Proper Propping (again!)

Simon B · 07 September 2004, 08:33

WOT RPM is ideal figure at the ne end of this band or bang in the middle or 3/4 through it? Also do you do your trials in calm conditions or moderate seas or the "expected conditions of operation".

Long running "club debate" as our boat has returned from diving having had expected heavy load on board but in good calm conditions it managed 5600 RPM. (5.5 Delta 100 hp Yam 4)

I felt this a little low given it doesnt take much to knock it off the plane and it labours to get back on it. Have looked at 4 blade props for better torque anyone out there running a four blade prop?

dodl · 07 September 2004, 08:49

Hello,

Tough decision, for me also. Currently i get 6000rpm instead of 5500. Of course not good for my 30 year old engine

On the other side, when i go with 4 adults, i have some trouble to get the boat on plane. But driving alone or with 4 persons, i always get nearly 6000rpm at WOT. Now what? Of corse i should go up with pitch at least 2". But then, will i be able to get the boat on plane when we are all in it? I doubt.

I'd say, for a diving boat i use a lower pitch. No need for the few mph more top end speed. I never tried 4 blade, but a lot of people i talked to, said they have much better performance out of the hole, but with the downside of considerably lower top end...

cu
martin

pellesmil · 06 October 2004, 18:59

Quote:

Originally Posted by dodl

Hello,

I'd say, for a diving boat i use a lower pitch. No need for the few mph more top end speed. I never tried 4 blade, but a lot of people i talked to, said they have much better performance out of the hole, but with the downside of considerably lower top end...

cu
martin

Agree,
have talked to some friends that went to a 4 blade, they where impressed by the cick and the low vibration, but in average the engine used more fuel and had a lower top speed.

do you have the instruction manual for your engine? if not you can view them here http://www.yamaha-motor.com/ under parts and service/view ovners manuals chpt 4 (i think) is about specs. rpm and so on

Pellesmil

PS.
bought a new Hustler prop. ( 3blade) with seperate prop ring and boss same dim. as my original well used but ok propeller 13-1/4" X 19"
Installed it and drove happy to the sea, but that was not the least funny, the rpm dropped maby as mutch as 800rpm

so there you have it, not all propellers at the same dim. and pitch are performing the same
My new prop was far more efficient with less slip (sorry for me with wrong pitch).
Enyone hwo have a 13-1/4" x 17 in the garage ?? ( PN: HU1317AM)

ashbypower · 06 October 2004, 19:30

Simon B, It's a question that is regularly asked.

Try Here http://go-fast.com/Prop_Slip_Calculator.htm for some more help.

Brian

Matt · 06 October 2004, 21:26

Yeh. The go-fast site is pretty good, especially if you have an existing set-up to compare against. Another useful one is http://www.mercurymarine.com/prop_selector

Turbodiesel · 07 October 2004, 19:55

You need to find out what the max Rpm is for the engine, then prop it to that. Your better off in calm seas when you doing it as you want to have a constant fast run to get the rpm right.
I have been doing some prop trials last week with a few four blade props. We found that you will loose some of you top end speed but your pick up will be better particularly if you have alot of weight in the boat and it will get you on the plane quicker.
You should also prop your boat to your usual load but you then need to be careful you dont over rev the engine if your out on your own in it!. The other thing to try is a prop that has a PVT system ( the small holes at the back of the prop) this helps get the engine rev up quicker by letting the prop cavitate slightly until it reaches enough speed to let the exhaust gases come through the flute of the prop (the bit in the middle). That system wont help you stay on the plane so much but it will help you get there a bit quicker.
ALternativly if your carrying alot of weight maybe a bigger engine is what you need!.

timboli · 08 October 2004, 04:44

My manual(yam 100-4 stroke) says full throttle operating range,5000-6000rpm.
A 19 inch prop sees 6100 rpm max,2 adults 1 child in calm conditions or 5800rpm,4 adults 1 child,a 21 inch prop sees 5600rpm(max before I get scared due to instability)2 adults 1 child and a max of about 5300,4 adults 1 child once again in calm conditions.
The bigger prop is more economical(slightly).
IMHO,if you intend to carry a high load occasionally but also drive lightly loaded,I would go for as close to 6000rpm as you can lightly loaded.
Both props BTW are standard 3 blade Yamaha.

Simon B · 08 October 2004, 07:35

Thanks chaps,

We have now bought a 15" 4 blade and can now test this is in identical conditions. I will post the results under a suitably descriptive title so they can be found by a search.

As a dive boat it runs heavily loaded and speed can be a disadvantage, we cruise about 20-25knots max. The previous 17" prop did not allow the engine to pick up quickly, this led to the boat wallowing off the plane and being pushed about by the stern in following seas.

We are hoping this can be reduced and that I've gone far enough in that we dont need the 13" that I'd initially looked at this would give about 6200rpm the 15" should give 5500(ish)