Prop to Crank HP rated engines?

[jedimaster]

I am just wondering what would the effect be of switching from an older 70hp(crank rated) to a newer 70hp(prop rated)?
Would it be a big engough HP increase to be able to go up a pitch or two in prop sizes? Or would the effect be negligable?
For discusion sake lets say a 1981 70hp johnson to a 1995 70hp johnson

 

[Dhadley]

Re: Prop to Crank HP rated engines?

Well, remember torque is what moves a boat (turns the prop) not hp. You're comparing a 49 cid motor to a 56 cid motor. And then you'll have to figure in set up. And hull design.

Lets assume you're talking about a good hull design that will respond to small increases in torque. If both are set up to the max you probably will see a little difference. If the older motor was set up poorly and the newer one is set up to the max, you might see a little more difference. However, the reverse can be true too.

Then don't forget the NMMA's 10% rule on advertised horsepower. That can have an effect either way too depending if you believe either motor's rating is conservative or optimistic.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

Do you know what year the 70hp johnson switched over? Was it '88 when the new regulation came out or earlier?

 

[Dhadley]

Re: Prop to Crank HP rated engines?

The 56 cid motor came out in 86 as a 65 and 70 hp motor. OMC did continue to use the 49 cid motor until 88 as a 75 hp motor. All of the 3 cylinder motors were so close to the same it really comes down to set up & hull design. If you're replacing a 49 cid 3 cylinder with a 56 cid 3 cylinder on the same boat with similar set ups, I wouldn't expect to see a night and day difference.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

The 56 cid motor came out in 86 as a 65 and 70 hp motor. OMC did continue to use the 49 cid motor until 88 as a 75 hp motor. All of the 3 cylinder motors were so close to the same it really comes down to set up & hull design. If you're replacing a 49 cid 3 cylinder with a 56 cid 3 cylinder on the same boat with similar set ups, I wouldn't expect to see a night and day difference.

What was the difference between the 49cid 70 and the 49 cid 75? Carbs, or just jets?

 

[Dhadley]

Re: Prop to Crank HP rated engines?

Depends on the year. Some had slightly better port timing but you'd have to have it on a pretty fast V bottom or a tunnel boat to see any difference. There was a difference in heads but again, it's hard to tell the difference.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

Cool thanks. Always nice to learn about our old motors.
As an aside would you say I am getting good performance of of my '81 70 hp johnson or poor?

I have a 17 pitch 3 blade prop, on a 15 1/2 foot stratos bass boat(1993)

at Wot on plane I am sitting at 33.6mph and 5400 rpms
This was over a 1 km run in both directions.

I am having problems getting on plane unless I move weight forward(passenger). The obvious thing would be to drop a pitch to ay a 15 but then I think my rpms would be too high.

I ordered a 4 blade 17 pitch aluminum and I am hoping the 4 blades will give me a bit more punch in the bottom side without changing much at teh top end.

Is it typicial for a setup like this to be so bad out of the hole or do I have motor issues? Compresion is even but low on my compresion tester at 107-110 accross the thre cylinders.

 

[Dhadley]

Re: Prop to Crank HP rated engines?

5400 is a bit low on top for a 3 cylinder looper. Therefore the bottom end / holeshot probably will be a bit less than what you'd like. You'll be a bit limited with aluminum props but try playing with height to gain top rpm. That should help the entire rpm range. If your prop(s) are not vented you might try that too. Generally speaking if the 4 blade is the same make & style as the 3, it'll probably like a little more height anyway.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

My problem is the whole shot stinks. I was told by an Iboats rep not to buy a Stainless prop, but to buy a 4 blade alluminum as a 70hp won't be able to spin the SS prop, and it needs the extra flex of the alluminum to be able to get slip to get out o the hole.
Do you think I should have gone to a SS prop?

 

[Dhadley]

Re: Prop to Crank HP rated engines?

That's a new one on me. There's no reason a 70 (either of them) won't turn a ss prop just fine. I sure hope he doesn't tell a lot of folks that.

Anyway, there's lots that can be done to improve a holeshot. First you want to make sure your motor is set up at or very near 6000 rpm. If it's not then we work on the set up to get it there. As the top end improves with set up, so will your holeshot.

If you're at max set up and still need more bottom end you can play with the venting of the prop. There are a couple other things to look at but always start with the basics. Make sure the motor is healthy and running as it should. Then move on to set up.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

Do you think my 70 should be turning more rpm's with a 17 pitch prop or should I just be running a 15 pitch, and go from there? I am sure if I drop to a 15 pitch my rpm's will pickup but I was a bit worried of running close to 6k rpm's on an older motor, I was under the impresion motors should stay in the mid range of the max rpm's but it sounds like these loopers should run at the high end?

 

[guy74]

Re: Prop to Crank HP rated engines?

A vented prop really helps out with the holeshot on those 3 cyl motors. It helps make up for the lack of low rpm torque. I run a vented 15" pitch prop on my 1988 60hp johnson with good results. On my 15 ft trihull it gives me enough holeshot to deepwater start slalom skiiers, ans still gives a WOT of 34mph @ 5950rpm.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

Sounds good. Since I have a 17 pitch alluminum 4 blade on order, I will drill my 3 blade 17 pitch and see what happens. What size holes should I make?

 

[Dhadley]

Re: Prop to Crank HP rated engines?

Like I said, 5400 is a bit low for top rpm. As you change your set up to get close to 6000 your holeshot should get better. I wouldn't drill any holes before moving the motor. But you'll be moving the motor up anyway since you're going to a 4 blade.

Or didn't the prop wizard mention that either?

 

[imported_Jimbo]

Re: Prop to Crank HP rated engines?

Hello guys!

Since I basically have the same problem (70hp Johnson-97, 5400 @WOT with 17 pitch on a 18 hull, doing 34 mph at WOT) I?ll add a question to your thread, hope thats ok.

If I trim the engine long enough the rpm goes to 5600 without any speed increase (prop starts to come close to surface?), how long should I be able to trim, on the gauge I?m way passed the horizontal line before the engine starts to increase RPM at WOT, is this a sign that the engine should be lifted?

Thanks!

Jimbo

 

[Dhadley]

Re: Prop to Crank HP rated engines?

Jimbo, we see that as a sign of a couple of things. First, yes that generally means the motor can go higher on that hull with that prop. Second, it indicates you need a lot of positive trim to help carry the bow. Generally a prop with bow lifting characteristics will help. Also some setback (changing the balance point) might help.

 

[imported_Jimbo]

Re: Prop to Crank HP rated engines?

Thanks Dhadley,

Do you recommend another prop, or should I try to lift the engine first? How high can I go? Generally the boat is not changing its performance much with load, if I have an empty tank and is driving by myself I might hit 5500@WOT and get 35 mph, and with full tank and four people in the boat I get 5300@WOT at 33mph... If I get another prop (running a Michigan 3-blade Vortex), do you recommend to test an ss 17 or should I go for 15?

Any risk that the engine is hurt by low top RPM at 5400? Like the way the boat performs in the water... BTW the engine is just broken in after re-bore (12 hours), but it should perform fully at this point in my mind...Still running some extra oil(1%), since I suspect that the VRO uses too little oil (Just filled up the oil-tank so I can run a full gas-tank and verify the oil-gas ratio).

Thanks!

/Jimbo

 

[jedimaster]

Re: Prop to Crank HP rated engines?

No problem I don't mind a other on topic stuff and since I ballooned my own thread to other things I really don't mind.

Since I only have one more hole to go up on the transom and thats only gonna give me 1/2 and inch or so, I have no choice but to go with a jack plate right?

 

[Dhadley]

Re: Prop to Crank HP rated engines?

We try to gain all the rpm we can with set up rather than go to a smaller prop. That way you're increasing efficiency rather than decreasing by going to less pitch. Sometimes we do have to go down in pitch but we always try to max out the set up first.

Jedi, I'd go up the last hole and see what happens. If you do indeed have to go to a jackplate that will open a whole nother world of opportunity.

 

[jedimaster]

Re: Prop to Crank HP rated engines?

So, I think I will going to pick up a jackplate tomorow. 6 inch setback 4 inch of vertical lift. Will be manual. According to the stratos tech guy I talked to he said the ideal height is 3 inches below the pad so I will start there. Anything else I can do?