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Author Topic:   Propeller for 70-HP Johnson
jtwhaler posted 02-21-2004 02:39 PM ET (US)   Profile for jtwhaler   Send Email to jtwhaler  
[Moved thread from another forum.] The 70-HP Johnson after overhaul turns a 13 3/4 x 15 prop somewhat over the 5500 max WOT. Previous owner installed this prop. The book says 13 1/4 x 17. I now do not have but about 1/4" clearance between blade tip and the antiventilation plate, after installing the Bob's Machine Stabalizer plate. I have studied prop data a lot, but data never takes the place of loads of experience such as you guys have. I have not seen any data on how close prop blade tips should run relative to the antiventilation plate. How close one can run the prop relative to the antiventilation plate?
Sal DiMercurio posted 02-21-2004 03:38 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
A quarter-inch is the closest you can be to your antivent plate without getting disturbed water to the prop and venting or added slip.
Also, if your engine is mounted high and your antivent plate is an inch or two above the bottom, a stock OEM prop will break loose and ventilate where a performance prop will still bite and not slip.

If your engine is mounted too low right now, you can raise it a hole or two and gain a couple more hundred RPM, then add single cupping to reduce the RPM back down, thus giving more thrust or compacting the thrust into a tighter blast (more push, like contricting the water flow of a fire hose), giving more top end speed but giving up a bit out of the hole.

If you are now twisting a 13.75 x 15 aluminum at over 5,500 RPM you might consider going to a 13.25 x 17 SS Stiletto for $285.00 and raising your engine up to be sure you get back up to 5500.


wetdogsoup posted 02-21-2004 11:01 PM ET (US)     Profile for wetdogsoup  Send Email to wetdogsoup     
How do you go about choosing the diameter of your prop?

Do you choose the largest diameter that still gives you roughly 1/2" of clearance?

Is there a performance gain (and if so, what kind) by maximizing the diameter?

Thanks, don

jimh posted 02-21-2004 11:46 PM ET (US)     Profile for jimh  Send Email to jimh     
Generally the propeller designer varies the diameter of the propeller in concert with the pitch and the intended motor it is designed for. The diameter is also varied with the intended application. For best speed the diameter is probably kept smaller, while for more low-speed power the diameter is increased. The propeller designer really picks the diameter.

In some cases with raked propeller blades you might have to change the sacrificial zinc anode to provide clearance, too.

whalersman posted 02-22-2004 12:22 AM ET (US)     Profile for whalersman  Send Email to whalersman     

I do not know what boat you are running this on but I had a 1985 70 hp Evinrude on a 1978 Montauk. I had 2 props... A 13 3/4 x 15 pitch which gives a better hole shot and a 13 1/4 x 17 pitch that gave better top end speed. The 15 pitch gave me 5500 rpm WOT and the 17 pitch gave me around 5300 rpm WOT... Both of these props were standard OMC which are not considered high performance props by any means...

The book for my Evinrude called for the 17 pitch but also had optional props to choose from which included the 15 pitch.. If you are over reving, then you should go to the 17 pitch with less diameter..

You cannot go with any larger diameter prop then the 13 3/4 Diameter as you and Sal mentioned but if you go with the 13 1/4 x 17, this will give you more clearance, not less....

Joe Kriz

Sal DiMercurio posted 02-22-2004 12:23 AM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Diameter equals power to get or hold the boat up, pitch equals forward thrust to push the boat forward, or speed.
For instance, a 10 inch dia prop wont be able to get a 21' revenge up on plane but would be fine on an 11 ft boat , a 13.75 to a 15" dia would go on an 18' or larger boat or an engine with the largest gear case & hp from 150 to 300 hp.
This is just basic rule of thumb because racing props are a different animal all together.

jtwhaler posted 02-22-2004 07:40 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
Jimh, whalerman, sal, etc. I thank you for the propeller inputs. I'm running this Johnson 70 on a 1966 BW Nauset. I had a good 1/2 inch clearance with the 13 3/4 x 15 Turning Point prop till' I added the Bobs Machine plate,(BT model) I hope the plate will help lift me out of the water sooner, and when the engines gets more time on it, meaning less oil in the gas I expect it will be over the max WOT with that prop. I am going to try the 13 1/4 x 17 turning point and will let you all know what happens. I know speed and power and money all go hand in hand. At my age a reliable, stable, 'good ole Whaler' with reasonable performance to get after catfish is ok. I do like this group of folks on continuous wave! jtwhaler
Sal DiMercurio posted 02-22-2004 10:16 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
JT, you mounted that plate on the wrong side of your antivent plate, it goes on the top of your antivent plate, not the bottom.
JohnJ80 posted 02-22-2004 11:26 PM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
Just for reference, I run a 15 pitch (I believe either 13 or 13.25") Stiletto SS prop on my 003 Evinrude FICHT/DFI 75HP on my Dauntless 15. At 5200 -5150 RPM, I get 41mph by GPS averaged over 2 one mile runs (on recipricol (sp?) headings). I actually think that with the rake, pitch and cupping, this is acting like a greater than 15" wheel.


Sal DiMercurio posted 02-23-2004 02:26 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
JT, did you see my last post to you ?
You mounted your fin on the wrong side of your antivent plate, it goes on top of the plate, not the bottom.
After you put it where it belongs, you will have plenty of clearance for your prop.
jtwhaler posted 02-23-2004 09:40 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
SalDiMercurio, before you make an ass of yourself, you might simply refer to the web site and look at the BT installation. It is obvious you know little or nothing about what you speak. I hate it when people put themselves up as something that they are not. I put the plate where it belongs! You have made other comments that are out of place and inaccurate at least. You need to review your approch to these forums and make sure the information you present is accurate.
jtwhaler, a long time boater!
Sal DiMercurio posted 02-23-2004 10:38 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
JT, I know of no application of any type of fin or tail that would attach to the lower or bottom of the antivent plate.
Every dole type tail i'v ever seen has been mounted on top of the AV plate, from what you explain, this one is on the bottom & obviously is causing you some concern, as it would me.
The reason is, it cuts down the clearance of the prop & could cause desturbed water to cause the prop to ventilate.
Why would you choose this type of fin over the more popular type.
No reason for you to loose your cool, looks like I'm going to learn something here to, just as I do everyday, because I most certainly do not know everything.
If you think I was trying to belittle you, your wrong, I thought I was helping you & thats what this board is all about.
I'v never heard of a Bobs machine stabilizer plate, but assume it's just another tail to put you on plane faster or stop porpoising.
Sorry you got upset, that isn't or wasen't the intent.
whalersman posted 02-24-2004 02:51 AM ET (US)     Profile for whalersman  Send Email to whalersman     

I looked on Bobs machine and saw the item you referenced... I have never seen one quite like that... Sal, it does mount underneath...

jt, I never had any problems with the hole shot on my Montauk with the 1985 Evinrude 70 hp... It was almost instant... Now a friend of mine has a 1980 70 hp Evinrude on his Montauk and his is much, much slower.. His engine was rated at the Power Head and mine was rated at the prop... We have tried the same props, different props, nothing seemed to help his hole shot...

What year engine are you running???

Joe Kriz

Bigshot posted 02-24-2004 11:20 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Doel-fins, etc on 15 7 17 Whalers are NOT necessary unless grossly underpowerd in my opinion....sell it.
jtwhaler posted 02-24-2004 02:06 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
Sal, thanks for the come back and sometimes this ole boater gits riled up. No offence ment, just a verbal uproar. The plate looks good on the motor-it is just that the 13 3/4 prop is not the stock prop. I'm in a learning mode too, even though in the 60's I rigged and sold lot of BW's out of Marin County, CA. Had some of the best teachers at that time includin Dick Fisher.

Whalersman, experimentation is my second middle name! If I see something that has possibilities in making my experiences more fun, I often go for it. I do think has some good products, and this BT plate is well made. By the way, the 1977 Johnson is a 70hp loop charged and getting stronger everyday.

Bigshot, if I sold everything I ever owned in my 70 plus years of life that did not seem to work well, I'd probably still be riding my mule! Thanks for the input anyway.

Bigshot posted 02-24-2004 02:10 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Yeah but you would have a crap load of cash my friend:)
jtwhaler posted 03-01-2004 08:57 PM ET (US)     Profile for jtwhaler  Send Email to jtwhaler     
Hey Bigshot, I do have a crap load of cash already! Just kidding of course. I have a reasonable enough education to enable me to make things work better, especially when other folks give up. For example. The Stabalizer from Bobs Machine works just great. What was a fast out of the hole for a 70hp johnson gets up there even quicker. Stays there too with less speed. So maybe old is not better, but old Whalers just need TLC. Works for me. I do enjoy the great things yo'all have to say.

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