spends only a fraction of it's life at WoT (and when the conditions allow), mid-range response and grip (and fuel economy from your thirsty 2T) are far more important imho.