prop selection question

[valkie]

Hi Guys

I have a 40 HP evinrude, 1989 model

It has a 12 1/4 x 15 prop on it currently.

The motor is in reasonable condition for a 20 year old motor, runs well and seems to have plenty of power
it fitted to a 4.3 m savage runabout with marine ply floor and a full bimini
Two up it also carries abot 200k of fat people.

The problem I have is that it will only get to 4500 rpm flat out.
If I tilt the motor while underway until the prop starts to break the surface it will rev over 6000, so I know it can get the revs up even if it cant while the prop is submerged.

I have heard that running a motor with a prop that does not allow the motor to reach the recommended wot rev range is not good for it, is this true?

It get to about 28 knotts flat out and will sit there all day.
It takes off pretty good out of the hole and gets on the plane at about 3500 rpm (abour 15 knotts)

The recommended OEM prop is 11 3/4 x 17
I have 12 1/4 x 15

I also have some cavitation issues while cornering and have checked the cavitation plate to the keel, its about 1 - 1 1/4 ' below the keel, which is the recommended depth.

Would a smaller dia prop help me?
I was thinking 11 1/2 x 17 or 11 1/2 x 15 to get some revs up and take some load off the engine.
Also being a smaller dia it may help mitigate the cavitation as it would sit even lower in the water.

Has anyone experienced, tried or seen this solution??

Any help/ suggestions would be appreciated.

 

[Lyle29464]

Re: prop selection question

sounds like you have a handle on the problem. You must have an unusual set up. On most boats the anti-ventilation plate is even with the bottom or 1 to 1 1/2 inches above. I do not mind the venting in a hard turn. It tells you that your not wasting HP and speed by being too low. Sounds like a smaller prop will be the answer as long as your are certain the engine is well tuned. I would drop the pitch. If you know the gear ratio google a prop calculator to find the best size. ( my guess is 12X13

 

[steelespike]

Re: prop selection question

According to the calculater 28 knots (32mph) with a 15" prop 2.42 ratio 6% slip(a guess) rpm is about 5800.
Is the speed by gps? Is the boat about 14 feet? Tell us the details best guess dry weight, Best guess loaded at
28 knots.People,fuel,motor (230 lbs) gear.
Small boats tend to struggle with a load. So I feel 28/32 may be optimistic.

 

[valkie]

Re: prop selection question

Ill have to have a look at the gear ratio when I get home and have the manual handy.

The speed is by GPS on slightly choppy water (very small chop) on a lake with no water movement (tide runout)
In dead smooth water 27 knotts is about the limit.

I up (just me by myself) I can achieve close to 30 knotts, and the revs get to about 4750.

The boat is 14.5 feet, alloy, windscreen and bimini about half the length of the boat.
It has forward conrols, which includes a trim so there is a litttle more weight.

At 3000 rpm I get about 8 knotts, but the boat is not o the plane.
At 3500 rpm I get about 20-22 knotts and is on the plane
At 4000 rpm I get about 24-25 knotts and is running qiute nice
at 4500 rpm I get about 27-28 knotts depending on the water surface

Im not looking at any more speed, the boat is quite uncomfortable at 28 knotts on even moderatly rough water and most of my crusing is at about 20-22 knotts

All I want to do is make sure that the motor is not overstressed and perhaps reduce ventalation/ cavitation.
Im not sure about the weight, Ill have to do a best guess, Ill get back to you later

 

[valkie]

Re: prop selection question

Cant find out waht the gear ratio is, there are sebveral for my motor and Im not sure which I have.

The boat is registered as 300 kilo with the motor I assume about 400

 

[steelespike]

Re: prop selection question

You can remove the plugs mark the flywheel and prop and in gear count the flywheel rotations to turn the prop once.
My unvarified numbers I checked 84-2000 2.42. 2001 said 2.27
Best way is the check I suggested.
Calculator 5800 rpm,2.42 ratio,15" prop,6% estimated slip =32 mph.
4500 rpm,2.42 ratio,15" prop,6% estimated slip = 24mph .
Slip can vary about 4% to about 12% depending on the setup.
Usually as prop slows slip goes up.Very heavy or hard to push boats raise slip.
4 of 5 variables are needed to estimate the 5th. slip is an estimate the only way to
get a calculated slip is with the 4 other numbers

 

[topglock]

Re: prop selection question

One recommendation. Verify the tach readings "before" spending a lot of cash...

 

[steelespike]

Re: prop selection question

Speed is the by product of getting the motor to rev freely within it rpm range.
Your motor is better off being able to rev close to the max wot rating.
No need to run at that speed You'll find the motor performs best when propped so.
If your speed is by gps it appears your motor is allready reving freely.
for it can't make 28/32 at 4500 rpm.
We need to confirn your speed.

 

[valkie]

Re: prop selection question

Thanks Guys;

I have tried to get a smaller prop, with no success.
I ahve decided to leave it as is.
i get reasonable spped.
I get reasonable gas milage.
It appears to have reasonable power, so I will just leave it be for the time being.

If it gives me any trouble, Ill have another go.

Thanks again.