1960 lark-40 ... what do I want my WOT RPM to be?

[surlyjoe]

I have been searching around for an hour, but i still can't find what RPM I am shooting or at WOT on my old '60 big twin.. TIA!!

 

[walleyehed]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

Paul Moir or Joe Reeves may know...let's see if we can catch them...

 

[jimmbo]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

4000 - 5000 was the full throttle range. Prop for the upper half of it.

 

[surlyjoe]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

Thanx Jimbo, thats what I was lloking for =)<br /><br />unfortunatly I got a free clymer manual, which sucks.. but I am too poor to own two...

 

[daveswaves]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

I run a 63 rude lark V 40 hp, 4200 rpm with 14p prop, seems to like it. Not much choice in these old props anyway, 13.25 P was the standard, I had a prop shop tweak mine up to the 14.

 

[walleyehed]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

4200 would be quite a bit low if the recommended RPM in 1960 was 4000-5000. I wouldn't run anything less than 5k at WFO....

 

[daveswaves]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

I agree, however these big twins wont rev over 4500 anyway, even when airborne! To prevent 2 cycle runaway one cyl is cut out if the intake vacuum gets too high (like when you are revving).<br />You would have to prop down to a 12 pitch to rev at 4500 and would be bouncing off the rev limiter (which is probably why you wont find a 12 pitch to fit these things.) Just my 2 cents, based on experience. Http://forums.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=28;t=036018#000012 another thread on props for this engine

 

[Paul Moir]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

12 pitch props are not uncommon. They were pretty well the standard prop for 28hp and 33hp outboards, and I think for a few years they even had 11 as the normal one for it. Heck, Iboats will sell you a new one 10"-14" in 1/2" increments.<br /><br />The problem is when 14.5" is not enough. Then you have to find (via ebay, etc) an old Michigan wheel 2 blade speed prop.<br /><br />I've often wondered about pressing the Big Twin hub into a modern prop. Even a thru-hub exhaust if necessary. When I've got the funds for it, I'll have to ask.

 

[walleyehed]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

A Competent prop shop could also "work" the prop for a higher amount of cup if the 14.5 is not enough.

 

[daveswaves]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

Glad you weighed in Paul, I have had similar thoughts about a modern prop re hubbed. Agreed Walleyhed, the thing is these props are really shaped differently than the new ones, apart from pitch, cupping etc the form of the blades is "unique".

 

[walleyehed]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

I understand the blades, that's why I suggested a "Competent" prop shop...a great amount of cup can be added to these if the right guy is doing it, in fact, they are very affective with only slight changes...just a thought if you run out of options.....

 

[Paul Moir]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

There was a major change in the propellers some time around the late 60s. Here are a pair of deadites.<br />

<br />The left one is the old style which normally came in 1/2" increments. That one is 12.5". The right one is the later style that come in even increments. That's a 12". I think the later ones would be much easier to cup, especially considering how much thinner and more flexible the blade material seems to be.<br /><br />Don't get me wrong, adding cup is a great and practical idea. But I think you're missing the real point here Walleyehed. We ,and by we I mean I, all want to spend the money we "saved" by ressurecting a Big Twin on shiny stainless propellers. <br /><br />These stainless props would be mounted to gearcases that posess the hydrodynamic properties of a loaf of bread, fed by engines producing up to 40hp. I thought I should be clear about that so you know the mentality of the people you're dealing with here.

 

[walleyehed]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

I guess I'm an idiot, Paul....I was suggesting a way to increase bite in what I assumed to be alum props, which also works in SS props.<br />Excuse me for being so blind, but I am not understanding the point I missed..????

 

[Paul Moir]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

Oh, jeeze, I'm sorry. Those last two paragraphs were ment to be some self-depricating humour and certainly no insult to you, or even in any sort of opposition. They were ment to be read tounge-in-cheek.<br /><br />What I was trying to get across was that although adding cup was by far the superior way to improve performance and pitch on one of those props, some of us were after shallower things. That is to say, being able to show ff a stainless prop. This in spite of serious lack in other departments. (I was thinking of the mechanical shift gearbox. The electric shift if a fine shape.)<br /><br />The self-depricating part is in that last sentence where I ment to suggest that I was really more interested in putting 19" chromed rear wheels on a K-car rather than any real performance gains.<br /><br />I was trying to be helpful with the first part of my post. I thought the later style props would be more practical to be worked. I'm truely sorry the following joke came across as being a smart-arse.

 

[daveswaves]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

Good one Paul, Im sure Walleyehead is not upset, and your right, unless you own one of these things .......well you know. Its just that we would like them to run more like a 50 horse even though by todays standards it would be a 30 horse, and wouldn't it be nice if we didn't have to stick the lower end so far in the water, oh well. Im lucky I have the electric shift which is more like a french loaf instead of the standard loaf of bread, it really slices through the water nicely. <br />Good picture of the later prop, the big difference is the straight trailing edge which if cupped would hopefully provide more bite. The other thing that is not obvious in the pic is that the prop rake kinda fades away by the time it gets to the tip of the blades. Im gonna try a "new" prop

 

[walleyehed]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

I'm following now...sorry I was so blind that I didn't understand what you were looking for...<br />How about a highly polished Turbo TXP on the bottom end......

 

[surlyjoe]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

well, I found an NOS 2/2 airgyide tach on e-bay, should be here today or monday. BUT I was out on teh boat yesterday and when putiing it away I noticed some severe cavitation burn on outside inch or so of the blades, I wont gave any RPM numbers till I get the tach tho.. should I just wait till it gets here before I do anything?

 

[surlyjoe]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

heres a couple pictures<br /><br /> <br />

<br /><br />

 

[gatorred]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

i think for a big twin it's good but more cyl like 4 or 6 have more rpm's and bigger prop's thus move's more water, one prop that is older has more area to grip the water then the new one it get's more thust then the newer one. I had a forty,I put to much money in mine i put top end and new gear's in lowerend to i got a bigger boat now with a v6 on it now, but there good old motor boy i wish mine was easyer on gas to. <br />on burn is the back of boat 15 or 20 in, is it to high thus short shaft engine on long shaft boat if it is they make a kit for that to, it makes the motor longer, that on cavation burn the cavation plate above the prop below the part that bolt's on it should be even with bottom of boat if it is that's right. don't over look simple stuff

 

[surlyjoe]

Re: 1960 lark-40 ... what do I want my WOT RPM to be?

well I got burned on the Tach, I don't think its gonna work for my motor. any ideas on the cav-burn problem till I find a Tach that will actually work? TIA!!