Prop choice for Evinrude 40p Lark3 '61

[LxAV]

Hi,

I destroyed my prop and need to find a replacement, sought out a dealer nearby and found schematics of parts he can order, now I need to choose between 4 props avaiable from these parts lists/drawings.

I'll try to give as much info as I can, so sorry if it's a long read...

Optional props:
10-1/2 x 12
10-3/8 x 11-1/2
10-3/8 x 13-1/4
10-3/8 x 14

Old prop was 10.2" (haven't got a clue how many x/xth that is, not used to inch system) also haven't got a clue wat pitch or if the original prop was a good choice to begin with..

Thru another post at this forum and the info I already know I gathered my engine specs are:
Evinrude Lark 3 40hp 1961 elec.start, & generator
132-159 lbs. (probably around 150lbs since I have both the starter and generator but no elec. shift)
WOT RPM is 4000-5000, with 40hp @ 4500RPM. Lower unit gear ratio is 12:21 (1:1.75).

My boat is at the waterline: 3,4m x 1,75m x 0,17m,
Mahogany plexwood with epoxy coated hull, seats 4,5 persons, carries max. 30liters of gasoline, 75Ah battery, tool box etc.....: total weight unknown.

With the old prop I could get to 36km/h max, but I don't have a tach so don't know at what rpm's...


Is there stil a way to advice which of the four above mentioned props I should go for?
Or maybe, can someone advice on more modern props that might be more efficient than these old models? -still needs to be mounted with the old-school (with the pin through the saft and plastic dome on top of that) method...

All advice or tips are greaty appreciated!!!

Cheers,
Alex

 

[steelespike]

Re: Prop choice for Evinrude 40p Lark3 '61

OK I'm not an expert but:: I'm pretty sure it will be difficult to find a special prop.And may be hard to find a regular one.A propeller shop may be able to repair yours.It's amazing what they can do.WE really need the full demensions on your old prop.diameter X pitch.Without rpms your gambling with the health of your motor Speeds outside your recomended RPM range( low or high) can damage your motor.Without knowing your present rpm I think I would go with the 10 1/2 X 12 prop.Then get yourself a TinyTach or borrow a shop tach and check your rpm.
Motor may run a little high but you can throttle back.Where as if your over propped you can't use the prop because regardless what speed you run at the motor will be lugging.I think the best bet is to try to get the prop rebuilt.
Then check your RPM and let us know your RPM and speed. GPS speed is best.

 

[LxAV]

Re: Prop choice for Evinrude 40p Lark3 '61

@steelespike:

Errr..... kinda late to get the prop rebuilt, I sort of put 15 parker screws through it so at least I could go cruising through the canals with 7km/h in this fine summer weather..

I know it's not the textbook thing to do but the prop was a gonner anyways.. lots of gaps and corrosion and sjeesh, with our summer season being so short over here I just couldn't stop myself... lol.. also, I live in Amsterdam and you'd be suprised how non-re-use or repair -willing the boat shops are here. they would've looked at it as if it were kryptonite.. Calling for a replacement to a dozen shops gave me " err.. that's really old, dunno, chech the dump." every single time.

I had to find the BRP part numbers myself and convince this shop he can order it like any other part before he told me I had to choose which one. I was hoping he could tell me but no such luck, that's when I came here where people enjoy sharing their knowledge..

So basically you'd recommend the 12" and getting a tachometer?

I'm installing a new instrument panel with a tachometer anyways but that could take a while, maybe even over this season..

I was thinking of going for the 13" but your story sounds convincing (could be because I'm easily convinced.. lol... )

2nd opinion? anyone?

 

[LxAV]

Re: Prop choice for Evinrude 40p Lark3 '61

Update:

Phoned the shop today and guess what... the 13"isn't available anymore, the 12" is, so I ordered that one.
Now I have a new question; how can I tell this is the right prop for my boat/motor combination ?

Do I definatly need a tachometer or are there engine sounds that I can go from?
I'm guessing of the pitch is too low the motor will be revving up too much, but how do I know when a lot is too much? If the pitch is to much, how can I tell it is so before overheating the engine?
Are there any trademark signs I should watch for when testing out this new prop and, how much of a difference is a 12" compared to a 13"?
You can tell I'm a newbie at this sorta thing but there's not a day that goes by without a thing or two to learn..

Cheers!

Alex

 

[steelespike]

Re: Prop choice for Evinrude 40p Lark3 '61

Really difficult to be accurate without a tach.Assuming the old prop was correct you could try to compare performance to the old prop as well as you can remember.If the boat seems really quick out of the hole you might want to be cautious about WOT.If it seems really sluggish then you may be over propped.Keeping in mind I'm not an expert.I feel the 12 in.pitch will be close perhaps a little on the light side.Perhaps the fact it is available may indicate it fits most applications.

 

[EricR]

Re: Prop choice for Evinrude 40p Lark3 '61

I ran a Lark of the same vintage on a 12 and a half foot runabout, I think I had a 10 3/8 X12" pitch on her, I recall it turned 4800 with two people in the boat. My top speed was 29mph and I think that was limited by the hull being a round chined design, but man oh man would it jump out of the hole!