prop choice 2 smaller and faster

floater · 13 March 2006, 23:09

hi i have a avon 400 with yam 30 running a 9&7/8 x 12 prop
i am getting max around 30mph on gps at a wot of 5000
and i am considering a pitch change up to a 13
what increase do you recon that this will give and will it efect plane time
as at the momement its almost instant, although speed drops 4 up to 24-26 mph

thanks andy

ps yes a 50 hp would be nice but missus wants new fire place

jwalker · 13 March 2006, 23:23

You'll go slower.

floater · 13 March 2006, 23:35

so what would i need to go faster as i seem to get conflicting reports on which way to pitch up or down
should i increase prop size and decrease pitch ?
or decrease prop size and increase pitch

Ta Andy

jwalker · 13 March 2006, 23:41

I don't think you can go faster. At least, not much. I can't remember what the max revs are for the 30 Yam but I'd guess at 5500. If that's a correct assumption, pitching down will gain you a couple of horsepower but it will also need some of the extra revs to make you go as fast as you are now. Then the few extra revs on top of that will be extra speed but it will be a small gain. I'll go and calculate for you...hold on.

Edit: I'll need your gear ratio cos It looks as though there is a bit of inaccuracy in your figures when I calculate backwards.

floater · 13 March 2006, 23:48

were would i find that in the manual or on the engine

Andy

found it 1.85

according to book wot is 4500-5500 with max hp @ 5000

jwalker · 13 March 2006, 23:52

The manual.

That's it, then. You're at max power as it is. End of story.

floater · 13 March 2006, 23:55

oh well thanks for help

guess itll be another cold winter then !

Andy

jwalker · 13 March 2006, 23:57

Still a problem with your figures cos at 1.85 you've got only about 3.4% prop slip.

I'd rough guess at about 20%.

jwalker · 14 March 2006, 00:00

Using your figures and 20%, it gives a boat speed of 24.5mph. If your GPS is accurate at 30mph, you must be pulling higher revs.

floater · 14 March 2006, 00:04

HI

No definently got 30+mph 1 up usually drops between 1&2 into wind but starts t o lift out of water more when fully trimmed up
main slowing is when laidened with wify,kids and toys

then as it says drops down to 24-26 on gps

Andy

but revs could and probably are out as only fitted tacho recently (bought of ebay)

jwalker · 14 March 2006, 00:08

Well you've got to check rev counter cos if your actually pulling higher revs, you may well get more speed with a higher pitch. You have to have accurate figures to do this stuff and my 20% is also just a guess.

floater · 14 March 2006, 00:12

well as i havnt got a bloody clue about pitch and revs etc

except that my dad once told me that the pitch was the distance that the prop would go through a sold substance in a vacum on 1 revolution

and judging by your threads you seem to have a wealth of info

thanks Again

Andy

roycruse · 14 March 2006, 00:13

JW - why do you estimate prop slip to be so high - i have done fairly extensive testing on my boat and found prop slip to be around between 8 - 11% when flat out but its the only boat ive ever done any testing on so i will bow to your greater experience if you have it.

based on this ive always used a nice round 10% when doing estimates...

jwalker · 14 March 2006, 00:41

10% is pretty good and with an efficient hull I'm sure it's realistic at high speeds.

For a general purpose motor pushing a wee boat I'm guessing 20%ish.

My previous rib had about 15-18% at cruising.

Actually, I have all the figures for my Volvo duoprops. I'll go and calculate....

Edit: approximately 15% slip. It's a bit tricky cos both props are not the same pitch.

That's at max speed. There's a good deal more slip at lower speeds.

ADS · 14 March 2006, 00:55

Floater is it an Avon Searider 4m or an Avon Sporstboat 4m?

regards

Alex

jwalker · 14 March 2006, 01:23

Quote:

Originally Posted by floater

well as i havnt got a bloody clue about pitch and revs etc...except that my dad once told me that the pitch was the distance that the prop would go through a sold substance in a vacum on 1 revolution

It's easy enough to understand and your dad was correct except I don't know why it would need to be in a vacuum.

With your gear ratio of 1.87, your engine will turn 1.87 times more than your propeller. So, for 5000 revs per minute (engine) your prop will spin at 2673.79rpm. (5000/1.87).

For a 12" pitch prop, it will move forward 12" for each revolution. That is a nice number because it equals 1ft forward for each revolution. We've just calculated that your prop is spinning at 2673.79 revolutions in one minute, so that must also be the distance the prop will move forward in one minute measured in feet. Since there are 5280 feet in one mile, the distance moved forward is 2673.79/5280 miles in one minute. In this case, by good luck, that's very close to ½mile in one minute. So, in one hour you will cover 30miles = 30mph.

Well, you would in a perfect world but the prop is not 100% efficient and also it will not screw through the water the 12" we have assumed.

My guess was that it would screw through the water approximatley 80% of its pitch and that is why I said it would have a slip of 20%.

It doesn't actually slip nor does it screw through the water but these terms are descriptive to aid the understanding of the calculations.

Actually, it might screw through the water a bit and it might slip a bit but let's not go there just now.

floater · 14 March 2006, 08:11

Hi Its an avon adventure 400 with rear bench, side console and seat and bow seat

we love it

iam have a few pals all with ribs 2 zodiac pro9s one with yam 60 4s and the other with suzi 50 4s , a bombard with tohatsu 40 and Zodiac medline 1 with suzi 2 s70hp

all great fun

also asumuing rev errors then of 10 % and on jw's calculations then with slip of 10% @ 5500 rpm using his figures then 30 mph is about right when at top of rev limit, so if i change to 13 pitch will this not give me about a 8% increase in top speed assumming that i can still hit max revs ?

Andy

jwalker · 14 March 2006, 12:11

Quote:

Originally Posted by floater

... with slip of 10% @ 5500 rpm

Andy, now you're into the realm of wishful thinking.

roycruse · 14 March 2006, 12:14

I have writen a prop calculator that you can download for free

See this thread http://www.rib.net/forum/showthread.php?t=13481

floater · 14 March 2006, 12:45

Quote:

Originally Posted by jwalker

Andy, now you're into the realm of wishful thinking.

or is it dreaming

just tryin to kep up with the pace

bigger engines then bigger boat then bigger car then smaller house bigger boat etc

Andy
used prop calc works a treat for me hopefull to get out this weekend and try out as just bought 9 7/8x13 from ebay for a tenner