Yamaha prop Nuts!!!

matt h · 07 December 2009, 18:05

i want to achieve a higher speed, at the moment i can get 40 knots at 5400, is that the limit??
currently running a 13x 19 pitch ally Yam K series Prop.

Handbook says 4500-5500 rpm, But its a lighter boat than the searider 54 by 80 kgs, (approx worked out).

what size stainless prop would achieve a higher top end,
I tried a 13x19 stainless and got 34 knots and 4900 max on the searider, so no go there.

if i tried a 13 or 13.3/4 by 17 pitch, would that do the trick??

does the larger prop help or hinder??

Downhilldai · 07 December 2009, 18:15

Did I read that you've just dropped the motor down on the transom by 1 hole?
That isn't going to help you achieve higher top speed.
If you're getting 5400rpm now, then it sounds like you're just about there, with your current set-up.

matt h · 07 December 2009, 18:38

brad, yes just dropped the motor one hole and handles so much better, i will refuse to break out under heavy cornering and trim down really pushes the nose down now, so i can get more speed in a heavier sea,

the speed has increased, weirdly?????

also it now revs to 5400 which is 100 short of its max stated rpm,

i just wonder if its to do with the engine running in, getting slacker.. now done 49 hours

9D280 · 08 December 2009, 08:06

If it's pure top speed you're after, and don't mind cr@p accel and the prop letting go at the slightest provocation, Yam used to do a series of props that were higher pitched but smaller diameter - the increase in torqure needed to turn the higher pitch was offset by the drag reduction of the smaller diameter.

Works better on a lighter boat, as it's not needing to push so much load. My old Yam 55 had one when I got it - it flew in a straight line on flat water, but was about as much use as a chocolate teapot for cornering & in big waves.

Hightower · 08 December 2009, 09:14

I'm no means an expert, but I'd have rough weather handling and speed over top end any day.

You're always going to compromise one set-up with another to a degree. On my BWM I used to run two different sized props. One for calm conditions and light load and one for a heavier load or where I needed the handling and acceleration but didn't need the top end.

As far as I can see there are two easy options for you.

A Jackplate or lighten the boat as weight is a big killer of speed. I lost 4 knotts from the first run to the time I had Venom kitted out.

PeterM · 08 December 2009, 16:57

Quote:

Originally Posted by 9D280

as much use as a chocolate teapot for cornering & in big waves.

No use at all to Matt then !

Poly · 08 December 2009, 17:31

Quote:

Originally Posted by Blackroady

No use at all to Matt then !

From the photos he seems to spend most of his time ot the water, in which case one of these might be better: http://en.wikipedia.org/wiki/File:Pr..._1500x2100.jpg

Seriously though if top speed is the priority is it better to get air borne (loosing drive but having no resistance from the water) or to keep the prop in the water (maintaining drive but with some drag)?

matt h · 08 December 2009, 17:54

now thats a question!!

if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???

bottom line is that if she is revving to near maximum RPM and 3-4 knots short of desired speed (love to acheive 44 knots on the flat) then i guess im looking at upping the motor hp, which has been discussed many times i know with limited results.

hmmmm , what to do!!

chewy · 08 December 2009, 18:01

Quote:

Originally Posted by matt h

now thats a question!!

if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???

bottom line is that if she is revving to near maximum RPM and 3-4 knots short of desired speed (love to acheive 44 knots on the flat) then i guess im looking at upping the motor hp, which has been discussed many times i know with limited results.

hmmmm , what to do!!

Twin 90's!

Chris · 08 December 2009, 20:25

Quote:

Originally Posted by matt h

hmmmm , what to do!!

Give up numb nuts maybe if you filled the boiler with some more coal you might get a few more revs outta that old stove.

Poly · 08 December 2009, 20:34

Quote:

Originally Posted by matt h

if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???

I'm no expert but that seems counter intuitive - if its a bigger diameter it will be harder to turn, and so the revs will drop? lower revs at same pitch = lower speed. So unless your current slip is quite bad then I'd guess no.

Downhilldai · 08 December 2009, 20:40

Quote:

Originally Posted by matt h

hmmmm , what to do!!

What's the max hp rating of the boat?

matt h · 08 December 2009, 21:52

Downhilldai · 08 December 2009, 22:42

Well, in terms of power to weight, at 90hp, it doesn't get any better than a 2-stroke Yam.
You could perhaps get in touch with the guys who race these motors in 3b. They may be able to offer you some tips on set up.

9D280 · 09 December 2009, 08:12

Quote:

Originally Posted by matt h

if i increased the dia a quarter of a inch to a 13 1/4 x 19 ally, would that give me more beans???

Increase in diameter will give you more grip on the water, but also will increase the drag.

Quote:

Originally Posted by matt h

(love to acheive 44 knots on the flat)

Drop the diameter, up the pitch. Unfortunately there are no easy guestimators for diameter / revs like the "1" pitch = approx 200rpm" rule of thumb for pitch.

jyasaki · 09 December 2009, 15:35

Quote:

Originally Posted by Polwart

Seriously though if top speed is the priority is it better to get air borne (loosing drive but having no resistance from the water) or to keep the prop in the water (maintaining drive but with some drag)?

Both, I think. As I recall, the big offshore racing powerboats try to get just the prop in the water, with the hull riding on a cushion of air. Probably easier to do with >1000hp on tap, though.

Quote:

Drop the diameter, up the pitch. Unfortunately there are no easy guestimators for diameter / revs like the "1" pitch = approx 200rpm" rule of thumb for pitch.

I've heard 100rpm per 1/4" change in diameter. Don't know how accurate that is, as I haven't found diameter adjustments that fine in props I can actually afford.

jky

9D280 · 10 December 2009, 08:04

Quote:

Originally Posted by jyasaki

I've heard 100rpm per 1/4" change in diameter. Don't know how accurate that is,

I think it depends on your starting diameter - as your prop gets bigger, 1/4" increase will give you a proportioanally higher area increase.......

jyasaki · 10 December 2009, 15:35

No doubt; same as pitch - it doesn't always follow the 200 - 250 per inch rule, especially when making large changes.

But, as a starting point, it's probably close. Closer than "no idea", anyway.

jky