Engine revs

Red Fox · 11 April 2004, 23:26

Handbook for the Suzuki 40 says engine revs between 5200-5800. On the rev counter it was showing 6000 at wot. Should I be concerned?

Turbodiesel · 12 April 2004, 15:52

Maybe if you want to make your engine last longer!.
I would think it proped wrong, would have thought going up a pitch will bring you to about 5800 rpm.

Red Fox · 12 April 2004, 17:24

Quote:

Originally Posted by Turbodiesel

I would think it proped wrong, would have thought going up a pitch will bring you to about 5800 rpm.

2 questions: 1) Does going up a pitch affect torque at the lower end or speed at the top end? The prop on is the one that came with the engine and is rated at 11.5 x 13, so 2) what would be the next one to go for? Thanks

behavin · 12 April 2004, 19:59

Upping your pitch will decrease your engine revs by roughly 150- 200rpm per inch of pitch.
Too much pitch will bog you down at take off and too little will give you massive acceleration but overrev the engine and riun your top speed.
You need to find a happy medium for the engine and boat.
try steel developments for prop advice they'll sort you out.

behavin · 12 April 2004, 20:02

sorry me again.
read your prop measurements try a 11.5 x 14 alloy or 11.5 x 15 stainless.

Red Fox · 12 April 2004, 23:34

What's the reason for two inch increase in pitch for stainless compared to one inch for alloy?

behavin · 13 April 2004, 20:06

I was told by my local dealer and it seems to ring true with the props i've swapped that the stainless prop has more momentum so will turn faster than an alloy one. so increasing the revs And the ss is also more efficient.
try steel developments they did mine and were very helpful.

xr770a · 17 April 2004, 09:42

My 60 Mercury Big foot has not reached peak revs yet with a standard 19pitch, stainless is it the way to go ?

Pete7 · 17 April 2004, 10:08

Quote:

Originally Posted by xr770a

My 60 Mercury Big foot has not reached peak revs yet with a standard 19pitch, stainless is it the way to go ?

Suggest you try a smaller Ali prop to get the revs up, then having found the right pitch you might want to think about S/S and keep the Ali for a spare. The boat will be faster and more responsive with the right size of prop.

There will be a gain in performance by going S/S but nothing like that gained by using the right size of prop.

Shame but just sold a S/S 17" prop that would have fitted.

Pete

roohairy · 17 April 2004, 13:33

I had a heavy 18' taskforce dory with 85 premix yamaha, came with 19" prop. It used to stall as you engaged gear when cold, it struggled to accellerate until it got to 1500rpm then was ok but wouldn't go above 5100rpm. Changed to 17" and all my problems gone, WOT 5400 (limited 5500) no change in top speed (not measureable anyway) but MUCH better accelleration and it didn't stall.
Incidentally the Yamaha dealer I was getting bits from used to race these engines, they used to remove the limiter and run them at 6500 rpm and didn't have any problems.

I got a propellor repairer to exchange my props for new pitched ones, cost £50 each but got the equivalent of brand new for old into the bargain! (and they were tatty)

swginn · 17 April 2004, 16:29

Have a look at this article on Mercurys website. Very helpful.

http://www.mercurymarine.com/everyth...ut_propellers6

JK, maybe it would be good to put this in the reference section of the forum. I don't know much about props, though so maybe you should check out the content first.

xr770a · 18 April 2004, 14:26

Quote:

Originally Posted by Pete7

Suggest you try a smaller Ali prop to get the revs up, then having found the right pitch you might want to think about S/S and keep the Ali for a spare. The boat will be faster and more responsive with the right size of prop.

There will be a gain in performance by going S/S but nothing like that gained by using the right size of prop.

Shame but just sold a S/S 17" prop that would have fitted.

Pete

Many thanks, if anyone has a 17" for a 60 Bigfoot Merc please contact, thanks for your help.

Red Fox · 18 April 2004, 18:20

Contacted dealer who sold me the boat, he said that they had tried a bigger pitch prop (the boat was ex-demo) but found it not to be as quick at acceleration and the top speed was the same so they switched the prop back to the smaller size. He said the rev limiter would cut in if there was going to be any engine damage and as long as I didn't run at WOT for too long there would be no harm done. I'll stick with what I have for now then. Interesting article on the Mercury link though.

steedthrust · 27 April 2004, 11:08

I have a 13 1/4" x 17" prop at the moment on a Honda BF130 on a ring 6.5m Rib.

The boat will only rev to between 3600-3800, speed 44mph. I have been advised to try a same sized prop with a 13" pitch. Owners handbook suggest a full throttle range of 5000-6000. Reading this thread that would give me a rev increase of (150-200 x 4") 800 rpm, to say 4600 rpm, should I be looking for higher revs or do you all think this may be enough.

Thanks for any comments. Steve B.

Pete7 · 27 April 2004, 16:39

Steve thats going to be an odd size of prop to find second hand. Most common second hand sizes for Merc / Honda boxes are like 17" upwards. If you buy a new one and get it wrong then its going to get expensive.

Also there is something wrong here. A Ribtec Camel which is much heavier 6.5m will reach WOTwith 17" prop using a Honda 130. Any chance that the throttle isn't opening fully ? or the engine needs a service? worth a check before you have to start buying props. It could even be the size of the fuel line restricting the flow, but my money is on the throttle not opening fully.

Pete

David B · 27 April 2004, 16:45

We have just tested two identical Parker 630 ribs fitted with mercruiser 1.7 TD in Poland one with a 21" prop and one with a 23" prop. Will post some of our findings later this week when i have had chance to analyise the data in full after speaking with Andre on this forum.

steedthrust · 27 April 2004, 17:00

Quote:

Originally Posted by Pete7

Steve thats going to be an odd size of prop to find second hand.

Also there is something wrong here. A Ribtec Camel which is much heavier 6.5m will reach WOTwith 17" prop using a Honda 130. Any chance that the throttle isn't opening fully ? or the engine needs a service? worth a check before you have to start buying props. It could even be the size of the fuel line restricting the flow, but my money is on the throttle not opening fully.

Pete

It was Steel Developments that suggested the prop size. Steel

I have one of the propulse props which i havent tried yet, was set on buying a new metal prop. I will try that on various pitches and see how i get on.

Engine has 100hrs and has been serviced twice. So discounting that maybe!

What diameter pipe should fuel line be?

Gear linkage is quite stiff so maybe routing for that and throttle cable is bad, stopping throttle from opening fully. How would I check (take engine cover off and see if throttle body turns to stops, if visible)?

Like you say the camel boats weigh alot more than my setup.

Thanks for the pointers.

Pete7 · 27 April 2004, 17:14

Steve, think the fuel line should be 3/8ths as you need to flow 45 lph with a honda engine at WOT.

Will the engine reach full revs in neutral ? or how about looking at the engine mechanism. Should be a lever with roller and a cam. You should be able to see if its not working fully or will go further.

Steel Developements have assumed that everything else is working correctly but something isn't quite right.

Pete

Matt · 27 April 2004, 17:23

Quote:

Originally Posted by Pete7

Will the engine reach full revs in neutral ?

Rather your engine than mine.

Dirk Diggler · 27 April 2004, 18:00

Quote:

Originally Posted by steedthrust

I have a 13 1/4" x 17" prop at the moment on a Honda BF130 on a ring 6.5m Rib.

The boat will only rev to between 3600-3800, speed 44mph. . Steve B.

The Honda Formula 4 Stroke race boats, which are a Ring 21 Sportscruiser use the 130hp Honda, and they ran around 49mph!